Temperature dependence of optical properties of aqueous dispersions of phosphatidylcholine

Aqueous dispersions of dipalmitoyl phosphatidyl choline exhibit a sharp decrease in turbidity at the crystal-liquid phase transition temperature of 41°C. The intensity of light scattered at 45°, 90°, and 135° also undergoes a sharp drop at the same temperature. Similarly, the refractive index of such dispersions decreases abruptly with the phase transition. Employing the relationship between light scattering intensity and specific refractive increment, it can be shown that about one half of the change in absorbancy and scattering are accounted for by the change in refractive index. The change in refractive index can be entirely accounted for by the known expansion and corresponding decrease in density of the bilayer. That part of the observed change in scattering and turbidity which is not accounted for by the observed change in average refractive index is apparently due to a decrease in the anisotropy of the bilayer during the melting process. Calculations based on a model which, although oversimplified, is consistent with the known thinning of the bilayer during the melting process, give quantitative agreement with experimental results. Below the phase transition temperature other changes in optical properties are observed; near 32°C, the light scattering envelope changes and the turbidity of dispersions drops markedly. The average refractive index remains constant in this region. For this and other reasons, it is postulated that these pre-transition changes indicate an aggregation-disaggregation phenomenon.