J. Aguiar

On the determination of the critical micelle concentration by the pyrene 1:3 ratio method

Abstract The aim of this paper is to establish a simple and accurate approach to the treatment of pyrene 1:3 ratio data in the context of critical micelle concentration determination in surfactant solutions. The procedure we propose is based on the assumption that pyrene 1:3 ratio data are properly fitted by a Boltzmann-type sigmoid. From the fitting parameters we consider two singular points as possible candidates to determine the critical micelle concentration of the surfactant, adopting objective criteria for the election of one or the other point. With the purpose of supporting our contention, numerous specific examples are presented and discussed, including single ionic and nonionic surfactants, micellization of surfactants in the presence of additives, and several mixed-surfactant systems. In all the cases examined the experimental data were well fitted by a decreasing sigmoid of the Boltzmann type, and the proposed approach worked in an appropriate way, providing critical micelle concentration values in good agreement with those in the literature obtained using different methods.

Mixed micellization of octaoxyethylene monododecyl ether and n-alkyltrimethylammonium bromides

The micellization process of binary mixtures formed by octaoxyethylene monododecyl ether (C12E8) and three different n -alkyltrimethylammonium bromides, including dodecyltrimethylammonium bromide (DTAB), tetradecyltrimethylammonium bromide (TTAB), and cetyltrimethylammonium bromide (CTAB), was examined by using the fluorescence probing method. The critical micelle concentration values were determined by the pyrene 1:3 ratio method. The experimental data were analyzed on the light of various mixing thermodynamic models within the framework of the pseudophase separation model. In all the cases, a negativ ed eviation from the ideal behavior was found. However, the interaction parameter (b12), as reported by the regular solution theory, was found to be dependent on the micellar composition. It was established that, in addition to the electrostatic interactions between the headgroups of the surfactants, secondary effects of steric character due to the different length of the alkyl chain of the n alkyltrimethylammonium bromides must be taken into account to justify the experimental data. The mixing thermodynamic functions for the C12E8/CTAB system were determined. The change in the micellization entropy was interpreted in connection with the hydration status of the mixed micelles. The increase observed in the micellar micropolarity with the content of the ionic component, which was consistent with the corresponding change in the micellization entropy, was attributed to the formation of more open micelles with a more hydrated structure. Data obtained in this study suggest that in the case of the C12E8/DTAB and C12E8/TTAB systems, with considerable differences in the critical micelle concentration values, pure non-ionic micelles are formed in the range of low proportion of the ionic component. # 2003 Elsevier B.V. All rights reserved.

Light scattering and fluorescence probe studies on micellar properties of Triton X-100 in KCl solutions

The effect of KCl on micelle formation and structure of Triton X-100 (TX-100) was investigated by using combined static and dynamic light scattering measurements, together with the fluorescence probe technique. An analysis of the light scattering data, including hydrodynamic radius and micellar aggregation number, accounted for both micelle growth and hydration. Fluorescence studies using pyrene as a probe were carried out to determine the critical micelle concentration (CMC) as a function of solution composition. In addition, with the aim of gaining information on the possible changes in the micro-environmental properties of TX-100 micelles, fluorescence probe studies, including intermolecular pyrene excimer formation and fluorescence polarization of coumarin 6 associated with micelles, were carried out. It was found that the addition of electrolyte induces a decrease in the CMC and an increase in both aggregation number and hydration. However, complementary data of partial specific volume and cloud point of the surfactant suggested that the main contribution to micellar hydration is due to water mechanically trapped in the micelle. Fluorescence measurements do not indicate changes in the micellar micropolarity, probably due to modifications of the solubilization site of the probe caused by the micellar growth. Both pyrene excimer formation and fluorescence polarization of coumarin 6 revealed an increase in microviscosity with electrolyte addition, which is consistent with increased micellar hydration.

Micellization of Sodium Dodecyl Sulfate in Glycerol Aqueous Mixtures

The effect of glycerol on both micellar formation and the structural evolution of the sodium dodecyl sulfate (SDS) aggregates in the context of the action mechanism of the cosolvent has been studied. The critical micelle concentration and the degree of counterion dissociation of the surfactant over a temperature range from 20°C to 40°C were obtained by the conductance method. The thermodynamic parameters of micellization were estimated by using the equilibrium model of micelle formation. The analysis of these parameters indicated that the lower aggregation of the surfactant is mainly due to a minor cohesive energy of the mixed solvent system in relation to the pure water. The effect of glycerol on the mean aggregation number of the micelles of SDS was analyzed by the static quenching method. It was found that the aggregation number decreased with the glycerol content. This reduction in the micellar size seems to be controlled by an increase in the surface area per headgroup, which was ascribed to a participation of glycerol in the micellar solvation layer. Studies on the micropolarity of the aggregates, as sensed by the probe pyrene, indicated that this microenvironmental parameter is almost unaffected by the presence of glycerol in the mixture. However, an increase in the micellar microviscosity at the surface region was observed from the photophysical behavior of two different probes, rhodamine B and auramine O. These results suggest a certain interaction of the cosolvent in the micellar solvation of SDS micelles.

Photophysical and light scattering studies on the aggregation behaviour of Triton X-100 in formamide—water mixed solvents

Micelle formation and structure of the non-ionic surfactant p-tert-octyl-phenoxy (9.5) polyethylene ether (Triton X-100) in mixed solvents consisting of water and formamide have been investigated. Changes in the critical micelle concentration of the surfactant upon the addition of formamide were examined by using the pyrene 1:3 ratio method. The observed increase in the critical micelle concentration was attributed to a rise in the solubility of the surfactant as the formamide content increased in the solvent system. Micelle structure parameters were obtained as a function of the co-solvent concentration by using combined static and dynamic light scattering measurements. It was found that the decrease in the micelle size, produced by the addition of formamide, is mainly due to a reduction in the mean aggregation number rather than to changes in the magnitude of the whole solvation of micelles. This fact was also supported by the observed trend in the partial specific volume of the TX-100 micelles, obtained by complementary density measurements. However, the postulated changes in the composition of the solvation layer of micelles were supported by the rise in both the surface area per head group and the cloud point of the surfactant. From the photophysical response of different fluorescent probes incorporated in the micellar phase, we obtained information on the changes in the microstructure of Triton X-100 micelles upon the addition of formamide. The pyrene 1:3 ratio index revealed an increasing micropolarity as the formamide content increases in the solvent system. On the other hand, studies based on both fluorescence polarization of coumarin 6 and intermolecular pyrene excimer formation have shown that the microviscosity of Triton X-100 decreases with the presence of co-solvent. These results were interpreted on the basis of considerable contact of the co-solvent with the inner region of the micelles.

Role of the head group on the mixed micellization process in binary systems containing a sugar-based surfactant: decanoyl- N -methylglucamide

The mixed micelles of nonionic decanoyl-N-methylglucamide (MEGA-10) with the anionic sodium dodecyl sulphate (SDS), the cationic dodecyltrimetylammonium bromide (DTAB), and the nonionic octaoxyethylene monododecyl ether (C12E8) have been studied using the fluorescence probe technique. The critical micelle concentration of the three mixed systems in the whole composition range were determined by the pyrene 1:3 ratio method, and the experimental results were analysed in the context of the pseudophase separation model, by using the regular solution theory. It was found that the mixed micelles containing the anionic surfactant are more stable than the pure micelles. This fact was attributed to the occurrence of ion–dipole interactions between the head groups of the component surfactants in the mixed micelle. The static quenching method was used to determine the mean aggregation number of pure and mixed micelles. It was found that whereas mixed micelles containing SDS show a positive deviation from the ideal behaviour, those constituted by DTAB deviate negatively. This different tendency was interpreted on the basis of both steric and electrostatic interactions. The evolution of the microstructure of the mixed micelles upon the participation of the co-surfactant was followed through the micropolarity and microviscosity of the mixed systems. Although the micropolarity studies do not allow definite conclusions, the microviscosity assays indicate that the participation of the co-surfactant induces the formation of less ordered micelles, this effect being more pronounced in the case of mixtures with the anionic surfactant.

Synergism in mixtures of n-octyl-β-d-thioglucoside and different n-alkyltrimethylammonium bromides: effect of the alkyl chain length

Mixtures of n-octyl-β-d-thioglucoside with three different n-alkyltrimethylammonium bromides (n = 12 (DTAB), 14 (TTAB), and 16 (CTAB)) have been studied by using fluorescence spectroscopic techniques. The critical micelle concentration values of pure and mixed systems were determined by the pyrene 1:3 ratio method. The experimental results were treated by using thermodynamic mixing approaches based on the pseudophase separation model. It was found that all the mixed systems show a negative deviation from ideal behavior, more pronounced as the larger the alkyl tail of the co-surfactant. It was also observed that the three mixed systems fulfill the conditions of synergism, this behavior being also dependent on the alkyl chain length. By using the static quenching method, the mean micellar aggregation numbers of mixed micelles were obtained. In all the cases, it was observed that the aggregation number is initially reduced with the participation of the cationic surfactant, remaining almost constant and close to the aggregation number of the pure cationic micelles. The local viscosity of pure and mixed micelles was examined by the photophysical response of the hydrophobic probe coumarin 6 solubilized in the micellar medium. It was found that the participation of the ionic component induces the formation of a less ordered structure than that of pure nonionic micelles. This effect being less pronounced as the chain hydrocarbon length of the co-surfactant increases.