Surfactants droplet size

Colloidal stability is usually controlled by the type and amount of surfactant employed. In miniemulsions, the fusion-fission rate equilibrium during soni-cation, and therefore the size of the droplets directly after primary equilibration, depends on the amount of surfactant. For styrene miniemulsions that use SLS as surfactant, droplet sizes between 180 nm down to 32 nm can be obtained. The polydispersity slightly increases with decreasing size, but is still quite low (see Table 11). Using similar molar amounts of the simple cationic surfac-... 

Surfactant Droplet size Ideal surfactant concentration Ctotal,ideal [mmol L ] Surfactant concentration in bulk phase Cb at Ctotal, ideal [mmol - ] Number of surfactant molecules in bulk phase n [molecules - ] Ratio r between number of surfactant molecules in bulk phase and droplets [molecules/droplet]... 

The energetics and kinetics of film formation appear to be especially important when two or more solutes are present, since now the matter of monolayer penetration or complex formation enters the picture (see Section IV-7). Schul-man and co-workers [77, 78], in particular, noted that especially stable emulsions result when the adsorbed film of surfactant material forms strong penetration complexes with a species present in the oil phase. The stabilizing effect of such mixed films may lie in their slow desorption or elevated viscosity. The dynamic effects of surfactant transport have been investigated by Shah and coworkers [22] who show the correlation between micellar lifetime and droplet size. More stable micelles are unable to rapidly transport surfactant from the bulk to the surface, and hence they support emulsions containing larger droplets. 

The phase inversion temperature (PIT) method is helpful when ethoxylated nonionic surfactants are used to obtain an oil-and-water emulsion. Heating the emulsion inverts it to a water-and-oil emulsion at a critical temperature. When the droplet size and interfacial tension reach a minimum, and upon cooling while stirring, it turns to a stable oil-and-water microemulsion form. " ... 

Water-in-oil microemulsions (w/o-MEs), also known as reverse micelles, provide what appears to be a very unique and well-suited medium for solubilizing proteins, amino acids, and other biological molecules in a nonpolar medium. The medium consists of small aqueous-polar nanodroplets dispersed in an apolar bulk phase by surfactants (Fig. 1). Moreover, the droplet size is on the same order of magnitude as the encapsulated enzyme molecules. Typically, the medium is quite dynamic, with droplets spontaneously coalescing, exchanging materials, and reforming on the order of microseconds. Such small droplets yield a large amount of interfacial area. For many surfactants, the size of the dispersed aqueous nanodroplets is directly proportional to the water-surfactant mole ratio, also known as w. Several reviews have been written which provide more detailed discussion of the physical properties of microemulsions [1-3]. 

Studies of flow-induced coalescence are possible with the methods described here. Effects of flow conditions and emulsion properties, such as shear rate, initial droplet size, viscosity and type of surfactant can be investigated in detail. Recently developed, fast (3-10 s) [82, 83] PFG NMR methods of measuring droplet size distributions have provided nearly real-time droplet distribution curves during evolving flows such as emulsification [83], Studies of other destabilization mechanisms in emulsions such as creaming and flocculation can also be performed. 

This concept allows the shape of the titration curves to be explained by postulating that the chloroform droplet size decreases as the interfacial tension (ift) between the aqueous and chloroform phases is decreased by the presence of active surfactant. As the endpoint in a titration is approached the amount of active SDBS decreases as it complexes with the injected hyamine. The reduction in the amount of active surfactant material results in an increase... 

It also suggests that an excess of either anionic or cationic surfactant causes a change in droplet size and an increase in light scattering. Therefore, it should be possible to mimic the two branches of the titration curve emanating from the equivalence point by starting with pure brine (35 cm3) and chloroform (10 cm3) and using either hyamine or SDBS as titrants. Experiments undertaken to examine this hypothesis are described below. 

A further difference is that the value of the transmittance minima of the titration curves is higher than that found for the water and chloroform system. This is believed to be due to the fact that in a titration the equivalence point will not correspond to an integer number of hyamine aliquots and so, even if the contents of the reaction vessel were allowed to equilibrate fully, there would still be a small amount of uncomplexed surfactant present which will be sufficient to decrease the droplet size and so increase the back-scattered light. 

A set of experiments was performed at variable droplet sizes. The graph in Fig. 4.7 shows the dependence of the normalized (by Kint/a) osmotic resistance as a function of the oil volume fraction. The normalized values fall onto a single curve within reasonable experimental uncertainty. The results were compared to the normalized data obtained by Mason et al. [7] in the presence of surfactants. These latter are represented as a solid line that corresponds to the best fit to the experimental points (Eq. (4.18)). It is worth noting that the normalized pressures in solid-stabilized emulsions are much larger than the ones obtained in the presence of surfactants. 

It is probable that numerous interfacial parameters are involved (surface tension, spontaneous curvature, Gibbs elasticity, surface forces) and differ from one system to the other, according the nature of the surfactants and of the dispersed phase. Only systematic measurements of > will allow going beyond empirics. Besides the numerous fundamental questions, it is also necessary to measure practical reason, which is predicting the emulsion lifetime. This remains a serious challenge for anyone working in the field of emulsions because of the polydisperse and complex evolution of the droplet size distribution. Finally, it is clear that the mean-field approaches adopted to measure > are acceptable as long as the droplet polydispersity remains quite low (P < 50%) and that more elaborate models are required for very polydisperse systems to account for the spatial fiuctuations in the droplet distribution. 

The integrated DLS device provides an example of a measurement tool tailored to nano-scale structure determination in fluids, e.g., polymers induced to form specific assemblies in selective solvents. There is, however, a critical need to understand the behavior of polymers and other interfacial modifiers at the interface of immiscible fluids, such as surfactants in oil-water mixtures. Typical measurement methods used to determine the interfacial tension in such mixtures tend to be time-consuming and had been described as a major barrier to systematic surveys of variable space in libraries of interfacial modifiers. Critical information relating to the behavior of such mixtures, for example, in the effective removal of soil from clothing, would be available simply by measuring interfacial tension (ILT ) for immiscible solutions with different droplet sizes, a variable not accessible by drop-volume or pendant drop techniques [107]. 

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