Emulsions steric stabilisation

Inverse (or water-in-oil) emulsions (315, 401) are emulsions in which an aqueous phase is dispersed within a continuous organic phase. This system is essentially the inverse of a conventional emulsion, hence the name inverse emulsion. The organic phase is typically an inert hydrocarbon (such as mixed xylenes or low-odour kerosenes), and the aqueous phase contains a water-soluble monomer such as acrylamide (268). The aqueous phase may be dispersed as discrete droplets or as a bicontinuous phase (335), depending upon the formulation and conditions of the inverse emulsion. The hydrophilic-lipophilic balance (HLB) value of the stabiliser determines the form and stability of an inverse emulsion, with HLB values of less than 7 being appropriate for inverse emulsions. Steric stabilisers such as the Span , Tween , and Plutonic series of nonionic surfactants are usually used in preparing inverse emulsions. Inverse emulsions, suspensions, miniemulsions (199), and microemulsions have been prepared, primarily as a function of the stabiliser concentration. Commercial products produced by inverse emulsion polymerisation include polyacrylamide, a water-soluble polymer used extensively as a thickener. 

This can occur if the energy barrier is small or absent (for electrostatically stabilised emulsions) or when the stabilising chains reach poor solvency (for sterically stabilised emulsions, that is if />0.5). For convenience, the flocculation of electrostatically and sterically stabilised emulsions will be discussed separately. 

Equation (12.10) also shows that when > 0.5 - that is, when the solvency of the medium for the chains becomes poor - Gj j will be negative and the interaction will become attractive. Thus, it is important to ensure that the solvent used to prepare the W/O emulsion is a good solvent for the PHS chains, otherwise flocculation of the water droplets (perhaps followed by their coalescence) may occur. Fortunately, the PHS chains are soluble in most hydrocarbon solvents used in most formulations. The condition = 0.5 is referred to as 0-solvent, and this denotes the onset of a change from repulsion to attraction. Thus, to ensure steric stabilisation by the above mechanism it must be ensured that the chains are kept in better than 0-solvent. 

Figure 12.9), which shows a shallow minimum, G (weak attraction) at h 28 that is, at h 20 nm for the present W/O emulsion based on PHS-PEO-PHS block copolymer. When h < 25, Gj is increased very rapidly with further decreases in h. The depth of the minimum, G , will depend on the adsorbed layer thickness. In the present W/O emulsion, based on a PHS layer thickness of about 10 nm, G j is very small (fraction of kT). This shows that, with the present sterically stabilised W/O emulsion, there is only a very weak attraction at a relatively long distance of... 

Steric Stabilisation and the Role of the Adsorbed Layer Thickness 297 Nano-emulsions 20 80 o/w - Silicone oils... 

Being surface active (375), surfactants lower the interfacial tension between the water and monomer phases. The decrease in interfacial tension allows smaller droplets to be formed more easily during dispersion of the monomer in the water phase. In addition, the thermodynamic driving force for coalescence is lowered as a consequence of the reduction in interfacial tension. For this reason, and especially because of the electrostatic and steric stabilisation mechanisms, emulsions prepared with surfactants are more colloidally stable. 

A rapid and low cost method was developed for direct analysis of residual monomer concentration of acrylamide from inverse-emulsion reactions. Inverse-emulsion polymerisations involve the dispersion of a water-soluble monomer in aqueous solution in a continuous organic phase. The addition of a low-medium hydrophilic-lyophilic balance steric stabiliser and continuous agitation is required to maintain emulsification. 19 refs. 

As will be described in the section on flocculation, the total energy-distance of separation curve for electrostatically stabilised shows a shallow minimum (secondary minimum) at relatively long separation between the droplets. By addition of small amounts of electrolyte such minimum can be made sufficiently deep for weak flocculation to occur. The same applies for sterically stabilised emulsions, which show only one minimum, whose depth can be controlled by reducing the thickness of the adsorbed layer. This can be achieved by reducing the molecular weight of the stabiliser and/or addition of a non-solvent for the chains (e.g. electrolyte). 

Rg. 6.30. Schematic representation of flocculation of sterically stabilised emulsions. 

The inherently high colloid stability of nano-emulsions can be well understood from a consideration of their steric stabilisation (when using nonionic surfactants and/or polymers) and how this is affected by the ratio of the adsorbed layer thickness to droplet radius, as will be discussed below. 

A-B, A-B-A block and BAn graft type polymeric surfactants are used to stabilise emulsions and suspensions [18]. B is the anchor chain that adsorbs very strongly at the 0/W or S/L interface, whereas the A chains are the stabilising chains that provide steric stabilisation. These polymeric surfactants exhibit surface activity at the 0/W or S/L interface. The adsorption and conformation of these polymeric surfactant at the interface has been described in detail in reference 18. 

The addition of smaller colloidal species (polymer molecules, micelles, microemulsions) to an emulsion may lead to destabilisation of the system due to depletion attraction or steric stabilisation if these species adsorb at the droplet interface. A model calculation, illustrating the depletion destabilisation, is presented in Figure 10.6. All the parameters are as in Figure 10.2a but in this case nonionic micelles are also present [see eqns. (10.17)-(10.19)]. The micellar diameter is chosen to be 10 nm and the volume fraction equals 0.1. 

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. 

Emulsions made by agitation of pure immiscible liquids are usually very unstable and break within a short time. Therefore, a surfactant, mostly termed emulsifier, is necessary for stabilisation. Emulsifiers reduce the interfacial tension and, hence, the total free energy of the interface between two immiscible phases. Furthermore, they initiate a steric or an electrostatic repulsion between the droplets and, thus, prevent coalescence. So-called macroemulsions are in general opaque and have a drop size > 400 nm. In specific cases, two immiscible liquids form transparent systems with submicroscopic droplets, and these are termed microemulsions. Generally speaking a microemulsion is formed when a micellar solution is in contact with hydrocarbon or another oil which is spontaneously solubilised. Then the micelles transform into microemulsion droplets which are thermodynamically stable and their typical size lies in the range of 5-50 nm. Furthermore bicontinuous microemulsions are also known and, sometimes, blue-white emulsions with an intermediate drop size are named miniemulsions. In certain cases they can have a quite uniform drop size distribution and only a small content of surfactant. An interesting application of this emulsion type is the encapsulation of active substances after a polymerisation step [25, 26]. 

In the formation of an emulsion, one of the two immiscible liquids is broken up into droplets which are dispersed in the other liquid. The dispersion of one liquid in another immiscible liquid leads to a large increase in interfacial free energy because of the increase in the area of the interface. The emulsifying agent stabilises the emulsion by adsorbing at the liquid-liquid interface as an oriented interfacial film. This film reduces the interfacial tension between the liquids and also decreases the rate of coalescence of the dispersed droplets by forming mechanical, steric and/or electrical barriers arormd them. 

In w/o emulsions the hydrocarbon chains of the adsorbed molecules protrude into the oily continuous phase. Stabilisation arises from steric repulsive forces as described in section 7.2.2. Emulsions are more complex than suspensions, because of the possibility (a) of movement of the surfactant into either the continuous or disperse phase, (b) micelle formation in both phases, and (c) the formation under suitable conditions of liquid crystalline phases between the disperse droplets. 

A series of well-defined A-B block copolymers of polystyrene-block-polyethylene oxide (PS-PEO) were synthesised [6] and used for the emulsion polymerisation of styrene. These molecules are ideal as the polystyrene (PS) block is compatible with the PS formed, and thus it forms the best anchor chain. The PEO chain (the stabilising chain) is strongly hydrated with water molecules and extends into the aqueous phase where it forms the steric layer necessary for stabilisation. 

Fig. 1.14. Mechanism of emulsion stabilisation a) electric repulsion, b) steric hindrance According to Hamaker (1937) the Free Energy of the attraction forces is approximately...

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