An STM investigation of the adsorption of mixtures of fatty acids and substituted acids at the solution-graphite interface

Scanning tunneling microscopy (STM) is used to investigate the adsorption on graphite of two-solute mixtures from phenyloctane solvent at room temperature. The first mixture is composed of the co-solutes hexadecanoic acid (HA) and 2-Br-hexadecanoic acid (2BrHA), while the second mixture is composed of co-solutes tetracosanoic acid (TA) and octadecanoic acid (OA). While the HA/2BrHA solution exhibits a miscible co-adsorbed surface structure, the TA/OA solution reveals a quasi-segregated surface structure where there is a sharp transition from full surface coverage by one solute to full coverage by the second as the composition of the solution mixture is varied. A one-dimensional Ising model is used to model the adsorption behavior of both systems, and it is shown that both the HA/2BrHA and OA/TA mixtures correspond to two different limits of this model. The HA/2BrHA mixture exhibits similar, weak interactions between adsorbed molecules, corresponding to one limit of the Ising model, which also corresponds to competitive Langmuir adsorption. Another limit of the Ising model is reached when there are significantly different interactions between the different co-adsorbates on the surface, a situation present in the OA/TA mixture resulting in a significantly different co-adsorption behavior.