Orientation of Methylguanidinium Ions at the Water–Air Interface

We use heterodyne-deteted vibrational sum-frequency generation (HD-VSFG) to determine the orientation of the molecular plane of methylguanidinium ions at the surface of aqueous solutions. We measure the VSFG response of the symmetric and asymmetric methyl stretch vibrations of the methylguanidinium ion with different polarization combinations. We find that for >80% of the methylguanidinium ions the molecular plane is at an angle >20 degrees with respect to the surface plane. Hence, for only a minor fraction of the ions the molecular plane has an orientation (near-)parallel to the surface plane, in contrast to the predictions of recent molecular dynamics simulation studies

Interplay of Electrostatic Interactions and Hydrophobic Hydration at the Surface of Tetra‑<i>n</i>‑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 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). 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