where θ(z) is the angle between the dipole moment of a water molecule and the normal to the solid surface at the position z. The bracket • θ denotes the ensemble average over all angles θ. The parameter S is equal to 1 when the molecules are perfectly aligned normally to the surface, S = -0.5 when the molecules are perfectly aligned horizontally to the surface, and S = 0 when the molecules are randomly aligned. For a translationally invariant, isotropic bulk system one thus expects S = 0. Calculations of S(z) for both NaCl and Na 2 SO 4 pores indicate that the water molecules within the first molecular layer are dominantly aligned with their dipole being parallel to the solid surface in the case of NaCl, and with their dipole pointing normal to the surface in the case of Na 2 SO 4 (Fig. ). The 'amount' of oriented water follows from the product of the density (ρ) and the orientation (S) shown in Fig. , which highlights even more pronounced differences between the two surfaces: water molecules of the first density layer are strongly polarized only in the case of Na 2 SO 4 .