Phospholipid vesicles vesicle dispersion

Table I shows the thermotropic transitions of phospholipids and vesicle dispersions of 1 and 2, along with enthalpies of transition. NCP-1 shows a sharp chain melting transition temperature at 37.5 C and a sharper transition peak at 32.8 C upon cooling. The AH at 37.5 C was determined to be 6.07 kcal/mole. Dispersions of the NCVD-1 show a chain melting transition at 25.9 C and a cooling transition at 23.6 C. CVD-1 shows a melting transition at 29.4 °C and a cooling transition at 25.9 C.

Polymerization in Bilayers. Upon irradiation with UV light the monomer vesicles are transferred to polymer vesicles (Figure 12.). In the case of the diyne monomers (2,5-9,12,13,14) the polyreaction can again be followed by the color change via blue to red except phospholipids (5,6), which turn red without going through the blue intermediate as observed in monolayers. The VIS spectra of these polymer vesicle dispersions are qualitatively identical to those of the polymer monolayers (Figure 13.). 

Liposomes are characteristic hollow spherical aggregates or vesicles that form spontaneously when phospholipids are dispersed in water. This is a function of their low solubilities in both oil and water and results from the hydrophobic nature of the twin acyl tails and the strongly hydrophilic polar head group which are the main characteristics of phospholipids (Figure 9.1). As a result, phospholipid molecules, when dispersed in water form double layers where the phospholipids align themselves, tail-to-tail and head-to-head (Figure 9.5)... 

Phospholipid vesicles form spontaneously when distilled water is swirled with dried phospholipids. This method of preparation results in a highly polydisperse array of multicompartment vesicles of various shapes. Extrusion through polymeric membranes decreases both the size and polydispersity of the vesicles. Ultrasonic agitation is the most widely used method for converting the lipid dispersion into single-compartment vesicles of small size. 

One of the several shapes that micelles can take is laminar. Since the ends of such micelles have their lyophobic portions exposed to the surrounding solvent, they can curve upwards to form spherical structures called vesicles. Vesicles are spherical and have one or more surfactant bilayers surrounding an internal pocket of liquid. Multi-lamellar vesicles have concentric spheres of uni-lamellar vesicles, each separated from one another by a layer of solvent [193,876] (Figure 14.1). The bilayers are quite thin (-10 nm) and are stabilized by molecules such as phospholipids, cholesterol, or other surfactants (Figure 14.2). Vesicles made from phospholipid bi-layers are called liposomes. Liposomes can be made by dispersing phospholipids (such as lecithin) into water and then agitating with ultrasound. 

The study of P relaxation times provides a specific method for observing dynamic events in the head group region of phospholipid dispersions and membranes [144]. Mn ions broaden only those resonances from nuclei on the outside of phospholipid vesicles, and therefore the percentages of lipids on the inside and outside surfaces can be calculated [145]. 

Although the detailed nature of the surface of costal strips is not known, evidence from binding studies indicate that cations such as Co and Fe + are preferentially adsorbed onto these structures (9). Similar interactions have been shown with organic and colloidal materials (9). For example, phospholipid vesicles were shown to be closely attached to the surfaces of costal rods incubated in aqueous dispersions of phosphatidylcholine for 24 hr. Similar observations were made for costal rods incubated in solutions of colloidal silica. These results indicate that a range of interactions can take place on the surface of biogenic silica and such events may serve important functional roles, such as inhibition of dissolution and adhesion of components in the construction of microscopic structures. 

Colloids can be broadly classified as those that are lyophobic (solvent-hating) and those that are lyophilic and hydrophilic. Surfactant molecules, because of their dual affinity for water and oil and their consequent tendency to associate into micelles, form hydrophilic colloidal dispersions in water. Proteins and gums also form lyophilic colloidal systems. Hydrophilic systems are dealt with in Chapters 8 and 11. Water-insoluble drugs in fine dispersion or clays and oily phases will form lyophobic dispersions, the principal subject of this chapter. While lyophilic dispersions (such as phospholipid vesicles and micelles) are inherently stable, lyophobic colloidal dispersions have a tendency to coalesce because they are thermodynamically unstable as a result of their high surface energy. 

Let us discuss briefly the main preparation methods of phospholipid vesicles (Figure 17.3). The formation of vesicles takes place when lipids are dispersed in water the different methods differ for the way this dispersion occurs. 

Emulsions are defined as dispersions of one liquid in another, stabilized by an interfacial film of emulsifiers such as surfactants and lipids. Emulsion formulations include water in od and oil in water emulsions, multiple emulsions, microemulsions, microdroplets, and liposomes. Microdroplets are unilamellar phospholipid vesicles that consist of a spherical lipid layer with an oil phase inside. 

Differential scanning calorimetry was performed to measure the chain melting temperature for non-cross-linked phospholipids (NCP) 1-3. Non-cross-linked vesicle dispersions (NCVD) and cross-linked vesicle dispersions (CVD) of 1 and 2 were also measured along with the chain melting transition of CVD-1 with a hydrophobic dye entrapped. Parent phospholipids DPPE and DLPE were used as references for NCP 1 and 2. Parent phospholipid EGGPE was used as reference for phospholipid 3. NCPs were weighed directly into the DSC pans. Vesicle dispersions were used for the non-cross-linked vesicle samples and freeze-dried powder was used for the cross-linked vesicle dispersion samples. 

NCP is an abbreviation for non-cross-linked phospholipids, NCVD is an abbreviation for vesicle dispersion non-cross-linked, CVD is an abbreviation for cross-linked vesicle dispersion. 

The interaction of water soluble polymers with lamellar dispersions has implications in many practical applications such as consumer products, drug delivery, bioseparations, and industrial processes. The osmotic stress method for studying membrane biophysics involves the addition of water soluble polymers such as poly (ethylene glycols) (PEGs) to phospholipid vesicles and measuring the changes in... 

Over 40 years since it what found that phospholipids can form closed bilayered structures in aqueous systems, liposomes have made a long way to become a popular pharmaceutical carrier for numerous practical applications. Liposomes are phospholipid vesicles, produced by various methods from lipid dispersions in water. Liposome preparation, their physicochemical properties and possible biomedical application have already been discussed in several monographs. Many different methods exist to prepare liposomes of different sizes, structure and size distribution. The most frequently used methods include ultrasonication, reverse phase evaporation and detergent removal from mixed lipid-detergent micelles by dialysis or gel-filtration. To increase liposome stability towards the physiological environment, cholesterol is incorporated into the liposomal membrane (up to 50% mol). The size of liposomes depends on their composition and preparation method and can vary from... 

W/O/W emulsions stabilized with soy lecithin-Span 80 mixtures have been used as the basis for the preparation of phospholipid vesicles [158]. A water-in-n-hexane emulsion was first prepared and the bulk of the hexane removed, the concentrate being dispersed in aqueous solution using a low concentration of hydrophilic surfactant which itself could then be removed leaving the phospholipid vesicles. 

Figure 6.1 Schematic representation of a vesicle dispersion made up of a phospholipid or double-chain surfactant.

Phospholipids e.g. form spontaneously multilamellar concentric bilayer vesicles73 > if they are suspended e.g. by a mixer in an excess of aqueous solution. In the multilamellar vesicles lipid bilayers are separated by layers of the aqueous medium 74-78) which are involved in stabilizing the liposomes. By sonification they are dispersed to unilamellar liposomes with an outer diameter of 250-300 A and an internal one of 150-200 A. Therefore the aqueous phase within the liposome is separated by a bimolecular lipid layer with a thickness of 50 A. Liposomes are used as models for biological membranes and as drug carriers. 

Cationic quaternary ammonium compounds such as distearyldimethylammonium-chloride (DSDMAC) used as a softener and as an antistatic, form hydrated particles in a dispersed phase having a similar structure to that of the multilayered liposomes or vesicles of phospholipids 77,79). This liposome-like structure could be made visible by electron microscopy using the freeze-fracture replica technique as shown by Okumura et al. 79). The concentric circles observed should be bimolecular lamellar layers with the sandwiched parts being the entrapped water. In addition, the longest spacings of the small angle X-ray diffraction pattern can be attributed to the inter-lamellar distances. These liposome structures are formed by the hydrated detergent not only in the gel state but also at relatively low concentrations. 

Hydration of Phospholipids with Solutions of Very Low Ionic Strength Very large unilamellar and oligolamellar vesicles can be prepared when a thin lipid film is dispersed in a solution of very low ionic strength (Reeves and Dowben, 1969). The formation of vesicles with diameters up to 300 pm enclosing latex beads with a diameter of 20 pm have been reported (Antanavage et al., 1978). 

Sonication of MLV dispersions above the Tc of the lipids results in the formation of SUV (Saunders, et al., 1962). Sonication can be performed with a bath sonicator (Papahadjopoulos and Watkins, 1967) or a probe sonicator (Huang, 1969). During sonication the MLV structure is broken down and small unilamellar vesicles with a high radius of curvature are formed. In case of SUV with diameters of about 20 nm (maximum radius of curvature), the outer monolayer can contain over 50% of the phospholipids and in the case of lipid... 

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