ABSTRACT
Both adhesion between two different solid materials immersed in water, and adsorption of solutes dissolved in water onto solid bodies are aspects of the same phenomenon and are governed by the same equations:
where ΔGLW1W2 + ΔGAB1W2 = ΔGIF1W2, representing the free energy of interfacial interaction (cf. eq. [III-16], based upon the Dupré equation):
γ γ γW )+ √ − √ −2 1 2 1 2 γ γ γ− ]
[XXIV-2]
and the expressions for ΔGEL1W2 are given in eqs. [V-3-6]; see also Chapter VII. For adhesion or adsorption to take place, ΔGTOT1W2 (eq. [XXIV-1]) must have a
negative value. For greater accuracy in predicting the degree of adhesion or adsorption
it is of course desirable to elaborate energy balance plots of ΔGTOT1W2 vs. distance () (cf. Chapters VII and XXIII). However, it usually suffi ces to know that ΔGTOT1W2 < 0, at = o. It should be kept in mind that whilst the negative value of ΔGTOT1W2 is a fair measure of the degree of adhesion or adsorption one may expect, the quantitative prediction of the amount adhering or adsorbed is linked to the equilibrium constant, Kass (cf. eqs. [XIX-9,10 and 10A]):
Sc ⋅ ΔG kT KWTOT ass1 2 / = − ln [XXIV-3]
where ΔGTOT1W2/kT is expressed in energy units per contactable surface area (Sc) per molecule (see eqs. [VI-6, 7 and 8]), and:
K S S S
[ ] [ ][ ]1 2
[XXIV-4]
(cf. eq. [XIV-8]) and where the symbols in brackets indicate concentrations (in mol fractions): Sb indicates bound sites and Sf1 and Sf2 free sites [for material (1) and solute or material (2)]. Kass can be derived from the initial slope of the Langmuir plot of the adhesion or adsorption system under study (Hiemenz, 1986, p. 401); thus ΔGTOT1W2 is proportional to the logarithm of that slope.
