where F (t)dt is the probability that a molecule leaves the first density layer for the first time between t and t + dt. We find the residence time to be larger in the case of Na 2 SO 4 than in the case of NaCl, respectively τ A ≈ 85 ps vs. τ A ≈ 52 ps (Fig. ). These longer surface residence times reveal a higher affinity between water and Na 2 SO 4 surface as compared to water and NaCl. The typical rotation time of a water molecule within the first density layer, which was calculated as