Interplay of Electrostatic Interactions and Hydrophobic Hydration at the Surface of Tetra-n-Alkylammonium Bromide Solutions

We use intensity and heterodyne-detected vibrational sum-frequency generation (VSFG and HD-VSFG) to study the structure of water at the surface of aqueous tetra-n-alkylammonium bromide (TAABr) solutions. We compare the water structure for four different n-alkyl chains (n = 1, 2, 3, 4). For solutions of tetra-n-alkylammonium bromides with short n-alkyl chains (n = 1, 2), we observe the structure of the surface water to be similar to the structure observed for simple inorganic salt solutions. For these solutions, the presence of Br− at the interface is observed to lead to a small decrease in the average strength of the hydrogen bonds. For solutions of tetra-n-alkylammonium bromides with long n-alkyl chains (n = 3, 4), we observe a strong ordering of the water molecules at the solution surface. The water molecules show a net orientation of their O-H group towards the bulk, which can be explained from the high surface propensity of positively charged tetra-n-alkylammonium ions with long alkyl chains (n = 3, 4). With increasing concentration of TAABr this ordering decreases and at very high concentrations (> 2 M) the orientation of the water molecules reverses. This latter finding can be explained from the formation of aggregated clusters of TAA⁺ cations and Br⁻ anions near the solution surface.