This indicates that the surface concentration of STS equal to about 2 M could be regarded as a lower limit for the binary STS system at high ionic strengths. For the case of binary STS at high RHs (see Figure ), deliberately setting θ ss > 0 does not lead to any better agreement between the model fit and experimental results than those corresponding to θ ss = 0, when taking the experimental errors into account. Thus, the present work assumes that any effect of surface ion depletion in these data of low ionic strengths is negligible.