Chemistry Reference
In-Depth Information
publications appearing on the subject, the area remains somewhat empirical in that
each system is highly specific. As yet there are few general rules to guide the inter-
ested formulator in the selection of the optimum surfactants for a given application.
Cubic bicontinuous phases discussed in Chapter 5 appear to offer some of the same
potential advantages sought in multiple emulsion systems. A great deal remains to
be done in understanding the colloidal stability of such complex systems and the
effects of the various components in each phase on overall multiple-emulsion pre-
paration and stability. A sound understanding of the role of surfactants in simple
emulsions and an intuitive feel for the effect of the multiple interfaces present on
system operation seem to provide the best guidance at the present time.
PROBLEMS
9.1. Tetradecane was emulsified at 25
C in two 0.5% (w/w) surfactant solutions:
(a) C
12
H
25
-(OCH
2
CH
2
)
5
OH and (b) C
12
H
25
OSO
3
Na. What class of emulsion
would you expect in each case? What would you expect to be the natures of
the two emulsions when heated to 50
C?
9.2. Two common surfactants, SDS and sodium dioctylsulfosuccinate, have been
found to occupy areas of 0.50 and 1.11 nm
2
, respectively, when adsorbed at
the oil-water interface. Calculate the amount of each surfactant required to
completely saturate the interface formed by the emulsification of 500 g of
tetradecane in one liter of aqueous surfactant solution. Assume that the
emulsion is monodisperse with a droplet diameter of 1000 nm.
9.3. One process by which an emulsion, like a foam, destroys itself is Ostwald
ripening—the diffusion of liquid from small to large droplets. Estimate the
time required for a benzene droplet to disappear when it is positioned near
much larger droplets at a distance comparable to its radius. Assume droplet
radii of 100 and 1000 nm. The solubility of benzene in water may be taken
as 0.2% (v/v); the diffusion constant of benzene in water D =10
5
cm
2
/ s, the
interfacial tension of water-benzene
s ¼
25 mN/m, and the molar volume of
100 cm
3
.
benzene V
m
¼
9.4. Calculate (to the nearest whole number) the maximum possible value for the
dispersed phase fraction
f
in an emulsion consisting of uniform spherical
particles.
9.5. A simple geometric theory for the stabilization of emulsions is that of the
''oriented wedge,'' in which the adsorbed surfactant molecules are assumed
to form a uniform structure of ''wedges'' around the emulsion droplet. If an
emulsion of 1000-nm-diameter droplets is stabilized by a surfactant whose
head group occupies a surface area of 0.45 nm
2
, what must be the cross-
sectional area of the hydrophobic tail for maximum effectiveness?
9.6. A mixture of 70% sorbitan monostearate and 30% sorbitan tristearate was
found to give optimum stability to a particular emulsion system. What
