E. V. Plotnikova

Dynamic Light Scattering Study of Cetyltrimethylammonium Bromide Aqueous Solutions

Cetyltrimethylammonium bromide (CTAB) aqueous solutions are studied by dynamic light scattering method in a wide concentration range covering the first and second critical micelle concentrations (CMC1 and CMC2, respectively). Nonmonotonic and ambiguous behavior of diffusion coefficients D with an increase in concentrations above CMC1 is revealed. An increase in the D values in the first decade of CTAB concentration above CMC1 agrees with known published data for aqueous solutions of ionic surfactants. It is shown that an increase in the ionic strength of solution with the addition of KBr leads to a decrease in the positive slope of the dependence of diffusion coefficients on CTAB concentration up to zero at 0.05 M KBr. Two relaxation processes corresponding to large and small D values are simultaneously observed in micellar solutions, beginning with 0.03 M CTAB concentration. The data obtained are compared with published data, as well as with the results of viscosity measurements. The performed analysis indicates that the observed relaxation processes are explained by the coexistence of spherical and nonspherical micelles. It is established that micelles acquire a cylindrical shape at CTAB concentrations ranging from 0.2 to 0.25 M. Hydrodynamic radii and lengths of micelles are calculated.

Diffusion coefficients and viscosities of aqueous solutions of alkyltrimethylammonium bromides

Dynamic light scattering and capillary viscometry have been employed to study aqueous micellar solutions of nonionic (pentaoxyethylene dodecyl ether) and ionic (decyl-, dodecyl-, and tetradecyltrimethyammonium bromides) surfactants in a wide concentration range covering the first and second critical micelle concentrations (CMC1 and CMC2, respectively). The concentration curve for the diffusion coefficient of the nonionic surfactant has been shown to correspond to a monotonically decreasing function. The diffusion coefficients of the ionic surfactants pass through maxima above CMC1. As the alkyl chain length increases, the slopes of the concentration curves rise in the range of the linear growth in the diffusion coefficient, while the values of the maxima increase, their positions shifting toward lower concentrations. The concentration dependences of the diffusion coefficients have been explained based on their relation to activity coefficients.

Diffusion coefficients of ionic surfactants with different molecular structures in aqueous solutions

Aqueous micellar solutions of dodecyl- and hexadecyltrimethylammonium bromides, hexade- cylpyridinium bromide and chloride, domiphen bromide ((dodecyldimethyl-N-2-phenoxyethyl)ammonium bromide), hexadecyltriphenylphosphonium bromide, and sodium dodecyl sulfate have been investigated using dynamic light scattering. A local increase in the diffusion coefficient has been discovered for all ionic surfactants at concentrations higher than the first critical micelle concentration (CMC1). It has been shown that, at the same alkyl chain length and counterion nature, the increase in the diffusion coefficient with surfactant content becomes more pronounced with a reduction in CMC1. The concentration dependences of the diffusion coefficients have been explained in terms of a previously proposed theory formulated for ideal micellar systems. The existence of the local minimum in the diffusion coefficient near CMC1 has been shown to be a general regularity for binary surface-active electrolytes.

Light absorption in solutions of cetyltrimethylammonium and cetylpyridinium bromides

Absorption of monochromatic light in solutions of cetyltrimethylammonium (CTAB) and cetylpyridinium (CPB) bromides has been studied in the wavelength λrange of 190–1000 nm. The investigation has been performed at 30°C and surfactant concentrations ranging from 5 × 10−5 to 3.5 × 10−3 M. Spectra of solutions of LiBr, KBr, and CTAB in the premicellar concentration region have been shown to coincide with each other. They are unimodal, and the heights of their maxima at λmax ≈ 193 nm depend on Br− concentration alone. In contrast to CTAB solutions, the UV spectra of CPB solutions are characterized by the presence of two absorption maxima near λmax1 = 191 nm and λmax2 = 259 nm, as well as a shoulder at 210–218 nm. Their existence is caused by the presence of both Br− anions and CP+ cations in the solutions. The dependences of the integral absorption in the examined wavelength range on CPB concentration exhibit inflections both in the premicellar region and upon the transition to micellar solutions, with these inflections characterizing the critical dimerization and micellization concentrations, respectively. For CTAB, this regularity has only been observed at the critical micellization concentration. The data on the concentration dependence of the light absorption have resulted in the proposal of a method for determining the degree β of counterion binding by micelles of ionic surfactants. The β values calculated for CTAB and CPB are equal to 0.89 ±0 0.1.

Calculation aspects of diffusion coefficients in micellar solutions of ionic surfactants

A method has been analyzed for calculating diffusion coefficients of 1 : 1 ionic surfactants as functions of their micellar solution concentrations within the framework of the quasi-chemical variant of the law of mass action. Relations have been presented for two- (with no allowance for micelles) and three-particle interaction (with allowance for micelles) formalisms. The methods for the introduction of initial calculation parameters and the calculation scheme for an ideal mixture of monomeric ions and micelles, as well as the correction for a deviation from ideality and a change in solution viscosity, have been considered. Numerical assessments have been performed by the examples of aqueous sodium dodecyl sulfate and alkyltrimethylammonium bromide solutions.

On the role of the activity coefficient in the processes of diffusion of ionic surfactants in micellar solutions

Theoretical relations are formulated to describe the effect of an activity coefficient reflecting interparticle interaction on the diffusion coefficient of a surfactant in a micellar system. The activity coefficient of hexadecyltrimethylammonium bromide in an aqueous micellar solution is measured with a bromide-selective electrode. Analysis of the data obtained shows that, above the critical micelle concentration, surfactant mobility and solution viscosity stronger affect an increase in the diffusion coefficient followed by its reduction than the interparticle interaction in a micellar system dose.

Electrical-percolation effects in micellar solutions of alkyltrimethylammonium bromides

AbstractSpecific conductivity K of aqueous solutions of alkyltrimethylammonium bromides has been studied in a wide range of concentrations c of surfactants containing 10, 12, 14, and 16 carbon atoms in alkyl chains. In general, three break points have been observed in the K(с) dependences. The first point observed upon increasing overall solution concentration corresponds to critical micelle concentration CMC1. The CMC1 values of alkyltrimethylammonium bromides decrease with an increase in the alkyl chain length. They are in satisfactory agreement with the published data. It has been supposed that the second break point in the K(с) dependences corresponds to the formation micellar structures as clusters and the appearance of channels with a higher specific conductivity, which is provided by the contribution from the overlap of electrical double layers existing in the vicinities of micelles. Surfactant concentrations corresponding to these break points have been called “critical percolation concentrations” (CPCs). The position of a CPC in the concentration scale strongly depends on alkyl radical length. All K(с) curves exhibit a third break, which corresponds to second critical micelle concentration CMC2, at which the properties of ionic-surfactant solutions may substantially change because of the appearance of supramicellar structures. The experimental data obtained have been used to evaluate the parameters of the model of electrical percolation for micellar solutions, i.e., effective conductivity

Sizes of Ionic Surfactant Micelles and Their Clustering According to Data on Viscosity of Micellar Solutions

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