Thermodynamic parameters of some partially fluorinated and hydrogenated amphiphilic enantiomers and their racemates in aqueous solution

Micellization properties of aqueous solutions of four enantiomeric/racemic pairs obtained from partially fluorinated octylesters (PFOEs) and hydrogenated dodecylesters (DDEs) of alanine and serine were investigated by electrical conductivity, surface tension and density measurements over the temperature range of (15 to 35)degrees C. The Krafft temperatures (T(K)s) of most of the esters were found to be below or near zero temperature. The critical micelle concentrations (cmcs) of the PFOEs were found to be larger than those of the DDEs from which it was calculated that a CF2 group in the partially fluorinated chain is approximately equivalent to 1.8 CH2 group in the hydrogenated dodecyl chain and there were no noteworthy differences in the cmcs of the L-and DL-forms of the studied amphiphiles. The thermodynamic quantities of micellization (Delta G(mic), Delta H-mic, Delta S-mic) and adsorption (Delta G(ad)), and the adsorption parameters at the air/water interface, the surface excess concentration (T-max), the minimum area per head group of a molecule (A(min)) and the efficiency in the surface tension reduction (pC(20)), were also calculated. It was found that the micellization for DDE of alanine and the adsorption for PFOE of serine were more favoured than the other L-and L-forms where the latter was in part as dimer at the air/water interface. Furthermore, except for the Delta H-mic values, there were no significant differences in these quantities for the L-and DL-forms of the studied systems. The mass density measurements showed that the apparent molal volumes (V-phi(cmc), V-phi(mic)) of the hydrogenated esters were strikingly larger than those of the partially fluorinated ones, whereas the change of the apparent molal volume upon micellization (Delta V-phi(mic)) exhibited reverse behaviour, from which it was concluded that the micelles of the partially fluorinated esters were larger than those of the hydrogenated ones. (C) 2011 Elsevier Ltd. All rights reserved.