Adsorption of ethylene oxide/propylene oxide copolymers on hydrophobicity gradient surfaces

Abstract

Adsorption of three commerciably available ethylene oxide(EO)/propylene oxide (PO) block copolymers with the general formula EO a PO b EO a , was studied with ellipsometry on a hydrophobicity gradient surface with contact angles ranging from 0 to 90°. Generally, higher adsorption values were obtained on the hydrophobic side than on the hydrophilic. The isotherm shapes were, strictly speaking non-Langmuir. However, within the range 0–10 ppm the isotherms could reasonably well be fitted to a Langmuir model. Generally, semi-plateau values of around 1.5–2.0 mg m −2 were measured on the hydrophobic side and around 0.0–0.5 mg m −2 on the hydrophilic, and a slowly progressing increase of the adsorption was observed at higher concentrations. The most hydrophilic surfactant, EO 148PO 56EO 148, gave slightly lower values on the hydrophobic side compared to the two others (EO 98PO 69EO 98 and EO 37PO 56EO 37). In contrast, comparatively higher affinity and adsorption capacity was found for EO 148PO 56EO 148 on the hydrophilic side of the gradient possibly indicating weak attraction between EO and surface SiOH groups. The adsorption capacity increased only slightly with temperature, but the adsorption affinity increased significantly between 20 and 38°C for EO 98PO 69EO 98, especially on the hydrophilic side. No spontaneous desorption was observed for any of the EO/PO copolymers. On the other hand, when exposed to an albumin solution, significant desorption and displacement occurred, in particular on the the hydrophobic end of the gradient as verified by ESCA analysis.