Thermodynamic and structural characterization of zwitterionic micelles of the membrane protein solubilizing amidosulfobetaine surfactants ASB-14 and ASB-16

Surface tension and isothermal titration calorimetry (ITC) were used to determine the critical micelle concentration (cmc) of the zwitterionic amidosulfobetaine surfactants ASB-14 and ASB-16 (linear-alkylamidopropyldimethylammoniopropanosulfonates) at 25 °C. The cmc and the heat of micellization were determined from 15 to 75 °C by ITC for both surfactants. The increase in temperature caused significant changes in the enthalpy and in the entropy of micellization, with small changes in the standard Gibbs energy (ΔG(mic)), which is consistent to an enthalpy?entropy compensation with a compensatory temperature of 311 K (ASB-14) and 314 K (ASB-16). In the studied temperature range, the heat capacity of micellization (ΔC(p)(mic)) was essentially constant. The experimental ΔC(p)(mic) was lower than that expected if only hydrophobic interactions were considered, suggesting that polar interactions at the head groups are of significant importance in the thermodynamics of micelle formation by these surfactants. Indeed, a NMR NOESY spectrum showed NOEs that are improbable to occur within the same monomer, resulting from interactions at the polar head groups involving more than one monomer. The ITC and NMR results indicate a tilt in the polar headgroup favoring the polar interactions. We have also observed COSY correlations typical of dipolar interactions that could be recovered with the partial alignment of the molecule in solution, which results in an anisotropic tumbling. The anisotropy suggested an ellipsoidal shape of the micelles, which results in a positive magnetic susceptibility, and ultimately in orientation induced by the magnetic field. Such an ellipsoidal shape was confirmed from results obtained by SAXS experiments that revealed aggregation numbers of 108 and 168 for ASB-14 and ASB-16 micelles, respectively. This study characterizes an interesting micelle system that can be used in the study of membrane proteins by solution NMR spectroscopy.     

Micelle formation by N-alkyl-N-methylpyrrolidinium bromide in aqueous solution

The micellization of the ionic liquid N-alkyl-N-methylpyrrolidinium bromide (C(n)MPB, n = 12, 14 and 16) in aqueous solutions was investigated by surface tension measurements, electrical conductivity and static luminescence quenching. The effectiveness of the surface tension reduction (Π(cmc)), maximum surface excess concentration (Γ(max)) and the minimum area (A(min)) occupied per surfactant molecule at the air/water interface can be obtained from the surface tension measurements at 25 °C. The critical micelle concentration (cmc) at different temperatures and a series of thermodynamic parameters (ΔG, ΔH and ΔS) of micellization were evaluated from electrical conductivity measurements in the temperature range of 25-45 °C. The thermodynamic parameters show that the micelle formation is entropy-driven at low temperature and enthalpy-driven at high temperature. Furthermore, the micelle aggregation number (N(agg)) of C(n)MPB was calculated according to the Turro-Yekta method through static luminescence quenching and found that N(agg) (49, 55, and 59) increased with the hydrophobic chain length of C(n)MPB.     

Investigations on mixed micelles of binary mixtures of zwitterionic surfactants and triblock polymers: a cyclic voltammetric study

Cyclic voltammetry has been employed to investigate the mixed micellar behavior of the binary mixtures of different zwitterionic surfactants such as 3-(N,N-dimethylhexadecylammonio)propane sulfonate (HPS), 3-(N,N-dimethyltetradecylammonio)propane sulfonate (TPS) and 3-(N,N-dimethyldodecylammonio)propane sulfonate (DPS) with three triblock polymers (L64, F127 and P65) by using 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) as an electroactive probe at 25 °C. Critical micellar concentration (cmc) has been determined from the plots of variation in peak current (i_p) versus the total concentration of surfactant/triblock polymer. Diffusion coefficient of the electroactive species has also been reported. The regular solution theory approximation has been used to determine various micellar parameters of ideal systems. The variation in micellar mole fraction (X₁) of the zwitterionic surfactant supports the formation of mixed micelles, which are rich in triblock polymer component in the surfactant rich region of the mixture and vice versa. The regular solution interaction parameter (β) suggests the formation of mixed micelles due to the synergistic interactions in case of HPS/TPS/DPS + F127/P65 systems and gets affected by EO/PO ratio of triblock polymers.     

Aggregation behavior and interaction of an amphiphilic drug imipramine hydrochloride with cationic surfactant cetyltrimethylammonium bromide: light scattering studies

The aggregation behavior and interaction of an amphiphilic antidepressant drug imipramine (IMP) hydrochloride with the cationic surfactant cetyltrimethylammonium bromide (CTAB) have been studied using light scattering (both static and dynamic) techniques. Due to rigid tricyclic hydrophobic moiety present in the molecule, the drug shows interesting association behavior. The static light scattering measurements show that the self-association of IMP commenced above a well-defined critical micellar concentration (CMC), which decreases with increasing the mole fraction of the CTAB surfactant. Both the excess Gibbs energy (ΔG(ex)) and the Gibbs energy of micellization (ΔG(M)°) are negative, and decrease with increasing mole fraction of the surfactant. The hydrodynamic diameters (d(h)) of the micellar aggregates were also evaluated using the dynamic light scattering measurements. The data indicate formation of larger aggregates by IMP and CTAB due to mixed micellization and subsequent micellar growth. The results have been analyzed using different models (viz., Clint, Motomura, Rosen, Rubingh, etc.) for mixed micelle formation.     

Effect of alcohols on aggregation behaviors of branched block polyether Tetronic 1107 at an air/liquid surface

The aggregation behaviors of branched block polyether Tetronic 1107 (T1107) at an air/liquid surface was investigated in mixed solvents consisting of water and one of the following polar cosolvents: ethanol, n-propanol, ethylene glycol (EG), or glycerol (GLY). Surface tension measurements provide information about the effects of cosolvents on the critical micellization concentration (cmc), the standard Gibbs energy (ΔG°(mic)), the maximum surface excess concentration (Γ(max)), the minimum area per polyether molecule at the air/liquid surface (A(min)), and the standard free energy of adsorption (ΔG°(ads)). The addition of ethanol and n-propanol to water disfavors the micellization and progressively increases the cmc of T1107, whereas the cmc decreases with the addition of EG and GLY. The values of ΔG°(mic) of T1107 are all negative in mixed solvents, and their absolute values become smaller as the ethanol or n-propanol content increases but become larger as the EG or GLY content increases. The cosolvents have a significant effect on the surface adsorption and cmc, and the order is as follows: n-propanol-water > ethanol-water > water > EG-water > GLY-water. The octanol/water partition coefficient (log P) of the cosolvent is used to correlate the effects, and it could capture the effect of cosolvents on the cmc qualitatively. The surface dilational rheological properties of T1107 in water and water-alcohol mixtures were also studied by surface dilational viscoelasticity and surface tension relaxation measurements. The dilational elasticity decreases monotonously in the presence of ethanol or n-propanol. With the increasing concentration of EG and GLY, the dilational elasticity of T1107 passes through a maximum that coincides with the change in Γ(max).     

Thermodynamic characterization of 3-[(3-cholamidopropyl)-dimethylammonium]-1-propanesulfonate (CHAPS) micellization using isothermal titration calorimetry: temperature, salt, and pH dependence

A systematic investigation of the micellization process of a biocompatible zwitterionic surfactant 3-[(3-cholamidopropyl)-dimethylammonium]-1-propanesulfonate (CHAPS) has been carried out by isothermal titration calorimetry (ITC) at temperatures between 278.15 K and 328.15 K in water, aqueous NaCl (0.1, 0.5, and 1 M), and buffer solutions (pH = 3.0, 6.8, and 7.8). The effect of different cations and anions on the micellization of CHAPS surfactant has been also examined in LiCl, CsCl, NaBr, and NaI solutions at 308.15 K. It turned out that the critical micelle concentration, cmc, is only slightly shifted toward lower values in salt solutions, whereas in buffer media it remains similar to its value in water. From the results obtained, it could be assumed that CHAPS behaves as a weakly charged cationic surfactant in salt solutions and as a nonionic surfactant in water and buffer medium. Conventional surfactants alike, CHAPS micellization is endothermic at low and exothermic at high temperatures, but the estimated enthalpy of micellization, ΔHM0, is considerably lower in comparison with that obtained for ionic surfactants in water and NaCl solutions. The standard Gibbs free energy, ΔGM0, and entropy, ΔSM0, of micellization were estimated by fitting the model equation based on the mass action model to the experimental data. The aggregation numbers of CHAPS surfactant around cmc, obtained by the fitting procedure also, are considerably low (nagg ≈ 5 ± 1). Furthermore, some predictions about the hydration of the micelle interior based on the correlation between heat capacity change, Δcp,M0, and changes in solvent-accessible surface upon micelle formation were made. CHAPS molecules are believed to stay in contact with water upon aggregation, which is somehow similar to the micellization process of short alkyl chain cationic surfactants.     

Conductivities, volumes, fluorescence, and aggregation behavior of ionic liquids [C4mim][BF4] and [C(n)mim]Br (n = 4, 6, 8, 10, 12) in aqueous solutions

Densities, conductivities, and polarity indexes of pyrene for aqueous solutions of a series of ionic liquids [C(n)mim]Br (n = 4, 6, 8, 10, 12) and [C4mim][BF4] have been determined at 298.15 K as a function of ionic liquid concentrations. It was shown that possible aggregation appeared for the ionic liquids in aqueous solutions except for [C4mim]Br. The critical aggregation concentration (CAC) of the ionic liquids, the ionization degree of aggregates (beta), the standard Gibbs energy of aggregation (Delta G(m)(o)), the limiting molar conductivity (Lambda(m)(o)), and the standard partial molar volume (V(m)(o)) for the ionic liquids were derived from the experimental data. The dependence of the CAC, Delta G(m)(o), Lambda(m)(o), and V(m)(o) on the length of the alkyl chain of the cations was examined. It was further suggested from volumetric data that a micelle was formed for [C8mim]Br, [C10mim]Br, and [C12mim]Br in aqueous solutions. Their apparent molar volumes at the critical micelle concentration (V Phi,CMC), apparent molar volumes in the micelle phase (V(Phi)(mic)), and the change of their apparent molar volume upon micellization (Delta V Phi,m) were calculated by application of the pseudophase model of micellization. In addition, the average aggregation number of [C(n)mim]Br (n = 8, 10, 12) has been determined by the steady-state fluorescence quenching technique, and predicted from a simple geometrical mode. It is found that the experimental values are in good agreement with the predicted ones.     

Synergistic interactions in the mixed micelles of cationic gemini with zwitterionic surfactants: the pH and spacer effect

Mixed micelle formation of binary cationic gemini (12-s-12, s=4, 6) and zwitterionic (N-dodecyl-N,N-dimethylglycine, EBB) surfactants has been investigated by measuring the surface tension of aqueous solution as a function of total concentration at various pH values from acidic to basic, under conditions of 298.15 K and atmospheric pressure. The results were analyzed by applying regular solution theory (RST), and Motomura's theory, which allows for the calculation of the excess Gibbs energy of micellization purely on the basis of thermodynamic equations. The synergistic interactions of all the investigated cationic gemini + zwitterionic surfactants mixtures were found to be dependent upon the pH of the solution and the length of hydrophobic spacer of gemini surfactant. The evaluated excess Gibbs free energy is negative for all the systems.     

Interfacial composition, thermodynamic properties, and structural parameters of water-in-oil microemulsions stabilized by 1-pentanol and mixed surfactants

The present study is focused on the evaluation of the interfacial composition, thermodynamic properties, and structural parameters of water-in-oil mixed surfactant microemulsions [(cetylpyridinium chloride, CPC+polyoxyethylene (20) cetyl ether, Brij-58 or polyoxyethylene (20) stearyl ether, Brij-78)/1-pentanol/n-heptane, or n-decane] under various physicochemical environments by the Schulman method of cosurfactant titration of the oil/water interface. The estimation of the number of moles of 1-pentanol at the interface (n(a)(i)) and bulk oil (n(a)(o)) and its distribution between these two domains at the threshold level of stability have been emphasized. The thermodynamics of transfer of 1-pentanol from the continuous oil phase to the interface have been evaluated. n(a)(i),n(a)(i), standard Gibbs free energy (ΔG(t)(0)), standard enthalpy (ΔH(t)(0)), and standard entropy (ΔG(t)(0)) of transfer process have been found to be dependent on the molar ratio of water to surfactant (ω), type of nonionic surfactant and its content (X(Brij-58 or Brij-78)), oil and temperature. A correlation between (ΔH(t)(0)) and (ΔS(t)(0)) is examined at different experimental temperatures. Bulk surfactant composition dependent temperature insensitive microemulsions have been reported. Associated structural parameters, such as droplet dimensions and aggregation number of surfactant and cosurfactant at the droplet interface have been evaluated using a mathematical model after suitable modifications for mixed surfactant systems. In light of these parameters, the prospect of using these microemulsion systems for the synthesis of nanoparticles and the modulation of enzyme activity has been discussed. Correlations of the results in terms of the evaluated physicochemical parameters have been attempted.     

Insights into electrocatalysis

When standard reversible potentials for bulk solution reactions, U(0), are known, the reversible potentials when the reactant and product are adsorbed on an electrocatalyst surface, U(surf)(rev), are given in terms of these potentials and the adsorption Gibbs energy bond strengths: U(surf)(rev) = U0 + D(ads)G (Ox)/F-Δ(ads)G (R)/F (i). When the Δ(ads)G (Ox) and Δ(ads)G (Red) values are known at potential U, this equation is exact. When the overpotential for a multi-electron transfer reaction is minimal, each electron transfer takes place at the standard reversible potential for the overall reaction. In the case of O(2) reduction to water via the intermediate step OOH(aq) → O(aq) + OH(aq), or via O(2)(g) → 2O(aq), the respective endergonic O-O dissociation Gibbs energies are shown to be 2.52 eV and 4.76 eV. When the oxygen product and water reactant adsorb weakly, as on platinum, the adsorption Gibbs energies, Δ(ads)G, for O, OH, and OOH intermediates can be uniquely predicted using these data. All of the above depend exclusively on experimentally determined data. Reversible potentials have been calculated for oxygen reduction steps on the platinum electrocatalyst surface using Interface 1.0, a comprehensive computational code for the potential dependence of the electrochemical interface. Using these results as benchmarks, is found to be accurate to around 0.1 V when the Δ(ads)G are values calculated for the potentials of zero charge, instead of 1.229 V, which is a significant simplification. The variation in Δ(ads)G values between the calculated potentials of zero charge and 1.229 V are found to be 0.2 eV V(-1) or less. Prior work, using internal adsorption energies calculated at the potential of zero charge in place of Gibbs energies in was found to be accurate to within about 0.2 V. On platinum Δ(ads)G of the reaction OOH(ads) → O(ads) + OH(ads) is calculated at the potential of zero charge for the reactant and product to be about 1.2 eV exergonic under Langmuir conditions, and this Gibbs energy loss reduces the 1.229 V four-electron reversible potential on the platinum surface to an effective reversible potential of about 0.93 V for this mechanism on platinum. The effective reversible potential is a consequence of efficiency loss, not kinetics. Based on these values, the onset potential for four-electron oxygen reduction will be less than or equal to the effective reversible potential and on pure Pt(111) it appears to be equal to it.     

Effects of organic solvent addition on the aggregation and micellar growth of cationic dimeric surfactant 12-3-12,2Br-

The micellization and micellar growth of cationic dimeric surfactant propanediyl-alpha-omega-bis(dodecyldimethylammonium) bromide, 12-3-12,2Br-, have been studied in several water-organic solvent mixtures. The organic solvents were ethylene glycol, glycerol, 1,2-propylene glycol, 1,3-propylene glycol, acetonitrile, dioxane, formamide, and N,N-dimethylformamide. Results showed that the aggregation process was less favored in the binary mixtures than in pure water, which was explained by considering the influence of the solvophobic effect on micellization. The addition of organic solvents was accompanied by a diminution in the average aggregation number, Nagg, of the dimeric micelles. This diminution was due to the decrease in the interfacial Gibbs energy contribution, Delta G0interfacial, to the Gibbs energy of micellization caused by the decrease in the hydrocarbon/bulk-phase interfacial tension. As a result of the micelle size diminution, the concentration at which the sphere-to-rod transition occurred, C*, was higher in the mixtures than in pure water. Micelle size reduction is accompanied by a decrease in the ionic interactions and in the extra packing contribution to the deformation of the surfactants tails, making the formation of cylindrical micelles less favorable.     

Conductometry and Thermodynamics Study of Metal Dodecyl Sulfates in Aqueous Solution

The aggregation behavior of metal dodecyl sulfates (MDS), [Na¹⁺, Mg²⁺, Mn²⁺, Co²⁺, and Ni²⁺] in water has been studied by electrical conductivity (at 293.15–333.15 K) and surface tension methods (at 303.15 K). Critical micelle concentrations (CMCs), degree of counterion dissociation (β) evaluated from conductivity data. Using law of mass action model, the thermodynamic parameters viz. Gibbs energy (ΔG_m ⁰), enthalpy (ΔH_m ⁰), and entropy (ΔS_m ⁰) were evaluated. The enthalpy of micellization decreases strongly with increasing temperature. ΔG is always negative (thermodynamically favored process) and slightly temperature and counterion dependent. Gibbs energy and entropy exploit micellization as thermodynamic favorable process. The electrostatic repulsions between ionic head groups, which prevent the aggregation, are progressively screened as the ionic character decreases with the size of the counterion. The plots of differential conductivity, (dk/dc) _T,P , versus the total surfactant concentration enables us to determine the CMC values more precisely than the conventional method. Surfactants with strong condense counterion are adapted to rodlike micelle better than to a spherical micelle. The data are explained in terms of molecular characteristics of surfactants viz. degree of dissociation, polar head group size and counterion.     

Micellization of anionic gemini surfactants and their interaction with polyacrylamide

A series of anionic gemini surfactants have been synthesized. The surface properties and micellization process of as-prepared sulfonate gemini surfactants (SGS) and carboxylate gemini surfactant (CGS) have been studied by surface tension measurement and isothermal titration microcalorimetry. Meanwhile, the interaction of these five surfactants with polyacrylamide (PAM) was investigated using surface tension, steady-state fluorescence measurement, and isothermal titration microcalorimetry. The results show that the critical micelle concentrations (CMCs) of above-mentioned surfactants are more than 1 order of magnitude lower than those of corresponding single chain surfactants. Moreover, the enthalpy of micelle formation (ΔH _mic) for the investigated gemini surfactants is negative. In the surfactant–PAM systems, the thermodynamic parameters of binding have also been determined. The conclusion may be drawn that the binding strength of SGS onto PAM is stronger than that of CGS, resulting from more compact structure of SGS aggregates. With increasing surfactant hydrophobicity, the values of ΔH _agg become more exothermic and a ΔS _agg decrease was observed. Therefore, the interaction between SGS and PAM is enthalpy-driven.     

Self-assembly of cetylpyridinium chloride in water-DMF binary mixtures: a spectroscopic and physicochemical approach

The aggregation behavior of cetylpyridinium chloride (CPyCl) in N,N-dimethylformamide (DMF)-water mixed solvents was investigated using electrical conductivity and spectroscopic techniques. Micellar and thermodynamic parameters (DeltaG(m)(0), DeltaH(m)(0), DeltaS(m)(0) and Delta(m)C(p)(0)) were obtained from the temperature dependence of critical micelle concentrations in various aqueous mixtures of DMF. The differences in the Gibbs energies of micellization of CPyCl between water and binary solvents were determined to evaluate the influence of the cosolvent. The effect of cosolvent on the Krafft temperature (K(T)) and on the aggregation number was also analyzed. Micellar micropolarity was examined spectrophotometrically using two different probes, methyl orange (MO) and methylene blue (MB), and was found to increase with DMF addition, accompanied by an enhanced solvation. The mechanism of docking of surfactant and the probe molecules in the system were obtained by using Fourier transform infrared spectroscopy (FTIR) and nuclear magnetic resonance (NMR) spectroscopy.     

Chloride ion effects on synthesis and directed assembly of copper nanoparticles in liquid and compressed alkane microemulsions

Microemulsions are effective media for solution-based synthesis of metallic nanoparticles where surfactants and other ionic species influence the directed assembly of the nanomaterials with specific sizes, geometries, and compositions. This study demonstrates the effects of chloride ion on the synthesis of copper nanoparticles within the sodium bis(2-ethylhexyl)sulfosuccinate (AOT) reverse micelle system utilizing both liquid isooctane and compressed propane as the bulk solvent. Copper nanoparticle synthesis can be achieved in the presence of HCl in the micelle core, taking advantage of the buffering action of the AOT surfactant. The concentration of chloride ions influence the particle growth rate and dispersion in liquid isooctane. The presence of chloride ions during particle synthesis in compressed propane has a significant effect on the geometry and structure of the copper nanomaterials produced. Chloride ion addition to the compressed propane/Cu(AOT)(2)-AOT/water reverse micelle system at 20 degrees C and 310 bar results in the formation of diamond-shaped copper nanoparticle assemblies. The copper nanoparticle assemblies exhibit unique structure and retain this structure through repeated solvent processing steps, allowing separation and recovery of the assembled diamond-shaped copper nanoparticle structures.     

Dissolution thermochemistry of alkali metal dianion salts (M2X1, M = Li+, Na+, and K+ with X = CO3(2-), SO4(2-), C8H8(2-), and B12H12(2-))

The dissolution Gibbs free energies (ΔG°(diss)) of salts (M(2)X(1)) have been calculated by density functional theory (DFT) with Conductor-like Polarizable Continuum Model (CPCM) solvation modeling. The absolute solvation free energies of the alkali metal cations (ΔG(solv)(M(+))) come from the literature, which coincide well with half reduction potential versus SHE data. For solvation free energies of dianions (ΔG(solv)(X(2-))), four different DFT functionals (B3LYP, PBE, BVP86, and M05-2X) were applied with three different sets of atomic radii (UFF, UAKS, and Pauling). Lattice free energies (ΔG(latt)) of salts were determined by three different approaches: (1) volumetric, (2) a cohesive Gibbs free energy (ΔG(coh)) plus gaseous dissociation free energy (ΔG(gas)), and (3) the Born-Haber cycle. The G4 level of theory, electron propagator theory, and stabilization by dielectric medium were used to calculate the second electron affinity to form the dianions CO(3)(2-) and SO(4)(2-). Only the M05-2X/Pauling combination with the three different methods for estimating ΔG(latt) yields the expected negative dissolution free energies (ΔG°(diss)) of M(2)SO(4). Salts with large dianions like M(2)C(8)H(8) and M(2)B(12)H(12) reveal the limitation of using static radii in the volumetric estimation of lattice energies. The value of ΔE(coh) was very dependent on the DFT functional used.     

Interaction Behavior Between an Amino Sulfonate Surfactant and Octylphenol Polyoxyethylene Ether (10) in Aqueous Solution

The micellization of binary mixtures of sodium dodecyl diamino sulfonate (C12AS) and nonionic octylphenol polyoxyethylene ether (10) (OP-10) was investigated in aqueous solution at a pH of about 6.0. Two techniques, UV–Vis spectroscopy using pyrene as a probe and surface tensiometry, were employed in this work to obtain information on the micellization behavior of the mixed C12AS/OP-10 system. The interaction parameters between the two components, activity coefficients in mixed micelles, compositions of mixed micelles, and thermodynamic parameters of micellization (calculated using Clint’s equation, Rubingh’s treatment based on regular solution theory, and Rodenas’s treatment considering the Gibbs–Duhem equation) were evaluated for this mixed surfactant system. The results show that the synergistic effect between C12AS and OP-10 in all mixed systems plays a vital role in the reduction of the overall critical micelle concentration (cmc) value in aqueous solution. In the process of micellization, both the steric effect of the head group and the charge density for C12AS affect the formation and stability of the mixed micelles, and the entry of a small amount of C12AS into the unconsolidated micelle of OP-10 is more favorable to the formation of the mixed micelle by promoting the reduction of the mixed micelle cmc value. Thermodynamic data show that micellization for the mixed C12AS/OP-10 system is both an enthalpy and entropy driven process.     

Synthesis and surface properties of semi-fluorinated gemini surfactants with two reactive bromo pendant groups

This paper presents a series of semi-fluorinated gemini surfactants with two bromo pendant groups. It reviews the effect of the number of methylene units in the spacer group between the two hydrophilic quaternary ammonium heads. Critical micelle concentration (cmc) and free energy of micellization (ΔG(M)(0)) of the title surfactants, in aqueous solution, have been investigated as a function of the number n of carbon atoms in the hydrocarbon spacer. We have pointed out a different behaviour as compared to Gemini hydrocarbon homologues. In the present study, when the number of methylene units (n) in the spacer increases, the cmc first decreases and reaches an optimum for (n=6), then it increases linearly from n≥6. Variations of cmc have been interpreted in terms of conformation changes of the surfactant ion and progressive penetration of the alkyl chain spacer in the micelle hydrophobic core. In this series, the increase of the hydrophobicity seems not to favour the micellisation process as expected, probably impacted by the mutual phobicity of the perfluorinated tails and the hydrocarbon spacer. A minimum is reached for a spacer with six methylene units which seems to be the optimal conformation. The free energy of micellization (ΔG(M)(0)) confirm this tendency.     

Effect of hydrocarbon structure of the headgroup on the thermodynamic properties of micellization of cationic gemini surfactants: an electrical conductivity study

A series of cationic gemini surfactants butanediyl-1,4-bis(dodecyldialkylammonium bromide), C(12)H(25)N(+)(C(m)H(2)(m)(+1))(2)C(4)H(8)N(+)(C(m)H(2)(m)(+1))(2)C(12)H(25)·2Br(-), where m=1, 2, 3, 4, referred to as C(12)C(4)C(12)(Me), C(12)C(4)C(12)(Et), C(12)C(4)C(12)(Pr), and C(12)C(4)C(12)(Bu), respectively, were synthesized, and their thermodynamic properties of micellization were studied by electrical conductivity measurements. There existed a minimum critical micelle concentration (cmc) in the curve of cmc versus temperature, and the temperature of the minimum of cmc (T(min)) increased with increasing the headgroup alkyl chain length. The values of log (cmc) depended linearly on carbon number of the alkyl chains, but that was not true for the carbon number of the headgroup substituents. The temperature dependence of cmc and degree of counterion association (β) were used to calculate the Gibbs free energy (Δ(mic)G°), enthalpies (Δ(mic)H°) and entropies (Δ(mic)S°) of micelle formation for these gemini surfactants, and well correlated enthalpy-entropy compensation was observed. The analyses showed C(12)C(4)C(12)(Me) and C(12)C(4)C(12)(Et) behaved similarly in terms of thermodynamics of micellization, but they behaved differently from C(12)C(4)C(12)(Pr) and C(12)C(4)C(12)(Bu), which could be ascribed to the hydrophobicity and the location of the headgroup alkyl chains in the aggregates. These initial results indicate the headgroup alkyl chain plays an important role in influencing the thermodynamic properties of gemini surfactants.