Reply to the 'Comment on "An explanation for the charge on water's surface"' by R. Vacha, D. Horinek, R. Buchner, B. Winter and P. Jungwirth, Phys. Chem. Chem. Phys., 2010, 12, DOI: 10.1039/c001492c

The claim that the behaviour of the fluoride ion at aqueous hydrophobic interfaces contradicts the fluctuation force model described in our paper is dismissed on four grounds. (1) The dielectric decrement of NaF solutions remains uncertain. (2) Even if it were as large as that of NaOH, this would not disprove our theory (as explained in our paper), partly because there is no reason to assume that it interacts with surrounding water molecules in the same way as does the hydroxide ion, despite their having similar ionic radii. In fact, experimental evidence shows the solvation structures around the two ions are quite different, a difference not captured by simulations of the type used by Vacha et al. (3) There is no experimental evidence that fluoride is not adsorbed at the interface at millimolar concentrations. (4) The models and methods of calculation used by Vacha et al. in their comment appear to be flawed, and this casts some doubt on the rather firm conclusions drawn from similar simulations in previous works.