or by removing it in layers from the sur-
face, or by removing it in layers by emulsi-
fication, or by penetrating the contamination
to form inverse micelles, followed by the
formation of liquid crystals and solubili-
zation or emulsification,
3) It disrupts the polyvalent cation links and
hydrogen bonds by pH control, ion ex-
change, and sequestrating the polyvalent
cation (forming coordination complexes),
and
4) It prevents redeposition by maintaining the
contaminants in suspension (Osipow 1962).
The concentration of surfactant at which mono-
mers assemble into aggregates (micelles) is termed
the critical micelle concentration (cmc) (Di Cesare
and Smith 1994). Many organic compounds that
are not very soluble in water dissolve to a consid-
erably greater extent in micellar solutions of both
ionic and nonionic surfactants (i.e., where the con-
centration is greater than the cmc); this process is
called solubilization (Osipow 1962). Direct evi-
dence that solubilized materials are present
within the micelles is provided by X-ray meas-
Figure 1. Effect of spray temperature on soil removal
urements (Osipow 1962). Solubility enhancements
(detergents at 0.5% concentration; 15-sec sprays)
of (nonionic) organic compounds at surfactant
(from Keys 1980).
concentrations below the cmc have only been re-
ported for extremely water-insoluble compounds,
The hottest water (49C or 120F) was more than
e.g., DDT (Di Cesare and Smith 1994).
In general, according to Osipow (1962), surface
twice as effective as the coolest water. Studies that
activity is due to nonmicellar surfactant, and the
have assessed the ability of detergent solutions to
micelles act as a reservoir for the unassociated
remove neat chemical warfare agents from vari-
surfactant molecules and ions. At concentrations
ous surfaces (an extreme example of contamina-
greater than the cmc value, the surface tension of
tion) have also generally found increased remov-
the solution does not decrease further with in-
al efficiencies using heated solutions (Mills et al.
creasing surfactant concentration (Osipow 1962).
1977, Bagley and Weatherhead 1978).
However, the ability of surfactant solutions to sol-
However, according to Osipow (1962), the ef-
ubilize water-insoluble materials starts at the cmc
fect of temperature on surfactant solutions is not
and increases with the concentration of micelles
straightforward. For ionic surfactants in the ab-
(Osipow 1962).
sence of other additives, the cmc generally increas-
Often, heating an aqueous cleaning solution can
es with increasing temperature. (Although for
improve cleaning efficiency. Generally, the solu-
some ionic surfactants, there is an optimum tem-
bility of organic chemicals is greater in heated so-
perature that produces a minimum cmc.) This
lutions. Heat also raises the surface temperature,
means that, generally, more surfactant will be re-
thereby facilitating volatilization of sorbed organic
quired in heated cleaning solutions to reach the
contaminants. In the case of gross contamination
cmc. However, temperature also affects the solu-
with pure product, heat would also reduce the
bility of the surfactants in a way that counters
viscosity of contaminants, thereby facilitating
this trend. The solubility of these (ionic) surfac-
physical removal from either the surface or with-
tants is low at low temperatures and increases
in the pore structure. Keys (1980) compared the
slowly and regularly with increasing temperature
effect of spray wash temperature (21C vs. 49C,
until, within a narrow temperature range, it in-
or 70F vs. 120F) on the ability of two detergent
creases very rapidly. According to Osipow (1962),
solutions to remove soil from a surface (Fig. 1).
this is because ionic surfactants in unassociated
4
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