Area Required For Initial Adsorption

At an air/water interface, the two-dimensional interfacial pressure (IT) can be easily monitored using an instrumented Langmuir trough. The initial adsorption rate at a clean surface is simply the rate of diffusion 

The work required is f fid A or flAA if fl is approximately constant. Then 

A simple linear plot of the data allows AA to be obtained. The results of a set of experiments (Table 5) are surprising. AA is independent of the size or molecular weight of the protein. Although the cross-sections of the proteins studied range from 1000 to 10,000 A2, AA is nearly constant at 100 to 200 A2. Conclusion . . only a small portion of the protein molecule needs to enter the interface in order for adsorption to then proceed spontaneously (Ref.3), p. 290). It is as if only a small foothold or handhold is required to stabilize the molecule against desorption. Now firmly planted at the interface, the molecule can optimize its interfacial interactions by time-dependent orientation and perhaps conformational changes. The size of the foot is obviously relevant to the exchange discussion in Sect. 4.5. 

An alternative interpretation of the data is that AA is proportional to the number of water molecules per protein participating in the adsorption and denaturation process and is related to water activity at the interface. 

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