Hydrophobically associated networks - Physical Gels from Biological and Synthetic Polymers - page 161

Table 6.1 Critical micellar concentrations of some common surfactants.

Surfactant CMC (mM)

Anionic (Sodium alkyl sulphates)
C 8 H 17 - SO 4 − Na + 130

C 10 H 21 - SO 4 − Na +

33.2

C 12 H 25 - SO 4 − Na + (SDS) 8.1

C 14 H 29 - SO 4 − Na + 2.1

Cationic (Alkyl trimethylammonium bromides)
C 12 H 25 - N(CH 3 ) 3 + Br −

15

N(CH 3 ) 3 + Br − (CTAB or HTAB) C 16 H 33 - 0.9

Non-ionic (alkyl polyoxyethylene monoethers)
C 12 H 25 - (OCH 2 CH 2 ) 6 OH (C 12 E 6 ) 0.087

between air and the aqueous solution. Surface tension decreases with increasing amphi-
phile concentrations, then shows a break at the CMC and remains constant above the
CMC. At the CMC an oriented monolayer of amphiphile molecules forms at the surface
of the water: the polar or ionic head of the molecules is incorporated into the aqueous
phase and the hydrocarbon tail is oriented away from the water.
Table 6.1 shows that, for a homologous series of surfactants bearing the same head
group, the CMC decreases with the length of the hydrocarbon chain, because of the
poorer solubility of the long hydrocarbon chain. For amphiphiles bearing the same length
of hydrocarbon chain, the CMC is much lower (by a factor of 100) for a polar head than
for an ionic head. The repulsive interactions due to charged head groups control micelle
formation; in other words, ionic surfactants are more soluble as single molecules. Above
the Krafft temperature the CMC varies very little with temperature (as shown by the
dashed line in Figure 6.2 ). That said, it does vary strongly with the ionic strength of the
aqueous solutions: for instance, adding 0.3 M of sodium chloride decreases the CMC of
SDS from 8.1 mM to 0.7 mM.
Associating polymers behave similarly to surfactants with respect to the formation of
micellar structures. The consequences of micelle formation for the rheological properties
of the aqueous solutions are, however, very important and lie at the origin of the use of
these polymers in numerous applications.

6.3 Hydrophobically modified water-soluble polymers

The notion of associative polymers was introduced in the early 1980s. These consist of a
water-soluble hydrophilic backbone and a small amount (< 2

5 mol%) of covalently
linked hydrophobic chains (Glass, 1986 , 1989 ; Shedge et al., 2005 ). The molecular
architecture of the grafted chains and the characteristics of the hydrophilic backbone
allow a great variety of hydrophobically modi

-

ed (HM) water-soluble polymers to be
synthesized, including random, block and end-capped molecules.