Interfacial Thermodynamics of Surfactants at the CO2−Water Interface

Abstract

Measurements of the interfacial tension γ at the water-CO2 interface with the surfactant perfluoropolyetherCOO-NH4+ (Mw = 2500) are utilized to determine the surfactant interfacial area, surface pressure vs area, critical microemulsion concentration, and the thermodynamic properties of microemulsion formation. The measurements were made at equilibrium vs surfactant concentration with a tandem variable-volume pendant drop tensiometer from 25 to 65 °C at a constant CO2 density of 0.842 g·mL-1. The experimental results along with a simple molecular surface equation of state indicate that the area per surfactant at the critical microemulsion concentration is much larger at the water-CO2 vs water−oil interface for two primary reasons. The first reason is that γο (without surfactant) is much smaller for the water−CO2 interface; thus, less surfactant is required to lower γ to a typical value for microemulsions, 1 mN/m. The second reason is the larger entropic contribution to the free energy of the monolayer, due to greater penetration of the small CO2 molecules in the tail region relative to larger oils. The critical microemulsion concentration varies from 0.26 to 1.5 mM for a temperature range of 25−65 °C. Microemulsion formation is driven by the favorable enthalpy of −37.6 kJ/mol.