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.