Use of the semi-equilibrium dialysis method in studying the thermodynamics of solubilization of organic compounds in surfactant micelles. Systemn-hexadecylpyridinium chloride-phenol-water

The semi-equilibrium dialysis method has been used to infer solubilization equilibrium constants or, alternatively, activity coefficients of solutes solubilized into micelles of aqueous surfactant solutions. Methods are described for inferring the concentrationa of monomers of the organic solute and of the surfactant on both sides of the dialysis membrane, under conditions where the organic solute is in equilibrium with both the high-concentration (retentate) and low-concentration (permeate) solutions. By using a form of the Gibbs-Duhem equation, activity coefficients of both phenol (the solubilizate) and n-hexadecylpyridinium chloride (the surfactant) are obtained for aqueous solutions at 25°C throughout a wide range of relative compositions of surfactant and solubilizate within the micelle. The apparent solubilization constant, K=[solubilized phenol]/([monomeric phenol][micellar surfactant]), is found to decrease significantly as the mole fraction of phenol in the micelle increases.     

Capillary electrophoresis study on the micellization and critical micelle concentration of sodium dodecyl sulfate. Influence of solubilized solutes

The influence of solubilized solutes on the micellization and critical micelle concentration (CMC) of sodium dodecyl sulfate (SDS) were investigated by means of capillary electrophoresis (CE). Three different structural types of test solutes, including chloropyridines. chlorophenols and cephalosporins with different binding strength to SDS micelles, were selected in this study. The variations of the effective electrophoretic mobility of these solutes as a function of SDS concentration in the premicellar and micellar regions were analyzed. Interestingly, the results indicate that, in the presence of these solubilized solutes, the micellization of SDS may occur over a range of SDS concentration, with the aggregate size increasing over this range. Depending on the nature of solubilized solutes and the extent of the interactions between solubilized solutes and SDS micelles, the CMC value of SDS may vary significantly. The incorporation of solubilized solutes into SDS micelles to form mixed micelles is proposed to interpret the migration behavior of solubilized solutes in CE.     

Micellar electrokinetic chromatography in coated capillaries: Determination of micelle–aqueous partition coefficients

A micellar electrokinetic chromatography method is presented for the determination of the partition coefficient for the distribution of nonpolar and moderately polar solutes between the micelle and the aqueous phases in aqueous micellar solutions. In comparison with the literature the method is, theoretically and experimentally, the most straightforward for this application. An equation is derived for the determination of the partition coefficient in a coated fused silica capillary, with neglible electroosmotic flow, from simple measurements of the migration times of solutes and a marker for micelle migration. The advantages and limitations of this method are discussed. The method is tested by using sodium dodecylsulfate (SDS) surfactant and naphthalene, benzene, toluene, and phenol solutes. Micellar electrokinetic chromatography in coated columns isideally suited for the separation of hydrophobic solutes in aqueous samples.     

Properties of polyethylene glycol (23) lauryl ether with cetyltrimethylammonium bromide in mixed aqueous solutions studied by self-diffusion coefficient NMR

NMR self-diffusion coefficient measurements have been used to study the properties of polyethylene glycol (23) lauryl ether (Brij-35) with cetyltrimethylammonium bromide (CTAB) in the mixed aqueous solutions with different mole fractions of CTAB. By fitting the self-diffusion coefficients to the two-state exchange model, the critical micelle concentrations of the two solutes in the mixed solutions (cmc*1 and cmc*2) were obtained. The critical mixed micelle concentrations (cmc*) were then evaluated by the sum of cmc*1 and cmc*2, which are in good agreement with the results measured by the surface tension method. The cmc* values are lower than those of the ideal case of mixing, which indicates that the behavior of the CTAB/Brij-35 system is nonideal. Moderate interactions between CTAB and Brij-35 in their mixtures can be deduced from the interaction parameters (betaM) based on the cmc* obtained by the NMR self-diffusion method. The compositions (x1) of the mixed micelles at different total surfactant concentrations were also evaluated. By using these results, a possible mechanism of mixed micellar formation and a picture of the formation of nonsimultaneous CTAB/Brij-35 binary mixed micelle were proposed. In contrast to the case of CTAB/TX-100 system, Brij-35 molecules have a tendency to form micelles first at any mole fraction of CTAB. The mixed micellar self-diffusion coefficients (Dm) increase slightly at lower CTAB molar ratios, and then speed up with increasing CTAB mole fraction.     

Solubility measurement of methane in aqueous solution of sodium dodecyl sulfate at ambient temperature and near hydrate conditions

Solubility data of methane in aqueous solutions of sodium dodecyl sulfate (SDS) with different concentrations were measured at ambient temperature and near hydrate conditions. The critical micelle concentration (CMC) and the number of methane molecules dissolved in each micelle of the methane + water + SDS system were calculated and compared with those of the ethylene + water + SDS system. The results demonstrated that the micelles could be formed in the SDS concentration range where an efficient promotion effect on hydrate formation was previously reported; the micelle solubilization to methane molecules was remarkable near hydrate conditions, and the ethylene molecules could be solubilized in micelles in preference to methane molecules.     

Aggregation of sodium dodecyl sulfate in poly(ethylene glycol) aqueous solution studied by <Superscript>1</Superscript>H NMR spectroscopy

¹H NMR self-diffusion coefficient, spin–spin relaxation and two-dimensional nuclear Overhauser enhancement spectroscopy measurements of sodium dodecyl sulfate (SDS) in poly(ethylene glycol) (PEG) aqueous solution show that SDS molecules start to self-aggregate at a concentration of 3.3 mM, which is well below the normal critical micellar concentration (cmc). SDS micelles are formed when the cmc is reached with PEG solubilized in their hydrophobic micellar cores. Electronic Publication     

Solubilization of Hydrophilic Compounds in Copolymer Aggregates

The solubilization of five hydrophilic water-soluble aroma compounds in self-aggregating triblock amphiphilic copolymers of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO), with similar percentages of PEO and different molecular weights, was studied. The five hydrophilic compounds (diacetyl, 2-methylpyrazine, pyrrole, furfural, guaiacol) were carefully selected to represent hydrophilic molecules with a similar molecular weight and molecular volume, but with different abilities to interact with the micellar core of PPO moieties and with the PEO palisade side chains. It was found that the solubilized solute mole fraction increased and the aggregate-water partition coefficients of the solutes decreased with increasing free solute concentration in the aqueous phase. The partition coefficients were smaller than those obtained for hydrophobic compounds and equilibrium was reached at lower solubilization values. Guaiacol was the least hydrophilic molecule and had the highest partition coefficient. Diacetyl was the most water-soluble compound and exhibited the smallest partition coefficient. The data reveal that the higher molecular weight polymers solubilized more solute than the low-molecular-weight polymers. Moreover it is supposed that at low solute concentrations, guaiacol (containing a hydroxyl electron acceptor group) penetrates the core of the micelle and displaces water while at more elevated concentrations it seems to be solubilized in the micelle corona. Diacetyl, the most hydrophilic solute investigated (consisting of electron donor groups), prefers mainly the corona since its affinity for the polymeric core is very weak. The solubilization occurs in the palisade layer and the partition coefficient is independent of the free solute concentration. Selective site (palisade vs core) solubilization of hydrophilic compounds in polymeric micelles can be a powerful tool to protect sensitive materials from reactants present in the continuous water phase and to conduct surface-sensitive organic reactions. Furthermore, selective release properties of reactants and products can be designed. Copyright 2000 Academic Press.     

头孢唑酮对CTAB胶束结构特性的影响

应用电导法和荧光法测定了头孢唑酮对阳离子表面活性剂十六烷基三甲基溴化铵（CTAB）胶束第一cmc1、第二cmc2、胶束聚集数及体系粘度的影响,测定了头孢唑酮在CTAB胶束中的分配系数.结果表明,头孢唑酮的加入使得CTAB胶束的第一cmc和第二cmc均上升,胶束的聚集数和体系的粘度降低.上述结果与头孢唑酮和CTAB分子的相互作用及其在CTAB胶束相和水连续相的分配有关.     

Effects on the micellar solubilization of organic compounds by surfactant micelles. I. Length of carboxylic side chains in aromatic acids

The semiequilibrium dialysis method has been used to determine solubilization equilibrium constants and activity coefficients of benzoic, phenylacetic, and hydrocinnamic acids (solubilizate) in micelles of the cationic surfactant hexadecylpyridinum chloride (cetylpyridinium chloride) in 0.1M HCl aqueous solutions. Methods described previously were employed to infer the concentrations of monomeric organic solute and surfactant on both sides of the dialysis cell. Values of the apparent solubilization constant K of the neutral acids have been correlated with mole fractions of the acid in the micelle X_A, where K=X_A/[monomeric acid]. The activity coefficients of both acid and surfactant were obtained, consistent with the Gibbs-Duhem equation. The solubilization constants of all three acids are nearly the same, indicating that there is no significant effect owing to the presence of one or more methylene groups between the carboxylate and the phenyl groups of benzoic acid. The solubilization constants also decrease appreciably, and the activity coefficients of the acids increase, as the mole fraction of the acid in the micelle increases.     

Diffusion of sodium dodecyl sulfate micelles in agarose gels

The gradient diffusion of ionic sodium dodecyl sulfate micelles in agarose gel was investigated at moderate concentrations above the CMC. Of particular interest were the effects of micelle, gel, and sodium chloride concentration on the micelle diffusivity. Holographic interferometry was used to measure the gradient diffusion coefficient at three sodium chloride concentrations (0, 0.03, 0.10 M), three gel concentrations (0, 1, 2 wt%), and several surfactant concentrations. Time-resolved fluorescence quenching was used to measure aggregation numbers both in solution and gel. The micelle diffusivity increased linearly with surfactant concentration at the two larger sodium chloride concentrations and all gel concentrations. In general, the strength of this effect increased with decreasing sodium chloride concentration and increased with gel concentration. This behavior is evidence of decreasing micelle-micelle electrostatic interactions with increasing sodium chloride concentrations, and increasing excluded volume effects and hydrodynamic screening with increasing gel concentration, respectively. The only exception was at 0.1M sodium chloride and 2 wt% agarose, which showed a slight reduction in the slope compared to 1 wt% agarose. It was found that the concentration effect is quite strong for charged solutes: at a NaCl concentration of 0.03 M in a 2% agarose gel, in a solution with 3% SDS micelles by volume, the micelle diffusion coefficient is doubled relative to its value in the same gel at infinite dilution. The extrapolated, infinite-dilution diffusion coefficients and the rate at which the micelle diffusivity increased with surfactant concentration were compared with predictions of previously published theories in which the micelles are treated as charged, colloidal spheres and the gel as a Brinkman medium. The experimental data and theoretical predictions were in good agreement.     

Use of Micellar-Enhanced Ultrafiltration at Low Surfactant Concentrations and with Anionic-Nonionic Surfactant Mixtures

Micellar-enhanced ultrafiltration is a separation technique which can be used to remove metal ions or dissolved organics from water. Metal ions bind to the surface of negatively charged micelles of an anionic surfactant while organic solutes tend to dissolve or solubilized within the micelles. The mixture is then forced through an ultrafiltration membrane with pore sizes small enough to block passage of the micelles and associated metal ions and/or dissolved organics. Monomeric or unassociated surfactant passes through the membrane and does not contribute to the separation. This paper considers advantages of addition of small concentrations of nonionic surfactant to an anionic surfactant; the resulting anionic-nonionic mixed micelles exhibit negative deviation from ideality of mixing which leads to a smaller fraction of the surfactant being present as monomer and a subsequently larger fraction present in the micellar form. The addition of nonionic surfactant improved the separation of divalent zinc substantially at total concentrations above the critical micelle concentration (cmc) of the anionic surfactant. Both zinc and tert-butylphenol (a nonionic organic solute) show unexpected rejection at surfactant concentrations moderately below the cmc, where micelles are absent. This is considered as due to a higher surfactant concentration in the gel layer adjacent to the membrane where micelles are present. Reduction of this rejection at lower transmembrane pressure drops supports this mechanism. Some rejection of zinc was observed in the absence of surfactant but not of tert-butylphenol, indicating an additional effect of membrane charge for ionic solutes. Copyright 1999 Academic Press.     

青霉素钾对CTAB胶束性质的影响

研究了青霉素钾对十六烷基三甲基溴化铵（CTAB）的cmc、CTAB胶束聚集数和扩散系数的影响.研究结果表明，青霉素钾(Pen K)的加入使得CTAB胶束的第一cmc、第二cmc上升；CTAB球形胶束的聚集数下降,扩散系数增加；CTAB棒状胶束的聚集数增加,扩散系数降低.     

Contribution of molecular pathways in the micellar solubilization of monodisperse emulsion droplets

It is often proposed that oil solubilization in anionic and nonionic micelles proceeds by different mechanisms, with diffusion of the oil molecule thought to control the former, and the latter interfacially controlled. In order to investigate this hypothesis, the effect of aqueous phase viscosity, salt, and surfactant concentration during the solubilization process was studied. The progressive decrease in average droplet size of nearly monodisperse emulsions during solubilization in SDS or Tween 20 micellar solutions was monitored by light scattering, and the change in turbidity was measured by UV-vis spectrophotometer. The solubilization rates were analyzed using a population balance approach to calculate the mass transfer coefficients. Increasing the aqueous viscosity by adding sucrose reduced the mass transfer coefficients of n-tetradecane and n-dodecane but had a smaller effect on n-hexadecane. The strong dependence of the solubilization rate for the shorter chain length alkanes on aqueous viscosity supported a mechanism in which the oil undergoes molecular diffusion before being taken up by micelles. The dependence of the solubilization kinetics on surfactant concentration appeared consistent with this mechanism but yielded a slower micellar uptake rate than previously predicted theoretically. As the solute chain length increased in nonionic surfactant solutions, an interfacial mechanism mediated by micelles appeared to contribute substantially to the overall rate. Addition of salt only slightly increased the solubilization rate of n-hexadecane in SDS solutions and, thus, indicated a weak role of electrostatic interactions for ionic surfactants on the overall mechanism.     

Mass transport in Triton X series nonionic surfactant solutions: a new approach to solute-solvent interactions

Densities, viscosities and tracer diffusion coefficients for solutions of the nonionic surfactants Triton X-45, X-114, X-100 and X-102 in water (except for Triton X-45) and methanol, and for Triton X-100 in three water-methanol mixtures have been measured at 298.15 K and 308.15 K. The activation energy for viscous flow and the contributions to it from solute and solvent have been calculated. Comparison of the Gibbs energies of these systems shows the roles of polyoxyethylene chains of Triton X molecules in the interactions between solute and solvent, and also the effects of solvents on the inter-actions for different solutes. The viscosityB coefficients are positive for all of the surfactant solutions, and the temperature coefficients ofB are negative. In the non-micellar solutions in methanol, values ofB are small and the temperature coefficient ofB is nearly zero. In water-methanol mixtures the critical micelle concentration becomes larger as the methanol content increases up to 40% w/w; micelles are not formed at higher methanol concentrations. TheB coefficient decreases with increasing proportion of methanol in the solvent, and the temperature coefficient ofB changes from a fairly large positive value at low methanol contents to a small negative value at 80% w/w methanol.     

Enhancement of Ostwald ripening by depletion flocculation

The time dependence of the dynamic mobility and the ultrasonic attenuation of octane and decane oil-in-water emulsions stabilized by sodium dodecyl sulfate (SDS) was measured. The emulsions grew to larger droplets due to Ostwald ripening. The growth rate measured by attenuation depends on the surfactant concentration and the polydispersity of the emulsion. At surfactant concentrations below the critical micelle concentration (cmc) of SDS, the growth was linear with time and the rate was dependent on the polydispersity of the drops; the rate was several times faster than that predicted on the basis of a diffusion growth mechanism. Above the cmc, however, as the droplets grew in size there was a point at which the rate of growth increased, which corresponds to the droplet size at which depletion forces due to the surfactant micelles become significant. Under these conditions both the electroacoustic dynamic mobility and the acoustic attenuation spectra displayed characteristics of flocs: a large decrease in the phase lag at higher frequencies in the dynamic mobility spectrum and a decrease in the attenuation coefficient at low-megahertz frequencies with an increase at higher frequencies. This depletion flocculation enhancement in ripening rates in the presence of SDS micelles provides another, alternative, and self-consistent mechanism for the effect of surfactant micelles on Ostwald ripening.     

Distribution of emulsified monomers with different water solubility

The location and distribution of acrylic acid and styrene in emulsions made with a cationic surfactant, cetyltrimethylammonium bromide (CTAB), or an anionic surfactant, sodium dodecylsulfate (SDS), were determined with ultra-violet spectroscopy, conductivity, and potentiometry. In these systems, the acrylic acid remains in the aqueous phase near the micelle surface, whereas the styrene is located in the micelles or in emulsified droplets. In the absence of acrylic acid, some of the styrene is solubilized in the micelle interior and some is adsorbed at the micelle inner surface. Upon addition of acrylic acid, all the styrene is displaced to the center of the micelles. The interaction between acrylic acid and CTAB micelles is stronger than that between acrylic acid and SDS micelles. With CTAB, acrylic acid is adsorbed at the micelle surface, whereas with SDS, acrylic acid remains in the intermicellar solution. These differences can account for the differences reported in the emulsion copolymerization of acrylic acid and styrene using CTAB or SDS.     

The unusual importance of activity coefficients for micelle solutions illustrated by an osmometry study of aqueous sodium decanoate and aqueous sodium decanoate + sodium chloride solutions

Freezing-point and vapor-pressure osmometry data are reported for aqueous sodium decanoate (NaD) solutions and aqueous NaD + NaCl solutions. The derived osmotic coefficients are analyzed with a mass-action model based on the micelle formation reaction qNa(+) + nD(-) = (Na(q)D(n))(q-n) and Guggenheim equations for the micelle and ionic activity coefficients. Stoichiometric activity coefficients of the NaD and NaCl components and the equilibrium constant for micelle formation are evaluated. Illustrating the remarkable but not widely appreciated nonideal behavior of ionic surfactant solutions, the micelle activity coefficient drops to astonishingly low values, below 10(-7) (relative to unity for ideal solutions). The activity coefficients of the Na(+) and D(-) ions, raised to large powers of q and n, reduce calculated extents of micelle formation by up to 15 orders of magnitude. Activity coefficients, frequently omitted from the Gibbs equation, are found to increase the calculated surface excess concentration of NaD by up to an order of magnitude. Inflection points in the extent of micelle formation, used to calculate critical micelle concentration (cmc) lowering caused by added salt, provide unexpected thermodynamic evidence for the elusive second cmc.     

Coupled tracer diffusion coefficients of solubilizates in ionic micelle solutions from liquid chromatography

The peak-broadening (Taylor dispersion) method is used to measure the diffusion of traces of alcohols (ethanol, n-butanol, n-hexanol, n-octanol, n-decanol) in aqueous solutions of sodium dodecylsulfate micelles at 25°C. A small quantity of each alcohol is injected into a long capillary tube containing a laminar stream of the micelle solution. The tracer diffusion coefficient is calculated from the broadened distribution of the eluted alcohol which is measured by differential refractometry. The fraction of each alcohol that is solubilized by the micelles is estimated from the drop in the diffusion coefficient relative to the value for the free alcohol molecules in pure water. The refractive index profiles across the dispersed samples are analyzed to obtain the cross-diffusion coefficient which gives the coupled flow of sodium dodecylsulfate produced by the tracer diffusion of each alcohol.     

Modulation of drug transport properties by multicomponent diffusion in surfactant aqueous solutions

Diffusion coefficients of drug compounds are crucial parameters used for modeling transport processes. Interestingly, diffusion of a solute can be generated not only by its own concentration gradient but also by concentration gradients of other solutes. This phenomenon is known as multicomponent diffusion. A multicomponent diffusion study on drug-surfactant-water ternary mixtures is reported here. Specifically, high-precision Rayleigh interferometry was used to determine multicomponent diffusion coefficients for the hydrocortisone-tyloxapol-water system at 25 degrees C. For comparison, diffusion measurements by dynamic light scattering were also performed. In addition, drug solubility was measured as a function of tyloxapol concentration, and drug-surfactant thermodynamic interactions using the two-phase partitioning model were characterized. The diffusion results are in agreement with a proposed coupled multicomponent diffusion model for ternary mixtures relevant to nonionic drug and surfactant molecules. Theoretical examination of diffusion-based drug transport in the presence of concentration gradients of micelles shows that drug fluxes and drug concentration profiles are significantly affected by coupled multicomponent diffusion. This work provides guidance for the development of accurate models of diffusion-based controlled release in multicomponent systems and for the applications of micelle concentration gradients to the modulation of diffusion-based drug transport.     

Determination of the Diffusion Coefficient for SDS Micelle with Different Shape and the Effects of Ethanol by Cyclic Voltammetry Without Probes

Micelle diffusion coefficient of SDS with different micelle shape in aqueous and water-ethanol solutions are determined by cyclic voltammetry without any probe. The diffusion coefficient decreases with as increasing SDS concentration. The first critical micellar concentration is 8.0xl0³, mol 1¹.corresponding to the transformation from premicelle to spherical micelle. The second critical micellar concentration is 5.60x10²and corresponding to transformation from spherical micelle to rod-like micelle. The less the weight ratio of SDS to ethanol is, the larger the diffusion coefficient is. The influence of added water to the micelle solution is almost the same for SDS-ethanol-HaO system with different micelle shape. Mechanism of electrochemical reaction for SDS at platinum electrode is discussed as well.