Monolayers, insoluble transition between

The difference between the static or equilibrium and dynamic surface tension is often observed in the compression/expansion hysteresis present in most monolayer Yl/A isotherms (Fig. 8). In such cases, the compression isotherm is not coincident with the expansion one. For an insoluble monolayer, hysteresis may result from very rapid compression, collapse of the film to a surfactant bulk phase during compression, or compression of the film through a first or second order monolayer phase transition. In addition, any combination of these effects may be responsible for the observed hysteresis. Perhaps understandably, there has been no firm quantitative model for time-dependent relaxation effects in monolayers. However, if the basic monolayer properties such as ESP, stability limit, and composition are known, a qualitative description of the dynamic surface tension, or hysteresis, may be obtained. 

The most familiar transitions between surface phases in fluid interfaces are those in the so-called insoluble monomolecular films that some higher alcohols and fatty adds form at a water-air interface. Distinct surface phases and transitions between them are frequently observed also in monolayers of adsorbed gases on solid substrates, and are the subject of an exuberant modem literature. ... 

Multilayers of Diphosphates. One way to find surface reactions that may lead to the formation of SAMs is to look for reactions that result in an insoluble salt. This is the case for phosphate monolayers, based on their highly insoluble salts with tetravalent transition metal ions. In these salts, the phosphates form layer stmctures, one OH group sticking to either side. Thus, replacing the OH with an alkyl chain to form the alkyl phosphonic acid was expected to result in a bilayer stmcture with alkyl chains extending from both sides of the metal phosphate sheet (335). When zirconium (TV) is used the distance between next neighbor alkyl chains is - 0.53 nm, which forces either chain disorder or chain tilt so that VDW attractive interactions can be reestablished. 

The terminology of L-B films originates from the names of two scientists who invented the technique of film preparation, which transfers the monolayer or multilayers from the water-air interface onto a solid substrate. The key of the L-B technique is to use the amphiphih molecule insoluble in water, with one end hydrophilic and the other hydrophobic. When a drop of a dilute solution containing the amphiphilic molecules is spread on the water-air interface, the hydrophilic end of the amphiphile is preferentially immersed in the water and the hydrophobic end remains in the air. After the evaporation of solvent, the solution leaves a monolayer of amphiphilic molecules in the form of two-dimensional gas due to relatively large spacing between the molecules (see Fig. 15 (a)). At this stage, a barrier moves and compresses the molecules on the water-air interface, and as a result the intermolecular distance decreases and the surface pressure increases. As the compression from the barrier proceeds, two successive phase transitions of the monolayer can be observed. First a transition from the gas" to the liquid state. 

It is well known that water dispersions of amphiphile molecules may undergo different phase transitions when the temperature or composition are varied [e.g. 430,431]. These phase transitions have been studied systematically for some of the systems [e.g. 432,433]. Occurrence of phase transitions in monolayers of amphiphile molecules at the air/water interface [434] and in bilayer lipid membranes [435] has also been reported. The chainmelting phase transition [430,431,434,436] found both for water dispersions and insoluble monolayers of amphiphile molecules is of special interest for biology and medicine. It was shown that foam bilayers (NBF) consist of two mutually adsorbed densely packed monolayers of amphiphile molecules which are in contact with a gas phase. Balmbra et. al. [437J and Sidorova et. al. [438] were among the first to notice the structural correspondence between foam bilayers and lamellar mesomorphic phases. In this respect it is of interest to establsih the thermal transition in amphiphile bilayers. Exerowa et. al. [384] have been the first to report such transitions in foam bilayers from phospholipids and studied them in various aspects [386,387,439-442]. This was made possible by combining the microscopic foam film with the hole-nucleation theory of stability of bilayer of Kashchiev-Exerowa [300,402,403]. Thus, the most suitable dependence for phase transitions in bilayers were established. 

There may be diverse causes for the dissipative part. In an ideally insoluble monolayer dissipation is exclusively caused by relaxation processes in the mono-layer, such as breaking and reforming bonds between the adsorbed molecules, phase transitions and reconformation of the molecules themselves. See sec. 3.6h. In Gibbs monolayers there is an extra dissipative term due to exchange processes between the bulk and the monolayer. For the case of continuous equilibrium between the monolayer and the bulk dy = 0 and, hence the surface rheological... 

A renewed interest in the structure and properties of amphiphilic molecules which reside at the air-water interface has occurred in the last decade due to the fact that the monolayers comprise an idealized two dimensional system which can be probed in terms of structure, composition and phase transitions, and due to the fact that the monolayers are used to form highly ordered coatings in Langmuir-Blodgett applications. Monolayers form at an air-water interface upon dissolution of amphiphiles in a water immiscible solvent, spreading of Ae solution on a water surface, and lastly, solvent removal. Such monolayers are insoluble in the aqueous phase. Alternately, monolayers which are soluble in the aqueous phase are formed by adsorption of amphiphiles from the bulk aqueous solution to the air-water interface. A primary difference between insoluble and soluble monolayers... 

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