Surface Potential of Mixed Micelles Composed of Prostaglandin B2 and Heptaethylene Glycol Dodecyl Ether by the Fluorescence Method

Abstract

Abstract The surface state of negatively charged mixed micelles prepared by addition of prostaglandin B2 (PGB2) to nonionic heptaethylene glycol dodecyl ether (HED) has been investigated and the surface potential (Δψ) of the micelles has been estimated by measuring the fluorescence intensity of ammonium 8-anilino-1-naphthalenesulfonate (ANS) as a fluorescent probe. Since the binding constant of ANS, k, is dependent on the surface potential of the micelle, the value of Δψ was determined according to the equation Δψ=−59.16log(k0⁄k) at 25°C, where k0 is the k of HED micelle. The value of |−Δψ| increases with increasing mole fraction of PGB2 (XPGB2) and decreases with increasing ionic strength (J). These changes are due to the effects of the negative charge of PGB2 and of ionic atmosphere of Na+, respectively. Surface charge density (σ) was, therefore, determined from Δψ according to the Gouy-Chapman theory, with a result that σ is nearly independent of J and proportional to XPGB2. These results have led to a conclusion that the state of the diffuse layer surrounding the Stern layer on the micelle surface is represented well by the Gouy-Chapman theory as well as a previous result for the system of sodium dodecyl sulfate (SDS) and HED. The value of |−σ| for PGB2 micelle is less than that for SDS micelle, which suggests that PGB2 forms a loosely packed micelle.