The energetics of surfactant adsorption at the air‐water interface

Abstract

AbstractThe standard free energy, enthalpy, and entropy changes (खG, खH, and खS, respectively) for the adsorption at the air‐water interface of a commercial ethylene oxide (EO) adduct of straight chain nonylphenol from monomer solution at the critical micelle concentration (CMC) have been calculated from surface tension‐concentration data at 21C舑45C using the Gibbs equation, the standard free energy change equation खG=蜢RT (In interfacial monomer concentration/CMC), and the Gibbs‐Helmholtz equation which gave खH directly from the slope of the (खG/T) vs (1/T) function. The CMC and surface tension at the CMC (ेCMC) decreased, and 蜢खG and molecular area increased slightly, with increasing temperature. The खH and खS values were positive, and appear explainable by the postulations applied to micellization. At ambient temperature (28C) an increase in the (EO) mol ratio of straight chain nonylphenol and C13 secondary alcohol ethoxylates resulted in increases of CMC, ेCMC, and molecular area, and slight decreases in 蜢खG. A comparison of the 9 (EO) mol ratio adducts of C13 straight chain primary and secondary alcohols showed that the CMC and molecular area of the secondary alcohol ethoxylate were larger, and the ेCMC and 蜢खG smaller, than the corresponding values of the primary alcohol ethoxylate.