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 + (M w = 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.

Effect of Compressed CO2 on the Critical Micelle Concentration and Aggregation Number of AOT Reverse Micelles in Isooctane

“…In recent years, microemulsions with supercritical CO 2 as the continuous phase have been studied extensively. [29][30][31][32][33][34][35] It is well known that some compressed gases, such as CO 2 , are quite soluble in many organic solvents and they can reduce the solvent strengths to such a degree that the solutes can be precipitated. [36][37][38] Our previous work showed that compressed CO 2 and ethylene can increase the solubilization capacity of water in sodium bis-2-ethylhexylsulfosuccinate (AOT) and Triton X-100 reverse-micellar systems at suitable pressures because the gases can insert into the interfacial region to enhance the rigidity of the interface layers and reduce the interdroplet attraction.…”

Concentrated CO2-in-Water Emulsions with Nonionic Polymeric Surfactants

“…A variety of studies involving W/C emulsions (10)(11)(12) and W/C microemulsions (13)(14)(15)(16)(17)(18)(19)(20)(21)(22)(23)(24) have been reported recently. Based on observations of the volume fractions of the upper and lower phases after shear, emulsions with a C/W morphology have been reported (25,26).…”

Microscopic phase behavior of supercritical carbon dioxide+non-ionic surfactant+water systems at elevated pressures

“…6. As shown in the figure, both the higher and lower phase transition pressures at 3 wt% of TMN-3 concentration were lower than those at 1 wt%, which would be caused by the effects of differences in surface properties that arise from the difference in TMN-3 concentration in scCO 2 , because it is reported that the interfacial tension between water and scCO 2 decreased with increasing concentration of surfactant for several types of surfactants [18,32,27]. On the other hand, the maximum value of W 0 obtained at 3 wt% of TMN-3 was lower than that obtained at 1 wt% of TMN-3, as shown in Fig.…”

“…In addition to investigations of engineering applications utilizing both W/CO 2 microemulsion and macroemulsion, basic studies from the viewpoint of thermodynamic stability and interfacial behavior were reported by Johnston et al [15][16][17][18][19][20][21][22][23]. Moreover, a phase diagram is also important for the development of novel processes utilizing W/CO 2 microemulsion and macroemulsion, therefore, the phase behaviors of the scCO 2 + surfactant + water systems were also measured.…”