Surface Chemistry of Surfactants and Polymers by Holmberg Krister Lindman Bjorn Kronberg Bengt

Author:Holmberg, Krister, Lindman, Bjorn, Kronberg, Bengt [Holmberg, Krister, Lindman, Bjorn, Kronberg, Bengt]
Language: eng
Format: epub
Published: 0101-01-01T00:00:00+00:00

238

Surface Chemistry of Surfactants and Polymers

Figure 12.6 shows some surface tension data obtained for the nonionic series C E . The

16 n

surfactants with shorter polyoxyethylene chains pack better at the liquid–air interface, as seen by their steeper slopes at concentrations just below the CMC. We also note that the effectiveness is higher, that is, the surface tension at and above the CMC is lowered, as the polyoxyethylene chains are shortened. The effect on the CMC is rather small, however.

50

mN/m

C

n/

16E21

ensio

C16E15

40

ace t

C16E9

Surf

C16E6

30–7

–6

–5

–4

–3

–2

Log (surfactant concentration / M)

Figure 12.6 Surface tension data for the nonionic series C E with the indicated values of n.

16 n

(With permission from Elworthy, P.H. and MacFarlane, C.B. (1962) Surface activity of a series of synthetic nonionic detergents. Journal of Pharmacology and Pharmacotherapeutics , 14, 100T–102T. Copyright 2011, John Wiley & Sons.)

Figure 12.7 shows the temperature dependence of the surface tension for C E . As the

12 6

temperature is increased, the polyoxyethylene chain compresses. The figure shows that this results in a higher effectiveness, that is, a lowering of the surface tension, as well as of the CMC. Careful analysis also shows that the adsorption at concentrations below the CMC

increases with temperature, that is, the surface area per molecule decreases with temperature.

40

C12E6

mN/m 36

n/

ensio

t = 15°C

32

ace t

t = 25°C

Surf

t = 35°C

28–4.5

–4.0

–3.5

–3.0

Log (Surfactant concentration / M)

Figure 12.7 Temperature dependence of the surface tension from solutions of C E .

12 6

(Reproduced from Corkhill, J.M., Goodman, J.F. and Ottewill, R.H. (1961) Micellization of homogeneous nonionic detergents. Transactions of the Faraday Society , 57, 1627–1636 with permission of The Royal Society of Chemistry.)

Chapter 12 Surface and Interfacial Tension

239

Ionic Surfactants

Also for ionic surfactants the surface tension becomes constant at the CMC. This is due to a constant unimer concentration at higher concentrations. However, ionic surfactants differ from nonionic surfactants in that the limiting value of the adsorption just below the CMC

is not limited by steric restriction but rather electrostatic restriction due to the entropy of the counterions. In the bulk solution the surfactant is totally dissociated, hence the counterions lose entropy when the unimers adsorb at the surface (counterion binding). This

entropy loss is higher the larger the surface charge density, that is, the higher the adsorption (Figure 3.2). A limiting adsorption is reached when this loss in entropy balances the driving force for adsorption. This limiting adsorption does not correspond to a close packing of the surfactant as for nonionic surfactants, that is, as in steric restriction, but is determined by the entropy loss of the counterions.

For monovalent ionic surfactants the constant n in Equation 12.8 ideally has a value of two, since the surfactant contains two species, namely the surfactant ion and the counterion. However, due to the constriction of electroneutrality, the counterion is not free to move around in the bulk solution, which means that this value is in reality less than two. If the counterion is bound to the amphiphile, as in a zwitterionic surfactant, the value of n would be one. Therefore n could have any value between one and two for a 1:1 surfactant.

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