Stabilisation mixed surfactants

The rate of coalescence is measured by following the droplet number n or average droplet size d (diameter) as a function of time. Plots of log(droplet number) or average diameter versus time give straight lines (at least in the initial stages of coalescence), from which the rate of coalescence k can be estimated using Eq. (6.73). In this way, one can compare the different stabilisers, e.g. mixed surfactant films, liquid crystalline phase and macromolecular surfactants. 

VCM and an emulsifier. These components are circulated through a mixing pump (homogeniser) which causes the mixture to disperse into very fine droplets, before being placed in the autoclave. The droplets are coated with surfactant which stabilises them during the reaction. Initiation and polymerisation occur within the droplets. After polymerisation, the autoclave contains a stable dispersion of fine particles of PVC in water. Thereafter the subsequent operations for obtaining the final product are similar to the emulsion polymerisation process. 

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. 

EMULSION POLYMERISATION OF STYRENE STABILISED BY MIXED ANIONIC AND NONIONIC SURFACTANTS Chem C S Lin S Y Chen L J Wn S C Taiwan,National Institnte of Technology Taiwan,National University... 

Seemingly, a minimum surfactant concentration is required to ensure the stability of the emulsion produced by dilution of the ECs. Mixed interfacial films with specific rheological properties are required for stabilisation of the emulsions. These films should provide high dilational viscoelasticity and they should prevent film thinning and drainage. This is discussed in more detail in Chapter 6. 

The second mechanism by which flocculation may be prevented is that of steric stabilisation. This is produced using nonionic surfactants or polymers that adsorb at the liquid/liquid interface with their hydrophobic portion, leaving a thick layer of hydrophilic chains in bulk solution, e.g. poly(ethylene oxide) (PEO) or poly( vinyl alcohol). These thick hydrophilic chains produce repulsion as a result of two main effects. The first, usually referred to as mixing interaction (osmotic repulsion), re-... 

When two immiscible liquids are mixed together, one of the phases can be dispersed as fine drops in the other phase (continuous phase). The dispersion produced is unstable because an increase in the interfacial area (due to mixing) results in an increase in the interfacial Iree energy, thus producing a thermodynamically imstable system. Consequently, the latter system cannot form an emulsion imless a third component is present to stabilise the system. This third component is often a surfactant (surface active agent) or a mixture of surfactants. The stability of such emulsions may last from a few minutes to many years, depending on the intended use. The nature of such an emulsion (OAV or W/O) depends mainly on the nature of the surfactant. The terms oil phase and water phase (aqueous phase) are used to differentiate between the two phases. If the... 

---