Mixed Micelle Formation between an Amino Acid-Based Anionic

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Mixed Micelle Formation between an Amino Acid-Based Anionic Gemini Surfactant and Bile Salts Célia M. C. Faustino,*,† Cláudia S. Serafim,† Inês N. Ferreira,† Mafalda A. Branco,† António R. T. Calado,† and Luis Garcia-Rio‡ †

Instituto de Investigaçaõ do Medicamento (iMed.ULisboa), Faculdade de Farmácia da Universidade de Lisboa, Av. Prof. Gama Pinto, 1649-003 Lisboa, Portugal ‡ Department of Physical Chemistry, Universidade de Santiago de Compostela, Av. das Ciencias s/n, 15706 Santiago de Compostela, Spain ABSTRACT: The mixed micelle formation in basic aqueous solutions between an anionic gemini amino acid-based surfactant derived from cysteine (C8Cys)2 and the bile salts sodium cholate (NaC) and sodium deoxycholate (NaDC) has been studied by conductivity, and the results have been compared with the ones obtained for binary solutions of (C8Cys)2 with the conventional anionic surfactant sodium dodecyl sulfate (SDS). A nonideal mixing pattern was observed for the mixed systems, and a striking behavior was found for the gemini surfactant−bile salt mixtures, with gemini−bile salt interactions changing from synergistic to antagonistic with increasing gemini surfactant composition, whereas for the (C8Cys)2/SDS mixed system, synergism was attained over all the molar fraction range studied. Regular solution theory (RST) was used to analyze the gemini surfactant−anionic surfactant binary mixtures, and the interaction parameter (β) has been evaluated, as well as mixed micelle composition. convex β-face, along with a flexible side chain ending on a carboxylate group (Figure 1), often conjugated with glycine or

1. INTRODUCTION Surfactants consist of a hydrophilic headgroup (polar or ionic) and a flexible, hydrophobic hydrocarbon chain (tail) that spontaneously aggregate in aqueous solution, forming supramolecular assemblies known as micelles once a certain critical concentration, called the critical micelle concentration (cmc), is reached.1−3 The amphiphilic nature of these compounds is responsible for their unique properties, such as adsorption at interfaces, self-association, and solubilization of hydrophobic molecules, underlying their wide use in the petrochemical, pharmaceutical, cosmetic, food, agrochemical, textile, paint, and coating industries as emulsifying, wetting and (anti)foaming agents, solubilizers, and suspension stabilizers.1−3 The interest in enhanced performances in these areas has driven research into mixed surfactant systems. Formulations using surfactant mixtures are often employed for many practical applications due to synergism, as the performance of the mixture is usually better than that attainable with the neat surfactants.4−6 Bile salts, which are among the major components of bile, are biological surfactants involved in the metabolism and excretion of cholesterol in mammals.7 Bile salts play important biological roles in the solubilization and transport of lipids, liposoluble compounds, and hydrophobic drugs from the gastrointestinal tract through mixed micelle formation;7−12 for example, sodium deoxycholate is used in the mixed micellar formulation of the poorly water-soluble polyene macrolide antibiotic amphotericin B, marketed as Fungizone, widely used in the treatment of systemic fungal infections.13 Unlike conventional surfactants, bile salts do not possess well-defined tail and headgroups, but instead are made of almost flat amphiphilic molecules composed of a rigid, hydrophobic steroid backbone with hydrophilic hydroxyl groups, varying in number, position, and orientation, located on the concave α-face and hydrophobic methyl groups on the © 2014 American Chemical Society

Figure 1. Three-dimensional structure of common bile salts. Conjugation of the carboxyl function (R4 = OH) with glycine or taurine leads to glycoderivatives (R 4 = NHCH2 CO2 − ) and tauroderivatives (R4 = NH(CH2)2SO3−), respectively.

taurine. As a consequence of their (almost) planar polarity, bile salts exhibit a complex self-assembly behavior with distinct aggregate properties that differ from those of conventional surfactants.7−9 Characterization of bile salt assemblies has been extensively carried out through several different methodologies motivated by their biological significance, and many reviews Received: Revised: Accepted: Published: 10112

January 26, 2014 May 30, 2014 May 30, 2014 May 30, 2014 dx.doi.org/10.1021/ie5003735 | Ind. Eng. Chem. Res. 2014, 53, 10112−10118

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understanding of the nature of the interaction of the gemini with bile salts of dissimilar hydrophobic moiety but of the same electrical charge. A complementary study of the interaction of the (C8Cys)2 with sodium dodecyl sulfate (SDS) has also been made for comparison purposes in terms of the nature of the interaction of the gemini with a conventional single-chain anionic surfactant. The chemical structures of the surfactants employed in the present study are shown in Figure 2.

based on their physicochemical properties have appeared.8,9,14−17 Mixed micelles comprising bile salts solubilize important biological compounds, such as cholesterol, bilirubin, and fatty acids, and represent promising systems for drug delivery.8−13 Bile salts are known to form mixed micelles with many drugs, which can act as drug reservoirs and have a profound influence in the drug bioavailability.10−13 Mixed micelle formation of bile salts with ionic surfactants and lipids has thus been the subject of considerable attention as these systems can be used to simulate biological processes and contribute to a better understanding of absorption phenomena within the intestine.18−21 Biobased surfactants, which are environmentally friendly surfactants that can be obtained from natural renewable sources,22−26 are an interesting class of biocompatible surfactants to study in mixed formulations with bile acids due to their potential pharmaceutical applications. Amino acidbased surfactants, as condensation products of amino acids with fatty acids or their derivatives, belong to such a class and offer the additional advantage of being amenable to biotechnological production as an alternative (or in combination with) chemical synthesis,26−28 or from protein hydrolysates from bioindustrial waste,23,29 thus turning secondary products into high addedvalue compounds. Our research group has recently synthesized and fully characterized an amino acid-based gemini surfactant derived from cysteine, (C8Cys)2,30,31 which was able to interact with biological (macro)molecules, such as membrane lipids, serum albumin proteins, and macrocyclic oligosaccharides.31−33 Dimeric or gemini surfactants are made of two hydrophilic headgroups and two hydrophobic hydrocarbon chains linked at the level of the headgroups by a rigid or flexible spacer.34,35 Gemini surfactants have attracted considerable attention as their special dimeric architecture renders them unique properties when compared to their monomeric counterparts, namely, superior efficiency in surface tension reduction and lower cmc values.35,36 Gemini surfactants have been used as biomembrane model systems and templates for nanoparticles and are also promising drug delivery and transfection agents.35,37−39 For the (C8Cys)2 gemini, the short disulfide bridge that acts as the spacer draws the two octyl chains of the gemini molecule close together, thus increasing alkyl chain density and also the charge density of the headgroups. Strong interaction between molecules is to be expected in mixtures containing this gemini. Synergism in mixed micelle formation was indeed found for surfactant mixtures between (C8Cys)2 and the phospholipid 1,2-diheptanoyl-sn-glycero-3-phosphocholine (DHPC), 32 which contrasts with the slight antagonism that characterized mixtures of DHPC with the gemini surfactant dimethylene-1,2bis(decyldimethylammonium bromide), 10−2−10, where a short methylene spacer links the two monomeric units.40 The present paper reports the behavior of (C8Cys)2, an amino acid-based anionic gemini surfactant derived from cysteine, in aqueous solutions containing bile salts. Information regarding amino acid-based gemini surfactant−conventional amphiphile mixed systems is scarce,41 and to our knowledge mixed micelle formation between gemini amino acid-based anionic surfactants and bile salts has not yet been published. Therefore, we report the conductometric study of the interaction of (C8Cys)2 with sodium cholate (NaC) and sodium deoxycholate (NaDC), aiming at a comparative

Figure 2. Chemical structures of amino acid-based gemini surfactant (C8Cys)2, bile salts sodium cholate (NaC), and sodium deoxycholate (NaDC), and conventional surfactant sodium dodecyl sulfate (SDS).

2. MATERIALS AND METHODS 2.1. Materials and Solutions. The amino acid L-cystine (99% purity) and octyl isocyanate (97% purity) were purchased from Aldrich and used as received. The surfactants sodium dodecyl sulfate, sodium cholate, and sodium deoxycholate with 99% purity were obtained from Sigma. Gemini surfactant (C8Cys)2 was synthesized from the condensation reaction of the amino acid L-cystine with the stoichiometric amount of octyl isocyanate, following literature procedures.30,31 The surfactant, with 99% purity, was obtained after purification by recrystallization from a water/acetone mixture, and it was characterized by conventional spectroscopic techniques, as fully described elsewhere.30,31 Binary mixtures of the gemini surfactant (C8Cys)2 with SDS or bile salt in the desired molar fraction range were prepared by mixing precalculated amounts of the stock solutions of both components in 1 mmol dm−3 sodium hydroxide (pH 11.0) to avoid bile salt hydrolysis.7,16 Pure surfactant stock solutions were prepared by accurately weighing the appropriate quantity of material and diluting with 1 mmol dm−3 sodium hydroxide to the final volume. 10113

dx.doi.org/10.1021/ie5003735 | Ind. Eng. Chem. Res. 2014, 53, 10112−10118


Article

compared to the free surfactant monomers, the micelles being worse charge carriers due to their higher weight and hydrodynamic radius, also reflecting the degree of counterion binding to the negatively charged micellar surface.42,43 The cmc values for each binary surfactant mixture were obtained from the breakpoint in the conductivity versus surfactant concentration plots, according to literature procedures,42,43 as exemplified in Figure 3. The degree of micelle dissociation, χ, was estimated from the ratio of the slopes in the post- and premicellar regions of the conductivity versus surfactant concentration curves, according to a simplified version of the method proposed by Evans.44 The accuracy of cmc determination from the conductivity versus surfactant concentration profile is dependent on an abrupt change of conductivity in the cmc vicinity,45 which occurred for all of the binary systems in the concentration ranges studied and also for pure SDS and pure gemini surfactant solutions, with CMC sharply defined owing to the highly cooperative nature of the aggregation process for dimeric as well as for most conventional amphiphiles. However, for pure bile salt solutions, the conductivity change in the cmc region was smooth, and the change in slope was small, which agrees with the high counterion binding typical of bile salt micelles.7,14,16 This can compromise the reliability of the cmc values thus determined, and surface tension measurements were performed to corroborate the cmc values obtained from the conductivity method. The cmc and surface tension at the cmc, γcmc, were determined from the breakpoint in the surface tension versus logarithm of bile salt concentration plots. These plots showed no minimum, which is a good indication of the purity of the bile salts. Cmc values determined from conductivity and surface tension measurements (Table 1) were in good agreement, and the former were used in further calculations. Literature values for the cmc of bile salts have been reported in the ranges of 4−20 and 2−5 mmol dm−3 for NaC and NaDC, respectively,7,14,16,17,46 and the average cmc values obtained in this study, 10.6 mmol dm−3 for NaC and 5.65 mmol dm−3 for NaDC, fall within these concentration ranges. Moreover, the experimental cmc values agree well with literature values of 12.5 mmol dm−3 for NaC and 6.4 mmol dm−3 for NaDC,16 obtained from surface tension measurements under the same experimental conditions employed in the present study, that is, 0.001 mol dm−3 NaOH at 25 °C. The experimental cmc values for the binary systems studied were found to deviate from the ideal mixing behavior as predicted by the Clint equation47

The composition of each binary solution was expressed in molar fraction of gemini surfactant, αG, defined as αG =

[(C8Cys)2 ] [(C8Cys)2 ] + [BS]

(1)

where [(C8Cys)2] and [BS] are the molar concentrations of gemini surfactant (C8Cys)2 and bile salt (or SDS) in the mixed solution, respectively. 2.2. Conductivity Measurements. Solution conductances were collected at 25.0 °C and 1 kHz with a Wayne-Kerr B905 automatic precision bridge (WKR, Runcton, UK) using an Ingold conductivity cell type 980-K19/120 with platinum electrodes and a cell constant of 1.09 cm−1. The cell was calibrated using standard Crison KCl solutions of the appropriate concentration range. All solutions were prepared with double-distilled deionized water (Milli-Q water purification system), yielding values always 0.3. Counterion binding to the surfactant ionic headgroups is known to reduce intramicellar electrostatic repulsions between the headgroups, thus promoting micellar growth.1−3 The degree of counterion binding for bile salts is typically lower than that found for conventional amphiphiles,14−16 and consequently micelle dissociation is higher, as shown in Figure 5. For (C8Cys)2−bile salt mixed systems, values of xG (Table 1) indicate that bile salts do not participate as significantly as

gemini surfactant in the formation of mixed aggregates, due to the bulky hydrophobic steroid skeleton, which inhibits incorporation in large extent of bile salts in the mixed micelles. Thus, mixed micelles of (C8Cys)2−bile salts are richer in the gemini surfactant when compared to the ideal values predicted by the Clint model for ideal mixing behavior, with xG > xGid for all of the molar fraction range studied. The same trend has been found for catanionic mixtures of bis(dodecyldimethylammonium bromide) cationic gemini surfactants of the type 12-s-12 with variable spacer length s and the bile salt sodium taurodeoxycholate (NaTDC).59 Participation of NaC in mixed micellar aggregates with gemini (C8Cys)2 is lower than that of NaDC, according to the lower micellar mole fraction of NaC (higher xG) when compared to NaDC, considering the xG values shown in Table 1. This behavior can be attributed to structural differences between the two bile salts, as NaC is bulkier and more hydrophilic than NaDC due to the presence of an extra hydroxyl group (Figure 2). The same trend has been observed for catanionic mixtures of these bile salts with alkyltrimethylammonium bromides, CnTABr, of variable chain length (n = 12, 14, 16).60 Similarly, micellar compositions for both nonionic surfactant octaethylene glycol monodecyl ether (C 10 E 8 ) and sodium glycochenodeoxycholate (NaGCDC) or sodium glycoursodeoxycholate (NaGUDC) mixed systems showed a tendency to change from bile salt-rich micelles to C10E8-rich micelles with the increase on the molar fraction of the nonionic surfactant, with mixed micelle formation being affected by the difference in bile salt molecular structure in terms of orientation of the hydroxyl group at the C7 position of the steroid skeleton.61 The cmc of the gemini surfactant−bile salt mixtures is dependent on the type of bile salt, and for the same molar fractions, (C8Cys)2−NaC binary solutions show higher cmc than the corresponding (C8Cys)2−NaDC binary mixtures, consistent with the higher hydrophilicity of NaC, which holds an extra hydroxyl group at C7, when compared with NaDC.7,14−16 Stronger synergism is thus to be expected for mixed gemini surfactant solutions with the more hydrophobic bile salt, NaDC, due to stronger hydrophobic interactions, similarly to binary mixtures of conventional surfactants.4−6 However, the inverse trend was observed, as lower β values were found for binary mixtures of the gemini surfactant with NaC, the more hydrophilic bile salt, than with NaDC. The same pattern has been described in the literature for binary mixtures of NaC or NaDC with nonionic polyoxyethylene-20sorbitan esters (polysorbates or Tweens) of different alkyl chain 10116



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lengths and saturation degrees, namely, monolaurate, monopalmitate, monostearate, and monooleate derivatives (Tween 20, Tween 40, Tween 6,0 and Tween 80, respectively),12,62,63 a class of biocompatible surfactants widely used as solubilizers in the pharmaceutical industry. Moreover, for these systems, stronger Tween−bile salt interactions were found for the more hydrophobic Tween, that is., the one with the longest hydrophobic tail, and for the more hydrophilic bile salt, NaC.62 Synergism is observed at gemini surfactant−bile salt compositions with αG < 0.3, and the stronger synergism for the (C8Cys)2−NaC binary system can be attributed to differences in the chemical structure of the bile salts, as NaC contains two α-axial hydroxyl groups located at positions C7 and C12, whereas NaDC has only one α-hydroxyl group at C12. The number (and position) of α-axial hydroxyl groups is known to be important for micelle stability,61−63 and for αG < 0.3, where synergism occurs, gemini mixed micelles with NaC are more stable than with NaDC, according to the more negative GE values of the former (Table 1). Negative GE values suggest that the mixed micelles formed are more stable than micelles of pure components,4−6 and mixed micelle stability increases with increase in bile salt concentration. The antagonism found for solution compositions with αG > 0.3 is also stronger for the more bulky bile salt NaC, which leads to increased steric interactions due to the presence of the extra hydroxyl group, when compared to NaDC. The bulky hydrophobic steroid core may induce incompatibility at the headgroup level due to packing geometry constraints.50,51 Incompatibility was removed when bile salts were replaced by SDS, with a long and flexible hydrocarbon chain, and (C8Cys)2−SDS mixtures showed synergism in the entire composition range, with β values always lower than the ones obtained for (C8Cys)2−bile salt mixtures with the same gemini molar fraction in the bulk.

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AUTHOR INFORMATION

Corresponding Author

*(C.M.C.F.) E-mail: cfaustino@ff.ul.pt. Notes

The authors declare no competing financial interest.

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REFERENCES

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4. CONCLUSION In this study the physicochemical properties of the mixed micelles formed between an amino acid-based anionic gemini surfactant, (C8Cys)2, and anionic bile salts NaC and NaDC were investigated and compared with binary mixtures of the gemini surfactant with conventional anionic surfactant SDS. Deviation from the Clint model47 describing ideal mixing behavior was found for all of the binary mixtures studied, and Rubingh’s regular solution theory4,5,48 was used to describe the binary systems. Determined physicochemical parameters included the cmc of ideal mixtures (cmcid), the gemini molar fraction in ideal mixtures (xGid) and in real mixed micelles (xG), and the interaction parameter (β). Synergism was found for the gemini−SDS system over the entire composition range studied, whereas the behavior of gemini−bile salt binary mixtures changed from synergistic to antagonistic for gemini molar fractions >0.3. The antagonistic mixing interactions between gemini surfactant and bile salts observed at solution compositions with αG > 0.3 were attributed to differences in the preferred curvature of the molecules in the mixed aggregate.50−52 Micellization in gemini surfactant−bile salt mixtures was found to depend not only on the hydrophobic effect, which aims at minimizing the hydrophobic surface, but also on hydrogen binding ability, which is determined by the number, position, and orientation of the hydroxyl groups of the bile salts7,14−16 as well as by packing geometry constraints.50,51 10117



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Mixed Micelle Formation between an Amino Acid-Based Anionic

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