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1590113 
Journal Article 
The Effects of Hofmeister Cations at Negatively Charged Hydrophilic Surfaces 
Flores, SC; Kherb, J; Konelick, N; Chen, Xin; Cremer, PS 
2012 
Yes 
Journal of Physical Chemistry C
ISSN: 1932-7447
EISSN: 1932-7455 
116 
9 
5730-5734 
WOS:000301315700053 
This work explores the interactions of cations with highly
negatively charged solid/aqueous interfaces. Vibrational sum frequency spectroscopy (VSFS) was
exploited to observe interfacial water structure on the surfaces of fused quartz and titanium
dioxide (TiO2) in the presence of nine chloride salts with different alkali, alkaline earth, and
transition metal cations. The results showed prominent specific cation effects at low
concentration. The cations followed a direct Hofmeister series. On quartz surfaces the series
was: Li+ > Cs+ > Rb+ > NH4+ > K+ > Na+ > Ca2+ > Mg2+ > Zn2+. As such, Zn2+ attenuated water
structure to the greatest degree and therefore gave rise to the smallest peaks in the OH stretch
region of the VSFS spectrum. The opposite was the case for Li+. Such results indicate that Li+
partitioned least to the surface, while Zn2+ partioned there to the greatest extent. We also
observed prominent specific cation effects on TiO2 surfaces. There were, however, some key
differences between these surfaces. On TiO2, Li+, which is the best hydrated monovalent cation,
behaved more similarly to K+ and Na+. In addition, the ordering of Mg2+ and Ca2+ was reversed on
TiO2 compared with quartz. Such reordering of the Hofmeister series should result from
differences in the charge density, polarizability, and basicity of the two oxide surfaces.
Finally, NH4+ was found to interact more strongly with TiO2 than any other monovalent cation.
This result is in line with the greater role that hydrogen bonding should play on TiO2 compared
with quartz under the conditions of the experiment.