Interaction of Charged Surfaces with Ions and Molecules

6 Interaction of Charged Surfaces with Ions and Molecules 

Unspecific adsorption refers to the electrostatic attraction of counter-ions and is, therefore, independent from ion size or chemical nature. Such an adsorption reduces the effective surface charge, but can never reverse its sign. That means that the isoelectric point (lEP), which refers to the zeta potential, agrees with the point of zero charge (PZC), which refers to the surface charge (Fig. 3.6). Since the attraction is purely Coulombic, the ions retain their hydration shells hence, they are located at the OHP. 

When the adsorption results not only from electrostatic, but (additionally) from physical adhesion (e.g. van-der Waals forces, hydrogen bonding) and/or chemical bonds (covalent or ionic) too, it is inevitably affected by the ion type (e.g. by mass and electronegativity) and solid phase (e.g. by atomic lattice). Such a specific adsorption can increase the value of the effective surface charge or may lead to a charge reversal. Specifically-adsorbed ions are partially or completely dehydrated and directly associated to the surface. They are, consequently, located at the IHP (cf. Fig. 3.3). 

According to Fuerstenau and Hunter (Hunter 1988, p. 233), the physical adsorption is relevant for counter-ions it does not affect the lEP, but can result in charge reversal at sufficiently high ion concentrations. In contrast, chemisorption can occur even when the surface charge has the same sign as the ion. Consequently, chemically adsorbing ions can affect the lEP. 

The adsorption of ionic species, or surface ionisation, is frequently portrayed as surface complexation (e.g. Osseo-Asare 1996 Kosmulski et al. 1999a). However, this terminology does not imply a particular bonding structure (Davis et al. 1978), as the ion bonding may be caused by physical or chemical interactions, yet it offers an opportunity to quantify the ionisation of surfaces as reversible chemical reactions in equilibrium. Appropriate calculation schemes for negatively charged surfaces and positive (hydrated) counter-ions are presented by Davis et al. (1978), Osseo-Asare (1996), Kosmulski et al. (1999a), and Jablonski et al. (2000). 

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