Drug induced micelle-to-vesicle transition in aqueous solutions of cationic surfactants

Abstract

The effects of the anti-inflammatory drug diclofenac sodium (DS) on the morphology of aqueous micellar aggregates of two ionic liquid-based surfactants (ILBSs), 1-hexadecyl-3-vinylimidazolium bromide, C₁₆VnImBr, 1-hexadecyl-3-methylimidazolium bromide C₁₆MeImBr, and (conventional) cetyltrimethylammonium bromide, C₁₆Me₃ABr (Vn, Im, Me, A = vinyl, imidazolium, methyl and ammonium, respectively) were studied at 25 °C. To probe the morphology changes of the formed aggregates, we employed turbidity, viscosity, dynamic light scattering, and transmission electron microscopy. Depending on [DS], the transitions observed were from spherical micelles → worm-like micelles → vesicles. Viscosity data indicated that the first transition occurred at lower [DS] for C₁₆VnImBr compared to C₁₆MeImBr and C₁₆Me₃ABr; indicating stronger interaction between (C₁₆VnIm)⁺ and DS⁻. Light scattering results revealed that the DS/C₁₆VnImBr system contained larger vesicles, as compared to DS/C₁₆MeImBr and DS/C₁₆Me₃ABr. The changes in morphology agree with the expected effects of DS on the packing parameter of the colloidal aggregates.

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Introduction

Vesicles are spherical or ellipsoidal particles formed by enclosing a volume of aqueous solution in a surfactant bilayer.¹ They are used as models for biological membranes and in drug delivery. In the latter application, vesicles act as carriers for hydrophilic or hydrophobic drugs, e.g., by encapsulating the molecules of the drug into the nano-aqueous pseudo-phases, or its intercalation into the aggregate hydrophobic domains.^2–4 Administration of drug-containing vesicles may reduce drug toxicity and the minimum dose because of its accumulation at targeted sites in the body.⁵ The limitations of phospholipid-based liposomes as drug delivery systems (hydrolysis and oxidative degradation) prompted investigations on using vesicles of other amphiphilic substances, e.g., polymers and surfactants.^6–9 Vesicles also act as a transdermal drug solubilization systems; this application reduces the toxicity and enhances the skin permeation of the drug.¹⁰

Vesicles prepared from cationic surfactants have advantages over liposomes and non-ionic surfactants (niosomes), because their formulation is simple and their chemical stability against hydrolytic and oxidative degradation in aqueous medium is high.^11,12 Vesicle formation of cetyltrimethylammonium bromide (C₁₆Me₃ABr) was induced using 5-methylsalicylic acid,¹³ cholesterol,¹⁴ and sodium deoxycholate.¹⁵

Ionic liquids (ILs) are composed only of ions and have, by operational definition, melting points < 100 °C. Due to their structural versatility, they have found applications as solvents, catalysts and for capturing of-, and sensors for carbon dioxide.^16–19 IL-based surfactants (ILBSs) possess one or more hydrophobic tails, usually attached to a heterocyclic ring (e.g., imidazolium, pyridinium, piperidinium, pyrrolidinium) or amino acid cation (glycine, alanine, valine, proline and glutamic acid).^20–42 Aggregation behavior of ILBSs that carry heterocyclic cations was investigated,^16–43 and the micellar properties were compared with those of conventional surfactants.⁴⁴

The critical micelle concentration, cmc, of ILBSs are usually lower than those of conventional surfactants with the same hydrophobic “tail” and counter-ion.²⁵ Few reports were published on vesicle formation by ILBSs, induced by addition of inorganic and organic electrolytes;⁴⁵ oppositely charged ionic surfactants (leading to formation of catanionic systems),^46–49 and cholesterol.⁵⁰ For the ILBSs 1-alkyl-3-methylimidazolium bromide series, C_nMeImBr (n = 10, 12, 14), the transition from spherical micelles to uni-lamellar vesicles was induced by increasing the concentration of the surfactant.⁵¹

The aim of the present contribution is to probe the effects of diclofenac sodium (DS) on the morphologies of micellar aggregates of 1-hexadecyl-3-vinylimidazolium bromide, C₁₆VnImBr, 1-hexadecyl-3-methylimidazolium bromide C₁₆MeImBr and C₁₆Me₃ABr (Vn, Im, Me, A = vinyl, imidazolium, methyl and ammonium, respectively) for potential therapeutic use, e.g., for drug delivery in topical applications. DS is a non-steroidal drug that is widely prescribed, especially due to its anti-inflammatory effect. Worldwide, it is the twelfth ranking generic prescription drug.⁵² The DS-induced micellar transitions were evaluated at 25 °C using turbidity, viscosity, dynamic light scattering (DLS), and transmission electron microscopy, TEM. The transitions observed were spherical micelles → worm-like micelles → vesicles, indicating potential application in drug delivery.

During the preparation of this manuscript, a publication appeared on the effects of the DS on the micellar morphology of the ILBSs 1-R-3-methylimidazolium bromides; R = C₁₂, C₁₂MeImBr and R = C₁₄, C₁₄MeImBr.⁵³ Our data for C₁₆MeImBr complete, therefore, this recently published work on the 1-R-3-methylimidazolium bromide series. Additionally, we probed the effects of DS on the micellar aggregates of C₁₆VnImBr, an ILBS whose head-ion carries an unsaturated (vinyl) group, and (conventional) C₁₆Me₃ABr. We have recently showed that the relative rigidity and less hydrophobic character of the vinyl group (relative to the ethyl group) lead to different micellar properties, including packing of the monomers in the micelles of C₁₆VnImBr relative to C₁₆EtImBr (Et = ethyl).⁵⁴

Experimental

Materials

The molecular structures of the materials employed in the present study are depicted in Scheme 1:

Scheme 1 Molecular structures of the compounds of interest in the present work.

The ILBSs C₁₆VnImBr and C₁₆MeImBr were from a previous study.⁵⁴ C₁₆Me₃ABr and DS were purchased from Spectrochem Pvt (Mumbai). Where appropriate, the starting solid materials were dried under reduced pressure. Double distilled, deionized water having conductivity of 6.1–6.4 μS cm⁻¹ was used throughout.

Methods

Notes: we performed all measurements at 25 ± 0.1 °C. We prepared solutions of DS in ILBS by mixing the appropriate volumes of aqueous stock solutions of both compounds. The pH value of all aqueous solutions (before mixing) was carefully adjusted to 6.2.

Turbidity measurements

We used Varian Carry 50 spectrophotometer, equipped with a thermostated cell compartment, using 1 cm path-length quartz cuvettes. Values of the absorbance (A) of the DS/surfactant solutions were recorded at variable [DS], at λ = 500 nm, i.e., where the surfactant and DS do not absorb.

Viscosity measurements

These were performed using Brookfield DV-2 + Pro viscometer.

Dynamic light scattering measurements

DLS measurements were performed using Zetasizer Nano ZS90 (Malvern). All aqueous solutions were filtered (0.45 micron filter) into the quartz cuvette; the latter was washed several times with the filtered solution before performing the DLS measurement. The distribution of the aggregate diameters was calculated by the cumulants method.

Transmissions electron microscopy measurements (TEM)

TEM images were recorded on JEM 2100 (JEOL), equipped with a LaB6 gun, operating at 200 kV acceleration voltage. The solution, 10 μL, was deposited on 200 mesh size Formvar/carbon-coated copper grid (10 nm Formvar/1 nm carbon film thicknesses) left for 10 min, the excess liquid on the grid was removed, and the residue was stained using 2% uranyl acetate solution.

Calculation of the micellar packing parameter

The packing factor (P) was calculated from the equation: P = v_t/(a_hl_c,t), where v_t is the volume of the surfactant tail, a_h is the (optimal) area occupied by the surfactant head-group, and l_c,t is the critical chain length of the tail.⁵⁵ We took n-hexadecane as a model for the hydrophobic tail of the surfactants studied. The calculated value of v_t was = 0.488 nm³, based on the molar mass and the density of this hydrocarbon (d = 0.7701 g cm⁻³ at 25 °C).⁵⁶ To calculate l_c,t, we used 0.9 × l_max, where l_max is the maximum length of the alkyl chain, as calculated using Tanford's equation: l_max = 0.15 + 0.1265N_C, where N_C is the number of carbons in the alkyl chain;⁵⁷ l_max = 2.174 nm and l_c,t = 1.9566 nm.

The values of a_h were calculated using two procedures: (i) for C₁₆Me₃ABr, we used literature values of the micellar hydrodynamic radius (2.3 nm) and aggregation number (N_agg = 76);⁵⁸ a_h = 0.875 nm² per molecule; (ii) the values of a_h for C₁₆MeImBr and C₁₆VnImBr was taken as minimum area occupied by the surfactant molecule at the water/air interface; a_h = 0.816 nm² per molecule for C₁₆MeImBr, and 0.80 nm² per molecule for C₁₆VnImBr.⁵⁴

Using the above-mentioned values, we calculated P = 0.285, 0.306 and 0.312 respectively for CTABr, C₁₆MeImBr and C₁₆VnImBr.

Results and discussion

Electrostatic attraction between ionic micelle and an oppositely charged substrate, as well as hydrophobic interactions between both species lead to accumulation of the substrate in the Stern layer of the micelle, a gradual decrease of the micellar surface potential, with concomitant changes of its geometry. These changes include micellar growth, e.g., into viscoelastic worm-like structures, accompanied with a large increase of viscosity and, eventually, formation of (low-viscosity) multi- or uni-lamellar vesicles. These morphology changes are most readily detected by following the dependence of some physical property of the system on [solubilizate]. The pK_a of DS in water is 4.16, i.e., the drug is essentially present as (DS⁻) under our experimental conditions. Indeed, when the pH of the DS/C₁₆VnImBr was decreased to 3 precipitation occurred as shown in Fig. SM-1 (Fig. 1 of ESI).† Consequently, we can safely assume that the above-mentioned micellar changes, if they occur, are due to interactions between the cationic micelle and the anionic drug, as indicated by ¹H NMR data of DS in cationic ILBSs.⁵³

In addition to conclusions based on the effect of DS on the packing parameter of the aggregates, vide infra, we followed the evolution of several properties of the solution, induced by adding increased concentrations of DS to 20 mmol surfactant. Values of the cmc of the surfactants employed are: 0.40 mmol, 0.54 mmol and 1.0 mmol, for C₁₆VnImBr; C₁₆MeImBr; C₁₆Me₃ABr, respectively,^54,58 i.e., all measurements discussed below were done on micellar solutions. The techniques that we employed were turbidity, viscometry, DLS, and TEM. Our results indicated clearly that incorporation of anionic DS into cationic micelles leads to the above-mentioned morphology changes; C₁₆VnImBr is more sensitive than the other two surfactants.

Packing parameter

Our calculated values of (P) for C₁₆Me₃ABr (0.285) and C₁₆MeImBr (0.306) agree with literature values, 0.29 and 0.27, respectively.⁵⁹ As expected, in the absence of DS, the three surfactants form micelles with P < 0.5. Vesicles form at P between 0.5 and 1. From the equation: P = v_t/(a_hl_c,t), this increase in P can result from a decrease in (a_h) due to the association between negatively charged DS and the surfactant cationic head-group to form contact ion pairs, with partial expulsion of the associated water of hydration (of the components of the catanionic species formed). This coulombic interaction, coupled with solute–surfactant hydrophobic interactions due to the presence of the two atomic rings of the former decrease (a_h), i.e., increase (P) and eventually lead to vesicle formation. We present below experimental evidence to corroborate our analysis of the expected change in (P) due to DS solubilization.

Turbidity measurements

As given in experimental, the solution absorbance was recorded at λ where the individual system components (DS and the surfactants) do not absorb light. Consequently, we can attribute any change in absorbance as a function of increasing [DS] to light scattering due to drug-induced aggregate morphology change. The results of these experiments are depicted in Fig. 1(a–c); we show images of the resulting solutions in Fig. 2–4.

Fig. 1 Dependence of the absorbance at 500 nm of surfactants solutions (20 mmol) on [DS], for C₁₆VnImBr, (a); C₁₆MeImBr, (b); C₁₆Me₃ABr, (c).

Fig. 2 Image of DS/C₁₆VnImBr solutions at a constant final surfactant concentration (20 mmol) and variable [DS] from 5 to 80 mmol. The final [DS] are written in red numbers.

Fig. 3 Image of DS/C₁₆MeImBr solutions at a constant final surfactant concentration and variable [DS]. The concentrations of both components are the same employed for C₁₆VnImBr, values of [DS] in mmol are written in red numbers.

Fig. 4 Image of DS/C₁₆Me₃ABr solutions at a constant surfactant concentration and variable [DS]. The concentrations of both components are the same employed for C₁₆VnImBr, values of [DS] in mmol are written in red numbers.

As shown in part (a–c) of Fig. 1, the absorbance remains practically constant until [DS] of ca. 10 mmol. At higher [DS], the solutions appear bluish (Tyndall effect),⁶⁰ there is an abrupt increase, and then decreases in absorbance. In all systems we observed precipitation, e.g., at 20 mmol DS. Similar changes were observed when the photo-responsive dye sodium 4-[(E)-phenyldiazenyl]benzoate was added to a micellar solution of C₁₆Me₃ABr,⁶¹ and are indicative of micelles to vesicles transition.⁶² Additionally in the case of ILBSs we observed phase separation, e.g., at 25 and 30 mmol DS (Fig. 2), 19 and 25 (Fig. 3).

As a function of increasing [DS], the following was observed: the order of [DS] necessary to produce the onset of turbidity was: C₁₆Me₃ABr (16 mmol) > C₁₆MeImBr (15 mmol) > C₁₆VnImBr (14 mmol). This result can be linked to the following order of surface activity (i.e., the surface tension at the cmc) C₁₆Me₃ABr (40.8 mN m⁻¹) > C₁₆MeImBr (34.2 mN m⁻¹) > C₁₆VnImBr (33.5 mN m⁻¹).⁵⁴

The oppositely charged DS⁻ and C₁₆Me₃ABr interact electrostatically as well as hydrophobically (between DS aromatic rings and the surfactant hydrophobic tail). The same mechanisms are operative for the ILBSs, in addition to interactions between π electrons of the drug, the surfactant heterocyclic ring and, for C₁₆VnImBr, the vinyl group.⁶³ The higher value of turbidity for the DS/C₁₆VnImBr system indicates the formation of larger size uni-lamellar vesicles as compared with the DS/C₁₆MeImBr and DS/C₁₆Me₃ABr counterparts.⁶⁴ This conclusion is corroborated by our DLS and TEM results, vide infra. We found that the precipitation and phase separation regions for the ILBSs, and the precipitation for C₁₆Me₃ABr occur close to equimolar ratio of DS/amphiphilic molecules as shown in Fig. 1. A similar behavior was observed when sodium-3-hydroxy-2-naphthoate was added to C₁₆Me₃ABr.⁶⁵

Viscosity measurements

Fig. 5 shows the changes in viscosity as a function of increasing [DS] for the three (drug/surfactant) systems.

Fig. 5 Viscosity changes as a function of increasing [DS] in aqueous solution of 20 mmol (a) C₁₆VnImB, (b) C₁₆MeImBr and (c) C₁₆Me₃ABr.

ILBSs generally form spherical micelles at concentrations above their cmc.^43,51 On comparing Fig. 1 and 5, we observe that with increasing the concentration of DS, viscosity and optical density data changed in similar manner. The increase in viscosity indicates the change in morphologies of micelles from spherical to elongated shape. At concentration of 13 mmol of DS, the viscosity reaches its maximum value, and the solution becomes viscoelastic, as indicated by trapping of air bubbles for long periods of time, Fig. 5. The steep increase in viscosity occurred at lower concentration of DS for C₁₆VnImBr (7 mmol) compared to C₁₆MeImBr (8 mmol) and C₁₆Me₃ABr (10 mmol) as shown in insets of Fig. 5. This indicates electrostatic and hydrophobic interactions of the drug with the head-group of C₁₆VnImBr. Further addition of DS above 13 mmol leads to a viscosity drop, followed by precipitation and phase separation (for ILBSs), or precipitation (for C₁₆Me₃ABr). Similar viscosity changes were previously reported for the addition of sodium tosylate to micellar C₁₆Me₃ACl.⁶⁶ Addition of DS to micellar solution of C₁₆VnImBr, C₁₆MeImBr and C₁₆Me₃ABr decreases electrostatic repulsion between the head-groups of the aggregated molecules, in addition to hydrophobic interactions between the alkyl chain of the amphiphile and the aromatic rings of DS. This two-site binding of DS leads to the tight packing of ILBSs and C₁₆Me₃ABr monomers, which results in the growth of micelles and, finally, to formation of vesicles. The size and shape of the micelles were further confirmed using DLS and TEM, vide infra.

Dynamic light scattering measurement (DLS)

The effect of DS on hydrodynamic diameter (D_h) of micellar aggregates of the three surfactants in aqueous solution were evaluated using DLS using the cumulants method. As an example, we show in Fig. 6 the size distribution in a solution containing 18 mmol DS and 20 mmol surfactant. The evolution of the average hydrodynamic diameters (D_h) as a function of increasing [DS] at a fixed [C₁₆VnImBr] is shown in Fig. 7.

Fig. 6 DLS results showing the effects of DS (18 mmol) on the average hydrodynamic diameters (D_h) of the amphiphilic aggregates present. Parts (A, B, C) are for C₁₆VnImBr, C₁₆MeImBr, and C₁₆Me₃ABr, respectively.

Fig. 7 Evolution of the average hydrodynamic diameters (D_h) as a function of increasing the concentration of DS in the presence of fixed [C₁₆VnImBr] of 20 mmol.

For comparison, the average values of D_h of the corresponding aggregates in the absence of DS are: 5.1 [this work], 2.2 [this work], 0.9 nm,⁶⁷ for C₁₆VnImBr, C₁₆MeImBr, and C₁₆Me₃ABr, respectively.

The increase in D_h of each aggregate with increasing [DS] indicates growth of spherical micelles into vesicles via the intermediate formation of wormlike micelles, vide infra the results of TEM. As an example, consider the results of C₁₆VnImBr. At low concentration of DS (5 mmol), the D_h of was 2.3 nm, which is a typical diameter of spherical micelles of C₁₆VnImBr.⁶⁴ An increase of DS from 5 mmol to 15 mmol is accompanied with an increase of D_h and a large concomitant increase in solution viscosity, indicating the formation of rod/wormlike micelles from spherical ones, see Fig. 5(a). Sharp increase in viscosity was observed at the same concentration range of DS. As [DS] was further increased, the drug–surfactant interactions increased, with concomitant decrease in the electrostatic repulsion between the surfactant head-groups. This resulted in tight packing of DS and the surfactant monomers, leading to an increase in (P) and vesicle formation. As shown in Table 1 for C_nMeImBr and C₁₆VnImBr, the effect of [DS] on the average D_h increases as a function of the length of the surfactant C_n, an indication of the importance of the drug–surfactant hydrophobic interactions.

Table 1 Evolution of the average hydrodynamic diameters of different ILBSs as a functions of the molar ration (DS)/(surfactant) for the series C_nMeImBr

ILBS	[DS], mmol	[DS]/[CnMeImBr], molar ratio	Average Dh, nm
C12MeImBr	50	0.5	164
C14MeImBr	25	0.5	190
C16MeImBr	18	0.9	161
C16VnImBr	14	0.7	189
	18	0.9	248
	35	1.75	310
	60	3.0	458

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Transmission electron microscopy (TEM)

Parts (A and B) of Fig. 8 show micrographs for worm-like micelle and vesicle, respectively. Addition of DS causes a change of aggregate morphology from spherical to worm like micelles and some small vesicles (13 mmol of DS; Fig. 8(A)) and finally into uni-lamellar vesicles (Fig. 8(B)), in agreement of the results of other techniques. The results of C₁₆MeImBr and C₁₆VnImBr shown in Table 2 indicate that vesicles are formed at lower {DS] with C₁₆VnImBr than with C₁₆MeImBr, probably due to interactions between the π-electrons of the drug and the vinyl group.

Fig. 8 TEM images for 20 mmol C₁₆VnImBr in the presence of 13 mmol (A, worm-like micelle) and 19 mmol DS (vesicle) in aqueous solution.

Table 2 Morphological changes induced by the dissolution of DS (mmol) in micellar ILBSs (20 mmol)

Technique	C16MeImBr			C16VnImBr
	Vesicle	Phase separation	Precipitation	Vesicle	Phase separation	Precipitation
Turbidity	15–18 and >30	19 and 25	20	14–19 and >35	20	25–35
Viscometry	14–18 and >30	19 and 25	20	14–19 and >35	20	25–35
DLS	18 and 60			14–19 and >35
TEM	60			19, 35 and 60

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Conclusions

The DS-induced morphological changes of the micellar aggregates depend on the concentration of (DS⁻) and the structure of the surfactant head-group. From viscosity data we concluded that transition of spherical micelles to rod/wormlike micelles occurs at lower [DS] for C₁₆VnImBr compare to DS/C₁₆MeImBr and DS/C₁₆Me₃ABr. This is a consequence of the presence of the vinyl group and possible π–π interactions between the imidazolium heterocycle and the aromatic rings of DS. Turbidity, viscosity, DLS and TEM analyses revealed that vesicles were formed for DS/C₁₆VnImBr mixtures at lower [DS], as compared to DS/C₁₆MeImBr and DS/C₁₆Me₃ABr. The TEM micrographs corroborated the conclusions drawn from other techniques.

Acknowledgements

Z. S. Vaid thanks Maulana Azad National Research Fellowship (grant MANF-2012-13-MUS-GUJ-10818); N. I. Malek acknowledges financial assistances through Department of Science and Technology, New Delhi (SR/FT/CS-014/2010), Institute Research Grants to the Assistant Professors by SVNIT and Council of Scientific and Industrial Research (CSIR), New Delhi (Grant No. 01 (2545)/11/EMR-II). O. A. El Seoud thanks FAPESP and CNPq for financial support (2014/22136-4) and Research Productivity fellowship (307022/2014-5), respectively. We thank Dr Paulo A. R. Pires and Mr Alfredo Duarte for help with TEM images.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra25577a

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