Emulsions, Microemulsions, and Lyotropic Liquid Crystals - Surface and Colloid Chemistry: Principles and Applications - page 181

Light scattering: Modern laser light scattering instruments are very advanced
devices for particle size distribution analyses. The laser light is scattered by
the small dispersed particles or drops. The latter is known to be dependent
on the radius of the particle.

9.2.3.1 emulsion Stability
Emulsions are stable as long as the drops are separated from each other. Flocculation
of an emulsion or dispersion takes place upon collision of the droplets, which is related
to the Brown motion, convective stirring, or gravitational forces. Actually, any emul-
sion can be separated into oil and water phases by suitable centrifugation treatment.
The dispersion force of attraction between two bodies i and j (molecules, par-
ticles, drops), E ij , is dependent on following parameters:

E ij = H ij /(12 Π R 2 ) (9.3)

where H ij is the Hamaker constant for i and j, and R is the distance between the par-
ticles. Since in emulsions we have oil (1) and the continuous medium water (2), then
the expression for E 121 is found to be

E 121 = ( a H 121 )/(12 R ) (9.4)

where a is the size of the oil drop. The Hamaker constant, H 121 , is found to be related
to the dispersion surface tension, γLD, such that, for oil/water emulsion,

H 121 = 3 10 −14 /ε 2 (γ LD 0.5 − γ 2D 0.5) 11/6 (9.5)

where γ LD (30 mN/m) and γ 2D (22 mN/m) are the dispersion surface tensions of oil
and water, respectively. From these equations, we find that, if oil/water

γ LD = 30 mN/m; ε 2 = 1.77 (9.6)

then H 121 is equal to 1.1 10 −14 ergs. For drops of size equal to 1 μcm (a = 5 10 −5 cm),
E 121 is equal to almost k B T (4 10 −14 erg, at 298 K [25°C]).
H 121 has been shown to be always positive, which suggests that, in two-phase
systems (such as oil-water), the particles will always be attracted to each other. This
means that even air bubbles will attract each other, as is also found from experi-
ments. A linear relation is found between H 121 6/11 and γ LD , as expected from Equation
Experimental values of A 121 as determined from flocculation kinetics showed that
this agreed with the theoretical relation.

9.2.3.2 electrical emulsion Stability
Those systems in which the emulsifier carries a charge would impart specific char-
acteristics to the emulsion. A double layer will exist around the oil droplets in an
O/W emulsion. If the emulsifier is negatively charged, then it will attract positive
counterions while repelling negative charged ions in the water phase. The change in