Droplet size, interfacial tension

HIPE stability depends greatly on a number of parameters, including the nature and concentration of the surfactant, the nature and viscosity of each liquid phase, system temperature, mean droplet size, interfacial tension between the phases, strength of the interfacial film and the presence of added electrolyte in the aqueous phase. The formation of a rigid interfacial film is thought to be of paramount importance to the stability of HIPEs. 

With Eqs. (21)-(24) it is, in principle, possible to obtain measurable properties of saturated droplet-type microemulsions the total volume fraction of the droplets the interfacial tension between the microemulsion phase and the excess phase the average size of the... 

As seen in the previous secliou, the shape of a liquid interlayer between interacting droplets depends on their size, interfacial tensions, and the shape of the disjoining pressure. The calculations presented in the following text describe the shape of an interlayer between droplets under equilibrium conditions until its possible rupture. 

The equilibrium droplet size is proportional to the Capillary number, Ca, at a given viscosity ratio. The drop size reduces with an increase of the shear stress and a decrease in the droplet/matrix interfacial tension. 00 Droplet breakup cannot occur if the viscosity ratio exceeds 3.5 (Ca goes to infinity). 

Finally, some general rules for the amount of surfactant appear to be vaHd (13). For anionic surfactants the average size of droplets is reduced for an increase of surfactant concentration up to the critical micellization concentration, whereas for nonionic surfactants a reduction occurs also for concentrations in excess of this value. The latter case may reflect the solubiHty of the nonionic surfactant in both phases, causing a reduction of interfacial tension at higher concentrations, or may reflect the stabilizing action of the micelles per se. 

Several components of the organic phase contribute greatly to the character of the final product. The pore size of the gel is chiefly determined by the amount and type of the nonsolvent used. Dodecane, dodecanol, isoamyl alcohol, and odorless paint thinner have all been used successfully as nonsolvents for the polymerization of a GPC/SEC gel. Surfactants are also very important because they balance the surface tension and interfacial tension of the monomer droplets. They allow the initiator molecules to diffuse in and out of the droplets. For this reason a small amount of surfactant is crucial. Normally the amount of surfactant in the formula should be from 0.1 to 1.0 weight percent of the monomers, as large amounts tend to emulsify and produce particles less than 1 yam in size. 

The mechanism of droplet deformation can be briefly summarized as follows. The factors affecting the droplet deformation are the viscosity ratio, shear stress, interfacial tension, and droplet particle size. Although elasticity takes an important role for general thermoplastics droplet deformation behavior, it is not known yet how it affects the deformation of TLCP droplet and its relationship with the processing condition. Some of... 

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. " ... 

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... 

Bansal, V.K. Chan, K.S. McCallough, R. Shah, D.O. The Effect of Caustic Concentration on Interfacial Charge, Interfacial Tension and Droplet Size A Simple Test for Optimum Caustic Concentration for Crude Oils, J. Canadian Petrol. Tech. 1978,17(1), 69. 

The model system used by Mabille et al. [149, 150] was a set of monodisperse dilute (2.5 wt% of dispersed oil) emulsions of identical composition, whose mean size ranged from 4 p.m to 11 p.m. A sudden shear of 500 s was applied by means of a strain-controlled rheometer for durations ranging from 1 to 1500 s. All the resulting emulsions were also monodisperse. At such low oil droplet fraction, the emulsion viscosity was mainly determined by that of the continuous phase (it was checked that the droplet size had no effect on the emulsion viscosity). The viscosity ratio p = t]a/t]c = 0.4 and the interfacial tension yi t = 6 mN/m remained constant. 

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 YBG equation is a two point boundary value problem requiring the equilibrium liquid and vapor densities which in the canonical ensemble are uniquely defined by the number of atoms, N, volume, V, and temperature, T. If we accept the applicability of macroscopic thermodynamics to droplets of molecular dimensions, then these densities are dependent upon the interfacial contribution to the free energy, through the condition of mechanical stability, and consequently, the droplet size dependence of the surface tension must be obtained. 

Note 2 Representative mechanisms for coarsening at the late stage of phase separation are (1) material flow in domains driven by interfacial tension (observed in a co-continuous morphology), (2) the growth of domain size by evaporation from smaller droplets and condensation into larger droplets, and (3) coalescence (fusion) of more than two droplets. The mechanisms are usually called (1) Siggia s mechanism, (2) Ostwald ripening (or the Lifshitz-Slyozov mechanism), and (3) coalescence. 

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]. 

Microemulsions are thermodynamically stable mixtures. The interfacial tension is almost zero. The size of drops is very small, and this makes the microemulsions look clear. It has been suggested that microemulsion may consists of bicontinuous structures, which sounds more plausible in these four-component microemulsion systems. It has also been suggested that microemulsion may be compared to swollen micelles (i.e., if one solubilizes oil in micelles). In such isotropic mixtures, short-range order exists between droplets. As found from extensive experiments, not all mixtures of water-oil-surfactant-cosurfactant produce a microemulsion. This has led to studies that have attempted to predict the molecular relationship. 

In a separate experimental study, Aronson and Petko [90] also observed an increase in yield value with increasing salt concentration, for w/o HIPEs. However, the interfacial tension of the emulsions was seen to decrease with addition of a number of electrolytes to the aqueous phase, in contrast to observations made by Pons et al. The reason for this discrepancy is not clear, but may be due to different interactions between the different surfactants and salts used by each group. It is hinted that the increase in yield value on addition of salt, as observed by Aronson et al., is due to a decrease in average droplet size however, this was not examined extensively, and determination of droplet diameters was by optical microscopy only. 

Increasing temperature has the effect of decreasing emulsion stability this has been demonstrated by Kunieda et al. [11,14], among others, and is due to the increase of the rate of coalescence of the dispersed phase droplets with increasing thermal energy. Pons et al. [100] also noted that a temperature increase caused an increase in average droplet size due to increasing interfacial tension. 

Another process which leads to HIPE instability is gravitational syneresis, or creaming, where the continuous phase drains from the thin films as a result of density differences between the phases. This produces a separated layer of bulk continuous phase and a more concentrated emulsion phase. The separated liquid can be located either above or below the emulsion, depending on whether the continuous phase is more or less dense, respectively, than the dispersed phase. This process has been studied by Princen [111] who suggests that it can be reduced by a number of parameters, including a high internal phase volume, small droplet sizes, a high interfacial tension and a small density difference between phases. 

It was previously shown that the formation of a stable emulsion of methylene chloride in water was vital for the successful formation of individual microspheres [4,9]. Two main factors played an important role in the emulsification of methylene chloride in water and influenced the microsphere size the interfacial tension of the methylene chloride droplets in the surrounding aqueous phase and the forces of shear within the fluid mass. The former tends to resist the distortion of droplet shape necessary for fragmentation into smaller droplets whereas the latter forces act to distort and ultimately to disrupt the droplets. The relationship between these forces largely determines the final size distribution of the methylene chloride in water emulsion which in turn controls the final size distribution of the solid microspheres formed. 

Eadie, in Ref 69, reports on a considerable amount of work done on the ability of beeswax and paraffin wax to remain coated on HMX surfaces when immersed in liq TNT. Thru measurements of contact angles, a technique used earlier on RDX/wax systems reported on by Rubin in Ref 23, it was determined that the TNT preferentially wets the HMX and the wax is stripped away. He concludes that the most important property of a desensitizing wax is that it should be readily dispersed uniformly thruout the TNT phase. He also suggests that a better desensitizer for investigation for use would be a wax or substituted hydrocarbon having a low interfacial tension with TNT. The smaller the wax droplet size the more efficiently it will be distributed and the more effectively it should desensitize. Williamson (Ref 64) in his examination of the microstructures of PETN/TNT/wax fusion-casts detected that wax is dispersed thru the cast as isolated descrete globules which he refers to as blebs or irregular or streak-like areas, surrounded by TNT (see also Ref 54)... 

Here, tr, is the liquid-to-surface interfacial tension l , u the vapor apparent velocity directing the droplets and S characterizes the wire mesh By ignoring the secondary effects, a simplified force balance can be written for the disengaging droplet size ... 

---