ANCHORING OF HYDROPHOBICALLY-MODIFIED POLY(SODIUM ACRYLATE)S INTO DDPVESICLE BILAYERS - HYDROPHOBIC MATCH AND MISMATCH

Differential scanning microcalorimetric thermograms have been recorded for aqueous solutions containing vesicles formed by sodium di-n-dohec yl phosphate, in the presence of different concentrations of poly(sodi um acrylate-co-n-alkyl methacrylate), where n-alkyl = C9H19, C12H25, C 18H37. The mole fraction of hydrophobic moieties in the copolymer is 0 .04. The main phase transition temperature (T-m) is hardly affected by the presence of poly(sodium acrylate)s bearing n-dodecyl chains, wher eas the anchoring of polymers bearing n-nonyl or n-octadecyl groups re duces the main phase transition temperature significantly from ca. 34 degrees C to ca. 32 degrees C. In parallel, the enthalpy of transition per mole of DDP monomer (Delta(m)H(int)) is lowered upon adding polym er. Again, the polymer containing n-dodecyl moieties hardly affects De lta(m)H(int). These patterns are explained by the notion that the exte nt of the disruptive effect of alkyl chains incorporated into the bila yer depends on the extent of the mismatch between the chain lengths of the intruding alkyl chains and the hydrophobic moieties composing the vesicle bilayer. Added hydrophobically modified polymers increase the cooperativity of the melting process, as shown by the increase of n(D DP). We suggest that the anchoring poly(sodium acrylate-co-n-alkyl met hacrylate) relieves the strain in the curved outer monolayer of a pure DDP bilayer by allowing the presence of larger ''patches'' characteri zed by low curvature.