Phase evaporation and

Szoka, F., Olson, F., Heath, T., Vail, W., Mayhew, E., and Paphadjopoulos, D. (1980) Preparation of unilamellar liposomes of intermediate size (0.1-0.2 pm) by a combination of reverse phase evaporation and extrusion through polycarbonate membranes. Biocbim. Biophys. Acta 601, 559-571. 

Analytical pervaporation is the process by which volatile substances in a heated donor phase evaporate and diffuse through a porous hydrophobic membrane, the vapour condensing on the surface of a cool acceptor fluid on the other side of the membrane. Surface tension forces withhold the fluids from the pores and prevent direct contact between them. A temperature difference that results in a vapour pressure difference across the membrane provides a strong driving force for the separation, which also occurs in the absence of a temperature gradient. Evaporation will occur at the sample surface if the vapour pressure exceeds that at the acceptor surface. One important feature of pervaporation modules used for analytical purposes is the air gap between the donor phase and the hydrophobic membrane, which avoids any contact between them and reduces the problems associated with fouling of the membrane. 

To 3-methyl-l,2,4,5-tetrazino[3,2-a]isoindol-6(4//)-one (14a 1.00 g, 5.0 mmol) in anhyd MeOH (500 mL) was added portion-wise with intensive stirring KMn04 (8.00 g, 50 mmol). To the red-violet solution was added Et20 (500 mL), the solution filtered and the filtrate evaporated. The residue was extracted with CHC1 the organic phase evaporated and the residue purified by column chromatography (silica gel, Et,0) yield 895 mg (78%) mp 85°C. 

With pressure fixed, for each value of the fraction of liquid L, the equilibrium temperature and the compositions in vapor and liquid phases can be computed by iteration. The results are given below and shown in Fig. 10.1-7. In this diagram each of the horizontal tie lines shown connecting the vapor and liquid compositions is labeled with its equilibrium temperature. Note that the first bubble of vapor occurs at 327.8 K and has an n-pentane mole fraction of 0.888. As the temperature increases, more of the liquid phase evaporates, and each of the phases becomes increasing more concentrated in n-heptane and less concentrated in /i-pentane. Of course, when all the liquid has evaporated (L = 0), the vapor will be of the same composition as the initial liquid. Also, the end points of this figure at L = 0 and L = 1 in fact can be computed somewhat more easily from bubble point and dew point calculations, respectively. 

Stability Testing. The boundaries of the humidity range in which an aqueous membrane is stable are determined on the low end by liquid phase evaporation and at... 

The 3-nitropyridine compound (10 mmol) in dimethyl sulfoxide (30 mL) was added dropwise to a stirred solution of 4-amino-l,2,4-triazole (35 mmol) and potasium tert-butoxide (20 mmol) in dimethyl sulfoxide (60 mL) under nitrogen atmosphere. The reaction mixture was stirred for 5 h at room temperature and then poured into water (200 mL) saturated with NH4CI. The aqueous phase was extracted with CH2CI2 (3 X 100 mL), the combined organic phases evaporated, and the residue recrystallized from aqueous methanol to give the 2-amino-5-nitropyridine compound. 

Two methods have been described in the literature for FTIR detection after SFC a flow-cell approach, in which the column effluent is monitored by the FTIR beam as it flows through the cell [21,22,23] and a solvent-elimination approach. With this interface the column effluent is sprayed on to an infrared transport support from a restrictor. The mobile phase evaporates and leaves the analyte as a concentrated spot on the surface, which is later analysed using an FTIR microscope [23]. 

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... 

Amorphous substances have a liquid-like supramolecular stmcture. Like liquid droplets, two amorphous particles in contact with each other tend to adopt a spherical shape. Accordingly, molecules are transported to the contact point between the two particles. The capillary and vapor pressure gradients between the particle volumes and the contact point between the two particles are the driving force for these transport processes. Linked to these local differences in capillary and vapor pressure different molecular transport mechanisms are observed. The molecules can be transported via the surrounding gas phase (evaporation and sublimation), diffusion on the surface (surface diffusion/grain boundary diffusion) or diffusion... 

The size and smface properties of liposomes vary with types of lipids, their compositions, their modification, and methods of preparation. For example, multilamellar vesicles (MLVs) several hundred nanometers in size can be produced by a reverse phase evaporation and extrusion, but smaller unilamellar vesicles (SUVs), whose size is less than 100 mn, can be produced by a sonication process [15]. Further, the membrane state of a bilayer is of primary interest not only for surface treatment but also for recognition of a cell surface and delivery of active ingredients. We will briefly review the microfluidity of bilayers and the interaction of liposomes with a cell surface. 

The physical chemist is very interested in kinetics—in the mechanisms of chemical reactions, the rates of adsorption, dissolution or evaporation, and generally, in time as a variable. As may be imagined, there is a wide spectrum of rate phenomena and in the sophistication achieved in dealing wifli them. In some cases changes in area or in amounts of phases are involved, as in rates of evaporation, condensation, dissolution, precipitation, flocculation, and adsorption and desorption. In other cases surface composition is changing as with reaction in monolayers. The field of catalysis is focused largely on the study of surface reaction mechanisms. Thus, throughout this book, the kinetic aspects of interfacial phenomena are discussed in concert with the associated thermodynamic properties. 

The fonnation of clusters in the gas phase involves condensation of the vapour of the constituents, with the exception of the electrospray source [6], where ion-solvent clusters are produced directly from a liquid solution. For rare gas or molecular clusters, supersonic beams are used to initiate cluster fonnation. For nonvolatile materials, the vapours can be produced in one of several ways including laser vaporization, thennal evaporation and sputtering. 

A drop of a dilute solution (1%) of an amphiphile in a solvent is typically placed on tlie water surface. The solvent evaporates, leaving behind a monolayer of molecules, which can be described as a two-dimensional gas, due to tlie large separation between tlie molecules (figure C2.4.3). The movable barrier pushes tlie molecules at tlie surface closer together, while pressure and area per molecule are recorded. The pressure-area isotlienn yields infonnation about tlie stability of monolayers at tlie water surface, a possible reorientation of tlie molecules in tlie two-dimensional system, phase transitions and changes in tlie confonnation. Wliile being pushed togetlier, tlie layer at... 

Extraction Eiltering limits particulate gravimetry to solid particulate analytes that are easily separated from their matrix. Particulate gravimetry can be extended to the analysis of gas-phase analytes, solutes, and poorly filterable solids if the analyte can be extracted from its matrix with a suitable solvent. After extraction, the solvent can be evaporated and the mass of the extracted analyte determined. Alternatively, the analyte can be determined indirectly by measuring the change in a sample s mass after extracting the analyte. Solid-phase extractions, such as those described in Ghapter 7, also may be used. 

Examination of possible systems for boron isotope separation resulted in the selection of the multistage exchange-distillation of boron trifluoride—dimethyl ether complex, BF3 -0(CH3 )2, as a method for B production (21,22). Isotope fractionation in this process is achieved by the distillation of the complex at reduced pressure, ie, 20 kPa (150 torr), in a tapered cascade of multiplate columns. Although the process involves reflux by evaporation and condensation, the isotope separation is a result of exchange between the Hquid and gaseous phases. 

Based on differences in melting points and Hquid-phase solubilities four modes of operation possible drown-out, isothermal evaporation, adiabatic evaporation, and cooling (choice depends on stream characteristics). 

Droplet Dispersion. The primary feature of the dispersed flow regime is that the spray contains generally spherical droplets. In most practical sprays, the volume fraction of the Hquid droplets in the dispersed region is relatively small compared with the continuous gas phase. Depending on the gas-phase conditions, Hquid droplets can encounter acceleration, deceleration, coUision, coalescence, evaporation, and secondary breakup during thein evolution. Through droplet and gas-phase interaction, turbulence plays a significant role in the redistribution of droplets and spray characteristics. 

Evaporation and Distillation. Steam is used to supply heat to most evaporation (qv) and distillation (qv) processes, such as ia sugar-juice processiag and alcohol distillation. In evaporation, pure solvent is removed and a low volatiUty solute is concentrated. Distillation transfers lower boiling components from the Hquid to the vapor phase. The vapors are then condensed to recover the desired components. In steam distillation, the steam is admitted iato direct coatact with the solutioa to be evaporated and the flow of steam to the condenser is used to transport distillates of low volatiHty. In evaporation of concentrated solutions, there may be substantial boiling poiat elevation. For example, the boiling poiat of an 80% NaOH solution at atmospheric pressure is 226°C. 

Processes involving oxygen and nitrogen oxides as catalysts have been operated commercially using either vapor- or Hquid-phase reactors. The vapor-phase reactors require particularly close control because of the wide explosive limit of dimethyl sulfide in oxygen (1—83.5 vol %) plants in operation use Hquid-phase reactions. Figure 2 is a schematic diagram for the Hquid-phase process. The product stream from the reactor is neutralized with aqueous caustic and is vacuum-evaporated, and the DMSO is dried in a distillation column to obtain the product. 

Preferential deposition of weaMy adsorbed dissolved components on the outer surface of the catalyst body is a similar phenomenon but can occur after impregnation during removal of the solvent. When the impregnated particle is heated, the Hquid phase expands and coats the exterior surfaces with dissolved species as the solvent evaporates. In particularly severe cases, the entire outer surface of the catalyst can become completely coated with a soHd that blocks access to the underlying pore stmcture. 

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