Direct and reverse hofmeister effects on interfacial water structure

The adsorption of Hofmeister anions to negatively charged hydrophilic, negatively charged hydrophobic, and positively charged hydrophilic aqueous/oxide interfaces was studied by analyzing changes in the adjacent interfacial water structure using vibrational sum frequency spectroscopy (VSFS). At the negatively charged quartz/water interface at pH 10.0, it was found that the introduction of sodium salts of weakly hydrated monovalent anions (i.e., SCN ^- and ClO ₄ ^-) led to less attenuation of the water structure than when NaCl was added to solution. Addition of NaBr and NaNO ₃ gave rise to intermediate behavior. These results are consistent with less exclusion of the more weakly hydrated anions from the quartz substrate and the counterion double layer. As such, the monovalent anions followed a direct Hofmeister series. When the quartz surface was modified with an octadecyltrichlorosilane (OTS) monolayer, the ordering of the anions remained the same. Again, this is consistent with preferential exclusion of Cl ^- over SCN ^- and ClO ₄ ^- in accordance with a direct Hofmeister series. The quartz surfaces were also modified with a thin layer of TiO ₂. At low pH, this created a positively charged hydrophilic interface. Under these conditions, SCN ^- and ClO ₄ ^- attenuated the VSFS water peaks to a greater extent than Cl ^-. This apparent monovalent anion series reversal, however, was also caused by the preferential adsorption of the more weakly hydrated anions in comparison to the more strongly hydrated anions. In fact, the reversal was the result of preferential anion adsorption and double layer formation at the positively charged oxide/aqueous interface as opposed to preferential Na ⁺ partitioning at the negatively charged oxide/aqueous interfaces.