may still be available as a reactant, although it will react only sluggishly. But
if no micelles were present, a very apolar ‘‘solute’’ would not be there and
thus would not react at all.
Other kinds of self-association occur. Several proteins associate to
form fairly large aggregates, thereby markedly decreasing their activity.
Fatty acids present in an oil phase always dimerize through hydrogen bonds:
jj jj jC 55OHO (^22)
j C
OHO 55
j jj jj
thereby greatly decreasing their activity and increasing their solubility (see
Question 2, below).
- Electric shielding.The activity of ions is diminished by electric
shielding. This is discussed in Section 2.3. The total ionic strength rather
than the concentration of the ions studied determines the activity coefficient;
see Figure 2.7.d.
Question 1
Consider an aqueous solution of 5 millimolar (pure) sodium dodecyl sulfate (SDS).
How large will the freezing point depression be? And the water activity? What will
happen if the solution is cooled down to 0 : 1 C?
Answer
SDS will fully dissociate at low concentration. A solution of 5 mmol?L^1 will thus
contain 0.01 mol ions per liter. Inserting m 2 ¼ 0 :01 in Eq. (2.16) leads to
4Tf¼ 0 :0186 K. Putting water activityaw¼x 1 then yieldsaw¼ 0 :99982. At
0 :1 K below the freezing point of water, we thus have significant freeze
concentration. According to Eq. (2.16), m 2 would then equal 0.054 molar,
corresponding to 27 mmolar SDS, assuming it to be fully dissociated. However,
Figure 2.8 shows that the critical micellization concentration (CMC) equals about
8 mmolar SDS. Hence micelles will be formed, andm 2 would hardly increase over the
CMC. This then would mean that by far the largest part of the water must freeze at
0 : 1 C. The little bit of concentrated solution left will be strongly nonideal, and
prediction of its properties would be quite difficult.