The role of local interactions in determining the orientation of adsorbate molecules: Adsorption of propargyl alcohol at the free surface of water and at the mercury/water interface

Abstract

The adsorption of propargyl alcohol (PA) has been studied by measuring the interfacial tension and the surface potential shift at the uncharged Hg/solution interface and at the free surface of the aqueous solution. The results have shown that PA is weakly surface active and that the triple bond does not interact specifically with the metal surface. The orientation is governed by the torque exerted by the hydrophilic—hydrophobic interactions between the ends of the molecule and the solvent. The different orientation found at the two interfaces is explained in terms of the fact that the air/solution interface can be penetrated by the adsorbate whereas the metal/solution interface cannot. The adsorption parameters have been derived by fitting the experimental points with different isotherms: Frumkin, Flory—Huggins and Bennes, with the aim to elucidate whether n (the size factor between adsorbate and solvent molecules) can bear on the understanding of the relative behaviour of PA at the two interfaces.