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

Abstract

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.

Keywords

• force–area isotherm
• Tris
• sodium
• potassium
• cation binding
• pH