Hydrophobic study of increasing alkyl chain length of platinum surfactant complexes: synthesis, characterization, micellization, thermodynamics, thermogravimetrics and surface morphology

Abstract

This paper contains details on the synthesis, characterization, physicochemical properties and surface morphology of five supramolecular metallosurfactants (SMMSs). The K₂PtCl₄ reaction with a series of cationic N,N,N-trimethyl-1-ammonium bromide (C_nTAB) surfactants yielded pink/green SMMSs. Electron deficient N⁺ of C_nTAB and electron rich [PtCl₄]²⁻ of K₂PtCl₄ coloumbically align to develop a chemical linkage. These SMMSs with increasing alkyl chains are abbreviated as MOTA, MDTA, MDDTA, MTDTA and MHDTA containing 8, 10, 12, 14 and 16 carbon atoms (n) respectively. The SMMSs are characterized by a CHN Analyzer, FTIR, UV-Vis and ¹H NMR. UV-Vis studies in DMSO, DMSO + water and DMSO + PBS (pH 7.2) predicted molecular sustainability. Their critcal micelle concentration (CMC) at 298.15, 308.15 and 318.15 K, calculated by conductivity and supported by surface tension measurements, has depicted self-aggregation which decreased on increasing alkyl chain length. Their increasing alkyl chain modulated their thermodynamic parameters which favours their micellization. Thermal decomposition and transition temperatures have facilitated their activation energy calculation with five different methods, which have depicted higher thermal stability. Differential Thermal Analysis (DTA) for heat flow predicted their endothermic and exothermic thermodynamics. Surface morphology and their particle size distribution analyzed by Scanning Electronic Microscopy (SEM), Atomic Force Microscopy (AFM) and Dynamic Light Scattering (DLS) respectively have confirmed their effective micellization on increasing alkyl chain length.

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1. Introduction

Currently, the functionalization of complex metal ions with surfactants has become a hot area of research due to their stable linkages. So, introduction of these effective functionalities develops a potential class of material termed metallosurfactant.¹

Metallosurfactants derived from transition and inner transition metals, have been functional materials in areas of biological and material sciences.¹ The origin of such materials with platinum-based halide salts (Magnus green and derivatives) has been of interest in the solid state, and catalyst chemistry with significant scientific advances.^2–6 These materials have been reported as potent sensitizing agents capable of inducing hypersensitivity in refineries⁶ due to their structural stabilities. Magnus green salt is a quasi-1-D [Pt(NH₃)₄]²⁺[PtCl₄]²⁻ material comprising linear arrays of Pt(II) containing both cationic and anionic parts for geometrical normalization. Later on, many soluble and processable derivatives of Magnus salts were synthesized by substituting hydrogen of ammonia by linear and branched amino alkanes.⁷ In some cases, the cationic part of Magnus salt was replaced by a quaternary ammonium (C_nQAB) group. The electronic linkages established between oppositely charged coordinating units lead to coordinate with salt into a quasi-1-D structure without bond formation.^1,8–11 Thereby, the C_nQAB is seen as an effective metal extracting agents of platinum group through an anion exchange mechanism.^12,13 The C_nQAB, a cationic detergent with surface-active configuration, facilitates its rapid and prolonged incorporation into cell lipid membranes.¹⁴ The C_nQAB as n-trimethylammoniumbromide (C_nTAB) has been frequently used as a preservative in several ophthalmic aqueous/non-aqueous solutions including nebulizer and nasal sprays.¹⁵ The C_nTAB is being considered useful for disinfectants, biocides, and detergents along with antielectrostatics having phase transfer catalytic activities.^11,16 Many advances about cobalt based surfactants, where, effect of hydrophobicity on protein binding, kinetic investigation of electron transfer reactions,¹⁷ interaction with calf thymus DNA,¹⁸ and their antiproliferative and apoptosis-induction biological activities against cancer cells are reported.¹⁹ Also, new class of metallosurfactants based on cobalt, nickel and copper and their thermogramatic studies are reported.²⁰ Recently, few studies of palladium, manganese and silver salts with cetyltrimethylammonium bromide and their material and biological applications are reported.^21–23 It creates a vacancy for designing larger studies with a series of surfactants. Metallosurfactants have different location for metals i.e. head group, tail or may act as counter ion with unique physicochemical properties.²⁴ Various metals based amphiphilic moieties are being explored for a numerous applications as catalysis, nanoparticle formation depending on the nature and position of metal present in metallosurfactants.^20–23 The studies reported till now about advancement of metallosurfactant only focused on single type of cationic surfactant with different metal ions.^20–24 No such studies are reported with a single metal ion along with a series of surfactants and their mode of linkage. The variation in C_nTAB are not reported by any researcher, thus a potential interface could not be explored. The conceptual studies about electronic linkage of metal ion vs. head group of C_nTAB have no clear understanding about their electronic neutralization and stability. So, all the reported study seems to be a set of assumptions where more new chemical entity formation is needed. Thus, to understand and explain a linkage, we have prepared a series of SMMSs with cationic surfactant (C_nTAB, n = 8, 10, 12, 14 and 16) containing platinum metal counter ion [PtCl₄]²⁻ via simple one step synthesis. The mixing of salts in a chosen stoichiometry (1 : 2, K₂PtCl₄ : C_nTAB) readily resulted a complex formation. However, it was refluxed and stirred for up to a 10 h reaction period at room temperature (rt) for [PtCl₄]²⁻·2[C_nTA]⁺ noted as SMMSs. We have attempted to look into an insight of a real mechanism for an establishment of linkage between [PtCl₄]²⁻ and 2[C_nTA]⁺. It seems that electron density of [PtCl₄]²⁻ is an integral one with a higher electron density whereas the C_nTA have another integral with a negative charge. So, both the integrals form columbic linkage leading to develop a normalized charge density acting as a bond for providing molecular stability. This new class of SMMSs with unique physicochemical parameters predicted stable micellization.²⁷ Despite effect of micellization and surface functionalities of C_nTA incorporating tetrachloroplatinate, no reports on SMMSs as ionic neutralized supramolecules are available. To our best understanding, no such studies with Pt salt are reported, except several transition metals with several cationic,^20–23 anionic^25,26 surfactants. So, we intend to report herein, synthesis of a series of [PtCl₄]²⁻·2[C_nTA]⁺ as SMMSs. Their characterizations are made with FTIR, ¹H NMR, UV-vis, DLS, SEM, AFM and TG-DTA. Their alkyl chain length increasing effect on micellization, kinetic and thermodynamic parameters are investigated. Our studies could pioneer a new research methodology and material with interacting engineering.

2. Experimental section

2.1. Materials used

Details of used chemicals K₂PtCl₄, OTAB, DTAB, DDTAB, TDTAB, HDTAB, DMSO and distilled water are reported in ESI Table S1.† All the reagents and organic solvents of analytical grade were used as received.

2.2. Methods

2.2.1. Synthesis of [PtCl₄]²⁻·2[C_nTA]⁺ (n = 8, 10, 12, 14 and 16). To obtain double alkyl chain containing [PtCl₄]²⁻·2[C_nTA]⁺, both the K₂PtCl₄ and C_nTAB constituents were separately dissolved in water. The C_nTAB solution was added drop wise in K₂PtCl₄ solution on constant stirring at rt up to 10 h. Their 1 : 2 molar ratio produced white/pink colour followed by precipitation but on completion of a reaction in ≈10 h no further precipitation was observed. Thereafter, the stirring was stopped and reaction mixture was kept overnight for a complete precipitation. The obtained pink/green precipitates (ppts) were filtered off and recrystallized several times with chilled water. The recrystallized products was kept overnight in vacuum oven for absolute dryness for characterization.

2.3. Instruments and detection methods

2.3.1. FTIR spectroscopy. Fourier transform infrared spectra were recorded on Perkin Elmer spectrum 65 instrument with KBr palate with polystyrene thin film of 50 nm as a calibration standard at 25 °C. The ≈2 mg SMMSs sample with 100 mg of KBr was mixed, grinded and pressed in a compressor for preparing pellets. The spectra were scanned against a blank KBr pellet background in 4000 to 400 cm⁻¹ with ±4.0 cm⁻¹ resolution.

2.3.2. ¹H NMR spectroscopy. ¹H spectra in DMSO-d₆ and D₂O (Sigma Aldrich, 99.99%) were recorded with a Bruker-Biospin Avance-III 500 MHz FT-NMR spectrometer for C_nTAB and SMMSs.

2.3.3. UV-Vis spectroscopy. Their molecular stabilities were determined with Analytical UV spectro 2060 plus spectrophotometer ranging from 200 to 600 nm in DMSO, DMSO + water and DMSO + phosphate buffer of pH 7.2 for a 1 × 10⁻³ M. Buffer solution was prepared using 70 mL of 0.1 M aqueous NaOH into 0.1 M aqueous KH₂PO₄. The pH of resultant buffer was checked with RS-232 modelled Cyber scan pH 2100, EUTECH pH meter. For analysis, 5 mL of 2 mM DMSO, DMSO + water and DMSO + PBS (pH = 7.4) solution of SMMSs were used for UV absorbance studies.

2.4. CMC determination

CMC values were determined conductometrically using a specific conductivity meter by calibrated conductivity cell with 0.1, 0.01 and 0.001 M KCl solutions.^28,29 The conductivity for OTAB, DTAB, DDTAB, TDTAB and HDTAB in 0.2 volume fraction of DMSO in DMSO–water were measured at 298.15 K. Also, the conductivity for each SMMSs in 0.2 volume fraction of DMSO in DMSO–water at 298.15, 308.15 and 318.15 K were measured within ±0.01 accuracy. The reason for using 0.2 volume fraction of DMSO in DMSO–water for a complete solubility of SMMSs as compared to water and 0.1 volume fraction of DMSO. The equilibrium establishment for a series of readings in 15 min intervals was checked until no significant change occurred.

2.5. Surface tension isotherm determination

Densities were measured using Anton Paar DSA 5000 M Densimeter, with the respective accuracies being ±10⁻⁶ g cm⁻³. The instrument was calibrated with water and dry air (DMA, manual; Anton Paar, Graz, 190 Austria). The surface tensions of pure surfactants in 20% DMSO were measured using Borosil Mansingh Survismeter (BMS), with ±0.01 mN m⁻¹ and ±1 × 10⁻⁴ mPa s accuracies respectively.^28,29 The constant temperature was maintained with Lauda thermostat with ±0.01 C accuracy. The accuracy and calibration of instruments were made with the corresponding literature values.

2.6. TG-DTA analysis

Their thermal stabilities were determined with a Mettler Toledo TGA 2 by heating samples in a 70 mL alumina pan from 30 °C to 1100 °C at a 10 °C min⁻¹ heating rate under 40 mL min⁻¹ an air flow.

2.7. Surface morphology and average size distribution

SEM images were obtained with EVO18-18-69 model ZEISS SEM. The sample was adhered to, to holder and then made conductive in a coater chamber of Au (80%) and Pd (20%) by plasma sputtering. Topographical studies were carried out with a XE-76 model, advanced scanning probe, Park system corp. Atomic force microscopy (AFM) was used with silicon cantilever in noncontact taping mode. The 0.1 M chloroform solution was put on a 10 × 2.5 cm² glass slide, and chloroform was evaporated in vacuum at rt for 30 min. On a complete evaporation, it was scanned for topography under a set phase and amplitude. The average size distribution made with a Microtrac Zetatrac, U2771, Dynamic Light Scattering (DLS) was used for SMMSs solution prepared in 0.2 volume fraction of DMSO in DMSO–water. Set-zero was done with 0.2 volume fraction of DMSO in DMSO + water, and then, SMMSs solution in 0.2 volume fraction of DMSO was filled in sample holder. For zeta potential and particle size standards, an auto-suspended alumina suspension solution (400206-100) was used.

3. Results and discussion

3.1. Structural characterization

The [PtCl₄]²⁻·2[C_nTA]⁺ formation is confirmed with CHN Analyzer, FTIR, UV/Vis and ¹H NMR and their structures are given in Fig. 1.

Fig. 1 Molecular structure of SMMSs.

The images of each SMMSs ppt are given in Table 1 and compared with literature.

Table 1 Literature of metallosurfactants and their comparison with SMMSs

Compound	Colour	σ (S cm^−1)	λmax	(M–Cl)	Reference
Magnus' green salt	Green	5 × 10^−6	290	311	7
Magnus' pink salt	Pink	n.a.	n.a	321
R = methyl green	Green	n.a.	290	n.a.
R = ethyl	Pink	n.a.	251	n.a.
R = octyl	Pink	n.a.	n.a.	319
R = 2-ethylhexyl	Violet	7 × 10^−10	292	306
R = 3,7-dimethyloctyl	Green	2 × 10^−7	310	303
Ic (C10 chain)	n.a.	n.a.	n.a.	n.a.	30
IIc (C12 chain)	n.a.	n.a.	n.a.	n.a.
IIIc (C14 chain)	n.a.	n.a.	n.a.	n.a.
Id (C10 chain)	n.a.	n.a.	n.a.	n.a.
IId (C10 chain)	n.a.	n.a.	n.a.	n.a.
IIId (C10 chain)	n.a.	n.a.	n.a.	n.a.
PdCTAC (C16 chain)	n.a.	γ = 34.5 (μN m^−1) at CMC		339, 360	21
MOTA (C8 chain)		2.026 × 10^−6 at CMC	275	—	Present work
MDTA (C10 chain)		3.138 × 10^−6 at CMC	270	—
MDDTA (C12 chain)		3.646 × 10^−6 at CMC	280	—
MTDTA (C14 chain)		4.003 × 10^−6 at CMC	275	—
MHDTA (C16 chain)		4.575 × 10^−6 at CMC	275	—

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Molecular formula, weight, % yield, reaction time and their colour are reported in Table 2. Also, their melting points increased on increasing chain length with their yields are found ≥80% (Table 2).

Table 2 Characterization data of SMMSs

Entry	Name	Molecular formula	Molecular weight	Reaction time (h)	Reaction temp. (°C)	Reaction solvent	Melting point	% yield	Colour	Elemental analysis
										Found (%)			Calculated (%)
										C	H	N	C	H	N
1	MOTA	C22H52Cl4N2Pt	681.55	10	rt	Aqueous	141.0	78	Green	38.77	7.69	4.11	38.88	7.82	4.33
2	MDTA	C26H60Cl4N2Pt	737.65	8	rt	Aqueous	141.2	81	Light green	42.33	8.20	3.80	42.51	8.42	3.98
3	MDDTA	C30H68Cl4N2Pt	793.76	7	rt	Aqueous	141.3	80	Pink	45.39	8.63	3.53	45.58	8.78	3.71
4	MTDTA	C34H76Cl4N2Pt	849.86	6	rt	Aqueous	141.2	88	Pale pink	48.05	9.01	3.30	48.34	9.23	3.48
5	MHDTA	C38H84Cl4N2Pt	905.97	6	rt	Aqueous	141.4	86	Dark pink	50.38	9.35	3.09	50.63	9.51	3.33

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On increasing alkyl chain length of C_nTAB their reaction time was decreased. A longer reaction time is observed for MOTA was decreased on increasing alkyl chain by an interval of two C atoms (ESI Fig. S1†). A shorter alkyl chain induces mild Brownian motions and needed longer time to develop adequate collisions for product formation. A decrease in reaction time and an increase in melting points on increasing alkyl chain length could be attributed to +I effect (induction) with hydrophobicity on [PtCl₄]²⁻·2[C_nTA]⁺ linkage (ESI Fig. S1†). ESI Fig. S1† depicts alkyl chain length effect on % yield and reaction time. An increase in reaction yield for longer alkyl chain and lower for shorter alkyl chain is observed due to effective collisions and Brownian motions in case of longer alkyl chain containing SMMSs. A shorter alkyl chain has lesser collisions. A higher reaction yield for SMMSs having shorter alkyl chain and lower for longer alkyl chain length could be attributed to less Brownian motion and higher collision and vice versa. The intersection point of ESI Fig. S1† is the point where we can develop a prediction that how the yield is related with the reaction time monitored by the length of alkyl chain. This point is named as reaction-yield point and explains which SMMSs gives the maximum yield in minimum reaction time.

3.1.1. FTIR spectral studies. FTIR stretching frequencies infer metal-surfactant linkage binding.³¹ IR stretching frequencies of C_nTAB are compared with that of their respective SMMSs. It is found minor differences between MOTA, MDTA, MDDTA, MTDTA, MHDTA as compared to OTAB, DTAB, DDTAB, TDTAB, HDTAB infer their nonbinding linkages. The 2915, 2848, 1468, 1380, 360, 339, 308, 294 [small nu, Greek, tilde]/cm⁻¹ of SMMSs with KBr confirm their structures (Fig. 2a–e).²¹

Fig. 2 (a–e). FTIR spectra of C_nTAB against SMMSs.

No shifts in stretching frequency of free surfactant as compared to their bound state with Pt are observed except a change in intensities. It strongly supports the non-bonding linkage with metal ion. Formation of new bands in 400–500 cm⁻¹ range of low intensity infer C_nTAB and metal ion linkages.²¹ As compared to C_nTAB, two intense and sharp bands at 2915 and 2848 cm⁻¹ infer asymmetric and symmetric stretching vibrations of –C–CH₂ of methylene chains, respectively. No change is seen in hydrophobic chains of C_nTAB on linkage with [PtCl₄]²⁻. So the –CH₂–CH₂– stretching frequencies at 1470 and 1481 cm⁻¹ remain unaffected. The 1392 and 1407 cm⁻¹ represent N–CH₃ symmetric stretching vibrations with a peculiar shifts confirming complexation.²¹ The lowering in the intensities of complex further infer non-bonding linkage between metal and surfactant. The lowering in intensities depicts hydrophobicity at stretching frequency of ⁺N(CH₃)³⁻ in free and bound state with metal ion.

3.1.2. ¹H NMR analysis. ¹H NMR spectra of C_nTAB and SMMSs were compared with TMS internal standard that confirmed their molecular structures. For example, a spectrum of C_nTAB showed a less intense singlet peak at 0.865–0.875 δ, assigned to its terminal –CH₃ protons of C_nTAB. This –CH₃ peak remains unchanged in SMMSs (ESI Fig. S2–S11†). It confirms a non-involvement of –CH₃ in complex formation due to its distant position from Pt–N linkage. ¹H NMR signals at 1.255–1.269 δ belongs to –CH₂ protons of –CH₂(CH₂)_nCH₂N(CH₃)₃ which remains unchanged in SMMSs contrary to decrease as compared to C_nTAB. The –CH₂ group of –CH₂(CH₂)_nCH₂N(CH₃)₃ is seen at 1.659–1.671 δ of C_nTAB which become less intense on linkage with metal ion and later remains unshifted. The –CH₂ proton of –CH₂(CH₂)_nCH₂N(CH₃)₃ in C_nTAB is observed at 3.264–3.304 δ which becomes less intense in SMMSs but remains upshifted (Table 3). Thus, a +I vis-a-vis comparatively lowering intensity with respect to C_n in SMMSs is weaker as compared to free C_nTAB.

Table 3 ¹H NMR of C_nTAB and SMMSs in D₂O and DMSO-d₆

Entry	^1H NMR	DMSO	D2O	DMSO	DMSO	DMSO	DMSO	DMSO	DMSO	DMSO	DMSO
		OTAB	MOTA	DTAB	MDTA	DDTAB	MDDTA	TDTAB	MTDTA	HDTAB	MHDTA
1	CH3	0.875	0.745	0.871	0.876	0.867	0.870	0.865	0.858	0.870	0.863
2	CH3–(CH2)–	1.671	1.208	1.666	1.661	1.668	1.670	1.659	1.666	1.675	1.669
3	(CH2)n	1.279	1.208	1.269	1.273	1.262	1.260	1.255	1.251	1.261	1.248
4	(CH2)n–N(CH3)3	3.304	3.181	3.271	3.226	3.270	3.267	3.275	3.264	3.287	3.266
5	N(CH3)3	3.069	2.982	3.049	3.038	3.043	3.048	3.047	3.038	3.056	3.044

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The weakening defines that the [PtCl₄]²⁻ links with 2[C_nTA]⁺ on attracting two units of ⁺N(CH₃)₃ for linkage, which has caused a much effect on electron cloud of C_nTAB. The proton of –CH₃ attached with –⁺N(CH₃)₃ in nTAB appears at 3.043–3.056 δ which is shifted to 3.038–3.044 δ on linkage with metal ion. An appearance of Zeeman effect is visible with SMMSs contrary to C_nTAB. It is excellent distribution of Pt charge effect with respect to +I which is never reported yet. It indicates an electronic communication between both cationic C_nTAB and anionic metal salt. The details of proton shift of pure C_nTAB and SMMSs are given in Table 3. Their spectrum is given in ESI Fig. S2–S11.†

3.1.3. Ultraviolet spectroscopy. UV-visible absorption spectra supports a metal surfactant linkage and as a results the SMMSs exhibit intense absorption peaks correspond to linkage formed between [PtCl₄]²⁻ and 2[C_nTA]⁺. In DMSO and DMSO + PBS, an increase in absorbance from longer to shorter alkyl chain length as compared to [PtCl₄]²⁻ proves an involvement of +I effect. Similarly, with DMSO + water, the trend is reversed with an increase in absorbance having shorter to longer alkyl chain indicates the role of water molecule with SMMSs. The Fig. 3a–c represents UV-spectra of SMMSs in several solvents.³²

Fig. 3 (a–c) UV spectra of SMMSs: (a) 0.001 M DMSO, (b) DMSO + PBS at pH 7.4 and (c) DMSO + water.

The UV spectra finds K₂PtCl₄ absorption bands for SMMSs appeared at approximately same wavelengths in several solvents with varying absorption. Their spectra in DMSO and DMSO + PBS are similar for both the SMMSs and Pt salt while in DMSO + water, the wavelength of SMMSs is shifted to a lower region as compared to Pt salt. It indicates a higher energy requirement for d-electrons transitions.³² The PBS optimizes the activities of SMMSs whereas H₂O induces the chemical activities. The absorbance with variable medium depicts the SMMSs, rather than their structural changes. The higher absorbance with DMSO indicates the dipole of DMSO with its two –CH₃ effectively homogenize SMMSs. The addition of H₂O and PBS with DMSO indicate an engagement of DMSO with their solvent that weakens the absorbance with SMMSs.

3.2. CMC values

The micelles formed at a very narrow concentration range are called CMC and have significance in different applications such as catalysis, drug delivery and emulsion polymerization.³³ A reduction in slope depicted micellization which were determined on fitting the data points above and below the reduced points to two straight line equations with two separate slopes. Both the equations were solved simultaneously to obtain an intersection point on concentration scale. For SMMSs, the electron affecting ability of metal ion vis-a-vis alkyl chain length reflects their effect on CMC. Such activity of Pt in SMMSs reflects binding activities of [PtCl₄]²⁻·2[C_nTA]⁺. It facilitates to determine a degree of counter ion binding (β) and CMC for C_nTAB and SMMSs. The CMC of C_nTAB in water are compared the literature values which were in close agreement (Table 4) whereas in 0.2 volume fraction of DMSO in DMSO + water are measured at 298.15 K.²⁶

Table 4 CMC and thermodynamic parameters of SMMSs in 0.2 volume fraction of DMSO

T (K)	CnTAB	CMC (mM)			0.2 volume fraction of DMSO
		CMCwater	CMC0.2 volume fraction of DMSO	Alfa (α)
298.15	OTAB	293	400.00	0.40	−0.442	—	—
	DTAB	66.3	124.82	0.38	−1.286	—	—
	DDTAB	14.6	28.03	0.35	−4.034	—	—
	TDTAB	3.72	9.37	0.33	−7.437	—	—
	HDTAB	0.96	2.11	0.31	−13.195	—	—
SMMSs
298.15	MOTA		0.400	0.320	−19.891	−29.55	−0.0324
	MDTA		0.372	0.267	−20.826	−33.01	−0.0409
	MDDTA		0.343	0.247	−21.418	−35.62	−0.0476
	MTDTA		0.332	0.239	−21.656	−37.73	−0.0539
	MHDTA		0.315	0.234	−21.956	−40.43	−0.0620
308.15	MOTA		0.501	0.340	−19.355	−31.21	−0.0385
	MDTA		0.493	0.297	−19.922	−34.62	−0.0477
	MDDTA		0.483	0.248	−20.593	−37.350	−0.0544
	MTDTA		0.457	0.242	−20.907	−40.54	−0.0637
	MHDTA		0.433	0.236	−21.220	−43.09	−0.0710
318.15	MOTA		0.643	0.358	−18.674	−33.01	−0.0451
	MDTA		0.622	0.298	−19.505	−37.06	−0.0552
	MDDTA		0.610	0.295	−19.623	−40.02	−0.0641
	MTDTA		0.595	0.259	−20.155	−42.49	−0.0702
	MHDTA		0.584	0.238	−20.485	−46.02	−0.0803

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The ≈2–3 folds higher CMC find as compared to literature (Table 4). On increasing concentration, the specific conductivity is increased, and their slope reduced after a specific SMMSs concentration at fixed temperature (Fig. 4a–c).

Fig. 4 (a–c) Variation of CMC of nTAB (a) OTAB, DTAB, (b) DDTAB, TDTAB, HDTAB and (c) SMMSs in 0.2 volume fraction of DMSO in DMSO–water at 298.15 K.

The breaking points in specific conductance vs. concentration of SMMSs presented in Fig. 4a–c at 298.15 K and ESI Fig. S14 and S15† at 308.15 and 318.15 K depicts a commencement of micelle formation. As compared to C_nTAB, the lower CMC values found for SMMSs are given in Table 4.

3.2.1. Effect of alkyl chain length on the CMC. On increasing the temperature and alkyl chain length, both the pre (S₁) and post (S₂) micellar slopes increase simultaneously. Thus, the kinetic activation energy enhances dispersion. The kinetic energy at higher temperature enables the surfactant to optimize the larger surfactant molecule in micelles. An increase in carbon chain length of C_nTAB at a particular temperature, reduces CMC value, probably due to an easier micelle formation on increasing solvophobicity.^34,35 An effect on CMC is observed on increasing alkyl chain length of C_nTAB and SMMSs in 0.2 volume fraction of DMSO in DMSO + water. So, on increasing chain length of C_nTAB, the CMC of SMMSs are decreased at a particular temperature (Fig. 4a–c and ESI Fig. S12 and S13†). A decrease in CMC on increasing alkyl chain length is observed. This may be due to increase in hydrophobic character of the molecule in the coordination sphere. It depicts that the weaker hydrophilic interaction supports Brownian motions to induce structured water effect. Such effect with higher hydrophobicity rearranges micellization at lower concentration. On increasing temperature, their degree of micellization (α) and CMC are increased. The increase in α with respect to the temperature could be due to the decrease in the charge density on the micellar surface and the CMC values increase with increasing temperature. This behaviour may be related to two opposing factors. First, a temperature increase causes a decrease in solvation of the hydrophilic group, which favours micellization. Second, a temperature increase also causes disruption of the solvent structure surrounding the hydrophobic group, which retards micellization. The relative magnitude of these two opposing effects will determine the CMC behaviour. The lowest CMC value of SMMSs is found at 298.15 K and the highest at 318.15 K. At a particular temperature, the specific conductivity of SMMSs are increased on increasing alkyl chain length with continuous decreases in CMC. On increasing the temperature, the specific conductivity and CMC of SMMSs increase simultaneously which could determine a contribution of alkyl chain length. An increase in temperature induces the motions of counter ions and hydrophilic part of C_nTAB manifolds. It results an increase in conductivity and its CMC in accordance with conventional surfactants due to a decrease in hydration of hydrophilic group, favours micellization on increasing temperature. The volumetric properties of C_nTAB in aqueous solution are in close agreement with our studies reported elsewhere³⁵ which monitored the increasing alkyl chain length effect with CMC trend similar as of our systems.³⁵ Many metallosurfactants with several metal ion and cationic surfactants and their results are very close to our findings.^21–23

3.2.2. Thermodynamics of micellization. Increasing alkyl chain length effect on thermodynamic parameters are in a close agreement with the literature.^34,35 The CMC vs. temperature relation furnishes information on hydrophobic and head group interactions, and derives thermodynamic parameters of micellization. A change in CMC with temperature is analyzed in terms of phase separation or equilibrium for micellization.^36–38 So standard Gibbs energy , enthalpy and entropy of micellization are calculated with following equations given aswhere R, T and α_ave are gas constant, absolute temperature and average degree of micellar ionization (micelle ionization degree) at CMC respectively as ratio between slopes of linear specific conductance vs. [SMMSs] plots above and below CMC is considered. The chosen thermodynamic parameters supporting micellization for SMMSs and C_nTAB in 0.2 volume fraction of DMSO in DMSO + water are compared with C_nTAB for in water from literature in Table 4.
3.2.2.1. Effect of on increasing alkyl chain length of SMMSs. The depicts a spontaneity of the micellization process. The more negative the values support spontaneity of micellization. A continuous negative increase in is observed on increasing alkyl chain length of SMMSs at a particular temperature. Since, hydrophobicity with shorter alkyl chain induces entropic motions with solvents that need more consumption of to induce higher . It could be attributed to stronger +I effect that develops London dispersive forces (LDF) to reorient the shared pairs of electron along with alkyl chain length. So, on increasing in temperature, the values slightly decreased for each SMMSs that indicates a lesser spontaneity of micellization at higher temperature. It could be attributed to agitation of the micelle due to thermal forces. The values of C_nTAB in 0.2 volume fraction of DMSO in DMSO + water were also calculated at 298.15 K and compared with its SMMSs at the same temperature. For C_nTAB, the less negative values are found for shorter alkyl chain. In case of shorter alkyl chain, less entropy of structured solvent occurs. On increasing carbon chain length the values further decrease. It favours micellization for higher carbon chain length surfactants (Fig. 5a–c).

Fig. 5 (a–c) Alkyl chain length effect on , and of SMMSs at chosen temperature.

The values of C_nTAB again compared with SMMSs at the same temperature which further decrease which directly reflect an influence of [PtCl₄]²⁻ counterpart on its linkage with [C_nTA]⁺ (Table 4).

3.2.2.2. Effect of on increasing alkyl chain length of SMMSs. The value of decreases on increasing the alkyl chain length of SMMSs at a particular temperature. A continuous negative increase of on increasing temperature reflects an effect of alkyl chain length on micellization (Table 4 and Fig. 5a–c). The decrease in on increasing temperature depicts that the more and more aggregates get disrupted. The aggregates remain together due to chemical binding forces like van der Waals and LDF caused to disparity of electron density in the SMMSs, hence disruption release chemical energy to the system as exothermic process. Also such disruptions seem stronger on increasing alkyl chain length because the values with longer alkyl chain at constant increase comparatively with higher magnitude. Thus, the negative values of predicted an exothermic nature of micellization process. The electronic interactions between head groups and the counterions involve in repulsion and attraction that leads to exothermic and endothermic nature of SMMSs.³⁹

A reverse trend between and with increasing temperature is noted. The gets higher to lower negative whereas the gets lower to higher negative values. Table 4 explains micellization as an exothermic process on increasing alkyl chain length and temperature.

3.2.2.3. Effect of on increasing alkyl chain length of SMMSs. On increasing alkyl chain length at a particular temperature, the values decrease. Also, on increasing temperature, the values decrease. This implies that disordering of water molecules becomes less pronounced due to the destruction of the iceberg water structure around the alkyl group with increasing temperature or due to the head group being is more hydrated than the hydrophobic tail. The negative value of may indicate the formation of the solid-phase aggregate (Fig. 5a–c and Table 4). Thus, along with disruption, it seems that the fragmented SMMSs could undergo intramolecular electronic changes or intramolecular oscillatory, electronic rotational, translational motions. The higher values for SMMSs depict unfavourable entropic changes for micellization at chosen temperature range while least values favouring micellization due to orderedness.

3.2.3. Enthalpy–entropy compensation phenomenon. The variation of both and values show a mutual relationship of enthalpy–entropy compensation phenomenon. When the enthalpy contributes its less share to , its counter part noted as , that contributes a larger share in order to lead to cause its lower values and vice versa. The Fig. 6a–e shows such relationship for SMMSs at chosen temperatures. A linear relationship is obtained for and is expressed with the help of eqn (4).where 1/T_c is slope and σ is intercept of linear plot.

Fig. 6 (a–e) Variation of vs. of SMMSs for compensation phenomenon: (a) MOTA, (b) MDTA, (c) MDDTA, (d) MTDTA and (e) MHDTA.

The T_c measures solvation part of micellization, the σ depicts solute–solvent interaction. It is considered as an index of the chemical part of micelle formation. The T_c values predict its effect on increasing alkyl chain length and the T_c results indicate a continuous increase. It also favours the micellization. A small deviation in T_c is seen in case of MDDTA which is lower than the MDTA having lower carbon chain length (Table 5). In literature it is reported that sometimes the T_c values deviate from their set trends.²²

Table 5 Values of T_c, σ and for SMMSs using conductivity measurement

Compensation parameters	SMMSs
	MOTA	MDTA	MDDTA	MTDTA	MHDTA
Tc (K)	270.27	285.71	270.27	294.12	303.03
σ	0.0757	0.0741	0.0853	0.0753	0.0702
	−173.2	−202.6	−220.0	−237.7	−279.3

---

Temperature dependence of hydrophobic effect expressed as heat capacity of micellization and estimated from slope of vs. temperature curve is noted as under

The values decrease on increasing alkyl chain length. A decrease in heat holding capacity favours micellization on increasing carbon chain length. The longer C_nTAB with higher due to entropically stronger Brownian motions favours micellization. The lower delays whereas the higher values favour the micellisation because intramolecular motions increase the activity of the SMMSs.

3.2.4. Surface tension isotherm and CMC. The surface tension isotherm and CMC of TDTAB and HDTAB have been recorded by measuring surface tension at 298.15 K in 20% DMSO to check a validity of CMC calculated from conductivity experiment and found a close agreement to each other (Fig. 7a and b). Thus, it seems, for the other surfactants and metallosurfactant used in this work, the CMC values from the conductivity experiments, will also be in good agreement with the values from surface tension isotherms. It is reported that surface tension isotherm can be found on plotting the graph between surface tension (γ) and concentration of surfactant (log C). A typical surface tension isotherm is plotted in Fig. 7a and b.

Fig. 7 (a and b) Schematic representation of typical surface tension isotherm of water/surfactant solution.

Fig. 7a and b shows that the at C > CMC, the surface tension is constant whereas at C < CMC the isotherm may be roughly divided into two regions. Region (I) the slope of curve γ decreases with log C while in region (II) γ is a linear function of log C. The isotherms depict impacts of alkyl chain length where C_nTAB with 14 and 16 carbon atoms produced 8 mM and 2 mM CMC values respectively in 4 : 2 ratio. It seems that the remaining surfactants and metallosurfactant also follow the same trend along with two isotherms.

3.3. TG-DTA analysis

TGA finds their thermal behaviour to determine various kinetic parameters of thermal decomposition and activation energy (E). Fig. 8 depicts TG curves of SMMSs that demonstrate a decomposition occurring in two steps. The SMMSs decomposition starts at ≈247–252 °C, with an initial breakdown of quaternary ammonium moieties continuing up to 500 °C. The second step eventually degrades the metal chloride into metal.^40,41 The % mass losses obtained experimentally are in agreement with theoretical values (Table 6). The TGA finds a thermal decomposition of SMMSs given as under Scheme 1.

Scheme 1

Fig. 8 Thermogravimetric (TG) analysis of SMMSs.

Table 6 TGA data and activation energy calculated for SMMSs^a

Methods	SMMSs
	MOTA			MDTA			MDDTA			MTDTA			MHDTA
	Transition temp. (°C)	Mass loss (%)		Transition temp. (°C)	Mass loss (%)		Transition temp. (°C)	Mass loss (%)		Transition temp. (°C)	Mass loss (%)		Transition temp. (°C)	Mass loss (%)
a R = regression coefficient.
Step 1	251	Calculated	Observed	245	Calculated	Observed	242	Calculated	Observed	245	Calculated	Observed	250	Calculated	Observed
		69.89	68.76		80.01	78.90		77.58	76.54		76.53	75.40		81.78	80.68
Step 2	360	—		360	—		330	—		340	—		365	—

---

	R	E/kJ mol^−1	R	E/kJ mol^−1	R	E/kJ mol^−1	R	E/kJ mol^−1	R	E/kJ mol^−1
CR	0.9993	153.37	0.9823	158.49	0.9948	179.20	0.991	193.589	0.9945	191.99
MKN	0.9993	153.71	0.9824	158.83	0.9948	179.55	0.991	193.941	0.9945	192.33
WYHC	0.9993	153.83	0.9824	158.93	0.9948	179.66	0.991	194.056	0.9944	192.45
VK	0.9972	153.67	0.9804	160.67	0.991	178.98	0.9897	197.091	0.9848	195.77
HM	0.999	131.11	0.9837	159.14	0.9999	171.37	0.9824	196.72	0.981	198.39

---

3.3.1. Kinetic and thermodynamic parameters of decomposition for first step. Thermal activities at a fixed heating rate are depicted in Fig. 8 for their decomposition pattern.

A small dip near 100 °C infer a loss of water molecules and from 247 to 252 °C onwards infers a decomposition by releasing two [C_nTA]⁺ chains and [PtCl₄]²⁻. Primarily, two major mass loss regions are observed: the first region between rt and ≈250 °C is associated with a breakdown of SMMSs and the decomposition of quaternary ammonium structure. TG results obtained experimentally are in excellent agreement with the calculated values (Table 6).

Table 6 reports the activation energy required for SMMSs decomposition of using five methods. Since the higher values for SMMSs are obtained due to a higher thermal stability than precursor.^21,22 The details of equations used and plots (linear fit) are given in ESI Fig. S14 and S18.†

The methods namely Coats–Redfern (CR), Madhusudanan Krishnan–Ninan (MKN), Wanjun–Yuwen–Hen–Cunxin (WYHC), Van Krevelen (VK) and Horowitz–Metzger (HM) for a single heating rate are used to calculate kinetic and thermodynamic parameters for a decomposition of step-1.²² The calculation for activation energy is made following the equations reported in literature.^20–23 In all the methods α (degree of reaction) and in turn, g(α) (integral function of conversion) is estimated with several parameters, discussed below.

3.3.1.1. Coats–Redfern (CR) method.

Taking natural log

A fractional mass loss (α) and corresponding (1 − α)ⁿ are calculated from TG curve, where n depends on reaction model.

Plotting the left hand side of eqn (5) against 1/T gives slope (−2.303E/R) and intercept (A).

3.3.1.2. Madhusudanan Krishnan–Ninan (MKN) method.
3.3.1.3. Wanjun–Yuwen–Hen–Cunxin (WYHC) method.
3.3.1.4. Van Krevelen (VK) method.
3.3.1.5. Horowitz–Metzger (HM) method. Parameters T = T_m + θ are used for an order of reaction is 1 where T_m is defined as the temperature at which (1 − α)_m = 1/e = 0.368 that enabled the following relationship.

Symbols β, T_m, E, A, R infer heating rate, DTG peak temperature, activation energy (kJ mol⁻¹), pre-exponential factor (min⁻¹) and gas constant (8.314 J mol⁻¹ K⁻¹), respectively. The excellent correlation coefficient indicates a good fit of linear function obtained for all the methods (ESI Fig. S14–S18†). The obtained activation energies (E) from five methods further analysed for a comparison of SMMSs (Table 6). Table 6 gives TGA data and value of activation energy obtained from five methods for first degradation step of SMMSs. So, on increasing alkyl chain of SMMSs, an increase in activation energy infers their higher thermal stability by using all five methods where hydrophobicity noted as stabilizing factor due to an increase in sigma bond in alkyl chain length. The activation energies for SMMSs are presented in Fig. 9 and increased on increasing alkyl chain of SMMSs in all the methods.

Fig. 9 Comparative activation energy of SMMSs on variable alkyl chain of surfactants.

The obtained E values from Coats–Redfern (CR) method are used to calculate ΔH, ΔS and ΔG using following equations and data are reported in Table 7.

ΔS = 2.303 log[Ah/kT]R (14)

ΔH = E − RT (15)

ΔG = ΔH − TΔS (16)

where, h is Planck constant, T is temperature in K, A is Arrhenius constant or frequency factor and k represents Boltzmann constant.

Table 7 Thermodynamic decomposition parameters for SMMSs using TGA data

MSSNs	Coats–Redfern (CR) method
	A (min^−1)	ΔG (kJ mol^−1)	ΔH (kJ mol^−1)	ΔS (J mol^−1 K^−1)
MOTA	572.472	98.969	−4.204	−196.839
MDTA	617.700	97.465	−4.149	−196.111
MDDTA	742.980	96.107	−4.104	−194.527
MTDTA	822.420	96.267	−4.114	−193.731
MHDTA	805.620	97.324	−4.157	−193.982

---

The thermodynamic parameters calculated from TGA data show a continuous decrease in ΔG, which somewhere deviated from the order (Table 7).

The ΔH value decreases negatively on increasing alkyl chain length with minor decrease in the value but are found very closer to each other. The ΔS found a continuous increase on increasing alkyl chain length. Many studies are reported for TGA studies of metallosurfactant; so, we have collected activation energies calculated from all five methods with the help of TGA data for similar reported metallosurfactants and their compared activation energy are given in Table 8.

Table 8 Some recent studies on similar metallosurfactants and their activation energies calculated by all five methods using TGA data are compared with our studies

Metallosurfactant		CR	MKN	WYHC	VK	HM	References
[M(CH3COO)4]^2− [C12H25NH3^+]2 M = Co(II), Ni(II) and Cu(II)	Co(II)	33.37	33.75	33.72	40.32	50.68	20
	Ni(II)	29.42	29.65	29.72	41.94	57.57
	Cu(II)	76.88	77.77	77.83	87.36	94.06
PdCTAC (C16 chain)		96.89	97.01	97.03	97.00	99.94	21
MnC I (C16 chain)		32.48	33.95	34.01	36.13	38.25	22
MNC II (C16 chain)		51.80	52.95	53.19	57.31	58.98
CTA-AgB (C16 chain)		28.05	28.21	28.26	32.59	32.58	23
MOTA (C8 chain)		153.37	153.71	153.83	163.99	128.55	Present work
MDTA (C10 chain)		158.49	158.83	158.93	165.96	107.51
MDDTA (C12 chain)		179.2	179.55	179.66	187.57	115.16
MTDTA (C14 chain)		193.59	193.94	194.06	201.95	137.46
MHDTA (C16 chain)		156.18	156.52	156.64	184.59	156.37

---

The Table 8 reflects a review study of changes in activation energies on changing different metal ions as counter part of particular surfactant whereas our data indicate an effect of alkyl chain length of the similar surfactant with a fix metal ion counterpart. An interesting trend came in the observation when we compared our activation energy data of Pt metallosurfactant to the literature data of Ni and Pd based metallosurfactant. These nickel (Ni), palladium (Pd) and Pt metals are belongs to nd⁸ system and different in their size. When these metals combine with cationic surfactants they form metallosurfactant and we find that their thermal stability increases when we move from Ni to Pt in the same group (Fig. 10).

Fig. 10 Compared thermal stability and activation energy plot for the Ni, Pd and Pt based metallosurfactant.

Fig. 10 infers the higher thermal stability of Pt based metallosurfactant as compared to Ni and Pd metallosurfactant. The reason for this higher thermal stability as well as activation energy could be due to their atomic size which facilitates stability. Here it is observed that the basic properties of periodic table could be a reason which makes the thermally stable Pt based metallosurfactant.

3.4. Differential thermal analysis (DTA)

Differential thermal analysis of SMMSs inferred their heat flow behaviour. The Fig. 11 obtained data suggests the two zone of heat flow first is endothermic and second is exothermic in nature.

Fig. 11 Differential thermal analysis (DTA) analysis of SMMSs.

Initially at ≈137 and 238 °C the heat flow is exothermic which becomes endothermic at around ≈270 and 370 °C for SMMSs. Further, reaction becomes complete exothermic up to 1100 °C. Thus, the reaction was exothermic before the transition temperature and after this, it becomes endothermic. These indicate structural transitions with variable heat capacities which could act as an excellent heat dissipater or heat sink in sophisticated electronic devices.

3.5. Surface morphology and size distribution of self-assembled SMMSs

The surface morphology and average hydrodynamic size of self-assembled SMMSs are examined with SEM, AFM and DLS respectively.^42–44 The MOTA was analyzed for surface morphology along with its particle size distribution (Fig. 12a–d) whereas the other members of the series expressed similar morphology and their DLS data are given in ESI Table 2.†

Fig. 12 (a–d) Surface morphological images of MOTA (a) SEM, (b) AFM, (c) 3D AFM and (d) DLS for particle size distribution.

Their SEM images reflect spherical polymeric nature.⁴⁵ On increasing alkyl chain length shape of SMMSs becomes more disperse in polymeric form. Therefore, after observing their peculiar molecular geometry it becomes essential to analyse their surface topographical images which was taken with AFM of a thin film coated on a glass slide in 20 μm scan area showed similar images. An orientation in arrays of coordination planes in [PtCl₄]²⁻·2[C_nTA]⁺ are depicted with AFM. The surface pattern in AFM shows reasonable dispersion in selected area. AFM images revealed that the aggregates were ellipsoid instead of spherical. In some areas an aggregation of MOTA is also observed which further supports its micellization. An average size distribution of MOTA is 350 nm with a 0.682 PDI.

4. Conclusion

The SMMSs on increasing alkyl chain lengths of C_nTAB have been investigated and their effect on hydrophobicity, CMC, thermodynamic parameters, kinetics, thermal decomposition and surface morphology are analysed. A series of MOTA > MDTA > MDDTA > MTDTA > MHDTA are formed the CMC values as 0.400 > 0.372 > 0.343 > 0.332 > 0.315 respectively on increasing alkyl chain lengths of SMMSs at a 298.15 K. On increasing temperature their CMC increased but the similar trend is observed for alkyl chain lengths. Their including other thermodynamic parameters have supported its micellization. Their transition temperatures vary from 242 to 251 °C and calculated activation energies which are comparable with literature. The activation energies are calculated with five different methods have produced a very close agreement with each other. The thermally stable SMMSs could be useful for heat capacitor devices as well as for the purpose of thermally stable material. Their thermal decomposition studies have shown exothermic behaviour before transition temperature whereas on and after the transition temperature, the SMMSs expressed the endothermic behaviour. The surface morphology of MOTA has shown a close relationship with the micellization along with a good dispersion pattern. These observations are greatly influenced by an effect of change in alkyl chain lengths and their effect of thermodynamic and thermogravimetric applications. Our study effectively reflects understanding of alkyl chain length in SMMSs. Such investigations could draw an interest for readers towards their biological studies and their interaction with biomolecules.

Acknowledgements

Authors are thankful to Central University of Gujarat, Gandhinagar for infrastructural support and experimental facilities. Mr N. K. Sharma is also thankful to Mr Baljeet Singh, Tata Institute of Fundamental research, Mumbai, India for thermogravimetric studies and Mr Abhishek Chandra, School of Chemical Sciences, Central University of Gujarat, Gandhinagar, India for his cooperation during the mathematical calculation of TGA. Dr A. Bhattarai and Prof M. Singh are thankful to The World Academy of Sciences (TWAS) for financial support and Mentorship respectively.

References

1. T. Owen and A. Butler, Metallosurfactants of bioinorganic interest: coordination-induced self-assembly, Coord. Chem. Rev., 2011, 255, 678–687.
2. M. Atoji, J. W. Richardson and R. E. Rundle, On the Crystal Structures of the Magnus Salts, Pt(NH₃)₄PtCl₄, J. Am. Chem. Soc., 1957, 79, 3017–3020.
3. P. B. Chock, J. Halpern, F. E. Paulik, S. I. Shupack and T. P. DeAngelis, Potassium Trichloro(ethene)palatinate(II) (Zeise's salt), Inorg. Synth., 1990, 28, 349–351.
4. G. Magnus, Ueber einige verbindungen des platinchlorrs, Ann. Phys., 1828, 90, 239–242.
5. G. Magnus, Ann. Chim. Phys., 1829, 40, 110–111.
6. D. S. Martin, R. M. Rush, R. F. Kroening and P. E. Fanwick, Crystal optics and polarized absorption spectra for tetraammineplatinum(II) tetrachloroplatinate(II), Magnus' green salt, Inorg. Chem., 1973, 12, 301–305.
7. W. Caseri, Derivatives of Magnus' Green Salt From intractable materials to solution-processed transistors, Platinum Met. Rev., 2004, 48, 9–100.
8. A. Hantzsch and F. Z. Rosenblatt, Über die Konstitution der Platintetramminsalze, Z. Anorg. Allg. Chem., 1930, 187, 241–265.
9. J. Bremi, V. Gramlich, W. Caseri and P. Smith, Synthesis, crystal structures and properties of quasi-one-dimensional platinum compounds, Inorg. Chim. Acta, 2001, 322, 23–31.
10. M. Fontana, H. Chanzy, W. R. Caseri, P. Smith, A. P. H. J. Schenning, E. W. Meijer and F. Gröhn, A Soluble Equivalent of the Supramolecular, Quasi-One-Dimensional, Semiconducting Magnus' Green Salt, Chem. Mater., 2002, 14, 1730–1735.
11. R. K. Ameta, M. Singh and R. K. Kale, Synthesis, characterization, EDX, thermal, antioxidant, antibacterial, topographical, and gas adsorption studies of supramolecular tetraammoniumplatinate, J. Coord. Chem., 2013, 66, 551–567.
12. A. D. Westland and L. Westland, A critical study of the determination of platinum with dimethylphenylbenzyl-ammonium chloride, Talanta, 1960, 3, 364–369.
13. L. M. Gindin, S. N. Ivanova, A. A. Mazurova and L. Y. Mironova, Extraction of Platinum Metals by Salts of Quaternary Ammonium Bases, Zh. Neorg. Khim., 1965, 10, 502.
14. K. Green, J. M. Chapman, L. Cheeks, R. M. Clayton, M. Wilson and A. Zehir, Detergent penetration into young and adult rabbit eyes: comparative pharmacokinetics, J. Toxicol., Cutaneous Ocul. Toxicol., 1987, 6, 89–107.
15. D. L. Dyer, A. Shinder and F. Shinder, Alcohol-free Instant Hand Sanitizer Reduces Elementary School Illness Absenteeism, J. Fam. Med., 2000, 32, 633–638.
16. B. Grillitsch, O. Gans, N. Kreuzinger, S. Scharf, M. Uhl and M. Fuerhacker, Environmental risk assessment for quaternary ammonium compounds: a case study from Austria, Water Sci. Technol., 2006, 54, 111–118.
17. K. Nagaraj, S. Sakthinathan and S. Arunachalam, Thermodynamics and kinetic investigation of electron transfer reactions of surfactant cobalt(III) complexes containing diimine ligands with iron(II) in the presence of liposome vesicles and amphiphilic salt media, RSC Adv., 2014, 4, 56068–56073.
18. (a) S. Veeralakshmi, S. Nehru, G. Sabapathi, S. Arunachalam, P. Venuvanalingam, P. Kumar, C. Anusha and V. Ravikumar, Single and double chain surfactant–cobalt(III) complexes: the impact of hydrophobicity on the interaction with calf thymus DNA, and their biological activities, RSC Adv., 2015, 5, 31746–31758; (b) N. K. Sharma, R. K. Ameta and M. Singh, Spectrophotometric and physicochemical studies of newly synthesized anticancer Pt(IV) complexes and their interactions with CT-DNA, J. Mol. Liq., 2016, 222, 752–761.
19. A. Riyasdeen, R. Senthilkumar, V. S. Periasamy, P. Preethy, S. Srinag, M. Zeeshan, H. Krishnamurthy, S. Arunachalam and M. A. Akbarsha, Antiproliferative and apoptosis-induction studies of a metallosurfactant in human breast cancer cell MCF-7, RSC Adv., 2014, 4, 49953–49959.
20. S. K. Mehta, R. Kaur and S. Singh, Thermogravimetric evaluation of decomposition kinetics of metalo surfactant complexes, J. Therm. Anal. Calorim., 2012, 107, 69–75.
21. (a) G. Kaur, S. Kumar, N. Dilbaghi, B. Kaur, R. Kant, S. Guru, S. Bhushan and S. Jaglan, Evaluation of bishexadecyltrimethyl ammonium palladium tetrachloride based dual functional colloidal carrier as an antimicrobial and anticancer agent, Dalton Trans., 2016, 45, 6582–6591; (b) E. P. Zhiltsovaa, S. S. Lukashenkoa, M. R. Ibatullinaa, M. P. Kutyrevab and L. Y. Zakharova, Complexation of 1-hexadecyl-4-aza-1-azoniabicyclo[2.2.2]octane bromide with nickel nitrate in acetone, Russ. J. Phys. Chem. A, 2016, 90, 1374–1378.
22. (a) G. Kaur, P. Garg and G. R. Chaudhary, Role of manganese-based surfactant towards solubilization and photophysical properties of fluorescein, RSC Adv., 2016, 6, 7066–7077; (b) Q. Zha, Q. Xie, Y. Hu, J. Han, L. Ge and R. Guo, Metallosurfactants C_n–Cu–C_n: vesicle formation and its drug-controlled release properties, Colloid Polym. Sci., 2016, 294, 841–849.
23. G. Kaur, S. Kumar, R. Kant, G. Bhanjana, N. Dilbaghi, S. K. Guru, S. Bhushan and S. Jaglan, One-step synthesis of silver metallosurfactant as an efficient antibacterial and anticancer material, RSC Adv., 2016, 6, 57084–57097.
24. C. Zhang, Y. Zhu, C. Zhou, W. Yuan and J. Du, Antibacterial vesicles by direct dissolution of a block copolymer in water, Polym. Chem., 2013, 4, 255–259.
25. K. Trickett, D. Xing, J. Eastoe, R. Enick, A. Mohamed, M. J. Hollamby, S. Cummings, S. E. Rogers and R. K. Heenan, Hydrocarbon Metallosurfactants for CO₂, Langmuir, 2010, 26, 4732–4737.
26. P. C. Griffiths, I. A. Fallis, T. Chuenpratoom and R. Watanesk, Metallosurfactants: interfaces and micelles, Adv. Colloid Interface Sci., 2006, 122, 107–117.
27. A. Cristaudo, F. Sera, V. Severino, M. De Rocco, E. Di Lella and M. Picardo, Occupational hypersensitivity to metal salts, including platinum, in the secondary industry, Allergy, 2005, 60, 159–164.
28. (a) J. F. Chambers, J. M. Stokes and R. H. Stokes, Conductances of Concentrated Aqueous Sodium and Potassium Chloride Solutions at 25°, J. Phys. Chem., 1956, 60, 985–986; (b) M. Singh, Survismeter – type I and II for surface tension, viscosity measurements of liquids for academic and research and development studies, J. Biochem. Biophys. Methods, 2006, 67, 151–161.
29. (a) J. Barthel, F. Feuerlein, R. Neueder and R. Wachter, Calibration of conductance cells at various temperatures, J. Solution Chem., 1980, 9, 209–219; (b) M. Singh, Survismeter, 3-in-1 instrument for simultaneous measurements of surface tension, inter facial tension (IFT) and viscosity, Pak. J. Anal. Environ. Chem., 2007, 8, 82–85.
30. A. I. Adawy and M. M. Khowdiary, Structure and Biological Behaviors of Some Metallo Cationic Surfactants, J. Surfactants Deterg., 2013, 16, 709–715.
31. S. M. Tawfik and H. H. Hefni, Synthesis and antimicrobial activity of polysaccharide alginate derived cationic surfactant-metal (II) complexes, Int. J. Biol. Macromol., 2016, 82, 562–572.
32. A. S. El-Tabl, R. M. El-Bahnasawy, M. M. E. Shakdofa and A. E. D. A. I. Hamdy, Synthesis of novel metal complexes with isonicotinoyl hydrazide and their antibacterial activity, J. Chem. Res., 2010, 34, 88–91.
33. S. Chandar, N. Caleb, D. Sangeetha and M. N. Arumugham, Synthesis, structure, CMC values, thermodynamics of micellization, steady-state photolysis and biological activities of hexadecylamine cobalt(III) dimethyl glyoximato complexes, Transition Met. Chem., 2011, 36, 211–216.
34. R. Zielinski, S. Ikeda, H. Nomura and S. Kato, Effect of temperature on micelle formation in aqueous solution of alkyltrimethylammonium bromides, J. Colloid Interface Sci., 1989, 129, 175–184.
35. R. S. Patil, V. R. Shaikh, P. D. Patil, A. U. Borse and K. J. Patil, Volumetric Properties of Alkyltrimethylammonium Bromides in Aqueous Solutions, J. Chem. Eng. Data, 2016, 61, 95–206.
36. P. Mukerjee, The thermodynamics of micelle formation in association colloids, J. Phys. Chem., 1962, 66, 1375–1376.
37. J. J. Galan, A. Gonzalez-Perez and J. R. Rodriguez, Micellization of dodecyldimethylethylammonium bromide in aqueous solution: thermal parameters, J. Therm. Anal. Calorim., 2003, 72, 465–470.
38. A. Gonzalez-Perez, J. C. Del Castillo, T. Czapkiewiez and J. R. Rodrigue, Micellization of decyl- and dodecyldimethylbenzylammonium bromides at various temperatures in aqueous solutions, Colloid Polym. Sci., 2002, 280, 503–508.
39. S. Shimizu, P. A. R. Pires and O. A. El Seoud, Thermodynamics of micellization of benzyl(2-acylaminoethyl)dimethylammonium chloride surfactants in aqueous solutions: a conductivity and titration calorimetry study, Langmuir, 2004, 20, 9551–9559.
40. H. R. C. Ouriques, M. F. S. Trindade, S. Prasad, P. F. A. Filho and A. G. Souza, Kinetics of decomposition of alkylammonium salts, J. Therm. Anal. Calorim., 2004, 75, 569–576.
41. R. Keuleers, J. Janssens and H. O. Desseyn, Thermal analysis and vibrational spectroscopy of Mn(II)–urea–halide complexes: comparative study of the metal–ligand bond strength, Thermochim. Acta, 2000, 354, 125–133.
42. X. Wu, Y. Tian, M. Yu, J. Han and S. Han, A targetable acid-responsive micellar system for signal activation based high performance surgical resolution of tumors, Biomater. Sci., 2014, 2, 972–979.
43. J. Yan, Z. Ye, H. Luo, M. Chen, Y. Zhou, W. Tan, Y. Xiao, Y. Zhang and M. Lang, Synthesis, characterization, fluorescence labeling and cellular internalization of novel amine-functionalized poly(ethylene glycol)-block-poly(ε-caprolactone) amphiphilic block copolymers, Polym. Chem., 2011, 2, 1331–1340.
44. T. Chang, M. S. Lord, B. Bergmann, A. Macmillan and M. H. Stenzel, Size effects of self-assembled block copolymer spherical micelles and vesicles on cellular uptake in human colon carcinoma cells, J. Mater. Chem. B, 2014, 2, 2883–2891.
45. S. Veeralakshmi, S. Nehru, S. Arunachalam, P. Kumar and M. Govindaraju, Study of single and double chain surfactant–cobalt(III) complexes and their hydrophobicity, micelle formation, interaction with serum albumins and antibacterial activities, Inorg. Chem. Front., 2014, 1, 393–404.

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Footnote

† Electronic supplementary information (ESI) available: ¹H NMR, CMC curves, linearization curves of TGA, and list of chemicals used. See DOI: 10.1039/c6ra20330b

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