title: pKa value and buffering capacity of acidic monomers commonly used in self-etching primers.
abstract: OBJECTIVE
  The aim of this investigation was to characterize acidic monomers used in self-etching primers/adhesives by determination of their pKa values and by calculation of their calcium dissolving capacity in comparison with phosphoric and hydrochloric acid.
  
  
  METHODS
  The following acidic monomers were included in this study: 4-methacryloyloxyethyl trimellitate anhydride (4-META), 10-methacryloyloxydecyl dihydrogen phosphate (MDP), dimethacryloyloxyethyl hydrogen phosphate (di-HEMA-phosphate), ethyl 2-[4-(dihydroxyphosphoryl)-2-oxabutyl]acrylate (EAEPA), 2-[4-(dihydroxyphosphoryl)-2-ox-abutyl]acrylic acid (HAEPA), and 2,4,6 trimethylphenyl 2-[4-(dihydroxyphosphoryl)-2-oxabutyl]acrylate (MAEPA). The pKa values were obtained by titration with 0.1 mol/l NaOH in aqueous solution. The inflection points of the resulting potentiometric titration curve were determined as pKa values. In the case of the sparingly water-soluble acidic monomers MAEPA and 4-META, the co-solvent method using different water/ethanol ratios for MAEPA or water/acetone ratios for 4-META was used. The dissolving capacity of each acidic monomer is defined as the amount of hydroxyapatite (HA) dissolved by 1 g of acid. For each monomer, the HA dissolving capacity was calculated bythe corresponding pKa value and the molecular weight. To confirm the calculated dissolving capacities, increasing amounts of HA powder (100 mg portions) were slowly added to 15 mmol/l aqueous solutions of the monomers to determine how much HA could be dissolved in the acidic solutions.
  
  
  RESULTS
  For all the investigated acidic monomers, pKal values between 1.7 to 2.5 were observed. The pKa2 values for the phosphate/phosphonate derivatives are between 7.0 and 7.3, and are comparable to phosphoric acid. For dicarboxylic acid derivatives, the pKa2 values are in the range of 4.2 to 4.5. Due to their comparable molecular weights and pKal values, the three tested acids di-HEMA phosphate, MDP and 4-META all possess comparable dissolving capacities for HA (ie, 0.26 to 0.28 g/g monomer). The same amount by weight of acidic monomer EAEPA dissolved more HA (ie, 0.35 g/g monomer). Due to their different molecular weights, phosphoric acid and hydrochloric acid dissolved three and ten times as much HA, respectively, as the first three acidic monomers. As a result, the ability of these acids to dissolve HA increases as their molecular weight decreases.
  
  
  CONCLUSIONS
  There are no significant differences among the investigated acidic monomers in terms of their pKa values and HA dissolving capacities. Monomer selection criteria for new self-etching adhesives must include properties such as calcium salt stability, wetting of the substrate, and copolymerization behavior.
authors.affiliation: lvoclar-Vivadent AG, Schaan, Liechtenstein. Ulrich.salz@ivoclarvivadent.com
authors.forename: Ulrich
authors.initials: U
authors.lastname: Salz
authors.forename: Angela
authors.initials: A
authors.lastname: Mücke
authors.forename: Jörg
authors.initials: J
authors.lastname: Zimmermann
authors.forename: Franklin R
authors.initials: FR
authors.lastname: Tay
authors.forename: David H
authors.initials: DH
authors.lastname: Pashley
mesh_terms: D000096:Acetone / Q000737:chemistry; D000143:Acids; D002021:Buffers; D002118:Calcium / Q000737:chemistry; D003764:Dental Materials / Q000737:chemistry*; D017886:Durapatite / Q000737:chemistry; D004563:Electrochemistry; D006801:Humans; D006851:Hydrochloric Acid / Q000737:chemistry; D006863:Hydrogen-Ion Concentration; D008422:Materials Testing; D008689:Methacrylates / Q000737:chemistry; D008970:Molecular Weight; D010756:Phosphoric Acids / Q000737:chemistry; D011199:Potentiometry; D019279:Resin Cements / Q000737:chemistry*; D012972:Sodium Hydroxide / Q000737:chemistry; D012995:Solubility; D017186:Titrimetry; D014867:Water / Q000737:chemistry
chemical_list: D000143:Acids; D002021:Buffers; D003764:Dental Materials; D008689:Methacrylates; D010756:Phosphoric Acids; D019279:Resin Cements; D014867:Water; D000096:Acetone; D012972:Sodium Hydroxide; C034139:4-methacryloxyethyltrimellitic acid anhydride; C069749:methacryloyloxydecyl dihydrogen phosphate; D017886:Durapatite; C030242:phosphoric acid; D006851:Hydrochloric Acid; D002118:Calcium