Composition of stearic and behenic acid monolayers from sodium-containing substrates

Monomolecular films of stearic and behenic acid were formed on substrates 0.1 M in sodium chloride, sodium bicarbonate, or sodium phosphate. The film was removed by skimming and its composition determined by infrared analysis. On all substrates only the pure acids could be detected by pH < 6. At pH 6–10 a mixture of the acid and the sodium salt was observed. The apparent pKa of the acids in the monolayer was 3–4 units higher than the estimated value in the bulk solution. The Gouy-Chapman theory was applied to these results and the true pKa was calculated to be 5.6–6.5, reasonably close to the bulk value. It was concluded that the binding of sodium ions to the monolayer is almost entirely ionic.