Amphiphiles insoluble monolayers

The attachment of pyrene or another fluorescent marker to a phospholipid or its addition to an insoluble monolayer facilitates their study via fluorescence spectroscopy [163]. Pyrene is often chosen due to its high quantum yield and spectroscopic sensitivity to the polarity of the local environment. In addition, one of several amphiphilic quenching molecules allows measurement of the pyrene lateral diffusion in the mono-layer via the change in the fluorescence decay due to the bimolecular quenching reaction [164,165]. 

The behavior of insoluble monolayers at the hydrocarbon-water interface has been studied to some extent. In general, a values for straight-chain acids and alcohols are greater at a given film pressure than if spread at the water-air interface. This is perhaps to be expected since the nonpolar phase should tend to reduce the cohesion between the hydrocarbon tails. See Ref. 91 for early reviews. Takenaka [92] has reported polarized resonance Raman spectra for an azo dye monolayer at the CCl4-water interface some conclusions as to orientation were possible. A mean-held theory based on Lennard-Jones potentials has been used to model an amphiphile at an oil-water interface one conclusion was that the depth of the interfacial region can be relatively large [93]. 

The adsorption of amphiphilic molecules at the surface of a liquid can be so strong that a compact monomolecular film, abbreviated as monolayer, is formed. There are amphiphiles which, practically, do not dissolve in the liquid. This leads to insoluble monolayers. In this case the surface excess T is equal to the added amount of material divided by the surface area. Examples of monolayer forming amphiphiles are fatty acids (CH3(CH2) c 2COOH) and long chain alcohols (CH3(CH2)nc iOH) (see section 12.1). 

Amphiphilic molecules form monomolecular films on liquid surfaces. Some amphiphiles, such as phospholipids, with a large hydrophobic tail are practically insoluble in water and therefore form insoluble monolayers at the air-water interface. 

Fainerman, V.B. Vollhardt, D. Melzer, V. Equation of state for insoluble monolayers of aggregating amphiphilic molecules. J. Phys." Chem. 1996, 100, 15478. 

For n-A isotherms of insoluble monolayes of amphiphilic molecules, assuming the association or dissociation of these molecules in the surface layer, a generalised Volmer equation was derived (based on Butler s and Gibbs equations33 35), which has the form... 

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. 

In section 6.2.4 we examined the case in which the surface of a solution containing an amphiphile became covered with a monomol-ecular film as a result of spontaneous adsorption from solution. The molecules in such films are in equilibrium with those in the bulk of the solution, i.e. there is a continuous movement of molecules between the surface and the solution below it. If, however, a surfactant has a very long hydrocarbon chain it will be insufficiently water-soluble for a film to be formed in this way. In such cases we can spread a film on the surface of the solution by dissolving the surfactant in a suitable volatile solvent and carefully injecting the solution on to the surface. The insoluble monolayer formed by this process contains all of the molecules injected on the surface there is no equilibrium with the bulk solution because of the low water solubility of the surfactant. Conse-... 

Insoluble amphiphilic compounds will also form films on water surfaces and these may be tightly packed, as in condensed films, or more loosely packed, as in expanded and gaseous films. We have seen that polymers and proteins may also form insoluble monolayers. 

Surface studies of insoluble monolayers of all the common unconjugated bile acids, including the unsubstituted cholanoic acid, have been carried out by a number of workers and thoroughly reviewed [5]. Being insoluble non-swelling amphiphiles with limited aqueous solubility, their surface pressure-area (v-A) isotherms can be measured satisfactorily with a Langmuir-Pockels surface balance on an aqueous subphase containing 3-6 M NaCl to salt out polar functions and at sufficient acidic pH (1-3) to prevent ionization [5,6). 

A typical effect related to surface relaxations is obtained in measurements of ti-A isotherms of insoluble monolayers. In most of the measurements with spread amphiphiles there are differences between the curve for compression and expansion of the surface films. Usually this characteristic behaviour is described as hysteresis. One experimental example of a spread dipalmitoyl lecithin is shown in Figs 3.12. This phenomenon corresponds to one or more of these surface relaxations. 

In contrast, there are practically insoluble amphiphiles, for example, a fatty acid molecule that consists of a hydrophilic part (carboxyl group) and a hydrophobic part (a long hydrocarbon chain), which prevents the molecule from dissolving in the aqueous phase. Such a molecule forms an insoluble monolayer on a water surface. Drops of the solution of the fatty acid in a volatile organic solvent are placed on an aqueous surface and after evaporation of the solvent a fatty acid s monolayer remains. This process is called spreading. The adsorbed molecules forming the insoluble monolayer are essentially isolated on the surface therefore, the surface excess F is equal to the added amount of material divided by the surface area. 

On the other hand, in case (b), an insoluble amphiphile is spread on one side of the trough. An insoluble monolayer... 

As with the water/oil interface, amphiphiles can specifically adsorb to the interface between water and air. One limiting case is complete adsorption resulting in the so-called insoluble monolayer. Experimental studies of insoluble monolayers at the water/air interface are routinely carried out using a film balance developed by Langmuir [1] over 70 years ago with this, the average area per molecule can be varied and the resulting change in surface... 

Monolayers at the Air—Water Interface. Molecules that form monolayers at the water—air interface are called amphiphiles or surfactants (qv). Such molecules are insoluble in water. One end is hydrophilic, and therefore is preferentially immersed in the water the other end is hydrophobic, and preferentially resides in the air, or in a nonpolar solvent. A classic example of an amphiphile is stearic acid, C H COOH, wherein the long hydrocarbon... 

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. 

Selected entries from Methods in Enzymology [vol, page(s)] Activation of lipolytic enzymes by interfaces, 64, 341 model for lipase action on insoluble lipids, 64, 345 interfacial enzyme inactivation, 64, 347 reversibility of the adsorption step, 64, 347 monolayer substrates, 64, 349 kinetic models applicable to partly soluble amphiphilic lipids, 64, 353 surface dilution model, 64, 355 and 364 practical aspects, 64, 357. 

The extensive studies of the behavior of mixed monolayers or bilayers of di-acetylenic lipids and other amphiphiles parallel to some degree the studies of dienoyl-substituted amphiphiles. Since the dienoyl lipids do not contain a rigid diacetylenic group in the middle of the hydrophobic chains, they tend to be miscible with other lipids over a wide range of temperatures and compositions. In order to decrease the lipid miscibility of certain dienoyl amphiphiles, Ringsdorf and coworkers utilized the well-known insolubility of hydrocarbons and fluorocarbons. Thus two amphiphiles were prepared, one with hydrocarbon chains and the other with fluorocarbon chains, in order to reduce their ability to mix with one another in the bilayer. Of course it is necessary to demonstrate that the lipids form a mixed lipid bilayer rather than independent structures. Elbert et al. used freeze fracture electron microscopy to demonstrate that a molar mixture of 95% DM PC and 5% of a fluorinated amphiphile formed phase-separated mixed bilayers [39]. Electron micrographs showed extensive regions of the ripple phase (Pb phase) of the DM PC and occasional smooth patches that were attributed to the fluorinated lipid. In some instances it is possible to... 

It is seen that Eq. (15), which follows approximately from Eq. (14) (assuming low monolayer coverage and neglecting entropy non-ideality), can also describe the behaviour of monolayers which comprise particles of any size. Similarly to Eq. (14), this equation involves not the geometric parameters of amphiphilic molecules (or particles), but only monolayer coverage by these entities. Equation (15) provides a good description of the experimental results obtained for various systems. For example, for some insoluble proteins in the liquid-expanded monolayer range, the value n = 20-100 was obtained.35... 

Tchoreloff, P., Boissonnade, M. M., Coleman, A. W., and Baszkin, A. (1995), Amphiphilic monolayers of insoluble cyclodextrins at the water/air interface. Surface pressure and surface potential studies, Langmuir, 11,191-196. 

A necessary prerequisite for the formation of microscopic foam films is the adsorption of surfactants at the solution/air interface. Different ways have been sought in order to obtain adsorption layers from insoluble surfactants at such interface. The easiest way to form a foam film is to blow a freely floating gas bubble at a liquid surface covered with a monolayer of insoluble amphiphile molecules. This approach has been used by various authors [138-142]. 

If it concerns a monolayer of an amphiphile that is insoluble in the bordering phases, the modulus is purely elastic (although at strong compression, i.e., large AA/A, the surface layer may collapse), and SD is constant in time and independent of the dilatation rate. If the surfactant is soluble, exchange of surfactant between interface and bulk occurs, and Esr> will be time dependent. This means that also an apparent surface dilatational viscosity can be measured ... 

Spherical vesicles (see Sec. 2.5.4) are made by the same kind of amphiphiles that form micelles. Highly soluble amphiphiles (e.g., sodium salts of fatty acids or soaps) form micelles badly soluble amphiphiles (e.g., free fatty acids) give vesicles or crystallize. Amphiphilic monomers with two or three long alkyl chains are often totally water insoluble as monomers but dissolve well as vesicular assemblies. Vesicles usually collapse upon drying (Fig. 1.5.8a), but one isolable monolayer vesicle made of rigid carotenoid bolaamphiphiles has also been reported (Fig. 5.5). Hydrogen bond chains convert spherical vesicles to tubules. Such tubules can again be isolated in the dry form and can be stored. They are particularly stable if monolayer membranes are used (Fig. 1.5.8b). 

The equilibrium and dynamic behaviour of mixed monolayers of soluble and insoluble amphiphiles at fluid/liquid interfaces plays an important role in various technological and biological processes, which was studied in numerous publications [139-157], However, even for very simple systems, say, gaseous mixed monolayers, the thermodynamic analysis is not trivial. For more complicated systems (the formation of two-dimensional domains) such an analysis is very cumbersome due to mathematical difficulties. 

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