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Article
Nitrosation Reactions in Water/AOT/Xylene Microemulsions
L. Garca-Ro, P. Hervs, J. C. Mejuto, and P. Rodrguez-Dafonte
Ind. Eng. Chem. Res., 2006, 45 (2), 600-606• DOI: 10.1021/ie050925t • Publication Date (Web): 15 December 2005
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600 Ind.Eng.Chem.Res.2006,45,600-606
Nitrosation Reactions in Water/AOT/Xylene Microemulsions
L. Garc´ıa-R´ıo,† P. Herve´s,‡ J. C. Mejuto,*,§ and P. Rodr´ıguez-Dafonte†
Departamento de Qu´ımica F´ısica, Facultad de Qu´ımica, UniVersidad de Santiago, 15782 Santiago, Spain,
Departamento de Qu´ımica, Facultad de Ciencias, UniVersidad de Vigo, Vigo, Spain, and
Departamento de Qu´ımica F´ısica, Facultad de Ciencias, UniVersidad de Vigo, Ourense, Spain
A kinetic study of the nitroso group transfer from N-methyl-N-nitroso-p-toluenesulfonamide (MNTS) to the
secondary amines piperazine (PIP) and N-methylbenzylamine (NMBA) in water/AOT/xylene and water/
AOT/isooctane microemulsions was carried out. The water/AOT/xylene system is nonpercolative by effect
of the decreased ability of the aromatic continuous medium to penetrate into the microemulsion interphase.
The kinetic study was conducted at 50 (cid:176) C (i.e., above the percolation temperature for water/AOT/isooctane
microemulsions).Thekineticparametersforthereactionsinvolvedweredeterminedbyusingthepseudophase
model.Thismodelassumesthatthereactantspartitionbetweenthethreepseudophasesinthemicroemulsion
(viz., water, oil, and the interphase) as a function of their solubility and provides a quantitative explanation
fortheexperimentalbehaviorobserved.Inallcases,thereactiontakesplaceatthemicroemulsioninterphase.
Also, the bimolecular rate constants, k i, differ markedly between the two types of microemulsions. The
2
difference can be ascribed to the differential polarity of the reaction medium. The activation parameters for
the reactions of MNTS with PIP and NMBA in water/AOT/isooctane microemulsions were calculated.
Introduction
Microemulsionsarethermodynamicallystablesystemscon-
sistingoftwoimmiscibleliquidsseparatedandstabilizedbya
monolayerofsurfactant.1,2Inrecentyears,microemulsionshave
increasinglybeenusedinawiderangeofchemicalfieldsfrom
nanoparticleresearch3,4toculturalheritageconservation.5Inthis
work, however, we focused on their use as chemical nano-
reactors.6,7 The extensive use of microemulsions as reaction
media has been promoted by (a) their similarity to biological
membranes(theinterfacialpropertiesofmicroemulsionsallow
a hydrophobic domain to be brought into contact with a
hydrophilic one at the nanoscale level and both polar and
nonpolarsubstancestobedissolvedbyconcentratingreagents
ofvariablesolubility)8,9and(b)theirkineticpotentialasscalable Figure 1. Conductivity vs temperature plot for a water/AOT/isooctane
reactors.10-13 The ability to have chemical reagents partition microemulsion(b)andawater/AOT/o-xylenemicroemulsion(O): [AOT]
)0.5M;W)12.
between, or concentrate in, different pseudophases in a mac-
roscopically homogeneous system is of high kinetic interest.
Also,thewatermoleculesconfinedinthewaterpooldifferin Percolation phenomena in microemulsions are widely
a number of ways from bulk water. Recently, the solvation documented.18-24Themicroemulsionsmostextensivelyexam-
dynamics of the confined water molecules in microemulsions ined in this respect are of the water-in-oil and water/AOT/
have been reported to exhibit a component that is rather slow isooctanetypes,whicharethermodynamicallystableoverawide
relative to bulk water.14,15 rangeofwaterproportions.Thesizeoftheseaggregatescanbe
Microemulsions are highly dynamic structures. Electrical controlledviaparameterW(W)[H O]/[AOT])asthedroplet
2
conductivity measurements provide a very useful method for radius is directly related to W (r ) 1.5W).2
characterizingmicellarinteractions.16,17Theelectricalpercola-
The influence of an aromatic continuous medium on the
tionphenomenonhasbeenexplainedbyassumingamechanism
percolation temperature has been studied to a comparatively
involving the formation of channels through which mass is
muchlesserextent.Inthiswork,weexaminedthewater/AOT/
exchanged between disperse water droplets in the continuous
xylene system, using both o-, m-, and p-xylene. This type of
phases (see Scheme 1). It is therefore necessary to have an
microemulsionisnonpercolative25(seeFigure1)butbecomes
effective collision between two water droplets in the micro-
percolativeinthepresenceof0.5Msodiumcholate.26However,
emulsion to cause the droplets to fuse together. Subsequently,
thepurposeofthisworkwastoconductacomparativekinetic
mass transfer between the water droplets must take place to
studyonpercolative(water/AOT/isooctane)andnonpercolative
allow charge to be conducted and the droplets to separate by
(water/AOT/xylene) microemulsions. The difference in com-
fission.
positionbetweenthetwoshouldresultindifferencesinstructure
and physical properties as the ease with which the oil can
*Towhomcorrespondenceshouldbeaddressed.Tel.: 34988387063.
penetratethesurfactantlayerwillvarymarkedlywiththesteric
Fax: 34988387001.E-mail: [email protected].
hindranceitencounters.47Westudiedthenitrosogrouptransfer
†UniversidaddeSantiago.
‡UniversidaddeVigo,Vigo,Spain. from N-methyl-N-nitroso-p-toluenesulfonamide (MNTS) to
§UniversidaddeVigo,Ourense,Spain. secondaryaminesN-methylbenzylamine(NMBA)andpipera-
10.1021/ie050925tCCC:$33.50 ©2006AmericanChemicalSociety
PublishedonWeb12/15/2005
Ind.Eng.Chem.Res.,Vol.45,No.2,2006 601
Scheme1
zine (PIP), which were chosen on the grounds of their stabilityofwater/AOT/o-xylenesystemsinthepresenceofthese
differential solubility in the three microemulsion domains. amines.Thewatersolubilityinthesemicroemulsionsislower
Because NMBA and MNTS are virtually insoluble in water, than that in water/AOT/isooctane microemulsions. In micro-
they should be present mainly in the continuous aromatic emulsions where isooctane provides the continuous medium,
medium.Ontheotherhand,PIPissolubleinwaterandinsoluble Wcanbeaslargeas80atasurfactantconcentrationof0.5M.
in oil. Therefore, the reaction should only take place to an In contrast, W can barely reach 20 at identical surfactant
appreciable extent in the surfactant film in both cases. concentrationsinwater/AOT/o-xylenemicroemulsionsat25(cid:176) C.
Raisingthetemperatureslightlyincreasesthesolubilityofwater
Experimental Section inthemicroemulsion.Thekineticsofthenitrosogrouptransfer
arefirst-orderinamineconcentration(PIPorNMBA)inwater/
AOT was purchased from Aldrich and used without further
AOT/o-xylene microemulsions at different temperatures (see
purificationafterdryinginavacuumdesiccatorfor2days.All Figure2).Wechosetouseafixedtemperatureof50(cid:176) Cinall
other reagents were supplied by Merck or Aldrich and were
kineticstudies.Thistemperaturewaswellabovethepercolation
used as received.
temperature for one of the systems (water/AOT/isooctane);
Electrical conductivity measurements were made with a
meanwhile, for the water/AOT/o-xylene system, no channels
RadiometerCDM3conductimeterwitha1.0cm-1cellconstant
areformedbetweendroplets,soitisofthenonpercolativetype.
thermostated to within (0.1 (cid:176) C. The percolation temperature
wasdeterminedfromthevariationoftheelectricalconductivity
of the microemulsions with temperature in 0.2 (cid:176) C steps in the
vicinityofthepercolationthresholdwhichwasinturncalculated
from conductivity data ((cid:20)/T).
The kinetics of the reaction were followed on a Kontron-
Uvikon spectrophotometer equipped with thermostated cell
holders and monitored via absorbance changes due to MNTS
consumption.Becausexylenesexhibitastrongabsorptionband
atca.250nm,thetemporalvariationoftheabsorbancehadto
berecordedat392nm.Inanycase,theconcentrationofMNTS
(2 (cid:2) 10-3 M) was always much lower than that of the amine
([PIP])5(cid:2)10-2Mand[NMBA])0.103M).Absorbance-
timedatapairswerefittedtoafirst-orderintegratedrateequation
by using linear regression. All correlation coefficients and Figure2. InfluenceoftheconcentrationofNMBAonitstransamination
residualswerequitesatisfactory(r>0.999).Thepseudo-first- reactionwith2(cid:2)10-3MMNTSinwater/AOT/o-xylenemicroemulsions
order constant is denoted by k below. ([AOT])0.5M;W)12)atavariabletemperature: (O)25(cid:176) C;(b)30
obs (cid:176) C;(0)40(cid:176) C;(1)50(cid:176) C.
ln(A -A ))ln(A -A )-k t (1)
t ¥ 0 ¥ obs (a) Nitrosation of Piperazine (PIP). The kinetic study of
PIPnitrosationwasconductedusingo-xyleneasthecontinuous
Results
medium in the microemulsions, variable AOT concentrations
1.Percolation.Thestudiedsystemhasbeenexperimentally from0.1to0.7M,andWvaluesovertherange7.4-15.7.The
showntobeofthenonpercolativetype.Theconductivityofa amine,PIP,andMNTSconcentrationswerekeptconstantat5
water/AOT/o-xylenemicroemulsion([AOT])0.5M,W)12) (cid:2)10-2and2(cid:2)10-3M,respectively,toexaminetheeffectof
exhibitednosubstantialchangesoverthetemperaturerange25- the medium composition on the nitrosation reaction.
90 (cid:176) C. Figure 1 compares it with the conductivity of a water/ Piperazineisreadilysolubleinwaterbutinsolubleinoil,so
AOT/isooctanemicroemulsionofidentical[AOT]andW.The it should partition between the aqueous pseudophase and the
figurerevealstheabsenceofmasstransferthroughtheformation surfactantfilm.Asaresult,thereactionshouldonlytakeplace
of channels between droplets above a given temperature. at the AOT film (see Scheme 2).
Replacingthealkanewithanaromaticmoleculeinthecontinu- If,byanalogywithwater/AOT/isooctanemicroemulsions,Z
ous medium probably hinders percolation to a considerable istakentobethe[Xylene]/[AOT]ratio,thepartitioncoefficients
extent. As noted earlier, the presence of sodium cholate can governing the distribution of the reactants between the
makeamicroemulsionpercolative.Theadditionoftwoamines pseudophases (viz., xylene and the interphase) can be defined
such as PIP and NMBA has not been found to alter the in terms of their mole per mole concentrations in the two
percolativenatureofthesemicroemulsions.Ontheotherhand, pseudophases:
the addition of PIP or NMBA to an isooctane-containing
microemulsion has been found to increase Tp by 0.4 (cid:176) C.27 KPIP) [PIP]iW KMNTS) [MNTS]iZ (2)
2.ReactivityinWater/AOT/o-XyleneMicroemulsions.The wi [PIP] oi [MNTS]
w o
nitroso group transfer between a secondary amine such as
NMBA or PIP and MNTS in water and various types of where subscripts o, i, and w denote quantities in the oil
microemulsions is well documented. We first examined the pseudophase, AOT film, and water, respectively. With these
602 Ind.Eng.Chem.Res.,Vol.45,No.2,2006
Scheme2
Figure3. Variationofk0withparameterZforthereactionbetweenMNTS
Scheme3 andPIPinwater/AOT/o-xylenemicroemulsions: (b)W)7.4;(0)W)
13.0;(2)W)14.8.
The pseudophase model, therefore, provides an expression,
eq 7, that relates k to the microemulsion composition
obs
parameters W and Z.
Equation 7 predicts a linear relationship between [PIP] /
T
(k Vh [AOT])andZateachWvalue.Figure3showsaplot
obs AOT
obtainedatdifferentWvalues.Ascanbeseen,therelationship
islinear;also,onthebasisofeq7,theratiooftheinterceptto
the slope will coincide with KMNTS (i.e., the partition constant
oi
ofMNTSbetweeno-xyleneandthesurfactantfilm).Fromthe
definitions, and on the basis of the model of Scheme 3, the mean of the results obtained, KMNTS was calculated to be 2.3.
oi
pseudo-first-order rate constant, kobs, will be given by Bysubstitutingthisvalueintoeq7,theotherkineticparameters
(k i and KPIP) were calculated (see Table 1). The consistency
k )k¢ KoMiNTS (3) be2tween thwei experimental and predicted values (see Figure 4)
obs iKMNTS+Z confirms the accuracy of the micellar pseudophase model.
oi
To compare the xylene and isooctane systems, we carried
k¢ being the pseudo-first-order rate constant at the interphase. outakineticstudyinwater/AOT/isooctanemicroemulsionsat
Siuch a rate constant can be expressed as a function of the 50 (cid:176) C. Kinetic parameters were determined by following the
bimolecular rate constant at the interphase, k i: sameprocedureasusedpreviously.AscanbeseenfromTable
2
1, the partition equilibrium constant of PIP, KPIP, between
wi
V [PIP] water and the AOT film was greater in the o-xylene micro-
k¢ )k i[PIP]i)k i t[PIP] )k i i (4)
i 2 i 2V i 2Vh [AOT] emulsions. Also, the reaction rate constant at the interphase,
i AOT k i,wassmallerinthesurfactantfilmofthewater/AOT/o-xylene
2
where [PIP]i is the PIP concentration at the microemulsion microemulsions.
i (b) Reaction with N-Methylbenzylamine (NMBA). The
interphase referred to the volume of the interphase and [PIP]
i
transnitrosation reaction between MNTS and NMBA was
is that referred to the overall volume of the system. The
studiedinawiderangeofwater/AOT/o-xylenemicroemulsions
interphase volume is given by that occupied by AOT. Thus,
V /V ) 1/(Vh [AOT]), where Vh is the molar volume of at 50 (cid:176) C. The influence of the microemulsion composition on
tot i AOT AOT
the surfactant (Vh ) 0.37 M-1(ref 28)) and [AOT] is the the reaction rate was examined in experiments using each
AOT
continuousmedium,keepingtheNMBAandMNTSconcentra-
total AOT concentration referred to the total microemulsion
tions([NMBA])0.103M;[MNTS])2(cid:2)10-3M)constant,
volume.Iftheamineconcentrationattheinterphaseisexpressed
byusingWvaluesbetween7and18andsurfactantconcentra-
as
tions over the range 0.2-0.7 M. As can be seen in Figure 5,
KPIP kobsincreasedmarkedlywithincreasingsurfactantconcentration
[PIP] ) wi [PIP] (5) ataconstantvalueofWandmoreslightlywithincreaseinW
i KwPIiP+W T at a constant AOT concentration.
The experimental data were processed quantitatively in the
substitution into eq 3 yields
light of the micellar pseudophase model. Both NMBA and
MNTSarevirtuallyinsolubleinwater,sothereactantsshould
k i KPIP KMNTS
k ) 2 wi [PIP] oi (6) partition between the continuous medium and surfactant film
obs Vh [AOT]KPIP+W TKMNTS+Z andbeabletoreactinbothpseudophases.However,thereaction
AOT wi oi
between MNTS and NMBA in the continuous medium was
which can be rearranged to found to be very slow and, hence, was discarded. The rate
constantsofnitrosationprocessesinalcanesasreactionmedia
[PIP] (KPIP+W)(KMNTS+Z) areseveralordersofmagnitudehigherthanthevaluesobserved
T ) wi oi (7) in this work.29 The pseudophase model used is depicted in
kobsVhAOT[AOT] k2iKwPIiPKoMiNTS Scheme 3.
Ind.Eng.Chem.Res.,Vol.45,No.2,2006 603
Figure6. VariationoftherateconstantwithparameterZforwater/AOT/
Figure4. Plotofexperimentalvaluesagainstthepredictionsofeq7for
o-xylenemicroemulsions.Thedashedlinecorrespondstothefittingofeq
thereactionbetweenMNTSandPIPinwater/AOT/o-xylenemicroemul-
9.
sions.
29.6 and 0.1-0.5 M, respectively. Application of the same
pseudophase model yielded and identical equation (eq 9), to
which the experimental values were fitted in order to obtain
thekineticparametersforthetransnitrosationreactionbetween
MNTSandNMBAinwater/AOT/isooctanemicroemulsionsat
50 (cid:176) C (see Table 1). As can be seen, the association constant
ofNMBAtothesurfactantfilm,KNMBA,wassimilarinthetwo
oi
typesofmicroemulsion.Worthspecialnoteisthevalueofthe
rate constant, ki, in the xylene microemulsions, which was
2
roughlyone-halfthatintheisooctanemicroemulsions.Thismay
be a result of the interphase in the former being less polar by
effectofthepenetrationofthecontinuousmedium(xylene)into
the surfactant film or of less extensive hydration of AOT in
Figure 5. Influence of the AOT concentration on the transnitrosation these systems.
reactionbetweenNMBAandMNTSinwater/AOT/o-xylenemicroemul- The study was completed by examining the reaction of
sions: [MNTS])2(cid:2)10-3M;(b)W)9.3;(0)W)11.1;(2)W) NMBA with MNTS in water/AOT/m-xylene and water/AOT/
13.0.
p-xylenemicroemulsions.Thechoiceofthisaminewasdictated
Onthebasisofthismodel,theequilibriumpartitionconstant by the fact that, as shown earlier, it partitioned between the
of NMBA between the AOT film and oil can be expressed as AOTfilmandoil,soitmusthavebeenmoremarkedlyaffected
by the structural difference between the two xylene isomers.
[NMBA] The pseudophase model used was identical with that depicted
KoNiMBA)[NMBA]iZ (8) inScheme3,sotheequationrelatingkobstoparametersWand
o Z for the microemulsion, eq 9, was also identical. The results
are shown in Table 1.
Theexpressionrelatingk tothemicroemulsioncomposition,
obs
[NMBA] (KNMBA+Z)(KMNTS+Z) Discussion
t ) oi oi (9)
k Vh [AOT] k iKNMBAKMNTS 1.Water/AOT/IsooctaneMicroemulsionsvsWater/AOT/
obs AOT 2 oi oi
o-Xylene Microemulsions. The interfacial rate constants ob-
wasobtainedsimilarlyasthatforPIP.Ontheassumptionofa tainedfromeqs7and9arelistedinTable1.Thek /ki ratiois
w 2
constant KMNTSswhich was previously determined for the a measure of inhibition of the reaction in a microemulsion by
oi
reactionwithPIPsthekineticparameters(KNMBAandki)were effect of the polarity of the interphase relative to water. The
oi 2
obtainedbyfittingtheexperimentalvaluestothisequation(see [iso(k /ki)]/[xyl(k /ki)] ratio allowed us to compare the degree
w 2 w 2
Figure 6 and Table 1). of inhibition resulting from water/AOT/isooctane and water/
Forcomparisonwithwater/AOT/isooctanemicroemulsions, AOT/o-xylenemicroemulsions.Onthebasisoftheresults,the
testsinvolvingtheuseofthesameamineandMNTSconcentra- reaction between MNTS and NMBA was found to be more
tions over a wide range of microemulsion compositions were markedly inhibited in the xylene-containing microemulsions.
performed at the same temperature (50 (cid:176) C). Values of W and Thus, the reaction was 2.5 times faster in the isooctane
the AOT concentration were changed over the ranges 11.1- microemulsionsthanintheo-xylenemicroemulsions.Also,the
Table1. KineticParametersObtainedbyFittingtheExperimentalDataforWater/AOT/XyleneandWater/AOT/IsooctaneMicroemulsionsat
50(cid:176) CtoEquations7and9
microemulsion amine KPIP KNMBA KMNTS ki (M-1(cid:226)s-1)
wi oi oi 2
water/AOT/o-xylene NMBA 20.1(2.6 2.3 (4.07(0.03)(cid:2)10-3
water/AOT/m-xylene NMBA 12.8(2.9 2.3 (4.44(0.07)(cid:2)10-3
water/AOT/p-xylene NMBA 12.5(2.6 2.3 (4.81(0.07)(cid:2)10-3
water/AOT/isooctane NMBA 13.8(2.0 7.7 (1.03(0.11)(cid:2)10-2
water/AOT/o-xylene PIP 10.6(2.3 2.3 (2.94(0.33)(cid:2)10-2
water/AOT/isooctane PIP 20.6(2.4 7.7 (4.48(0.03)(cid:2)10-2
604 Ind.Eng.Chem.Res.,Vol.45,No.2,2006
Table2. KineticParametersObtainedbyFittingtheExperimental
DataforWater/AOT/IsooctaneMicroemulsionstoEquations7and
9
amine T((cid:176) C) KPIP KNMBA KMNTS ki (M-1(cid:226)s-1)
wi oi oi 2
PIP 25 9.5(0.7 11.0 (5.81(0.03)(cid:2)10-3
PIP31 35 13.1 10.1 1.18(cid:2)10-2
PIP 50 20.6(2.4 7.7 (4.48(0.03)(cid:2)10-2
NMBA 25 25.0(1.0 11.0 (1.62(0.04)(cid:2)10-3
NMBA31 35 30 10.1 2.55(cid:2)10-3
NMBA 50 13.8(2.0 7.7 (1.03(0.11)(cid:2)10-2
reactionwithPIPwas1.5timesfasterintheformerthaninthe
latter.Theincreasedinhibitoryeffectofthexylenemicroemul-
sions was a result of the decreased polarity of the reaction
medium (viz., the AOT film between the continuous medium
Figure7. Plotofexperimentalvstheoreticaldataforthetransnitrosation
and the water). The polarity of the interphase is a function of reactionbetweenMNTSandNMBAinwater/AOT/o-xylene(b),water/
theamountofwaterandoilpresent.Structuralevidencesuggests AOT/m-xylene(0),andwater/AOT/p-xylene(2)microemulsions.
that penetration of the continuous medium into the AOT film
iseasierforthearomaticmediumthanforisooctane.However,
the polarity change induced by an increase in droplet size is
slightly more substantial in isooctane microemulsions than in
xylene microemulsions. Probably, as W increases, the number
of water molecules at the interphase in the isooctane micro-
emulsion also increases and the interphase polarity is slightly
higher, as reflected in the kinetic data. Also, the fact that the
working temperature used was higher than the percolation
temperatureforwater/AOT/isooctanemicroemulsionsmayhave
altered the polarity of the system.
The equilibrium constants (KPIP, KNMBA, and KMNTS) are
wi oi oi
suggestiveofthewaythereactantspartitionbetweenthethree
pseudophases in the two reactions. Worth special note is the
rathersmallvalueoftheequilibriumpartitionconstantofMNTS Figure8. Influenceoftemperatureonki forthereactionsofMNTSwith
2
between the continuous medium and the AOT film, KMNTS, PIP(O)andNMBA(b)inwater/AOT/isooctanemicroemulsions.
oi
whichmaybearesultoftheincreasedrigidityofthesurfactant
filminthewater/AOT/xylenemicroemulsions.Inanycase,an
However, the results (Table 1) revealed the virtual absence of
increase in temperature similarly decreases KMNTS for water/
oi differencesbetweenthethreexylenesusedascontinuousmedia.
AOT/isooctanemicroemulsions.Theequilibriumpartitioncon-
Figure7testifiestothegoodfitoftheexperimentaldatatothe
stant between water and PIP, KPIP, exhibits a different behav-
wi equationprovidedbythepseudophasemodelforthethreetypes
ior;thus,anincreaseintemperaturefacilitatesthemigrationof
of xylene microemulsions.
PIP to the pseudo-reaction phase in water/AOT/isooctane
3. Activation Parameters. The study was completed with
microemulsions(seeTable2).Thisresultalsoseemstoconfirm
the presence of an amount of water at the interphase that the determination of the activation parameters for the nitroso
increasesasthetemperatureisraised.Inanycase,theseresults group transfer reaction from MNTS to the secondary amines
should be taken with some reservations as the equilibrium NMBA and PIP in water/AOT/isooctane microemulsions. We
constantvaluesobtainedbyfittingtheexperimentalvaluesdiffer examinedtheinfluenceoftemperatureonthebimolecularrate
less markedly than the rate constants. constant for the two processes and found a linear relationship
2.Water/AOT/o-Xylene,Water/AOT/m-Xylene,andWater/ between ln(k/T) and 1/T in both cases (Figure 8), consistent
AOT/p-XyleneMicroemulsions.Stereochemicalchangesinthe with the Eyring equation (r2 ) 0.9928 and r2 ) 0.9600,
aromaticadditiveorinthenatureoftheoilofamicroemulsion respectively,forPIPandNMBA).Thus,¢H(cid:176) wasfoundtobe
are known to alter its percolation temperature.26,30-32 Thus, 63.3(5.4kJ(cid:226)mol-1forpiperazineand57.8(11.8kJ(cid:226)mol-1
Mouliketal.30foundthehydrotropes2-methoxyphenol,4-meth- forN-methylbenzylamine,and¢S(cid:176) )-75.6(10.6J(cid:226)mol-1(cid:226)K-1
oxyphenol, 5-methoxyresorcinol, and 3-methoxycatechol to for the former and -105.5 ( 43.3 J(cid:226)mol-1(cid:226)K-1 for the latter,
influence the dynamic percolation process. These compounds allinwater/AOT/isooctanemicroemulsions.33Thesevaluesare
caneitherassistpercolationbywayofbridgingapairofdroplets consistent with those previously obtained in aqueous media34
forefficientfusionandiontransferordelayitthroughblocking forthereactionsofMNTSwith4-hydroxyproline(¢H(cid:176) )55.3
andresistingfusionofthedropletstohindermasstransfer.The kJ(cid:226)mol-1,¢S(cid:176) )-81J(cid:226)mol-1(cid:226)K-1),3-quinuclidol(¢H(cid:176) )55.5
position of the hydroxyl groups in the phenolic hydrotrope kJ(cid:226)mol-1,¢S(cid:176) )-87J(cid:226)mol-1(cid:226)K-1)andazideion(¢H(cid:176) )60.2
molecule is the decisive factor for its action on the dynamics kJ(cid:226)mol-1, ¢S(cid:176) ) -78 J(cid:226)mol-1(cid:226)K-1), which confirms the
of the process. Thus, the monohydroxy compounds resist accuracy of the pseudophase model in predicting kinetic
percolation, whereas the dihydroxy compounds assist it. The parametersformicroemulsions.Thevaluesofthermodynamic
percolationefficiencyincreasesinthefollowingsequence: ortho parameters for our reactions prove that changes in the rate
<meta<para.Thethresholdtemperatureisdirectlyrelatedto constant are due to compartmentalization of reactants and
the hydrotrope concentration. changesintherelativeconcentrationinthedifferentpseudophases
Our purpose was to examine the kinetic implications of and that there is no effect upon the chemical mechanism of
changes in the continuous medium of the microemulsion. nitrosation.
Ind.Eng.Chem.Res.,Vol.45,No.2,2006 605
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