Study of the nature of surfaces with polar molecules I. Adsorption of lauric acid on aluminum oxides and hydroxides

Abstract

Lauric acid is adsorbed on aluminum hydroxides and oxides from solutions in nonpolar solvents, giving isotherms with a pronounced saturation character. Especially from solutions in pure n-pentane, a complete monolayer of lauric acid on alumina is formed at very low equilibrium concentrations. The specific lauric acid adsorption f on the various hydroxides amounts to 0.43 mmole/100 m 2, on the aluminas heated above 800 °C to 0.617 mmole/100 m 2. Aluminas obtained just above the decomposition temperature of the hydroxides give low values for f, which are attributed to steric hindrance in narrow pores. It could be shown that the lauric acid molecules adsorbed adapt themselves to the periodicity of the oxygen atoms forming the surface of the aluminas, each lauric acid molecule being bound to four oxygen atoms. The adsorption of lauric acid provides a simple method for the determination of the surface area of aluminas in the absence of steric hindrance in narrow pores. Comparison of the lauric acid adsorption with the Brunauer-Emmett-Teller (BET) surface area will give useful information about the presence of narrow pores.