Chemistry Reference
In-Depth Information
11 New Experimental Tools and
Modelling Techniques in PTC Research
was that interfacial TBAB promotes the reaction via
ion-pair formation but at the same time it disturbs
the mass transfer across the interface. The optimal
catalyst concentration therefore should be deter-
mined by taking into account the interfacial cover-
age while adjusting the conflicting processes of
ion-pair formation and mass transfer.
Another novel physical method for analysis of
surface properties is metastable-induced electron
spectroscopy (MIES). This technique was applied by
Oberbrodhage for the measurement of surface activ-
ity of different onium salts and for determining the
nature of their surface layer [484]. Spectroscopic
properties in the second harmonic generation (SHG)
were used for direct measurements of adsorption
layers of 4-dimethylaminopyridine derivatives at
liquid/liquid interfaces [485]. Another method-
scanning electrochemical microscopy-was used to
induce and follow the rate of charge-transfer cou-
pling between two ion-transfer processes across
liquid/liquid interfaces [486].
Transport of ionic species across interfaces could be
driven also by potential differences between phases.
This was demonstrated recently by Kakiuchi
et al
.
[487], who carried out biphasic azo coupling reac-
tions promoted by the phase boundary potential
across the dichloroethane/water interface. The
kinetics of the coupling reaction of the hydrophilic
Fast Red TR (4-chloro-2-methylbenzene diazonium
chloride, hemi zinc chloride) and other diazonium
salts in water with
N
,
N
-dimethyl-1-naphthylamine
and other coupling agents in dichloroethane was
monitored using cyclic voltammetry and potential-
step chronoamperometry. The value of the standard
ion transfer potential of each diazonium salt could
be a measure of its lipophilicity [488]. It was con-
firmed also that the transfer of arenediazonium ions
from water to dichloroethane was diffusion con-
trolled. However, contradicting the common percep-
tion by PTC researchers, this work concluded that
adsorbed reactants did not play a role in this process.
The authors advocated this method as being highly
useful for the control of PTC processes by application
of the phase boundary potential and also for the elu-
cidation of complex PTC mechanisms. It should be
noted that in an earlier publication Srivastava
et al.
observed the oscillations of electric potential differ-
ences across the liquid/liquid interface in phase-
transfer systems and correlated it with the Starks
extraction mechanism [489]. In a later work these
The last couple of years have witnessed a step change
in the refinement of the physical means available
for visualisation and analysis of the phase-transfer
phenomenon. Numerous exciting new instruments
and methodologies brought about deeper and more
accurate perception of the molecular dynamics of
extraction and reaction in multiphase systems.
The dynamic interfacial behaviour of TBAB at
the water/nitrobenzene interface was monitored by
Swada
et al
. using time-resolved quasi-elastic laser
scattering (QELS) measurements [481]. Analysing
the capillary wave frequencies as a function of
concentration of different surfactants the authors
were able to determine the surface tension g and
the interfacial number density G. The TBAB-
catalysed reaction between sodium phenolate and
diphenylphosphoryl chloride (DPC) to yield triph-
enyl phosphate was selected as a model reaction. It
was found that the capillary wave frequencies were
independent of C
6
H
5
ONa and DPC concentrations,
indicating that these substrates do not react in
the absence of a phase-transfer catalyst. The
authors concluded that the concentration ratio of
TBAB/C
6
H
5
ONa at the interface was unity when the
bulk concentration of TBAB exceeded 50 mM. At
lower bulk TBAB concentration this ratio was lower
than unity. The authors concluded that the site of
the reaction between the two species to form the ion
pair TBA
+
C
6
H
5
O
-
was changing with concentration.
Above 50 mM it is an aqueous phase reaction,
whereas below 50 mM it is an interfacial process.
By inspecting the time courses of the QELS spec-
trum [482] after injecting various phase-transfer cat-
alysts to the interface, it was determined that the
desorption rate of TBAB was very low in compari-
son with tetra-
n
-ethylammonium bromide (TEAB)
and TPAB. As a result, a stationary state was quickly
established with this catalyst at high interfacial mol-
ecular density, leading to a more effective catalysis.
Evidently, the interface is the main reaction site
in this process. Remarkably, with increasing TBAB
concentration from 1 mM to 25 mM the amount
of TBA
+
C
6
H
5
O
-
transferred to the organic phase
decreased
[483]. The interfacial molecular number
density of TBA
+
C
6
H
5
O
-
for 1 mM TBAB was 8.2 ¥
10
-10
mol cm
-2
and with 25 mM TBAB it was only 11
¥ 10
-10
mol cm
-2
. Therefore, the explicit conclusion
