Effects of Solvents and pH Values on the Chemical Affinity of 10-Methacryloyloxydecyl Dihydrogen Phosphate toward Hydroxyapatite

Abstract

This study aimed to investigate the effects of solvents and pH values on the chemical interaction between 10-methacryloyloxydecyl dihydrogen phosphate (MDP) and hydroxyapatite (HAp). The chemical affinity of MDP toward HAp dissolved in different solvents (E-MDP: 10 wt % MDP and 90 wt % ethanol; E-W-MDP₁: 10 wt % MDP, 75 wt % ethanol, and 15 wt % water; A-W-MDP: 10 wt % MDP, 75 wt % acetone, and 15 wt % water; and E-W-MDP₂: 10 wt % MDP, 45 wt % ethanol, and 45 wt % water) was investigated. The pH of E-W-MDP₂ was increased from 2.04 to 5 (E-W-MDP_2/5) and to 7 (E-W-MDP_2/7). The reaction products were characterized by Fourier transform infrared spectroscopy, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis (TGA), and nuclear magnetic resonance (NMR). XRD and NMR results revealed that no MDP-calcium salt formed in E-MDP. XRD, TGA, and XPS results indicated that MDP interacted with HAp, producing the MDP-calcium salt in all groups except E-MDP. NMR results revealed that the dicalcium salt of the MDP dimer (DCS-MD) and the MDP tripolymer (DCS-MT) and the monocalcium salt of the MDP monomer and the MDP dimer were formed in E-W-MDP₁. DCS-MD and DCS-MT were also formed in E-W-MDP₂ and A-W-MDP. In E-W-MDP_2/5 and E-W-MDP_2/7, DCS-MD was obtained. Both the solvents and pH values affect the chemical interactions between MDP and HAp and the types of reaction products formed. MDP and HAp do not form any MDP-calcium salt in pure ethanol; the structural stability of MDP-calcium salts is dependent on the solvent water content and the pH value. The ethanol/water mixture is recommended as the main solvent in an MDP-containing primer, and the ideal pH value is 2-7; if these conditions are satisfied, sufficient amounts of MDP-calcium salts with stable structures are expected to be formed, thus improving the longevity of dentin/enamel bonding.

Figure 1

FTIR absorbance spectra of MDP-primer-treated HAp and untreated HAp. Solvents were (A) 90 wt % ethanol, (B) 75 wt % ethanol with 15 wt % water, (C) 75 wt % acetone with 15 wt % water, (D) 45 wt % ethanol with 45 wt % water (pH = 2.04), (E) 45 wt % ethanol with 45 wt % water (pH value was increased to 5 using NaHCO₃), and (F) 45 wt % ethanol with 45 wt % water (pH value was increased to 7 using NaHCO₃).

Figure 2

Infrared spectra and the peak-fitting results of specific wavenumber domains. Solvents were (A) 90 wt % ethanol, (B) 75 wt % ethanol with 15 wt % water, (C) 75 wt % acetone with 15 wt % water, (D) 45 wt % ethanol with 45 wt % water (pH = 2.04), (E) 45 wt % ethanol with 45 wt % water (pH value was increased to 5 using NaHCO₃), and (F) 45 wt % ethanol with 45 wt % water (pH value was increased to 7 using NaHCO₃).

Figure 3

TGA results showing weight loss of HAp samples, including the untreated HAp. Solvents were (A) 90 wt % ethanol, (B) 75 wt % ethanol with 15 wt % water, (C) 75 wt % acetone with 15 wt % water, (D) 45 wt % ethanol with 45 wt % water (pH = 2.04), (E) 45 wt % ethanol with 45 wt % water (pH value was increased to 5 using NaHCO₃), and (F) 45 wt % ethanol with 45 wt % water (pH value was increased to 7 using NaHCO₃).

Figure 4

TG-DTG results of HAp samples, including the untreated HAp. Solvents were (A) 90 wt % ethanol, (B) 75 wt % ethanol with 15 wt % water, (C) 75 wt % acetone with 15 wt % water, (D) 45 wt % ethanol with 45 wt % water (pH = 2.04), (E) 45 wt % ethanol with 45 wt % water (pH value was increased to 5 using NaHCO₃), and (F) 45 wt % ethanol with 45 wt % water (pH value was increased to 7 using NaHCO₃).

Figure 5

XRD patterns of MDP-primer-treated HAp and untreated HAp. Solvents were (A) 90 wt % ethanol, (B) 75 wt % ethanol with 15 wt % water, (C) 75 wt % acetone with 15 wt % water, and (D) 45 wt % ethanol with 45 wt % water (pH = 2.04).

Figure 6

XRD patterns of MDP-primer-treated HAp and untreated HAp. Solvents were (D) 45 wt % ethanol with 45 wt % water (pH = 2.04), (E) 45 wt % ethanol with 45 wt % water (pH value was increased to 5 using NaHCO₃), and (F) 45 wt % ethanol with 45 wt % water (pH value was increased to 7 using NaHCO₃).

Figure 7

(A) Wide-scan XPS spectra of HAp powder samples treated with four experimental primers and of untreated HAp. (B) Untreated HAp powder exhibited a peak at 288.5 eV corresponding to −COO binding. (C) HAp treated with 10 wt % MDP and 90 wt % ethanol exhibited a peak at 288.9 eV assigned to −COO binding. (D) HAp treated with 10 wt % MDP, 75 wt % ethanol, and 15 wt % water showed a peak at 288.5 eV corresponding to −COO binding. (E) HAp treated with 10 wt % MDP, 75 wt % acetone, and 15 wt % water exhibited a peak at 288.6 eV representing −COO binding. (F) HAp treated with 10 wt % MDP, 45 wt % ethanol, and 45 wt % water showed a peak at 288.5 eV assigned to the −COO binding.

Figure 8

Typical ³¹P NMR spectra of HAp powder samples. Solvents were (A) 90 wt % ethanol, (B) 75 wt % ethanol with 15 wt % water, (C) 75 wt % acetone with 15 wt % water, (D) 45 wt % ethanol with 45 wt % water (pH = 2.04), (E) 45 wt % ethanol with 45 wt % water (pH value was increased to 5 using NaHCO₃), and (F) 45 wt % ethanol with 45 wt % water (pH value was increased to 7 using NaHCO₃). The arrows denote the NMR peaks assigned to the phosphorus atoms of the MDP-calcium salts. Peak α was assigned to HAp. The numbered peaks were assigned to the phosphorus atoms in the corresponding salts in Table 2.

Figure 9

Curve-fitting results corresponding to the observed ³¹P NMR spectra (black lines) for HAp powder samples. Solvents were (A) 90 wt % ethanol, (B) 75 wt % ethanol with 15 wt % water, (C) 75 wt % acetone with 15 wt % water, (D) 45 wt % ethanol with 45 wt % water (pH = 2.04), (E) 45 wt % ethanol with 45 wt % water (pH value was increased to 5 using NaHCO₃), and (F) 45 wt % ethanol with 45 wt % water (pH value was increased to 7 using NaHCO₃). The green lines show the simulated α peak for HAp. The sky blue lines are the simulated peaks 3, 4, 5, and 6 for the four MDP-calcium salts. The red line is the resulting overall synthetic spectrum.

Figure 10

Reaction pathways of MDP dissociation in vacuum, water, ethanol, and acetone.