A R T I C L E S
Fallis et al.
the use of water-soluble peroxide activators19 or catalysts such
as bicarbonate,20 molybdate,21 or heteropolytungstates.22 This
illustrates a fundamental dichotomy in HD decontamination,
namely that water-soluble oxidants and catalysts can yield
effective rates of oxidation of an essentially water immiscible
substrate (HD).
Reactivity in micellar or microemulsion systems is often
interpreted using the pseudophase model, in which the aqueous
continuous phase and micelles or droplets are considered to be
a discrete phases23 while the ‘compartmentalization’ of reaction
media can often have profound effects on rates and product
speciation.24 In this paper we further explore the concept of
compartmentalized reaction media further by addressing the
following questions: is the locus of decontamination within a
microemulsion important in determining reaction rate, and thus,
can catalysts be engineered to be targeted to specific sites within
microemulsions so as to achieve selective decontamination and
higher reaction rates?
here containing hydrogen peroxide, catalysts and simulants were
clear, one-phase systems. The oil-soluble catalysts [MnCl(1)] and
[MnCl(2)] were prepared from the parent ligands by adapting the
method of Jacobsen et al.27 [MnCl(3)] was synthesized by the
condensation of ethylenediamine and the appropriate salicylalde-
hyde, which was prepared by adapting the procedure of Miller et
al.28 The simulants 2-chloroethyl ethyl sulfide (here denoted EtSim,
also known as “CEES”), 2-chloroethyl phenyl sulfide (here denoted
PhSim, also known as “CEPS”), and thiodiglycol (TDG) were
purchased from Aldrich and used as received. 2-Chloroethyl
2-naphthyl sulfide (here denoted NpSim) was prepared under basic
conditions from 2-mercaptonaphthalene and a large excess dichlo-
roethane. Sulfoxides and sulfones for diffusion coefficient measure-
ments were prepared by oxidation of the sulfide using 1 (sulfoxide)
or 2 equivalents (sulfone) of sodium perborate by the method of
McKillop and Tarbin.29
Results and Discussion
HD and Simulant Oxidation: Water- and Oil-Soluble
Catalysts. In order to probe the site of sulfide oxidation within
a microemeulsion, a number of catalysts and sulfides of varying
lipophilicity were prepared. In such an approach, it is possible
to match or mismatch the sites to which the catalysts and
substrates predominantly distribute themselves and hence
determine the importance of partitioning phenomena in reactiv-
ity. The rates (expressed as half-lives, t1/2) of sulfide oxidation
of HD and a range of simulants in the toluene-butanol-sodium
dodecylsulfate (SDS)-water microemulsion (here denoted
SDS5T) by dilute hydrogen peroxide, were quantified by in situ
FT-IR measurements via the growth of sulfoxide and sulfone
peaks (Table 1). The microemulsion formulation was chosen,
as it has a track record of being able to disperse HD under
conditions of low shear. Furthermore, Bunton et al. have
suggested that in sulfide oxidations, the reaction transition state
are likely to bear a fractional positive charge on sulfur,30 and
hence an anionic microemulsion is a logical choice for catalytic
oxidations. Group VI oxo-anions are known to be efficient
hydrogen peroxide activators in sulfide oxidation,21 while oil-
soluble alkene epoxidation catalysts31 such as [MnCl(2)] have
not been used to date in this role have been shown to be effective
in the asymmetric H2O2 oxidation of sulfides to sulfoxides.32,33
Hence, tungstate (as K2WO4) and the hydrophobic Mn(III) salen
complexes [MnCl(1)] and [MnCl(2)] (Scheme 1c) were used
as water and oil soluble hydrogen peroxide activation catalysts,
respectively. Tungstate was chosen over the more widely applied
molybdate21 to facilitate heavy atom fluorecscence quenching
(vide infra). It should be noted that in water/cosolvent mixtures
(e.g., MeO(CH2)2OH) the catalysts employed, where sufficiently
soluble, were of low activity.
Experimental Section
HD is listed in Schedule 1 of the Chemical Weapons Convention
(CWC) and its production, acquisition, retention or use is strictly
controlled. The appropriate CWC National Authority must be
informed and any legislative requirements met before working with
this chemical. Caution: HD and chloroethyl simulants (hemisulfur
mustards) are severe vesicants and must be handled with great care
in an efficient fume hood.
Solvents were purified by standard literature methods.25 Deu-
terated materials were purchased from Cambridge Isotope Labo-
ratories. Full characterization of the microemulsion, synthesis of
materials, PGSE NMR and neutron/X-ray scattering measurements,
and all other experimental protocols are described in full in the
Supporting Information.
Catalysis. Rates of sulfide oxidation in microemulsion solutions
of HD and selected simulants were tested at DSTL Porton Down
with a gently stirred 20 mL sample volume and monitored in situ
for the growth of sulfoxide and sulfone peaks using a Mettler Toledo
ReactIR 4000 FT-IR spectrometer with a DiComp (diamond) probe
running ReactIR 3.0 software. Details of an analogous experimental
procedure have been described elsewhere.26 Potassium tungstate
(Aldrich, 99.99+ %) was used as received. All systems presented
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9748 J. AM. CHEM. SOC. VOL. 131, NO. 28, 2009