Energetics of adsorption of neutral and charged molecules at the air/water interface by second harmonic generation: Hydrophobic and solvation effects

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Abstract
Surface second harmonic generation has been used to study the adsorption of alkyl phenols and anilines and their respective ions to the air/water interface. The free energies of adsorption of the neutral molecules were obtained using a simple Langmuir model. The quantitative effects of the opposing hydrophobic and solvations forces on molecular adsorption were investigated by varying the chain length of alkylphenolate and alkylanilinium ions. An alkyl chain of at least five carbons was found to be necessary to balance the bulk ionic solvation and thereby bring the ion interface population to a level above the water background. The adsorption of the ions can be described quantitatively by using a modified Langmuir model that includes the electrostatic free energy of the electric double layer as given by the Gouy–Chapmann theory. From the excellent fit of the second harmonic data to this treatment, the chemical and electrostatic free energies of adsorption, interface electric potential, and interface pH were obtained.