Solvent, selection

Solvents, like contaminants, may be polar or nonpolar. As ageneral rule, polar solvents dissolve polar residues while nonpolar solvents dissolve nonpolar residues. Thus, ionie residues such as chlorides, salts, acids, acid fluxes, and alkalis are best dissolved and removed with polar solvents such as water, isopropyl alcohol, ethanol, or methylethyl ketone. Greases, oils, silicones, rosin flux, and low-molecular-weight monomers are best dissolved and removed with solvents such as hydrocarbons, Freons , hydrochloro-fluorocarbons, xylene, terpenes, and naphtha. To remove both polar and 

Organic solvents are most efifective as blends of polar and nonpolar solvents that have a constant boiling point known as azeotropes. Azeotropes are mixtures of two or more solvents used primarily in vapordegreasing equipment. 

and HFE solvents are available as azeotropes with alcohols such as ethanol. Besides their effectiveness as solvents, these hydrohalocarbons are desirable because oftheir low toxicity, nonflammability, heat stability, and inertness to components and materials on circuit-board assemblies. Examples of azeotropes include DuPont sHFC 43-lOmee (2,3-dihydrodecafluoropentane) and 3M s HFE-7100 (methylnonafluorobutyl ether) both of which are comparable or better than CFC-113. 

Lenium ES/ Petrofemi n-propyl bromide, isopropyl alcohol, and stabilizers Vapor degreasing, deiluxing, or general precision cleaning 150 68 25.9 58.8 None 110.8 

Megasolv NOC/ Petroferm Propoxy propsuiol, methoxy and method ethyloxy propanol, and N-methyl pyrrol idone Room temperature cleaning of electronic assemblies, surface-mount adhesive cleaning (misprints and stencils). Used in pressure spray, immersion, and ultrasonic applications. N/A 180-202 Low N/A 82 1 (20°C) 

The ideal solvent should contain no protons and be inert, low boiling, and inexpensive. Deuterated solvents are necessary for modem instruments because they depend on a deuterium signal to lock or stabilize the B0 field of the magnet. Instruments have a deuterium channel that constantly monitors and adjusts (locks) the B ) field to the frequency of the deuterated solvent. Typically, H NMR signals are in the order of 0.1 to several Hz wide out of 300,000,000 Hz (for a 300 MHz system), so the B ) field needs to be very stable and homogeneous. 

The deuterium signal is also used to shim the B0 field. Instruments use small electromagnets (called shims) to bend the main magnetic field (Bu) so that the homogeneity of the field is precise at the center of the sample. Most modern instruments have approximately 20-30 electromagnetic shims they are computer controlled, and can be adjusted in an automated manner. 

Deuterated chloroform (CDC13) is used whenever circumstances permit—in fact most of the time. The small sharp proton peak at 8 7.26 from the CHC13 impurity present rarely interferes seriously. For very dilute samples, CDC13 can be obtained in 100% isotope purity. A list of common, commercially available 

Traces of common laboratory solvents can be annoying. See Appendix H for an extensive list of common solvent impurities. Other offenders are greases and plasticizers (phthalates in particular). NMR solvents should be kept in a desiccator. 

Only a single proton peak should be expected from the interaction of rf energy and a strong magnetic field on all of the protons in accordance with the basic NMR equation (Section 3.2.2)  

Proper selection of a solvent for preparing lignin solutions is of great importance in ultraviolet spectroscopy since only true solutions yield maximum absorptivity 

organosolv lignins Lignosulfonates Milled wood lignins Lignin model compounds Dimethylformamide, 2-methoxyethanol/water (8 2, v/v) Water, ethanol/water (2 8, v/v) Dimethylformamide, ethanol/water (8 2, v/v), 2-methoxyethanol/water (8 2, v/v) Water, cyclohexane, ethanol, 2-methoxyethanol/water (8 2, v/v) 

Lignin and lignin-related model compounds are subject to air oxidation and photodegradation, particularly in a dilute, alkaline solution. It is strongly recommended that a fresh solution of the sample be prepared immediately before making spectral measurements. The following precautions should be taken in the preparation of solutions  

Dissolve the lignin and lignin model compounds in a good solvent at room temperature do not heat the solution to dissolve the sample. 

Avoid exposing the solution to strong daylight or ultraviolet light flush the solution with nitrogen if possible. 

Water is almost exclusively used as the solvent for the industrial crystallization of inorganic substances from solution. This fact is quite understandable because, apart from the relative ease with which a very large number of chemical compounds dissolve in it, water is readily available, cheap and innocuous. For these reasons water is used whenever possible even for the industrial crystallization of organic compounds, although for a variety of reasons other solvents may have to be used in this particular field. 

The selection of the best solvent for a given crystallization operation is not always an easy matter. Many factors must be considered and some compromise must inevitably be made several undesirable characteristics may have to be accepted to secure the aid of one important solvent property. There are several 

A mixture of two or more solvents will occasionally be found to possess the best properties for a particular crystallization purpose. Common binary solvent mixtures that have proved useful include alcohols with water, ketones, ethers, chlorinated hydrocarbons or benzene homologues, etc. and normal alkanes with chlorinated hydrocarbons or aromatic hydrocarbons. 

A second liquid is sometimes added to a solution to reduce the solubility of the solute, cause its precipitation/crystallization and maximize the yield of product. It is necessary, of course, for the two liquids (the original solvent and the added precipitant) to be completely miscible with one another in all proportions. The process is commonly encountered, for instance, in the crystallization of organic substances from water-miscible organic solvents by the controlled addition of water. The term watering-out is often used in this connection. This approach is also used to reduce the solubility of an inorganic salt in aqueous solution by the addition of a water-miscible organic solvent in salting-out precipitation processes (section 7.2.5). 

Based on the nature of their intermolecular bonding interactions solvents may be conveniently divided into three main classes  

Efforts have also been made to correlate solvent properties, namely hydrophobic-ity, dipole moment, and dielectric constant with enanhoselectivity, but such correlations have only been demonstrated in some systems [31-33]. 

Overall biotransformation rate in biphasic systems depends on both mass transfer and biochemical reaction. In this section, we focus on the parameters 

Once the reagents are selected, compatible solvents are chosen. Some solvents avoided for scale-up are shown in Table 2.2. 1,2-Dichloroethane, a Class 1 solvent with toxicity similar to benzene, should be avoided. Solvents more commonly used are shown in Table 2.3. Characteristics considered include toxicity, polarity, boihng point, freezing point, miscibihty with water, and ability to remove water (and other compounds) by 

Hexane Electrostatic discharge neurological toxicity Heptane 

CHCI3 Mutagenicity, toxicity (COCI2), environmental CH2CI2 PhCHa with CH3CN, n-BuOH or DMF 

Water and carbon dioxide are the most used solvents due to their low price and environmental friendliness. The critical temperature of water is 473 K and it is used for reactions under extreme conditions.Carbon dioxide, on the other hand, presents a very low critical temperature and it is adequate for reactions carried out under mild conditions, for example, selective hydrogenations. Unfortunately, it is well known that CO2 is not a good solvent for high molar mass organic compounds. Liquid carbon dioxide is miscible with alkanes with up to approximately 10 carbon atoms, while the range of miscibility increases for ethane up to 18 carbon atoms, and for propane up to 30 carbon atoms. Thus, the application of CO2 as reaction media is limited to low molar mass hydrocarbons if a homogenous operation is desired, while ethane and propane are a better option for higher molar mass hydrocarbons. 

The phase behaviour of the binary mixtures between the potential solvents and the system components (reactants and reaction products) should be studied first later the study should be extended to the multicomponent reactive mixtures for definition of feasible operating regions. 

The vapour-hquid-hquid equilibrium lines are limited by the upper critical (UCEP) and the lower critical (LCEP) end points, where a hquid phase becomes identical to the vapour phase or two liquids become identical and merge into one phase. Each type of phase diagram could be recognized by its UCEP and LCEP. For instance, type 1 has neither UCEP nor LCEP type 1 / and l AlnQ have only one UCEP (in the case of type l AlnQ the UCEP is above the light component critical temperature) type 2 1 is the only system with two UCEP and one LCEP and type 2 has one UCEP and one LCEP. 

Molecular size asymmetry is always present in gas-liquid reactions performed under supercritical media. It is important to have prior knowledge of the potential type of phase behaviour that a system can present. In general, type l AlnQ phase behaviour should be avoided because it presents liquid-liquid immiscibility even at extremely high pressures. In contrast, if a type 2 phase behaviour is found, the region of partial hquid miscibility can be avoided by increasing the pressure to reach an homogenous region. Peters et present 

This section deals with solvent selection in general terms with respect to the basic principles on which the choice is based. Solvents and their effects are dealt with in more detail in the chapters dealing with the applications of solvent based coatings. 

The principle factors affecting solvent selection for a specific polymer and end use are cost, solvent power, evaporation rate, flash point, toxicity and customer requirements. 

The popular, cheap solvents are hydrocarbons such as xylene, alcohols such as the isomers of butanol and propanol, and ketones such as methyl ethyl ketone and methyl isobutyl ketone. The more expensive solvents used tend to be esters such as acetates, ethers and ethoxylated ethers such as butyl glycol (butoxy ethanol). Other speciality solvents are also occasionally encountered, but generally the quantities used are small due to the cost factor. 

The evaporation rate of the solvent is an important factor in some film forming mechanisms. In general, the evaporation rate depends upon the vapour pressure of the solvent which will decrease with increasing boiling point. Solvents generally evaporate more slowly than their vapour pressure would suggest 

The flammability of a solvent is normally measured by its flash point which is defined as the temperature at which the mixture of air and solvent vapour above the solvent will just ignite. There are two distinct types of apparatus used to measure this parameter. One is a closed cup, which contains the vapour as it evaporates, and the other is an open cup where the vapour is free to escape into the atmosphere. Both methods are frequently employed and it is normal to quote the method used when recording the results of flash point determinations because different values result from the different methods. 

This section is designed to give guidance to mobile phase selection from first principles. The choice of mobile phase composition is critical because of the widely differing selectivities which can be obtained. In certain cases it may be that trivial factors such as availability and expense govern the final selection. 

The molecular formula of the solute may suggest the type of solvent which maybe selective for its extraction, based on probable affinities between related functional groups. Thus, to extract organic acids or alcohols from water, an ester, ether, or ketone (of sufficient molecular weight to have very limited solubility in the aqueous phase) might be chosen as the solvent. The pH of aqueous phase feeds may also be very important. The sodium or potassium salts of an organic salt may well prefer the aqueous media at pH 10, but in the acidulated form may readily extract into the organic phase if the pH is low. 

Specific factors taken into consideration in the selection of a solvent include  

51e/ecftv/Yy-the ability to remove and concentrate the solute from the other components likely present in the feed liquor. 

Availability-the inventory of solvent in the extraction system can represent a significant capital investment. 

Immiscibilitywiihihe feed-otherwise there will need to be recovery of the solvent from the raffinate, or a continual and costly replacement of solvent as make up. 

There are two principal ways in which solvent selection can influence waste minimisation  

The science of solvent selection has not yet developed to the state where a simple set of rules or a selection flowchart can be provided to give an optimum choice. This chapter seeks to address the principles involved in determining the performance of the solvent at the reaction stage. It will provide an outline of the properties of solvents relevant to the solvation of solutes and reactive intermediates, and show how these relate to reaction rate and selectivity. Solvent-dependent regioselectivity effects are due to selective solvation of incipient reactive sites on a multidentate reactant, and some general predictive principles are available. An outline of the scope and mechanistic principles of two-phase reaction systems is presented, and their potential for providing simpler solvent recycle is emphasised. An alternative approach to easier solvent recovery via the use of volatile inorganic solvents is also discussed. 

The importance of including a consideration of workup aspects early in the process design is emphasised. 

2 Solvent effects on reaction rates - the transition state approach 

Parker and Cox [1] measured the rate of a simple nucleophilic substitution reaction (equation 12.1) in a variety of solvents. Relative rates are shown in Table 12.1. 

One of the biggest challenges in oxidation catalysis is the development of clean technologies that can operate without standard laboratory organic solvents [120]. Here, we describe some of the solutions to this problem. 

The Delaney amendment to the Food and Drug Act prohibits the use in food production of materials that exhibit any evidence of carcinogeniciry. Therefore, evea though great efforts are made to reduce solvent 

The use of alcohols and alcohol-water mixtures for extracting vegetable oil has attracted attention recently. These solvents can ptovida greater selectivity than hexane, which is currently used for most vegetable oil extractions. Alcohols and alcohol-water mixtures can also be separated from extracted oil more madjly and with less expenditure of energy. 

As is usually the case, it is desirable for solvants to be cheap, noncorrosive, annflammable, tionexpto-sive, nontoxic, ensily removable, and easily recoverable. In the case of heterogeneous extractions, the external solvent and the occluded solvent must be immiscible. In soma cases the solvent should be selective bet when extracts with belauced flavors are desired, selectivity with respect to important flavor components is undesirable. It obviously may be impossible to meet all these objectives. 

If possible, flammable solvents should be used at lempa ratines slightly below their hoiling point. This prevants pressure-induced ontleakage of solvent vapor air inleakage yields air-vapor mixtures that are above the upper flammable limit. Automatic flame and explosion arrestor systems should be used whenever highly flammable or explosive solvents are used. 

Carbon tetrachloride Dipentyl ether Methylene chloride 

From the theoretical equations presented in the foregoing section, we note that BEF is a function of the nature of the solvent used, as well as the ratio a of its volume to that of water. It is always tempting to formulate rules for selecting the best solvent from both of these points of view. This is seldom possible, however, and resort to trial-and-error selection seems unavoidable. Even so, some useful guidelines can be suggested. 

An important criterion is the solvent s capacity for solvophobic interactions, of which hydrophobic interactions are a specific case (Ray, 1971 see also Cramer, 1977 Klibanov et al., 1978 Tanford, 1978 Kauzmann, 1979 Hildebrand, 1979). According to this criterion, solvents can be grouped in the following three classes in decreasing order of solvolytic interactions, that is, of biphasic effectiveness  

Class 1 Water, glycerol, ethylene glycol, amino ethanol, and formamide Class 2 Methyl formamide and dimethyl formamide Class 3 Methanol, ethanol, and toluene 

A weak, negative correlation exists between the polarity of the solvent and the retention of activity by the enzymes. This indicates that the less polar the solvent, the more suitable it would be for the biphasic system. 

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Precautions, (i) The above tests must be carried out with discretion. If the substance is only moderately soluble in the solvent selected, and a comparatively large volume of the latter is required, the consequent dilution of the acid in the reagent may cause the separation of the free 2,4 dinitrophenylhydrazine (although this is more likely to happen with Reagent B than with A). Furthermore, if the compound under investigation should have basic properties, the neutralisation of part of the acid in the reagent may have the same result. 

In the separations (2) and (3) above, it is often advisable to dissolve the original mixture in a water-insoluble solvent. Select a solvent which will dissolve the entire mixture, and then shake the solution with either (i) dil. NaOH or (ii) dil. HCl. Separate the aqueous layer, and to it add either (i) dil. HCl or (ii) dil. NaOH to liberate the organic acid or the organic base, as the case may be. The non-aqueous layer now contains the neutral component. Reextract this layer with either (i) dil. NaOH or (ii) dil. HCl to ensure removal of traces of the non-neutral component. 

Physical Equilibria and Solvent Selection. In order for two separate Hquid phases to exist in equiHbrium, there must be a considerable degree of thermodynamically nonideal behavior. If the Gibbs free energy, G, of a mixture of two solutions exceeds the energies of the initial solutions, mixing does not occur and the system remains in two phases. Eor the binary system containing only components A and B, the condition (22) for the formation of two phases is... 

Selectivity. Solvent selectivity is intimately linked to the purity of the recovered extract, and obtaining a purer extract can reduce the number and cost of subsequent separation and purification operations. In aqueous extractions pH gives only limited control over selectivity greater control can be exercised using organic solvents. Use of mixed solvents, for example short-chain alcohols admixed with water to give a wide range of compositions, can be beneficial in this respect (6). 

UCAR Solvents Selection Guide for Coatings, Brochure F-7465 y Union Carbide Corp. 

Industrial solvent appHcations are broad, varied, and complex and each has its own set of characteristics and requirements. Proper solvent selection and blend development have a large impact on the success of the operation in which the solvent is used, from the perspectives of economic effects, technical adequacy, safety issues, and environmental impacts. 

Practical Solubility Concepts. Solution theory can provide a convenient, effective framework for solvent selection and blend formulation (3). When a solute dissolves in a solvent, a change in free energy occurs as a result of solvent—solute interactions. The change in free energy of mixing must be negative for dissolution to occur. In equation 1,... 

Solvent Selection. A thorough knowledge of the requkements of each solvent appHcation is necessary to formulate a solvent system successfully and meet all needs at the lowest possible cost. The most important properties are solvency, evaporation rate, flash poiat, and solvent balance. In nearly every appHcation, these properties are important even though the specific requkements differ greatly from one appHcation to another. Each potential solvent has a particular set of properties, and the solvent chosen and the amount of each depend on the specific appHcation requkements. 

Agricultural Products. Pesticides are frequendy appHed as emulsiftable concentrates. The active insecticide or herbicide is dissolved in a hydrocarbon solvent which also contains an emulsifier. Hydrocarbon solvent selection is critical for this appHcation. It can seriously impact the efficacy of the formulation. The solvent should have adequate solvency for the pesticide, promote good dispersion when diluted with water, and have a dash point high enough to minimise dammabiUty ha2ards. When used in herbicide formulas, low solvent phytotoxicity is important to avoid crop damage. Hydrocarbon solvents used in post-harvest appHcation require special testing to ensure that polycycHc aromatics are absent. 

The problem of solvent selection is most difficult for high molecular-weight polymers such as thermoplastic acryHcs and nitrocellulose in lacquers. As molecular weight decreases, the range of solvents in which resins are soluble broadens. Even though solubihty parameters are inadequate for predicting ah. solubhities, they can be useful in performing computer calculations to determine possible solvent mixtures as replacements for a solvent mixture that is known to be satisfactory for a formulation. 

Final adjustment of solvent selection must be done under actual field conditions. In many baking coatings a significant fraction of the solvent is lost in the baking oven, yet the tables of relative evaporation rates are based on 25°C air. Information on evaporation rates for a small number of solvents as a function of temperature up to 150°C has been published (73). 

The most common method for screening potential extractive solvents is to use gas—hquid chromatography (qv) to determine the infinite-dilution selectivity of the components to be separated in the presence of the various solvent candidates (71,72). The selectivity or separation factor is the relative volatihty of the components to be separated (see eq. 3) in the presence of a solvent divided by the relative volatihty of the same components at the same composition without the solvent present. A potential solvent can be examined in as htfle as 1—2 hours using this method. The tested solvents are then ranked in order of infinite-dilution selectivities, the larger values signify the better solvents. Eavorable solvents selected by this method may in fact form azeotropes that render the desired separation infeasible. 

Extractive distillation works by the exploitation of the selective solvent-induced enhancements or moderations of the liquid-phase nonidealities of the components to be separated. The solvent selectively alters the activity coefficients of the components being separated. To do this, a high concentration of solvent is necessaiy. Several features are essential ... 

Solvent selection ana screening approaches can be divided into two levels of analysis. The first level focuses on identification of functional groups or chemical famihes that are hkely to give favorable solvent-key component molecular interactions. The second level of analysis identifies and compares individual-candidate solvents. The various methods of analysis are described briefly and illustrated with an example of choosing a solvent for the methanol-acetone separation. 

Deviations from Raonlt s law in solution behavior have been attributed to many charac teristics such as molecular size and shape, but the strongest deviations appear to be due to hydrogen bonding and electron donor-acceptor interac tions. Robbins [Chem. Eng. Prog., 76(10), 58 (1980)] presented a table of these interactions. Table 15-4, that provides a qualitative guide to solvent selection for hqnid-hqnid extraction, extractive distillation, azeotropic distillation, or even solvent crystallization. The ac tivity coefficient in the liquid phase is common to all these separation processes. 

Choice of Solvent The solvent selected will offer the best balance of a number of desirable characteristics high saturation hmit and selec tivity for the solute to be extrac ted, capability to produce... 

Temperature The temperature of the extraction should be chosen for the best balance of solubility, solvent-vapor pressure, solute diffusivity, solvent selectivity, and sensitivity of product. In some cases, temperature sensitivity of materials of construction to corrosion or erosion attack may be significant. 

Solvent Selection The choice of a particular solvent is most important. Frequently, water is used, as it is very inexpensive and plentiful, but the following properties must also be considered ... 

Note that H is simply Henry s constant corrected for units. When the solute gas is readily soluble in the liquid solvent, Henry s law constant (H or H ) is small and Kj approximately equals k, and the absorption process is controlled by the gas film resistance. For systems where the solute is relatively insoluble in the liquid, H is large and K( approximately equals k, and the absorption rate is controlled by the liquid phase resistance. In most systems, the solute has a high solubility in the solvent selected, resulting in the system being gas film resistance controlled. 

Odele, O., and Macchietto, S. (1993). Computer aided molecular design A novel method for optimal solvent selection. Fluid Phase Equilibria, 82,47-54. 

Solvent selection and conversion chart for Waters Styragel columns. (Courtesy of... 

Successful recrystallization of an impure solid is usually a function of solvent selection. The ideal solvent, of course, dissolves a large amount of the compound at the boiling point but very little at a lower temperature. Such a solvent or solvent mixture must exist (one feels) for the compound at hand, but its identification may necessitate a laborious trial and error search. Solvent polarity and boiling point are probably the most important factors in selection. Benzhydrol, for example, is only slightly soluble in 30-60 petroleum ether at the boiling point but readily dissolves in 60-90° petroleum ether at the boiling point. 

A. Wierzbicki, and J. H. Davis, Jr., Proceedings of the Symposium on Advances in Solvent Selection and Substitution for Extraction, March 5-9, 2000, Atlanta, Georgia. AIChE, New York, 2000. 

Depending on the chemical structure of the MAI, a suitable solvent is sometimes needed to get a homogenous state of reaction mixture. Even if using the same combination of comonomers, for example, to prepare PMMA-b-poly(butyl acrylate) (PBA), the selection of the using order of comonomers for the first step or second step would affect the solvent selections, since PMMA is not easily soluble to BA monomer, while PBA is soluble to MM A monomer [28]. 

Cobalt, sepn. of from nickel, (cm) 532 Codeine and morphine, D. of 740 Coefficient of variation 135 Colloidal state 418 See also Lyophilic, Lyophobic Colorimeters light filters for, 661 photoelectric, 645, 666 Colorimetric analysis 645 criteria for, 672 general remarks on, 645, 672 procedure, 675 solvent selection, 674 titration, 652... 

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