Kinetic evidence for the solubilization of pyridine-2-azo-p-dimethylaniline in alkanediyl-α,ω-bis(dimethylcetylammonium nitrate) surfactants. Role of the spacer chain length

Article abstract of DOI:10.1039/b315422j

The incorporation of the bidentate ligand pyridine-2-azo-p-dimethylaniline (PADA) into micellar aggregates of the dimeric cationic surfactants propanediyl-, hexanediyl- and dodecanediyl-α,ω- bis(dimethylcetylammonium nitrate) (16-3-16,2NO₃^-, 16-6-16,2NO₃^- and 16-12-16,2NO₃^-, respectively) has been studied at 25°C by examining the kinetics of the complexation reaction of the Ni(II) ion with this ligand. For comparison, cetyltrimethylammonium nitrate (CTAN), which can be considered as the monomeric surfactant of 16-3-16,2NO₃^-, has also been used. The kinetic data have shown that, for 16-3-16,2NO ₃^- and CTAN, at a surfactant concentration below the critical micelle concentration (cmc) the rate of the complex formation reaction does not significantly depend on the surfactant concentration, while it slightly decreases in the presence of the other two gemini surfactants. Beyond this critical value, in all cases examined, the rate constant is conspicuously inhibited by the presence of surfactant. The results below the cmc have been explained in the light of conductometric measurements, which have evidenced that both 16-6-16,2NO₃^- and 16-12-16,2NO₃ ^- form premicellar aggregates while 16-3-16,2NO₃ ^- and CTAN do not. The kinetic data above the cmc conform to a reaction mechanism that implies partitioning of the ligand only between the aqueous and the micellar pseudo-phases. The quantitative analysis of the kinetic data allows us to estimate the binding of PADA to the cationic micellar aggregates used. Solubilization of PADA in the micelles markedly depends on the nature of the surfactant used and, in particular, decreases on either increasing the spacer chain length or changing the surfactant type, namely conventional or gemini. These trends have been ascribed to the change in the shape of the micellar aggregates and, consequently, the hydrophobic character of the micelles, which can be modulated either by insertion of the spacer in the micellar interior or by using a conventional surfactant. The incremental free energy of transfer of a methylene group in the spacer chain from the aqueous to the micellar pseudo-phase has also been determined. The present data evidence that binding of PADA to micellar aggregates is primarily governed by hydrophobic interactions and the solubilization capability of gemini aggregates is superior to that of conventional micelles.

Full text of DOI:10.1039/b315422j

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P A P E R
Kinetic evidence for the solubilization of pyridine-2-azo-p-
dimethylaniline in alkanediyl-a,x-bis(dimethylcetylammonium
nitrate) surfactants. Role of the spacer chain length
Maria L. Turco Liveri,*a Renato Lombardo,a Carmelo Sbriziolo,a Guido Viscardib and
Pierluigi Quagliottob
a
`
Dipartimento di Chimica Fisica ‘‘F. Accascina’’, Universita degli Studi di Palermo, Viale delle
Scienze, Parco d’Orleans II, 90128, Palermo, Italy. E-mail: tliveri@unipa.it;
Fax: +39 091 590015; Tel: +39 0916459849
`
Dipartimento di Chimica Generale ed Organica Applicata, Universita di Torino,
C.so Massimo d’Azeglio 48, 10125, Torino, Italy
b
Received (in Montpellier, France) 27th November 2003, Accepted 25th February 2004
First published as an Advance Article on the web 8th June 2004
The incorporation of the bidentate ligand pyridine-2-azo-p-dimethylaniline (PADA) into micellar aggregates of
the dimeric cationic surfactants propanꢀediyl-, hexanediyl- anꢀd dodecanediyl-a,o-bis(dimethylcetylammonium
nitrate) (16-3-16,2NO3ꢀ, 16-6-16,2NO3 and 16-12-16,2NO3 , respectively) has been studied at 25 ꢁC by
examining the kinetics of the complexation reaction of the Ni(II) ion with this ligand. For comparison,
cetyltrimethylammonium nitrate (CTAN), which can be considered as the ‘‘monomeric’’ꢀsurfactant of
16-3-16,2NO3ꢀ, has also been used. The kinetic data have shown that, for 16-3-16,2NO3 and CTAN, at a
surfactant concentration below the critical micelle concentration (cmc) the rate of the complex formation
reaction does not significantly depend on the surfactant concentration, while it slightly decreases in the presence
of the other two gemini surfactants. Beyond this critical value, in all cases examined, the rate constant is
conspicuously inhibited by the presence of surfactant. The results below the cmc have been explained in tꢀhe light
ꢀ
of conductometric measurements, which have evidenced that both 16-6-16,2NO3 and 16-12-16,2NO3 form
ꢀ
premicellar aggregates while 16-3-16,2NO3 and CTAN do not. The kinetic data above the cmc conform to a
reaction mechanism that implies partitioning of the ligand only between the aqueous and the micellar
pseudo-phases. The quantitative analysis of the kinetic data allows us to estimate the binding of PADA to
the cationic micellar aggregates used. Solubilization of PADA in the micelles markedly depends on the nature of
the surfactant used and, in particular, decreases on either increasing the spacer chain length or changing the
surfactant type, namely conventional or gemini. These trends have been ascribed to the change in the shape
of the micellar aggregates and, consequently, the hydrophobic character of the micelles, which can be
modulated either by insertion of the spacer in the micellar interior or by using a conventional surfactant. The
incremental free energy of transfer of a methylene group in the spacer chain from the aqueous to the
micellar pseudo-phase has also been determined. The present data evidence that binding of PADA to micellar
aggregates is primarily governed by hydrophobic interactions and the solubilization capability of gemini
aggregates is superior to that of conventional micelles.
hydrophobic spacer lengths, respectively. These surfactants
Introduction
differ in their chemical nature from the above mentioned
surfactants having a single chain and a polar head group,
so-called ‘‘conventional’’ ones. Gemini surfactants consist
of two hydrophobic alkyl chains and two hydrophilic head
groups covalently connected by a hydrocarbon spacer, which
may be rigid or flexible. These surfactants form in aqueous
solution different types of aggregated systems5 whose struc-
ture and properties significantly depend on the nature of
the alkyl chain, head group and spacer. These parameters
can be independently varied and, as a consequence, both
the shape and size of the aggregated systems can be easily
modulated.
While quite a large amount of work dealing with the deter-
mination of the physico-chemical properties4c,4i,6 of surfactant
aggregates of gemini surfactants has been done, solubiliza-
tion4f,4h in gemini micelles has not been examined in detail.
In addition, scarce attention7 has so far been paid to the effects
of these organized assemblies on the rate and mechanism of
chemical reactions. To our knowledge no kinetic studies on the
influence of gemini surfactants on the kinetics of complexation
Among the different applications1 of aqueous micelle-forming
surfactant solutions, such as in the soap industry, for mineral
flotation, medical and pharmaceutical applications and reme-
diation of aquifer basins contaminated by apolar liquids, one
of the most exploited is their capability to enhance the solubi-
lity of hydrophobic compounds that are otherwise only slightly
soluble in water. Several physical methods2 have been used to
study the solubilization process of substrates of widely differ-
ent characteristics in both ionic and non-ionic micellar aggre-
gates and to determine the predominant binding sites of the
organic compounds in such aggregates. The study of the solu-
bilization in aqueous micellar solutions3 of sparingly soluble
substances dates back to the beginning of the last century.
Only recently a new class of surfactants,4 the alkanediyl-a,o-
bis(dimethylalkylammonium bromides) of general chemical
formula (CH3)2CmH2m þ 1Nþ(CH2)sNþCmH2m þ 1(CH3)2 ,2Brꢀ,
namely gemini surfactants, have attracted researchers’
attention. Usually, they are represented by the abbrevia-
tion m-s-m,2Brꢀ, where m and s are the alkyl chain and the
T h i s j o u r n a l i s Q T h e R o y a l S o c i e t y o f C h e m i s t r y a n d t h e
C e n t r e N a t i o n a l d e l a R e c h e r c h e S c i e n t i f i q u e 2 0 0 4
N e w . J . C h e m . , 2 0 0 4 , 2 8 , 7 9 3 – 7 9 9
793

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reactions exist. A systematic kinetic study in the presence of
aggregated systems offers the twofold advantage of providing
valuable information on both reactivity and reaction mechan-
isms in these media, as well as on the solubilization of reagents,
and, in some cases, location of the solute in the micellar
structure.
In pursuit of these goals we have studied the effects of the
dimeric cationic surfactants propanediyl-, hexanediyl-
and dodecanediyl-a,o-bis(dimethylcetylammꢀonium nitrate)
referred to as 16-3-16,2NO3ꢀ, 16-6-16,2NO3 and 16-12-16,
2NO3ꢀ, respectively, on the kinetic of the complexation
reaction (1) of the nickel(II) ion and the ligand pyridine-2-
azo-p-dimethylaniline (PADA):
from methanol. Elemental analysis of the final product
revealed the absence of impurity as AgBr. The dimeric cationic
surfactants propanediyl-, hexanediyl- and dodecanediyl-a,o-
bis(dimethylcetylammonium nitrate) have been synthesized
accordingly to literature methods12 from the dimethylhexade-
cylamine and the appropriate dibromoalkane. The ionic
exchange from bromide to nitrate anion was attempted using
the same procedure reported above for CTAN but in these
cases the elemental analysis revealed the presence of AgBr
impurities. Thus, the ionic exchange has been carried out by
using an ionic hydroxide exchanger (Merck) and titrating the
final solution with HNO3 . This second method proved to be
better than the first one. In fact, no impurities were present
as the elemental analysis showed.13
Pyridine-2-azo-p-dimethylaniline [C5H5N–N2–C6H4–N(CH3)2 ,
PADA] was used as received from Sigma. Stock solution
containing Ni(II) ion was prepared from Ni(NO3)2ꢂ6H2O
(Carlo Erba) and its concentration was estimated by com-
plexometric titration with EDTA. Stock solutions of all
chemicals used in the present work were prepared by weight
before use. Deionized water from reverse osmosis (Elga,
model Option 3), having a resistivity higher than 1 MO
cm, was used to prepare all solutions. Solubilization of
16-3-16,2NO3ꢀ in pure water required heating, while the other
two surfactants could be readily dissolved in pure water.
In all cases, once the surfactant was solubilized the final
ð1Þ
ꢀ
solutions were optically transparent. The 16-3-16,2NO3
aqueous soluꢀtions were stable for 1 day while those of
ꢀ
16-6-16,2NO3 and 16-12-16,2NO3 were stable for several
weeks. However, for all measurements carried out in this
work we have used freshly prepared solutions. In addition,
it was not possible to perform measurements with 16-3-16,
ꢀ
2NO3 at concentrations higher than 1 ꢃ 10ꢀ3 mol dmꢀ3
For comparison, the effects of the cationic surfactant cetyltri-
methylammonium nitrate (CTAN), which can be considered
the ‘‘monomeric’’ analog of 16-3-16,2NO3ꢀ, have also been
examined. In reaction (1), kf and kd represent the formation
and dissociation rate constants, respectively. The kinetic
experiments have been carried out at 25 ꢁC over a wide range
of surfactant concentrations. This reaction has been chosen
because the mechanism in water,8 in other solvent media9
and in the presence of conventional micelle-forming surfac-
tant10 systems is well-established.
In addition to the rate experiments we have performed con-
ductivity measurements to estimate both the critical micelle
concentration (cmc) of the surfactants and the micelle ioniza-
tion degree (a), which allow us to gain very helpful information
for the interpretation of the kinetic data.
because the solutions were viscous and this caused practical
difficulties in both their preparation and for the kinetic
measurements.
Kinetic measurements
A HI-TECH SF-61 stopped-flow spectrophotometer has been
used to carry out the kinetic measurements. The surfactants
were added to both solutions of PADA and Ni(II) before their
mixing in the stopped-flow apparatus, which was equipped
ꢁ
with thermostatted compartments at 25.0 ꢄ 0.1 C and inter-
faced to a computer for both data collection and analysis.
In order to verify the stability of the reactants and the pro-
ducts of reaction in the presence of the surfactants used in this
work, UV-vis spectra of the solutions as a function of both the
surfactant concentration and time have been monitored. It has
been found that, in all cases examined, the spectra do not show
any significant changes with respect to those obtained in aqu-
eous solutions. The rates of complex formation were followed
at a wavelength of 550 nm.
The findings of this work allow for a broad insight into
the effects of both gemini and conventional surfactants on
the rates of reaction and, as a consequence, on the solubiliza-
tion ability of the two different classes of micelle-forming
surfactants.
To avoid shape transitions of the micellar aggregates
induced by salts the kinetic measurements in the presence of
surfactants were performed without keeping the ionic strength
constant, such as by adding supporting electrolyte (NaNO3).
Preliminary kinetic measurements were carried out in aqueous
solution in the presence of increasing amounts of NaNO3
(1 ꢃ 10ꢀ3–0.1 mol dmꢀ3). In this salt concentration range the
rate constants do not appreciably depend on the concentration
of added salt. This result is reliable because an uncharged
reacting species is involved in the chemical process.
All reactions were studied under pseudo-first-order condi-
tions with the nickel ion concentration (in the range 5 ꢃ 10ꢀ5
to 1 ꢃ 10ꢀ3 mol dmꢀ3) greater than the concentration of the
PADA (5 ꢃ 10ꢀ6 mol dmꢀ3). The observed pseudo-first-order
rate constants, kobs , were reproducible to within ꢄ5%. Under
the experimental conditions used in the present work kobs
is related8–10 to the formation (kf) and dissociation (kd) rate
Experimental
Materials and general methods
The cationic surfactant cetyltrimethylammonium nitrate
(CTAN) has been obtained11 by slowly adding in the dark
an AgNO3 methanol–water (1:1 ratio) solution to an aqueous
solution of cetyltrimethylammonium bromide (Merck). The
obtained suspension has been stored in the dark, for one week
at low temperature, and, then, to remove the AgBr, has been
filtered with a MilliporeTM system with a pore diameter of 1
micron. The solvents from the final solution have been par-
tially removed by means of a rotating evaporator and then
completely eliminated by using a vacuum oven. The surfactant,
obtained as a white powder, has been recrystallized two times
T h i s j o u r n a l i s Q T h e R o y a l S o c i e t y o f C h e m i s t r y a n d t h e
C e n t r e N a t i o n a l d e l a R e c h e r c h e S c i e n t i f i q u e 2 0 0 4
794
N e w . J . C h e m . , 2 0 0 4 , 2 8 , 7 9 3 – 7 9 9

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constants in reaction (1) by eqn. (2):
kobs ¼ kf ½NiðiiÞꢅ þ kd
ð2Þ
This equation enables both kf and kd to be evaluated by plot-
ting kobs as a function of the stoichiometric nickel(II) concen-
tration and, hence, the equilibrium constant Keq is obtained
from the ratio kf/kd . In all cases examined, the plots derived
from the experimental data were good straight lines. These
trends are not shown for the sake of space.
Conductivity measurements
The critical micelle concentration of the surfactants and the
micelle ionization degree were determined in the absence and
presence of Ni(II) and PADA, under the same experimental
conditions used for the kinetic runs, by means of a digital
Amel 160 conductimeter (cell constant 0.998 cmꢀ1). The
critical micelle concentrations of the surfactants used were
obtained from the plots of the specific conductivity (k) as a
function of the surfactant concentration ([surf]). The cmc
values were taken from the intersection of the two straight
lines drawn before and after the inflection point in the k vs.
[surf] plot, while a was calculated as the ratio of the slopes
of the conductivity lines above and below the cmc.
The measurements were performed in a temperature-con-
trolled double-walled glass cylinder with circulation of water
connected to a thermostat at 25.0 ꢄ 0.1 ꢁC. For the conductiv-
ity runs the aqueous surfactant solutions at the desired concen-
tration were obtained by adding appropriate aliquots of a
stock solution to 15 ml of pure water. Additions were made
by a burette with an accuracy of 1 mL. The final solution were
stirred for 5 min to allow the system to equilibrate and then the
conductivity was measured. The conductivity was measured
to within 0.01 mS for conductivity values lower than 20 mS
and to within 0.1 mS for higher values. As a consequence of
the experimental method used for increasing the surfactant
concentration the conductivity measurements are not suffi-
ciently accurate to be analyzed in a quantitative manner.14
However, the conductivity data show qualitative differences
on varying the surfactant nature and this allows us to draw
significant conclusions regarding their behaviour in aqueous
solution, which were very helpful for the interpretation of
the kinetic data.
Fig. 1 Plot of the specific conductivity k as a function of 16-3-16,
ꢀ
2NO3ꢀ 0c.5oncentration and of the molar conductivity L vs. [16-3-16,
2NO3
]
at T ¼ 25.0 ꢁC.
L vs. [surf]0.5 indicates that premicellar association of the sur-
factant in aqueous solution takes place. For CTAN and 16-3-
16,2NO3ꢀ, the absence of a maximum in the plot of L on
increasing [surf]0.5 demonstrates that premicellar association
does not occur for these surfactants. Premicellar aggregates
of anionic and cationic gemini surfactants have been evidenced
by Menger et al.4a,4e and similar trends to those shown in the
present work have also been obtained by Zana15b for a wide
number of dimeric cationic surfactants.
The cmc and a values obtained from the conductivity mea-
surements are collected in Table 1. It has also been found that
both the cmc and a values are not significantly affected by the
presence of the NiPADA2þ complex (data not shown).
We can observe that a monotonically increases as the length
of the spacer increases whereas the cmc value first increases
with increasing spacer chain length (s) and then decreases with
a further increase in the s value. Similar trends4c,5c were
Results and discussion
Conductivity data
The complete course of the specific conductivity (k) for the
three gemini surfactants examined, as a function of the surfac-
tant concentration, is illustrated in Figs. 1 to 3. In addition, in
the same figures the variation of the molar conductivity
L ¼ (k ꢀ k0)/[surf] with [surf]0.5 is also shown. For compara-
tive purpose, the results for the surfactant CTAN are shown
in Fig. 4.
Perusal of these figures evidences different regions of beha-
viour. At low surfactant concentrations ([surf] < cmc) the k
value sharply increases on increasing surfactant concentration
and, then, after the inflection point, a further increase in the
surfactant concentration leads to an increase in the k values
but with a lower slope with respect to that observed at low sur-
factant concentration. These trends are indicative of micelle
formation. In the plots of L as a function of [surf]0.5 striking
differences in the behaviour of thꢀe surfactants used can be seen.
In particular, for 16-6-16,2NO3 and 16-12-16,2NO3ꢀ, the L
values increase on increasing [surf]0.5 and reach a maximum,
this maximum being more pronounced for 16-6-16,2NO3ꢀ, at
a concentration value very close to that corresponding to the
inflection point in the plots of k vs. [surf]. It is now well-estab-
lished14b,15 that the appearance of a maximum in the plot of
Fig. 2 Plot of the specific conductivity k as a function of 16-6-16,
ꢀ
2NO3ꢀ 0c.5oncentration and of the molar conductivity L vs. [16-6-16,
2NO3
]
at T ¼ 25.0 ꢁC.
T h i s j o u r n a l i s Q T h e R o y a l S o c i e t y o f C h e m i s t r y a n d t h e
C e n t r e N a t i o n a l d e l a R e c h e r c h e S c i e n t i f i q u e 2 0 0 4
N e w . J . C h e m . , 2 0 0 4 , 2 8 , 7 9 3 – 7 9 9
795

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Table 1 Critical micelle concentration (cmc) and dissociation degree
(a) for the CTAN and 16-s-16,2NO3 micelles in aqueous solution
ꢀ
(T ¼ 25.0 ꢁC)
Surfactant
104 cmc/mol dmꢀ3
a
CTAN
16-3-16,2NO3ꢀ
8.1
0.29
0.54
0.61
0.68
ꢀ
0.18
0.25
0.16
16-6-16,2NO3
16-12-16,2NO3
ꢀ
16-3-16,2Brꢀ micelles are disk-like in shape while gemini
surfactants having a spacer chain length greater than 4 form
prolate ellipsoidal aggregates. We reasonably believe that the
ꢀ
shape of aggregates formed by the 16-s-16,2NO3 gemini
surfactants do not differ significantly from those formed by
the 16-s-16,2Brꢀ series.
Kinetic data
The kinetics of reaction (1) have been extensively studied and it
has been found that eqn. (2) is valid8–10a in both aqueous
media and in the presence of surfactant. Under the experimen-
tal conditions used in the present work, the estimated rate and
equilibrium constants in the absence of surfactant are:
Fig. 3ꢀ Plot of the specific conductivity k as a function of 16-12-16,
2NO3ꢀ 0c.o5 ncentration and of the molar conductivity L vs. [16-12-16,
2NO3
]
at T ¼ 25.0 ꢁC.
kf,w ¼ (12.1 ꢄ 0.1) ꢃ 102 dm3 molꢀ1
s
ꢀ1, kd,w ¼ (7.1 ꢄ 0.4) ꢃ
10ꢀ2 sꢀ1 and Keq ¼ (1.7 ꢄ 0.1) ꢃ 104 dm3 molꢀ1. If allowance
is made for the different experimental conditions, these values
are in good agreement with those previously10a reported.
The kinetics of complex formation inꢀreaction (1) has been
studied in the presence of 16-s-16,2NO3 over a wide surfac-
tant concentration range. For comparison, a kinetic study in
the presence of CTAN has also been carried out. For all sur-
factants used, it has been found that eqn. (2) is valid. In the
presence of surfactant we use the notation kf,app and kd,app
for the apparent (overall) formation and dissociation rate con-
stants, respectively, to distinguish them from the correspond-
ing rate constants in water denoted by the w subscript. The
apparent rate and equilibrium constants obꢀtained at varying
concentrations of CTAN and 16-s-16,2NO3 are collected in
Tables 2 to 5.
obtained previously for the same dicationic surfactant with
bromide as counter ion. Comparison among the cmc and a
values obtained in this work and those reported in the litera-
ture reveals a slight difference, probably due to the experimen-
tal conditions used (lower temperature) and not to the diverse
counter ions (nitrate instead of bromide).
The observed cmc and a dependence on the spacer chain
length have been previously4c,5c attributed to conformational
changes of the polymethylene spacer chain within the dimeric
surfactant ion and gradual penetration of a significant part
of the polymethylene segment into the micellar interior. Con-
sequently, this insertion causes a progressive change in the
shape of the micellar aggregates and implies that micelles
formed by surfactants with long and flexible spacers have a less
hydrophobic character with respect to those formed by surfac-
tants with short spacers. Small angle neutron scattering studies
and microviscosity measurements have evidenced5c,5f that
These data show that the complex formation rate appreci-
ably depends on both the surfactant nature and surfactant
concentration, while the dissociation rate constant is almost
unaffected by the presence of surfactant. As a consequence,
the equilibrium constant varies with the surfactant concentra-
tion in the same way as kf,app does.
The observed absence of significant effects of both types of
cationic surfactants, that is conventional and gemini, on the
dissociation rate of the NiPADA2þ complex can be easily
Table 2 Rate and equilibrium constants for reaction (1) in the
presence of CTAN (T ¼ 25.0 ꢁC)
103 [CTAN]/
mol dmꢀ3
10ꢀ2 kf,app
/
102 kd,app
sꢀ1
/
10ꢀ3 Kapp
/
dm3 molꢀ1
dm3 molꢀ1 sꢀ1
0.01
0.10
0.30
0.70
0.85
1.0
12.0
12.1
11.7
8.2
9.0
7.2
7.6
7.9
9.2
8.1
7.3
7.1
7.0
7.0
7.2
6.5
13.3
16.8
15.3
10.3
7.4
6.8
6.0
7.4
4.2
2.0
3.5
3.07
1.80
1.28
1.01
0.80
0.74
2.55
1.83
1.45
1.12
1.14
5.0
6.5
Fig. 4 Plot of the specific conductivity k as a function of CTAN
concentration and of the molar conductivity L vs. [CTAN]0.5 at
T ¼ 25.0 ꢁC.
8.5
10
T h i s j o u r n a l i s Q T h e R o y a l S o c i e t y o f C h e m i s t r y a n d t h e
C e n t r e N a t i o n a l d e l a R e c h e r c h e S c i e n t i f i q u e 2 0 0 4
796
N e w . J . C h e m . , 2 0 0 4 , 2 8 , 7 9 3 – 7 9 9

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Table 3 Rate and equiꢀlibrium constants for reaction (1) in the
presence of 16-3-16,2NO3 (T ¼ 25.0 ꢁC)
Table 5 Rate and equiliꢀbrium constants for reaction (1) in the
presence of 16-12-16,2NO3 (T ¼ 25.0 ꢁC)
103 [16-3-16,2NO3ꢀ]/
mol dmꢀ3
10ꢀ2 kf,app
/
102 kd,app
sꢀ1
/
10ꢀ3 Kapp
/
103 [16-12-16,2NO3ꢀ]/
mol dmꢀ3
10ꢀ2 kf,app
/
102 kd,app
sꢀ1
/
10ꢀ3 Kapp
/
dm3 molꢀ1
dm3 molꢀ1 sꢀ1
dm3 molꢀ1
dm3 molꢀ1 sꢀ1
0.001
0.01
0.05
0.10
0.30
0.50
0.70
0.85
1.0
11.3
10.1
10.1
9.2
8.1
11
13.9
9.2
8.5
10.4
9.5
8.4
4.9
4.7
4.1
0.002
0.01
0.02
0.05
0.10
0.15
0.50
1.0
9.0
8.7
1.0
7.8
9.4
1.3
7.1
7.6
6.5
8.5
7.6
7.5
7.2
7.5
8.7
11
12
8.3
8.2
8.8
11
8.9
7.3
6.3
7.9
7.3
7.4
6.9
5.3
8.2
7.6
11
10
3.8
3.4
6.6
4.9
10
5.7
3.2
2.42
1.89
1.15
2.99
3.0
5.0
2.41
1.82
1.37
0.86
7.0
10
understood by considering that both the complex and the
micelle are positively charged, as a consequence electrostatic
repulsions confine the complex in the bulk aqueous region.
Moreover, it has been previously10a found that, despite the
strong hydrophobicity of the coordinated ligand (PADA), no
interactions between the NiPADA2þ complex and non-ionic
Triton X-100 [C8H17C6H4(OC2H4)9,10OH] micelles occur.
Thus, the hydrophilic interactions predominate over the
hydrophobic ones.
We can also see from Tables 2 to 5 that the inhibitory effect
depends on both the type of surfactant and the spacer chain
length. In particular, it can be calculated that the 16-3-16,
ꢀ
2NO3 gemini surfactant causes a two times rate retardation
with respect to CTAN over the whole surfactant concentration
range examined. In addition, at a given gemini surfactant con-
centration, the rate constant kf,app decreases following the
From the kinetic data obtained in the presence of CTAN
and 16-3-16,2NO3ꢀ, it is seen that the kf,app values remain
substantially unchanged with increasing surfactant concentra-
tion until a given surfactant concentration is reached. This
surfactant concentration value is very close to the cmc value
conductometrically estimated. Beyond this critical surfactant
concentration value the formation rate constant sharply
decreases, tending to attain a limiting value at high surfactant
concentrations. A different behaviour of kf,app as a function of
surfactant concentration can be noticed when the other two
ꢀ
ꢀ
ꢀ
order 16-3-16,2NO3 > 16-6-16,2NO3 > 16-12-16,2NO3 .
For instance, addition of 1 ꢃ 10ꢀ3 mol dmꢀ3 of surfactant
causes the formation rate constant to decrease with respect
to kw ꢀby a factor ca. 4, 3 and 2.5 for 16-3-16,2NO3ꢀ, 16-6-16,
2NO3 and 16-12-16,2NO3ꢀ, respectively. These inhibitory
effect trends can be ascribed to the change5c in the shape of
the micellar aggregates. In fact, by modulating the hydropho-
bic micellar character, either by using the conventional CTAN
surfactant that forms spherical micelles or by increasing the
spacer chain length in the gemini dicationic surfactant, the rate
of complex formation is inhibited to an extent that reflects the
capability of the aggregated systems to solubilize the hydro-
phobic PADA in their interior. Thus, the higher the solubiliza-
tion of PADA the less favoured is complex formation.
ꢀ
gemini surfactants, 16-6-16,2NO3 and 16-12-16,2NO3ꢀ, are
used. In fact, the apparent formation rate constants always
decrease on increasing surfactant concentration, even though
at low surfactant concentration the inhibitory effect is less
marked. The trends in rate at surfactant concentrations below
the cmc can be explained in the light of the conductometric
results (see above). In fact, we have found that 16-3-16,
The observed rate-retarding effects of the surfactant are con-
sistent with the reaction scheme (Scheme 1) proposed previous-
ly10a for the same reaction in the presence of other aggregated
systems. This reaction scheme implies that only the hydropho-
bic ligand PADA is partitioned between water (w) and the
micellar pseudo-phase (M), with a ‘‘binding’’ constant KPADA
([PADA]M/[PADA]wꢂC, where C ¼ [surf] ꢀ cmc is the micel-
lized surfactant concentration), while the hydrophilic Ni(II) is
confined in the aqueous pseudo-phase. Thus, two reaction
paths may contribute to the overall rate of complex formation,
namely the reactions of the Ni(II) ion in the bulk water with the
PADA solubilized in both the aqueous and micellar pseudo-
ꢀ
2NO3 surfactant and its corresponding monomer (CTAN)
do not form premicellar aggregates while the other two
gemini surfactants do. Thus, the slight decrease of kf,app on
increasing surfactant concentration is consistent with the pre-
sence of premicellar aggregates that interact with the reacting
species, that is the hydrophobic PADA and the hydrophilic
Ni(II) ions.
Table 4 Rate and equiꢀlibrium constants for reaction (1) in the
presence of 16-6-16,2NO3 (T ¼ 25.0 ꢁC)
phases. The rate constants kf,w and k00
of these pathways
f,M
are related to the apparent (overall) formation rate constant
and to the micellized surfactant concentration by eqn. (3):
103 [16-6-16,2NO3ꢀ]/
mol dmꢀ3
10ꢀ2 kf,app
/
102 kd,app
sꢀ1
/
10ꢀ3 Kapp
/
dm3 molꢀ1
dm3 molꢀ1 sꢀ1
kf;w þ kf00;MKPADA
1 þ KPADAC
C
kf;app
¼
ð3Þ
0.01
0.05
0.10
0.30
0.50
0.70
0.85
1.0
10.7
9.3
8.2
8.9
9.5
6.9
7.7
6.5
8.0
7.2
6.8
7.4
8.4
7.6
7.4
13
10
8.7
6.8
9.1
9.8
5.1
4.6
6.7
7.1
3.9
3.7
4.9
5.1
3.0
5.0
1.42
0.93
0.58
0.50
0.42
2.07
1.25
0.69
0.67
0.57
7.0
8.5
10
Scheme 1
T h i s j o u r n a l i s Q T h e R o y a l S o c i e t y o f C h e m i s t r y a n d t h e
C e n t r e N a t i o n a l d e l a R e c h e r c h e S c i e n t i f i q u e 2 0 0 4
N e w . J . C h e m . , 2 0 0 4 , 2 8 , 7 9 3 – 7 9 9
797

View Article Online
Table 6 Binding constants of the ligand PADA in the presence of
The particular variation of kf,app as a function of C depends
upon the relative importance of each term in eqn. (3) and can
be easily weighed up by considering the plot of 1/kf,app vs. C.
In particular, such a plot will show a linear trend if the term
kf,w > > k00f,MKPADAC, while a curved plot, tending to reach
a plateau at high C values, will be obtained when the term
ꢀ
CTAN and 16-s-16,2NO3 micelle-forming surfactants (T ¼ 25.0 ꢁC)
Surfactant
103 KPADA/dm3 molꢀ1
CTAN
16-3-16,2NO3ꢀ
1.7 ꢄ 0.1
3.1 ꢄ 0.2
2.8 ꢄ 0.1
1.2 ꢄ 0.2
ꢀ
k00f,MKPADAC is not negligibly small compared with kf,w
.
16-6-16,2NO3
16-12-16,2NO3
Non-linear least squares analysis of the kinetic data has shown
that, for all surfactants used, the term k00f.MKPADAC can be
neglected with respect to kf,w . Thus, we can rearrange eqn.
(3) and obtain:
ꢀ
The available KPADA data as a function of the spacer chain
length allow us to calculate the standard transfer free energy
Dmꢁ(CH2) of a methylene group in the spacer chain from water
to micelles. As it is known,17 the binding constant KPADA is
related to Dmꢁ by the expression Dmꢁ ¼ ꢀRTln(55.5ꢂKPADA),
where 55.5ꢂKPADA indicates the binding constant in the mole
fraction scale. The plot of Dmꢁ as a function of the number
of methylene groups in the spacer chain (Fig. 6) is linear with
a positive slope.
1
1
KPADA
¼
þ
C
ð4Þ
kf;app kf;w
kf;w
We can see from Fig. 5 that the plots of 1/kf,app versus C are
linear. This behaviour indicates that the second-order rate
constant k00
is markedly smaller than that (kf,w) of the
f,M
complexation path occurring in water, consistent with the
difficulty of approach of the Ni(II) cation to the positively
charged micellar surface. Similar inhibiting effects have also
been previously2d,2e,10,16 obtained for some chemical reactions
involving one of the reactant species solubilized in the micellar
pseudo-phase and the other one in the aqueous pseudo-phase.
The plots in Fig. 5 allow us to evaluate kf,w from the inter-
cept (1/kf,w) and the binding constants (KPADA) from the
slopes (KPADA/kf,w). The kf,w value evaluated from the
intercept is in fair agreement with that obtained by indepen-
dent kinetic measurements. The estimated KPADA values are
reported in Table 6.
The data show that the solubilization of the PADA in the
micellar pseudo-phase markedly depends on the type of
surfactant used. In particular, it decreases inꢀ the order
16-12-16,2NO3ꢀ < 16-6-16,2NO3ꢀ < 16-3-16,2NO3
and
CTAN < 16-3-16,2NO3ꢀ. The decreased solubilizing capacity
of the cationic micelles towards PADA, in line with the micro-
viscosity results5c, reflects the less hydrophobic character of
the aggregated system achieved on either increasing the spacer
chain length or changing the surfactant type, either conven-
tional or gemini. It has also been previously4f found that
within the gemini 12-s-12,2Brꢀ series, the capacity to solubilize
toluene decreases with increasing spacer chain length. In addi-
tion, the ability of gemini aggregates to solubilize organic com-
pounds is higher than that of conventional micelles formed by
surfactants having the same alkyl chain length.
Fig. 6 Plot of Dmꢁ as a function of the number of carbon atoms (s) in
the spacer chain.
Previous2b,2d kinetic determinations of the distribution con-
stant of substituted ferrocenes as a function of the number of
carbon atoms (ns) in the alkyl chain of some alkyltrimethylam-
monium nitrate and alkylpoly(oxiethyleneglycol) monoethers
have shown that the plots of Dmꢁ vs. ns are linear with negative
slopes. The opposite sign of the slope highlights that, even
though a CH2 group is added to both kind of surfactants,
the effects brought about the methylene group depends on
whether it is added to the surfactant alkyl chain or to the
spacer chain. In particular, addition of a CH2 group in the sur-
factant chain leads to micelles having a more hydrophobic
character while in the spacer chain of the gemini surfactant
it favours a smaller and less hydrophobic aggregated system.
Conclusions
The effects of both conventional and gemini cationic micelle-
forming surfactants on the rate of the complexation reaction
of Ni(II) ion with the bidentate ligand PADA have been stu-
died over a wide surfactant concentration range. Some of the
observations from this work are the following. (i) The presence
of surfactant influences only the complex formation rate to
an extent that depends on both the surfactant nature and
Fig. 5 Plot of 1/kf,app vs. C for reaction (1) in the presence of: (˘) 16-
3-16,2NO3ꢀ (m ¼ 0, n ¼ 4), (/) 16-6-16,2NO3ꢀ (m ¼ 0.8, n ¼ 3), (L)
ꢀ
16-12-16,2NO3 (m ¼ 0.3, n ¼ 3) and (S) CTAN (m ¼ 0.5, n ¼ 3).
The solid lines represent the least-squares fits. The parameter m is
added to the ordinate axis to avoiding overlapping of lines.
T h i s j o u r n a l i s Q T h e R o y a l S o c i e t y o f C h e m i s t r y a n d t h e
C e n t r e N a t i o n a l d e l a R e c h e r c h e S c i e n t i f i q u e 2 0 0 4
798
N e w . J . C h e m . , 2 0 0 4 , 2 8 , 7 9 3 – 7 9 9

View Article Online
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The present work shows that a systematic kinetic study is a
powerful tool for obtaining information on the solubilization
capabilities of microstructured systems.
6
Acknowledgements
This work was supported by M.I.U.R. (Italy) and by the Uni-
versity of Torino (Funds for Selected Research Topics) grants.
7
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T h i s j o u r n a l i s Q T h e R o y a l S o c i e t y o f C h e m i s t r y a n d t h e
C e n t r e N a t i o n a l d e l a R e c h e r c h e S c i e n t i f i q u e 2 0 0 4
N e w . J . C h e m . , 2 0 0 4 , 2 8 , 7 9 3 – 7 9 9
799