Estimation of molecular diffusivity in liquid phase systems by the Wilke–Chang equation

Dynamic light scattering (DLS) is routinely used to measure the diffusion of surfactant micelles. Theory and experiment suggest, however, that the microscopic concentration fluctuations monitored by DLS obey the same mutual diffusion equations that describe the decay of macroscopic concentration gradients. According to this interpretation, DLS provides mutual diffusion coefficients for the total surfactant components, including contributions from micelles, free surfactant monomers, and counterions. An attempt is made to decide the correct interpretation of DLS measurements by comparing DLS diffusion coefficients (DDLS) with mutual diffusion coefficients measured by macroscopic gradient techniques for binary aqueous solutions of ionic and zwitterionic surfactants. Possible contributions to DDLS from free surfactant monomers are investigated by extending DLS measurements into the critical micelle (cmc) region where substantial portions of the surfactants diffuse as free monomers. The widely held assumption ...

Diffusion in Solutions of Micelles. What Does Dynamic Light Scattering Measure

Experimental data on ordinary diffusion coefficients and thermal diffusion factors for binary liquid systems containing hexane, heptane, benzene, toluene and carbon tetrachloride are derived from steady-state and non-steady-state separation measurements in a thermal diffusion column. The data extraction method uses a time evolution theory previously developed by the authors that accounts for the forgotten effect. Comparison of the diffusion data with those of the literature provides a test of the derived thermal diffusion factors.

http://iopscience.iop.org/article/10.1088/0953-8984/2/9/017/pdf

Fickian and thermal diffusion coefficients from liquid thermogravitational columns

Peak parking method for measurement of molecular diffusivity in liquid phase systems.

The present study shows that dynamic light scattering (DLS) is capable of measuring mutual diffusion coefficients for binary mixtures of ionic liquids (ILs) with different molecular liquids over the complete composition range. Evidence is given that the light scattering signals are related to true molecular binary diffusion. The method stands out due to its ability to work non-invasively in macroscopic thermodynamic equilibrium with reasonable accuracy and within convenient measurement periods. Compared with other techniques, mixtures with distinctly higher viscosities can be probed. For exemplary binary mixtures of 1-ethyl-3-methylimidazolium ethylsulfate ([EMIM][EtSO4]) with acetone, acetonitrile, dichloromethane, ethanol, or water as well as of 1-ethyl-3-methylimidazolium methanesulfonate ([EMIM][MeSO3]) with acetone, water, or methanol, mutual diffusivity data were measured over a wide range of composition at a temperature of 293.15 K. In general, the mutual diffusivity increases with increasing mole fraction of the molecular liquid and similarities to aqueous solutions of classical inorganic salts can be found. The characteristic behavior of the mutual diffusion coefficients is influenced by the nature of the chosen molecular liquid. For IL water mixtures, low light scattering intensities were observed despite the large refractive index difference of the pure components. The reason for this behavior may be the existence of water clusters in the mixtures. Additional measurements for IL acetone mixtures at temperatures ranging from 278.15 K to 323.15 K showed that the temperature dependence of the mutual diffusivity can be represented by Arrhenius functions and is increasing for decreasing mole fractions of acetone.

Klaus Lucas

Papers

Light scattering measurements of mutual diffusion coefficients in binary liquid mixtures