Relationships between adsorption and wetting of surfactant solutions

Abstract

Advancing contact angles, Θ, for aqueous solutions of the nonionic surfactants, penta(oxyethylene) dodecyl monoether, C 12E 5, and penta(oxyethylene) decyl monoether, C 10E 5, have been measured on surfaces prepared from polystyrene, PS, and poly(methyl methacrylate), PMMA, latex particles, as well as on paraffin. Whereas with paraffin, wetting of surfactant solutions is the same as with pure liquids at the same surface tension, wetting is increasingly less efficient relative to pure liquids for PS and PMMA. From adhesion tension plots of γ LV cos Θ vs γ LV and application of the Gibbs and Young equations it appears, for paraffin, that Γ SL, the amount adsorbed to the solid—liquid interface, is essentially equal to Γ LV, the amount adsorbed to the vapor—liquid interface over the entire concentration range, whereas for PS and PMMA, the ratio of Γ SL to Γ LV becomes increasingly less than 1, indicative of increasingly less efficient wetting as the solid becomes more polar. Experimental determinations of Γ SL and Γ LV and application of the Gibbs and Young equations allowed calculation of γ LV cos Θ and the construction of “estimated” adhesion tension plots. Excellent agreement between plots using contact angle and adsorption data was obtained up to concentrations of surfactant corresponding to surface tensions of about 40 mN/m, thus confirming the important role of Γ SL relative to Γ LV to this point in affecting wetting efficiency. At higher surfactant concentrations, but still below the cmc, ratios of Γ SL to Γ LV exceed 1, predicting significantly lower contact angles than are actually measured. Such differences are much greater for the more polar solid surface of PMMA than of PS. These latter results are discussed in the context of possible changes in the structure of the adsorbed monolayer at the solid—liquid interface at higher surfactant concentrations.