Sol–gel preparation of non-hygroscopic siliceous thin films enriched with alkaline-earth ions

Abstract Thin film layers of silica with calcium or magnesium ions were prepared by the sol–gel method in order to obtain protective coatings with low water affinity for historical glasses. The films were synthesised by pre-polymerised solutions of tetraethoxysilane [Si(OC 2 H 5 ) 4 ] (TEOS) and deposited by dipping soda-lime glasses. Then they were immersed in saturated solutions of Ca(OH) 2 or Mg(OH) 2 for time intervals of 1, 24 and 36 h to adsorb calcium or magnesium ions. No heat treatment was done, to apply this method to cover real historical glasses, which could be irreversibly damaged by high temperature. The X-ray photoelectron spectroscopy and secondary ions mass spectrometry analyses show that the films present an almost homogeneous in depth distribution of calcium or magnesium. Magnesium adsorbed films also present a decrease of free non-bridging oxygen (SiOH groups). By atomic force microscopy it was observed that the average roughness is low (around 10 A). Contact angle measurements reveal a degree increase starting from soda-lime uncoated glasses, through silica coated glasses up to calcium or magnesium adsorbed silica coated glasses. The data obtained revealed a real decrease of water affinity, strictly important to improve the protective performances of the films.