Acid-base behavior of carboxylic acid groups covalently attached at the surface of polyethylene: The usefulness of contact angle in following the ionization of surface functionality

Abstract

Oxidation of polyethylene with chromic acid/sulfuric acid generates a material (PE-C02H) having a high density of carboxylic acid and ketone functionalities in a thin surface layer on the polymer. This paper determines the extent of ionization of the surface and near-surface carboxylic acid groups of these materials in contact with water as a function of pH using three experimental techniques: measurement of attenuated total reflectance infrared (ATR-IR) spectra, measurement of contact angles, and direct potentiometric titration. On the basis of correlations between results obtained by these three techniques, we propose an equation relating the contact angle of an aqueous solution having a given value of pH and the extent of ionization (a;) of those carboxylic acid groups that are directly exposed to the solution. These carboxylic acid groups have broad titration curves and have C02H groups that are less acidic than soluble carboxylic acids. The initial ionization of these carboxylic acid groups occurs when the solution is approximately pH 6. The detailed structures of these oxidized polymer surface layers and the nature of the interactions between the carboxylic acid and carboxylate ions in them are still not completely defined. Salt effects on the extent of ionization a; at a particular value of pH are unexpectedly small and suggest that charge-charge interactions between carboxylate ions may not dominate the titration curves. This work demonstrates the usefulness of contact angle in following chemical changes occurring in organic functional groups on surfaces. Comparisons of wetting behavior of buffered and unbuffered solutions establishes the importance of using experimental protocols for measuring contact angles in which the reactive groups present in the interfacial region do not outnumber the reactive groups present in the small drop of dilute aqueous solution.

Influence of epiphytic micro‐organisms on leaf wettability: wetting of the upper leaf surface of Juglans regia and of model surfaces in relation to colonization by micro‐organisms

“…Contact angle titrations were used to obtain information about the chemical nature of functional groups occurring at interfaces (Holmes-Farley et al, 1985 ;Chau & Porter, 1991). Polar functional groups such as OH groups introduced to the interface of polyethylene render the surface more polar and, consequently, contact angles lower (Holmes-Farley et al, 1985). The introduction of ionizable groups like COOH or NH $ into the interface results in a further increase of wetting and a pH dependence of wetting, leading to lower contact angles if the functional groups occur in an ionized and consequently more polar form.…”

Activation of lymphocyte proliferation by boronate-containing polymer immobilised on substrate: The effect of boron content on lymphocyte proliferation

“…Contact angle titration results have also been correlated with the degree of ionization determined directly by ATR-IR spectroscopy (Holmes-Farley et al, 1985), fluorescence spectroscopy (Holmes-Farley and Whitesides, 1986), and conventional titration (Holmes-Farley et al, 1985). Holmes-Farley et al (1985) reported that the contact angle of water on polyethylene film derivatized with ionizable carboxylate groups changes systematically with the pH of the water, correlating nicely with the ionization degree of the carboxylate groups. However, this study was performed using dry films, and thus, the measured interface may not be in thermodynamic equilibrium with the water phase, resulting in hysteresis.…”

“…pH-dependent change in contact angle of the charged surface determined by contact angle titration provides data on the dissociation state and concentration of charged functional groups on the surface, and correlates with material properties, at the macroscopic level, such as wettability and adhesion strength (Holmes-Farley et al, 1985;Holmes-Farley and Whitesides, 1986). Contact angle titration results have also been correlated with the degree of ionization determined directly by ATR-IR spectroscopy (Holmes-Farley et al, 1985), fluorescence spectroscopy (Holmes-Farley and Whitesides, 1986), and conventional titration (Holmes-Farley et al, 1985).…”

Fundamental studies of the interactions of adsorbates on organic surfaces

“…To prepare model substrates, we have employed a synthetic method based on the molecular self-assembly of organosulfur compounds on a Au(111) single-crystal surface (9)(10)(11)(12). This chemisorption system, using either alkyl thiols or dialkyl disulfides as the adsorbates, leads to the formation of very densely packed organic monolayers (10)(11)(12).…”

“…There is also a growing appreciation that a surface can significantly alter the properties of a liquid relative to the bulk (7,8). The reactivity of organic functional groups present at these interfaces is susceptible, as well, to strong perturbation as compared to their character in solution (9).…”