Emulsions small molecule liquid

Inorganic Particles in Small Molecule Liquid Emulsion A Model System for Filled Polymer Blends ... 

The practical industrial knowledge on stabilization effects of inorganic particles on emulsions has been apparently known since the nineteenth century [11]. Scientific analyses in this field followed up to the early twentieth century [12,13]. A few words are in order to elaborate the intended meaning of the term stabilization. In small molecule liquid emulsions, the inorganic particles may locate themselves at the interfaces of the immiscible phases, if the affinity for any of the phases is not strong. 

Statistical mechanics was originally formulated to describe the properties of systems of identical particles such as atoms or small molecules. However, many materials of industrial and commercial importance do not fit neatly into this framework. For example, the particles in a colloidal suspension are never strictly identical to one another, but have a range of radii (and possibly surface charges, shapes, etc.). This dependence of the particle properties on one or more continuous parameters is known as polydispersity. One can regard a polydisperse fluid as a mixture of an infinite number of distinct particle species. If we label each species according to the value of its polydisperse attribute, a, the state of a polydisperse system entails specification of a density distribution p(a), rather than a finite number of density variables. It is usual to identify two distinct types of polydispersity variable and fixed. Variable polydispersity pertains to systems such as ionic micelles or oil-water emulsions, where the degree of polydispersity (as measured by the form of p(a)) can change under the influence of external factors. A more common situation is fixed polydispersity, appropriate for the description of systems such as colloidal dispersions, liquid crystals, and polymers. Here the form of p(cr) is determined by the synthesis of the fluid. 

Emulsion stability is required in many dairy applications, but not all. In products like whipped cream and ice cream, the emulsion must be stable in the liquid form but must partially coalesce readily upon foaming and the application of shear. The structure and physical properties of whipped cream and ice cream depend on the establishment of a fat-globule network. In cream whipped to maximum stability, partially coalesced fat covers the air interface. In ice cream, partially coalesced fat exists both in the serum phase and at the air interface also, there is more globular fat at the air interface with increasing fat destabilization. Partial coalescence occurs due to the collisions in a shear field of partially crystalline fat-emulsion droplets with sufficiently-weak steric stabilization (low level of surface adsoiption of amphiphilic material to the interface per unit area). To achieve optimal fat crystallinity, the process is very dependent on the composition of the triglycerides and the temperature. It is also possible to manipulate the adsorbed layer to reduce steric stabilization to an optimal level for emulsion stability and rapid partial coalescence upon the application of shear. This can be done either by addition of a small-molecule surfactant to a protein-stabilized emulsion or by a reduction of protein adsorption to a minimal level through selective homogenization. 

Particles larger than small molecules may form mixtures with solvents. If gravity does not cause these particles to settle out of the mixture over time, the mixture is called a colloidal system, or colloid. The term colloid can also refer to the colloidal particles.) Colloidal particles are larger than solute particles, and can even be single large molecules such as hemoglobin. A colloidal system can be any combination of phases except gas in gas). Some examples of colloidal systems are an aerosol (liquid or solid particles in a gas like fog or smoke), a. foam gas particles in a liquid like whipped cream), an emulsion (liquid particles in a liquid or solid like milk or butter), or a sol (solid particles in a liquid like paint). 

Although finely divided insoluble solid particles constitute an important class of emulsifying agents [44-46], the preparation of liquid-liquid dispersions traditionally involves the use of ionic and nonionic small-molecule surface-active agents. Mixtures of surfactants can also be used to achieve a desirable viscosity of emulsions [12] and to enhance the stabilization properties compared to the effect of one of the emulsifiers [47-49], although evidence of synergistic effects are not always found. 

The structures of the interfacial layers in emulsion droplets might be expected to be simple when small-molecule emulsifiers are used, but this is not necessarily the case, especially when not one but a mixture of surfactant molecules is present. Although simple inter-facial layers may be formed where the hydrophobic moieties of the surfactants are dissolved in the oil phase, and the hydrophilic head groups are dissolved in the aqueous phase, it is also possible to form multilayers and liquid crystals close to the interface (78). These, of coiuse, depend on the natiue and the concentrations of the different siufactants. Interactions between surfactants generally enhance the stability of the emulsion droplets, because more rigid and structured layers tend to inhibit coalescence. Also, mixtiues of different surfactants having different HLB numbers appears to provide structured interfacial layers, presumably because of the different affinities of the siufactants for the oil-water interface (79). 

Because of the importance of ice cream as a product, much has been written on its stmcture and for mation (171), and the process can only be summarized here. In toppings and ice cream (and indeed simply in whipping cream), it is first necessary to produce a stable emulsion. Ice-cream mix is a complex mixtiue, but the initial emulsion is basically homogenized milk, containing an admixture of small-molecule siufactants as well in whipped toppings, the emulsion is made with oil and a surfactant mixture, which may or may not contain protein and in cream, the natural membrane of phospholipid and protein surrounds the milk fat. In all of these, it is necessary to have some small-molecule emulsifiers so as to exchange with, and weaken the rigidity of, the adsorbed layer of protein (118). The second essential is fliat the fat or oil in the formulation is partly crystalline neiflier completely liquid nor completely solid oil will perform optimally. If the oil is partly crystalline, then the emulsion droplets may not be truly spherical but may have protrusions of crystals on their surfaces. 

There are fundamental similarities between small molecule immisdble liquid-liquid dispersions, for example, emulsions, and immisdble polymer blends. It is obvious to the reader that the basic laws of equilibrium thermodynamics applies to all physical systems, whether composed of small molecules or long-diain polymers. The thermodynamic considerations can be relatively easily visualized in terms of a simple principle such as like dissolves like (Figure 8.1). 

Nature has provided us with various food-dispersed components some are liquids (water-in-oil or oil-in-water emulsions), and some are semisolids or solids (dispersed fats, proteins, and carbohydrates). Some components are dissolved in continuous phases, and others are dispersed in various complex molecular and physical macro- or microstructured networks. This infinite number of structural combinations are organized and arranged in very complex types of assemblies such as dispersions, emulsions, foams, and gels. In addition, foods contain hundreds of small molecules as minor ingredients and various other added compounds termed food additives. Additives act as vitamins, antioxidants, preservatives, acidulants, enzymes, flavors, colorants, amphiphiles, and so on. 

Colloids are classified according to the phases of the substances involved (Table 15.10). A colloid that is a suspension of solids in a liquid is called a sol, and a suspension of one liquid in another is called an emulsion. For example, muddy water is a sol in which tiny flakes of clay are dispersed in water mayonnaise is an emulsion in which small droplets of water are suspended in vegetable oil. Photographic emulsions also contain solid colloidal particles of light-sensitive materials such as silver bromide. Foams are suspensions of a gas in a liquid or solid. Foam rubber, Styrofoam, soapsuds, and aerogels are foams. Zeolites (Box 13.4) are a type of solid foam in which the openings in the solid are comparable in size to molecules. 

A dispersion Is a system made of discrete objects separated by a homogeneous medium In colloidal dispersions the objects are very small In at least one dimension. Colloidal sizes range from 1 to 100 nm however these limits are somewhat arbitrary, and It Is more useful to define colloids as dispersions where surface forces are large compared to bulk forces. Here we are concerned with systems where the dispersion medium Is a liquid examples are droplets In emulsions or mlcroemulslons (oll/water or water/oll), aggregates of amphiphilic molecules (surfactant micelles), foams, and all the dispersions of solid particles which are used as Intermediates In the manufacture of ceramics. At this stage we are not too concerned with the nature of the constituents, but rather with the structures which they form this Is a geometrical problem, where the system Is characterized by Its surface area A, by the shapes of Its Interfaces (curvatures - b ), and by the distances between opposing surfaces (d — concentration parameter). 

---