Apparent molar volumes and heat capacities of various adsorbed additives on polystyrene latexes

Flow densimetry and flow microcalorimetry have been used to study the effect of electrolytes, surfactants, and alcohols on positively and negatively charged polystyrene latexes. The thermodynamic volumes and heat capacities of transfer of the latex from water to the aqueous solutions and of the additives from water to the latex dispersions were measured as a function of the additive concentration. With some systems, the adsorption isotherms were also determined. These thermodynamic properties are largely insensitive to electrostatic interactions but depend significantly on changes in hydrophobic interactions resulting from adsorption of the additive on the latex. Negligible changes in the quantities of transfer are thus observed for electrolytes, methoxyethanol, and methanol, but ionic and nonionic surfactants are adsorbed on both types of latexes. The adsorption of medium-chain-length alcohols and alkoxyethanols increases with their hydrophobic character. A simple model based on the Langmuir isotherm, the thermodynamic properties of the additives in water, and the properties of the adsorbed additives correctly predicts the general trends of the transfer functions of the additive and of the latex. However, a detailed analysis of the data indicates that the adsorption isotherm is more complex than expected from the Langmuir equation and that the thermodynamic properties of the adsorbed species depend on the extent of coverage. At high concentrations, the volumes and heat capacities of adsorbed surfactants are of the same order of magnitude as the corresponding values of the surfactants in micellar form.