Hydroxide nanoparticles for deacidification and concomitant inhibition of metal gall ink corrosion of paper

Historically valuable paper objects are rapidly deteriorating and their long-term conservation requires a full understanding of the chemical degradation mechanisms to conceive appropriate methodologies for conservation. The main cellulose degradation pathways are the acid hydrolysis of glycosidic bonds and oxidation. Low pH values can lead to cellulose depolymerization even at room temperature. Ancient manuscripts were usually executed by using iron-gall-inks. These were obtained by reaction of iron (II) sulfate (i.e. vitriol, as reported in old recipes) with tannins extracted from gall-nuts, to give a pyrogallate complex of iron (III) and sulfuric acid. However, acid-catalyzed hydrolysis of cellulose is not the only paper degradation pathway due to the presence of iron-gall ink. In fact, transition metal ions usually catalyze cellulose oxidation through a free radical mechanism, which produces hydrogen peroxide in situ. This paper reports an investigation on the use of magnesium hydroxide nanoparticles dispersed in alcohols to inhibit two different and synergistic degradation processes usually affecting historically valuable manuscripts and common paper documents. We show that the preservation of paper from acid hydrolysis and oxidative ink corrosion can be achieved by stabilizing the final pH of deacidified paper around 6.5-7.5. PH control may allow to reduce the catalytic action of metals and minimize radical production. The inhibiting action of magnesium hydroxide nanoparticles are compared to magnesium oxide particles present in one of the best mass deacidification method (Bookkeeper).