Concentration Fluctuations and Surface Adsorption in Hydrogen-Bonded Mixtures

Concentration fluctuations of the 1,4-butanediol + 1-dodecanol mixture have been characterized by laser light-scattering experiments at two temperatures. The results show that both components have a clear tendency toward heterocoordination that increases with temperature. The combination of the concentration fluctuation results with the surface tension measurements have provided precise values of the excess surface adsorption Γ 21 as a function of the composition. Γ 21 shows a maximum for a mole fraction of 1-dodecanol of x 2 ≈ 0.02; the maximum of Γ 21 decreases with increasing temperature. The position of this maximum contrasts with that at which the local compositions show extrema in the bulk. The surface entropy versus the composition curve shows maxima and minima throughout the composition range. This behavior resembles that found for some water−alcohol mixtures. Using the experimental surface tension data, we predicted the excess surface adsorption using a lattice-fluid model that takes into account the existence of hydrogen bonds between the two components. In spite of the fact that the model accounts reasonably well for the volumetric properties of the system, poor predictions are obtained for Γ 21 .