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.