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Hydroxyapatite-poly-L-aspartic acid (HA-PASP) and hydroxyapatite-polyacrylic acid (HA-PAA) composite crystals have been prepared by direct synthesis in aqueous solution. The polyelectrolytes are quantitatively incorporated into the crystals up to about 8-9 wt%, as a function of their concentration in solution. The structural and morphological properties of the crystals vary as a function of polyelectrolyte content. TEM images show that the composite crystals display a greater length/width ratio with respect to the control HA crystals. The broadening of the X-ray diffraction reflections on increasing polyelectrolyte content was investigated using three different methods: (i) the Scherrer method, (ii) the Warren-Averbach approach, and (iii) a whole pattern analysis approach. Both polyelectrolytes induce a greater reduction of the mean crystallite size along a direction perpendicular to the c-axis direction, suggesting a preferential interaction of the polymers with the crystal faces parallel to the c-axis. PASP interaction with HA structure provokes a greater increase of strain in comparison to PAA. The data indicate that anionic polyelectrolytes can be usefully applied to modulate the structural and morphological properties of hydroxyapatite crystals.

Mechanical mixing of solid dicarboxylic acids of variable chain length HOOC(CH(2))(n)COOH (n = 1-7) with solid 1,4-diazabicyclo[2.2.2]octane generates the corresponding salts or co-crystals of the formula [N(CH(2)CH(2))(3)N]-H-[OOC(CH(2))(n)COOH] (n=1-7). Preparation of the same systems from solution has been instrumental for a full characterization of the mechanochemical products by means of single-crystal and powder-diffraction X-ray analyses, as well as by solid-state NMR. The acid-base adducts, whether involving proton transfer from the COOH group to the N-acceptor, that is having ((-))O...H-N((+)) interactions, or the formation of neutral O-H...N hydrogen bonds, show a melting point alternation phenomenon analogous to that shown by the neutral carboxylic acids. The carbon chemical shift tensors of the COOH group obtained from the sideband intensity of low speed spinning NMR spectra provide a reliable criterion for assigning the protonation state of the adducts.