Emulsion drawbacks

The shake-flask method has some drawbacks, however. One problem that occurs occasionally is the formation of micro-emulsions which remain stable for hours or days, and prevent the two solvent layers from separating. In this case it is not possible to measure the concentration of analyte in each phase. More generally, the upper and lower range of log D values is limited by problems of solution handling. For very high log D values the sample is much more soluble in octanol than in water. For example, if log D is 4 the sample is 10,000 times more soluble in octa-... 

The drawbacks are that latex has to be coagulated filtered and dried to obtain a solid polymer. There is contamination with soap. But this is the preferred method for obtaining synthetic rubbers which requires a high Molecular weight. By this method emulsion paints are also obtained. 

In the development of a reactive non-chrome post-treatment, a variety of phenolic resins were synthesized and commercial phenolic resins evaluated. It was found that phenol-formaldehyde resins, creso1-forma1dehyd e condensates, ortho-novo 1 ak resins, and phenol-formaldehyde emulsions gave positive results when employed as post-treatments over zinc and iron phosphate conversion coatings. The above materials all possessed drawbacks. The materials in general have poor water solubility at low concentrations used in post-treatment applications and had to be dried and baked in place in order to obtain good performance. The best results were obtained with poly-4-vinylphenol and derivatives thereof as shown in the following structure (8,9,10)... 

A serious drawback of solution polymerization is the need to remove the residual solvent. This problem is also present in emulsion polymerization techniques, where the water must be removed and moreover cleaned. 

Time. It has been said that time will resolve anv emulsion. This is an arguable point, hut it is ptobably true for the tangeof oil field emulsions. Unfortunately, lime is a very expensive c ominoditv. It translates into lai ger vessels, more space and greater heat loss, to mention only a few drawbacks... 

The protocol described here is based on the relationship between the density of the dispersed phase, the density of the continuous phase, the emulsion density, and the volume fraction, which is a measure of the oil droplet concentration. An essential part of the measurement is the precise determination of density. The method introduced below is the most inexpensive method to accurately determine density, but has the drawback that a relatively large sample volume is required. 

One drawback with NMR is that the liquid signal from water in o/w emulsions has to be subtracted to obtain the true SFC. This can be done by analyzing emulsions and fat blends with no tendency to supercooling and making calibration curves. Another possibility is to measure reference samples without fat and calculate the true SFC by subtracting the signal from the water and water-soluble components9. 

The use of an emulsion of PDMS particles is attractive because its low viscosity makes it suitable for preimpregnation of fibers to produce composite materials. A possible drawback is the emulsion stability which has to be controlled in order to avoid particle coalescence. 

Carbonate has one important drawback. When a large amount is used carbonic gas bubbles can form in the emulsion of either film or paper when transferred to an acetic stop bath, resulting in pinholes and/or reticulation. Using a less acid stop bath, such as Kodak SB-1 Nonhardening Stop Bath or a running water bath, should prevent this from occurring. 

The concept of using reactive surfactants in emulsion polymerization processes is relatively recent and aims at eliminating the drawbacks associated with the use of conventional, monomeric, nonreactive surfactants. Despite the demonstrated advantages and the availability of commercial products, reactive surfactants have not yet reached a widespread... 

There is no need for a stirrer as the reacting liquids are mixed by the natural flow resulting from the difference in specific gravities of the components. Because of certain drawbacks inherent in the difficulty of removing the heat of reaction, and as an emulsion may be formed, the method has not found any permanent practical uses. 

An innovative way to perform a sample cleanup that is fast, accurate, and easy, is the use of solid-phase extraction (SPE) columns. In many ways these devices resemble miniature HPLC columns upon which a preseparation is done. Using these minicolumns for sample clean-up eliminates many of the drawbacks associated with traditional liquid-liquid extraction, such as (1) the use of large amounts of expensive organic solvent, (2) low recovery due to solvent emulsion formation, and (3) large requirements for labor, time, glassware, and bench space. 

Another major drawback of classical extractions is that additional clean-up procedures are frequently required before chromatographic analyses. Solid phase extraction (SPE) avoids the emulsion problems often encountered in liquid-liquid extraction. A wide range of adsorbents are commercially available and may be divided into three classes polar, ion-exchange, and nonpolar adsorbents. Solid-supported liquid-liquid extraction on Extrelut columns is frequently reported for efficient cleanup of crude tropane alkaloid mixtures. Basifled aqueous solutions of alkaloids maybe transferred to Extrelut columns and the bases recovered in dichloromethane-isopropanol mixture [13]. 

The first drawback in the use of water (the solubility problem) may be overcome by using surfactants, which solubilize organic materials or form emulsions with them in water. Indeed, surfactants have been occasionally used in organic synthesis [3-6]. A successful example is emulsion polymerization [7]. Some late transition metal-catalysed reactions in water have also been conducted in the presence of surfactants or surfactantlike ligands [8-15]. In many other cases, however, large quantities of surfactant molecules compared with the reaction substrates are needed for the desired reactions to proceed efficiently, and thus, the systems are impractical even if water can be used as a solvent. From the viewpoints of practicability and applicability, the surfactant-aided organic synthesis is still at the preliminary stage. 

The main drawback in the use of water (low solubility of most organic substances in water) would be overcome by using surfactants, which solubilize organic materials or form emulsions with them in water. To address this solubility issue, therefore, we planned to use surfactants, hopefully small amounts of them, for the Lewis acid-catalysed reactions in water. 

A major drawback of conventional microemulsion polymerization is the high surfactant-to-monomer ratio usually needed to form the initial microemulsion. Surfactant can be used more efficiently in semi-continuous or fed polymerization processes. Several polymerization cycles can be run in a short period of time by stepwise addition of new monomer. After each cycle of monomer addition, most of the surfactant is still available to stabilize the growing hydro-phobic polymer particles, or to forms microemulsion again when a polar monomer is used. For instance, in the polymerization of vinyl acetate (VA) by a semi-continuous microemulsion process [21], latexes with a high polymer content of about 30 wt% were obtained at relatively low AOT concentrations of about 1 wt%. Moreover, their particle sizes and molecular weights were much smaller than those obtained by conventional emulsion polymerization. 

Dispersive Liquid-Liquid Microextraction The aforementioned SDME method, although it significantly reduces solvent consumption, is not free from drawbacks such as low extraction efficiency and slowly reached equilibrium. In many cases, the extraction efficiency can be increased by using dispersive systems such as the emulsion of organic solvent in an aqueous sample. In dispersive liquid-liquid microextraction (DLLME), a mixture of two solvents (extraction solvent and disperser) is injected by syringe into an aqueous sample. The extraction solvent is a water-insoluble and nonpolar liquid such as toluene, chloroform, dichloro-methane, carbon tetrachloride, or carbon disulfide. A water-miscible, polar solvent, typically acetonitrile, acetone, isopropanol, or methanol, is used as disperser. The typical concentration of extractant in such a mixture is in the range 1-3 %. 

There are other drawbacks to conventionally fatliquored leather. When it is used in automobile upholstery, it is often heated by sunlight in the unstirred air of the parked vehicle to temperatures high enough to sublime or distill the fat or oil onto the windows. This deposited material is troublesome to remove and causes some lots of leather to be rejected by the automobile manufacmrer. Another drawback is flammability, which can be obviated by a judicious choice of oil in the emulsion. Further, the oil sometimes tends to migrate to the surface of the leather, causing an unpleasant film, or spew, to form. Under some conditions it can cause separation of the hnish or coating from the leather substratum. 

Different methods [120] such as volume variation, internal phase droplet size variation, viscosity variation, density variation, tracer method, Karl-Fischer method, and electrical conductivity have been employed in the measurement of emulsion swelling. The data obtained tends to be as varied as the methods used [120]. One major drawback is that none of the above methods is capable of determining both emulsion swelling and membrane breakage in the same experiment. Further, osmotic swelling cannot be differentiated from entrainment swelling. 

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