Adsorption of Macromolecules at Oil
Water Interfaces during Emulsification - Food Colloids: Self-Assembly and Material Science - page 445

The emulsifying activity of proteins shows a weak positive correlation with
their protein surface hydrophobicity. 38 The emulsifying activity is also strongly
related to the ability to unfold at the interface, so that partially unfolded proteins
often have higher emulsifying activity that is related to the higher surface
hydrophobicity and the greater flexibility. 39,40 The amount of spreading and
its timescale depend on the protein structure, the solubility, and the nature of the
interface itself. The timescale for interfacial spreading ranges from 10 s for

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10 3 s for globular proteins 41 (although
values as small as 10 2 s have been reported for bovine a-lactalbumin 42 ).
The importance of the hydrophobic block character for the adsorption of
other surface-active macromolecules such as triblock copolymers has been
reported. 43,44 The results of these studies show that a longer coherent block of
monomers with a high affinity for the interface tends to give greater surface-
activity and higher surface loads. Griths et al. 45 has studied the role of
copolymer architecture on the adsorption at interfaces and found that a cyclic
triblock copolymer (in which the two hydrophilic blocks were covalently bound
together) had surprisingly similar adsorption behaviour to a linear triblock
copolymer of the same composition. This is indeed an interesting result that
could to some extent be related to the efficiency of the adsorption of proteins.
Furthermore, the heterogeneity in the distribution of hydrophobic substituents
in modified celluloses has been shown to influence the clouding behaviour of
such derivatives. 46 Thus the distribution of hydrophobic groups plays a large
role in determining the amphiphilic character and surface activity of water-
soluble macromolecules, and we suggest that the Kyte Doolittle plots in
Figure 10 reflect this concept.

flexible proteins such as b-casein up to

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30.4.3 Equilibrium-Controlled Adsorption
What we mean by 'equilibrium-controlled adsorption' is the situation in which
the rates of adsorption and desorption are equal, e.g., as described by the
Langmuir isotherm. Our results for the adsorption of OSA-starch show that
the highest molar mass species are preferentially adsorbed at the interface while
the lower molar mass species reside to a larger extent in the bulk solution. The
interfacial displacement of low molar mass polymers by higher molar mass
polymers, in dilute solutions at equilibrium conditions, is well known and can
be well described for homopolymers by the Scheutjens Fleer (SF) theory. 19
The shorter chain polymers adsorb more rapidly at the bare surface because of
their higher diffusion coefficients, but they are then displaced by the slower
diffusing long chains because the entropy loss upon adsorption is lower for the
latter whereas the gained adsorption energy is roughly the same. Naturally, this
interpretation assumes reversible adsorption, but this is very unlikely to be the
case when the adsorption is due to the hydrophobic effect and the macromol-
ecules are large. Care should therefore be taken when interpreting experimental
results from equilibrium theories on systems like these with strong evidence for
nonequilibrium effects. 47