Amphiphile bilayers

Most characteristics of amphiphilic systems are associated with the alteration of the interfacial stnicture by the amphiphile. Addition of amphiphiles might reduce the free-energy costs by a dramatic factor (up to 10 dyn cm in the oil/water/amphiphile mixture). Adding amphiphiles to a solution or a mixture often leads to the fomiation of a microenuilsion or spatially ordered phases. In many aspects these systems can be conceived as an assembly of internal interfaces. The interfaces might separate oil and water in a ternary mixture or they might be amphiphilic bilayers in... 

Flelm C A, Israelachvili J N and McGuiggan P M 1989 Molecular mechanisms and forces involved in the adhesion and fusion of amphiphilic bilayers Science 246 919-22... 

Den Otter, W. K. and Briels, W. J. (2003). The bending rigidity of an amphiphilic bilayer from equilibrium and nonequilibrium molecular dynamics, J. Chem. Phys., 118, 4712-4720. 

Tolpekina, T.V., den Otter, W.K., Briels, W.J. Nucleation free energy of pore formation in an amphiphilic bilayer studied by molecular dynamics simulations. J. Chem. Phys. 2004, 121, 12060-6. 

Vesicles are capsules in which the shells are composed of amphiphilic small molecules or polymers. Generally, the shell is an amphiphilic bilayer with an aqueous interior. These differ fundamentally from capsules generated in a water-in-oil emulsion because the oil phase in the vesicle system is only in the shells, which are surrounded by an outer aqueous phase. 

Maoz, R., Cohen, H. and Sagiv, J., Specific nonthermal chemical structural transformation induced by microwaves in a single amphiphilic bilayer self-assembled on silicon, Langmuir, 1998,14, 5988. 

As already discussed in Section 3.4, these two states of black films are, respectively, common black (CBF) and Newton black (NBF) films. Initially, bilayer films were named Perrin films by Scheludko later Jones, Mysels and Scholten called them primary and "secondary films. It was not until the issuing of IUPAC nomenclature that they were termed CBF and NBF. It is rather arguable, however, whether it is fairer to name them after the scientist who observed them first or after the one that characterised them quantitatively. In many cases NBF are also called amphiphile bilayers . 

As already noted, the NB foam films, the bilayer emulsion films and the BLMs, are amphiphile bilayers, and their stability in respect to rupture and their permeability can be considered from a unified point of view. 

Let us turn to results obtained on the basis of the molecular model of amphiphile bilayer illustrated in Fig. 3.82. The basic idea in the theoretical description [402,403] is to regard the bilayer as consisting of two monolayers of amphiphile molecules mutually adsorbed on each other. Each of the monolayers can be filled with a maximum of Nm = 1M0 -... 

The molecular model of amphiphile bilayers can also be used for describing the process of hole nucleation by the classical nucleation scheme [408,409] as resulting from a series of bimolecular reactions characterised by the nucleation rate J (s 1) which is the frequency with which the / -sized nucleus holes become supemucleus holes of size / +1. For steady-state nucleation, J is known to be [408,409]... 

The molecular model of amphiphile bilayers with holes in them is a good basis also for the description of the rupture of NB foam films by a-particle irradiation [331,415,416]. The mean lifetime ra of the foam bilayer shortens dramatically under irradiation, and probability considerations [416] show that only a small area Sh S of the bilayer is active for the passage of the a-particles. Assuming that 5 is the overall area of those holes in the foam bilayer which are large enough to be irradiation-active makes it possible to represent Sh as... 

In conclusion, let us outline some more important aspects of the hole-nucleation theory for stability of amphiphile bilayers of Kashchiev-Exerowa and its experimental support. The outlined theoretical and experimental investigations of the stability and permeability of foam, emulsion and membrane bilayers represent a new approach towards... 

The good agreement between theoretical and experimental results of hole-mediated permeability of foam bilayers to air allows the determination of the permeability coefficient of bilayers of both ionics and nonionics. Though the mechanism of hole-mediated permeation of foam bilayers is not entirely clarified, its efficiency for lower surfactants concentrations in a wide range of temperatures is firmly established. This finding is in strong support of the basic idea of the existence of randomly nucleated microholes in the amphiphile bilayer. 

The theoretical and experimental investigations of rupture and permeability of amphiphile bilayers are valuable also for the understanding of some microstructural effects in interfacial layers and phases of small volumes. The interpenetration of macroscopically measured quantities, e.g. r and W, by means of molecular statistical models seems to be most interesting and useful. As first attempts in this respect, a molecular statistical lattice model of such bilayers has been proposed [427] and a lattice model of such bilayers has been studied by means of Monte Carlo simulation by Chowdhury and Stauffer [429]. The results obtained have been compared with some experimental data presented in this Section. Clearly, the combination of macro and micro considerations is a promising way to obtain a deeper insight into the properties of matter and, especially, of interfacial layers and phases of small volumes. 

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. 

The relationship between the lifetime rof amphiphile bilayer and the hole edge energy % is described by Eqs. (3.123), (3.124) and (3.126). This relationship is applicable to bilayers both free of or containing foreign bodies if in the latter case % is described by a numerical factor accounting for the ability of the foreign body to stimulate hole formation [403]. The latter case is of special interest, since in enables the rupture of BLMs containing proteins or... 

Hole-nucleation rupture of foam bilayers was considered on the basis of formation of nucleus-holes from molecular vacancies existing in the film in Section 3.4.4. The experimentally determined parameters of film rupture along with the hole-nucleation theory of rupture of amphiphile bilayers of Kashchiev-Exerowa [300,301,354,402] made it possible to evaluate the coefficient of lateral diffusion of vacancies in foam bilayer. 

Constants Dv and Co are determined as free parameters in the non-linear regression of the experimental J(t) dependence along with the theoretical one calculated by the least square root method. The theoretical curve calculated at Co = 1. 7-10 4 mol dm 3 and >v = 4-1 O 6 cm2 s 1 is presented with solid line in the figure. The approximation of the lattice model of the amphiphile bilayer Dv is related to the coefficient of lateral diffusion of surfactant molecules building up the bilayer by the degree of filling 0 (respectively, of vacancies 6V)... 

The results on formation and stability of black foam films, on the first place those on bilayer foam films (NBF) (see Sections 3.4.1.2 and 3.4.4) have promoted the development of methods which enable lung maturity evaluation. The research on stability of amphiphile bilayers and probability for their observation in the grey foam films laid the grounds of the method for assessment of foetal lung maturity created by Exerowa et al. [20,24]. Cordova et al. [25] named it Exerowa Black Film Method. It involves formation of films from amniotic fluid to which 47% ethanol and 7-10 2 mol dm 3 NaCl are added [20,24]. In the presence of alcohol the surface tension of the solution is 29 mN m 1 and the adsorption of proteins from the amniotic fluid at the solution/air interface is suppressed, while that of phospholipids predominates. On introducing alcohol, the CMC increases [26], so that the phospholipids are present also as monomers in the solution. The electrolyte reduces the electrostatic disjoining pressure thus providing formation of black foam lipid films (see Sections 3.4.1.2 and 3.4.4). 

Let us summarise the conditions of formation of a microscopic foam film in order to serve the in vivo situation. These are film radius r from 100 to 400 pm capillary pressure pa = 0.3 - 2.5-102 Pa electrolyte (NaCl) concentration Ce 0.1 mol dm 3, ensuring formation of black films (see Section 3.4) and close to the physiological electrolyte concentration sufficient time for surfactant adsorption at both film surfaces. Under such conditions it is possible also to study the suitable dependences for foam films and to use parameters related to formation and stability of black foam films, including bilayer films (see Section 3.4.4). For example, the threshold concentration C, is a very important parameter to characterise stability and is based on the hole-nucleation theory of bilayer stability of Kashchiev-Exerowa. As discussed in Section 3.4.4, the main reason for the stability of amphiphile bilayers are the short-range interactions between the first neighbour molecules in lateral and normal direction with respect to the film plane. The binding energy Q of a lipid molecule in the foam bilayer has been estimated in Section 11.2. 

In the above expression, e and a (with <7 being the tail-tail collision diameter) are the energy and the length units respectively, rc is the cut-off length, while the decay range is determined by the parameter wc. The later was shown to be of key importance for the model and can be used to tune the amphiphile phase diagram and the material properties such as area per amphiphile, bilayer structure, elastic constants, and diffusivity. The functional form of the pair potential between the lipid tails given by the combination of the WCA potential with the attractive interaction of (9) is reproduced by the solid line in the inset on the left of Fig. 8, while the main panel shows the phase behavior of the amphiphiles. 

Fig. 17. Longitudinal and cross-sectional views of the proposed molecular models for the structure of bile salt-lecithin (BS-L) mixed micelles. All recent experimental data for BS-swelling amphiphile micelles are consistent with model B. In this model, reverse BS aggregates are present in high concentrations within the hydrophobic domains of L or other swelling amphiphile bilayers. BS also coat the perimeter of the disks as a bilayered ribbon . (From ref. 102 with permission.)...

P-substituted porphyrins in organic solvents or membranes is the shifted face-to-face dimer found in crystal structures (Fig. 6.2.15a). Here one usually finds that an electron-rich pyrrole ring of one porphyrin sits atop the electron-poor center of the other porphyrin. Water-soluble substituents, in particular the most common acetic and propionic acid side chains, lead to amphiphilic bilayers (Fig. 6.2.15c) (Fuhrhop, 1976 Hunter and Sanders, 1990). 

Figure 1.2 Surfactant molecule with polar head group and hydrocarbon chain. Membranes composed of an amphiphilic monolayer, separating water and oil, and an amphiphilic bilayer, separating two water regions.

Figure 8.4 Schematic representation of spongelike bicontinuous structure in water-oil microemulsions compared with an ordered lamellar structure. A similar picture applies to an amphiphilic bilayer separating "inside and "outside water or oil domains for systems with a single solvent (L3 phases).

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