Adsorption on Hydrophobic Surfaces

Lagaly, G. and I. Dekany. 2005. Adsorption on hydrophobized surfaces Clusters and selforganization. Arfv. Coll. Interface Sci. 114-115 189-204. 

Petroleum reservoirs can exhibit the full range of wettabilities from water-wet to oil-wet (53). Adsorption of crude oil heavy ends modifies solid surface properties and is thought to change reservoir wettability toward more oil-wet. Surfactant adsorption on hydrophobic surfaces takes place by hydrophobic interactions between surfactant hydrocarbon chains and the solid surface (35, 54—58). At low surfactant concentrations, surfactant molecules are oriented parallel to the surface. As the surfactant concentration increases, hydrophobic interactions between surfactant hydrophobes become significant. The surfactant molecules become oriented vertically to the surface with the polar groups toward the aqueous phase. 

Surfactant—solid and surfactant—surfactant hydrophobic interactions lead to minimization of solid—water and surfactant-chain—water contact and are energetically favorable. Unlike hydrophilic surfaces, hydrophobic surfaces do not lead to significant structuring of interfacial water, and the interfacial water is displaced from the surface relatively easily by the surfactant molecules. Consequently, surfactant adsorption on hydrophobic surfaces has often been found to be higher than adsorption on the corresponding hydrophilic surfaces (39, 54, 56, 57, 59—62), provided aqueous phase salinity is low. 

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. 

One of the most commonly used dispersants for aqueous media are nonionic surfactants. The most common nonionic surfactants are the alcohol ethoxylates R-0-(CH2 CH2 0)jj H, e.g. Ci3/i5(EO)jj with n being 7, 9,11 or 20. These nonionic surfactants are not the most effective dispersants since the adsorption by the C13/15 chain is not very strong. To enhance the adsorption on hydrophobic surfaces a polypropylene oxide (PPO) chain is introduced in the molecule giving R 0 (PPO) -(PEO) H. These nonionic surfactants can also be used for stabilization of polar solids in nonaqueous... 

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