Fluid interfaces lipids

Chiu, S. W., Clark, M., Balaji, V., Subramaniam, S., Scott, H. L. and Jakobsson, E. (1995). Incorporation of surface tension into molecular dynamics simulation of an interface a fluid phase lipid bilayer membrane, Biophys. J., 69,1230-1245. 

Proteins and Lipids Can Alter the Thermodynamic and Dynamic Characteristics of Water at Fluid Interfaces... 

Nino, Ma.R.R., Patino, J.M.R., Sanchez, C., Cejudo, M., and Navarro, J.M. Physicochemical characteristics of food lipids and proteins at fluid-fluid interfaces, Chem. Eng. Commun., 190,15, 2003. 

Figure 12. Epifluorescence (fluorescent probe, 23) photomicrograph of a mono-molecular film of the phospholipid dipalmitoyl phosphatidyl choline (10, R = R = n-CisHsi) at the air-water interface. The black regions are composed of solid-phase lipid, and the white (fluorescent) regions are fluid-phase lipid containing about 1 mol% of a fluorescent lipid probe. (Top) Micrograph showing the onset of solid phase formation bar, 50 pm. Middle) Micrograph showing formation of chiral solid domains when the phospholipid is one of the enantiomeric forms (R) bar, 50 pm. Bottom) Micrograph showing spiral forms of enantiomeric lipid when 2 mol% of cholesterol is included in the monolayer so as to reduce the line tension bar, 30 pm. Reproduced from ref. 146 (McConnell and Keller, Proc. Natl. Acad. Sci. USA 1987, 84,4706) with permission of the Academy of Sciences of the USA.

All fluid interfaces, including lipid membranes and surfactant lamellas, are involved in a thermal fluctuation wave motion. The configurational confinement of such thermally exited modes within the narrow space between two approaching interfaces gives rise to short-range repulsive surface forces, which are considered below. 

Benvegnu DJ and McConnel HM (1992) Line tension between liquid domains in lipid monolayers. J Phys Chem 96 6820-6824 Chou T and Nelson DR (1994) Surface wave scattering at nonuniform fluid interfaces. J. Chem. Phys 101 9022-9032... 

Cells exploit bilayer structures to create anatomical boimdaries, eg in the case of cell membranes which are composed of lipids, proteins, and carbohydrates. During the early 1960s researchers demonstrated that certain classes of lipids, especially phospholipids, could be used to form protein- and carbohydrate-free model membranes. Methods were developed for the preparation of supported bilayer lipid membranes (1), and it was discovered that dried thin films of phospholipids spontaneously hydrate to yield lipid vesicles (2). Vesicles have since then been used as model systems for fluid interfaces and biomembranes (3). Practical applications involving vesicles are in the area of cosmetics and pharmaceutics. 

The authors review the theoretical analysis of the hydrodynamic stability of fluid interfaces under nonequilibrium conditions performed by themselves and their coworkers during the last ten years. They give the basic equations they use as well as the associate boundary conditions and the constraints considered. For a single interface (planar or spherical) these constraints are a Fickean diffusion of a surface-active solute on either side of the interface with a linear or an erfian profile of concentration, sorption processes at the interface, surface chemical reactions and electrical or electrochemical constraints for charged interfaces. General stability criteria are given for each case considered and the predictions obtained are compared with experimental data. The last section is devoted to the stability of thin liquid films (aqueous or lipidic films). 

The absolute values of the interfacial tensions varied between different amphi-philes and solvents (Table 1). AOT, which is well known in the literature for the formation of microemulsions, showed the lowest surface tension at the interface of both solvents. The other nonionic snrfactants mentioned here. Span 80 and Brij 72 showed shghtly higher valnes. This was also observed for Lecithine, but this lipid precipitated partly during the spinning-drop measurements. Due to this phenomenon, it was not possible to measure accurate data for this emulsifying compound. The interfacial tension had also some influence on the mean size of the emulsion droplets and on the stability of the vesicles (Table 3). In addition to the stationary values of the surface tension, dynamic processes as the surfactant diffusion represented another important factor for the process of stimulated vesicle formation. If an aqueous droplet passed across the fluid interface it carried-over a thin layer of emulsifiers and thereby lowered the local surfactant concentration in the vicinity of the oil-water interface. In the short time span, before the next water droplet approached the interface, the surfactant films should entirely reform and this only occurred, if the surfactant diffusion was fast enough. 

The gel-to-fluid chain-melting transition in pseudo-two-dimensional lipid bilayer membranes induces formation of lipid domains of gel-like lipids in the fluid phase and and fluid-like lipids in the gel phase. The average domain size and in particular the average length of the one-dimensional interfaces between lipid domains and bulk have a dramatic temperature dependence with anomalies at the transition temperature. These anomalies are related to similar anomalies in response functions. The interfacial area may be modulated by intrinsic impurities which are interfacially active molecules such as cholesterol [1,2]. The properties of the interfacial area provide a means for understanding aspects of the functioning of certain biological membrane processes like the passive permeability of small ions and the activity of some membrane enzymes. 

We also discussed additional factors that have an effect on the polymer adsorption and grafted layers the quahty of flic solvent, undulating and flexible substrates such as fiuid/fluid interfaces or lipid membranes adsorption and grafted layer of charged polymers (polyelecfrolytes) and adsorption and grafting on curved surfaces such as spherical colloidal particles. 

These are molecules which contain both hydrophilic and hydrophobic units (usually one or several hydrocarbon chains), such that they love and hate water at the same time. Familiar examples are lipids and alcohols. The effect of amphiphiles on interfaces between water and nonpolar phases can be quite dramatic. For example, tiny additions of good amphiphiles reduce the interfacial tension by several orders of magnitude. Amphiphiles are thus very efficient in promoting the dispersion of organic fluids in water and vice versa. Added in larger amounts, they associate into a variety of structures, filhng the material with internal interfaces which shield the oil molecules—or in the absence of oil the hydrophobic parts of the amphiphiles—from the water [3]. Some of the possible structures are depicted in Fig. 1. A very rich phase... 

Another interesting class of phase transitions is that of internal transitions within amphiphilic monolayers or bilayers. In particular, monolayers of amphiphiles at the air/water interface (Langmuir monolayers) have been intensively studied in the past as experimentally fairly accessible model systems [16,17]. A schematic phase diagram for long chain fatty acids, alcohols, or lipids is shown in Fig. 4. On increasing the area per molecule, one observes two distinct coexistence regions between fluid phases a transition from a highly diluted, gas -like phase into a more condensed liquid expanded phase, and a second transition into an even denser... 

Lung surfactant is a mixture of proteins and amphipathic lipids that acts like a detergent or soap to greatly decrease the surface tension forces at the alveolar fluid-air interface. 

The structure of biological and model membranes is frequently viewed in the context of the fluid mosaic model [4], Since biological membranes are composed of a mixture of various lipids, proteins, and carbohydrates the supra-structure or lateral organization of the components is not necessarily random. In order to model biological membranes, lipid assemblies of increasing complexity were studied. Extensive investigation of multicomponent monolayers (at the air-water interface) as well as bilayers have been reported. 

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). 

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