1682
logk
TATARCHUK et al.
degree bound by the surfactant to form ion pairs (Ag+,
i
–O3S–) forAOT and [AuCl4]–···ç+···é< hydrogen bonds
for Triton N-42. Free water that appears inside micellar
cavities enhances the dissociation of these bonds, mak-
ing Ag+ and [AuCl4]– free to participate in reaction (2).
0.5
–1.0
–2.5
2
The dissolution of silver particles by nitric acid in
the system with Triton N-42 was studied for 100Vs/Vo ≥
1%, the conditions providing the occurrence of free
water inside micellar cavities; this water likely largely
cancels out the change in the properties of the medium
in response to Vs/Vo variations. Moreover, Triton N-42
cannot bind silver(I) cations to form ion pairs or hydro-
gen bonds. Apparently, these features of the system
under consideration are responsible for the noninflu-
ence of Vs/Vo on the effective rate constants k1 and k2.
1
0
3
100 Vs/Vo, %
In summary, our new data support that the oxidative
solubilization of nanoparticles, regardless of the metal
(silver or gold), oxidizer (nitric acid or hydrated chlo-
rine), and surfactant (ionic AOT or nonionic Triton
N-42), follows the same kinetic mechanism, which
involves the interaction of surface metal atoms with the
oxidizer in two routes: either with (autocatalysis) or
without ionic species of the oxidized metal as reaction
products. The reaction medium changes, following
variations in solubilization capacity, on account of a
change in the state of water inside polar cavities of
micelles. The process kinetics are influenced by both
the integral medium effect and specific interactions
between water, metal ions, and the surfactant. The
medium effect is more significant for the noncatalytic
route, whereas the autocatalytic route is more susceptible
to the occurrence of free water in micelles if there are rel-
atively strong specific interactions between newly formed
metal atoms and the surfactant. In the absence of such
interactions and given that an aqueous core exists, effec-
tive rate constants for both routes are independent of Vs/Vo,
as observed for silver and Triton N-42.
Fig. 5. Effect of the solubilization capacity of the micellar
solution on effective rate constants (1) k and (2) k .
1
2
'
cH O ≈ 55 mol/L and csurf = 0.25 mol/L) changes not
2
only the micelle size but also the properties of the reac-
tion medium inside micellar cavities. This primarily
pertains to the major component of the solubilized
aqueous solution, i.e., water, for which two states can
roughly be distinguished: water structured on account
of interactions of its molecules with one another and
with polar fragments of surfactant molecules and free
(bulk) water inside the cavities (cores) of micelles.
Comprehensive analysis of the literature on the struc-
ture of inverted surfactant micelles and the state of
water inside them is found in Bulavchenko’s disserta-
tion [14]. Estimates of the limiting Vs/Vo value above
which the properties of bulk water are observed depend
on the nature of the surfactant, solvent, property mea-
sured, and the sensitivity of the measurement tech-
nique. In micelles of nonionic surfactant Triton N-42,
free water appears when 100Vs/Vo > 1% as judged from
characteristic IR bands [7]. Ionic surfactant AOT binds
water more strongly. Therefore, aqueous cores appear
at higher values of the solubilization capacity (>4%);
when 100Vs/Vo < 4% (csurf = 0.25 mol/L) or wo < 10, all
solubilized water is structured due to hydrogen bonds
stabilized by high dipole moments of the sulfo groups
of AOT [15].
ACKNOWLEDGMENTS
This work was supported by the Russian Foundation
for Basic Research (project no. 05-03-32308).
REFERENCES
1. S. P. Gubin, G. Yu. Yurkov, and N. A. Kataeva, Precious
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Comparison of the laws governing the influence of
Vs/Vo on effective rate constants and the state of the
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for gold and Triton N-42, shows that the integral
medium effect is significant for k1, whereas k2 is less
responsive to medium effects but responds to the
appearance of free water. The latter is likely due to the
fact that, unlike in route (1), the products of nanoparti-
cle dissolution (Ag+ and [AuCl4]–) are involved in auto-
catalytic route (2) apart from the oxidizer. In the
absence of free water, these product ions are to some
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RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 54 No. 10 2009