Physical Chemistry of Foods

when adding NaCl. From the discussion in Section 2.3, it follows that the
activity is given byg+(cþ 6 c
)0.5, whereg+is the ion activity coefficient,
cþis the molar concentration of the cation (Na) andc
that of the anion
(dodecyl sulfate). Since the molar concentration of SDS is very small,
addition of NaCl will greatly increase the ion concentration product; on the
other hand, the higher ionic strength diminishes the ion activity coefficient
[Eq. (2.28b)], but the former effect is predominant.

Note In principle, the Gibbs adsorption equation as given [Eq.

(10.2)] should be modified for ionic adsorbates. A well dissociated

surfactant like SDS can be considered as consisting of two

adsorbing species: Naþand dodecyl sulfate
. Since electroneutral-

ity must be maintained, the counterion Naþ can be said to

FIGURE10.11 Surface pressurePat the A–W interface of aqueous sodium
dodecyl sulfate solutions as a function of concentration c, for various molar
quantities of NaCl added (indicated near the curves). The curves end at the CMCs.
(From data by E. Matijevic, B.A. Pethica. Trans. Faraday Soc. 58 (1958) 1382.)