Solubilization-emulsification mechanisms of detergency

C.A. Miller and K.H. Raney/Colloids Surfaces A: Physicochem. Eng. Aspects 74 (1993) 169-215 187
have recently been performed. Experiments
were designed to investigate the effects of
changes in temperature on the dynamic
phenomena which occur when aqueous
solutions of pure non-ionic surfactants contact
hydrocarbons such as tetradecane and
hexadecane [18,72]. These oils can be
considered to be models of non-polar soils such
as lubricating oils. The dynamic contacting
phenomena, at least immediately after contact,
are representative of those which occur when a
detergent solution contacts an oily soil on a
synthetic fabric surface. The following is a
summary of the observed behavior interpreted
through the use of schematic diffusion paths.
Detailed phase behavior in such systems has
been reported previously and was used in
construction of the diffusion paths [47].
With C12E5 as the non-ionic surfactant at a 1
wt.% level in water, quite different phenomena
were observed below, above, and well above the
cloud point when tetradecane or hexadecane
was carefully layered on top of the aqueous
solution. Below the cloud point temperature of
31ºC, very slow solubilization of oil into the
one-phase micellar solution was observed. An
interesting phenomenon was observed at 20ºC
involving a "volcanolike" instability which
caused flow of the aqueous solution to the oil
interface. This flow column, which is believed
to have resulted from an adverse density
gradient within the aqueous phase, is shown in
Fig. 19 [72]. The upper tip of the column was
observed to oscillate, probably due to gradients
in interfacial tension along the oil-water
interface (Marangoni flow). Of more importance
to a detergency process, the schematic diffusion
path shown in Fig. 20(a) explains why no
intermediate phase formed between the water
and oil. Also, due to the low solubility of oil in
the dilute aqueous surfactant solution in this
region of the ternary phase diagram, it predicts
the quite slow solubilization of oil into the
surfactant solution. At temperatures just below
the cloud point temperature, an intermediate
phase depleted in surfactant did form between
the micellar solution and the oil. The schematic
diffusion path in this case is shown in Fig.
20(b). Once again, instabilities in the aqueous
Fig. 19. "Volcano" instability in C 12E 5-water-ntetradecane
system at 20ºC. The image is out of focus
to allow observation of refractive index variations
[72]. Reprinted with permission of Academic Press.
Fig. 20. (a) Diffusion path well below the cloud point
showing no intermediate phase formation; (b)
diffusion path slightly below the cloud point showing
the formation of intermediate phase W [72].
Reprinted with permission of Academic Press.
ous phase occurred, in this case due to a density
difference between the original micellar solution
and the intermediate phase, causing flow of
surfactant solution to the oil interface.
Nevertheless, at all temperatures studied below
the cloud point, only very slow solubilization of
oil into the surfactant solution was observed.
At 35ºC, which is just above the cloud point, a
much different behavior was observed. The
surfactant-rich L1 phase separated to the top of
the aqueous phase prior to contacting by
hexadecane.