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case of skin emulsions, their type and characteristics are of much importance, as in
the case of day creams and night creams.
9.2.1.1 oil-in-Water emulsions
The main criteria for O/W emulsions is that, if water is added, it is miscible with the
emulsion. Further, after water evaporates, the oil phase will be left behind. Thus, if
the oil phase is needed on the substrate (such as skin, metal wood, etc.), then an O/W
type emulsion should be used.
9.2.1.2 Water-in-oil emulsions
The criteria for W/O emulsions are that it is miscible with oil; that is, if the emul-
sion is added to an oil, a new but diluted W/O emulsion is obtained. In some skin
creams, the W/O type emulsion is preferred, especially to obtain an oil-like feeling
after application.
9.2.1.3 hydrophilic-lipophilic (hlb) values of emulsifiers
The emulsifiers commonly used exhibit varying solubility in water (or oil). This
property will have consequences on the emulsion. Let us consider a system where we
have oil and water. If we add an emulsifier to this system, then the latter will be dis-
tributed both in the oil and water phases. The degree of solubility in each phase will
of course depend on its structure and HLB character. The emulsifiers used in making
emulsions are characterized with regard to their molecular structure. For example,
amphiphile molecules have HLB characteristics. Thus, each emulsifier needed for a
given system (such as an O/W or a W/O emulsion) will need a specific HLB value.
The data in Table 9.2 give a rough estimation of the HLB needed for a given system
of emulsion. In general, it may be expected that, if the emulsifier dissolves in water,
then, on adding oil, an O/W emulsion will be obtained. Conversely, if the emulsifier
is soluble in oil, then, on adding water, a W/O emulsion will result.
W/O emulsions are formed by using HLB values between 3.6 and 6, thus suggest-
ing that emulgators that are soluble in the oil phase are generally used. O/W emul-
sions need HLB values around 8 to 18. This is only a very general observation; it
must be noted that HLB values alone do not determine emulsion type. Other param-
eters, such as temperature, properties of the oil phase, and electrolytes in the aqueous
phase also have their effect. However, HLB values have no relation to the degree
table 9.2
hlb values of different emulsifiers
commonly used in emulsions
emulsifier Solubility
hlb
application in Water
Very low solubility
0-2
W/O
Low solubility
4-8
W/O
Soluble
10-12
Wetting agent
High solubility
14-18
O/W
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