About temperature dependency of surface tensions in mixtures of lecithin in 3-hydroxypropionitrile

Abstract

The surface tension temperature dependency of lecithin in 3-hydroxypropionitrile (3-HPN) was studied by means of the BP-2 bubble pressure tensiometer developed by KRUESS. The dynamic surface tension curves of dilute. lecithin solutions show a surprising temperature dependency within the range of 5-40 degreesC at 5 degreesC intervals and an "inversion point" was noticed. At higher concentrations of lecithin the observed inversion point disappears. The surface tensions recorded for long time periods are in agreement with selected data determined by means of the Wilhelmy plate method. The. dynamic surface tensions at 50 s were fitted by the general exponential function y =A exp (-x/ b) + c with x as the concentration. The obtained parameter b hereby equals the concentration intersection. point of the asymptotes ( x and x --> 0) and yields the value of the; critical aggregation concentration:. (cac). The strong linear temperature dependency of the determined cac values within the range of 0.22 mmol/kg (5 degreesC) to 0.716 mmol/ kg (40 degreesC) is remarkable. Diffusion coefficients of the dilute solutions were calculated by the Ward and Tordai equation. assuming a diffusion controlled adsorption mechanism. Similar to the determined surface tensions at 50 s, the diffusion coefficients of the dilute solutions show itself an unusual temperature dependency. This may support the assumption of a change in orientation of the lecithin lipid at the surface. Above the cac the temperature dependency of the diffusion coefficient follows the general Arrhenius law. The calculated molar fractions and absolute amounts of lecithin on the surface show an expected temperature dependency [1] in distinction of the surface excess amounts.