Foam Bilayers from Amniotic Fluid:  Formation and Phase State

Abstract

The temperature effect on the threshold dilution (e.g., threshold concentration) for formation of foam bilayers (Newton black films) from amniotic fluid is investigated. Samples of amniotic fluid are collected during the 39th gestation week. Microscopic foam films are formed from diluted amniotic fluid and studied interferometrically by the method of Scheludko and Exerowa within the temperature range 10-30°C. The threshold dilution is determined from the experimental dependence of the probability for observation of foam bilayer on the dilution for each sample at different temperatures. It is shown that the threshold dilution is inversely proportional to the threshold concentration for formation of foam bilayer. This parameter is crucial for assessment of the risk for respiratory distress syndrome in newborns. The binding energy Q of a lipid molecule in the foam bilayer is calculated from the Arrhenius dependence of the threshold dilution applying the hole-nucleation theory of bilayer stability. The so-obtained values of Q for the various samples are very close to each other and vary from 5.3 × 10 -20 to 9.4 × 10 -20 J (about 13-22 kT), which indicates that in all cases the foam bilayers are in the liquid-crystalline state. Comparison is made with the phase states of dipalmitoylphosphatidylcholine foam bilayers. IntroductionRecently a new approach was introduced for clarification of amphiphile bilayer formation and stability. A holenucleation theory which explains the amphiphile bilayer stability and permeability on the basis of short-range molecular interactions was developed. 1-3 This theory was confirmed experimentally for foam 4-6 and emulsion bilayers. 7 The parameters (for example the critical concentration C c for formation of foam bilayer) and relations set up in the theory are convenient for investigation of the formation and phase state of foam bilayers from various synthetic and natural surfactants. They present a method of obtaining some molecular characteristics of foam bilayers such as the binding energy of a phospholipid molecule in the bilayer and the specific edge free energy of the nucleus hole for different phase states of the bilayers.This new approach was applied successfully for studying foam bilayers from amniotic fluid (AF), as well as from some of its components 8 in relation to the physicochemical clinical method for diagnostics of lung maturity of newborns, proposed by us. 9 This method includes the detection of the threshold bulk concentration of phosphatidylcholines necessary for foam bilayer formation. This threshold concentration is determined from the experimental dependence of the probability for observation of foam bilayer from AF on the concentration. 9 Numerous methods for prediction of lung maturity by examination of amniotic fluid 10-16 and lung surfactant 17,18 have been developed. The air/solution interface is essential in the methods related to monolayer, 17 bubble, 18 and foam. 11 Each of these methods exploits a certain surface parameter for determination of the prop...