No abstract

Self-assembled monolayers (SAMs) of omega-functionalized long-chain alkanethiolates on gold films are excellent model systems with which to study the interactions of proteins with organic surfaces. Monolayers containing mixtures of hydrophobic (methyl-terminated) and hydrophilic [hydroxyl-, maltose-, and hexa(ethylene glycol)-terminated] alkanethiols can be tailored to select specific degrees of adsorption: the amount of protein adsorbed varies monotonically with the composition of the monolayer. The hexa(ethylene glycol)-terminated SAMs are the most effective in resisting protein adsorption. The ability to create interfaces with similar structures and well-defined compositions should make it possible to test hypotheses concerning protein adsorption.

This paper describes the preparation of oligo(ethylene glycol)-terminated alkanethiols having structure HS-(CH2)n(0CH2CH2)m0H (m = 3-7) and their use in the formation of self-assembled monolayers (SAMs) on gold. A combination of experimental evidence derived from X-ray photoelectron spectroscopy (XPS), measurement of contact angles, and ellipsometry implies substantial disorder in the oligo(ethylene glycol)-containing segment. The order in the -(CH2)ngroup is not defined by the available evidence. The SAMs are moderately hydrophilic: 0a(H2O) = 34-38°; 0r(H2O) = 22-25°. A study of monolayers containing mixtures of HS(CH2)nCH3 and HS(CH2)u(OCH2CH2)6OH suggests that the oligo(ethylene glycol) moieties are effective at preventing underlying methylene groups from influencing wetting by water. A limited study demonstrates that these oligo(ethylene glycol)-containing SAMs resist the adsorption of protein from solution and suggests that SAMs will be a useful model system for studying the adsorption of proteins onto organic surfaces.

The study of the ionization of carboxylic acid groups at the interface between organic solids and water demonstrates broad similarities to the ionizations of these groups in homogeneous aqueous solution, but with important systematic differences. Creation of a charged group from a neutral one by protonation or deprotonation (whether -NH3+ from -NH2 or -CO2-from -CO2H) at the interface between surface-functionalized polyethylene and water is more difficult than that in homogeneous aqueous solution. This difference is probably related to the low effective dielectric constant of the interface (ε≃9) relative to water (ε≃80). It is not known to what extent this difference in ε (and in other properties of the interphase, considered as a thin solvent phase) is reflected in the stability of the organic ions relative to their neutral forms in the interphase and in solution, and to what extent in differences in the concentration of H+ and OH-in the interphase and in solution. Self-assembled monolayers (SAMs)-especially of terminally functionalized alkanethiols (HS(CH2)nX) adsorbed on gold-provide model systems with relatively well-ordered structures that are useful in establishing the fundamentals of ionization of protic acids and bases at the interface between organic solids and water. These systems, coupled with new analytical methods such as photoacoustic calorimetry (PAC) and contact angle titration, may make it possible to disentangle some of the complex puzzles presented by proton-transfer reactions in the environment of the organic solid-water interphase.

Model polyelectrolyte networks having controllable and independently known structural characteristics were prepared from different molecular weights of a, -amino-terminated poly(acrylic acid), by cross-linking them with a trifunctional isocyanate coupling agent. The density of cross-links, average molecular weight between cross-links, and the functionality of the cross-link points of these networks have been characterized. The swelling behavior of these novel networks was studied as a function of bath pH and ionic strength and the results were interpreted in term of structure-property relationships of the polymeric compounds. The maximum hydration was achieved at values of pH greater than 7. Hydration was linearly dependent upon the average molecular weight between cross-links, Mc. The hydration relative to this maximum value, flrei = H/fímax, was a function of pH but not of Mc. The relative hydration, HK\, and fraction of carboxyl groups ionized, a, were similar functions of the pH of the equilibrating solution. In previous studies, on randomly cross-linked polyelectrolyte networks, Mc could not be directly characterized by experimental measurements and has been inferred using theoretical models. In this study, we have directly related the network structural parameters, which were imposed by the synthetic route used and confirmed experimentally, to the macroscopic swelling behavior observed without invoking any theoretical models.