Crossover from normal to inverse temperature dependence in the adsorption of nonionic surfactants at hydrophilic surfaces and pore walls

The adsorption of the nonionic surfactant C8E4 from its aqueous solutions to the pore walls of four controlled-pore silica glass (CPG) materials of different mean pore widths (10-50 nm) has been studied in a temperature range from 5 to 45 degrees C, that is, close to the lower critical temperature of liquid-liquid phase separation of the bulk system (T-c approximate to 40 degrees C). Pronounced S-shaped isotherms, with a normal temperature dependence of the adsorption in the initial low-affinity region but an inverse temperature dependence in the plateau region, are found with all CPG materials. The experimental adsorption isotherms are compared with predictions of a theoretical model (Phys. Rev. Lett. 2004, 92, 135701), which takes into account H-bonding and micelle formation in the bulk and at the surface. It is found that this model reproduces all peculiarities of the adsorption in the present systems. The following conclusions emerge from this analysis: (1) At low bulk concentrations, the surfactant is adsorbed only in monomeric form (Henry's law behavior). In this regime, there is an energetic driving force for surfactant adsorption (in spite of the fact that nonselective pore walls are assumed), in agreement with the observed normal temperature dependence of the adsorption in this regime. (2) Surface aggregation observed at higher concentrations involves an energy penalty due to the loss of H bonds, which is overcompensated by the gain of (rotational) entropy of the H-bonding sites of the surfactant heads. Accordingly, surface aggregation is entropy-driven and endothermic and thus shows inverse temperature dependence. Hence, the model accounts for the observed crossover in the temperature dependence of the adsorption at the critical surface aggregation concentration.