Microstructural morphologies of CTAB micelles modulated by aromatic acids

Abstract

The morphological changes in cetyltrimethylammonium bromide (CTAB) micelles caused by the presence of three phenylalkanoic acid analogues viz. 3-phenylpropanoic acid (PPA), 3-phenylprop-2-enoic acid (PPE) and 3-phenylprop-2-ynoic acid (PPY) have been examined by ¹H NMR, light and neutron scattering and rheology. Spheroidal CTAB micelles transform into vesicles in the presence of PPY and elongated ellipsoidal micelles with PPE and PPA. The CTAB/PPY system showed an interesting phase transition from vesicles to long rod-like micelles at a temperature of ∼45 °C or at pH ∼9 while CTAB/PPE and CTAB/PPA systems even in alkaline pH (∼9) showed little effect. pH- and temperature-dependent self-assembly of CTAB with analogous aromatic acids behaves differently and may have applications in microfluidics, drag reduction, and encapsulation processes.

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1 Introduction

Quaternary ammonium compounds (QACs)-based cationic surfactants are known for their antibacterial/antistatic/corrosion inhibitory activity besides their cleansing properties and are thus extensively used in personal care and household products.^1,2 These surfactants form a variety of molecular self-assemblies, i.e. spherical, rod-like, worm-like, disk-like, vesicles, vesicle in the nanometre length scale based on the same mechanism of noncovalent interactions. Several studies have been reported in the literature for cetyltrimethylammonium salt type surfactants with different anion size and polarizability.^3–6 Cetyltrimethylammonium bromide (CTAB) is a cationic surfactant that has been studied extensively owing to its most useful physicochemical properties. At lower concentrations, CTAB forms spheroidal micelles which transform to ellipsoidal, rod-like, worm-like and vesicle microstructures in the presence of various types of additives.

Several authors have reported inorganic salt-induced ellipsoidal,⁷ rod-like⁸ and worm-like^9,10 micelles of CTAB. Organic salts (aromatic hydrotropes) are more effective than inorganic salts and generally lead to formation of viscoelastic solutions containing wormlike micelles.^11–14 Weakly polar aliphatic compounds with medium size (C₆–C₈) alkyl chains, viz. alcohols,^15–17 amines^16,18 and carboxylic acids,¹⁶ induce sphere-to-rod transition of CTAB. Nevertheless, vesicle formation of CTAB by long alkyl chain alcohols (>C₈ chain) shows a good incidence.^19,20 As compared with weakly polar aliphatic compounds, polar aromatic substances are competent to boost the aggregation morphology of CTAB. Micellar transition of CTAB has been extensively studied in the presence of polar aromatic compounds e.g. phenols,^21–24 aromatic amines,^24–26 and aromatic acids.^24,27–31 From the literature it is found that the effectiveness of these compounds to induce micellar transition of CTAB follows the trend: phenols > aromatic acids > aromatic amines. Agarwal et al.²¹ observed vesicle formation of CTAB by phenol derivatives. Bahadur and coworkers²⁴ reported that p-cresol leads to formation of CTAB vesicles while p-toluic acid and p-toluidine induce sphere-to-short-rod and ellipsoidal micelles, respectively. Several studies have been reported in which aromatic acids and salts induced formation of rod-like and worm-like micelles of CTAB.^29,30,32,33 However, spontaneous formation of CTAB vesicles in the presence of aromatic acids and their salts has been only marginally addressed. Vesicle-type self-assembled structures of CTAB were observed in the presence of 5-methyl salicylic acid²⁷ and sodium salicylate.³⁴ Furthermore, vesicle formation of CTAB has also been reported in the presence of azobenzene dye,³⁵ the plant sterol β-sitosterol,³⁶ the ionic liquid 1-butyl-3-methylimidazolium octyl sulfate³⁷ and anionic surfactants.^38,39

The change-over in the aggregation morphology of CTAB with additives has been observed by tuning temperature,^27,38 light^40–42 and pH.^{13,24,28,43,44} Raghavan and coworkers²⁷ described vesicles transforming into viscoelastic worm-like micelles upon heating CTAB solution containing 5-methyl salicylic acid. Light-tunable micellar transition in CTAB micelles was seen with photoresponsive solubilizates ortho-methoxycinnamic acid⁴¹ and sodium cinnamate.⁴² pH-Dependent aggregate morphologies of CTAB have been investigated in the presence of potassium phthalic acid,¹³ sodium salicylate,⁴⁵ anthranilic acid,²⁸ o-coumaric acid⁴⁴ and naphthols.⁴³ Our group has also examined pH-induced micellar transition of surfactants with solubilized p-cresol, p-toluidine and p-toluic acid.^24,46,47

Vesicular aggregates of cationic surfactants have great importance as model biological membranes⁴⁸ and encapsulation systems for drugs,⁴⁹ DNA,⁵⁰ liquid food,⁵¹ and dyes.⁵² Viscoelastic worm-like micelles from cationic surfactants are useful in personal care products, fracturing fluids in oil fields, heat-transfer fluids, drag reduction agents and hard-surface cleaners.^53,54 For that reason, in this paper we have investigated the formation of vesicles and elongated micelles and their morphological changes with pH and temperature for CTAB micelles in the presence of aromatic acids.

We observed that CTAB micelles show markedly different solution behaviour by introducing unsaturation into the propyl chain of phenylpropanoic acid (PPA). The acid with a double bond (3-phenylprop-2-enoic acid, PPE) showed some increase in solution viscosity. Highly viscoelastic solution changes into vesicles were observed in the presence of the acid with a triple bond (3-phenylprop-2-ynoic acid, PPY). This prompted us to investigate CTAB micelles in the presence of acids. Shah and coworkers⁵⁵ studied the solubilization of PPA and PPE in CTAB solution but they did not reveal information regarding micellar changes.

In this paper, we have taken three acids, PPA, PPE and PPY, having an alkane –(H₂C–CH₂)–, alkene –(H₂C=CH₂)– and alkyne group –(C≡C)– respectively in the side chain. Thus, these aromatic acids have different polarity and geometry, which facilitates their dissimilar interactions with CTAB micelles resulting in various aggregation morphologies. These CTAB/aromatic acid systems are highly responsive to both pH and temperature. The changes in micellar geometry have been investigated using viscometry, rheology, zeta potential, UV-visible spectroscopy, nuclear magnetic resonance (NMR), dynamic light scattering (DLS) and small angle neutron scattering (SANS) measurements.

2 Material and methods

2.1. Materials

Cetyltrimethylammonium bromide (CTAB), tetradecyltrimethylammonium bromide (TTAB), dodecyltrimethylammonium bromide (DTAB), cetyltrimethylammonium chloride (CTAC), cetyltributylphosphonium bromide (CTBPB) and cetyltriphenylphosphonium bromide (CTPPB) were supplied by Sigma-Aldrich. 3-Phenylpropanoic acid (hydrocinnamic acid, PPA), 3-phenylprop-2-enoic acid (cinnamic acid, PPE) and 3-phenylprop-2-ynoic acid (phenylpropiolic acid, PPY) were also purchased from Sigma-Aldrich [Scheme 1]. All chemicals have high purity (>99.9%). Deionized water from a Millipore Milli-Q system was used for viscosity, rheology and DLS measurements while D₂O (Sigma) was used for SANS and NMR. CTAB solution (50 mM) was used for all measurements. The pH of the solutions was adjusted by using HCl/NaOH.

Scheme 1 Structures of aromatic acids.

2.2. Methods

2.2.1. Viscosity. The relative viscosities were measured using calibrated Cannon-Ubbelohde viscometers in a temperature-controlled water bath. The viscometers were cleaned thoroughly before each measurement.

2.2.2. Dynamic light scattering. The micelle hydrodynamic diameter and zeta potential were determined by dynamic light scattering (DLS) using a Horiba SZ-100 instrument at 30 °C. Each measurement was repeated at least three times.

2.2.3. Rheology. An Anton Parr Physica MCR 101 rheometer with double gap concentric cylinder geometry (DG 26.7) was used to study rheology. The shear rate was applied in the range 0.1 to 100 s⁻¹. Dynamic shear measurements were also carried out between 0.01 and 100 rad s⁻¹. The torque was directly converted into G′ (storage modulus) and G′′ (loss modulus).

2.2.4. Small-angle neutron scattering (SANS). SANS experiments were carried out at Dhruva reactor, Bhabha Atomic Research Centre (BARC), Mumbai, India. The coherent differential scattering cross-section (dΣ/dΩ) per unit volume was plotted as a function of wave vector transfer Q (=4π sin(θ/2)/λ, where θ is the scattering angle and λ is the wavelength of the incident neutrons). The background correlation was done for the measured SANS data. The details of SANS measurements and analysis can be found elsewhere.²⁴

2.2.5. ¹H NMR. The ¹H NMR spectra for CTAB solutions in D₂O in the presence and absence of aromatic acids were recorded on a Bruker AVANCE II 400 MHz spectrometer at the Central Salt and Marine Chemicals Research Institute (CSMCRI), Bhavnagar, India.

3 Results and discussion

3.1. Structural effect of aromatic acids on CTAB micelles

The effect of acids on the change in aggregate morphology in aqueous solutions was initially observed by viscosity studies. Fig. 1(a) shows the viscosity for 50 mM CTAB solution as a function of acid concentration at 30 °C. The effect of concentration of PPE and PPY on CTAB solution was studied in a limited range because of their low solubility in water and surfactant solution. Moreover, PPE and PPY undergo phase separation at 50 mM and PPA shows such behaviour at 200 mM concentration. Among these three aromatic acids, PPE and PPY showed a noticeable increase in relative viscosity while PPA was least effective. PPE showed a considerable increase in the viscosity, while a viscosity peak at around 35 mM PPY concentration reflects substantial changes in aggregation morphology. In such solutions, the decrease in the viscosity after the maximum is attributable to several phenomena, such as breaking of micelles, micellar transition from linear to branched network geometry or phase transition from micelles to vesicles.^27,56,57

Fig. 1 (a) Relative viscosity of 50 mM CTAB at varying concentration of PPA, PPE and PPY at 30 °C. (b) Absorbance of 50 mM CTAB at varying PPY concentration at 30 °C.

Generally, changes in the appearance of a surfactant solution indicate morphological transition for surfactant aggregates.^58,59 In this work, the appearance of CTAB solution altered from clear and viscous to bluish and less viscous with the addition of PPY (>40 mM) [Fig. 1(b)]. According to the Tyndall effect, the bluish appearance of surfactant solution manifests the presence of large nanostructures in solution, and usually it is observed for solutions containing vesicles.²⁷ We examined the change in colour of solution by measuring absorbance. The CTAB solutions do not show any noticeable change in absorbance up to 40 mM PPY, but above this concentration the absorbance drastically increases [Fig. 1(b)]. Such behaviour reflects the formation of large nanostructures. CTAB solutions remain transparent with addition of PPE and PPA even up to high concentrations, indicating their ineffectiveness to change aggregate morphology. Photographs of CTAB solution in the presence of acids are shown in Scheme 2.

Scheme 2 Solution behaviour of 50 mM CTAB with 45 mM acids.

To further explicate the microstructures of CTAB aggregates, we carried out rheology, DLS and SANS measurements. Fig. 2 demonstrates the zero-shear viscosity, size distribution (DLS) and SANS intensity curves for the CTAB solutions with varying [PPY]. The variation in the viscosity with shear rate for the CTAB/PPY mixed system is shown in Fig. 2(a). The viscosity is independent of shear rate, for CTAB solution with 15 and 45 mM PPY showing Newtonian behavior. With 30 and 35 mM PPY, CTAB solutions exhibit strong shear thinning and increased zero-shear viscosity by a factor of about 100 as compared with solutions with 15 and 45 mM PPY.

Fig. 2 (a) Zero-shear viscosity, (b) DLS and (c) SANS intensity curves of micelles for 50 mM CTAB with varying concentration of PPY at 30 °C.

DLS measurements show that as concentration of PPY increases the aggregate size increases and reaches a maximum (∼542 nm) with 45 mM PPY [Fig. 2(b)]. The SANS spectra for 50 mM CTAB samples containing various concentrations of PPY are shown in Fig. 2(c). The data for 20 and 30 mM PPY both change from an asymptote to a plateau at low Q and basically correspond to micelles. The data for 45 mM PPY show high scattering intensity at low Q without any plateau, and the slope of −2 indicates vesicular structures. The SANS analysis confirms that CTAB forms elongated micelles at 30–35 mM PPY and vesicles at 45 mM PPY.

The analogous dynamic rheological response as plots of the elastic G′ and viscous G′′ moduli versus frequency ω was measured for the different solutions of the CTAB/PPY system [Fig. 3]. The surfactant solution behaving elastically (G′ > G′′) at high frequency and switching to a viscous behaviour (G′′ > G′) at low frequency discloses the presence of elongated micelles.²⁷

Fig. 3 Dynamic rheological response of 50 mM CTAB solution with different concentrations of PPY at 30 °C.

Here, PPY triggers such a viscoelastic response owing to the formation of elongated micelles over the concentration range of 30 to 35 mM. The solution containing 35 mM PPY shows longer relaxation time (inverse of the frequency at which G′ and G′′ cross) than that with 30 mM PPY. This indicates that CTAB micelles become more elongated with 35 mM PPY. However, the solution demonstrates a purely viscous response (G′′ > G′) in the whole range of frequencies in the presence of 45 mM PPY. Thus, the dynamic rheological response discloses the transition from a viscoelastic fluid to a less viscous one (vesicles). This again supports the relative viscosity results [Fig. 1(a)].

The effect of high concentration of the three acids on CTAB micelles was compared using zero-shear viscosity and SANS [Fig. 4]. The solution 50 mM CTAB + 45 mM PPE shows higher shear viscosity at low shear rates than PPY and PPA at the same acid concentration [Fig. 4(a)]. PPA does not show any significant effect on the shear viscosity of CTAB solution. The rheology results have good correlation with the relative viscosities.

Fig. 4 (a) Zero-shear viscosity and (b) SANS intensity curves of micelles for 50 mM CTAB with 45 mM PPA, PPE and PPY at 30 °C.

The SANS plots of CTAB solutions with high concentration (45 mM) of PPA and PPE show a plateau at low Q which usually corresponds to micelles. Moreover, the scattering intensity of PPE is higher than PPA in the low Q region. SANS analysis indicates that PPA and PPE produce elongated ellipsoidal micelles and PPY induces the formation of vesicles [Table 1].

Table 1 SANS analysis for CTAB aggregates and mixed system with PPY, PPE and PPA^a

System	Ra, nm	Rb, nm	Axial ratio	Rt, nm	Aggregate morphology
a Ra, semimajor axis; Rb, semiminor axis; Rt, vesicle thickness.
No additive	2.4	3.4	1.4	—	Ellipsoidal
30 mM PPY	2.0	8.3	4.1	—	Elongated ellipsoidal
35 mM PPY	2.0	8.6	4.4	—	Elongated ellipsoidal
45 mM PPY	—	—	—	2.4	Vesicle
45 mM PPE	2.0	10.9	5.4	—	Elongated ellipsoidal
45 mM PPA	1.9	7.9	4.1	—	Elongated ellipsoidal

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We now proceeded to study the question of why only PPY causes the transition of CTAB micelles to vesicles while PPE and PPA do not show such a significant type of effect. Usually, organic and inorganic additives can induce the micellar transition. These acids can adsorb onto the micelle–water interface and/or penetrate into the interior of micelles. Moreover, polar aromatic compounds or hydrotropes bind more strongly to cationic micelles than non-penetrating inorganic counter ions. This type of binding provides shielding of the head group charge and thus the effective head group area decreases whereas the volume of the non-polar tail portion increases. This binding lowers the surface charge of micelles and increases the surfactant packing parameter. Hence, the surfactant packing parameter can be changed by an alteration in solution conditions such as the addition of inorganic and organic salts/compounds to the micellar solution. Morphology of surfactant aggregates depends on the packing parameter of the surfactant.

To acquire more information about aggregation of CTAB in the presence of acids, we measured zeta potential. Shukla and Rehage⁶⁰ and Ge et al.⁶¹ have measured the zeta potential of cationic micelles in the presence of anionic hydrotropes. The hydrotrope molecules bind with cationic micelles and reduce the charge of the micelles. The decrease of zeta potential depended on electrostatic interactions between cationic surfactant and anionic hydrotropes.^60,61

Fig. 5 shows zeta potentials of the three CTAB/aromatic acid systems. The zeta potential of CTAB micelles is 69 mV, which decreases with the addition of aromatic acids. The tendency of acids to reduce the zeta potential of micelles is in the order: PPY > PPE > PPA. The zeta potential showed a marked decrease for PPY even at 20 mM concentration while PPA and PPE exhibited a lesser decrease in zeta potential. This decrease is due to acid-induced shielding (charge neutralization) of the head group. PPY has an alkyne group in addition to a carboxylic group (–COOH); therefore it may provide greater electrostatic interaction with CTAB than with PPE and PPA. Thus, PPY offers more shielding of the CTAB head group than PPE while PPA is least effective. Shah and coworkers⁵⁵ reported that the carbon–carbon double bond present in PPE exhibits more polar character in addition to the carboxylic group and exhibits greater electrostatic interaction than the less polar PPA. However, PPY has a carbon–carbon triple bond and is a linear molecule; so it shows greater electrostatic interaction than PPE and PPA.

Fig. 5 Zeta-potentials of CTAB aggregates in the presence of PPY, PPE and PPA at 30 °C.

¹H NMR has been widely used to identify the interactions between additives and micelles.^62,63 In this work, the interaction of CTAB micelles with aromatic acids was investigated. Scheme 3 displays the structural formula with proton labelling of CTAB. ¹H NMR spectra of 50 mM CTAB in D₂O with varying concentrations of acids are shown in Fig. 6. The shift in the NMR signal peaks towards higher frequency reflects the increase in the hydrophobic environment of micelles and the broadening of peaks reveals the micellar growth.^24,26

Scheme 3 Proton labelling for CTAB.

Fig. 6 Chemical shift of CTAB (50 mM) protons in D₂O with and without aromatic acids at 30 °C.

A small shift of CTAB protons by PPA and PPE is observed without peak broadening at low concentration (15 mM), while PPY exhibits a considerable shift as well as broadening of peaks even at low concentration. The resonance signal of the C₅ protons of CTAB is merged with that of the head group protons (C₆), and the chain proton (C₁ to C₄) peaks are merged in the presence of 15 mM PPY and get more pronounced above this concentration. PPE shows a similar effect but at higher concentration; no such behavior is observed for PPA. These results absolutely agree with viscometry, rheology, absorbance, zeta potential and scattering measurements. The results confirm that PPE causes a sphere-to-elongated-ellipsoidal transition and PPY changes micelles to vesicles while with PPA only a small growth is seen.

The micellar behaviour of different cationic surfactants with varying alkyl chain, polar head group and counter ion with CTAB in the presence of PPY was compared. DTAB (with 12C chain) does not show any increase in viscosity while TTAB (with 14C) shows a considerable increase in the relative viscosity with addition of PPY [ESI Fig. 1†]. The results show that micellar growth caused by PPY is favoured for the surfactant with the longer alkyl chain. To compare the effect of polar head group, CTBPB and CTPPB were used. These surfactants with bulky head group do not show any change in the relative viscosity with PPY. Phase separation at a small concentration (<10 mM) of PPY is also observed [data not shown]. Thus, the smaller the polar head group size, the more favoured is micellar growth. CTAC was compared with CTAB to observe the effect of counter ion, which showed similar behaviour but at higher PPY concentration [ESI Fig. 1†]. Therefore, among all the cationic surfactants used, CTAB was most efficient for micelle-to-vesicle transition.

3.2. Effect of temperature

The three aromatic acids have dissimilar effects on the micellar behaviour of CTAB. Consequently, they may also provide something unlike micellar behaviour with changes in temperature. Thus, we have also investigated the aggregation behaviour of CTAB/acid solutions upon heating. Fig. 7(a) demonstrates the changes in the relative viscosities of 50 mM CTAB + 45 mM acid systems with temperature. For the CTAB/PPE system, the relative viscosity decreased as temperature increased and significant reduction in the viscosity was found at 40 °C. The CTAB/PPA mixed system also showed a decrease in the viscosity with increase in temperature. Thus, CTAB solutions with PPA and PPE show the usual viscosity behaviour. Interestingly, for the CTAB/PPY system, an initial increase was observed with rise in temperature; a maximum in viscosity was seen at ∼40–45 °C.

Fig. 7 (a) Effect of temperature on the relative viscosity of 50 mM CTAB with 45 mM concentration of PPA, PPE and PPY. (b) Absorbance of CTAB/PPY system at different temperatures.

The changes in solution upon heating were visually observed. The mixture of 50 mM CTAB with 45 mM PPY was whitish-blue and less viscous at ∼30 to 35 °C, indicating the presence of vesicles. This solution became clear and viscous, signifying a noticeable change in aggregate morphology above 40 °C. The absorbance vs. temperature plot shown in Fig. 7(b) supports the visual observation. Here also, the absorbance measurements have good correlation with the relative viscosity.

Rheology and scattering studies were done to determine the influence of temperature on the aggregation behaviour of the CTAB/PPY system. The rheological responses of the CTAB/PPY system are displayed in Fig. 8(a). As discussed before, the CTAB/PPY system shows low viscosity with shear thinning behaviour at room temperature (30 °C). At 45 °C, this system shows high viscosity with shear thinning behaviour. Moreover, the viscosity decreases and turns out to be shear-independent at ∼60 °C. These rheological changes support the relative viscosity and absorbance measurements to confirm vesicle-to-micelle transition.

Fig. 8 (a) Zero-shear viscosity, (b) DLS and (c) SANS intensity curves of micelles for 50 mM CTAB/45 mM PPY solution with varying temperature.

Fig. 8(b) demonstrates the hydrodynamic size distributions of micelles for the CTAB/PPY system at 30, 45 and 60 °C. The corresponding SANS spectra are shown in Fig. 8(c). The hydrodynamic size of aggregates decreases upon heating. SANS results demonstrate that the scattering intensity decreases in the low Q region with increase in temperature. This trend discloses the perceptible structural transformations of surfactant aggregates. The data show a change from asymptote to plateau at low Q, which fundamentally corresponds to micelles, at high temperature (>45 °C). The analysis reveals that vesicles (at 30 °C) convert into rod-like micelles at 45 °C [Table 2]. The size of the micelles decreases as the temperature is further increased. The DLS and SANS results support our hypothesis of transition from vesicles to micelles with increasing temperature. Moreover, this transition is thermo-reversible, so that vesicle-to-micelle conversion upon heating can be reversed upon cooling.

Table 2 SANS analysis for 50 mM CTAB/45 mM PPY mixed system at different temperatures^a

Temperature, °C	Rt, nm	R, nm	L, nm	Ra, nm	Rb, nm	Axial ratio	Aggregate morphology
a R, cross-sectional radius; L, length of rod; Rt, vesicle thickness; Ra, semimajor axis; Rb, semiminor axis.
30	2.3	—	—	—	—	—	Vesicle
45	—	2.2	17.6	—	—	8.0	Rod-like
50	—	2.1	14.6	—	—	7.0	Rod-like
60	—	—	—	2.2	8.8	4.0	Elongated ellipsoidal

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Usually polar aromatic acids have a strong tendency to bind to surfactant aggregates. The resultant binding of these acids changes the surfactant packing parameter.^24,64 Self-assemblies in the dilute surfactant solutions show packing parameters in the range 0 < p < 1/3, 1/3 < p < 1/2 and 1/2 < p < 1 for spherical micelles, cylindrical micelles and vesicles respectively.⁶⁵

Generally, the temperature causes a reversible binding (adsorption) process of polar additive to the surfactant aggregates. However, the bound acid molecules may desorb from the surfactant self-assembly owing to their enhanced solubility in bulk water. Therefore, the temperature effect causes an increase in the effective head group area and decrease in the nonpolar chain volume, and results in decreased critical packing parameter p, as proposed by Israelachvili et al.⁶⁵ For PPE and PPA, the size of the elongated ellipsoidal micelles decreases with temperature; as a result only a decrease in viscosity was observed. In the case of the CTAB/PPY system, vesicles first convert into rod-like micelles (at 45 °C) and then to ellipsoidal micelles at higher temperature (60 °C). Accordingly, an initial increase in viscosity followed by a decrease in viscosity with increase in temperature reflects such behaviour.

3.3. Effect of pH

The acids show pH sensitivity because they convert into anionic hydrotropes (carboxylates) at alkaline pH. The extent of ionization of these acids depends on pH. Therefore, the interaction of these acids with CTAB micelles is pH-dependent.

The relative viscosities of CTAB/acid solutions as a function of pH are shown in Fig. 9(a). Mixed aqueous solutions of 50 mM CTAB and aromatic acids (45 mM) have pH ∼2.3. Results indicate that PPY and PPE leads to a considerable increase of the relative viscosity, but PPA demonstrates intriguing behaviour. The CTAB/PPA system shows a viscosity peak around pH ∼4.0 while lower viscosity is observed at pH ∼2 and 6. Moreover, the zero-shear viscosity of CTAB/PPA is very low and independent of shear thinning at pH ∼9.0 [Fig. 10(a)].

Fig. 9 (a) Effect of pH on the relative viscosity of 50 mM CTAB with 45 mM concentration of PPA, PPE and PPY at 30 °C. (b) Absorbance of CTAB/PPY system at different pH and 30 °C.

Fig. 10 (a) Zero-shear viscosity and (b) SANS intensity curves of micelles for 50 mM CTAB/45 mM acids at pH ∼9.0 and 30 °C.

Fig. 9(b) shows absorbance changes with pH. High absorbance at pH ∼2–3 is seen for solutions that are bluish at pH ∼3 to 4. These solutions become clear and viscous showing low absorbance above pH ∼4. As discussed for the effect of temperature, this type of behaviour indicates vesicle-to-micelle transformation for the CTAB/PPY system as pH increases from 3 to 4.

The relative viscosities of CTAB solution in the presence of PPE and PPY increase with pH. Zero-shear viscosity at pH ∼9.0 for all CTAB/acid systems are shown in Fig. 10(a). Here, CTAB solutions containing PPY and PPE exhibit shear thinning behaviour and increased zero-shear viscosity by a factor of 1000 as compared with PPA. Accordingly, PPY and PPE give elongated micelles with CTAB while PPA gives small ellipsoidal micelles at pH ∼9.0. The rheology measurements have good correlation with the viscosity results.

The effect of pH on the microstructural changes in these systems was also determined by SANS measurements [Fig. 10(b)]. The results demonstrate higher scattering intensity in the low Q range for CTAB/PPY and CTAB/PPE whereas the CTAB/PPA system showed lower scattering intensity in the low Q range. The data for PPY have a typical slope of (−1) in the low Q region and clearly designate the presence of rod-like micelles in the solution. SANS analysis confirms that PPY, PPE and PPA result in long rod, elongated ellipsoidal and ellipsoidal types of morphologies of CTAB micelles at pH ∼9 [Table 3].

Table 3 SANS analysis for 50 mM CTAB/45 mM acid mixed system at pH ∼9.0^a

Additive	R, nm	L, nm	Ra, nm	Rb, nm	Axial ratio	Aggregate morphology
a R, cross-sectional radius; L, length of rod; Ra, semimajor axis; Rb, semiminor axis.
PPY	2.1	23.7	—	—	11.3	Long rod
PPE	—	—	2.0	10.0	5.0	Elongated ellipsoidal
PPA	—	—	2.0	2.7	1.3	Ellipsoidal

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If we compare the aggregate morphology for these systems at pH ∼2.3 and pH ∼9, we observe that the CTAB/PPY system provides vesicle-to-long-rod transition with increase in pH. CTAB/PPE has elongated ellipsoidal type of geometry at both the pH values. The CTAB/PPA system has elongated ellipsoidal micelles at pH ∼2–3 which transform into small ellipsoidal micelles at pH ∼9.0. This is very thought-provoking as the solubilized acids show dissimilar pH-dependent effects on the aggregation behaviour of CTAB. However, owing to the dissociation of the carboxyl group, the acids get deprotonated and more negatively charged with increase in pH. This enhances electrostatic interaction with cationic micelles. Interaction between acids and micelles is dependent on the pK_a value of the acids used, which is in the range 4–5.⁶⁶ Thus, at pH conditions above and below the pK_a value the acids may behave differently.

As pH increases, PPA begins to ionize, and ionized PPA has more interaction with cationic micelles at pH ∼4, which leads to micellar growth. Low viscosity at higher pH is due to ionized acid, which acts as a hydrotrope. Accordingly, some carboxylate ions are expelled from micelles and transferred into the bulk phase. This process reduces the packing parameter of CTAB micelles and decreases micelle size above pH ∼4.0 for CTAB/PPA systems. The 50 mM CTAB/45 mM PPA system has ellipsoidal micelles (ellipticity ∼ 4.1) at pH ∼2–3 and more elongated micelles at pH ∼4.0. However, small ellipsoidal micelles with ellipticity ∼1.3 form at pH ∼6–10.

The CTAB/PPE system shows pH-induced changes in the relative and zero-shear viscosity; nevertheless the SANS data disclose the existence of elongated micelles at lower as well as higher pH. Therefore, the changes in viscosity may be due to alteration in the inter-micellar interaction. For CTAB/PPY the viscosity was less at pH ∼2–3 owing to the presence of vesicles and increased with increasing pH, indicating vesicle-to-rod transitions as evidenced from SANS results. In this way, a variety of aggregate morphology for CTAB/aromatic acid systems can be produced by changing pH and temperature.

4 Conclusion

The present paper reports concentration-, temperature- and pH-induced transitions in the aggregates formed in CTAB solutions in the presence of three aromatic acids. Doping with polar aromatic acids offers a method of controlling the shape and size of CTAB self-assembled microstructures (viz. ellipsoidal, rod-like and vesicles), which can be altered by temperature and pH. Rheology, scattering and spectral studies indicate that PPY induces vesicle formation while elongated ellipsoidal micelles form in the presence of PPE and PPA. A new insight gained in this study is that polarity of aromatic compounds increases as the unsaturation increases in the nonpolar chain of aromatic acids; more-polar compounds have a significant effect on the aggregation behaviour. Accordingly, PPY shows the greatest electrostatic interaction with CTAB, resulting in spontaneous phase transformation from micelles to vesicles. CTAB/PPY vesicles can be converted into long rod-like micelles beyond a critical temperature or pH. The CTAB/PPE system produces elongated ellipsoidal micelles over a wide pH range but increase in temperature decreases micelle size. PPA does not show a noticeable effect on the micellar growth except at particular pH values. The changes in the morphologies of CTAB aggregates are due to the temperature- and pH-dependent solubilization of the aromatic acids. These aggregate systems form reversible morphologies. The present studies have prospective applications in encapsulation, micellar catalysis, surfactant-templated synthesis and drag reducing processes.

Acknowledgements

SP and PB thank UGC, New Delhi for fellowships.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra24271e

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