Effect on composition of dry mechanical grinding of calcium phosphate mixtures.

For diverse reasons, calcium phosphates used to prepare hydraulic calcium phosphate cements can be ground mixed. The grinding with a rotating micromill of monocalcium phosphate monohydrate or anhydrous, dicalcium phosphate dihydrate or anhydrous with calcium oxide, calcium hydroxide, calcium carbonate, tetracalcium phosphate, or alpha- or beta-tricalcium phosphate was studied for different calcium to phosphate (Ca/P) ratios, rotating rates, masses of balls, and environmental conditions. During dry grinding by ball milling, anhydrous or hydrated acid calcium phosphates can mechanochemically react with anhydrous or hydrated basic calcium salts to form dicalcium phosphate dihydrate or anhydrous, noncrystalline calcium phosphate, and/or calcium deficient or stoichiometric hydroxyapatite, depending on the Ca/P ratio in the mixture and the time of grinding. The reaction rate is a function of the rotation rate and the mass of the balls. Water is not necessary to initiate the reaction but facilitates it because hydrated salts react faster than the corresponding anhydrous salts. Neither carbon dioxide nor carbonate ions seem to have any influence on the transformation kinetics. The transformations that occur during grinding influence the final mechanical properties of hydraulic calcium phosphate cements prepared from these materials. Thus, if a grinding step of the starting materials is planed, the grinding conditions will have to be particularly well defined to obtain reproducible results.

Physical properties and self-setting mechanism of calcium phosphate cements from calcium bis-dihydrogenophosphate monohydrate and calcium oxide.

An apatitic calcium phosphate cement was developed from calcium bis-dihydro-genophosphate monohydrate (or monocalcium phosphate monohydrate, MCPM) and calcium oxide (CaO). The powder had a Ca/P molar ratio of 1.67, and the liquid was either pure water or 0.25 M-1 M sodium phosphate buffer, pH 7.4. The influence of the powder-to-liquid (P/L) ratio on the setting time and the mechanical strength were studied. The best results were obtained for the 1 M phosphate buffer with a P/L ratio of 1.53; the setting time was 7 min and the compressive strength was 25 MPa after 24 h and 33 MPa after 11 d. The mechanism and kinetics of the setting reaction were investigated by X-ray diffraction, differential scanning calorimetry, 31P magic angle spinning-nuclear magnetic resonance and infrared spectrometry. The setting reaction was found to be biphasic: in the first step, during the mixing time, MCPM reacted with CaO immediately to give calcium hydrogenophosphate dihydrate (or dicalcium phosphate dihydrate, DCPD) which, in the second step, reacted more slowly with the remaining CaO to give hydroxyapatite. The conversion of the starting materials to hydroxyapatite was complete within 24 h when the liquid was water, but was slower and incomplete with the phosphate buffers. Of the starting materials, 30% remained after 3 d.

Chemical characterization of in vivo aged zinc polycarboxylate dental cements.

The chemical composition of zinc polycarboxylate dental cements aged in vivo was studied. Thirty samples aged from one to 17 years were investigated using X-ray diffraction, infrared spectroscopy, thermogravimetric analysis and differential scanning calorimetry. Evidence for the presence of zinc oxide, amorphous zinc polycarboxylate and water of hydration was found. No correlation with age concerning either the chemical structure of the components or their relative amounts was found. Zinc polycarboxylate dental cements show very good chemical stability on long-term use.

Chemical and structural changes in zinc polycarboxylate cements after immersion in dilute organic acid solutions.

The behaviour of zinc polycarboxylate cements in contact with dilute aqueous solutions of organic acids at concentrations close to those existing in buccal medium, was studied. The organic acids were acetic, citric, tartaric and lactic acids, at 0.01 M and 0.001 M. The elution of zinc and magnesium was 10-1000 times greater in acid than in pure water, and correlated with the concentrations and the dissociation constants, pK1, of the acids tested. In all cases, important water losses were observed. In the 0.01 M acids, the cement structure collapsed to form a viscous, compact and homogeneous layer on the cement surface. In this layer, the polymeric carboxylic chains were regenerated from the zinc and magnesium polycarboxylate cement. Comparison with pure water showed that even the smallest concentration of the weak acids greatly modified the cement behaviour. This could explain the well-known differences in erosion processes between theoretical erosion predicted by standard specification tests and the in vivo situation.

Infrared, raman, and 13C NMR spectra of two crystalline forms of (1R,3S)-3-(p-thioanisoyl)-1,2,2-trimethylcyclopentanecarboxylic acid.

A comparative structural analysis of the two polymorphic forms of (1R,3S)-3-(p-thioanisoyl)-1,2,2-trimethylcyclopentanecarboxylic acid has been performed with infrared, Raman, and 13C NMR spectroscopy. The results are compared with those of the crystallographic and thermal studies on the two forms published in previous papers. The enantiotropism of the polymorphs as well as the differences in the conformation of the carboxyl group and the resulting intermolecular hydrogen bonds were confirmed by the infrared and Raman studies.

Comparative study of the two polymorphic forms of p-(1R,3S) 3-thioanisoyl-1,2,2-trimethylcyclopentane carboxylic acid.

The crystal structure of polymorph I of p-(1R,3S)3-thioanisoyl-1,2,2-trimethylcyclopentane carboxylic acid has been determined and is compared with that of polymorph II that was previously described. Polymorph I is very different at the level of the carboxyl group. It does not present disorder and the values found for the C-O bonds correspond exactly to single and double bond lengths. In addition, the carboxyl groups of the two molecules in the cell packing are involved in symmetric hydrogen bonds [2.662(6) A] leading to the formation of a dimer around the twofold axis following x with a shift on z. The different conformations on the carboxylic group between the two polymorphs are in good agreement with the thermodynamic study.

[Spectral and thermal characterization of cephalosporins. I. Cefadroxil and cefalexin]. / Caractérisation spectrale et thermique de céphalosporines. I--Céfadroxil et céfalexine.

Cefadroxil and cefalexine were characterized by thermal and spectral analysis. A vibrational study by infrared and Raman spectroscopies was made to connect the structural data with the antibacterial activity.

Physicochemical and structural study of sulfamethazine.

We have shown by several physicochemical methods that the existence of different crystal habits for a given molecule does not necessarily imply polymorphism. For sulfamethazine, we found one polymorph, form 1, whose crystalline structure is published herein, and a methanol solvate.