Emulsions electrolyte effects

Figure 3 Schematic representation of the influence of surfactants on the curvature of the interface. The hydrophilic parts of the surfactant molecules repel each other sideways, tend to curve the interface around the oil side, and promote O/W emulsions. This effect is most pronounced with long and/or bullg nonionic polar groups or, in the case of ionic surfactants, at low electrolyte content, where the double layers are extended. On the other hand, the mutual repulsion of the hydrophobic parts of the surfactants tend to curve the interface around the water side and promote W/O emulsions. Here, long hydrocarbon tails and/or close packing, as in combinations with cosurfactants, make the effects more pronounced. (After Refs. 11 and 12.)...

C. D. Black and N. D. Popovich. A study of i. v. emulsion compatibility effects of dextrose, amino acids, and selected electrolytes. Drug Intelt. Clin. Phann., 15. 184-193, 1981. 

Polymer Areas Emulsion polymerization. Effective in styrene-butadiene, poly(vinyl chloride), poly(vinyl acetate), acrylic, and styrene-acrylic latexes. Imparts excellent mechanical, thermal and electrolyte stability, low coagulum and small particle latexes. 

The repulsion between oil droplets will be more effective in preventing flocculation Ae greater the thickness of the diffuse layer and the greater the value of 0. the surface potential. These two quantities depend oppositely on the electrolyte concentration, however. The total surface potential should increase with electrolyte concentration, since the absolute excess of anions over cations in the oil phase should increase. On the other hand, the half-thickness of the double layer decreases with increasing electrolyte concentration. The plot of emulsion stability versus electrolyte concentration may thus go through a maximum. 

Fig. XIV-9. Effects of electrolyte on the rate of flocculation of Aerosol MA-stabilized emulsions. (From Ref. 35.)...

For example, van den Tempel [35] reports the results shown in Fig. XIV-9 on the effect of electrolyte concentration on flocculation rates of an O/W emulsion. Note that d ln)ldt (equal to k in the simple theory) increases rapidly with ionic strength, presumably due to the decrease in double-layer half-thickness and perhaps also due to some Stem layer adsorption of positive ions. The preexponential factor in Eq. XIV-7, ko = (8kr/3 ), should have the value of about 10 " cm, but at low electrolyte concentration, the values in the figure are smaller by tenfold or a hundredfold. This reduction may be qualitatively ascribed to charged repulsion. 

The reaction product of sahcylaldehyde and hydroxylamine, sahcylaldoxime, has been found to be effective in photography in the prevention of fogging of silver hahde emulsions on copper supports (96). It also forms the basis for an electrolytic facsimile-recording paper (97) and in combination with a cationic polymer, is used in another electrolytic dry-recording process (98) (see Electrophotography). 

Closely akin to the subject of emulsions is the field of foams, mentioned only in passing. The two fields are similar, in that their properties both depend on surface effects, changes in interfacial tension, electrolyte composition, and manner of preparation. 

The nickel oxide electrode is generally useful for the oxidation of alkanols in a basic electrolyte (Tables 8.3 and 8.4). Reactions are generally carrried out in an undivided cell at constant current and with a stainless steel cathode. Water-soluble primary alcohols give the carboxylic acid in good yields. Water insoluble alcohols are oxidised to the carboxylic acid as an emulsion. Short chain primary alcohols are effectively oxidised at room temperature whereas around 70 is required for the oxidation of long chain or branched chain primary alcohols. The oxidation of secondary alcohols to ketones is carried out in 50 % tert-butanol as solvent [59], y-Lactones, such as 10, can be oxidised to the ketoacid in aqueous sodium hydroxide [59]. 

W/O emulsions are formed by using HLB values between 3.6 and 6, thus suggesting that emulgators that are soluble in the oil phase are generally used. O/W emulsions need HLB values around 8 to 18. This is only a very general observation it must be noted that HLB values alone do not determine emulsion type. Other parameters, such as temperature, properties of the oil phase, and electrolytes in the aqueous phase also have their effect. However, HLB values have no relation to the degree... 

Colloidal suspensions and emulsions acquire their charge owing to preferential adsorption of cations or anions, thus a negative suspension or emulsion will be precipitated on the addition of an electrolyte containing a readily adsorbable cation, the added electrolyte however contains an anion as well, and tends to counteract the effect of the cation, hence the amounts of various electrolytes containing a common cation necessary to produce... 

What is clear, however, is the effect of salt addition on the change in yield stress of the emulsions with time. Those prepared without added electrolyte show a marked decay on storage, over as little as 24 hours, whereas emulsions of salt solutions in oil retain their initial yield values for a number of days. This is attributed to the inhibition of coarsening of the HIPEs, caused by the presence of electrolyte in the aqueous phase the effect is clearly shown in photomicrographs by Aronson and Petko. The stabilisation of emulsions (HIPEs) by salt addition will be discussed in greater detail in the section on HIPE stability. 

Steric stabilization differs from electrostatic stabilization in not being a function of a net force, but of the thickness of an adsorbed layer. When < >, equals 5-10%, stabilizing and destabilizing forces extend beyond the length of the electrostatic, interparticle barrier (Cabane et al., 1989). At this distance, attraction and repulsion are inconsequential, and electrolytes therefore have little effect. Bergenstahl (1988) proposed that the steric stabilization of emulsions by gums in the presence of a surfactant involves adsorption of the gum on the surfactant to form a combined structure constituted by a primary surfactant layer covered by an adsorbed polymer layer. 

This concept is, however, quite simplified and takes no account of the real conformation of the surfactant molecules adsorbed at the interface, which depends on variables such as electrolyte concentration, particularly the temperature or effects of further ingredients. The significance of the temperature in influencing the emulsion type can be illustrated by a system of equal amounts of water and hydrocarbon containing a certain concentration of the surfactant C12E5 (Figure 3.23). 

Generally speaking, for a stable emulsion a densely packed surfactant film is necessary at the interfaces of the water and the oil phase in order to reduce the interfacial tension to a minimum. To this end, the solubility of the surfactant must not be too high in both phases since, if it is increased, the interfacial activity is reduced and the stability of an emulsion breaks down. This process either can be undesirable or can be used specifically to separate an emulsion. The removal of surfactant from the interface can, for example, be achieved by raising the temperature. By this measure, the water solubility of ionic surfactants is increased, the water solubility of non-ionic emulsifiers is decreased whereas its solubility in oil increases. Thus, the packing density of the interfacial film is changed and this can result in a destabilisation of the emulsion. The same effect can happen in the presence of electrolyte which decreases the water solubility mainly of ionic surfactants due to the compression of the electric double layer the emulsion is salted out. Also, other processes can remove surfactant from the water-oil interface - for instance a precipitation of anionic surfactant by cationic surfactant or condensing counterions. 

Numerical Calculations. In what follows, the equations from section II will be employed to calculate the effect of the hydration force on the stability of colloids or emulsions in water in the presence of NaCl and SDS, which represent a common electrolyte and surfactant, respectively. 

The effect of Na+ on the stability of water-in-oil emulsions is exercised mainly through its influence on sodium caseinate. It has been shown that as the surface concentration of casein on oil droplets is increased, the oil-in-water emulsion becomes less susceptible to flocculation/coalescence in the presence of electrolyte. Added NaCl broadens the droplet size distribution at a low casein content (0.25%) but causes this effect at a high casein content (0.5%) only when CaCl2 is added (Dickinson et al., 1984). 

Vakaleris, D.G., Sabharwal, K. 1972. Stability of fluid food emulsions. II. Interacting effects of electrolytes, sodium caseinate and emulsifiers. J. Dairy Sci. 55, 283-288. van den Berg, G. 1982. Developments in buttermaking. Proc. XXI Int. Dairy Congr. (Moscow) 2, 153-159. 

Electrophoresis — Movement of charged particles (e.g., ions, colloidal particles, dispersions of suspended solid particles, emulsions of suspended immiscible liquid droplets) in an electric field. The speed depends on the size of the particle, as well as the -> viscosity, -> dielectric permittivity, and the -> ionic strength of the solution, and it is directly proportional to the applied electric field. In analytical as well as in synthetic chemistry electrophoresis has been employed to separate species based on different speeds attained in an experimental setup. In a typical setup the sample is put onto a mobile phase (dilute electrolyte solution) filled, e.g., into a capillary or soaked into a paper strip. At the ends of the strip connectors to an electrical power supply (providing voltages up to several hundred volts) are placed. Depending on their polarity and mobility the charged particles move to one of the electrodes, according to the attained speed they are sorted and separated. (See also - Tiselius, - electrophoretic effect, - zetapotential). 

The process of expansion of an emulsion film is also quite similar to that of black spots in a foam film at low electrolyte concentrations the spots in the emulsion film expand slowly, at high concentrations the process is very fast (within a second or less) and ends up with the formation of a black film with large contact angle with the bulk phase (meniscus). In the process of transformation of the black spots into a black film, the emulsion film is very sensitive to any external effects (vibrations, temperature variations, etc.) in contrast to the equilibrium black foam film. 

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