Effect of ionization and cation selectivity on the expansion of stearic acid monolayers

Force-area isotherms of stearic acid and stearic acid-stearyl alcohol mixtures were investigated on alkaline subphases that contained Tris, Na(+), or K(+) cations and that varied in pH and ionic strength. The monolayer behaved as though ionization was effectively complete in the expanded region of the force-area isotherm. Surface pressure in this region was independent of pH and varied inversely with ionic strength as predicted by the Davies equation. The monolayer behaved as a partially ionized film in the plateau region of the force-area isotherm. Surface pressure in this region varied directly with pH and ionic strength as predicted by a modified Davies equation for partially ionized monolayers. The neutral molecule, stearyl alcohol, exerted a large condensing effect on the ionized film at pH 12.8, and this condensing effect also supported the concept that a partially ionized stearic acid film existed in the plateau region of the force-area isotherm. A greater binding affinity for Na(+) than for K(+) showed that the stearate anion surface behaved as a strong field at pH 10 and above, and a greater binding affinity for K(+) than for Na(+) showed that the stearate anion surface behaved as a weak field at pH 9. The weak field explained in part the anomalous binding affinity of the large Tris cation for the stearate monolayer at pH 9.