Pka Value and Buffering Capacity of Acidic Monomers Commonly Used in Self-etching Primers

Objective: Modern adhesive formulations attempt to simplify bonding steps by using selfetching primers. These are optimized based on acidic monomers like MDP, MA154 or 4-META in a high content which are responsible for hybrid layer and etch pattern formation. Therefore their acidity has to be sufficient to dissolve hydroxyapatite. A measurable dimension for the dissolving capacity is the pka-value. In this study the pka-values and dissolution capacity of acidic monomers commonly used in selfetching primers were determined. The pka-value depends on the kind and structure of the acidic monomers. The capacity to dissolve hydroxyapatite can be calculated from the pka-value and the chemical structure of the monomers and was compared to the acidity of phosphoric acid. Methods: Solutions of MDP, MA154, di-HEMA-phosphate and 4-META (0.08mol/l) were titrated with 0.1mol/l NaOH. The pH-values were determined in dependence of the NaOH consumption. The inflection point of the resulting titration curve was determined as pka-value. The dissolution capacity was calculated in consideration of the pka-value and the molecular weights of the acidic monomers. Results: MDP (pka1: 2.2; pka2: 7.0), MA154 (pka1: 1.9; pka2: 7.3), and di-HEMA-phosphate (pka: 1.7) possess an acidity comparable to phosphoric acid (pka1: 2.0; pka2: 7.2).The pka-values of 4-META are pka1: 2.8; pka2: 4.8. Compared to phosphoric acid the acidic monomers possess much higher molecular weights. Due to this, the dissolving capacity (g hydroxyapatite/g monomer) of the particular monomers, rise from di-HEMA-phosphate (1.6g/g), MDP (1.6g/g), MA154 (2.1g/g), 4-META (2.5g/g), to phosphoric acid (5.1g/g). Conclusion: The pka-value and molecular weight of acidic monomers are helpful parameters for the prediction of the efficiency to dissolve hydroxyapatite.