EFFECT OF PRESSURE ON THE COLLOIDAL SOLUTION BEHAVIOR OF AQUEOUS LOCAL-ANESTHETIC TETRACAINE SOLUTIONS

The solution behavior of two local anesthetics, tetracaine hydrochlori de (TC . HCl) and tetracaine hydrobromide (TC . HBr), in water was stu died. The temperature-pressure diagrams of aqueous TC . HCl and TC . H Br solutions of different concentrations were determined by observing the sudden change in transmittance accompanying the phase transition f rom the coagel phase to the micellar solution under high pressure up t o 170 MPa. The transition temperature was linearly elevated by applyin g pressure. It was seen that the temperature-pressure slopes of both a nesthetics increase with increasing concentration and become constant above 0.3 mol kg(-1). We obtained the concentration-temperature phase diagram of TC . HCl solution from the solubility vs. temperature curve s at different pressures, which were determined from the concentration dependence of the temperature-pressure diagrams, and the critical mic elle concentration (CMC) vs. temperature curves. The solubility curve intersects the CMC curve at a certain temperature under atmospheric pr essure, which is regarded as the Krafft temperature of TC . HCl. The v olume change associated with the phase transition from coagel to micel le was estimated from the Clapeyron-Clausius equation using the values of the temperature-pressure slope above 0.3 mol kg(-1) and the transi tion enthalpy taken from the DSC study. The values were determined to be 8.4 cm(3) mol(-1) for TC . HCl and 9.5 cm(3) mol(-1) for TC . HBr, respectively. Further, by considering the volume change of transition for TC . HCl among three states of anesthetic assembly, it was conclud ed that the solution behavior of these anesthetics in water bears a st riking resemblance, with respect to volume, to that of typical cationi c surfactants such as alkyltrimethylammonium salts.