Thermodynamics vesicles

The formation of ordered two- and three-dimensional microstructuies in dispersions and in liquid systems has an influence on a broad range of products and processes. For example, microcapsules, vesicles, and liposomes can be used for controlled drug dehvery, for the contaimnent of inks and adhesives, and for the isolation of toxic wastes. In addition, surfactants continue to be important for enhanced oil recovery, ore beneficiation, and lubrication. Ceramic processing and sol-gel techniques for the fabrication of amorphous or ordered materials with special properties involve a rich variety of colloidal phenomena, ranging from the production of monodispersed particles with controlled surface chemistry to the thermodynamics and dynamics of formation of aggregates and microciystallites. 

Liposomes — These are synthetic lipid vesicles consisting of one or more phospholipid bilayers they resemble cell membranes and can incorporate various active molecules. Liposomes are spherical, range in size from 0.1 to 500 pm, and are thermodynamically unstable. They are built from hydrated thin lipid films that become fluid and form spontaneously multilameUar vesicles (MLVs). Using soni-cation, freeze-thaw cycles, or mechanical energy (extrusion), MLVs are converted to small unilamellar vesicles (SUVs) with diameters in the range of 15 to 50 nm. ... 

Vesicles [10, 11] these aggregates of insoluble natural or artificial amphiphiles in water can have various shapes (spherical, cylindrical). Depending on the preparation conditions, small unilamellar or large multilamellar vesicles can be produced. The structures meet the self-organization criterion, because they are, albeit on a long time scale, dynamic and not in thermodynamic equilibrium, which would in many cases be a macroscopically phase separated lamellar phase. 

Flewelling, R. F., Hubbell, W. L., Hydrophobic ion interactions with membranes. Thermodynamic analysis of tetraphenylphosphonium binding to vesicles, Biophys. J. 1986, 49, 531— 540. 

In model systems for bilayers, one typically considers systems which are composed of one type of phospholipid. In these systems, vesicles very often are observed. The size of vesicles may depend on their preparation history, and can vary from approximately 50 nm (small unilamellar vesicles or SUVs) up to many pm (large unilamellar or LUV). Also one may find multilamellar vesicular structures with more, and often many more than, one bilayer separating the inside from the outside. Indeed, usually it is necessary to follow special recipes to obtain unilamellar vesicles. A systematic way to produce such vesicles is to expose the systems to a series of freeze-thaw cycles [20]. In this process, the vesicles are repeatedly broken into fragments when they are deeply frozen to liquid nitrogen temperatures, but reseal to closed vesicles upon thawing. This procedure helps the equilibration process and, because well-defined vesicles form, it is now believed that such vesicles represent (close to) equilibrium structures. If this is the case then we need to understand the physics of thermodynamically stable vesicles. 

The total curvature energy of a spherical vesicle is given by 4tt(2/cc + k). As all experimental data on phospholipids indicate that kc is not small, one is inclined to conclude that the vesicles are thermodynamically unstable the reduction of the number of vesicles, e.g. by vesicle fusion or by Ostwald ripening, will reduce the overall curvature energy. However, such lines of thought overlook the possibility that k is sufficiently negative to allow the overall curvature free energy of vesicles to remain small. 

Above, we argued that a portion of a finite-sized membrane can close upon itself to remove edge effects. In this way, vesicles are formed. The thermodynamic stability of vesicles is still a topic of hot debate in the literature, primarily because there are so many scenarios. The SCF analysis of vesicles leads to information on the mechanical parameters for a particular membrane system. 

Anderson, N. H., Davis, S. S., James, M. and Kojima, I. (1983). Thermodynamics of distribution of p-substituted phenols between aqueous solution and organic solvents and phospholipid vesicles, J. Pharmaceut. Sci., 72, 443 148. 

The tendency of apolar side chains of amino acids (or lipids) to reside in the interior nonaqueous environment of a protein (or membrane/micelle/vesicle). This process is accompanied by the release of water molecules from these apolar side-chain moieties. The effect is thermodynamically driven by the increased disorder (ie., AS > 0) of the system, thereby overcoming the unfavorable enthalpy change (ie., AH < 0) for water release from the apolar groups. 

The MARTINI model effectively replaces three to four heavy atoms with a bead, parameterized to reproduce condensed-phase thermodynamic data of small molecules [23]. The MARTINI model has been used to investigate many biological processes, such as lung surfactant collapse [24], nanoparticle permeation in bilayers [25], large domain motion of integral membrane proteins [26], vesicle fusion [27,28], and lateral domain formation in membranes [29]. 

M Hong, BS Weekley, SJ Grieb, JP Foley. Electrokinetic chromatography using thermodynamically stable vesicles and mixed micelles formed from oppositely charged surfactants. Anal. Chem. 70 1394-1403 (1998). 

New glycolipids have to be synthesized to get further insights into liquid crystal properties (mainly lyotropic liquid crystals), surfactant properties (useful in the extraction of membrane proteins), and factors that govern vesicle formation, stability and tightness. New techniques have to be perfected in order to allow to make precise measurements of thermodynamic and kinetic parameters of binding in 3D-systems and to refine those already avalaible with 2D-arrays. Furthermore, molecular mechanics calculations should also be improved to afford a better modeling of the conformations of carbohydrates at interfaces, in relation with physical measurements such as NMR. 

If the electrostatic forces between charged surfactant head groups are sufficiently high, vesicles can also be a thermodynamically stable phase and be... 

Aqueous micelles have diameters ranging typically from 0.5 to 5 nm, and being so small, do not scatter visible light and form transparent solutions. Figure 9.6 shows some basic parameters for aqueous micelles, relative to the well-known SDS (sodium dodecylsulfate). Micelles are thermodynamically stable, and this is a signihcant difference with respect to most large vesicle aggregates. 

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