Abstract
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 toward 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)...
Published Version
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