The adsorption of water and amino acids onto hydrophilic and hydrophobic quartz surfaces

Abstract

The adsorption of (a) serine and leucine and (b) water onto hydrophilic and hydrophobic surfaces was investigated using the radio tracer technique. (a) Films only a fraction of a monolayer (serine 0.45 and leucine 0.30 monolayers as maximum) in thickness were formed. The free energy of adsorption of leucine was found to be 1.147 times greater than that of serine on a hydrophobic surface, and about 1.38 times greater on a hydrophilic surface, i.e. leucine showed a higher affinity of binding onto both surfaces than serine did. Independent of whether serine or leucine is used, the surface concentration is higher on hydrophilic than on hydrophobic surfaces. Furthermore, both amino acids showed a decrease in the adsorption rate at high bulk concentration except for leucine (on a hydrophobic surface) where the surface density increased linearly with bulk concentration. This linear relation was probably an effect of the medium that screened the electrostatic and hydrophobic side-chain interactions. (b) An accelerating rate of adsorption was observed at surface coverage >0.04 of a monolayer indicating strong lateral interactions among adsorbed water molecules. Both isotherms show a high sticking probability coefficient and a saturation level at high bulk concentration. The surface concentration of water (≈ 5–6 monolayers) on a hydrophilic surface was found to be approximately 1.7 times greater than that on a hydrophobic surface.