A small part of a phase diagram is given in Figure 10.10a. An
important characteristic is the temperature at which the apparent solubility
of the surfactant markedly increases, called theKraft point,T. Above that
temperature micelle formation can occur. BelowT, the molecules can form
a-crystals, which have a very small solubility in water. The Kraft
temperature increases with increasing chain length and can be as high as
608 C for some surfactants. Mesomorphic phases only form above the Kraft
temperature. Figure 10.9b illustrates that at lower temperature a so-called
a-gelcan be formed. Ana-gel is not in thermodynamic equilibrium; it tends
to change into a dispersion of crystals in water.
The latter change can be quite slow if the surfactant is not pure, and
virtually all technical preparations constitute a mixture. Chain length, chain
saturation, and type of head group can vary. This is of considerable
practical importance. For example, when a fairly pure preparation of
saturated monoglycerides is added to a liquid food at, say, 60 8 C, it will
readily disperse, forming micelles. On cooling to below the Kraft point,
however, crystals will form, and very little monoglyceride is left in solution.
If then oil is added and the mixture is agitated to form an emulsion, the
monoglyceride will barely or not at all reach the O–W interface and is thus
inactive. If a less pure preparation is used, crystal formation can be delayed
TABLE10.2 Some Small-Molecule Surfactants and Their Hydrophile–Lipophile
Balance (HLB) Values
Type Example of surfactant HLB value
Nonionics
Aliphatic alcohols Hexadecanol 1
Monoacylglycerols Glycerol mono stearate 3.8
Monoacylglycerol esters Lactoyl monopalmitate 8
Spans Sorbitan mono stearate 4.7
Sorbitan mono oleate 7
Sorbitan mono laurate 8.6
Tweens Poly(oxyethylene) sorbitan mono oleate 16
Anionics
Soaps Na oleate 18
Lactic acid esters Na stearoyl-2-lactoyl lactate 21
Teepola Na dodecyl sulfate 40
Cationicsa Palmityl trimethyl ammonium bromide large
Phospholipids(zwitterionic) Lecithin * 9
aNot used in foods but as detergents.