C. Carnero Ruiz

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.

Thermodynamics of micellization of tetradecyltrimethylammonium bromide in ethylene glycol–water binary mixtures

Abstract The aggregation behaviour of tetradecyltrimethylammonium bromide in ethylene glycol–water mixtures across a range of temperatures has been investigated by electrical conductivity measurements. The critical micelle concentration (cmc) and the degree of counterion dissociation of micelles were obtained at each temperature from plots of differential conductivity, (κ/c)T,P, versus the square root of the total concentration of the surfactant. This procedure not only enables us to determine the cmc values more precisely than the conventional method, based on plots of conductivity against total concentration of surfactant, but also allows straightforward determination of the limiting molar conductance and the molar conductance of micellar species. The equilibrium model of micelle formation was applied to obtain the thermodynamics parameters of micellization. Only small differences have been observed in the standard molar Gibbs free energies of micellization over the temperature range investigated. The enthalpy of micellization was found to be negative in all cases, and it showed a strong dependence on temperature in the ethylene glycol poor solvent system. An enthalpy–entropy compensation effect was observed for all the systems, but whereas the micellization of the surfactant in the solvent system with 20 wt% ethylene glycol seems to occur under the same structural conditions as in pure water, in ethylene glycol rich mixtures the results suggest that the lower aggregation of the surfactant is due to the minor cohesive energy of the solvent system in relation to water.

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.

Micelle formation and microenvironmental properties of sodium dodecyl sulfate in aqueous urea solutions

Abstract The critical micelle concentrations (CMC) of sodium dodecyl sulfate (SDS) and the degrees of counterion dissociation in a wide range of urea concentrations have been determined by the electrical conductivity method. While the CMC values are in good agreement with literature values, the values of the degree of counterion dissociation at high urea concentration are greater than those recently published. The micellization process of SDS in different aqueous urea solutions has also been followed through the fluorescence behaviour of pyrene-3-carboxaldehyde (PCA) solubilized in the micellar surface. From this study an attempt was made in order to extract quantitative conclusions about changes in the micropolarity in the micellar surface induced by the presence of urea. Although no definitive conclusions could be derived due to the desolvation of the probe caused by the urea action, the results obtained were interpreted as evidence of a direct interaction of urea with the micellar surface. Changes in the structure of the hydrophobic region of SDS micelles upon addition of urea were studied by using polarized fluorescence measurements of diphenylbutadiene (DPB) solubilized in the micellar phase. The effect of urea on the hydrophobic region of SDS micelles was clearly reflected in the DPB steady-state fluorescence anisotropy measurements, which indicated a more rigid microenvironment around the probe as urea concentration increased. The results obtained in this study support a direct mechanism of urea action, whereby urea participates in the solvation of the hydrophobic chains and the polar headgroups of the amphiphile.

A photophysical study of the urea effect on micellar properties of sodium dodecylsulfate aqueous solutions

With the aim of studying the effect of urea on micellar properties of aqueous solutions of sodium dodecylsulfate (SDS), steadystate fluorescence experiments were carried out with different luminescence probes incorporated into the micellar phase. The increase of critical micelle concentration (CMC) of the surfactant with urea addition was followed by changes in the relative intensities of the vibrational fine structure of the pyrene fluorescence spectra. Micellar aggregation numbers were obtained from the analysis of fluorescence quenching data using ruthenium tris(bipyridyl) chloride and 9-mehylanthracene as a donorquencher pair. It was found that the decrease in the aggregation number is mainly controlled by rise in the surface area per headgroup of the surfactant. From fluorescence measurements, using several ionic probes (8-anilino-1-naphthalen-sulfonic acid, rhodamine B, and auramine O), it was found that urea decreases the polarity and increases the microviscosity of the micellar interface. These effects, which are dependent on the concentration of urea, can be explained according to a direct interaction of urea at the micellar surface.

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.

Intramicellar energy transfer in aqueous CTAB solutions

Abstract We report an experimental study on the energy transfer process between perylene and fluorescein in cetyltrimethylammonium bromide (CTAB) micelles. The spectral behaviour and the dynamic and polarization data of dyes confirmed that perylene is solubilized in the inner hydrocarbon region of the micelle, while fluorescein is micellized in the inner part of the Stern region. The results obtained from the quenching experiments demonstrated that a static mechanism operates for the perylene-fluorescein pair in CTAB micelles. The study of the influence of the acceptor concentration on the energy transfer efficiency suggested that only intramicellar energy transfer contributes to the overall process. Moreover, the influence of the addition of 0.01 M NaOH on CTAB micelles was investigated by the changes in both the critical micelle concentration (CMC) and the micellar aggregation number. CMC values were obtained from the pyrene 1:3 ratio method, and micellar aggregation numbers were determined by using a procedure based on the energy transfer process.

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.

Rotational dynamics of coumarin 153 in non-ionic mixed micelles of n-octyl-β-D-thioglucoside and Triton X-100

Rotational diffusion of the neutral probe coumarin 153 has been examined in a mixed surfactant system containing n-octyl-β-D-thioglucoside (OTG) and Triton X-100 (TX100), two non-ionic surfactants belonging to the alkyl glucoside and polyoxyethylene alkyl ether families, respectively. Both steady-state and time-resolved fluorescence measurements indicate that the polarity of the dye decreases slightly as the amount of ethoxylated surfactant in the mixed micelle increases. This behaviour can be attributed to migration of the probe towards the inner region of the micellar palisade layer as a result of the increasing hydration induced by the presence of TX100. The anisotropy decay curves, r(t), were well fitted to a biexponential function, characterized by two reorientational times of the probe in the micellar pseudophase. The experimental data were analyzed on the basis of the two-step and wobbling-in-a-cone model, and the results obtained correlated with the changes that occur in the palisade layer of the mixed micelles due to the different structure and nature of the head groups of both surfactants. It was found that, although the average reorientation time of the probe molecule is almost unaffected with the participation of the ethoxylated surfactant, the observed increase in the generalized order parameter suggests a certain rise in the compactness of the mixed micelles.

On the urea action mechanism: a comparative study on the self-assembly of two sugar-based surfactants.

Studies on the effect of urea on micelle formation and structure of n-octyl-beta-D-thioglucoside (OTG) and N-decanoyl-N-methylglucamide (MEGA-10) were carried out by using the steady-state and time-resolved fluorescence techniques, together with combined static and dynamic light scattering measurements. A similar increase in the critical micelle concentration with the urea addition was observed for both surfactants. This behavior was attributed to a rise in the solubility of hydrocarbon tails and the increase of solvation of the headgroup of the surfactants in the presence of urea. Structural studies mainly based on the analysis of the hydrodynamic radius and aggregation number of micelles revealed that urea induces changes much more significant on micelles of OTG. Particularly, it was found that, whereas the surface area per headgroup of OTG increases with the urea concentration, it does decrease in the case of MEGA-10. This fact suggests that different action mechanisms operate for both surfactants. Accordingly, investigations on the micellar microstructure based on the study of microenvironmental properties such as micropolarity and microviscosity also indicated a more pronounced effect in the case of OTG. Although changes were not observed in the hydrophobic inner region of both micellar systems, a significant increase of polarity and viscosity in the micellar interface of OTG suggests a direct participation of urea in the micellar solvation layer. The differences between the observed behaviors for both micellar systems were interpreted on the basis of two features: the weaker hydration and greater rigidity of the OTG headgroup as compared with MEGA-10.