Reactivity in micelles - Are we really able to design micellar catalysts?

Article abstract of DOI:10.1135/cccc20080127

Coordination of lipophilic alkyl pyridin-2-yl ketoximes 1 to Ni ²⁺ ions, reduction of lipophilic 3-alkoxyacetophenones 2 with sodium borohydride, and alkaline hydrolysis of 4-nitrophenyl diphenyl phosphate (PNPDPP) were employed as probes in the investigation which factors may influence the reactivity of organic compounds in micellar systems. In all these reactions, a lipophilic substrate solubilized in micellar core was attacked by a hydrophilic reagent from the bulk aqueous phase. To evaluate the contribution of electrostatic interactions between the micellar surface charge and the reagent to the observed reactivity, we combined reactions involving the reagents with opposite polarity (Ni²⁺ cations and borohydride or hydroxide anions) with positively charged micelles of hexadecyltrimethyl-ammonium chloride (CTAC) or bromide (CTAB) and negatively charged micelles of sodium dodecyl sulfate (SDS). Non-ionic micelles (Triton X-100 or Brij 35) served as a reference. The results of the kinetic studies give evidence that each of the investigated systems has unique properties going in particular aspects beyond the scope of the generally accepted concepts of reactivity in micelles.

Full text of DOI:10.1135/cccc20080127

Reactivity in Micelles
127
REACTIVITY IN MICELLES – ARE WE REALLY ABLE TO DESIGN
MICELLAR CATALYSTS?
1
2
3
4,
Radek JUROK , Eva SVOBODOVÁ , Radek CIBULKA and František HAMPL *
Department of Organic Chemistry, Institute of Chemical Technology, Prague,
1
2
Technická 5, 166 28 Prague 6, Czech Republic; e-mail: jurokr@vscht.cz, svobodoe@vscht.cz,
cibulkar@vscht.cz, hamplf@vscht.cz
3
4
Received July 2, 2007
Accepted Jan uary 18, 2008
Publish ed on lin e February 19, 2008
Coordin ation of lipoph ilic alkyl pyridin -2-yl ketoxim es 1 to Ni2+ ion s, reduction of
lipoph ilic 3-alkoxyacetoph en on es 2 with sodium boroh ydride, an d alkalin e h ydrolysis of
4-n itroph en yl diph en yl ph osph ate (PNPDPP) were em ployed as probes in th e in vestigation
wh ich factors m ay in fluen ce th e reactivity of organ ic com poun ds in m icellar system s. In all
th ese reaction s, a lipoph ilic substrate solubilized in m icellar core was attacked by a h ydro-
ph ilic reagen t from th e bulk aqueous ph ase. To evaluate th e con tribution of electrostatic
in teraction s between th e m icellar surface ch arge an d th e reagen t to th e observed reactivity,
we com bin ed reaction s in volvin g th e reagen ts with opposite polarity (Ni2+ cation s an d
boroh ydride or h ydroxide an ion s) with positively ch arged m icelles of h exadecyltrim eth yl-
am m on ium ch loride (CTAC) or brom ide (CTAB) an d n egatively ch arged m icelles of sodium
dodecyl sulfate (SDS). Non -ion ic m icelles (Triton X-100 or Brij 35) served as a referen ce. Th e
results of th e kin etic studies give eviden ce th at each of th e in vestigated system s h as un ique
properties goin g in particular aspects beyon d th e scope of th e gen erally accepted con cepts of
reactivity in m icelles.
Keyw ord s: Cation ic m icelles; An ion ic m icelles; Non -ion ic m icelles; Micellar catalysis; Coor-
din ation ; Boroh ydride reduction ; Alkalin e h ydrolysis; Oxim es; Hydrolysis; Ph osph ates.
Th e ph en om en on of m icellar catalysis h as been exten sively studied in th e
course of th e past four decades1. Neverth eless, th e n um ber of curren tly pub-
lish ed papers an d reviews2 dealin g with reaction s perform ed in m icelles an d
m icroem ulsion s gives eviden ce th at th is research area h as still been an ever-
green in colloid ch em istry. Th e substan ce of m icellar catalysis lies in brin g-
in g togeth er th e reactan ts in a sm all volum e of colloid particle – m icelle,
in verse m icelle, vesicle, or in an oil or water droplet in o/w or w/o m icro-
em ulsion s. Th e surface ch arge of ion ic m icelles can also attract ion ic re-
agen ts from th e bulk aqueous ph ase, th us in creasin g th eir con cen tration at
th e boun dary of th e n an oaggregate1f. Th e in verse case, i.e. th e separation of
th e lipoph ilic reactan t solubilized in a colloid particle from ion ic reagen ts
Collect. Czech. Chem. Commun. 2008, Vol. 73, No. 2, pp. 127–146
© 2008 Institute of Organic Chemistry and Biochemistry
doi:10.1135/cccc20080127

128
Jurok, Svobodová, Cibulka, Hampl:
by coulom bic repulsion , called m icellar in h ibition , occurs also in m an y in -
stan ces. Th us, th e ph en om en on of m icellar catalysis is predom in an tly a re-
sult of an in crease in th e effective con cen tration of reactan ts. Th erefore,
Rom sted an d Bun ton 3 pertin en tly bran ded m icellar catalysis as a useful
m isn om er.
Colloid particles (m icelles, vesicles, or m icroem ulsion s) can be doped by
reactive fun ction al surfactan ts or m etallosurfactan ts. Such system s often ex-
h ibit rem arkable reactivity to various substrates an d are con sidered as en -
zym e m im ics. Am on g th em , m odels of h ydrolytic en zym es h ave been of
con siderable in terest1e,1g,4; in vestigation of h ydrolytic m icellar catalysts h as
been stim ulated by th e n eed for agen ts for fast an d efficien t h ydrolysis of
n eurotoxic organ oph osph ates un der m ild con dition s4,5. Much effort h as
been spen t on attem pts to in crease th e efficien cy of such system s; th e re-
search h as been focused predom in an tly on th e reactive fun ction design an d
on optim ization of colloid system properties1,2,4,5
.
Kin etic data obtain ed from th e reaction s perform ed in n an oaggregates
un der pseudo-first-order con dition s can be treated by th e pseudoph ase
m odels1c,1d. Ion exch an ge between th e Stern layer an d bulk aqueous ph ase
in th e case of ion ic m icelles is in volved in th e pseudoph ase ion -exch an ge
m odel1f.
However, despite th e fact th at we un derstan d basic prin ciples of m icellar
catalysis an d th at we are able to fit experim en tal kin etic data usin g th e
above-m en tion ed m odels, we are still far from a ration al design of m icellar
catalysts. Th e problem lies in th e fact th at m icelles are dyn am ic aggregates
with low degree of organ ization of surfactan t m olecules an d with in defin ite
ph ase in terface6. Th e beh avior of m icelles depen ds on th e type of
surfactan t, its con cen tration , tem perature, con cen tration of electrolytes
presen t in th e system , etc. No doubt, dyn am ic ch aracter of m icelles affects
m ass exch an ge between th e aggregate (“n an oreactor”) an d th e bulk ph ase
as well as th e location an d orien tation of th e solubilized organ ic m olecules.
Con sequen tly, wh en studyin g organ ic reaction s in colloid system s, we usu-
ally observe various addition al factors affectin g th e reactivity in particular
cases; all th ese factors are subtle an d h ard to predict. For exam ple, we h ave
recen tly foun d a relation sh ip between th e structure an d lipoph ilicity of iso-
m eric am ph iph ilic pyridin ium ketoxim es an d th eir ability to h ydrolyze
ph osph ates in m icellar solution s. We con cluded th at th e observed relation -
sh ip is a con sequen ce of th e location of pyridin ium salts an d orien tation of
th eir m olecules in m icelles7.
A great in terest in th e n an oaggregates dyn am ics, m ass exch an ge between
th e bulk ph ase an d colloid particles an d location of th e reactan ts is well
Collect. Czech. Chem. Commun. 2008, Vol. 73, No. 2, pp. 127–146

Reactivity in Micelles
129
docum en ted by th e n um ber of papers dealin g with th ese topics. In addition
to a th eoretical approach represen ted by various m ath em atical m odels of
solubilization an d m ass exch an ge in colloid system s8, several an alytical
m eth ods h ave been em ployed as tools in th is research , th e m ost im portan t
bein g tim e-resolved lum in escen ce quen ch in g9 an d m easurem en t of relax-
ation tim es in NMR 10. However, th e results an d con clusion s en suin g from
th ese studies are n ot yet an applicable tool in th e design of m icellar cata-
lysts. An oth er approach to th e in vestigation of in teraction s between a
colloid particle an d solubilized reactan ts is kin etic studies of various m odel
reaction s used as probes in to n an oaggregates11. Th e advan tage of th is ap-
proach con sists in m ore straigh tforward in terpretation of th e obtain ed re-
sults; h owever, th eir gen eralization is m ore difficult.
In th e course of our previous studies focused on h ydrolytic activity of
m etallom icellar catalysts based on ch elates of various lipoph ilic alkyl
h eteroaryl ketoxim es12 we observed th at th e rate of coordin ation of lipo-
ph ilic alkyl pyridin -2-yl ketoxim es 1 to Ni2+ ion s (Sch em e 1) in h exadecyl-
trim eth ylam m on ium brom ide (CTAB) m icelles is extrem ely slow com pared
with an alogous reaction s with oth er tran sition m etal ion s. Th is ph en om e-
n on can n ot be explain ed sim ply by coulom bic repulsion between Ni2+ ion s
an d positive surface ch arge of CTAB m icelles sin ce th e form ation of Cu2+
com plexes un der th e sam e con dition s proceeds im m ediately. Most proba-
bly, slow com plexation of Ni2+ ion s with lipoph ilic ligan ds 1 en sues from
th e geom etry of th e resultin g com plex wh ich disfavours its form ation at
th e water/organ ic ph ase in terface as well as at th e boun dary of th e colloid
particle13
.
N
Ni2+, L
n
(6-2n)Ni2+
R
L
N
N
R
n
N
OH
OH
R
n = 2, 3
L = H2O or some of the anions present in the system
1a
1b
1c
1d
1e
1f
n-C6H13
n-C7H15
n-C8H17
n-C9H19
n-C10H21
n-C16H33
SCHEME 1
Th e h igh sen sitivity of th e observed rate of coordin ation of 1 to Ni2+ ion s
in m icelles to th e lipoph ilicity of th e ligan d in spired us to use th is reaction
as a probe in th e in vestigation of th e in fluen ce of th e type of m icellar sys-
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130
Jurok, Svobodová, Cibulka, Hampl:
tem on th e course of organ ic reaction s. For th at purpose we decided to
study th e kin etics of th is com plexation in cation ic, an ion ic, an d n on -ion ic
m icellar system s prepared from th e m ost frequen tly used surfactan ts CTAB,
sodium dodecyl sulfate (SDS), an d Triton X-100 (4-(1,1,3,3-tetram eth yl-
butyl)ph en yl–oligo(eth ylen e glycol); n = 9, 10). In th e n ext series of experi-
m en ts we plan n ed to switch th e polarity of coulom bic in teraction s between
m icelles an d th e h ydroph ilic reagen t in th e bulk aqueous ph ase. As m odel
reaction s to be perform ed in CTAB, SDS, an d Triton X-100 or Brij 35 (poly-
(eth ylen e glycol) dodecyl eth er; n ~ 23) m icelles, we ch ose th e reduction of
lipoph ilic 3-alkoxyacetoph en on es 2 with sodium boroh ydride an d alkalin e
h ydrolysis of 4-n itroph en yl diph en yl ph osph ate (PNPDPP) (Sch em e 2). We
expected th ese experim en ts to h elp to estim ate th e con tribution of cou-
lom bic forces to th e overall observed effect of m icelles on th e reaction rate;
th e reaction s carried out in n on -ion ic m icelles (Triton X-100 or Brij 35)
sh ould serve as a referen ce.
PhO
O
OH
Me
O
P
OH-
-
PhO
PhO
O
O
PhO O-
BH4
Me
P
NO2
HO
NO2
OR
OR
2a
PNPDPP
R
n-C4H9
3
2b
2c
2d
2e
2f
n-C6H13
n-C8H17
n-C10H21
n-C12H25
n-C16H33
SCHEME 2
RESULTS AND DISCUSSION
Model Compounds Used in the Study
Alkyl pyridin -2-yl ketoxim es 1 an d PNPDPP were prepared previously for
oth er studies13,14. 3-Alkoxyacetoph en on es 2 were prepared an alogously to
th e procedure described in th e literature15 by alkylation of 3-h ydroxy-
acetoph en on e sodium salt with correspon din g alkyl brom ide (Sch em e 3).
Th e prepared substan ces gave 1H NMR spectra wh ich were in accordan ce
with th e publish ed data16. Th e followin g requirem en ts were taken in to con -
sideration in th e design of keton es 2: (i) th e possibility of tun in g th eir
lipoph ilicity, (ii) th e possibility to m on itor th e course of th eir reduction by
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Reactivity in Micelles
131
UV spectroph otom etry. 3-Alkoxyacetoph en on es 2 m eet both requirem en ts:
th eir lipoph ilicity can be con trolled by th e h ydroph obic alkyl ch ain len gth
an d th e ban ds in UV spectra of keton es 2 an d th e resultin g secon dary alco-
h ols 3 do n ot overlap as sh own on th e exam ple of keton e 2f (absorption
m axim a at 250 an d 309 n m in all in vestigated m icellar system s) an d alco-
h ol 3f (absorption m axim a at 280 an d 273 n m in all in vestigated m icellar
system s). An auth en tic sam ple of alcoh ol 3f was prepared by th e reduction
of keton e 2f with sodium boroh ydride. Th e requirem en t for th e possibility
of m on itorin g th e reduction by UV spectroscopy excluded th e isom eric
4-alkoxyacetoph en on es sin ce both th e keton es an d th e alcoh ols, th eir re-
duction products, were expected to h ave practically iden tical absorption
m axim a in UV spectra17. Th e sam e problem can be assum ed with th eir
orth o isom ers possessin g sim ilar electron distribution .
R
O
O
2a
2b
2c
2d
2e
2f
n-C4H9
n-C6H13
n-C8H17
n-C10H21
n-C12H25
n-C16H33
NaOMe, RBr
DMSO
Me
Me
OH
OR
SCHEME 3
Coordination of Alkyl Pyridin-2-yl Ketoximes 1 to Ni2+ Ions
in Micellar Solutions
Job plots perform ed with Ni2+ ion s an d ketoxim es 1 give eviden ce th at
th e m etal ion :ligan d stoich iom etry in th e com plexes form ed in all of th e
exam in ed m icellar solution s is 1:2. Th e absorban ces were m easured at th e
absorption m axim um of th e resultin g ch elate wh ich ran ged from 338 to
342 n m in th e in vestigated m icellar system s. All th e prepared m ixtures were
left to stan d for 5 h after m ixin g th e reactan ts to en sure th e attain m en t of
equilibrium . As an exam ple, Job plots for ketoxim e 1b in various types of
m icelles are sh own in Fig. 1.
Th e kin etics were m easured at pH 6.3 (0.05 M 2-(N-m orph olin o)eth an e-
sulfon ic acid (MES) buffer) an d 25 °C with an excess of Ni2+ (cNi2+ = 5.0 ×
10–4 m ol l–1) with respect to ligan d 1 (c1 = 1.0 × 10–4 m ol l–1). Th e course of
th e coordin ation was m on itored by m easurin g th e UV absorban ce of th e
resultin g com plex in th e reaction m ixtures. Regardless of th e alkyl ch ain
len gth , th e rate of Ni2+ com plexation with ketoxim es 1 in an ion ic m icelles
of SDS was very fast an d we were n ot able to follow th e reaction s even by
th e stopped-flow m eth od with th e scan n in g tim e of 12.5 m s (allowin g th e
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132
Jurok, Svobodová, Cibulka, Hampl:
determ in ation of th e first-order rate con stan ts up to 20 s–1). In cation ic m i-
celles of CTAB an d n on -ion ic m icelles of Triton X-100 we were able to fol-
low th e kin etics of com plexation of ligan ds 1 with h ydroph obic alkyl ch ain
len gth C4 an d lon ger.
Th e on ly ban ds observed in UV spectra of th e reaction m ixtures were
th ose of th e startin g ketoxim e 1 an d th e resultin g 1:2 com plex. Th e over-
laid UV spectra of th e reaction m ixture recorded in an y kin etic run were in -
tersected in an isosbestic poin t th us givin g th e eviden ce th at th e 1:1
com plex is form ed in a pre-equilibrium step wh ile th e form ation of th e 1:2
com plex is th e rate-determ in n in g step of th is reaction sequen ce (Sch em e 4;
L stan ds for ketoxim e 1). Th e proposed kin etic sch em e is in accordan ce
with th e assum ption th at th e coordin ation of m ore th an on e m olecule of
lipoph ilic biden tate ligan d to Ni2+ ion is disfavoured if it proceeds at th e
aqueous/organ ic ph ase in terface13. Sch em e 4 disregards th e acid-base equi-
libria.
SCHEME 4
Assum in g th e applicability of th e Boden stein steady-state approxim ation
for th e 1:1 com plex of ketoxim e 1 an d Ni2+, we get Eq. (1). Th e rate of th e
1:2 com plex form ation is described by Eq. (2). Com bin in g Eqs (1) an d (2)
we obtain Eq. (3). Excess of Ni2+ ion s allows to sim plify Eq. (3) to Eq. (4)
(k2k1[Ni2+] = k′).
k1[Ni2+][L] = k–1[NiL2+] + k2[NiL2+][L]
(1)
(2)
(3)
(4)
d[NiL22+]/dτ = k2[NiL2+][L]
d[NiL22+]/dτ = k2k1[Ni2+][L]2/(k–1 + k2[L])
d[NiL22+]/dτ = k′[L]2/(k–1 + k2[L])
If k–1 << k2[L] (th is con dition can be assum ed at h igh er ligan d con cen tra-
tion s), th e secon d-order kin etics (Eq. (4)) can be approxim ated by th e
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Reactivity in Micelles
133
pseudo-first-order kin etics (Eqs (5) an d (6); [L0] stan ds for th e in itial con -
cen tration of ketoxim e 1).
d[NiL22+]/dτ = k[L]
(5)
d[NiL22+]/dτ = k([L0] – 2 [NiL22+])
(6)
Th erefore, first we attem pted to fit th e obtain ed absorban ce-versus-tim e
data (up to 60% con version ) usin g th e pseudo-first-order kin etic m odel.
Th e fact th at th e fit error did n ot exceed 3% in an y case verified th at th e
above-m en tion ed approxim ation was correct.
Not surprisin gly, th e observed pseudo-first order rate con stan t kobs values
decrease with in creasin g surfactan t con cen tration (an d, con sequen tly, dilu-
tion of th e ligan d in m icellar ph ase) as sh own on th e exam ple of ligan d 1b
coordin ation in CTAB m icelles (Fig. 2a). Neverth eless, th e effective rate
con stan ts keff takin g in to accoun t real con cen tration s of th e reactan ts
solubilized in m icelles18 (Fig. 2b) were practically in depen den t of th e
surfactan t con cen tration csurf. Effective rate con stan ts keff can be calculated
from th e observed rate con stan ts kobs usin g Eq. (7)
FIG. 1
Coordination of ligand 1b to Ni2+ ions in micelles of CTAB (᭹), SDS (ᮀ), and Triton X-100 (ꢀ) –
Job plots. Con dition s: pH 6.3 (0.05 M MES buffer), 25 °C, con cen tration of th e surfactan ts
csurf = 1.0 × 10–2 m ol l–1. Con cen tration s cNi2+ of th e Ni2+ ion s an d c1c of th e ligan d: cNi2+ + c1c
=
8.0 × 10–4 m ol l–1
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134
Jurok, Svobodová, Cibulka, Hampl:
FIG. 2
Coordination of ligand 1b to Ni2+ ions in CTAB m icelles. Dependence of the observed pseudo-
first-order rate con stan t kobs a) an d th e effective rate con stan t keff b) on CTAB con cen tration .
Con dition s: pH 6.3 (0.05 M MES buffer), 25 °C, cNi2+ = 5.0 × 10–4 m ol l–1, c1 = 1.0 × 10–4 m ol l–1
FIG. 3
Coordin ation of ligan ds 1 to Ni2+ ion s in CTAB (᭺) an d Triton X-100 (᭹) m icelles. Depen d-
en ce of th e effective rate con stan t keff on th e lipoph ilicity of ligan d 1. Con dition s: pH 6.3
(0.05 M MES buffer), 25 °C, cNi2+ = 5.0 × 10–4 m ol l–1, c1 = 1.0 × 10–4 m ol l–1, cCTAB = 5.0 × 10–3
m ol l–1, cTriton = 4.0 × 10–4 m ol l–1
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Reactivity in Micelles
135
keff = kobs csurf (VM)surf
(7)
wh ere (VM)surf stan ds for th e m olar volum e of th e m icellized surfactan t an d
oth er sym bols h ave th e above-defin ed m ean in g. Th e advan tage of th e effec-
tive rate con stan ts keff con sists in th e fact th at th ey are in depen den t of th e
surfactan t con cen tration un less th e in creased con cen tration of th e surfact-
an t causes ch an ges in th e type of th e aggregate. Hen ce, usin g keff, it is possi-
ble to com pare reaction s perform ed in system s of differen t con cen tration s
of th e m icelle-form in g surfactan t. Th e m olar volum e values used in th is
study were taken over from Scrim in et al.18 for CTAB, Brij 35, an d Triton
X-100 ((VM)CTAB = 0.37 l m ol–1, (VM)Brij = 1.1 l m ol–1, (VM)Triton = 0.5 l m ol–1)
an d from Vass et al.19 for SDS ((VM)SDS = 0.25 l m ol–1).
Th e depen den ce of th e effective rate con stan ts keff on th e lipoph ilicity of
ligan d 1 in CTAB an d in Triton X-100 m icelles is sh own in Fig. 3. Partition
coefficien ts P of ketoxim es 1 between octan -1-ol an d water were used as
a gen erally accepted m easure of lipoph ilicity; th eir values were predicted
usin g a software package20 Pallas 1.2. Both in CTAB an d in Triton X-100,
th e keff values decrease with in creasin g lipoph ilicity of th e ligan d 1 reach -
in g a plateau at log P values aroun d 5.5, correspon din g to C10 alkyl ch ain s.
We assum e th at in th e plateau region ligan ds 1 are alm ost com pletely
solubilized in m icellar core an d th e reaction rate is con trolled by th e diffu-
sion of Ni2+ ion s th rough a Stern layer in th e case of CTAB or th rough a pal-
isade layer of oligo(oxyeth ylen e) ch ain s in th e case of Triton X-100. Th e
in crease in th e com plexation rates observed for lower h om ologues gives
eviden ce th at th e ligan ds of lower lipoph ilicity can m ore easily cross th e
in terface between th e m icellar pseudoph ase an d bulk aqueous ph ase an d
adopt th e arran gem en t suitable for com plex form ation .
Con siderin g th e com plexation in n on -ion ic m icelles of Triton X-100 as
a referen ce, th e reaction rate en h an cem en t caused by th e n egative m icellar
surface ch arge on th e on e h an d, an d th e deceleration of th e reaction due to
th e positive m icellar surface ch arge on th e oth er h an d, are vastly differen t.
Wh ile th e com plexation in an ion ic SDS is faster by m ore th an six orders of
m agn itude th an com plexation in Triton X-100, th e differen ce between
n on -ion ic Triton X-100 an d cation ic CTAB is less th an on e order of m agn i-
tude on ly.
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136
Jurok, Svobodová, Cibulka, Hampl:
Reduction of 3-Alkoxyacetophenones 2 with Sodium Borohydride
in Micellar Solutions
In th e n ext part of th is study we h ave focused on reduction s of lipoph ilic
3-alkoxyacetoph en on es 2 with sodium boroh ydride in CTAB, SDS, an d
Brij 35. Th e aim of th ese experim en ts was to switch th e polarity of cou-
lom bic in teraction s between m icelles an d a h ydroph ilic reagen t located in
th e bulk aqueous ph ase. Hith erto, m icellar system s h ave been used as a re-
action m edium for boroh ydride reduction s in several in stan ces21,22. Al-
th ough cation ic m icelles h ave been expected to facilitate th ese reaction s
due to coulom bic attraction between th e reagen t an d th e m icellar sur-
face21b, th e reactivity of sodium boroh ydride h as also been exam in ed in th e
aggregates of an ion ic21c an d n on -ion ic21c surfactan ts. However, th e pub-
lish ed data do n ot allow direct evaluation of th e m icellar surface ch arge ef-
fect on th e observed reactivity.
Th e obtain ed results for acetoph en on es 2 reduction s with sodium
boroh ydride were som ewh at surprisin g. Th e effective rate con stan ts keff ob-
tain ed in CTAB m icelles were lower th an th ose foun d in n on -ion ic Brij 35
(Fig. 4). It is eviden t th at th ese results are in con tradiction with th e an tici-
FIG. 4
–
Reduction of keton es 2 with NaBH4 in SDS (᭿), CTAB (ꢀ), Brij 35 (᭹), an d with Me4N+BH4 in
Brij 35 (᭺). Depen den ce of th e effective rate con stan t keff on th e lipoph ilicity of keton e 2.
Con dition s: pH 12.3 (1.9 × 10–2 M NaOH), 25 °C, cNaBH4 = 6.25 × 10–3 m ol l–1, c2 = 1.0 ×
10–4 m ol l–1, cCTAB = 1.00 × 10–2 m ol l–1, cBrij = 5.00 × 10–3 m ol l–1, cSDS = 5.00 × 10–2 m ol l–1. Th e
keff value for keton e 2f in Brij 35 was n ot determ in ed due to low solubility of th e substrate in
th e reaction m ixture
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Reactivity in Micelles
137
pated coulom bic attraction between th e positive surface ch arge of CTAB
m icelles an d th e n egatively ch arged boroh ydride an ion .
First, we assum ed th e form ation of alkoxyboroh ydrides of gen eral
–
form ula [C12H25(OCH2CH2)23O]nBH4–n (n = 1–3) in th e reaction system .
Th ese species were expected to be form ed by th e reaction of boroh ydride
an ion with term in al h ydroxy group23 of Brij 35. Alkoxyboroh ydrides
–
[C12H25(OCH2CH2)23O]nBH4–n represen tin g surfactan ts with a covalen tly
boun d reductive fun ction could in crease th e observed reactivity of th e
system . However, in 11B NMR spectra of th e 0.01 M sodium boroh ydride
solution in 0.1 M sodium h ydroxide an d 0.1 M Brij 35 (th e con cen tration s
of boroh ydride an ion an d Brij 35 were in creased adequately to en able NMR
experim en t) m easured with in th e in terval of 10 h we did n ot fin d an y oth er
sign al except th ose of th e startin g boroh ydride an ion (42.1, qi, JBH = 80 Hz)
an d of borate an ion (1.3, s), th e fin al product of th e boroh ydride an ion
degradation . Moreover, th e kin etics of th e keton es 2 reduction s do n ot
exh ibit an y in duction period correspon din g to th e in itial form ation of
alkoxyboroh ydrides (all th e reaction m ixture com pon en ts, i.e. sodium
boroh ydride, Brij 35, an d keton e 2 were m ixed just before startin g th e reac-
tion in all cases).
An oth er plausible explan ation of h igh er reaction rate in n on -ion ic m i-
celles lies in n on -covalen t bin din g of sodium boroh ydride to oligoeth ylen e
ch ain s of Brij 35. Ion -pair extraction s by oligo(eth ylen e glycols) or by th eir
m on oalkyl eth ers h ave been widely utilized in ph ase-tran sfer catalysis24
(oligo(eth ylen e glycols) are som etim es n am ed as “poor ch em ist’s crown s”24c).
If it was really extraction of sodium boroh ydride from bulk aqueous ph ase
by oligoeth ylen e ch ain s of Brij 35 wh ich in creased th e rate of th e keton es 2
reduction , th is reaction sh ould be m arkedly decelerated by replacin g
sodium for an y cation wh ich can n ot be coordin ated. To verify th is as-
sum ption , we also perform ed reduction s of keton es 2 with tetram eth yl-
am m on ium boroh ydride in tetram eth ylam m on ium h ydroxide un der
an alogous con dition s. As sh own in Fig. 4, th e cation replacem en t led in -
deed to a reaction rate decrease approxim ately by on e order of m agn itude.
Hen ce, n on -ion ic m icelles seem to facilitate reduction s of keton es 2 with
sodium boroh ydride due to th e reducin g agen t extraction in to m icellar
pseudoph ase. Poten tial application s of th is ph en om en on will be a topic of
furth er research .
As expected, th e reduction of keton es 2 in SDS m icelles was m arkedly
slower com pared with th e reaction s perform ed in CTAB an d Brij 35.
If all th e in dividual com pon en ts of th e reaction system (SDS solution ,
solution of th e substrate, an d alkalin e solution of sodium boroh ydride)
Collect. Czech. Chem. Commun. 2008, Vol. 73, No. 2, pp. 127–146

138
Jurok, Svobodová, Cibulka, Hampl:
were m ixed at zero tim e of th e reaction we observed an in duction period of
ca. 2 h as eviden t from th e exam ple of th e keton e 2e reduction (Fig. 5a).
Th erefore, th e kin etics of th e keton es 2 reduction s were followed after th e
in jection of a substrate solution in to m icellar system s of SDS an d sodium
boroh ydride prepared 10 h before th e reaction . In th ese in stan ces th e
absorban ce-versus-tim e data followed th e first-order kin etics (Fig. 5b): th ese
data were used for th e calculation of keff values given in Fig. 4.
Th e absorban ce-versus-tim e data obtain ed from reduction s of keton es 2
in SDS m icelles (Fig. 5) give eviden ce th at th e boroh ydride an ion in teracts
with SDS form in g a reducin g agen t boun d to th e n an oaggregate. Due to th e
repulsive coulom bic forces between th e boroh ydride an d dodecyl sulfate
an ion s it seem s un likely th at sodium boroh ydride is boun d to SDS m icelles
by an y kin d of n on -covalen t in teraction . Despite very low n ucleoph ilicity
of dodecyl sulfate an ion it is reason able to assum e th at species of gen eral
–
form ula [C12H25OSO2O]nBH4–n m igh t be form ed un der th e reaction con di-
tion s. Th e existen ce of a sim ilar type of com poun ds (sulfon atoboran es) h as
already been reported25. Sim ilarly as in th e case of Brij 35 we followed th e
fate of boroh ydride an ion in SDS m icelles by m ean s of 11B NMR spectro-
scopy. However, as in th e previous case we did n ot fin d an y oth er sign al
except th ose of boroh ydride an d borate an ion s. Th us, th e above-stated
explan ation of th e observed in duction period is on ly speculative.
FIG. 5
Reduction of keton es 2e with NaBH4 in SDS m icelles – typical exam ples of th e reaction course.
a) All th e com pon en ts of th e reaction system were m ixed at zero tim e of th e reaction . b)
Keton e 2e was added to m icellar system s of SDS an d sodium boroh ydride prepared 10 h before
th e reaction . Con dition s: pH 12.3 (1.9 × 10–2 M NaOH), 25 °C, cNaBH4 = 6.25 × 10–3 m ol l–1, c2e
1.0 × 10–4 m ol l–1, cSDS = 5.00 × 10–2 m ol l–1
=
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Reactivity in Micelles
139
As follows from th e aforem en tion ed discussion of th e results of keton es 2
reduction s in cation ic, n on -ion ic, an d an ion ic surfactan ts, th e beh avior of
boroh ydride an ion in m icellar system s is m ore com plex th an it h as been
expected so far21,22. Hen ce, th ese reaction s can h ardly serve as a probe for
th e in vestigation of n an oaggregates properties.
Hydrolysis of 4-Nitrophenyl Diphenyl Phosphate (PNPDPP) in
Micellar Solutions
Th e un expected beh avior of th e boroh ydride an ion in m icellar system s
m ade us to turn our atten tion to an oth er reaction in volvin g an ion as a re-
agen t. Of th e reaction s bein g worth of con sideration we ch ose alkalin e h y-
drolysis of 4-n itroph en yl diph en yl ph osph ate (PNPDPP) for several reason s.
Th is reaction h as been used in n um erous studies1–7 as a m odel for testin g
th e efficacy of m icellar catalysts. PNPDPP as a lipoph ilic com poun d (log P =
4.7) is expected to be located in m icellar core. Con sequen tly, th e reaction
rate sh ould be con trolled by th e h ydroxide an ion con cen tration at m icellar
surface an d by its tran sport in to m icellar in terior. We assum ed th at th e pH
depen den ce of th e pseudo-first order rate con stan t kobs of th e PNPDPP h y-
drolysis sh ould allow qualitative evaluation of th e m icellar surface ch arge
effect on th e above-m en tion ed factors in fluen cin g th e observed reactivity.
We decided to study th e alkalin e h ydrolysis of PNPDPP in cation ic m i-
celles of CTAB an d h exadecyltrim eth ylam m on ium ch loride (CTAC), in
n on -ion ic m icelles of Triton X-100 an d Brij 35, an d in an ion ic m icelles of
SDS. In all in stan ces, th e surfactan t con cen tration was a double of th e pub-
lish ed critical m icelle con cetration 26. Modified Britton –Robin son buffers
were used to set th e pH values in reaction m ixtures. Th eir con cen tration
(0.1 m ol l–1) was h igh en ough to m ain tain pH in th e bulk aqueous ph ase
even if all th e h alide an ion s of cation ic surfactan ts were exch an ged for h y-
droxide an ion s. In all cases, th e course of th e PNPDPP h ydrolysis was m on i-
tored by UV spectroph otom etry, m on itorin g th e con cen tration of th e
resultin g 4-n itroph en ol.
Th e obtain ed pH depen den ces of th e observed rate con stan ts kobs are
sh own in Fig. 6. For com parison , th e pH depen den ce of alkalin e h ydrolysis
of PNPDPP in h om ogen eous aqueous solution is in cluded as well; th e de-
pen den ce was calculated usin g th e publish ed27 secon d-order rate con stan t
(0.4 l m ol–1 s–1).
As expected, th e PNPDPP h ydrolysis was in h ibited in SDS m icelles an d
th e h igh est reaction rates were observed in cation ic surfactan ts.
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140
Jurok, Svobodová, Cibulka, Hampl:
Both in CTAB an d in CTAC, th e observed log kobs vs pH depen den ce is
flat up to pH 10. In th e pH ran ge from 7 to ~9.5 th ere is even a plateau
wh ere th e rate of PNPDPP h ydrolysis is in depen den t of th e h ydroxide an -
ion con cen tration . Hen ce, cation ic m icelles beh ave as an ion exch an gers
m ain tain in g th e local pH in th e Stern layer at practically con stan t value –
in th e span of six pH un its (5–11), th e observed rate con stan t kobs value in -
creases on ly by on e order of m agn itude. At h igh er pH values th e “buffer-
in g” capacity of th e surfactan t cation ic h ead groups is exh austed an d th e
slope of th e log kobs vs pH depen den ce is approxim ately un ity, just as in th e
case of th e reaction in h om ogen eous aqueous solution .
Un der sligh tly alkalin e con dition s (pH < 9.5), th e PNPDPP h ydrolysis in
n on -ion ic m icelles of Triton X-100 an d Brij 35 is slower th an th at in
cation ic m icelles but faster th an th e reaction in h om ogen eous aqueous so-
lution . Most probably, th e rate en h an cem en t in com parison with h om oge-
n eous reaction results from th e in creased effective PNPDPP con cen tration
in m icellar pseudoph ase. Th e log kobs vs pH depen den ce is alm ost lin ear;
its slope is low (ca. 0.2) th us givin g eviden ce of th e resistan ce of th e oligo-
(oxyeth ylen e) ch ain palisade to th e h ydroxide an ion tran sport. Th is resis-
tan ce seem s to be broken at pH aroun d 10; at h igh er pH, th e rates of
FIG. 6
Alkalin e h ydrolysis of PNPDPP – pH depen den ce of th e observed rate con stan t kobs in SDS (ꢀ),
Triton X-100 (ᮀ), Brij 35 (᭿), CTAC (᭺), an d CTAB (+) m icelles. Con dition s: 25 °C, cPNPDPP
=
,
2.0 × 10–5 m ol l–1, cSDS = 1.6 × 10–3 m ol l–1, cTriton = 4.4 × 10–4 m ol l–1, cBrij = 2.0 × 10–4 m ol l–1
cCTAC = 3.2 × 10–3 m ol l–1, cCTAB = 2.0 × 10–3 m ol l–1. Lin e: alkalin e h ydrolysis of PNPDPP in
h om ogen eous aqueous solution (calculated usin g th e publish ed27 secon d-order rate con stan t
value)
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Reactivity in Micelles
141
PNPDPP h ydrolysis in n on -ion ic m icelles an d in h om ogen eous aqueous
solution s are practically th e sam e.
In con trast with coordin ation of ketoxim es 1 to Ni2+ ion s, th e effect of
coulom bic attraction or repulsion on th e observed rate of th e PNPDPP h y-
drolysis in m icellar system s is com parable, con siderin g th e reactivity in
n on -ion ic m icelles as a referen ce.
It is worth y to m en tion an un usual course of th e PNPDPP h ydrolysis
observed in Brij 35 at pH below 10. As eviden t from th e exam ple given in
Fig. 7, th e h ydrolysis was very fast up to 25–30% con version . Th en , th e re-
action rate decreased substan tially. At th e secon d stage of th e h ydrolysis
th e absorban ce-versus-tim e data followed th e first-order kin etics; th ese data
were used for th e calculation of th e rate con stan ts given in Fig. 6. So far we
h ave h ad n o plausible explan ation of th e observed ph en om en on . Most
likely, its substan ce lies in th e depth of th e oligo(oxyeth ylen e) ch ain pali-
sade of Brij 35 m icelles wh ich is h igh er by a factor of ~2 com pared with
Triton X-100. Sin ce th e reaction s were started by th e in jection of a sub-
strate solution in to a buffered surfactan t solution , th e fast in itial stage of
th e h ydrolysis m igh t in dicate slow tran sport of PNPDPP in to m icellar core.
Localized in th e oligo(oxyeth ylen e) ch ain palisade, PNPDPP can be m ore
easily attacked by h ydroxide an ion s.
FIG. 7
Th e course of alkalin e h ydrolysis of PNPDPP in Brij 35 at pH 9.0. Con dition s: 25 °C, cPNPDPP
2.0 × 10–5 m ol l–1, cBrij = 2.0 × 10–4 m ol l–1
=
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142
Conclusion
Jurok, Svobodová, Cibulka, Hampl:
Th e curren t study represen ts an attem pt to evaluate th e in fluen ce of prop-
erties of th e in terface between th e bulk aqueous ph ase an d m icellar
pseudoph ase on th e course of ch em ical reaction s. We tried to utilize vari-
ous ch em ical reaction s as probes in to th e m ost frequen tly used types of m i-
celles. Th e results of th e perform ed kin etic studies dem on strated th at each
of th e in vestigated system s h as un ique properties goin g in particular as-
pects beyon d th e scope of gen erally accepted con cepts of reactivity in m i-
celles assum in g on ly th e effect of th e in creased local con cen tration of
reactan ts an d th e effect of coulom bic forces. For exam ple, con siderin g th e
reactivity in n on -ion ic m icelles as a referen ce, th e apparen t effect of th e
m icellar surface ch arge on th e reaction rate varies m arkedly from case to
case. Th is fact in dicates th at th e con tribution of coulom bic forces between
th e m icellar surface an d th e reagen t approach in g th e aggregate m ay be
overestim ated in m an y cases an d th at th e observed reactivity in m icelles
can be stron gly in fluen ced by various oth er factors.
Th us, bein g used as a reaction m edium , m icellar solution s m ay provide
con siderable ben efits in com parison with h om ogen eous solution s in partic-
ular cases; h owever, th e tools for a ration al design of m icellar catalysts are
still lim ited.
EXPERIMENTAL
Gen eral
Th e tem perature data were un corrected. 1H NMR spectra were recorded on a Varian Mercury
Plus 300 spectrom eter operatin g at 299.97 MHz for 1H. Ch em ical sh ifts (δ) in ppm are re-
ported relative to Me4Si as in tern al stan dard. All couplin g con stan ts (J) are in Hz. TLC an aly-
ses were carried out on DC Alufolien Kieselgel 60 F254. Preparative colum n ch rom atograph y
was perform ed on silica gel 60, 0.040–0.063 m m . Kin etic m easurem en ts were perform ed
with a Hewlett–Packard HP 8452 spectroph otom eter with a th erm ostatted m ulticell tran s-
port cell h older HP 89075C or with Varian Cary 50 UV-VIS equipped with a th erm ostatted
m ulticell tran sport cell h older in 1-cm spectroph otom etric cells.
Ch em icals
All th e ch em icals used in th e syn th eses (purum or pract.) were used as received. Th e sol-
ven ts were purified an d dried usin g th e described procedures28. 4-Nitroph en yl diph en yl
ph osph ate (PNPDPP) an d alkyl pyridin -2-yl ketoxim es 1 were prepared previously12d,13
.
Hexadecyltrim eth ylam m on ium brom ide (CTAB) puriss., h exadecyltrim eth ylam m on ium
ch loride (CTAC)i 25 wt.% solution in water were obtain ed from Aldrich , Triton X-100 from
Fluka, an d Brij 35, 30 wt.% solution in water from Sigm a. 2-(N-Morph olin o)eth an e-
1-sulfon ic acid (MES) was purch ased by Sigm a. Nickel(II) n itrate (an alytical grade) was th e
Collect. Czech. Chem. Commun. 2008, Vol. 73, No. 2, pp. 127–146

Reactivity in Micelles
143
product of Lach em a Brn o; its con cen tration in stock solution s was determ in ed by EDTA
titration followin g stan dard procedure29
.
1-(3-Hexadecyloxyph en yl)eth an ol 3f
Sodium boroh ydride (100 m g, 2.64 m m ol) was added in several portion s over 0.5 h in to a
solution of 3-(h exadecyloxy)acetoph en on e (2f) (250 m g, 0.7 m m ol) in th e m ixture of eth a-
n ol (10 m l) an d tetrah ydrofuran e (2 m l). Th e reaction m ixture was stirred at room tem pera-
ture overn igh t; th e com pletion of th e reaction was ch ecked by TLC (dich lorom eth an e).
Th en , th e solven ts were evaporated, water (5 m l) was added to th e obtain ed residue an d th e
product was extracted in to dich lorom eth an e (2 × 3 m l). Th e com bin ed extracts were wash ed
with water (5 m l) an d dried with m agn esium sulfate. Th e crude product obtain ed after th e
evaporation of th e solven t was purified by colum n ch rom atograph y (dich lorom eth an e–tolu-
en e 1:1). Yield 190 m g (76%), wh ite crystals. M.p. 39–40 °C. 1H NMR (CDCl3): 0.88 t, 3 H,
J(16′,15′) = 6.6 (CH3); 1.26 bs, 24 H (CH2)12; 1.45 m , 2 H (3′-CH2); 1.49 d, 3 H, J(2,1) = 6.3
(CH(OH)CH3); 1.78 m , 2 H (2′-CH2); 3.96 t, 2 H, J(1,2) = 6.6 (OCH2); 4.86 qa, 1 H, J(1,2) =
6.3 (CH(OH)CH3); 6.80 d, 1 H (ArH); 6.90–6.98 m , 2 H (ArH); 7.25 m , 1 H (ArH). For
C
24H42O2 (362.6) calculated: 79.50% C, 11.67% H; foun d: 79.29% C, 12.08% H. UV, λm ax:
280 an d 273 n m in all in vestigated m icellar solution s un der th e con dition s given in th e cap-
tion to Fig. 4.
Coordin ation of Alkyl Pyridin -2-yl Ketoxim es 1 to Ni2+ Ion s
Reaction m ixtures were prepared directly in spectroph otom etric cells by m ixin g th e appro-
priate am oun ts of m icellar solution of ligan d 1 in CTAB, Triton X-100 or SDS, re-destilled
water, an d aqueous buffer solution . Th e cells were th erm ostatted for 20 m in in th e cell
h older to 25.0 ± 0.1 °C. Th e reaction s were in itiated by th e addition of an appropriate
am oun t of 5.0 × 10–2 M aqueous solution of n ickel(II) n itrate. No ch an ges in pH were ob-
served durin g th e kin etic run s. Th e reaction s were m on itored at th e m axim um of th e result-
in g com plex absorption (from 338 to 342 n m ). Th e pseudo-first-order rate con stan ts kobs
were obtain ed by n on -lin ear regression an alysis of th e absorban ce-versus-tim e data up to
60% con version usin g th e equation At = (Ain f – A0)[1 – exp(–kt)] + A0. Regression an alysis
was perform ed usin g program Origin 6.1 (ref.30). In all cases, th e correlation coefficien t r >
0.999. Th e kin etic run s were perform ed in duplicate an d th e rate con stan ts were calculated
as a m ean of th e values obtain ed from th ese parallel determ in ation s. Th e differen ces be-
tween th ese results did n ot exceed 5%.
Reduction of 3-Alkoxyacetoph en on es 2 with Sodium Boroh ydride
Reaction m ixtures were prepared directly in spectroph otom etric cells by m ixin g an appropri-
ate am oun t of th e surfactan t solution , fresh ly prepared 0.1 M NaBH4 solution in 0.3 M NaOH,
an d water. Th e cells were th erm ostatted for 20 m in in th e cell h older to 25.0 ± 0.1 °C. In
th e case of SDS, m icellar solution s of sodium boroh ydride were prepared 10 h before startin g
th e reaction . Th e reaction was in itiated by th e addition of appropriate am oun t of 0.01 M so-
lution of keton e 2 in aceton itrile in to spectroph otom etric cell. Th e reaction s were m on i-
tored at th e m axim um absorption of keton e 2 (308 n m ). Th e pseudo-first-order rate
con stan ts kobs were obtain ed by n on lin ear regression an alysis of absorban ce-versus-tim e data
usin g th e equation At = A0 exp(–kt) + A0. Regression an alysis was perform ed usin g program
Collect. Czech. Chem. Commun. 2008, Vol. 73, No. 2, pp. 127–146

144
Jurok, Svobodová, Cibulka, Hampl:
Origin 6.1 (ref.30). In all cases, th e correlation coefficien t r > 0.999. Th e kin etic run s were
perform ed in duplicate an d th e rate con stan ts were calculated as a m ean of th e values ob-
tain ed from th ese parallel determ in ation s. Th e differen ces between th ese results did n ot ex-
ceed 5%.
Hydrolysis of 4-Nitroph en yl Diph en yl Ph osph ate (PNPDPP)
Reaction m ixtures were prepared directly in spectroph otom etric cells by m ixin g of an appro-
priate am oun t of an aqueous surfactan t solution , buffer, an d water. Th e cells were th erm o-
statted for 20 m in in th e cell h older to 25.0 ± 0.1 °C. Th e reaction s were in itiated by th e
addition of appropriate am oun t of 5.0 × 10–3 M PNPDPP solution in aceton itrile. No ch an ges
in pH were observed durin g th e kin etic run s. Th e reaction s were m on itored at th e m axim um
absorption of th e resultin g 4-n itroph en oxide an ion (400 n m ) or, in th e case of reaction s per-
form ed at pH below 6.5, at 317 n m (m axim um of 4-n itroph en ol absorption ). Th e pseudo-
first-order rate con stan ts kobs were obtain ed by n on lin ear regression an alysis of th e
absorban ce-versus-tim e data usin g th e equation At = (Ain f – A0)[1 – exp(–kt)] + A0. Regression
an alysis was perform ed usin g program Origin 6.1 (ref.30). In all cases, th e correlation
coefficien t r > 0.999. Th e kin etic run s were perform ed in duplicate an d th e rate con stan ts
were calculated as a m ean of th e values obtain ed from th ese parallel determ in ation s. Th e
differen ces between th ese results did n ot exceed 5%.
The authors thank Prof. L. Bartovská from the Department of Physical Chemistry, Institute of
Chemical Technonology, Prague for help with the treatment of kinetic data from coordination
reactions and Dr. T. Martinů from the Department of Organic Chemistry, Institute of Chemical
Technonology, Prague for helpful discussion. This study was supported by the Czech Science
Foundation (Grant No. 203/04/0488).
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