Quantifying the Hydrophobic Effect per CF2 Moiety from Adsorption of Fluorinated Alcohols at the Water/Oil Interface

Abstract

Amphiphilic fluorocarbon substances are a trending topic of research due to their wide range of applications accompanied by an alarming environmental and health impact. In order to predict their fate in the environment, use them more economically, develop new water treatment methods, etc., a better understanding of their physicochemical behavior is required. Their hydrophobicity in water/oil systems is particularly sensitive to one key thermodynamic parameter: the free energy of transfer of a perfluoromethylene group from oil to water. However, for the –CF2– moiety, the transfer energy values reported in the literature vary by more than ±25%. Due to the exponential relationship between this energy and the adsorption constants or the partition coefficients, such an uncertainty can lead to orders of magnitude error in the predicted distribution of fluorinated species. We address this problem by presenting an experimental determination of the hydrophobic effect of a –CF2– moiety with a greater certainty than currently available. The transfer energy is determined by measuring the interfacial tension of water|hexane for aqueous solutions of short-chained fluorotelomer alcohols. The obtained results for the free energy of transfer of a –CF2– moiety from oil to water are 1.68±0.02×𝑅𝑇0 , 1.75±0.02×𝑅𝑇0 , and 1.88±0.02×𝑅𝑇0 at 288.15 K, 293.15 K, and 303.15 K, respectively.