Monolayers of synthetic lecithins as well as phosphatidic acid at different stages of ionization were studied with the film balance technique at pressures above the lateral vapour pressure. Pressure (π) versus area (a) curves (isotherms) and, by application of a special technique, area (a) versus temperature (T) curves (isobars) were recorded. From these data, plots of the lateral compressibility, χ, as a function of lateral lipid density (a-1) and of the thermal expansion coefficient, α, as a function of temperature (T) were obtained. The well known transition (at T = TM) between the expanded (fluid) and the condensed (crystalline) states of the films, called the main transition, is characterized by a non-horizontal deflection of the isotherms (and isobars) in the coexistence region. A pronounced hysteresis shows that it is a first order transition. Two additional phase transitions, one at T > TM (fluid region) and one at T < TM (crystalline region), were revealed by breaks in the slopes of the isotherms and the isobars. Discontinuities in the compressibilities and in the expansion coefficients show that these transitions are of second order. A flow experiment showed : that the fluid → fluid transition is also characterized by a pronounced discontinuity in viscosity. In fatty acid monolayers the transition (at T < TM) between two crystalline states is of weak first order ( — 10 J./Mole). The signs of the discontinuities, in χ and a, were analysed using the Landau theory of phase transitions. This provided information on the symmetry of the polymorphic states of the monolayers. The symmetry is described 1) in terms of the de Gennes stretching vector J, characterizing the orientation within both the hydrophobic and the hydrophilic region of the monolayer and 2) in terms of the density wave, p, characterizing the symmetry of the lateral organization of the lipid molecules. For α-Dipalmitoyllecithin a phase diagram is established. For π > 15 dyn./cm the polymorphic states of phospholipid monolayers observed at decreasing areas are : fluid isotropic (I) (hydrocarbon chains normal to water surface) ; fluid anisotropic (II) (tilted chains) ; tilted crystalline (III) ; non tilted crystalline (IV). At π ≦ 10 dyn./cm phase II and IV are not observable. Heats of transition ( QM) of the main transition were obtained from the Clausius Clapeyron equation. QM is constant (37 kJ./Mole) at low temperatures while it converges towards zero at higher temperatures, according to QM = Q0M(T - Tc), thus defining a critical temperature. It is proposed that this is a tricritical point above which the main transition becomes of second order. This allows for a symmetry break at the fluid-to-crystalline (main) transition above this critical temperature. The behaviour is explained by the Landau theory in terms of the strength in coupling between the lateral order (density wave p) and the chain orientational order (stretching vector J). In analogy to the Rodbell-Bean effect in magnetism a decrease in coupling leads from a first to a second or...

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Atomic force microscopy (AFM) has been used to characterize the formation of a phospholipid bilayer composed of 1,2-dimyristyl-sn-glycero-3-phosphocholine (DMPC) at a Au(111) electrode surface. The bilayer was formed by one of two methods: fusion of lamellar vesicles or by the combination of Langmuir-Blodgett (LB) and Langmuir-Schaefer (LS) deposition. Results indicate that phospholipid vesicles rapidly adsorb and fuse to form a film at the electrode surface. The resulting film undergoes a very slow structural transformation until a characteristic corrugated phase is formed. Force-distance curve measurements reveal that the thickness of the corrugated phase is consistent with the thickness of a bilayer lipid membrane. The formation of the corrugated phase may be explained by considering the elastic properties of the film and taking into account spontaneous curvature induced by the asymmetric environment of the bilayer, in which one side faces the gold substrate and the other side faces the solution. The effect of temperature and electrode potential on the stability of the corrugated phase has also been described.

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