SURFACE EQUATION OF STATE AND TRANSITIONAL BEHAVIOR OF ADSORPTION LAYERS OF SOLUBLE AMPHIPHILES AT FLUID INTERFACES

Generally, the equilibrium surface tension vs concentration isotherm o f any amphiphile has to be described by a surface equation of state th at makes allowance for a transition state in the adsorption layer. The transition state is assumed to continuously connect two adsorption st ales attributable to two different surface configurations of the adsor bed amphiphile. By the investigation of various types of nonionic amph iphiles, it is shown that the transition range sets on at rather low s urface pressures, i.e., in the Henry region. The width of the transiti on range depends on the amphiphilic structure. The transition behavior can be described mathematically by various functions. Here, we used a polynomial (POLY), the tangens hyperbolicus (TANH), and two analytica l expressions related to the distribution of the configurations' diffe rent cross-sectional area (AREA) and/or surface tension (SELF) values in the transition region. These functions lead To almost identical res ults. It turns out that from the evaluation of the AREA transition in terms of molecular space constraints, only the surface accommodation o f straight-chain surfactants with small terminal head group can reason ably well be understood. Rising head group size, addition of more hydr ophilic entities, and nonterminal binding of the hydrophilic group(s) result in area ratios far beyond those derived from the molecules' coo rdinates. Obviously, amphiphiles of such molecular structure possess a comparatively high mobility in the adsorption layer. Thus, it is conc luded that there is no uniform mechanism for the amphiphiles' surface accommodation at the onset of the transition region.