HLB values of some commonly used emulsifying agents are given in Table 35.1. Two or more surfactants can be combined to achieve a suitable HLB value and often give better results than one surfactant alone. It is found that the increase in voltage and/or temperature can significantly reduce the separation time and the residual water in the emulsion. The continental method is used to prepare the initial or primary emulsion from oil, water, and a hydrocolloid or gum type emulsifier (usually acacia). Liquid paraffin, for example, has a required HLB value of 4 to obtain a w/o emulsion and 12 for an o/w emulsion. Oils and waxy materials have a ‘required HLB number’ which helps in the selection of appropriate emulsifying agents when formulating emulsions. These are actually double emulsions of O/W emulsion and W/O emulsion. In these types of emulsions, oil droplets enclosing water droplets are dispersed in water. Low numbers (3–6) indicate a lipophilic molecule which produces a w/o emulsion. Water- in-oil-in-water emulsion These are also called W/O/W emulsions. Higher numbers (8–18) indicate a hydrophilic molecule which produces an o/w emulsion. The lower the number, the more oil soluble the emulsifying agent. An HLB number, usually between 1 and 20, is allocated to an emulsifying agent and represents the relative proportions of the lipophilic and hydrophilic parts of the molecule. The HLB (hydrophilic–lipophilic balance) system. Examples of the main types include glycol and glycerol esters, macrogol ethers and esters, sorbitan esters and polysorbates. The type of emulsion formed depends on the balance between hydrophilic and lipophilic groups which is expressed as the HLB (hydrophilic–lipophilic balance) number (see below).
The non-ionic surfactants are compatible with both anionic and cationic substances and are highly resistant to pH change. They are used to produce either o/w or w/o emulsions for both external and internal use. These are synthetic materials and make up the largest group of surfactants.
Examples include cetrimide and benzalkonium chloride. The cationic surfactants also have antimicrobial activity. Emulsions formed by a cationic surfactant are generally stable at acidic pH. They are used in the preparation of o/w emulsions for external use and must be in their ionized form to be effective. These are usually quaternary ammonium compounds which have a surface-active cation and so are sensitive to anionic surfactants and drugs. Soaps of divalent and trivalent metals such as calcium oleate (w/o) (see Example 35.3)Īmine soaps such as triethanolamine oleate (o/w)Īlkyl sulphates such as sodium lauryl sulphate (o/w).Ĭationic surfactants. Alkali metal and ammonium soaps such as sodium stearate (o/w)
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