Tee and Yazbeck
1107
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Nonlinear fitting of eq. [6] was carried out with GraphPad
Prism software. When using eqs. [5] and [6] in data analysis,
one has to decide how to handle the concentration of
surfactant [Surf] involved in micelles. This quantity is re-
duced from the total surfactant by the amount of the critical
micellar concentration (cmc): [Surf] = ([Surf]o – cmc) (1).
For CTAB, cmc is 0.92 mM at zero ionic strength, but it is
appreciably lower at high salt concentrations (18) and the
lowering effect is greater still for less hydrophilic anions like
thiolate ions (19). For example, the addition of only 40 µM
thiophenolate ion, in a 0.010 M borate buffer at pH 10, low-
ers the cmc of CTAB to 0.08 mM (14). In the present work,
taking [Surf] = ([Surf]o – cmc), and treating cmc as a fitting
parameter in eq. [6] for the data obtained for each of the
seven esters reacting with 20 mM ME in 0.10 M carbonate
buffer, gave values of cmc that were close to an average of
0.05 mM. Accordingly, for consistency we have taken
the cmc to be equal to this value throughout.
According to the literature, the pKas of the thiols at zero
ionic strength are: (i) cysteine, 8.39 (20); (ii) 2-
mercaptoethanol, 9.72 (21); (iii) mercaptoacetic acid, 10.56
(20);6 (iv) 3-mercaptopropionic acid, 10.84 (21), but they are
lower at high ionic strength. For example, at µ = 1.0 M, the
pKa of ME is 9.61 (22), and at µ = 0.2 M, the second pKa of
MAA drops from 10.56 to 9.84 (20). Thus, at the working
pH of 10.60, in the 0.10 M carbonate buffer, the thiols will
exist to a considerable extent as their reactive thiolate an-
ions. The exact fractions of the anions are not important to
the discussion above because they are the same for all the
esters in the series, and they cancel out in the ratios kcN/kN
and in KTS = kNKS/kcN (eq. [7]). The relevant pKa of glycine
is 9.78 (23), and that of TFE is 12.4 (22, 24).
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Acknowledgements
We thank the Natural Sciences and Engineering Research
Council of Canada for operating grants and an equipment
grant, and Mr. A.A Fedortchenko for technical advice. We
have also benefited from correspondence with Prof. A. Wil-
liams, University of Kent, Canterbury (U.K.). OST is partic-
ularly grateful to Dr. Chris Easton and the Research School
of Chemistry at the Australian National University for their
hospitality during a sabbatical leave spent in Canberra.
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© 2000 NRC Canada