SynopsisThe thermal diffusion coefficient DT has been obtained for 17 polymer-solvent combinations, each of them spanning a range of polymer molecular weights, using thermal field-flow fractionation. The polymers examined include polystyrene, poly(alpha-methyl)styrene, polymethylmethacrylate, and polyisoprene. The solvents include benzene, toluene, ethylbenzene, tetrahydrofuran, methylethylketone, ethylacetate, and cyclohexane. Although D, was confirmed as essentially independent of polymer molecular weight, it was found to vary substantially with the chemical composition of polymer and solvent. The results were used to evaluate several thermal diffusion theories; the agreement with theory was generally found to be unsatisfactory. Attempts were then made to correlate the measured thermal diffusion coefficients with various physicochemical parameters of the polymers and solvent. A good correlation was found in which D, increases with the thermal conductivity difference of the polymer and solvent and varies inversely with the activation energy of viscous flow of the solvent.

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The thermophoresis of homopolymer chains dissolved in a pure nonelectrolyte solvent is theoretically examined. Using a similar approach to that used for suspended particles, thermophoresis is related to the temperature-dependent osmotic pressure gradient in the solvent layer surrounding the monomer units (mers). The gradient is produced by small changes in the concentration of solvent molecules (i.e., solvent density) as a result of the mer−solvent interaction energy. The resulting expression contains the interaction energy as well as solvent thermodynamic parameters, including the cubic coefficient of thermal expansion, the isothermal compressibility and its temperature coefficient. Using the general dependence of dipole−dipole potentials on the distance between interacting objects, an expression for thermophoretic mobility that contains a characteristic Hamaker constant is obtained. The resulting expression is used to calculate interaction constants for polystyrene and poly(methyl methacrylate) in several organic solvents using thermophoresis data obtained from thermal field-flow fractionation. The calculated constants are compared to values in the literature and found to follow the same order among the different solvents. Furthermore, the model is consistent with laboratory measurements of polymer thermophoresis, which is weak in water compared to less polar solvents, and which correlates with monomer size. In nonelectrolyte solvents, London dispersion forces must play a major role since other dipole−dipole interactions are insufficient to produce the required interaction energies. Finally, the model predicts that to have a measurable thermophoretic mobility in a given solvent, the polymer should have a Hamaker constant that is greater than 10−15 kT, as calculated by simple but commonly used theoretical models

The retention of polystyrene and silica colloids that have been chemically modified is measured in several aqueous carrier liquids. Retention levels are governed by particle size and composition but are also sensitive to subtle changes in the carrier. Size-based selectivities are higher in aqueous carriers compared to acetonitrile. In aqueous carriers, retention varies dramatically with the nature of the additive, and for a given additive, retention increases with ionic strength, regardless of modifications to the particle surface. The role played by electrostatic effects in retention is studied by varying the ionic strength of the carrier, estimating electrical double layers, determining particle-wall interaction parameters, and calculating the coefficients of mass diffusion and thermal diffusion. Although electrostatic phenomena can affect mass diffusion and particle-wall interactions in carriers of low ionic strength (<10(-3) M), such effects are not great enough to explain the dependence of retention on ionic strength. Therefore, thermal diffusion must be affected directly. Thermal diffusion is found to increase with pH, and at a given pH with the surface tension of the suspended particle. Finally, while the addition of the surfactant FL-70 generally decreases retention, greater retention levels can ultimately be achieved with FL-70 because larger temperature gradients can be used without particle adsorption to the accumulation wall.

The movement of molecules and homopolymer chains dissolved in a nonelectrolyte solvent in response to a temperature gradient is considered a consequence of temperature-induced pressure gradients in the solvent layer surrounding the solute molecules. Local pressure gradients are produced by nonuniform London-van der Waals interactions, established by gradients in the concentration (density) of solvent molecules. The density gradient is produced by variations in solvent thermal expansion within the nonuniform temperature field. The resulting expression for the velocity of the solute contains the Hamaker constants for solute-solvent and solute-solute interactions, the radius of the solute molecule, and the viscosity and cubic coefficient of thermal expansion of the solvent. In this paper we consider an additional force that arises from directional asymmetry in the interaction between solvent molecules. In a closed cell, the resulting macroscopic pressure gradient gives rise to a volume force that affects the motion of dissolved solutes. An expression for this macroscopic pressure gradient is derived and the resulting force is incorporated into the expression for the solute velocity. The expression is used to calculate thermodiffusion coefficients for polystyrene in several organic solvents. When these values are compared to those measured in the laboratory, the consistency is better than that found in previous reports, which did not consider the macroscopic pressure gradient that arises in a closed thermodiffusion cell. The model also allows for the movement of solute in either direction, depending on the relative values of the solvent and solute Hamaker constants.