DIHEXADECYLPHOSPHATE BILAYERS - INTERBILAYER INTERACTIONS AND INTRABILAYER STRUCTURE

The initial flocculation rate of sodium dihexadecylphosphate (v0) (DHP) vesicles is determined as a measurement of intervesicles interaction for a range of NaCl concentrations (C) or pH values. Also, the stability of the membrane structure is assessed by turbidimetrically determining the gel-to-liquid-crystalline phase transition temperature (Tc) of the DHP vesicles. The stability of the vesicles with respect to interactions taken as log 1/v0 displays a bell-shaped profile as a function of pH for a given NaCl concentration. The temperature-induced phase transition of the bilayer occurs for a broad range of temperatures with midpoints between 59 and 73°C, depending on the pH value. The maximal Tc value is at pH 5.5-6.0, the apparent pK value of the phosphate polar head at the membrane water interface. The pH range of maximal structural stability (Tc) coincides with the pH range of maximal colloidal stability ( 1 v0) and with the maximal values of d log( 1 v0) d log C, a parameter representative of the Stern Potential. In close analogy to phosphatidic acids and other lipids with a dissociative polar head, an increase in the total charge of the DHP headgroup from zero to half caused an increase in Tc. Factors governing the stability of the bilayer structure in the gel state seem to be the same as those governing the stability relative to interbilayer interactions. © 1990.