Interactions of cationic gemini surfactants with hydrophobically modified poly(acrylamides) studied by fluorescence and microcalorimetry

Steady-state fluorescence, time-resolved fluorescence quenching, and isothermal titration microcalorimetry have been used to study the interactions of cationic gemini surfactants alkanediyl-alpha,omega-bis(dodecyldimethylammonium bromide) (C(12)C(S)C(12)Br(2), S = 3, 6, and 12) with hydrophobically modified poly(acrylamide) (HMPAM) and unmodified poly(acrylamide) (PAM). Without addition of gemini surfactant, 0.2 wt % HMPAMs except PAM have already self-aggregated into hydrophobic aggregates. Different from single-chain surfactants, C(12)C(S)C(12)Br(2) have stronger interactions with HMPAMs to form surfactant/polymer aggregates, even with PAM. Addition of C(12)C(S)C(12)Br(2) may cause the disruption of HMPAM hydrophobic aggregates and the formation of mixed micelles. It is found that HMPAMs generate lower micropolarity of mixed micelles, larger values of enthalpy of interaction (DeltaH(ps)), and nearly constant values of Gibbs free energy of interaction (DeltaG(ps)). On the other hand, C(12)C(S)C(12)Br(2) with longer spacer brings out slightly lower micropolarity of mixed micelles, owing to the lower electrostatic repulsion between surfactant headgroups. Especially for C(12)C(12)C(12)Br(2), the values of DeltaH(ps) are much more endothermic and the values of DeltaG(ps) are much less negative. The weaker interactions of C(12)C(12)C(12)Br(2) with HMPAMs arise from the marked reduction of attraction between surfactant headgroups and polymer hydrophilic groups induced by its longer spacer.     

Salt effect on the complex formation between cationic gemini surfactant and anionic polyelectrolyte in aqueous solution

Salt effect on the interaction of anionic polyelectrolyte sodium carboxymethylcellulose (NaCMC) with cationic gemini surfactant hexamethylene-1,6-bis(dodecyldimethylammonium bromide) [C12H25(CH3)2N(CH2)6N(CH3)2C12H25]Br2 (C12C6C12Br2) has been investigated using turbidimetric titration, steady-state fluorescence, and mobility measurement. It is found that the critical aggregation concentration(cac) for C12C6C12Br2/NaCMC complexes depends little on addition of sodium bromide (NaBr). However, in the presence of nonionic surfactant Triton X-100 (TX100), the critical ionic surfactant mole fraction for the onset of complex formation (Yc) increases markedly with increasing NaBr concentration. These salt effects are supposed as the overall result from competition between the increase of interaction and the screening of interaction. The increase of interaction is referred to as the effect that the larger micelle with higher surface charge density induced by salt has a stronger interaction with oppositely charged polyelectrolyte. The screening of interaction is referred to as the salt screening of electrostatic attraction between the polymer chain and the surfactant. For complex formation between C12C6C12Br2 and NaCMC, the increase of interaction probably compensates the screening of interaction, leading to constant cac values at different salt concentrations. For complex formation between the C12C6C12Br2/TX100 mixed micelle and NaCMC, the screening of interaction probably plays a dominant role, leading to higher suppression of electrostatic binding of micelles to polyelectrolyte.     

Effect of the nature of the spacer on the aggregation properties of gemini surfactants in an aqueous solution

The aggregation properties of three dicationic quaternary ammonium gemini surfactants with the same structure, except the spacer group, diethyl ether, six methylene, and p-xylyl, have been studied using electrical conductivity and fluorescence. The critical micelle concentration (cmc) and the micelle aggregation number (N) were determined, and the micropolarity and the microviscosity of the micelle were characterized. The micelle ionization degree (alpha) was obtained by a combination of the electrical conductivity data and the micelle aggregation number. Furthermore, the Gibbs free energy of micellization (deltaGmic) was studied. These results have shown that the nature of the spacer has an important effect on the aggregation properties of gemini surfactants in an aqueous solution. A hydrophilic, flexible spacer prompts micelle formation, which leads to a smaller cmc, smaller alpha, larger N, and more negative deltaGmic. Meanwhile, the microviscosity study indicates that the gemini surfactant with a hydrophilic, flexible spacer forms a more closely packed micelle structure than the one with a hydrophobic, rigid spacer.     

Micellization of dissymmetric cationic gemini surfactants and their interaction with dimyristoylphosphatidylcholine vesicles

The micellization process of a series of dissymmetric cationic gemini surfactants [CmH2m+1(CH3)2N(CH2)6N(CH3)2C6H13]Br2 (designated as m-6-6 with m = 12, 14, and 16) and their interaction with dimyristoylphosphatidylcholine (DMPC) vesicles have been investigated. In the micellization process of these gemini surfactants themselves, critical micelle concentration (cmc), micelle ionization degree, and enthalpies of micellization (DeltaHmic) were determined, from which Gibbs free energies of micellization (DeltaGmic) and entropy of micellization (DeltaSmic) were derived. These properties were found to be influenced significantly by the dissymmetry in the surfactant structures. The phase diagrams for the solubilization of DMPC vesicles by the gemini surfactants were constructed from calorimetric results combining with the results of turbidity and dynamic light scattering. The effective surfactant to lipid ratios in the mixed aggregates at saturation (Resat) and solubilization (Resol) were derived. For the solubilization of DMPC vesicles, symmetric 12-6-12 is more effective than corresponding single-chain surfactant DTAB, whereas the dissymmetric m-6-6 series are more effective than symmetric 12-6-12, and 16-6-6 is the most effective. The chain length mismatch between DMPC and the gemini surfactants may be responsible for the different Re values. The transfer enthalpy per mole of surfactant within the coexistence range may be associated with the total hydrophobicity of the alkyl chains of gemini surfactants. The transfer enthalpies of surfactant from micelles to bilayers are always endothermic due to the dehydration of headgroups and the disordering of lipid acyl chain packing during the vesicle solubilization.     

混合胶束中Gemini阳离子表面活性剂14-s-14与常规表面活性剂的协同作用:连接臂及反离子效应(英文)

The mixed micelle formation of binary cationic 14-s-14 gemini with conventional single chain surfactants was studied by conductivity measurements.The critical micelle concentration(cmc) and the degree of counterion binding values(g) of the binary systems were determined.The results were analyzed by applying regular solution theory(RST) to calculate micellar compositions(X),activity coefficients(f1,f2),and the interaction parameters(β).The synergistic interactions of all the investigated cationic gemini+conventional surfactant combinations were found to be dependent upon the length of hydrophobic spacer of the gemini surfactant.The excess Gibbs free energy of mixing was evaluated,and it indicated relatively more stable mixed micelles for the binary combinations.     

Steady-state fluorescence investigations of aqueous binary mixtures of myristyl alcohol based bis-sulfosuccinate anionic gemini surfactant and effect of different conventional surfactants therein

The present research work is associated with the fluorescence investigations of binary aqueous mixed surfactants solutions of anionic bis-sulfosuccinate gemini surfactant (BSGSMA_1,8) and three different conventional surfactants—anionic viz. sodium dodecyl sulfate (SDS), cationic viz. cetyl trimethyl ammonium bromide (CTAB), and nonionic surfactant viz. Triton X 100. Steady-state fluorescence spectroscopy technique has been utilized to examine the micellization behavior of aqueous solution of pure myristyl alcohol-based BSGSMA_1,8 having flexible methylene chain [(CH₂)₈] as spacer group. Critical micelle concentration (CMC), aggregation number (N), and micropolarity of pure and mixed surfactants systems were explored during the investigations. The results revealed the best synergism behavior of prepared gemini BSGSMA_1,8 with SDS as compared to CTAB and Triton X 100. The maximum reduction in the value of pyrene intensity ratio (I₁/I₃) was observed for gemini and SDS mixed surfactant solution. On the other hand, the increased I₁/I₃ value of mixed gemini with Triton X 100 exhibited that mixed surfactant system of anionic gemini BSGSMA_1,8 with non-ionic Triton X 100 is not as compact as other mixed surfactant systems. Aggregation number increased and micropolarity decreased with increased concentration of gemini surfactants.     

Role of added counterions in the micellar growth of bisquaternary ammonium halide surfactant (14-s-14): 1H NMR and viscometric studies

The change in the morphology of a series of dicationic gemini surfactants C(14)H(29)(CH(3))(2)N(+)-(CH(2))(s)-N(+)(CH(3))(2)C(14)H(29), 2Br(-) (14-s-14; s=4-6) on their interaction with inorganic (KBr, KNO(3), KSCN) and organic salts (NaBenz, NaSal) have been thoroughly investigated by means of (1)H NMR spectral analysis and the results are well supported by viscosity measurements. The presence of salt counterions results in structural transition (spherical to nonspherical) of gemini micelles in aqueous solution. With an increase in salt concentration all the three gemini surfactants showed changes in their aggregate morphology. This change is dependent on the nature and size of the added counterion. The effect of inorganic counterions on the micellar growth is observed to follow the Hofmeister series (Br(-) < NO(3)(-) < SCN(-)). The roles of organic counterions are discussed on the basis of probable solubilization sites of the substrate molecule in the gemini micelles, showing more growth in case of Sal(-) than Benz(-). The results are confirmed in terms of the obtained values of chemical shift (δ), line width at half height (lw), and relative viscosity (η(r)). Also, the growth of micelles was most pronounced for the gemini surfactant with the shortest spacer (s=4). This was attributed to the unique molecular structure of gemini surfactant micelles having flexible polymethylene spacer chain linking the twin polar headgroups.     

Effect of surfactant structure on the stability of carbon nanotubes in aqueous solution

The suspending behaviors of multiple-wall carbon nanotubes (MWNTs), including pristine MWNTs (p-MWNTs) and acid-mixture-treated MWNTs (MWNTCOOH), stabilized by cationic single-chain surfactant, dodecyltrimethylammonium bromide (DTAB), and cationic gemini surfactant hexyl-alpha,beta-bis(dodecyldimethylammonium bromide) (C 12C 6C 12Br 2) were studied systematically. The surfactant structure influences the suspendability of MWNTs dramatically as well as the surfactant adsorption behavior on the nanotubes. Although both the surfactants can disperse the MWNTs effectively, they actually show different stabilizing ability. DTAB is not capable of stabilizing these two MWNTs below critical micelle concentration (CMC). However, C 12C 6C 12Br 2 can suspend both the nanotubes effectively even well below its CMC. Moreover, the adsorption of these two surfactants reaches equilibrium at twice the CMC with the original MWNT concentration of 2 mg/mL, 2 mM for C 12C 6C 12Br 2, and 30 mM for DTAB. After the adsorption equilibrium, the maximum amounts of the two suspended MWNTs in C 12C 6C 12Br 2 solution are about twice as much as those in DTAB solution. The strong hydrophobic interaction among the C 12C 6C 12Br 2 molecules and between the C 12C 6C 12Br 2 molecules and the nanotubes as well as the high charge capacity of C 12C 6C 12Br 2 lead to its much stronger adsorption ability on the MWNTs and result in its superior stabilizing ability for the MWNTs in aqueous phase. The gemini surfactant provides a possibility to effectively stabilize the MWNTs in aqueous solutions even at very low surfactant concentration well below its CMC.     

Thermodynamics of micellization of multiheaded single-chain cationic surfactants

The energetics of micelle formation of three single-chain cationic surfactants bearing single (h = 1), double (h = 2), and triple (h = 3) trimethylammonium [(+)N(CH(3))(3)] headgroups have been investigated by microcalorimetry. The results were compared with the microcalorimetric data obtained from well-known cationic surfactant, cetyl trimethylammonium bromide (CTAB), bearing a single chain and single headgroup. The critical micellar concentrations (cmc's) and the degrees of counterion dissociation (alpha) of micelles of these surfactants were also determined by conductometry. The cmc and the alpha values increased with the increase in the number of headgroups of the surfactant. The relationship between the cmc of the surfactant in solution and its free energy of micellization (DeltaG(m)) was derived for each surfactant. Exothermic enthalpies of micellization (DeltaH(m)) and positive entropies of micellization (DeltaS(m)) were observed for all the surfactants. Negative DeltaH(m) values increased from CTAB to h = 1 to h = 2 and decreased for h = 3 whereas DeltaS(m) values decreased with increase in the number of headgroups. The DeltaG(m) values progressively became less negative with the increase in the number of headgroups. This implies that micelle formation becomes progressively less favorable as more headgroups are incorporated in the surfactant. From the steady-state fluorescence measurements using pyrene as a probe, the micropolarities sensed by the probe inside various micelles were determined. These studies suggest that the micelles are more hydrated with multiheaded surfactants and the micropolarity of micelles increases with the increase in the number of headgroups.     

表面活性剂双水相的性质及其应用 Ⅰ. 表面活性剂双水相的微环境性质

阴、阳离子表面活性剂混合体系, 在一定浓度及混合比范围内, 可以形成两个互不相溶、平衡共存的水相, 称为表面活性剂双水相。其中阳离子表面活性剂过量的双水相体系, 称为阳离子双水相。本文分别以芘和罗丹明B作为探针, 用荧光探针法研究了摩尔比为1.6:1的C12NE和SDS混合体系成形成的阳离子双水相,测定其上层和下层的胶束微环境的极性和微粘度, 取得了有意义的结果。     

Gemini阳离子表面活性剂在水溶液中胶团化行为的温度效 应与焓、熵补偿

测定了Cemini阳离子表面活性剂C~m-----s-----C~m·2Br(m=8,10,12,;s=2,6及m=12;s=3,4)水溶液的电导,从电导(k)～表面活性剂浓度(c)曲线的转折点可求得临界胶团浓度cmc.实验发现,Gemini阳离子表面活性剂的胶团化倾向明显强于其“单体分子”)即单离子头基单烷烃链表面活性剂)。根据质量作用模型计算了胶经过程的吉布氏能、焓和熵的改变。结果表明Gemini表面活性剂聚集机理和其对应的“单体分子”类似,主要来自熵驱动。所有的焓/熵补偿图均呈现良好的线性关系,补偿直线在γ轴的截距随s减小而变小,这意味着具有较小s的Gemini表面活性剂倾向于生成稳定的胶团。     

Properties of binary surfactant systems of nonionic surfactants C12E10, C12E23, and C12E42 with a cationic gemini surfactant in aqueous solutions

Properties of binary surfactant systems of nonionic surfactants poly(ethylene oxide) (PEO) lauryl ethers (C(12)E(10), C(12)E(23), C(12)E(42)) with a cationic gemini surfactant, butanediyl-α,ω-bis(tetradecyldimethylammonium bromide) (14-4-14), have been investigated by Steady-state Fluorescence (FL), zeta potential, Dynamic Light Scattering (DLS), Transmission Electron Microscopy (TEM), Cryogenic Transmission Electron Microscopy (CryoTEM), and X-ray Diffraction (XRD). Through FL measurements, critical micelle concentration (CMC) of the three binary systems for different mixing mole fractions is determined and the values fall between those of pure constituent surfactants. Ideal CMC (CMC(ideal)), mole fraction in aggregates (X), interaction parameter (β), activity coefficients (f(1) and f(2)), and excess free energy of mixing (ΔG(ex)) have been calculated. All these parameters indicate nonideal behavior and synergistic interactions between the constituent surfactants, which is explained in terms of electrostatic attraction between headgroups of constituent surfactants and reduction of electrostatic repulsion between headgroups of 14-4-14 due to the presence of nonionic surfactants. DLS, TEM and CryoTEM results show that nonionic surfactants facilitate the formation of larger aggregates. Micelles and vesicles in larger size compared with those of 14-4-14 coexist in the mixed solutions. Both surfactant composition and PEO chain length are found to play a strong effect on the properties of the binary systems.     

Specific ion pairing and interfacial hydration as controlling factors in gemini micelle morphology. Chemical trapping studies

Results from chemical trapping experiments in micellar solutions containing 1.5-5 mM aqueous solutions of three didodecyl dicationic dibromide gemini surfactants with different methylene spacer lengths (12-n-12 2Br where n = 2-4 CH(2) groups) gave quantitative estimates of the molarities of interfacial bromide (Br(m)) and water (H(2)O(m)), the fractions of free and paired headgroups and counterions, and the net headgroup charge. These results are one of the most detailed compositional studies of an association colloid interface to date. Br(m) increases and H(2)O(m) decreases as n decreases and the two cationic charges are closer together. The 12-2-12 2Br gemini (the only one of the three geminis known to form threadlike micelles) shows a marked increase in Br(m) (from 2.3 to 3.6 M) and a decrease in H(2)O(m) (from 35 to 17 M) at the exceptionally low surfactant concentration in the vicinity of the previously reported sphere-to-rod transition or second cmc concentration. Rod formation occurs because of an increase in headgroup-counterion association and dehydration at the micelle surface that depend on both the free energies of hydration and specific ion interactions and surfactant and counterion concentrations. These and other recent chemical trapping results support a new model for the balance of forces controlling morphological transitions of association colloids. The hydrophobic effect drives the formation of headgroup-counterion pairs, which have a lower demand for water of hydration. Release of water permits tighter packing and formation of cylindrical aggregates.     

Polymerizable cationic gemini surfactant

A polymerizable cationic gemini surfactant, [CH(2)=C(CH(3))COO(CH(2))(11)N(+)CH(3))(2)CH(2)](2).2Br(-), 1 has been synthesized and its basic interfacial properties were investigated (in water and in the presence of 0.05 M NaBr). For comparison, the properties of monomeric surfactant corresponding to 1, CH(2)=C(CH(3))COO(CH(2))(11)N(+)(CH(3))(3).Br(-), 2, were also investigated. Parameters studied include cmc (critical micelle concentration), C(20) (required to reduce the surface tension of the solvent by 20 mN/m), gamma(cmc) (the surface tension at the cmc), Gamma(cmc) (the maximum surface excess concentration at the air/water interface), A(min) (the minimum area per surfactant molecule at the air/water interface), and cmc/C(20) ratio (a measure of the tendency to form micelles relative to adsorb at the air/water interface). For the polymerizable gemini surfactant, 1, the methacryloxy groups at the terminal of each hydrophobic group in a molecule have no contact with the air/water interface in the monolayer, whereas for the corresponding monomeric surfactant, 2, the methacryloxy group contacts at the interface forming a looped configuration like a bolaamphiphile. Polymerized micelles of the gemini surfactant are fairly small monodisperse and spherical particles with a mean diameter of 3 nm.     

Effect of hydrocarbon parts of the polar headgroup on surfactant aggregates in gemini and bola surfactant solutions

Cationic gemini surfactant homologues alkanediyl-alpha,omega-bis(dodecyldiethylammonium) bromide, [C12H25(CH3CH2)2N(CH2)SN(CH2CH3)2C12H25]Br2, where S = 4, 6, 8, 10, or 12, referred to as C12CSC12(Et), and cationic bolaamphiphiles BPHEAB (biphenyl-4,4'-bis(oxyhexamethylenetriethylammonium) bromide), PHEAB (phenyl-4,4'- bis(oxyhexamethylenetriethylammonium) bromide) were synthesized, and their aggregation behaviors in aqueous solution were studied and compared by means of dynamic light scattering, fluorescence entrapment, and transmission electron microscopy. Spherical vesicles were found in the aqueous solutions of these gemini and bola surfactants, which can be attributed to the increase of the hydrocarbon parts of the polar headgroup of the surfactants. In combination with the result of the other gemini with headgroup of propyl group, the increase of the hydrophobic parts of the surfactant polar headgroup will be beneficial to enhance the aggregation capability of the gemini and bola surfactants. Both of the vesicles formed in the gemini and bola systems showed good stabilities with time and temperature, but different stability with salt due to the different membrane conformations of surfactant molecules in the vesicles.     

Studies of mixed surfactant solutions of cationic dimeric (gemini) surfactant with nonionic surfactant C12E6 in aqueous medium

The interaction between the alkanediyl-alpha,omega-type cationic gemini surfactant, [(C(16)H(33)N(+)(CH(3))(2)(CH(2))(4)N(+)(CH(3))(2)C(16)H(33))2Br(-)], 16-4-16 and the conventional nonionic surfactant [CH(3)(CH(2))(10)CH(2)(OCH(2)CH(2))(6)OH], C(12)E(6) in aqueous medium has been investigated. The critical micelle concentrations of different mixtures have been measured by surface tension using a du Nouy tensiometer in aqueous solution at different temperatures (303, 308, and 313 K). Maximum surface excess (Gamma(max)) and minimum area per molecule (A(min)) were evaluated from a surface tension vs log(10)C (C is concentration) plot. The cmc value of the mixture was used to compute beta(m), the interaction parameter. The beta(sigma), the interaction parameter at the monolayer air-water interface, was also calculated. We observed synergism in 16-4-16/C(12)E(6) system at all concentration ratios. The micelle aggregation number (N(agg)) has been measured using a steady state fluorescence quenching method at a total surfactant concentration approximately 2 mM at 25 degrees C. The micropolarity and the binding constant (K(sv)) of mixed systems were determined from the ratio of intensity of peaks (I(1)/I(3)) of the pyrene fluorescence emission spectrum. The micellar interiors were found to be reasonably polar. We also found, using Maeda's concept, that the chain-chain interactions are very important in this system.     

Effect of head group polarity and spacer chain length on the aggregation properties of gemini surfactants in an aquatic environment

The aggregation behavior of cationic gemini surfactants with respect to variation in head group polarity and spacer length is studied through conductance, surface tension, viscosity, and small-angle neutron-scattering (SANS) measurements. The critical micellar concentration (cmc), average degree of micelle ionization (beta(ave)), minimum area per molecule of surfactant at the air-water interface (A(min)), surface excess concentration (gamma(max)), and Gibb's free energy of micellization (delta G(mic)) of the surfactants were determined from conductance and surface tension data. The aggregation numbers (N), dimensions of micelles (b/a), effective fractional charge per monomer (alpha), and hydration of micelles (h(E)) were determined from SANS and viscosity data, respectively. The increasing head group polarity of gemini surfactant with spacer chain length of 4 methylene units promotes micellar growth, leading to a decrease in cmc, beta(ave), and delta G(mic) and an increase in N and b/a. This is well supported by the observed increase in hydration (h(E)) of micelles with increase in aggregation number (N) and dimension (b/a) of micelle.     

Aggregation numbers of cationic oligomeric surfactants: a time-resolved fluorescence quenching study

The micelle aggregation numbers (N(agg)) of several series of cationic oligomeric surfactants were determined by time-resolved fluorescence quenching (TRFQ) experiments, using advantageously 9,10-dimethylanthracene as fluorophore. The study comprises six dimeric ("gemini"), three trimeric, and two tetrameric surfactants, which are quaternary ammonium chlorides, with medium length spacer groups (C(3)-C(6)) separating the individual surfactant fragments. Two standard cationic surfactants served as references. The number of hydrophobic chains making up a micellar core is relatively low for the oligomeric surfactants, the spacer length playing an important role. For the dimers, the number decreases from 32 to 21 with increasing spacer length. These numbers decrease further with increasing degree of oligomerization down to values of about 15. As for many conventional ionic surfactants, the micelles of all oligomers studied grow only slightly with the concentration, and they remain in the regime of small micelles up to concentrations of at least 3 wt %.     

Spontaneous aggregate transition in mixtures of a cationic gemini surfactant with a double-chain cationic surfactant

Controllable aggregate transitions were realized by mixing two kinds of cationic surfactants, hexylene-1,6-bis(dodecyldimethylammonium bromide) (C(12)C(6)C(12)Br(2)) and didodecyldimethylammonium bromide (DDAB). It was found that two parameters are the main factors determining the aggregation behavior of the mixed system, the total concentration of DDAB and C(12)C(6)C(12)Br(2) (C(T)), and the mole fraction of DDAB in the mixtures of DDAB and C(12)C(6)C(12)Br(2) (X(DDAB)). How these two parameters act on the aggregate transitions was studied in detail by various measurements including surface tension, turbidity, electrical conductivity, ζ potential, isothermal titration microcalorimetry, dynamic light scattering, cryogenic transmission electron microscopy, and (1)H NMR. When C(T) was constant, spontaneous vesicle-to-micelle transitions were found with decreasing X(DDAB) at high C(T). When X(DDAB) was constant, aggregate transitions were generated by gradually increasing C(T), depending on different X(DDAB) ranges. At X(DDAB) < 0.6, small spherical aggregates formed first and then transferred to vesicles, and finally the vesicles transitioned to micelles. At X(DDAB) ≥ 0.6, the progressive increase in C(T) led to aggregate transitions on the order of the arising of vesicles, the continuous growth of vesicles, the disruption of vesicles into micelles, and the final coexistence of vesicles and micelles. The hydrophobic interaction and electrostatic repulsion between DDAB and C(12)C(6)C(12)Br(2) together with the related degree of ionization and hydration of the surfactants were gradually adjusted by changing the ratio and the total concentration of these two surfactants, which should be responsible for the complicated aggregation behavior.     

Mixed micelle formation between amino acid-based surfactants and phospholipids

The mixed micelle formation in aqueous solutions between an anionic gemini surfactant derived from the amino acid cystine (C(8)Cys)(2), and the phospholipids 1,2-diheptanoyl-sn-glycero-3-phosphocholine (DHPC, a micelle-forming phospholipid) and 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC, a vesicle-forming phospholipid) has been studied by conductivity and the results compared with the ones obtained for the mixed systems with the single-chain surfactant derived from cysteine, C(8)Cys. Phospholipid-surfactant interactions were found to be synergistic in nature and dependent on the type of phospholipid and on surfactant hydrophobicity. Regular solution theory was used to analyse the gemini surfactant-DHPC binary mixtures and the interaction parameter, β(12), has been evaluated, as well as mixed micelle composition. The results have been interpreted in terms of the interplay between reduction of the electrostatic repulsions among the ionic head groups of the surfactants and steric hindrances arising from incorporation of the zwitterionic phospholipids in the mixed micelles.