Adsorption and Aggregation of C8E4 and C8G1 Nonionic Surfactants on Hydrophilic Silica Studied by Calorimetry

Abstract

The critical micelle concentrations and the enthalpies of micelle formation of n-octyl tetraethylene glycol monoether (C8E4) and n-octyl β-d-monoglucoside (C8G1) in water have been determined at 298.15 K by titration microcalorimetry. The adsorption and thermal behavior of these nonionic surfactants at the hydrophilic silica glass/aqueous solution interface have been investigated by simultaneous measurements of the adsorption isotherm and the calorimetric enthalpies of displacement in a flow system. Both surfactants display pronounced cooperative adsorption behavior (S-shaped isotherms). While in the low-affinity region the amount adsorbed and the integral enthalpies of displacement are only slightly smaller for C8G1 than for C8E4, the deviation increases to 1 order of magnitude in the aggregative adsorption region. The observed difference in the adsorption behavior is interpreted in terms of the different hydrophilic head groups of the two amphiphiles. The differential molar enthalpies of displacement are comparable in magnitude in both the low-affinity (exotherm) and aggregative (endotherm) regimes. The molar enthalpies of aggregate formation at the silica/water interface were found to be close to those in the bulk solution, indicating that surface aggregation and bulk micellization are very similar phenomena. The enthalpy/entropy balance of the formation of the adsorption layer has been described in terms of thermodynamic potential functions.