The Role of Electrophilic Metal Ions AluminumIII and MagnesiumII in Paper Degradation and Deacidification

While conservation scientists acknowledge the oxidative degradation of cellulose in paper to be complex, they overwhelmingly present hydrolysis as proceeding in just one way in paper degradation, via a proton-catalyzed mechanism. Researchers report in the scientific literature, however, an important additional mechanism of degradation where aluminumIII (AlIII), the metal cation in papermaker’s alum, is present. This competing mode of degradation has a differing pH dependence and temperature dependence, thus affecting deacidification measures and influencing accelerated aging tests, respectively. At the 38th Annual Meeting of the American Institute for Conservation (AIC) in Milwaukee (spring 2010), Heritage Science for Conservation presented an ongoing study targeting these competing reactions. Innovative approaches include systematic incorporation of macromolecular structure and phase interactions into aging studies, and sample formulation techniques that better isolate mechanisms and improve ionic strength and pH control. Results show unambiguous destabilization of samples by AlIII, controlling for critical variables including pH. Preliminary pH dependence results suggest that the effect may disappear between pH 5.0 and 6.9, supporting deacidification as an effective means of limiting both mechanisms of degradation. Also suggested is that magnesiumII, though chemically similar to AlIII, does not itself catalyze hydrolysis. MagnesiumII is in some deacidification agents, e.g. magnesium carbonate and oxide. heritage science for conservation Funded by the Andrew W. Mellon Foundation to build a bridge between conservators and conservation scientists, Heritage Science for Conservation is located at Johns Presented at a joint session of the Book and Paper Group and the Research and Technical Studies Group, AIC’s 38th Annual Meeting, May 11 –14, 2010, Milwaukee, Wisconsin. 114 The Book and Paper Group Annual 29 (2010) hydrolysis of cellulose in the presence of ironIII chloride, enabled by suspected ironIII coordination with the leaving group. Baty and Sinnott did find efficient electrophile-catalyzed hydrolysis under the conditions tested, but the pH range was limited to where the sample could remain a homogenous aqueous solution (3.0 –3.5), avoiding crystal entrainment of either products or reactants preferentially, which would skew results. Further limitations of the study were that while the cellulose model compound mimicked cellulose perfectly in terms of steric and inductive effects around the glycosidic linkage—the site of hydrolysis—it did not incorporate the macromolecular structure that is retained in paper, nor the particular way in which water is present adsorbed onto the paper fibers in heritage collections. Periodically, however, reviewers have suggested that the picture is more complex for papers containing AlIII salts such as papermaker’s alum. Parks concluded in 1971 that destabilization of cellulose by aluminum sulfate is qualitatively different from destabilization by sulfuric acid, which should not be the case if the AlIII is acting exclusively as a Brønsted acid. From a kinetics study, Barański, Dziembaj, and coworkers (2004) concluded that in addition to proton-catalyzed hydrolysis, some other mechanism must be simultaneously at work. Gurnagul, Howard, and coworkers noted in a 1993 review that “it is a matter for some astonishment that we still do not know for certain . the exact mechanism by which aluminium compounds affect permanence . ” A previous study (Baty and Sinnott 2004; 2005) tested the hypothesis that AlIII is directly catalyzing cellulose hydrolysis relevant to paper-based heritage materials degradation. Such catalysis could proceed from the coordination pattern illustrated in figure 2. Notice that the C6'-OH group is well positioned to behave as a second ligand to chelate, “bite,” the AlIII in place, though this coordination has not been proven. The 2004–2005 Baty and Sinnott study was prompted by the realization that physical organic chemists have known of an electrophile-catalyzed hydrolysis of glycosides with relatively stable leaving groups (the part of the molecule that breaks off) for some time (Clark and Hay 1970; Clark, Hay, and Dea 1973), although the same hypothesis could equally have been prompted by Popoola’s (1991) observation of enhanced Fig. 1. Proton-catalyzed hydrolysis of cellulose, a.k. a. “acid-catalyzed” hydrolysis of cellulose in paper, originating with protonation of the glycosidic oxygen atom Fig. 2. Proposed coordination of AlIII by the glycosidic oxygen and C6' hydroxyl, from which coordination pattern AlIII-catalyzed hydrolysis of cellulose in paper could proceed 115 The Role of Electrophilic Metal Ions AluminumIII and MagnesiumII in Paper Degradation and Deacidification Baty, Minter, and Lee dT (Dawson, Elliott et al. 1986). Figure 3 shows the approximate useful ranges of four buffering systems selected for this study: succinic acid pKa1/succinate, acetic acid/acetate, monobasic/dibasic phosphate, and dibasic/tribasic phosphate. Catalysts and Catalytic Controls Aluminum sulfate, a.k. a. papermaker’s alum (20 mM Al2(SO4)3) was the first salt studied in this project for its hypothesized role as a catalyst. In order to see the effects of the AlIII metal center, sulfate had to be present in the same amount in the control. Accordingly lanthanum sulfate was used, since the lanthanumIII cation (LaIII) is much larger and heavier than AlIII, thus not a candidate for electrophilic attack. The addition of this salt to the control series also controlled for the kinetic ionic strength effect, which is the phenomenon whereby reactions are observed to proceed faster with a higher overall concentration of salts (that do not necessarily participate in the degradation mechanism). While MgII opposite a number of anions is of continuing interest in this study given the many MgII compounds used over the years for aqueous and non-aqueous deacidification (Baty, Maitland et al. 2010), the first step was to study MgII opposite sulfate to draw the analogy from the aluminum/lanthanum series—isolating the effects of the cation. Still, this comparison cannot be a direct one because MgII is divalent and AlIII, LaIII are trivalent.