Changes in wetting properties of alumina surface treated with DPPC in the presence of phospholipase A2 enzyme

Wetting properties of commercial Al2O3 plates contacted with dipalmitoylphosphatidylcholine (DPPC) or DPPC + enzyme (phospholipase PLA(2)) in NaCl solution were determined by thin layer wicking and with the help of Washburn equation. Van Oss et al.'s approach to interfacial free energy interactions was applied to determining the solid surface free energy components. Wicking experiments were performed both for bare and alumina plates precontacted overnight with the probe liquid saturated vapours, as well as the untreated and DPPC (or DPPC + PLA2) treated alumina plates. For this purpose the penetration rates of n-octane, water and formamide were measured. From these experiments it resulted that original alumina surface is strongly polar with electron-donor interactions originating from the surface hydroxyl groups. Adsorption of DPPC on Al2O3 plates slightly increased the hydrophobic character of the alumina surface (considerable decrease of the electron-donor, gamma(-)(s) parameter and gamma(AB)(s) component was visible) in such a way that the hydrocarbon chains were directed outwards and the polar part towards the alumina surface. However, after the enzyme action the products of DPPC hydrolysis by PLA(2) (palmitic acid and lysophosphatidylcholine) increased again the hydrophilic character of Al2O3 surface (a minor increase in gamma(AB)(s) component and drastic increase of the electron-donor gamma(-)(s) parameter was noticeable). After treatment with DPPC or DPPC+ enzyme PLA(2) solution the changes of the total surface free energy of alumina and its Lifshits-van der Waals (gamma(LW)(s)) component were in the range 7-10 mJ/m(2), but the most considerable and delivering more interesting information were the changes of the electron-donor (gamma(-)(s)) parameter ranging from 27 to 35 mJ/m(2). Moreover, the changes of the alumina surface wettability were dependent on the time of the enzyme contacting with DPPC in NaCl solution. On the basis of the obtained results it seems that the thin layer wicking method can be an additional useful tool in investigations of the effect of phospholipid and PLA(2) action on the hydrophilic-hydrophobic character of alumina surface. (C) 2011 Elsevier B.V. All rights reserved.