Page 30 - Nanotechnology an introduction

of Lewis acids and Lewis bases constituting the two interacting substances. Superscript ⊖ will be used to denote electron-donating (Lewis base)
and superscript ⊕ will be used to denote electron-accepting (Lewis acid) moieties; van Oss has proposed that one might again take the geometric
mean, namely

(3.8)
Two monopolar substances of the same sign will repel each other; attraction depends on the presence of cross-terms. By analogy with equation
(3.5),

(3.9)
Hence the ab combining law is

(3.10)

It takes account of the fact that ⊖ interacts with ⊕, which is why the ab interaction can be either attractive or repulsive. In typical biological and related
systems, the Lewis acid/base interaction accounts for 80–90% of the total interactions. The most familiar manifestation is hydrogen bonding (e.g.,
double-stranded DNA (the double helix), globular proteins containing α-helices); π–π interactions (stacking of alternately electron-rich and electron-
deficient aromatic rings) are frequently encountered in synthetic organic supermolecules.
Let us now consider two solids 1 and 3 in the presence of a liquid medium 2 (e.g., in which a self-assembly process, cf. Section 8.2.1, takes place).
ΔG 123 is the free energy per unit area of materials 1 and 3 interacting in the presence of liquid 2. Using superscript || to denote the interfacial
interaction energies per unit area between infinite parallel planar surfaces,

(3.11)
and

(3.12)
From the above equations we can derive:

(3.13)
where ΔG is the free energy per unit area of materials 1 and 3 interacting directly. It follows that:
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• LW forces (anyway weak) tend to cancel out;
• the so-called “hydrophobic force” is a consequence of the strong cohesion of water ΔG . Attraction of suspended solids is only prevented by
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their hydrophilicity. The sign of ΔG with 2 = water, provides an unambiguous measure of the hydrophobicity of substance 1: ΔG < 0 ≡
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12
hydrophilic; ΔG > 0 ≡ hydrophobic (see also Section 3.2.4).
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can be used to provide a rapid first estimate of whether adhesion between materials 1 and 3 will take place in the presence of medium 2.
Table 3.1 and Table 3.2 give some typical values (see Section 3.2.3 for information about their determination).
Table 3.1 Surface tension parameters of some solids [127]
⊖ ⊖
⊕ ⊕
Material γ (LW ) /mJ m −2 γ /mJ m −2 γ /mJ m −2
Synthetic polymers
Nylon 6,6 36 0.02 22
PMMA 41 0 13
Polyethylene 33 0 0
Polyethylene oxide 43 0 64
Polystyrene 42 0 1.1
Polyvinylpyrrolidone 43 0 30
PVC 43 0.04 3.5
Teflon 18 0 0
Carbohydrates
Cellulose 44 1.6 17
Dextran T-150 42 0 55
Metal oxides
39 0.8 41
SiO 2
31 2.9 8.5
SnO 2
42 0.6 46
TiO 2
35 1.3 3.6
ZrO 2
Table 3.2 Surface tensions of some liquids (data mostly from [127])
⊖ ⊖
⊕ ⊕
Liquid γ (LW ) /mJ m −2 γ /mJ m −2 γ /mJ m −2
Water a 22 25.5 25.5
Glycerol 34 3.9 57
Ethanol 19 0 68
Chloroform 27 3.8 0
Octane 22 0 0
n-hexadecane 27.5 0 0
Formamide 39 2.3 40
α-bromonaphthalene 44 0 0
Diiodomethane 51 0 0
⊖
⊕
a Absolute values of γ and γ are not known at present; values are arbitrarily assigned to ensure that the known overall γ is correct (equation 3.8).