Unexpected efficiency boosting in CO2-microemulsions: a cyclohexane depletion zone near the fluorinated surfactants evidenced by a systematic SANS contrast variation study

Abstract

Microemulsions with supercritical CO₂ are promising alternatives for organic solvents, especially if both polar and non-polar components need to be dissolved. However, only fluorinated surfactants, which are known to be environmentally unfriendly, are appropriate to formulate well-structured microemulsions. While most approaches to increase the environmental performance of CO₂-microemulsions deal with the design of new surfactants with a reduced degree of fluorination, we discovered that the partial substitution of CO₂ by cyclohexane enables a considerable reduction of fluorinated surfactants. Thereby, the most efficient solubilization of the CO₂/cyclohexane mixture, which turned out to be pressure-dependent, was found at a cyclohexane-to-CO₂ mass ratio between 1 : 6 and 1 : 4. In order to elucidate this unexpected effect a systematic Small Angle Neutron Scattering (SANS) contrast variation study was performed. The analysis of the recorded scattering curves by the Generalized Indirect Fourier Transformation (GIFT) clearly shows that the scattering length density profiles differ considerably from CO₂-microemulsions without cyclohexane. Instead of a nearly constant scattering length density, a density profile that varies systematically over half of the droplet radius was detected. These results clearly indicate that the observed efficiency boosting is caused by the formation of a depletion zone of cyclohexane close to the fluorinated amphiphilic film.