Contact interactions hydrophobic surfaces

Water does not spread spontaneously on hydrocarbons and the aqueous films on hydrocarbons are rather unstable. " The cause for these effects is a strong attractive hydrophobic force, which is found to appear in aqueous films in contact with hydrophobic surfaces. The experiments showed that the nature of the hydrophobic surface force is different from the van der Waals and double layer interactions. - It turns out that the hydrophobic interaction decays exponentially with the increase of the film thickness, h. The hydrophobic free energy per unit area of the film can be described by means of the equation " ... 

Association and dissociation of protein molecules on hydrophobic surfaces may also take place. Small proteins comprised of a single pol) eptide chain, such as lysozyme, associate to form dimers upon adsorption on hydrophobic surfaces, especially near the pi of 11. Proteins that have a quaternary structure, such as hemoglobin, can dissociate into subunits upon contacting a hydrophobic surface, as the subunits are held together by hydrophobic interactions. 

Wu Y, Rosen MJ (2002) Superspreading of trisiloxane surfactant mixtures on hydrophobic surfaces 2. Interaction and spreading of aqueous trisiloxane surfactant -n-alkyl pyrrolidinone mixtures in contact with polyethylene. Langmuir 18 2205-2215... 

What occurs when two hydrophobic molecules, susceptible to reaction, are put together in aqueous solution Due to the hydrophobic interaction, they have a tendency to aggregate, but this association is not sufficient to explain the rate enhancement. Noteworthy is the high surface tension of water (72 dynes/cm), a consequence of its high c.e.d. (550 cal/cm or 22000 atm), which tends to induce a diminution of the surface of contact between hydrophobic and water molecules. This process is facilitated by a decrease of the volume of the reactants along the reaction coordinate. 

For the polymer surface, on the other hand, it was assumed that only a fraction (73/73), is effective for the hydrophobic interaction. Peppas et al. also assumed that 24 cal/(A)2 is released when an exposed hydrophobic group of the protein comes into contact with a hydrophobic surface. Thus, they obtained, Eq. 2.13 for the evaluation of g23. 

When a protein adsorbes on a surface, the above interactions will be perturbed. For example, if the surface is hydrophobic the protein usually unfolds to bring its hydrophobic grous into contact with the surface. This often leads to a denaturing of the protein it will be changed irreversibly and no longer work properly. 

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