treating said coated substrate to harden said solution into a porous silicon oxide film, wherein said {HEAD ~ nonionic surfactant} is first purified to remove alkali metal impurities; and wherein said ionic additive is a tetramethylammonium salt chosen from a group of compounds consisting of tetramethylammonium formate, tetramethylammonium nitrate, tetramethylammonium oxalate, tetramethylammonium acetate, tetramethylammonium hydroxide and other salt exhibiting complete decomposition to volatile species upon heating to a temperature between 300 and 450{TAIL ~ ° C} 