Stabilization by ionic surfactants

M.P. Aronson and H.M. Princen Contact Angles in OU-in-Water Emulsions Stabilized by Ionic Surfactants. Nature 286, 370 (1980). 

Electric double layers can be present at the gas/liquid interfaces between bubbles in foams. In this case, since the interfaces on each side of the thin film are equivalent, any interfacial charge will be equally carried on each side of the film. If a foam film is stabilized by ionic surfactants then their presence at the interfaces will induce a repulsive force opposing the thinning process. The magnitude of the force will depend on the charge density and the film thickness. 

Figure 1. Schematic picture of a thin liquid film stabilized by ionic surfactant. The ion bulk and surface diffusion fluxes are j/ and j,s, respectively. The surface convective flux is j,c.

Salad dressings and mayonnaise can be stabilized by ionic surfactants, which provide some electrostatic stabilization as described by DLVO theory, or by nonionic surfactants which provide a viscoelastic surface coating. The protein-covered oil (fat) droplets tend to be mostly stabilized by steric stabilization (rather than electrostatic stabilization) [34,126,129], particularly at very high levels of surface protein adsorption, in which case the adsorption layer can include not just protein molecules but structured protein globules (aggregates). In some cases, lipid liquid crystal layers surround and stabilize the oil droplets, such as the stabilization of O/W droplets by egg-yolk lecithins in salad dressing [34,135]. 

Foams and emulsions are achieved due to adsorption of foam stabilizing agents like surfactants at the interface between the dispersed and continuous phases. The foam stability is often related to the stability of thin liquid films formed between two air bubbles. All considered foam films are stabilized by ionic surfactant. 

Using steady-state absorption studies, several other authors examined the micropolarity of confined IL in microemulsions stabilized by ionic surfactants [64,85,87], For example, Sarkar and coworkers examined [bmim][BF ]/benzene mixtures stabilized by the anionic SAIL surfactant [bmim][AOT] and observed that, within the studied range, the A for solubilized MO continued to undergo redshift with increasing R [85, 87], In another work, Falcone and coworkers compared the micropolarities of [bmim][BF4]/benzene mixtures stabilized by cationic BHDC and nonionic TX-lOO surfactants using l-methyl-8-oxyquinolinium betaine (QB), a dye that locates mainly at the surfactant interfacial layer [64]. When [bmim][BF ] was added to both BHDC/benzene and TX-lOO/benzene systems, a larger hypsochromic shift was sensed by the probe in the former. This implies that the local environments in BHDC/benzene system are more polar. The authors ascribed this phenomenon to the strong electrostatic interactions between the [BFJ anion and the BHD moiety of the cationic surfactant. 

Electrostatic stabilization, arising from surface electrical potential and counterion cloud [19]. These electrostatic forces are sensitive to the ionic nature and type of electrolytes present in the system. As a result, latices stabilized by ionic surfactants are easily coagulated by the addition of electrolytes to the system. 

It is seen above that the areas for microemulsions stabilized by ionic surfactants are decisively dependent on the structure of the cosurfactant to cause the necessary disorder in the system. Microemulsions stabilized by polyethylene glycol adduct nonionic surfactants, on the other hand, are characterized by the fact that co surfactant is not used. Instead, the areas of stability now rely on temperature (Figure 1.11) although the relation with the liquid crystal structure is still the essential element [14]. 

Electrostatic Stabilization If the emulsion is electrostatically stabilized by ionic surfactants, then the potential energy curve can be calculated using the measured quantities of... 

Other experiments performed by Bergeron [34] on air foams stabilized with ionic surfactants reveal that the so-called Gibbs or dilatational elasticity e may play an important role in the coalescence process. The Gibbs elasticity measures the variation of surface tension yi t associated to the variation of the surfactant surface concentration F ... 

The first two components are the active surfactants, whereas the other components are added for a variety of reasons. The polyphosphate chelate Ca ions which are present (with Mg ions also) in so-called hard waters and prevents them from coagulating the anionic surfactants. Zeolite powders are often used to replace phosphate because of their nutrient properties in river systems. Sodium silicate is added as a corrosion inhibitor for washing machines and also increases the pH. The pH is maintained at about 10 by the sodium carbonate. At lower pH values the acid form of the surfactants are produced and in most cases these are either insoluble or much less soluble than the sodium salt. Sodium sulphate is added to prevent caking and ensures free-flowing powder. The cellulose acts as a protective hydrophilic sheath around dispersed dirt particles and prevents re-deposition on the fabric. Foam stabilizers (non-ionic surfactants) are sometimes added to give a... 

Finally, some studies have been performed on the addition of salt to the aqueous phase of oil-in-water HIPEs [109]. For systems stabilised by ionic surfactants, increasing salt concentration reduces the double-layer repulsion between droplets however, stability is more or less maintained, probably due to steric and polarisation repulsions. Above a sufficiently high salt concentration, emulsions become unstable due to salting-out of the surfactant into the oil-phase. For nonionic surfactants, the situation is similar, except that there are no initial double-layer forces. In addition, Babak [115] found that increasing the electrolyte concentration reduced the barrier to coagulation between emulsion droplets, and therefore increased coalescence. Generally, therefore, stability of o/w HIPEs is not enhanced by salt addition. 

The data given above definitely prove also the metastable character of the CBF/NBF transition and provide the experimental base for further quantitative explanation of this process. The transition to NBF can be regarded as a transition to bi-dimensional state. For instance, the process of grey film/NBF transition in films stabilised by non-ionic surfactants and CBF/NBF transition in films stabilised by ionic surfactants can be presented as a nucleation process of a new phase. So far this approach is applied only to analyse the stability of NBF (see Section 3.4.4). 

Another observation Monteiro et al. made was that a red layer was observed during Interval II, consisting of low molecular weight dormant chains, swollen with monomer. At the crossover to Interval III, this red layer coalesced, forming red coagulant. The same red layer was also observed by De Brouwer et al. [290] in miniemulsions stabilized with ionic surfactants. When polymer was used as the so-called cosurfactant, this polymer was not present in the red layer, indicating that this layer was not due to droplet coalescence. Also, the use of an... 

Classical theories of emulsion stability focus on the manner in which the adsorbed emulsifier film influences the processes of flocculation and coalescence by modifying the forces between dispersed emulsion droplets. They do not consider the possibility of Ostwald ripening or creaming nor the influence that the emulsifier may have on continuous phase rheology. As two droplets approach one another, they experience strong van der Waals forces of attraction, which tend to pull them even closer together. The adsorbed emulsifier stabilizes the system by the introduction of additional repulsive forces (e.g., electrostatic or steric) that counteract the attractive van der Waals forces and prevent the close approach of droplets. Electrostatic effects are particularly important with ionic emulsifiers whereas steric effects dominate with non-ionic polymers and surfactants, and in w/o emulsions. The applications of colloid theory to emulsions stabilized by ionic and non-ionic surfactants have been reviewed as have more general aspects of the polymeric stabilization of dispersions. ... 

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. 

The DLVO theory does not explain either the stability of water-in-oil emulsions or the stability of oil-in-water emulsions stabilized by adsorbed non-ionic surfactants and polymers where the electrical contributions are often of secondary importance. In these, steric and hydrational forces, which arise from the loss of entropy when adsorbed polymer layers or hydrated chains of non-ionic polyether surfactant intermingle on close approach of two similar droplets, are more important (Fig. 4B). In emulsions stabilized by polyether surfactants, these interactions assume importance at very close distances of approach and are influenced markedly by temperature and degree of hydration of the polyoxyethylene chains. With block copolymers of the ethylene oxide-propylene oxide... 

In the chapter on Pigmentation (Chapter 8), under the heading Dispersion, we considered how to make stable colloidal dispersions of solid and found that, for stability, it was necessary to keep the particles apart. This could be done by using polymer molecules, anchored strongly to the particle, but also extending out into the solvent, in which they were soluble. These polymer molecules provide a steric barrier around the particle and this method of stabilization is called steric stabilization. We also learnt that aqueous pigment dispersions could be stabilized by adsorbed surfactant molecules, which ionized in the water to produce an electrical charge barrier around the particle ionic stabilization). Exactly the same techniques are used to stabilize emulsions. 

The monomers are dispersed in water in the presence of surfactants. The surfactants adsorb on the surface of the monomer droplets, stabilizing them. Ionic surfactant stabilizes the droplets by electrostatic repulsion, whereas non-ionic surfactants provide steric stabihzation [23 ]. In most formulations, the amount of surfactant exceeds that needed... 

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