Solubility, balancing

Effects of Surfactants on Solutions. A surfactant changes the properties of a solvent in which it is dissolved to a much greater extent than is expected from its concentration effects. This marked effect is the result of adsorption at the solution s interfaces, orientation of the adsorbed surfactant ions or molecules, micelle formation in the bulk of the solution, and orientation of the surfactant ions or molecules in the micelles, which are caused by the amphipathic structure of a surfactant molecule. The magnitude of these effects depends to a large extent on the solubility balance of the molecule. An efficient surfactant is usually relatively insoluble as individual ions or molecules in the bulk of a solution, eg, 10-2 to 10-4 mol/L. 

Other components of the design [138—142] include the choice of the crown size or even the use of open-chain metalloclefts, not necessarily polyether armed. The issue of special importance for extraction and transport applications is the selection of substituents ensuring a necessary lipo-philicity-solubility balance. For example, use of a cyclohexano moiety as shown above, instead of the more common o-phenylene bridge between nitrogens, enhances solubility in the membrane phase. The modification of the polyether chain with binaphthyl or calixarene substituents provides high membrane transport rates due to increased ionophore lipophilicity [138,142]. Some representative examples (initial fluxes, in 10 mol cm h , through o-nitrophenyloctyl ether-impregnated Accurel membrane 1 M source urea [138]) are as follows. 

Further, the ability to synthesize random copolymers with various hydrocarbon monomers allows the anchor-soluble balance to be tuned while maintaining solubility even with high incorporations of hydrocarbon comonomers [29]. Because of the amphiphilic nature of such copolymers, it was predicted that these materials would selfassemble into micelles consisting of a highly fluorinated corona segregating the lipophilic core from the compressed CO2 continuous phase. Thus, PFOA-F-PS block copolymers were synthesized via controlled free-radical techniques (Fig. 9.3), and it was confirmed (by smaU-angle neutron scattering) that these copolymers spontaneously assemble into multimolecular micelles in solution [40]. In addition to amphiphilic materials, which physically adsorb to the surface of polymer particles in dispersion polymerizations, fluorinated acrylates can be utihzed as polymerizable comonomers in the stabilization of C02-phobic polymer colloids [41]. 

An unknown value x is assigned to the mass flow rate of the solubles in the outflow of washed solids from the final stage (the system underflow)— see Figure 15.14. This unknown x will carry through the system and appear as a factor in the solubles balance around all stages in the system. The difference... 

Although the data for the silver halides suggest that silver(I) fluoride is likely to be more soluble than the other silver halides (which is in fact the case), the hydration enthalpies for the sodium halides almost exactly balance the lattice energies. What then is the driving force which makes these salts soluble, and which indeed must be responsible for the solution process where this is endothermic We have seen on p. 66 the relationship AG = — TAS and... 

A precipitation reaction occurs when two or more soluble species combine to form an insoluble product that we call a precipitate. The most common precipitation reaction is a metathesis reaction, in which two soluble ionic compounds exchange parts. When a solution of lead nitrate is added to a solution of potassium chloride, for example, a precipitate of lead chloride forms. We usually write the balanced reaction as a net ionic equation, in which only the precipitate and those ions involved in the reaction are included. Thus, the precipitation of PbCl2 is written as... 

Gravimetric methods based on precipitation or volatilization reactions require that the analyte, or some other species in the sample, participate in a chemical reaction producing a change in physical state. For example, in direct precipitation gravimetry, a soluble analyte is converted to an insoluble form that precipitates from solution. In some situations, however, the analyte is already present in a form that may be readily separated from its liquid, gas, or solid matrix. When such a separation is possible, the analyte s mass can be directly determined with an appropriate balance. In this section the application of particulate gravimetry is briefly considered. 

Emulsifiers are classified by the hydrophilic—lipophilic balance (HLB) system. This system indicates whether an emulsifier is more soluble in water or oil, and for which type of emulsion (water-in-oil or oil-in-water) it is best suited. Emulsifiers having alow HLB value are more oil soluble, and are better suited for water-in-oil appHcations such as margarine. Conversely, emulsifiers having a high HLB value are more water soluble, and function more effectively in oil-in-water emulsions such as ice cream (34). The use of this system is somewhat limited because the properties of emulsifiers are modified by the presence of other ingredients and different combinations of emulsifiers are needed to achieve a desired effect. The HLB values of some common emulsifiers are given (35). 

An a priori method for choosing a surfactant was attempted by several researchers (50) using the hydroph i1 e—1 ip oph i1 e balance or HLB system (51). In the HLB system a surfactant soluble in oil has a value of 1 and a surfactant soluble in water has a value of 20. Optimum HLB values have been reported for latices made from styrene, vinyl acetate, methyl methacrylate, ethyl acrylate, acrylonitrile, and their copolymers and range from 11 to 18. The HLB system has been criticized as being imprecise (52). 

Although p oly (a-olefin) s (PAO) and esters are the prominent synthetic base stocks for automotive appfications, combinations of the two are becoming the choice in offering a balance of properties such as additive solubility, sludge control, and elastomer compatibility (34). 

Emulsifiers. Removing the remover is just as important as removing the finish. For water rinse removers, a detergent that is compatible with the remover formula must be selected. Many organic solvents used in removers are not water soluble, so emulsifiers are often added (see Emulsions). Anionic types such as alkyl aryl sulfonates or tolyl fatty acid salts are used. In other appHcations, nonionic surfactants are preferred and hydrophilic—lipophilic balance is an important consideration. 

Ethoxylation of alkyl amine ethoxylates is an economical route to obtain the variety of properties required by numerous and sometimes smaH-volume industrial uses of cationic surfactants. Commercial amine ethoxylates shown in Tables 27 and 28 are derived from linear alkyl amines, ahphatic /-alkyl amines, and rosin (dehydroabietyl) amines. Despite the variety of chemical stmctures, the amine ethoxylates tend to have similar properties. In general, they are yellow or amber Hquids or yellowish low melting soHds. Specific gravity at room temperature ranges from 0.9 to 1.15, and they are soluble in acidic media. Higher ethoxylation promotes solubiUty in neutral and alkaline media. The lower ethoxylates form insoluble salts with fatty acids and other anionic surfactants. Salts of higher ethoxylates are soluble, however. Oil solubiUty decreases with increasing ethylene oxide content but many ethoxylates with a fairly even hydrophilic—hydrophobic balance show appreciable oil solubiUty and are used as solutes in the oil phase. 

Di- and Triisobutylcncs. Diisobutylene [18923-87-0] and tnisobutylenes are prepared by heating the sulfuric acid extract of isobutylene from a separation process to about 90°C. A 90% yield containing 80% dimers and 20% trimers results. Use centers on the dimer, CgH, a mixture of 2,4,4-trimethylpentene-1 and -2. Most of the dimer-trimer mixture is added to the gasoline pool as an octane improver. The balance is used for alkylation of phenols to yield octylphenol, which in turn is ethoxylated or condensed with formaldehyde. The water-soluble ethoxylated phenols are used as surface-active agents in textiles, paints, caulks, and sealants (see Alkylphenols). 

The hydrophile—hpophile balance (HLB) is an empirical system based on the fact that oil—water (o/w) emulsions are best stabilized by water-soluble-emulsifiers and water—oil (w/o) emulsions are best stabilized by oil-soluble ones (3). The HLB scale mns from 0—20 and is based on the ratio of the saponification number of ester, A, to the acid number of recovered acid, where HLB = 20 1-Sj A). The dispersibiUty of an emulsifier in water is related to HLB value. 

In most cases, these active defoaming components are insoluble in the defoamer formulation as weU as in the foaming media, but there are cases which function by the inverted cloud-point mechanism (3). These products are soluble at low temperature and precipitate when the temperature is raised. When precipitated, these defoamer—surfactants function as defoamers when dissolved, they may act as foam stabilizers. Examples of this type are the block polymers of poly(ethylene oxide) and poly(propylene oxide) and other low HLB (hydrophilic—lipophilic balance) nonionic surfactants. 

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