Adsorption of nicotinic and isonicotinic acid derivatives by hydroxyapatite from aqueous solutions.

Abstract

Adsorption of nicotinic and isonicotinic acid derivatives from aqueous solution by hydroxyapatite (HAP), the main component of tooth enamel, was investigated, and the effects of various factors on the adsorption were studied. These nicotinic and isonicotinic acid derivatives are similar in chemical structure to nicotine which is contained in tobacco, and which might be adsorbed by teeth during smoking. The amounts adsorbed at pH 6.0 and 7.0 at 37°C were determined by measurement of concentration differences (before and after adsorption). Six compounds, that is, nicotinic acid, isonicotinic acid, nicotinamide, isonicotinamide, nicotinic acid hydrazide, and isonicotinic acid hydrazide were used as drugs. All the drugs used were determined by ultraviolet absorption spectrophotometry. Adsorption isotherms were essentially of almost Langmuir type for every drug. The order of the amount adsorbed was 10-6 mol/g, and this value was smaller than that of chlorhexidine adsorption by HAP reported previously. It is suggested that the size of adsorbate molecules affects the adsorbability. The amounts adsorbed of isonicotinic acid derivatives were larger than those of the corresponding nicotinic acid derivatives. The order of the amounts adsorbed for both nicotinic and isonicotinic acid derivatives was as follows : acid>amide>hydrazide. This order was opposite to that of the solubility. The adsorption behavior of HAP was different from that of carbon black, a hydrophobic adsorbent. The amount adsorbed decreased upon addition of sodium chloride and sodium fluoride.