INTERACTION OF PROCAINE WITH STEARIC-ACID MONOLAYERS AT THE AIR-WATER-INTERFACE

The adsorption characteristics of procaine have typically been studied by measuring the surface tension of aqueous procaine solutions and by recording compressional isotherms, i.e., surface pressure as a function of molecular area, for stearic acid monolayers in the presence of various procaine subphase concentrations at both pH 2 and pH 8. The presence of the stearic acid monolayers promotes the enhanced adsorption of procaine in liquid or fluid states of the stearic acid monolayers. On compression of the stearic acid monolayer the procaine adsorption increases and, after attaining a maximum value, decreases and vanishes near the collapse of the stearic acid monolayer. The surface characteristics derived from the compressional isotherms, as well as the area increase recorded at constant surface pressure, are interpreted by taking into account models both for the mechanism of procaine penetration into stearic acid monolayers and for the protolytic equilibria occurring in the system. The penetration numbers, i.e., the ratio of procaine molecules to stearic acid molecules in the mixed penetrated films, are derived from the molecular area increments and are in good agreement with the values obtained by using the Gibbs' adsorption equation. Molecular species of procaine are initially thought to be adsorbed at the air/water interface in a horizontal orientation. On compression of the spread stearic acid film, the procaine is gradually forced to adopt a vertical position. At higher surface pressure values procaine is squeezed out from the monolayer and is accumulated in an adjacent layer, thus causing a significant increase in the collapse pressure of the stearic acid monolayers. At pH 2 this latter effect has been interpreted terms of ion-dipole interactions between the positively charged procaine molecules and the uncharged carboxyl groups of the stearic acid monolayers and in terms of hydrogen bonding between the carboxyl group and primary amino group of the procaine monocation. At pH 8 electrostatic interactions between the stearate anions and the procaine monocation are also taken into account. Procaine penetration occurred preferentially in the liquid or fluid phase of stearic acid monolayers and was found to be dependent on the surface pressure. Maximum penetration was established at around 10 mN/m for monolayers on pH 2 and at 5 nN/m for stearic acid monolayers at pH 8. The maximum values are much higher at pH 8 than at pH 2 due to the protolytic equilibria in which both the stearic acid and the procaine participate. The physical insertion of procaine between the stearic acid molecules is presumably primarily responsible for the expansion effect found. At the same time, electrostatic interactions or ion-dipole interactions will modify such interchain interactions, although the latter are increasingly important in the solid state of the monolayer as collapse is approached.