Surfactants droplet deformation

For two drops to coalesce, the two surfactant films must first come into molecular contact. Due to the, always present, van der Waals attraction, two neighboring droplets deform, giving a flat contact area. Depending on the repulsive forces between the two surfactant films the lamella is more or less stable. 

The droplet deformation increases with increases in the Weber number which means that, in order to produce small droplets, high stresses (i.e., high shear rates) are require. In other words, the production of nanoemulsions costs more energy than does the production of macroemulsions [4]. The role of surfactants in emulsion formation has been described in detail in Chapter 10, and the same principles apply to the formation of nanoemulsions. Thus, it is important to consider the effects of surfactants on the interfacial tension, interfacial elasticity, and interfacial tension gradients. 

The first question is, What is the effective interfacial tension during droplet deformation and breakup This depends on surface load T, which depends on the adsorption rate of the surfactant. Actually, this rate can be very high, since surfactant is not transported by diffusion but by convection. This would imply that T can be fairly high. However, every breakup event occurs... 

For the above regimes, a semi-quantitative theory is available that can give the time scale and magnitude of the local stress (Text, the droplet diameter d, time scale of droplets deformation raef, time scale of surfactant adsorption, Tads and mutual collision of droplets [9, 10]. 

Stability of W/OAV multiple emulsion containing Span 80 and Tween 80 was evaluated with respect to sodium chloride and sodium salicylate concentrations in the inner water phase (Jiao and Burgess, 2002). In this study we observed that the multiple emulsion droplets deformed and there was coalescence of the inner aqueous droplets as we applied an external force (i.e., a microscopic covershp) to multiple emulsion samples on a microscope slide. Under certain conditions (e.g., lipophihc surfactant concentration and internal phase osmotic pressure) the destabilized multiple emulsions formed unique metastable structures that had a dimpled appearance. The formation of these metastable structures correlated with the real time instability of the W/O/W multiple emulsions investigated. Our study revealed that emulsions with a salt concentrations closer to the optimal value calculated by using (1.7) had maximum stability. 

Equilibrium will occur when the presence of a surface-active agent does not lower Y any further. This concentration is typically at the critical micelle concentration [47]. Modification of fhe confacf angle (and fherefore emulsion sfability) can be achieved by a modification of the aqueous, oil, or solid phase so as to alter Yow, Yos, or Ysw- As described later, this can be achieved with the use of surfactants. The interfacial tension and surface tension are measurements of droplet deformation. Neither Ysw nor Yso can be directly measured, because the solid cannot be deformed. To solve Young s equation and to determine the solid/water and solid/oil interfacial surface tensions, the equation-of-state approach for interfacial surface tensions is required [48] ... 

Other important aspects are the liquid drainage between approaching emulsion droplets, droplet deformation, transport of molecules in the bulk, rate of macromolecular adsorption, competition between surfactants, and possible chemical reactions. 

During droplet deformation, its interfacial area is increased. The drop will commonly have acquired some surfactant, and it may even have a F value close to equilibrium at the prevailing (local) surfactant activity. Will this surfactant be able to distribute itself evenly over the enlarged interface in the very short times available Evening out can occur by surface diffusion or by spreading. 

The most widely studied deformable systems are emulsions. These can come in many forms, with oil in water (O/W) and water in oil (W/O) the most commonly encountered. However, there are multiple emulsions where oil or water droplets become trapped inside another drop such that they are W/O/W or O/W/O. Silicone oils can become incompatible at certain molecular weights and with different chemical substitutions and this can lead to oil in oil emulsions O/O. At high concentrations, typical of some pharmaceutical creams, cosmetics and foodstuffs the droplets are in contact and deform. Volume fractions in excess of 0.90 can be achieved. The drops are separated by thin surfactant films. Selfbodied systems are multicomponent systems in which the dispersion is a mixture of droplets and precipitated organic species such as a long chain alcohol. The solids can form part of the stabilising layer - these are called Pickering emulsions. 

It is cavitation in a heterogeneous medium which is the most studied by sonoche-mists. When produced next to a phase interface, cavitation bubbles are strongly deformed. A liquid jet propagates across the bubble towards the interface at a velocity estimated to hundreds of metres per second. At a liquid-liquid interface, the intense movement produces a mutual injection of droplets of one liquid into the other one, i. e. an emulsion (Fig. 3.3). Such emulsions, generated through sonication, are smaller in size and more stable than those obtained conventionally and often require little or no surfactant to maintain stability. It can be anticipated therefore that Phase Transfer Catalysed (PTC) reactions will be improved by sonication. Examples are provided later in this chapter. 

By fitting the conductivity data to the above equations, one usually finds a theoretical limit of 0.29. At this volume fraction, charge transfer laetween w/o globular micelles submitted to attractive interactions take place. Moreover, as we will see later, such systems contain easily deformable and flexible interfaces. 

It is argued that the kinetics of the limited coalescence process is determined by the uncovered surface fraction 1 - t and by the rate of thinning (drainage) of the films formed between the deformable droplets [46,47], The homogeneous and monodisperse growth generated by limited coalescence is intrinsically different from the polydisperse evolution observed for surfactant-stabilized emulsions. As noted by Whitesides and Ross [48], the mere fact that coalescence halts as a result of surface saturation does not provide an obvious explanation of the very... 

Bibette has used this method to study the effect of osmotic pressure on the stability of thin films in concentrated o/w emulsions [96], by means of an osmotic stress technique. The emulsion is contained in a dialysis bag, which is immersed in an aqueous solution of surfactant and dextran, a water-soluble polymer. The bag is permeable to water and surfactant, but impermeable to oil and polymer. The presence of the polymer causes water to be drawn out of the emulsion, increasing the phase volume ratio and the deformation of the dispersed droplets (Fig. 10). 

Changing the shape of the dispersed species while flowing also has an impact. Since emulsion droplets and foam bubbles are not rigid spheres, they may deform in shear flow. In the cases of electrostatically interacting species, or those with surfactant or polymeric stabilizing agents at the interface, the species will not be noninteracting, as is assumed in the theory. Thus, Stokes law will not strictly apply and may underestimate or even overestimate the real terminal velocity. 

In concentrated emulsions and foams the thin liquid films that separate the droplets or bubbles from each other are very important in determining the overall stability of the dispersion. In order to be able to withstand deformations without rupturing, a thin liquid film must be somewhat elastic. The surface chemical explanation for thin film elasticity comes from Marangoni and Gibbs (see Ref. [199]). When a surfactant-stabilized film undergoes sudden expansion, then immediately the expanded... 

In order to obtain emulsification, a premix of the fluid phases containing surface-active agents and further additives is subjected to high energy for homogenization. Independent of the technique used, the emulsification includes first deformation and disruption of droplets, which increase the specific surface area of the emulsion, and second, the stabilization of this newly formed interface by surfactants. 

Measurements of forces between probe particle and sessile oil drops in water were first reported by Mulvaney et al. [54] and Snyder et al. [55], with a number of studies following by Hartley et al. [56], Aston et al. [57,58], Attard and co-workers [59-61], Nespolo et al. [62] and Dagastine et al. [63]. As an example of the types of typical forces observed between an alkane droplet and a silica probe in the presence of an anionic surfactant, refer to Figure 4.4 [63]. The general shape of the force curve is similar, at first glance, to the behaviour at rigid surfaces, but as discussed below, this is a product of changes in separation and interface deformation. Also note that at low surfactant concentrations, jump-in does occur, but with an increase in surfactant concentration (which leads to a decrease in interfacial tensions) only repulsive forces are observed. 

We have developed new reaction systems based on colloidal dispersions [23, 24], namely highly concentrated water-in-oil (gel) emulsions, which could overcome most of the disadvantages of the aqueoussolvent mixtures such as inactivation of the aldolase and incomplete aldehyde solubilization in the medium. These emulsions are characterized by volume fractions of dispersed phase higher than 0.73 [25] therefore, the droplets are deformed and/or polydisperse, separated by a thin film of continuous phase. Water-in-oil gel emulsions of water/Ci4E4/oil 90/4/6 wt%, where C14E4 is a technical grade poly(oxyethylene) tetradecyl ether surfactant, with an average of four moles of ethylene oxide per surfactant molecule and oil can be octane, decane, dodecane, tetradecane, hexadecane, or squalane, were typically chosen as reaction media [23, 26]. 

The DLVO theory, which was developed independently by Derjaguin and Landau and by Verwey and Overbeek to analyze quantitatively the influence of electrostatic forces on the stability of lyophobic colloidal particles, has been adapted to describe the influence of similar forces on the flocculation and stability of simple model emulsions stabilized by ionic emulsifiers. The charge on the surface of emulsion droplets arises from ionization of the hydrophilic part of the adsorbed surfactant and gives rise to electrical double layers. Theoretical equations, which were originally developed to deal with monodispersed inorganic solids of diameters less than 1 pm, have to be extensively modified when applied to even the simplest of emulsions, because the adsorbed emulsifier is of finite thickness and droplets, unlike solids, can deform and coalesce. Washington has pointed out that in lipid emulsions, an additional repulsive force not considered by the theory due to the solvent at close distances is also important. 

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