Semi-Solid Emulsions

A number of azetidine-based compounds have been disclosed in patent applications from Aventis Pharma for CBi-modulated treatment of diseases such as obesity, Parkinson s disease, schizophrenia, respiratory and neurological diseases [330-334]. Compound (556) was specifically claimed for use in two formulation patent applications [330, 331] for a stable semi-solid composition and oral emulsion composition, respectively. The optional coadministration of an agent that activates norepinephrinergic and se-rotoninergic neurotransmission (for example, sibutramine) or dopaminergic neurotransmission was also claimed for the treatment of obesity. The optional use of a dopamine agonist (for example, levodopa) was claimed... 

Figure 5.17 A Pickering O/W emulsion that had sufficient stability to permit partial drying at 40°C to this semi-solid disc shape. From Arditty [298]. Copyright 2004, Elsevier.

Many industrial and consumer products are formulated as emulsions including numerous surface coatings (Chapter 12), foods, pesticides and herbicides (Chapter 13), pharmaceutical products (Chapter 14), and personal care products such as creams and lotions (Chapter 15). Some emulsions are made to reduce viscosity so that an oil can be made to flow. Emulsions of asphalt, a semi-solid variety of bitumen dispersed in water, are formulated to be both less viscous than the original... 

Some special problems arise at sea. When crude oil is spilled on the ocean, a slick is formed which spreads out from the source with a rate that depends on the oil viscosity. With sufficient energy an O/W emulsion may be formed, which helps disperse oil into the water column and away from sensitive shorelines. Otherwise, the oil may pick up water to form a water-in-oil emulsion, or mousse ( chocolate mousse ). These mousse emulsions can have high water contents and have very high viscosities, with weathering they can become semi-solid and considerably more difficult to handle, very much like the rag-layer emulsions referred to above. The presence of mechanically strong films makes it hard to get demulsifiers into these emulsions, so they are hard to break. See Chapter 9. 

The structure of the interfacial layers in food colloids can be quite complex as these are usually comprised of mixtures of a variety of surfactants and all are probably at least partly adsorbed at interfaces which even individually, can form complex adsorption layers. The layers can be viscoelastic. Phospholipids form multi-lamellar structures at the interface and proteins, such as casein, can adsorb in a variety of conformations [78]. Lecithins not only adsorb also at interfaces, but can affect the conformations of adsorbed casein. The situation in food emulsions can be complicated further by the additional presence of solid particles. For example, the fat droplets in homogenized milk are surrounded by a membrane that contains phospholipid, protein and semi-solid casein micelles [78,816], Similarly, the oil droplets in mayonnaise are partly coated with granular particles formed from the phospho and lipo-protein components of egg yolk [78]. Finally, the phospholipids can also interact with proteins and lecithins to form independent vesicles [78], thus creating an additional dispersed phase. 

A waxy semi-solid, functions as an O/W emulsifier for mineral oil, fats, and waxes. It is a good fiber-to-metal textile lubricant and is used as a co-emulsifier with EMSORB 2505 in paraffin wax emulsions for textiles, paper and wallboard coatings. It is also used as an emulsifier in household products. 

In general, the preparation of such formulations as poultices and pastes is extemporaneous, and it is unlikely that the industrial pharmaceutical formulator will be required to develop stable, safe and efficacious products of this type. Solutions and powders lack staying power (retention time) on the skin and can only afford transient relief. In modern-day pharmaceutical practice, semi-solid formulations are preferred vehicles for dermatological therapy because they remain in situ and deliver their drug payload over extended periods. In the majority of cases, therefore, the developed formulation will be an ointment, emulsion or a gel. Typical constituents for these types of formulations are shown in Table 14.5. 

In addition to the traditional dermal delivery formulations discussed above, several other pharmaceutical semi-solid and liquid formulation types have been the subject of a considerable amount of R D. These include sprays, foams, multiple emulsions, microemulsions, liposomal formulations, niosomes, cyclodextrins, glycospheres, dermal membrane structures and microsponges. Although some of these formulations form part of the pharmaceutical armamentarium, they are yet to achieve widespread application and are not within the scope of this chapter. The interested reader is referred to the excellent coverage by Osborne and Amann (1990), Kreuter (1994) and Liu and Wisniewski (1997). 

The official definition (FDA Standard of Identity) describes mayonnaise as a semi-solid food prepared from vegetable oil (no less than 65%), egg yolk and vinegar. Most mayonnaise in the US contains 75-82% oil which is usually soybean oil. Other salad oils that have undergone winterization (including partially hydrogenated soybean oil) can also be used in mayonnaise. The production of mayonnaise is partly an art due to the difficulty of producing the o/w emulsion in which the dispersed phase is seven times more than the continuous phase. Egg solids and processing conditions play critical roles in mayonnaise quality. 

Acetylaied lanolin, Acetadcps, Aloduian. Almost odorless, pale yellow semi-solid unctuous mass, mp 36". Sol in mineral oils in some vegetable oils. Readily dispersed in oil-in-water emulsions- Will not emulsify to form water-in-oil emulsions. 

Water-in-oil emulsions, are commonly exemplified by systems such as butter, spreads and margarine. These consist of water droplets formed in an oil phase of a liquid oil or a semi-solid partially crystalline oil phase. These systems are typically stabilized by oil-soluble emulsifiers and by the presence of solid particles. The water solubility in a triglyceride, in contrast to the triglyceride solubility in water, is about 0.5%, thus making the ripening processes fast unless the droplets are stabilized by the inclusion of a totally oil-insoluble material such as salt. 

Micelles are nano or micro structures with a hydrophobic core and a hydrophilic shell. They form above a eritieal eoneentration called the eritical micellar concentration (CMC) as a result of the self-association of amphiphilic molecules to minimize hydrophobic interactions. Micellar surfactants and polymers are widely used in the pharmaceutical industry as formulation aids in solids, semi-solids, suspensions, emulsions, and solutions. One of the... 

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