ENTROPIES OF COMPRESSION OF CHARGED MONOLAYERS AT AQUEOUS INTERFACES.

Entropies of compression calculated from surface pressure-area isotherms for anionic or cationic monolayers of the same chain length (18 carbons) that are spread at n-heptane/water (O/W) or air/water (A/W) interfaces or for their equimolar mixture at the O/W interface are negative. When plotted against the logarithm of the area they give two lines - one for the A/W interface and a steeper one for the O/W interface. As well as being independent of head group, the entropies do not depend on ionic strength. A model that treats the monolayer as an ensemble of discs that interact only through hard sphere and coulombic forces explains the size of the entropies at A/W but not at O/W where an extra chain-conformation term is needed.