Emulsifiers Tween

The microemulsions were identical to those examined in our previous dielectric studies ( l, j2). The emulsifier (Tween and n-pentanol) was mixed with the oil (hexadecane) in the proportion (by weight) 0.594 Tween 0.306 1-pentanol 0.100 hexadecane. Water or electrolyte (0.1 N NaCl for the conductivity measurements) was then added to give the final composition. The physical characterization of this system is given in References 5,11,13, and 14 of our previous paper ( 1 ). In the discussion to follow, the compositional phase volume p is the volume fraction of oil and emulsifier, and is equal to (1 - wg), where w and g are the weight fractions of water and the specific gravity of the microemulsion, respectively. 

The phenomenon of cyclic dimpling was first oserved ° with xylene films intervening between two water droplets in the presence of the noifionic emulsifier Tween 20 or Tween 80 (initially... 

Whipping preparations are used with different background formulas and sometimes contain sucrose, sodium hexametaphosphate, or a polysorbate emulsifier (Tween 60) to improve stability. Many of these preparations can whip to twice the volume... 

Figure 6 Yield of formation of double emulsions of W/OAV and OAV/0 as a function of the w/w ratio of the internal hydrophobic emulsifier, Span 80, and the external hydrophilic emulsifier. Tween 80. (From Ref. 29.)...

The maximum aetivity of the isolated enzyme was observed at 30 °C and pH 6.5 in a buffer system with 5% (v/v) DMSO as a eosolvent. The enzyme was very stable at pH 7.5 and retained full activity after ineubation at 40 °C for 6 h. Interestingly, when the eosolvent DMSO was replaced by an emulsifier (Tween-80, 0.5% w/v) as an alternative modulator to disperse the water-in-soluble substrate, the apparent activity of the epoxide hydrolase significantly increased by 1.8-fold, while the optimum temperature shifted from 30 to 40 °C and the half-life of the enzyme at 50 °C increased by 2.5 times (Figure 2.5). The enzymatic hydrolysis of rac-PGE was highly enantioselective, with an B-value (enantiomeric ratio) of 69.3 in the Tween-80 emulsion system, which is obviously superior than that (41.2) observed in the DMSO-modulated system. ... 

A major problem in LOX assays is adequate dispersion of the fatty acid substrates. Although the usual substrate, linoleic or linolenic acid, forms soluble soaps at pH 9 (optimal for the soybean type-1 enzyme), most LOXs require a pH between 5.5 and 7.0, and in this range fatty acid dispersions are unstable. Ammonium or K" " salts, with or without added emulsifiers (Tween-20, Triton X-100, etc.) are normally used. However, reproducibility of substrate preparations is difficult to achieve, and some variation in apparent substrate concentration may be experienced. In the authors laboratories 5 X 10 M linoleic acid and Triton X-100 (250 /xg/ml) have normally been used in the assay medium. Oxygen consumption (in moles per/minute) is calculated from initial rates, assuming a dissolved oxygen concentration of 240 fiM at 25°C in air-saturated solutions. 

Figure 10.2. Common food emulsifiers Tweens (polyoxyethylene mono fatty aeid esters of sorbitan or sorbitol anhydrides), Spans (partial fatty acid esters and sorbitol anhydrides), lecithin (phosphatidylcholine), monoglycerides (monoaeylglyeerols), propylene glyeol mono fatty aeid esters (1,2-propanediol esters), suerose mono fatty aeid esters, stearyl-2-lactylate. Tween/Span 20, monolaurate Tween/Span 40, monopalmitate Tween/Span 60, monostearate.

For inducing coalescence, o/w emulsions were prepared using the different emulsifiers Tween 20, Tween 80, and Tween 65. The emulsifiers were used at a concentration of 1% m/m (above the CMC). Vegetable oil (Floreal Haagen, Germany) was used as dispersed phase. The hydrophiUc-Upophilic balance (HLB) values of each emulsifier, as well as the characteristics of the emulsions, are shown in Table 1. 

Fig. 8 Creaming during storage left) and freeze-thaw (right). Emulsions stabilized by Tween 20 and Tween 80 are very stable, in contrast to emulsifier Tween 65...

Figure 13.9 gives an example of droplet volume density distributions of emulsions obtained by pressing an emulsion premix through a membrane at transmembrane pressure differences varying from 3 bar to 11 bar. These pressure differences are 7.5- to 27.5-fold the minimum pressure difference required (capillary pressure). A hydrophilic polyamide membrane with a mean pore size of 0.8 pm was used. The emulsion premix consisted of 20% dispersed phase (vegetable oil). As continuous phase water containing emulsifier Tween 80 at a concentration of 2% was used. The Sauter diameter of the emulsion premix was X3,2 = 25 pm. 

In the example given in Figure 13.11, polyamide membranes with different mean pore sizes were used (0.2 pm, 0.45 pm, 0.8 pm). A small disperse fraction of 5%, and a high emulsifier concentration (2% of emulsifier Tween 80) was chosen in order to avoid coalescence. Figure 13.11 gives Sauter diameters of the emulsions produced with membranes of different mean pore sizes after the first, second and third pass at 12 bar through the membrane. The Sauter diameter of the emulsion premix was 12 pm. 

Influence of Emulsifier Concentration Emulsions of small droplet sizes and narrow droplet size distributions can be obtained at high emulsifier concentration. Figure 13.12 shows the volume density distributions of emulsions of a disperse phase fraction of cp = 72% and two different emulsifier (Tween 80) concentrations, 2.4% and 4.6%. The production parameters were trans-membrane pressure difference of 12 bar, three passes and membrane mean pore size of 0.8 pm. In both cases the emulsifier concentration is above the critical micelle concentration (CMC). 

Figure 13.12 Volume density distributions for emulsions with different emulsifier concentrations. Disperse phase fraction

This phenomenon is driven by surfactant transfer fi-om the continuous phase toward the droplets. It manifests itself as a cyclic dimpling of emulsion films. The phenomenon was first observed [490] with aqueous films between two xylene droplets in the presence of the nonionic emulsifier Tween 20 or Tween 80 (initially dissolved in water, but also soluble in oil). The same phenomenon has been observed also with other emulsion systems [491]... 

Notation A-potato starch, B-glycerol, E-ether BRI) 35, S-emulsifier Tween 20, W-H2O... 

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