Preferential adsorption on porous glass near the liquid/liquid coexistence curve of a binary liquid mixture

Abstract

A differential refractometric method is used to study the temperature and composition dependence of preferential adsorption of water on the surface of pore-controlled glass out of 2-butoxyethanol (abbreviated C4E1)/water mixtures. The experiments are carried out close to the liquid/liquid coexistence curve in the vicinity of the lower critical point of the system. When the temperature of phase separation Tp of the mixture is approached at (almost) constant bulk composition from temperatures below Tp the amount of substance of adsorbed water ñsw per unit mass of adsorbent extrapolated to Tp increases along the liquid/liquid coexistence curve with increasing values of x° (x°, mole fraction of C4E1 of the mixtures before adding the absorbent). In experiments with mixtures of composition x°≳xc the quantity ñsw is about 10 times larger than that found in experiments with mixtures of compositions x°<xc (xc, critical composition). The data are interpreted by presuming that the extent of the preferential adsorption of water and its temperature and composition dependence is primarily caused by the presence of −SiOH and −SiO− groups at the hydrophilic surface of the adsorbent [and H+(aq) counter ions] in combination with concentration fluctuations occurring in a wide range of temperatures and compositions along the binodal curve.