Solubilization-emulsification mechanisms of detergency

C.A. Miller and K.H. Raney/Colloids Surfaces A: Physicochem. Eng. Aspects 74 (1993) 169-215 197
observed during the experiment. When the drop
composition reaches point F at the end of the
coexistence curve, the driving force for
diffusion of the surfactant and water into the
drop remains. gut because the drop cannot
remain in the L2 region, according to the phase
diagram, the lamellar phase begins to form.
A theory has been developed which predicts
that the time t from the start of the experiment
until the liquid crystal starts to form is given by
the following expression [60]
2
t = Ks (R0 /Dsws∞) (6)
Here R 0 is the initial radius of the drop, D s and
are the diffusivity and bulk concentration of the
surfactant in the aqueous solution and K s is a
constant that depends only on the shape of the
coexistence curve and the location of point F.
The predicted proportionality between t and Ro
2 has been confirmed by experiment (see Fig.
34), as has the inverse relationship between t and
the bulk surfactant concentration w s∞. With a
Fig. 34. Plot of the square root of the time t required
to initiate liquid crystal formation as a function of
initial drop size for the system of Fig. 32. The
surfactant concentration in the aqueous phase is 1.0
wt.% [74]. Reprinted with permission of Plenum
Press.
value of 0.514 for K. calculated using the phase
behavior of Fig. 32, Eq. (6) and the measured
values of t were used to estimate Ds. A value of
about 4 x 10 -11 m 2 s -1 was obtained, which is
reasonable for a micellar solution.
A similar equation has been developed for the
case of a uniform layer of oil on a flat solid
surface immersed in a stirred aqueous surfactant
solution [74]
t = K p (h 0d/D sw s∞) (7)
where ho is the initial thickness of the oil layer
and d is the thickness of the diffusion boundary
layer adjacent to the oil. Like Ks in Eq. (6), Kp depends only on the shape and terminal point of
the coexistence curve between the L1 and L2 phases.
For the C12E8-water-n-decanol system at
temperatures above 14ºC, the three-phase
triangle bounding the L1-L2 region has D'
instead of Lα as the additional phase [59]. When
a contacting experiment was conducted at 27ºC
in this system, with a drop initially about 60 µm
in diameter, a liquid intermediate phase
developed after about 14 min and surrounded
the initial alcohol drop [55]. As the intermediate
phase grew during the next 8 min to a diameter
of about 83 mm, the alcohol drop shrank slightly
to a diameter of about 83 µm. Presumably, this
growth process could be analyzed using a
"shrinking core" model with the quasi-steady
state approximation. However, as the available
data on phase behavior [59] do not include
coexistence curves for the L1-D' and L2-D' two-phase regions, it is not currently possible to
make quantitative comparisons between
predictions of the analysis and the experimental
results.
7.3. Experiments with mixtures of hydrocarbons
and long-chain alcohols
More interesting for detergency applications
than pure alcohols are mixtures of polar and
nonpolar oils, which are representative of
sebum-like soils. Here we discuss a series of
experiments in which drops of various mixtures
of n-hexadecane and oleyl alcohol, typically