The hydrophobic interaction underlies numerous critical processes including protein folding, micellar assembly at the molecular scale and separation processes at the macro scale. Ions, positive and negative, have been shown to impact hydrophobic force but mechanistic insights are not entirely clear. While common hydrophobic surfaces, e.g., hydrocarbons and perfluorocarbons, are commonly believed to be electrically neutral, new experiments have revealed that these materials have got significant surface-bound electronic charges. Therefore, it is important to understand how the surface charge, negative or positive, influences hydrophobic force, especially in the presence of ions. Here, we realized charged hydrophobic self-assembled monolayers (SAMs) by mixing 1-undecanthiol + 11-mercapto-undecanoic acid (7:3 v/v) for negatively charged surfaces, and 1-undecanthiol + 11-amino-1-undecanthiol hydrochloride (7:3 v/v) for positively charged surfaces. Next, we utilized AFM to compare the effects of potassium salts of the following ions of varying sizes and polarizabilities on forces between the various surfaces described above: Cl-, Br-, I-, and ClO4-. In comparison to 1-undecanthiol surfaces, the charged counterparts showed dramatic differences. We will present our experimental results along with molecular dynamics simulations to facilitate mechanistic insights.

Effects of anions on hydrophobic forces between charged and uncharged hydrophobic surfaces

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