No abstract

We propose a theoretical study of phase transitions in long hydrocarbon chain monolayers. lying on aqueous substrate. We use a model taken from that applied by DiMarzio to liquid crystals: each molecule can occupy several aligned sites on the holder (I to 5 for us). We also take into account the attractive interactions between the hydrocarbon chains in the framework of a mean field approximation: it gives a first order phase transition in a diagram showing surface pressure variations in terms of the molecular density. noted (7T, p) diagram. We also introduce an order-disorder transition and we show that it is either of the first order, or of the second order, according to the physical conditions. The (7T, p) isotherms are in good qualitative agreement with experimental results.

A model for aliphatic molecules monolayers phase transitions at air–water interfaces is proposed: the molecules are treated as rectangles lying on a square lattice, as in a previous paper. But in this work one extremity of the rectangle is discriminated as the polar head of a molecule by means of an interaction energy different from that between aliphatic chains— (Π−A) isotherms and coexistence curves are calculated in the framework of this model and we point out some improvements brought to the results obtained with our previous model, in particular, a third phase tranisition is given by a polar head orientation.

W h e n lipid molecules are deposited on an air-water interface, they spread spontaneously. Because of the amphiphily of lipidic molecules (one end h y d r o p h o b i c , the other hydrophilic), a film is f o r m e d at the interface. T h e balance between attraction and repulsion for the water maintains, m o r e or less tightly, these molecules in a potential well lying at the interface. T h e strength of the repulsion due to h y d r o p h o b i c part of the molecules leads soluble, slightly soluble or quasi insoluble monolayers to be formed.T h e dissolution -or best desorptionkinetics of a slightly soluble m o n o ] a y e r can be studied at a constant surface pressure ~ (1, 2, 3). Generally, a solution of the amphiphilic p r o d u c t in a volatile solvent is used to spread monolayers.T h e aim of this paper is to study the influence of a residual a m o u n t of solvent remaining in the m 0 n o l a y e r (4). the barrier. The trough is placed in a plexiglass cover. It is completely isolated from the mechanical devices and dusts. The water and the atmosphere of the cover are kept at a consant temperature ( T = 22 °C). The products:We use n-decanol puriss furnished by Flucka; tridistilled water; and as solvents: benzene, hexane and chloroform which we have distilled twice. 10--50 /A of a 5 mg/ml solution are dropped onto the surface of the water with an Agla micrometer syringe.

No abstract