Sorption and Desorption of Simple Ions

The specific ion effects at the water/air interface have recently been reviewed by Jungwirth and Tobias (2006). Since the surface region of water with respect to air or water vapor differs in properties and structure from bulk water (Sect. 4.1), it is expected that ions react to these different environments by being either attracted to or repelled from the surface. In other words, ions may be sorbed at or desorbed from the surface of an electrolyte solution. The excess concentration Te (the amount of an electrolyte sorbed per unit increase of the surface energy) at the surface relative to the bulk is governed by the Gibbs adsorption law  

Cation dy/dci (Marcus 2010) dy/dci improved Anion dy/dci (Marcus 2010) dy/dci improved 

Theoretical discussions of the surface tension increments try to explain the trends noted for the various ions. Aveyard and Saleem (1976) related the 577 of an electrolyte solution to the product of its molality and its osmotic coefficient (p  

The main point to consider in a theory of the 577 by ions is the unequal surface/bulk concentration ratios of individual ions (Randles 1977). Pegram and Record (2007, 2008) arrived at an equation similar to Eq. (4.17), namely  

Theoretical considerations showed that at moderate concentrations the dispersion forces dominate over the electrostatic ones and that dy/dcj should be proportional to where Bj, in turn, is proportional to the static excess polarizability of the ion in water, R -Rw (Bostrbm et al. 2001). For multivalent anions (not considered in (Bostrom et al. 2001)) Bj/zi needs to be taken into account to obtain the excess polarizability, in terms of the molar refraction Ri/zi -Rw, where Bw = 3.67 cm mol and some Rj values (taken from (Marcus 1998)) are shown in Table 2.1. The correlation is far from perfect, as seen in Fig. 4.1, but the general trend is confirmed. 

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