Emulsions, Microemulsions, and Lyotropic Liquid Crystals - Surface and Colloid Chemistry: Principles and Applications - page 186

complex (1 mol:1 mol) is formed, which is sparingly soluble in water. The reason is
that positively and negatively charged moieties interact and produce a neutral com-
plex (which is insoluble in water). This complex is oil soluble.
The complex component is one in which the anionic and cationic moieties include
hydrophilic portions or substituents in addition to the complex-forming portions
thereof. The anionic detergent is a mixture of higher paraffin sulfonate and higher
alkyl polyoxyethylene sulfate. The cosurfactant is a polypropylene glycol ether, a
poly-lower-alkylene glycol lower-alkyl ether, or a poly-lower-alkylene glycol lower
alkanoyl ester, and the organic solvent is a nonpolar oil, such as an isoparaffin, or an
oil having polar properties, such as a lower fatty alkyl chain. Such a light-duty, clear
microemulsion liquid detergent composition has been reported to be useful for the
removal of greasy soils from substrates, in both concentrated form and in a diluted
form with water.
9.3.2 c h a r a c T e r I S T I c S a n d S T a b I l I T y o f e m u l S I of n S
The stability of any emulsion is dependent on needs and the application area. In some
cases, the emulsion need to be stable for longer time than in other cases. As in the case
of hair cream, the emulsion should destabilize as soon as it is applied to the hair, as
otherwise, the hair will be white with emulsion droplets. On the other hand, any emul-
sion used in spraying on plants needs to be stable for longer time. Further, if one needs
to clean oil spills on oceans, the emulsion formation then needs to be destabilized .
There are different processes that are involved in the stability and characteristics
of emulsions.
They are as follows:
Creaming or flocculation of drops: This process is described in those cases
where oil drops (for oil-water) cling to each other and grow in large clus-
ters. The drops do not merge into each other. The density of most oils is
lower than that of water. This leads to instability as the oil drop clusters rise
to the surface (Figure 9.5).
This process can be reduced by
1. Increasing the viscosity of the water phase and thereby decreasing the
rate of movement of the oil drops
2. Decreasing interfacial tension and thus the size of the oil drops
The ionized surfactants will stabilize O/W emulsions by imparting the surface
electrical double layer ( EDL ).
9.4 lIquId cryStalS
Liquid crystals (LC) are phase structures that are intermediate between liquid and
crystal phases. They have also been mentioned as mesophases (Greek mesos = mid-
dle). Liquid crystals have an intermediate range of order between liquid and crystal
phases (Soltis et al., 2004; Friberg, 1976). LC may be described as follows. If a pure
substance, such as stearic acid, is heated, it melts at a very specific temperature.
Heating a pure solid shows the following behavior: