Dynamic adsorption and tension of mixed nonionic surfactants at the air/water interface
Abstract
Most surfactant formulations involve multicomponent systems of molecules of different adsorption capacities, surface activities, and nonideal interactions at the interface. To study the dynamic and equilibrium adsorption and tension a new general model, Nonideal Adsorbed Solution (NAS) model, is developed. This model is inherently thermodynamically consistent. Other binary isotherms such as the generalized Langmuir isotherm are shown to be viable only under specific conditions. The NAS model is extended to the air/water interface for determining surface tension of aqueous binary surfactant mixtures with molecules of different size, surface activity, and nonideal interactions. The regular solution theory is used for modeling the nonideal interactions between the adsorbed molecules and in the micelles. Two mixing parameters $\beta\sp{\sigma}$ and $\beta\sp{m}$ are used for fitting data for pre-micellar and micellar concentrations. The surface coverages and compositions for two nonionic mixtures are calculated with the NAS model. These calculations show that the larger molecules adsorb at the interface more favorably at low concentrations than at high concentrations. The phenomenon of synergism in tension or adsorption for binary surfactants in water, for concentrations above and below the cmc, is studied using the NAS model. Ranges of concentrations, compositions, and surface tensions are determined for which a mixture is synergistic below or above the cmc. Although tension synergism is predicted when $\beta\sp\sigma < 0,$ adsorption synergism is possible even when $\beta\sp\sigma = 0.$ These results can be useful in designing surfactant formulations of controlled tension or surface composition with minimal amounts. A model for dynamic tension is developed for aqueous solutions of nonionic surfactants. Local equilibria between the subsurface concentrations and the adsorbate densities are modeled using the NAS theory. The model shows that the larger molecules tend to be preferentially adsorbed at low times and the smaller molecules at longer times. This adsorption selectivity is reduced when the larger molecules have a much larger adsorption equilibrium constant, or when there are negative deviations from ideality in the monolayer. This model's predictions are compared to tension data for two nonionic surfactants, C$\sb{12}$E$\sb5$ (dodecyl pentaethylene glycol ether) and Triton X-100 (polyethylene tert-octylphenyl ether) at 25$\sp\circ$C.
Degree
Ph.D.
Advisors
Franses, Purdue University.
Subject Area
Chemical engineering|Chemistry
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