Surfactant concentration effects emulsion stability

Figure 10. Effect of surfactant type and surfactant concentration on emulsion stability Span 20 o Span 80 V Span 85 breaking time 20 s.

Perhaps the most interesting findings experimentally have arisen from investigating the effect of surfactant concentration on emulsion stability. Various surfactants, when present at a critical concentration above their CMC, cause emulsions to flocculate leading to rapid creaming/sedimentation. The presence therefore of free (i.e. not emulsified) surfactant is implicated in this process. The systems include 0/W emulsions stabilised by either nonionic " or ionic surfactants, " "" and flocculated by aqueous micelles, vesicles " or water-soluble... 

Effect of Surfactant Concentration. Raising the surfactant concentration increases the stability of adsorption layer which enhances resistance of the emulsion to coalescence. The radius of the aqueous internal phase droplets in the emulsion initially diminishes with the increase in surfactant concentration and then remains constant. This results from the tendency of the dispersed phase to minimize the interfacial area for a given amount of surfactant. Despite a high degree of dispersion of the internal phase, the droplets coalesce until they attain a surfactant content at the interface which is close to the limiting value of adsorption. When the surfactant is present in an amount sufficient for stabilization of all of the drops of internal phase formed as a result of the dispersion, a maximum stability of the emulsion (Figure 1) and a minimum coalescence rate constant (Figure 2, curve 1) are attained. 

Figure 1.10 Effect of surfactant concentration on the stability to creaming of 25 wt% alkane (CJ-in-water emulsions stabilised by pure nonionic surfactants (C Em) at25 C...

Experiments on the stability of the HIPEs indicated that one of the most important factors was the solubility of the emulsifier in the continuous (formamide) phase. Thus, the higher the surfactant solubility, the more stable the emulsion. The emulsifier concentration was also important stability increased to a maximum, then decreased, with increasing surfactant concentration. Surprisingly, the HLB number did not appear to have much effect on the stability of the emulsions, over the range studied (11 to 14). This was attributed to the high concentration of emulsifier in the continuous phase, although the narrow HLB value range is probably also a factor. 

It seems that increasing the surfactant concentration causes thinning of the films between adjacent droplets of dispersed phase. Above a certain level, the films become so thin that on polymerisation, holes appear in the material at the points of closest droplet contact. A satisfactory explanation for this phenomenon has not yet been postulated [132], It is evident, however, that the films must be intact until polymerisation has occurred to such an extent as to lend some structural stability to the monomer phase if not, large-scale coalescence of emulsion droplets would occur yielding a poor quality foam. In general, vinyl monomers undergo a volume contraction on polymerisation (i.e. the bulk density increases) and in the limits of a thin film, this effect may play a role in hole formation, especially at higher conversions in the polymerisation process. 

As discussed earlier, it is known that the surfactant concentration present during emulsification can affect the particle size of an emulsion. It has also been shown that the stability of an emulsion can be affected in rather unexpected ways by changing the concentration of the surfactant (16). The techniques presented in the last section allow the researcher to follow the full particle-size distribution of the emulsion system rather than just an average diameter. Using several oil/water emulsion systems as examples, we demonstrate the ability of these techniques to determine the effect of emulsifier concentration on the particle-size distribution produced by an inversion method of emulsification. Some of the benefits of obtaining the full distribution will also be discussed. 

Using this model, two important conclusions may be made concerning the stability of a foam. Firstly, after a certain surfactant concentration (above the cmc) the stability of the foam (or emulsion) film will increase due to the additional stabilizing force of the micellar periodic structure in the film. Secondly, increasing the electrolyte concentration will decrease the repulsive forces between the micelles in the film thus inhibiting the formation of a micellar periodic structure in the film core. The effect of electrolyte will of course differ for anionic and nonionic surfactants. 

Very few studies have focused on the stability of OAV emulsions in porous media. Sarbar et al. 24) conducted a study to determine the effect of chemical additives on the stability of OAV emulsion flow through porous media. They injected 1, 5, and 10% OAV emulsions in sand packs with varying pH and surfactant concentrations and found that there was an optimal value of the surfactant concentration at which emulsions were the most stable. Addition of sodium chloride to the aqueous phase had a detrimental effect on the stability of the emulsion. For their system they found that there... 

Apart from the fact that the use of the HLB system is limited as it is based on the observation of creaming or separation of the emulsions, as an index of instability the HLB system also neglects the effects of surfactant concentration on stability (26) and of course it is irrelevant to the particular problems with multiple emulsion systems. Nevertheless, it provides a useful approach to the choice of optimal surfactant system. In general, in a w/o/w emulsion, the optimal HLB value of the primary surfactant will be in the range 2-7 and in the range 6-16 for the secondary surfactant. Equilibration of the systems after mixing will undoubtedly result in the transfer of surfactant between the aqueous and nonaqueous components. Saturation of the phases with the two surfactants used should prevent instability during this equilibration. 

Effect of Surfactant Type and Concentration. Surfactant concentration and type is of great importance for the stability of thin liquid films and for emulsion stability. Type and concentration of surfactants are responsible for the degree of lowering the interfacial tension and for the viscoelastic properties of droplet surface, as well as for the film thickness between two droplets. 

With increasing surfactant concentration, the stability of the emulsion increases continuously until a certain value. Increasing surfactant concentration above this point yields no more emulsion stability. This means that there is a saturation concentration of adsorbed surfactant molecules at the droplet surfaces corresponding to the molecular size (12). An increase in bulk phase concentration has therefore no more effect on emulsion stability. 

Most of the traditional adsorption studies of surfactants correspond to dilute systems without aggregation in the bulk phase. At the same time micellar solutions are much more important from a practical point of view. To estimate the equilibrium adsorption, neglecting the effect of micelles can usually lead to reasonable results. However, the situation changes for nonequilibrium systems when the adsorption rate can increase by orders of magnitude when the of surfactant concentration is increased beyond the CMC. Current interest in the adsorption from micellar solutions is mainly caused by recent observations that the stability of foams and emulsions depends strongly on the concentration in the micellar region [1]. This effect can be explained by the influence of the micellisation rate on the adsorption kinetics. 

These data proof the need of using surfactants at C CMC. The excessive bulk concentration of surfactants is required not only for an effective decrease of y but also for the formation of a protective adsorption layer after the abrupt increase in interfacial area caused by the growing number of small size droplets. Thus, emulsions formed with typical surfactants behave towards sedimentation stability similar to those stabilized by natural surfactants. In this respect the advantage becomes better understandable of the so-called true , i.e. micelle-forming, surfactants as compared to the surfactants unable to form aggregate. True surfactants not... 

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