WATER PENETRATION AND PHASE-BEHAVIOR OF SURFACTANT GEL PHASES AND LIPID BILAYERS

Two derivatives of stearic acid (as potassium salts) containing a ketone functionality either at position 5 or 12 (5-oxo-KSA and 12-oxo-KSA) and an aliphatic ketone (9-heptadecanone, 9HP) were mixed with potassium stearate (KSA). Infrared spectra of aqueous dispersions of these mixtures (D2O, 10 wt %) were recorded at various temperatures to obtain spectra characteristic of the coagel, gel, and micellar phases of potassium stearate. The C=O stretching vibrations of the ketone functionalities of 5-oxo-KSA, 12-oxo-KSA, and 9HP were used to determine the extent of solvent (D2O) penetration into the different mesomorphic phases. In the lamellar coagel phase the solvent penetration depth is estimated to be greater than ∼5 Å but less than ∼7 Å; in the lamellar gel phase it is larger than ∼7 Å but less than 10 Å. In micelles of potassium stearate most of the keto functionalities of 5-oxo-KSA and 12-oxo-KSA are in contact with the solvent as a result of extensive chain looping. The behavior of 9HP is different; in micelles of KSA most of the guest aliphatic ketone is dissolved in the micellar core while in the lamellar coagel and gel phases 9HP adopts a bent conformation whereby its carbonyl group is immobilized at the surfactant-water interface. Comparative experiments were performed with bilayers formed by the ether-linked phospholipid dihexadecylphosphatidylcholine (DHPC); solvent penetration is not as pronounced in bilayers of this lipid compared to the gel phase of KSA.