Fundamentals of Sum-Frequency Spectroscopy (Cambridge Molecular Science) by Y. R. Shen

Author:Y. R. Shen [Shen, Y. R.]
Language: eng
Format: azw3, epub
Publisher: Cambridge University Press
Published: 2016-02-28T16:00:00+00:00

8.3.2 Emergence of ions from solutions at water/vapor interfaces

In early days, it was believed that ions would not come to the water/vapor interface because they would experience repulsive Coulomb interaction from its image charge.46 Recent molecular dynamic simulations, however, predicted that polarizable anions, such as I–. would actually prefer to emerge at the surface and form an EDL with cations, such as Na+.47 Large polarizable ions are energetically favorable to reside at the interface mainly because it reduces the cavitation energy in water while polarization deformation on the ion allows only small increase in electrostatic energy.48 The negative field acts to reorient water molecules in the EDL with H toward the vapor. Accordingly, the spectrum of the interface in the bonded OH stretching region would appear more positive (or less negative).This was experimentally observed by PS-SFVS. As seen in Fig. 8.13, the spectrum for a 2.1M NaI solution, compared to that of the neat water, is significantly more positive in the 3,100–3,500 cm–1 range even though the surface density of I– is estimated to be only a few percent of a monolayer.26 Because the small surface density of I– is not likely to disturb much of the surface water structure, the spectral change is believed to come mainly from the reorientation of water molecules in the EDL, described by in Eq. (8.1), which is directly proportional to the potential of the EDL if the phase factor in the integral is negligible. A number of different salt solutions have been measured by PS-SFVS, allowing ranking of surface propensity of ions at the water/air interface according to the EDL potential they create, that is, a Hofmeister series describing which ions are more likely to emerge at the interface.49,50

Download

Copyright Disclaimer:
This site does not store any files on its server. We only index and link to content provided by other sites. Please contact the content providers to delete copyright contents if any and email us, we'll remove relevant links or contents immediately.