Enhanced surface coverage of anchoring quaternary ammonium salts (AQAS) on oxygen-plasma treated quartz substrates

Abstract

Oxygen plasma treatment was applied to atomically flat quartz surfaces in order to study the effect of surface activity on the adsorption of a silyl anchored quaternary ammonium salt biocide. Reduction of contact angle of the quartz surfaces were achieved from 72° to between 14° and 20° by applying oxygen plasma at increasing power between 100 W and 500 W. XPS analysis of the plasma-treated quartz surfaces indicated an increase in the presence of oxygen, implying the conversion of quartz surface siloxane groups into surface hydroxyl groups, consistent with the reduction in contact angle. Surfaces were examined by AFM after adsorption of silyl anchoring quaternary ammonium salts (AQAS). The surface profile of samples indicated an increase in the thickness of the adsorbed AQAS layer from 2.9 ± 0.1 nm for pristine quartz surfaces to 19.0 ± 2.0 nm for oxygen plasma treated surfaces, which is approximately the vertical dimension of 6 oriented molecular layers. The covalently bonded AQAS layer on the quartz surface exhibits a discontinuous film structure consisting of micro-zones of AQAS layers separated by voids or gaps, defined as regions on the quartz surface free of bound AQAS layers. These zones and voids are a consequence of the varying hydrophilicity/adsorption capability of the quartz surface. Plasma treated surfaces also showed a 3-fold reduction of void dimension from 3 µm to 1 µm between the AQAS adsorbed zones on the quartz slide owing to the increased hydrophilicity and adsorption capability of the surface. A leaching study of the AQAS adsorbed-quartz surfaces resulted in the reduction of film thickness from 19.0 ± 2.0 nm to 8 ± 1.0 nm. This reduction indicates the removal of some loosely bound outer layers, but confirms the retention of the surface-anchored layer of AQAS as well as a second and perhaps third layer adsorbed on the anchored first layer. • Oxygen plasma treatment increases surface hydrophilicity of quartz surfaces. • Reduced contact angle of quartz due to conversion of surface siloxane groups into surface silanol groups. • AFM shows increased adsorption of AQAS and multilayering (19 nm ~6 molecular layers) on plasma treated quartz surfaces. • Leaching studies indicate removal of loosely bound higher AQAS layers, but retention of surface anchored AQAS layers.