AQUEOUS PHASE-DIAGRAM AND CRYSTAL-STRUCTURE OF SODIUM DIOCTYLPHOSPHINATE

The sodium di-n-octylphosphinate-water system was studied by several methods and a phase diagram drawn. A micellar solution was found to exist at very low concentrations. There are two lamellar mesophases, Lam 1 and Lam 2; the former is present at temperatures below almost-equal-to 330 K and the latter at high temperatures. Lam 2 liquid crystals become isotropic liquid at about 380 K. The anhydrous solid SDOP is monoclinic. The elementary cell parameters are a = 1.7221 +/- 0.0013 nm, b = 1.0584 +/- 0.0012 nm, c = 2.345 +/- 0.017 nm, and beta = 61.15 +/- 0.45-degrees, the number of surfactant molecules per elementary cell is 12, the crystallographic density is 1667 +/- 12 kg.m-3, and the chains are tilted 47.63 +/- 0.39-degrees with respect to the (001) plane. Three kinds of water are present: two water molecules are bound to a surfactant molecule so tightly that it is impossible to detect them by differential scanning calorimetry; about 9.6 water molecules are bound to the polar group and the Na+ counterion in such a way that their melting point is about 263 K, and the remaining water in the system behaves as pure water.