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The action of n-alkanols upon the transition temperature of phospholipid membranes between liquid-crystalline and solid-gel phases was not a monotonic function of alkanol concentrations. All n-alkanols tested (1 -nonanol, 1 -decanol, 1 -undecanol, 1 -dodecanol, and 1-tridecanol) depressed the transition temperature at low concentrations and elevated it at high concentrations. There were no n-alkanols that exclusively depressed or elevated the transition temperature. The observed phase equilibria were interpreted according to phase diagrams. The diagrams expressed a minimum point, suggesting that the mixing was not ideal. Although a possibility cannot be entirely ruled out that the minimum point has a finite width to form a eutectic mixture in a narrow concentration range, it appears that all n-alkanols form uniform membranes with the phospholipids under the present experimental conditions. The shape of these phase diagrams was similar to that of liquid-gas equilibria, which form azeotropes. The position of the minimum in phase diagrams was shifted to lower alkanol concentrations when longer alkanols were used; when the chain length exceeded 10-12 carbon atoms, the effect appeared to be predominantly temperature elevation.

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