J. CHEM. RESEARCH (S), 1998 575
Table 3 Effect of [SDS]
promoted Cr oxidation of DMSO in aqueous acidic media.
T
and [CPC]
T
on kobs for the PA-
VI
VI
�3
�3
�3
,
[
[
Cr ] 2Â10 mol dm , [DMSO]
T
0.2 mol dm
�
3
PA]
T
0.03 mol dm , 35 8C
2
0 [SDS]
a
�3
�3
1
1
1
1
T
�1
/mol dm
:
:
:
:
0.0 1.0 2.0 3.0 4.0
3.4 4.7 6.0 7.2 8.9 10.2 11.8
0.0 1.0 2.0 3.0 4.0
4.9 4.3 3.6 3.2 2.8
5.0 6.0
4
0
0 [CPC]
kobs/s b
3
T
/mol dm
5.0
2.0
4
�1
kobs/s
0
a
] 0.25 mol dm�3; I 1.5 mol dm�3
�
.5 mol dm .
b
[
HClO
4
.
[H SO
2
4
]
3
0
the increase in kobs. Inhibition by the cationic micelle (i.e.
CPC) is due to the fact that DMSO is distributed preferably
in the micellar phase due to hydrophobic interaction, but
the approach of the other reactive species II (positively
charged) is repelled. The process is catalysed by H , whose
approach to the micellar phase is prevented due to the
repulsion from the cationic surfactant.
Lastly it is worth mentioning that the oxidation of DMSO
involves the nucleophilic attack of `S' of DMSO on the oxi-
dant species II and it leads to the build-up of positive charge
on S (cf. species III, IV) which will be disfavoured by the cat-
ionic micellar head groups. On the other hand, development
of such positive charge on S in DMSO is coulombically
favoured in the anionic surfactants. Thus the micellar eects
support the proposed mechanism involving positively
charged reactive oxidant species II and the proposed inter-
mediate III/IV where there is a build-up of positive charge
on S of DMSO due to the nucleophilic attack by S.
Scheme 1
3
[
®
H ] (0.35±1.5 mol dm ), eqn. (12) explains the observed
Experimental
rst order dependence on [H ]. From the [H ] dependence,
�
3
3
�1 �1
the calculated values of k
s
(3.9Â 10 dm mol
�1 �1
s
s
) and
) at 35 8C nicely agree with
those obtained from [DMSO] and [PA] dependence,
respectively. The catalytic eciency of PA in the present
PA was used after repeated recrystallisation from methanol
mp 136 8C). DMSO was puri®ed and standardised as described
�
3
3
(
k
p
(12.8Â 10 dm mol
9
previously. All other chemicals used were of analytical or general
reagent grade or puri®ed by standard procedures. The rate of
disappearance of Cr was followed by the titremetric quenching
T
T
VI
2
VI
IV
system is argued as being mainly due to enhanced Cr /Cr
technique using excess standard Mohr's solution and the unreacted
Fe was determined by standard Ce solution using ferroin indi-
cator. The pseudo-®rst-order rate constants were obtained from the
IV
IIII
II
9
IV
and Cr /Cr reduction potentials in the presence of PA.
To circumvent the solubility problem, dierent acids, i.e.
HClO and H SO , have been used for the anionic (SDS) and
cationic (CPC) surfactants respectively. It is evident that SDS
catalyses (cf. Table 3) the title reaction while CPC inhibits
VI
slopes of ln[Cr
t
] versus time (t). Initial slopes were used for the
4
2
4
T
sets with very low [PA] .
Thanks are due to CSIR and UGC, New Delhi, for
®nancial support.
(
cf. Table 3) the process. The results can be explained by con-
8
sidering the pseudo-phase ion exchange (PIE) model which
considers the micellar and aqueous phases as two distinct
phases and in the present case the redox reaction occurs in
the both phases. In the presence of SDS, the rate acceleration
Received, 4th February 1998; Accepted, 19th May 1998
Paper E/8/00993G
VI
is due to the preferential partitioning of the Cr ±PA com-
References
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2
3
4 J. R. Pladziewicz and J. H. Espenson, Inorg. Chem., 1971, 10,
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5
6
2
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T
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7
8
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9
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Scheme 2