Thermodiffusion in a multicomponent lyotropic mixture in the vicinity of the critical micellar concentration by using the Z -scan technique.

Thermodiffusion in a lyotropic mixture of water and potassium laurate is investigated by means of an optical technique (Z scan) distinguishing the index variations due to the temperature gradient and the mass gradients. A phenomenological framework allowing for coupled diffusion is developed in order to analyze thermodiffusion in multicomponent systems. An observable parameter relating to the mass gradients is found to exhibit a sharp change around the critical micellar concentration, and thus may be used to detect it. The change in the slope is due to the markedly different values of the Soret coefficients of the surfactant and the micelles. The difference in the Soret coefficients is due to the fact that the micellization process reduces the energy of interaction of the ball of amphiphilic molecules with the solvent.

Colloid transport in nonuniform temperature.

The aim of this paper is to set up a theoretical framework for obtaining the thermodiffusion (or Soret) coefficient of a colloid in a carrier liquid. It is first argued that the expression of the particle-current density in nonuniform temperature cannot be derived from a theoretical formula valid for an isothermal solution. Then the kinetic theory of Brownian motion is used to derive an expression for the current density properly accounting for thermodiffusion. The cases of free and interacting particles are treated, and the thermodiffusion current pertinent to an ideal solution adds up with a current driven by a temperature- and concentration-dependent potential. Accordingly, a general explicit formula for the thermodiffusion coefficient is obtained. Practical use of the framework is illustrated on simple specific models of a colloid in a solvent. Large Soret coefficients of both signs are calculated for realistic values of the physicochemical parameters, in qualitative agreement with published experimental data.

Nonequilibrium statistical mechanics of drifting particles

This paper describes a method for obtaining nonequilibrium one-particle energy distributions of fermions or bosons. For the program to be carried out, particle transport should occur in the drifting mode in which the average velocity is much lower than the instantaneous velocity. Under this condition, the spectral current density has a drift-diffusion structure involving a mobility-diffusion relationship unrelated to statistics. When a local-equilibrium energy distribution is used, the linear response theory is recovered. Next, the particle-medium energy exchange is treated within a Fokker-Planck framework in order to obtain the nonequilibrium energy distribution; a nonlinear framework is used to account for the quantum-statistical correlations. Explicit formulas are obtained for homogeneous distributions at steady state. The rate of change of entropy is a simple generalization of the second law of thermodynamics. The positivity of the total entropy production stems from the positive definiteness of the diffusion tensors. Minimal entropy production is not necessarily achieved in the stationary state.