One of the earliest results of surface-sensitive vibrational sum-frequency generation (VSFG) experiments was the observation that soft anions impact the vibrational lineshape in the O-H stretching region, but that hard anions do not . The term "soft" is chosen carefully here, as an alternative to "polarizable", and can be roughly interpreted as "monovalent and polarizable", equivalent to having a low surface charge density . (Such ions are sometimes called "chaotropic" .) Although the surface activity of certain anions is often discussed in terms of polarizability , it should be noted that polyvalent anions such as SO 2- 4 (aq) are quite polarizable , however the presence of such ions in solution does not affect the O-H lineshape measure in VSFG experiments . Molecular dynamics (MD) simulations suggest that hard anions, including polyvalent species but also fluoride, are repelled from the air/water interface .