  I consider the effect of a small concentration of a molecule, such as a
short-chain alcohol, on the miscibility transition temperature of a giant
plasma membrane vesicle. For concentrations sufficiently small such that the
system can be treated as a weak solution, the change in transition temperature
is known to depend upon the extent of the molecule's partition into the
coexisting liquid-disordered and liquid-ordered phases. Preferential
partitioning into the former decreases the miscibility temperature, while
preferential partitioning into the latter causes an increase. The analysis,
combined with calculated values of the partition coefficient of saturated
chains, illuminates the results of recent experiments on the change in
miscibility transition temperatures with changing alcohol chain length, and
makes several testable predictions.
