Influence of the Emulsifier

The choice of the emulsifiers and their concentration has a great impact on the quality of the SLN dispersion [13,30]. Siekmann and Westesen determined that 2 w% tyloxapol was insufficient to stabilize a 10 w% tripalmitin dispersion. Increasing the tyloxapol concentration to 10 w% resulted in 85-nm particles with unimodal size distribution [27], 

However, it is not recommended that one use rapidly distributing surfactants exclusively because surfactant mixtures (e.g., Lipoid S75/poloxamer 188 [30] or tyloxapol/lecithin [27]) might lead to lower particle sizes and higher storage stability compared with formulations with only one surfactant. The addition of sodium glycocholate to the aqueous phase as coemulsifying agent decreases the particle size, too [27], 

Different emulsifier compositions might require different homogenization parameters. For example, the maximal degree of dispersing was obtained with 500 bar and 3 cycles for poloxamer 188-stabilized systems [38], Homogenization with pressures of 1000 or 1500 bar did not result in further reduction of the particle size. In contrast, pressures of 1500 bar proved to be the best for lecithin- (Lipoid S75) stabilized systems. A possible explanation for this observation is the different velocity of the coverage of the new lipid surfaces. 

The choice of emulsifiers might also influence the crystallization behavior. For example, glycocholate was able to preserve the alpha polymorph of the lipid tripalmitin for long times, whereas transitions to more stable polymorphs occurred more rapidly for the other surfactants [40], 

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Sjostrom B., Bergenstahl B., and Kronberg B., A method for the preparation of submicron particles of sparingly water-soluble drugs by precipitation in oil-in-water-emulsions. II. Influence of the emulsifier, the solvent, and the drug substance, J. Pharm. ScL, 82, 584, 1993. 

In the case of oil sprays a direct relationship has been shown by Cressman and Daw-sey (7) between the amount of oil deposited and insect kill or control. A similar relationship has been shown by Chapman ei al (4) using oil as an ovicide in control of the fruit tree leaf roller. This relationship appears to hold regardless of the emulsifier used. The literature indicates that much attention has been given to the infiuence of emulsifiers on differences in effectiveness of oils. In many cases differences in effectiveness are attributed directly to the influence of the emulsifier. In spite of the various ways in which effects of emulsifiers have been interpreted, it appears that the basic influence of the emulsifier is on amount of oil deposited. 

Sherman, P. 1955c. Studies in oil-in-water emulsions. IV. The influence of the emulsifying agent on the viscosity of water-in-oil emulsions of high water content. J. Colloid Sci. 10,... 

Fig. 9. Influence of the emulsifier concentration on the size profiles diamonds C= 15%, triangles C=25%, circles C= 30% and squares C=45% of Ifralan 205. The dispersed phase is kept constant at 75% of a 350 mPa.s silicone oil. The lines are guides for the eye...

For emulsifled fuel droplet combustion, the kinetic criteria for the onset of nucleation, and hence the possible rupturing of the parent droplet, are required. If nucleation is likely to occur at the interface between the micro-droplets and the bulk liquid medium, then the influence of the emulsifying agents on the onset of nucleation should be assessed also. At the surface of the parent droplet, the vapor concentration depends on the accessibility of the molecules of the hquid phases to those of the gas phase. It is conceivable that the emulsifying agents and/or surface tension effects can prevent the dispersed hquid-phase micro-droplets from being in contact with the gas, hence effectively inhibiting its vaporization. 

In the case of griseofulvin, administration in a fatty medium enhances absorption. Fat is emulsified by the bile salts, and the administration of an already emulsified form increases the opportunity for solubilisation and hence transport across the microvilli by fat absorption pathways. The influence of the emulsifier on membrane permeability is one factor that must be considered. Knowledge that particles may be absorbed from the gut by the gut-associated lymphoid tissue suggests that we may have to revise our views on the nature of absorption of many dmgs from the gastrointestinal tract. 

The influence of the emulsifier (SHS) concentration on Np is more pronounced in the conventional emulsion polymerization system (Rp°c[SHS]y, y= 0.68) than in mini-emulsion polymerization (y=0.25). This result is caused by the different particle formation mechanism. While homogeneous nucleation is predominant in the conventional emulsion polymerization, monomer droplets become the main locus of particle nucleation in mini-emulsion polymerization. In the latter polymerization system, most of the emulsifier molecules are adsorbed on the monomer droplet surface and, consequently, a dense droplet surface structure forms. The probability of absorption of oligomeric radicals generated in the continuous phase by the emulsifier-saturated surface of minidroplets is low as is also the particle formation rate. 

Ostberg et al. [210] have investigated in detail the influence of the emulsifier type and alkyd properties on droplet size and stability of alkyd emulsions. Alkyd S-68 (Bergvik Kemi AB, Sweden) was mainly used as an alkyd resin, containing 67.8% of oil, acid value=10.6, viscosity=680 mPa s. Other oils were also used, with acid values 8-9 and viscosities from 180 to 970 mPa s. The surfactants used were ... 

Generally, the factors that affect the interfacial shear viscosity can also affect the IFPYV of the interfacial film. In most cases, the influence of the factors on both the interfacial shear viscosity and IFPYV is identical. The following gives a brief description of the influence of the emulsifier concentration and temperature on the IFPYV of the interfacial film. 

Emulsifier. The influence of the emulsifier on the viscosity of an emulsion can be of two types for instance, it can decrease the viscosity, when it adsorbs on the droplet and it increases the particle size, or it can increase the viscosity when it develops a network and forms a gel-like structure. 

The research of the influence of the emulsifier type and concentration on the emulsifier reaction order x(N oc [E] ) is summarized in Table 3, In most cases, the reaction order x is much higher than 0.6 (expected). Thus, the strong particle association at low emulsifier concentration and/or trapping of emulsifier in the interface is discussed. The reaction order above 1.0 is taken as an indication of low stability of particles. By trapping of emulsifier, the amount of emulsifier available for the formation of particles is lower than the initially charge. Under such conditions, the association increases and the rate decreases. Table 3 shows that sodium alkyl sulfonates should be the most efficient emulsifiers for stabilization of P VC particles especially at low emulsifier concentration. 

Table 3. Influence of the emulsifier type and concentration on the emulsifier reaction order x (N cx [E] ) [88]...

Attempts have also been made to take into consideration the influence of the emulsifier layer around the globules. All equations show that rj increases with increase of disperse phase volume fraction, . When 0 exceeds 0.4 to 0.5 the emulsions tend to become pseudo-plastic and the viscosity increases significantly for small changes in 0. Like all equations which deal with emulsions higher orders of appear in the equation when > 0.05. A more general equation for concentrated systems takes the form [186, 187] ... 

Any conclusion that a low interfacial tension per se is an indication of enhanced emulsion stabiUty is not rehable. In fact (8), very low interfacial tensions lead to instabiUty. The stabiUty of an emulsion is influenced by the charge at the interface and by the packing of the emulsifier molecules, but the interfacial tension at the levels found in the common emulsion has no influence on stabiUty. 

The final factor influencing the stabiHty of these three-phase emulsions is probably the most important one. Small changes in emulsifier concentration lead to drastic changes in the amounts of the three phases. As an example, consider the points A to C in Figure 16. At point A, with 2% emulsifier, 49% water, and 49% aqueous phase, 50% oil and 50% aqueous phase are the only phases present. At point B the emulsifier concentration has been increased to 4%. Now the oil phase constitutes 47% of the total and the aqueous phase is reduced to 29% the remaining 24% is a Hquid crystalline phase. The importance of these numbers is best perceived by a calculation of thickness of the protective layer of the emulsifier (point A) and of the Hquid crystal (point B). The added surfactant, which at 2% would add a protective film of only 0.07 p.m to emulsion droplets of 5 p.m if all of it were adsorbed, has now been transformed to 24% of a viscous phase. This phase would form a very viscous film 0.85 p.m thick. The protective coating is more than 10 times thicker than one from the surfactant alone because the thick viscous film contains only 7% emulsifier the rest is 75% water and 18% oil. At point C, the aqueous phase has now disappeared, and the entire emulsion consists of 42.3% oil and 57.5% Hquid crystalline phase. The stabilizing phase is now the principal part of the emulsion. 

The influence of pH, Emulsifier, and Accelerated Ageing upon Preservative Requirements of O/W Emulsions, by Gene Jacobs, M.S. at all Soc. Cosmet. Chem., 26,105-117 (February, 1975)... 

With increasing temperature the CMC passes through a minimum (Fig. 15). The initial small decrease at low temperatures is due to a positive enthalpy of the micelle formation whereas the stronger increase of CMC towards higher temperatures is caused by a thermal perturbation of the emulsifier molecules in the micelles. The smaller influence of the temperature on the CMC in case of EUP indicates that these micelles are thermally more stable than SDS-micelles. 

Figure 2 shows the influence of the different phase transitions on the rheological characteristics of a vaginal cream formulation containing cetostearyl alcohol and Poly-sorbate 60 as the emulsifying wax. Incomplete transition of two observable polymorphs... 

It is accepted that the radical entry rate coefficient for miniemulsion droplets is substantially lower than for the monomer-swollen particles. This is attributed to a barrier to radical entry into monomer droplets which exists because of the formation of an interface complex of the emulsifier/coemulsifier at the surface of the monomer droplets [24]. The increased radical capture efficiency of particles over monomer droplets is attributed to weakening or elimination of the barrier to radical entry or to monomer diffusion by the presence of polymer. The polymer modifies the particle interface and influences the solubility of emulsifier and coemulsifier in the monomer/polymer phase and the close packing of emulsifier and co emulsifier at the particle surface. Under such conditions the residence time of entered radical increases as well as its propagation efficiency with monomer prior to exit. This increases the rate entry of radicals into particles. 

In addition to influencing stability, the nature of the emulsifier can also have an influence on droplet size distribution, the mean droplet size, and also on emulsion viscosity [215] (see also Section 6.5.5). 

For electrostatically or sterically interacting drops, emulsion viscosity will be higher when droplets are smaller. The viscosity will also be higher when the droplet sizes are relatively homogeneous, that is, when the drop size distribution is narrow rather than wide. The nature of the emulsifier can influence not just emulsion stability but also the size distribution, mean droplet size, and therefore the viscosity. To describe the effect of emulsifiers on emulsion viscosity Sherman [215] has suggested a modification of the Richardson Equation to the following form ... 

The physical chemical properties of the phases can influence droplet formation as well as their stability in the bulk. For example, the viscosity of the continuous phase influences both the shear stress at the membrane wall and the adsorption kinetics of the emulsifier. 

Since emulsifiers determine the particle size of the latex, we studied the influence of the distribution ratio of the emulsifiers between monomer feed emulsion and initial reactor charge on the latex properties. We found that small amounts of the nonionic emulsifier in the initial reactor charge caused coagulation during latex preparation. Hence, the total... 

The above-mentioned influence of the total amount of anionic emulsifier on the particle size of the latex was checked by varying this total amount, the 40/60 distribution between monomer feed emulsion and initial reactor charge being kept constant. The results of these experi-... 

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