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Effect of various surfactants and aluminum solutions on in vitro acid dissolution of dental enamel.

Various surfactants were investigated for compatibility and stability in aqueous dispersions with aluminum (Al) and for their ability to inhibit enamel acid dissolution both with and without Al present. A total of 39 samples, representing anionic, cationic and nonionic surfactant classes, were dispersed at a concentration of 1% in water or a 0.005 mol/l AlK(SO4)2 solution. After 24 h the dispersions were evaluated for visual appearance, pH and ability to reduce enamel acid dissolution (EDR). Because reactions involving Al are sometimes very slow to equilibrate, the surfactants that were compatible with Al and did not suppress the 80% EDR of the Al solution were tested again after storage for 12 months. Most surfactant samples formed limpid aqueous dispersions that had little or no EDR activity. However, there were substantial differences in the effect of surfactants on Al EDR activity. Under the conditions of this study, nonionic samples from every subclass were compatible with Al. Anionics usually resulted in the formation of insoluble products that inactivated Al, although a polyethoxy carboxylate and a sulfated ester were exceptions which formed clear, stable dispersions that yielded high EDR values. The cationics varied in their response to Al; the amines reacted with and deactivated Al, but quaternary ammonium compounds were more compatible. No surfactants from any class enhanced the capacity of Al to reduce enamel acid dissolution. The long-term stability data revealed that 21 of 22 Al-surfactant combinations tested retained a significant EDR activity. However, some dispersions exhibited visual signs of instability, and the pH of many nonionic samples decreased. This study showed that, in the presence of Al, nonionic surfactants were relatively stable for long periods and did not block the ability of Al to inhibit enamel acid dissolution. However, certain anionic and cationic surfactants also displayed comparable properties.