Lipids water-fluid interfaces

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

In another approach to functionalize surfaces we made use of the Langmuir-Blodgett-Kuhn technique a dimyristoyllecithin monolayer at the water/air-interface doped (by co-spreading) with 5 raole% biotinylated lipid (Fig. 4(a)) shows the usual pressure-area (tt-A) isotherm (if compressed) (Fig. 4(b)), with the coexistence of fluid and ordered domains... 

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. 

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... 

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 viscous drag felt at the bilayer-water interface can be large when the bounding fluid viscosity approaches the viscosity of the membrane (53) or when the fluid lipid bilayer is associated with a rigid substrate (54). 

Stratum corneum, the nonliving layer of skin, is refractory as a substrate for chemical reactions, hut it has a strong physical affinity for water. The chemical stability of stratum corneum is evident in its mechanical barriers which include insoluble cell membranes, matrix-embedded fibers, specialized junctions between cells, and intercellular cement. The hygroscopic properties of stratum corneum appear to reside in an 80 A-thick mixture of surface-active proteins and lipids that forms concentric hydrophilic interfaces about each fiber. This combination of structural features and surface-active properties can explain how stratum corneum retains body fluids and prevents disruption of living cells by environmental water or chemicals. 

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.

Major applications of SFE-SFC are somewhat limited at the moment to the analysis of lipids and pesticides from foods and similar matrices and different types of additives used in the production of polymers [79,146,188-194]. The approaches used cover a wide range of sophistication and automation from comprehensive commercial systems to simple laboratory constructed devices based on the solventless injector [172,174,175,188]. Samples usually consist of solid matrices or liquids supported on an inert carrier matrix. Aqueous solutions are often analyzed after solid-phase extraction (SPE-SFE-SFC) to minimize problems with frozen water in the interface [178,190]. The small number of contemporary applications of SFE-SFC reflects a lack of confidence in supercritical fluid chromatography as a separation technique and competition for... 

It is interesting to note that during digestion of triglyceride oils, all the substances discussed above are present as well as the phases mentioned. This was nicely illustrated by Patton and Carey [10] in an in vitro study where the fate of a drop of soybean oil in simulated intestinal fluid was monitored under the microscope. The cubic phase is formed at the oil/water interface, and due to its bicontinuous structure it is capable of both delivering the water molecules necessary for hydrolysis and taking care of the resulting fatty acids. The role of lamellar and micellar phases is to act as carriers of lipids to the intestinal wall [11,12]. 

Lipids and phospholipids The study of phospholipid monolayers adsorbed on a mercury electrode and the interaction between phospholipids and proteins has been an active research topic for a number of years. The reason for this is obvious when one considers the currently accepted fluid mosaic model of the bilayer lipid membrane (BLM). In addition to its role as a structural element in cells, etc., the BLM is also important in some foods. Since there is enough phospholipid in milk to form a film on a greatly expanded oil-water interface, this lipid undoubtedly plays an important role in stabilizing dairy and other food products that utilize homogenized milk [123]. 

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