regular solution theory
Springer Online Journal Archives 1860-2000
Chemistry and Pharmacology
Summary The effect of 1-alkanols upon the main phase-transition temperature of phospholipid vesicle membranes between gel and liquid-crystalline phases was not a simple monotonic function of alkanol concentration. For instance, 1-decanol decreased the transition temperature at low concentrations, but increased it at high concentrations, displaying a minimal temperature. This concentration-induced biphasic effect cannot be explained by the van't Hoff model on the effect of impurities upon the freezing point. To explain this nonlinear response, a theory is presented which treats the effect of 1-alkanols (or any additives) on the transition temperature of phospholipid membranes in a three-component mixture. By fitting the experimental data to the theory, the enthalpy of the phase transition ΔH * and the interaction energy, ɛ AB * between the additive and phospholipid molecules may be estimated. The theory predicts that when ɛ AB * 〉2 (where ɛ AB * = ɛAB,/RT o,T o being the transition temperature of phospholipid), both maximum and maximum transition temperatures should exist. When ɛ AB * = 2, only one inflection point exists. When ɛ AB * 〈 2, neither maximum nor minimum exists. The alkanol concentration at which the transition temperature is minimum (X min) depends on the ɛ AB * value: the larger the ɛ AB * values, the smaller theX min. When ɛ AB * is large enough,X min values become so small that the plot ΔT vs.X shows positive ΔT in almost all alkanol concentrations. The interaction energy between 1-alkanols and phospholipid molecules increased with the increase in the carbon chain-length of 1-alkanols. In the case of the dipalmitcylphosphatidylcholine vesicle membrane, the carbon chain-length of 1-alkanols that caused predominantly positive ΔT was about 12.
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