Changes in Cloud Point of Nonionic Surgactant Surfactant Solution by Thiobarbiturates

Choe, Huhn

Abstract

The micelle solution of Triton X-100 has been used as one of the experimental analogues of biological membrane in the study of the mechanism of anesthesia. It is well known that the cloud point(Tc) of micelle solution is increased by the unionized species of local anesthetics which can penetrate and move into the micelle, and decreased by the ionized form of local anes- thetics which can not penetrate the membrane. Barbiturates(HA) ionize to H+ and A- . The degree of ionization of any drug depends on its own pKa and pH of the solution. The effect of thiobarbiturates on the Tc of micelle solution was studied to see if an unionized form is necessary for the drug to penetrate cell membrane as in local anesthetics. Thiobarbiturates decreased the Tc at low pH and increased at high pH. From the data obtained at pH around 4, the decrease in Tc(dT1) was regarded to be proportional to the concentration of [HA] in micelle, and the data obtained at pH around 10, the increase in Tc(dT2) was regarded to be proportional to the concen- tration of [A-] in aqueous phase. Assuming that the observed changes in Tc(dTc) at pH ranging from 4 to 10 were the summation of dT1 and dT, dTc was expressed as theoretical equation. The experimental data showed a perfect fit to the curve derived from the equation. This confirmed that thiobarbiturates, as local anesthetics, penetrate the micelle(membrane) in an unionized form.