15390 J. Phys. Chem. B, Vol. 108, No. 39, 2004
Jiang et al.
1. Dependence on the Nature of the Counterion. The results
in Table 2 show that the CAC is 2 to 6 times lower than the
corresponding CMC, indicating that interaction between DNA
and these surfactants starts at very low surfactant concentrations.
Despite the difficulty of determining the CAC accurately because
of its early occurrence in the titration curves, the values of the
cross linking of the DNA by the attached micelles, that is, the
degree of attachment of DNA to the micelles increases with
DNA concentration. As the DNA concentration increases, ∆Gagg
becomes less negative and ∆Hagg becomes more negative, but
in all cases T∆Sagg > -∆Hagg, showing that the aggregation of
the C12C6C12X2 with DNA is driven by entropy in all the
circumstances investigated here.
2-
CAC at a given DNA concentration vary in the order of SO4
-
< NO3 < Br- < Ac- < Cl- < F-, exactly the same as the
Conclusions
CMCs in the absence of DNA. However, whereas the enthalpies
of micellization of the monovalent anions also followed the same
order as the CMCs, apart from the anomalous chloride, the
pattern of the behavior of both the enthalpy of aggregation and
the thermodynamic properties of binding is less obvious.
In this study, we report the results of the first systematic
investigation of the effect of counterions on the micellization
of the C12C6C12X2 gemini surfactants in aqueous solution as
well as on their interaction with DNA. The various thermody-
namic parameters of the two processes have been obtained from
the results of isothermal titration microcalorimetry and con-
ductivity measurements. The values of enthalpy changes for
dilution of these surfactants with monovalent counterions into
pure water and into DNA are all negative, whereas those for
C12C6C12SO4 are all positive. This is interpreted tentatively as
being associated with changes of hydration during the associa-
tion. The counterion has a marked influence on both micelli-
zation and aggregation. The CMC, CAC, and free energies of
aggregation generally change closely parallel the Hofmeister
series, but the pattern of behavior of the enthalpies is often more
complex, revealing features that are probably associated with
different levels of hydration. The binding of micelles to DNA
is dominated by the large gain in entropy on release of the small
counterions from the micelles and DNA.
The aggregation process can be thought of as consisting of a
contribution from micellization and a contribution from the
binding of the micelle to DNA. The thermodynamic properties
of binding, for example, ∆GDS, can be obtained from the
differences of ∆Gagg - ∆Gmic. The values of ∆GDS are all
negative, confirming the strong binding of the micelles to DNA,
but there is no obvious pattern that correlates with the Hofmeis-
ter series. The contribution from micellization should parallel
that for micellization in the absence of DNA. The contribution
from micelles binding to DNA will depend mainly on electro-
static interactions, that is, the degree of ionization of the micelle
and the binding strength of the counterions to the micelle and
to DNA. Displacement of counterions could be endothermic or
exothermic, depending on changes in hydration, but will
generally be expected to be accompanied by a gain in entropy.
Comparison of the enthalpies and entropies of aggregation with
those of micellization indicates that the enthalpies ∆HDS are
generally small and exothermic, but the entropies are all large
at typical T∆SDS values of 10 kJ mol-1. Thus, the binding of
the micelles to DNA is more or less completely determined by
the large gain in entropy resulting from release of the counter-
ions. This explains why the binding of the micelles to DNA is
the weakest for the sulfate complex, because only half the
number of ions is released from the micelle. Also, among the
monovalent ions the highest gain of entropy is for the least
dissociated micelle, that of the nitrate ion. The pattern of free
energies of binding in the monovalent ion series follows the
Hofmeister series, except for the bromide system, which is out
of place. This is the only system for which the enthalpy of
binding to DNA is significantly above the experimental error
and it is distinctly exothermic even at the lowest DNA
concentration. The gain in entropy is also unusually low in
comparison with the rest of the series. These two parameters
seem to have combined to displace the bromide system from
its place in the Hofmeister series. It is also interesting that,
although the enthalpy of micellization for the chloride system
was anomalous, its binding to DNA is much more closely in
line with that expected from the Hofmeister series. Indeed, apart
from the bromide, the binding of the other four monovalent
systems follows the Hofmeister series exactly.
Acknowledgment. We are grateful for financial support from
the Royal Society, the Chinese Academy of Sciences, the
National Natural Science Foundation of China, and the National
Science and Technology Committee and CNPC Innovation Fund
(Grants Q810, 20233010, 20173067, and 2001AA602014-2).
References and Notes
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2. Dependence on DNA Concentration. The calorimetric
titration experiments were carried out at three DNA concentra-
tions, 0.33, 0.65, and 1.63 mM phosphates.
The titration curves at various DNA concentrations for each
of the C12C6C12X2 into DNA at 298.15 K are generally similar
in shape and, in all cases, the exothermic peak shifts to higher
surfactant concentration as the DNA concentration increases,
showing that there is an increase in the interaction with DNA
with increasing concentration. However, the results in Table 2
show that the increase in both the CAC and C2 is not linear in
DNA concentration, suggesting that there may be a degree of