Re-evaluating the Gibbs analysis of surface tension at the air/water interface.

Molecular areas of soluble films at the air/water interface have traditionally been calculated by applying the Gibbs equation to the steep linear decline in surface tension as the bulk concentration increases. This approach presupposes that the interface is saturated in the "Gibbs region," thereby allowing a single unique area to be calculated. We show that the areas derived by the Gibbs equation (typically 50-60 A(2)/molecule) are much too large to account for the abrupt surface tension decline. Moreover, a surface tension/concentration plot was observed for a system where micelle formation does not interfere with the Gibbs region. Nonetheless, the surface tension plot leveled off, ostensibly owing to saturation, when the Gibbs approach predicted a continued linear decline, proving that the interface in the Gibbs region is not saturated as generally assumed. This conclusion means that the hundreds of published molecular areas obtained by the Gibbs approach should be reconsidered.