for quite long times, and the monoglycerides do adsorb at interfaces. Once
adsorbed, they remain so, even at low temperature.
The region in the phase diagram denoted ‘‘crystalsþwater’’ often
contains other structures as well. For phospholipids, which are generally
mixtures and are very poorly soluble in water, vesicles and ‘‘liposomes’’, i.e.,
fragments of liquid crystalline phases (or possibly ofa-gel), have been
observed.
Chain Crystallization. At the O–W interface it is often observed—
especially for surfactants containing a saturated aliphatic chain—that
lowering of the temperature leads to a marked decrease in interfacial
tension. A fairly sharp critical temperature can be noted, which is called the
chain crystallization temperatureTc. AboveTcthe chains of the adsorbed
surfactant are presumed to be flexible, i.e., as in a liquid. BelowTcthe
chains would be rigid, as in a crystal. Chain crystallization can only occur if
the Gvalue is high. Upon heating the system, heat is taken up atTc
and the amount per mole is of the same order of magnitude as, though
somewhat smaller than, the molar melting enthalpy ofa-crystals of the
surfactant.Tcis lower by 20 to 50 K than thea-crystal melting temperature.
FIGURE10.10 Micellization. (a) Simplified phase diagram of a surfactant and
water.Tis temperature,T* Kraft temperature,cis molar concentration,cmis CMC.
As an example,T* may be 20 8 C,cm0.3 mol?m^3 (or about 0.01% w/w). (b)
Concentration of surfactant in free or in micellar form as a function of total
concentrationc.