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

Abstract

The pressure−area isotherms obtained in the initial stages after the spreading of fatty primary amines on the aqueous subphase has been investigated as a function of pH, length of hydrocarbon chain, basicity of the amine, nature of the acid counterion, etc. The main emphasis is on the high-area liquidlike features observed prominently at low pH. The dependence on the specific nature of the acid counteranion or the nature of the amine (aliphatic or aromatic) highlights the importance of the interactions at the interface in deciding the nature of the pressure−area isotherm in addition to the well-known tail−tail interactions between the hydrocarbon chains. The marked influence of small amounts (∼2 × 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 subphase ionic species, repulsive limits between charged −NH3+ head groups, and ion-pair interaction distances, as well as the valence of the subphase acid species is examined. The role of the area-expanding head-group/subphase interactions vis-a-vis the area-condensing tail−tail interactions is discussed in terms of recent two-state models for liquid monolayers.