Using Mixed Solvents

This is an oversimplified treatment of the concentration effect that can occur on a thin layer plate when using mixed solvents. Nevertheless, despite the complex nature of the surface that is considered, the treatment is sufficiently representative to disclose that a concentration effect does, indeed, take place. The concentration effect arises from the frontal analysis of the mobile phase which not only provides unique and complex modes of solute interaction and, thus, enhanced selectivity, but also causes the solutes to be concentrated as they pass along the TLC plate. This concentration process will oppose the dilution that results from band dispersion and thus, provides greater sensitivity to the spots close to the solvent front. This concealed concentration process, often not recognized, is another property of TLC development that helps make it so practical and generally useful and often provides unexpected sensitivities. 

While water has been used as a solvent more than any other media, nonaqueous solvents [e.g., acetonitrile, propylene carbonate, dimethylformamide (DMF), dimethyl sulfoxide (DMSO), or methanol] have also frequently been used. Mixed solvents may also be considered for certain applications. Double-distilled water is adequate for most work in aqueous media. Triple-distilled water is often required when trace (stripping) analysis is concerned. Organic solvents often require drying or purification procedures. These and other solvent-related considerations have been reviewed by Mann (3). 

Mel poisoning in isobutylene polymerization with r-BuX/Me3Al systems was studied using mixed solvent systems containing various proportions of MeG and Mel. Results, together with experimental details and materials used are reported in Fig. 2. 

Currently, graphite-based lithium ion batteries use mixed solvent electrolytes containing highly viscous ethylene carbonate (EC) and low viscosity dilutants such as dimethyl carbonate (DMC) or diethyl carbonate (DEC) as main solvents. EC is indispensable because of its excellent filming characteristics. DMC and/or DEC are required to get the low temperature... 

Elderfield and Greaves [629] have described a method for the mass spectromet-ric isotope dilution analysis of rare earth elements in seawater. In this method, the rare earth elements are concentrated from seawater by coprecipitation with ferric hydroxide and separated from other elements and into groups for analysis by anion exchange [630-635] using mixed solvents. Results for synthetic mixtures and standards show that the method is accurate and precise to 1% and blanks are low (e.g., 1() 12 moles La and 10 14 moles Eu). The method has been applied to the determination of nine rare earth elements in a variety of oceanographic samples. Results for North Atlantic Ocean water below the mixed layer are (in 10 12 mol/kg) 13.0 La, 16.8 Ce, 12.8 Nd, 2.67 Sm, 0.644 Eu, 3.41 Gd, 4.78 Dy, 407 Er, and 3.55 Yb, with enrichment of rare earth elements in deep ocean water by a factor of 2 for the light rare earth elements, and a factor of 1.3 for the heavy rare earth elements. 

The SP procedure of water-soluble vitamins from multivitamin tablets is particularly challenging due to the diverse analytes of varied hydrophobicities and pfC. Water-soluble vitamins (WSVs) include ascorbic acid (vitamin C), niacin, niacinamide, pyridoxine (vitamin B ), thiamine (vitamin Bj), folic acid, riboflavin (vitamin B2) and others. While most WSVs are highly water soluble, riboflavin is quite hydrophobic and insoluble in water. Folic acid is acidic while pyridoxine and thiamine are basic. In addition, ascorbic acid is light sensitive and easily oxidized. The extraction strategy employed was a two-step approach using mixed solvents of different polarity and acidity as follows ... 

Other hydrazine solutions using mixed solvent systems (MeOH-EtOH-THF) were susceptible to precipitate formation and did not display adequate swelling properties. 

Commercial solvent dyes for leather are employed in a great variety of solvents. It is common practice to use mixed solvents to obtain the requisite physical properties. They are also applied to correct off-shade dyeings and improve the brilliance of shades. Alcohols or glycols are the most common solvents but esters and ketones are also convenient. The colors are normally applied to dyed or undyed tanned leather by spraying from solvent solution or aqueous/organic emulsions. Nowadays, water-based finish recipes, which are combinations of sol-... 

Mixed Solvents Effect. Using mixed solvents can improve selectivity. For example, adding small amounts of water has improved the selectivity of furfural in separating C4 hydrocarbons (24). Baumgarten and Gerster (25) have studied how various solvents affect the selectivity of furfural for the pentane-pentene pair. They concluded that for only a few solvents some improvement was observed. The resulting selectivity lies between the selectivity of the pure solvents (see Table III). To avoid immiscibility at high solvent concentrations, a second solvent is sometimes added (25). 

Truly quantitative confirmation is not to be expected, even if the model were more rigorous, the reason being that changes in D are usually achieved by using mixed solvents. In such a situation there is no reason to expect the microscopic behavior to be dictated by the single average macroscopic variable D,... 

The fonnulas quoted in this section have been used to estimate absolute rates of a considerable number of outer-sphere electron-transfer reactions. The reactions Rn(OH2)6 +/2+ and Ru(NH3)6 +/2+,3 and Mn04- are particnlarly well-studied examples. The dependence of rate on solvent polarity implied by eqnation (6) has proved more difficult to test and it has only recently become clear that some reactions, such as [Ru(hfac)3]°/, fit the equation well, while others, such as [Mn(NC(C6Hn)6)] + +, do not. The variation in polarity is usually achieved by using mixed solvents in varying proportions, and selective solvation by one component may be one reason for discrepancies. 

The aim of this appendix is to evaluate the sensitivity of the integral in Eq. (12) to the ideality assumption of the molar volume. For this purpose, the composition dependence of (In y )/(V ) for the mixture water/l,4-dioxane at 25 °C was calculated for two cases (1) y = 0 (V being the excess molar volume) and (2) V 0 (the mixture water/1,4-dioxane was selected because it is the most frequently used mixed solvent considered in the present paper). The activity coefficient of water in water/1,4-dioxane mixture was calculated using the Wilson equation with the parameters provided by the Gmehling VLE compilation (Gmehling and Onken, 1977). The molar volume of the mixed solvent was calculated using the expression ... 

In using mixed solvents, it is important to dissolve the cellulose acetate phthalate in the solvent with the greater dissolving power, and then to add the second solvent. Cellulose... 

The device of using mixed solvents is resorted to when drug solubility in one solvent is limited or perhaps when the stability characteristics of soluble salts forbid the use of single solvents. Many pharmaceutical preparations are complex systems. Common water-miscible solvents used in pharmaceutical formulations include glycerol, propylene glycol, ethyl alcohol and polyoxyethylene glycols. As can be imagined, the addition of another component complicates any system and explanations of the often complex solubility patterns... 

EFFECT OF PHASE BEHAVIOUR ON THE RATE OF DIELS-ALDER REACTION USING MIXED SOLVENTS CONTAINING COj ... 

Using mixed solvents for recrystallization - never assume that a mixed solvent system is a mixture of equal volumes of the two solvents. The ratio of the two solvents is established practically during the recrystallization experiment. 

Kumamoto, M., Sonda, T., Takedomi, K., and Tabata, M. 2000. Enhanced separation and elution of catechins in HPLC using mixed-solvents of water, acetonitrile and ethyl acetate as the mobile phase. Anal. Sci. 16 139-44. 

The range of permittivities can be extended by using mixed solvents. Many studies of the behaviour of electrolyte solutions have been carried out in mixed solvents. But it is important to realise that there may be preferential solvation by one of the solvents under these conditions, and this could affect the correctness or otherwise of the interpretations which can be made. 

Bis(indolyl)nitroethanes have been obtained readily in 7-10 min in high yields (70-86%) on fine TLC-grade silica gel (5-40 pm) via Michael reaction of 3-(2 -nitrovinyl) indole with indoles. The same reaction is reported to require 8-14 h for completion at room temperature [86]. Several functionalized resins have been prepared from Merrifield resins via a MW-assisted procedure that used mixed solvent system to facilitate swelling of the resins and coupling with microwaves [87]. These resins can function as solid supports or polymeric scavengers in solid-phase synthesis. 

Our results clearly point out the critical role of sample cleanup in development of an immunoassay for lipophilic compounds. The methods should be simple, rapid, and capable of being exported into the field if the full potential of immunoassays are to be realized. The sample cleanup method we describe here meets these criteria for heptachlor, aldrin and chlordane. However, recovery experiments clearly indicate that the more polar epoxides of these insecticides are retained on the column. Thus, while the antibody binds all cyclodiene insecticides with roughly equal affinity, the cleanup method will not equally present the cyclodienes to the assay. Attempts at using mixed solvent elution systems such as ether/hexane remove the more polar insecticides but also remove more of the starting fat and interfering compound(s). Thus, further efforts in the area of sample cleanup are needed. 

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