(179y) The Influence of Bilayer Composition On the Gel to Liquid Crystalline Transition

We report molecular dynamics simulations of bilayers using a united atom model with explicit

solvent molecules. The bilayer consists of

single tail cationic surfactant, behenyl trimethyl ammonium chloride

(BTMAC) with stearyl alcohol (SA) as the co-surfactant.

We study the gel to liquid crystalline transitions in the bilayer

by varying the amount of water at fixed BTMAC to SA ratio as well as by varying

the BTMAC to SA ratio at fixed water content. The bilayer is found to exist in the tilted, L_β' phase at low temperatures and

for the compositions investigated in this study,

the L_β' to L_α melting transition occurred in the temperature range of

330-338 K. The larger area per head group associated with the

quarternary amine surfactant BTMAC decreases the transition temperature with an increase

in the BTMAC composition.

For the highest BTMAC to SA composition (2:3 molar ratio) we observe an increase in

the d-spacing prior to the melting

transition. This increase in d-spacing is

accompanied by a sharpening in the water density variation across the

head group region of the bilayer. Signatures of this swelling effect

which can be observed in the alkane density distributions, area per head group and membrane thickness,

are attributed to the hydrophobic effect.

At a fixed bilayer composition, the transition temperature (>338 K) from the L_β' to

L_α transition obtained for

the high water content bilayer (80 wt %) is similar to that

obtained with low water content (54.3 wt %), confirming that the

melting transition at these water contents, is dominated by chain melting.