Association in fluoroacids. Spectral charakteristics of added dyes

Abstract

Association has been found to occur in solutions of perfluoroacids as determined by the spectral dye method. Whereas micelles are usually not found in solutions of normal paraffin chain salts with less than six carbon atoms in the chain, there is definite evidence for a critical micelle concentration (CMC) in perfluoroacetic acid solutions. There is a complete loss of color of pinacyanol chloride solutions at concentrations of added perfluoroacids at and above the CMC which is seen to be due to the environment at the surface of the micelles where the dye molecules are bound. Data are presented to show the effect of substitution ofω-hydrogen and chain chlorines on the CMC of fluoroacids and salts. Using a method proposed byDebye, the calculated contribution to the attraction energy (van der Wools' forces) in micelle formation is about 1200 cal per mole per CH2 in paraffin chain salts and about 2000 cal per mole per CF2 in the perfluoroacids, but these values, particularly the latter ones, must be considered to be too high for no correction has been applied for the role of the water molecules and other factors in these calculations. Using some more recent equations, it is possible to obtain values of about 1300 cal/mole per CF2 which is in good agreement with other available data.