Ehj

atom-slab

Adamson, hv0 = 18 eV (= 18 x 10~12 ergs), a (polar-izability) = 1.46 x 1CT24 cm3. Since pw density of liquid water g cm~3 and n = N0pw/18 = 3.34 x 1022 molecules/cm3 using these figures C1 = 28.8 x 10~6° ergs cm6.

The dispersion energy between two infinite flat surfaces after integration is:

where s(Msiab-siab is in ergs cm 2.

Fowkes has equated s(hx) of eqn [5] to yf, the dispersion component of the total energy for water. As will be shown later, he has linked this dispersion energy with the physical adhesion energy at the interface between two different phases.

The dispersion energy for liquid water, yW, is calculated from eqn [5] to be 35.8 ergs cm"2 using the values given above and a value of 2.76 A for the average intermolecular distance. This is somewhat higher than the experimentally determined value of 21.8 ergs cm"2 from contact angle measurements, but nevertheless it is valuable as it gives a theoretical basis for the forces of attraction at the interface between two immiscible phase surfaces. This is relevant to studies where the adsorption of a surfactant (collector) on a solid surface is used to modify the surface's hydrophobicity.

Eqn [5] is usually written in terms of, A, the Hamaker constant, which is specific for a given separation; for water A = n2n2C1 = 0.3 x 10"12, and on average is of the order of 10"12 ergs:

s(hj slab-slab

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