Solubilization-emulsification mechanisms of detergency

C.A. Miller and K.H. Raney/Colloids Surfaces A: Physicochem. Eng. Aspects 74 (1993) 169-215 171
between the surfactant solution and oil (γ ow), oil
and solid substrate (γ os) and surfactant solution
and solid substrate (gws) to the equilibrium
contact angle q measured through the soil
cos θ = γ ws − γ os
γ ow
(1)
For quite hydrophilic surfaces like cotton, gws
is smaller than g0s, and a contact angle greater
than go is commonly achieved. Anionic
surfactants which adsorb on the fabric with their
negatively charged head groups oriented toward
the detergent solution are particularly effective
in reducing gws. in this case, the roll-up
mechanism is operative: the water preferentially
wets the fabric, causing the oily stains to be
entirely lifted off the fibers into the washing
solution. This behavior, shown schematicaily in
Fig. 1(b) for soil removal from a flat surface. is
enhanced on cotton fabric due to swelling of the
cotton fibers with water which increases the
hydrophilicity of the fabric surfaces [9,13].
For low surface energy, i.e. hydrophobic,
materials such as polyester, a contact angle of
less than 90º is usually observed, and small
portions of the oily soil may be removed by
hydraulic currents at the soil-water interface, as
shown in Fig. 1(a). In Fact, if the fabric surface
is initially completely covered by oily soil, no
location is available for the surfactant solution
Fig. 1. Mechanisms of liquid soil removal: (a)
emulsification; (b) roll-up.
to reach the fiber surface and undercut the soil.
Observation of this "necking" or emulsification
mechanism has been made by many
investigators for mineral oils and mineral
oil-polar soil mixtures on hydrophobic flat films
and fibers [8-12]. Removal in this manner is
enhanced by low interfacial tension at the
oil-water interface which allows the oil film to
be deformed easily to form small emulsion
droplets.
Several factors have been studied with regard
to their effect on the emulsification mechanism
for the removal of mixtures of mineral oil and
polar organic alcohols or acids from polyester
[8-10]. Such model systems, depending on the
ratio of the non-polar and polar constituents, can
be considered to be representative of sebum
soils from the skin. The rate of emulsification of
mineral oil-oleic acid mixtures from polyester
(Mylar) films was found to change as the oleic
acid content was varied [8], Other factors such
as electrolyte concentration and temperature
were also found to have large effects on the rate
of soil removal by this mechanism [8,9]. In
some situations, emulsification of
non-polar-polar soil mixtures without external
agitation, i.e. spontaneous emulsification, has
been observed [ 13,14]. Emulsification, roll-up,
and other adhesion and detachment phenomena
involving oily soils and solid surfaces are
reviewed in the accompanying paper [ 15].
Another mechanism of oily soil removal
involves the formation of intermediate phases at
the detergent solution-oil interface
[11,13,14,16]. Apart from the recent studies
described below, this mechanism has most often
been reported for the removal of soils containing
large quantities of polar constituents. The
growth of liquid crystal occurs in these systems
due to interaction at the interface between the
polar soil constituents and the adsorbed
surfactants. After growing to a sufficient extent,
the intermediate phase is broken off by agitation
and emulsified into the aqueous solution
allowing fresh contact of the remaining soil with
the detergent solution.
Direct solubilization of oily soils into
surfactant micelles can also occur to a
significant extent if a large excess of surfactant