ROLE OF TAIL-TAIL INTERACTIONS VERSUS HEAD-GROUP SUBPHASE INTERACTIONS IN THE PRESSURE-AREA ISOTHERMS OF FATTY AMINES AT THE AIR-WATER-INTERFACE .1. INFLUENCE OF SUBPHASE ACID COUNTERIONS/

The pressure-area isotherms obtained in the initial stages after the s preading of fatty primary amines on the aqueous subphase has been inve stigated as a function of pH, length of hydrocarbon chain, basicity of the amine, nature of the acid counterion, etc. The main emphasis is o n the high-area liquidlike features observed prominently at low pH. Th e dependence on the specific nature of the acid counteranion or the na ture of the amine (aliphatic or aromatic) highlights the importance of the interactions at the interface in deciding the nature of the press ure-area isotherm in addition to the well-known tail-tail interactions between the hydrocarbon chains. The marked influence of small amounts (similar to 2 x 10(-4) M) of alcohol such as n-butanol in influencing the stability as well as the nature of the pressure-area isotherms of fatty acids and amines is demonstrated. The relation between the lift -off areas and length scales, such as the size of the solvated subphas e ionic species, repulsive limits between charged -NH3+ head groups, a nd ion-pair interaction distances, as well as the valence of the subph ase acid species is examined. The role of the area-expanding head-grou p/subphase interactions vis-a-vis the area-condensing tail-tail intera ctions is discussed in terms of recent two-state models for liquid mon olayers.