Research Article Kinetics and Mechanism of Nanoparticles ... · M 1 s 1 forNa V =Na ,Na2,Na...

Research ArticleKinetics and Mechanism of Nanoparticles-CatalyzedPiperidinolysis of Anionic Phenyl Salicylate

Norazizah Abd. Razak and M. Niyaz Khan

Department of Chemistry, Faculty of Science, University of Malaya, 50603 Kuala Lumpur, Malaysia

Correspondence should be addressed to M. Niyaz Khan; [email protected]

Received 20 May 2014; Accepted 9 September 2014; Published 13 November 2014

Academic Editor: Feng Ding

Copyright © 2014 N. Abd. Razak and M. N. Khan. This is an open access article distributed under the Creative CommonsAttribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work isproperly cited.

The values of the relative counterion (𝑋) binding constant 𝑅Br𝑋(=𝐾𝑋/𝐾Br, where 𝐾𝑋 and 𝐾Br represent cetyltrimethylammonium

bromide, CTABr, micellar binding constants of 𝑋V− (in non-spherical micelles), V = 1, 2, and Br− (in spherical micelles)) are 58,68, 127, and 125 for 𝑋V− = 1−, 12−, 2−, and 22−, respectively. The values of 15mM CTABr/[NaV𝑋] nanoparticles-catalyzed apparentsecond-order rate constants for piperidinolysis of ionized phenyl salicylate at 35∘Care 0.417, 0.488, 0.926, and 0.891M−1 s−1 forNaV𝑋= Na1, Na

21, Na2, and Na

22, respectively. Almost entire catalytic effect of nanoparticles catalyst is due to the ability of nonreactive

counterions,𝑋V−, to expel reactive counterions, 3−, from nanoparticles to the bulk water phase.

1. Introduction

Research on nanoparticles has now become a cutting-edgearea of chemical research [1]. Mono- and bilayer surfactantaggregates are nanoparticles which have been known for theircharacteristic physicochemical properties for more than 100years [2]. The effects of surfactant aggregates/nanoparticlesof different structural features on reaction rates have beenextensively studied for the past nearly six decades [3–5].Thesestudies reveal very complex mechanistic aspects of micel-lar/nanoparticles catalysis of reaction rates [4–6]. Effects ofcounterionic salts on ionic surfactant as well as biomolecularstructural transitions have been under extensive study since1887whenHofmeister first reported specific salt effects on thesalting-out proteins [7]. But the mechanistic aspects of thesespecific salt effects are not yet fully understood [8–10].

Effects of inert salts of moderately hydrophobic coun-terions, such as benzoate and substituted benzoate ions,on ionic surfactant micellar growth have become veryimportant for various industrial applications [9–11]. How-ever, mechanistic details of such inert salt effects on ionicmicellar growth are almost nonexistent. Effects of inertcounterionic salts on pseudo-first-order rate constants (𝑘obs)for the ionic surfactant nanoparticle-catalyzed semi-ionic

bimolecular reactions, where ionic reactant is also a coun-terion, have been explained quantitatively by the use ofpseudophase ion-exchange (PIE) model. But the use ofPIE model involves mostly counterionic salts of highly andmoderately hydrophilic counterions [12]. However, someinherent weaknesses of PIE model have been also realized[13, 14]. The increase in [MX] (MX = 3- and 4-FBzNa withBz− = C

6H4CO2

−) has caused nonlinear increase in 𝑘obsfor piperidinolysis of anionic phenyl salicylate (PSa−) ata constant [CTABr]T ≫ cmc where [CTABr]T and cmcrepresent total concentration of cetyltrimethylammoniumbromide and critical micelle concentration of CTABr, respec-tively [15]. The magnitudes of the gradient of the plot of𝑘obs versus [MX] show continuous decrease with increasing[MX] [15]. The values of 𝑘obs remained almost independentof [MX] within its range where the presence of 5mM CTABrresulted in more than 10-fold increase in 𝑘obs. Thus, 5mMCTABr/[MX] nanoparticles act as catalyst because, in theabsence of CTABr, the values of 𝑘obs remained independentof [MX] within its range covered in the study [15]. More than10-fold catalytic effects of CTABr/MX nanoparticles were notemphasized and discussed in the report [15]. The catalyticeffects of CTABr/MX/H

2O nanoparticles catalyst (MX = 4-

methoxy and 4-methyl salicylates) on 𝑘obs for piperidinolysis

Hindawi Publishing Corporatione Scientific World JournalVolume 2014, Article ID 604139, 7 pageshttp://dx.doi.org/10.1155/2014/604139

2 The Scientific World Journal

O N

O−

Na5

NH

4

OH

6

O OY

OZ

X

2H, X = Me, Y = Z = H3H, X = Z = H, Y = C6H5Na1, X = OMe, Y = Na+ , Z = HNa21, X = OMe, Y = Z = Na+

Na2, X = Me, Y = Na+ , Z = HNa22, X = Me, Y = Z = Na+

Na3, X = H, Z = Na+ , Y = C6H5

1H, X = OMe, Y = Z = H

Figure 1: Molecular structures of compounds 1H, Na1, Na21, 2H, Na2, Na

22, 3H, Na3, 4, Na5, and 6.

of PSa− have been studied in the present study. The resultsand their probable explanations are described in this paper.

2. Materials and Methods

2.1. Materials. Reagent-grade 4-methoxysalicylic acid (1H),4-methylsalicylic acid (2H), cetyltrimethylammonium bro-mide (CTABr), phenyl salicylate (3H), and piperidine (4)(Figure 1) were commercial products of highest availablepurity. Other common chemicals used were also of reagentgrade. The stock solutions of 0.50M MV𝑋 (=NaV1 and NaV2with V = 1 and 2) were prepared by adding 0.52 and1.25M NaOH to the corresponding 0.50M solutions of 1Hor 2H. The stock solutions of 0.01M 3H were prepared inacetonitrile.Throughout the text, the symbol [𝑋]T representsthe total concentration of𝑋.

2.2. Kinetic Measurements. The rate of CTABr/NaV𝑋 nano-particles-catalyzed nucleophilic substitution reaction of 4with Na3 was studied spectrophotometrically at 35∘C bymonitoring the disappearance of Na3 at 365 or 370 nm.The products of the reaction of 4 with Na3 are sodium N-piperidinyl salicylate (Na5) and phenol (6) (Figure 1). Thedetails of the kinetic procedure and product characterizationhave been described elsewhere [16]. Absorbance values (𝐴ob)at different reaction time (𝑡) were found to fit to (1) for ∼8half-lives of the reactions. In (1), [𝑅

0] represents the initial

concentration of 3H, 𝛿ap is the apparent molar absorptivityof

𝐴ob = [𝑅0] 𝛿ap exp (−𝑘obs𝑡) + 𝐴∞ (1)the mixture, 𝑘obs is the pseudo-first-order rate constant, and𝐴∞

= 𝐴obs at 𝑡 = ∞. Throughout the study, the initialconcentrations of 3H or Na3 were kept constant at 0.2mM.The choice of this specific concentration was governed bythe need to keep it sufficiently low so that it is less than theother salicylate counterions but high enough to measure theabsorption spectrophotometrically.

3. Results

3.1. Effects of [𝑁𝑎V𝑋] (V = 1, 2) on 𝑘obs for the Reaction of4 with Na3 at a Constant [CTABr]T and 35∘C. A series of

0

2

4

6

8

10

12

14

16

18

0 0.05 0.1 0.15 0.2 0.25 0.3

[Na1] (M)

103k

obs

(−1)

s

Figure 2: Plots showing the dependence of 𝑘obs upon [Na1] forpiperidinolysis of 3H at 0.2mM 3H, 0.1M 4, 0.03M NaOH, and35∘C.The solid line is drawn through the calculated data points using(2) with kinetic parameters (𝑘cat and 𝐾

𝑋/𝑆), listed in Table 2. Thedotted line is drawn through the predicted data points assuming thepresence of WM at [Na1]op

0< [Na1] ≤ 300 mM.

kinetic runs was carried out at the constant 15mM CTABr,0.2mM 3H, 0.1M 4, and varying values of [NaV𝑋] (V = 1, 2)within the range 0 ≤ [NaV𝑋] ≤ 0.30 M for NaV𝑋 = NaV1(V = 1, 2). The values of 𝑘obs versus [NaV1] at [NaOH]/[1H] =1.04 are shown in Figure 2. Similar plot of 𝑘obs versus [NaV1]was also obtained at [NaOH]/[NaV1] = 2.50. The plot ofFigure 2 shows initial segment where the values of 𝑘obs arealmost independent of [NaV1] at the initial low values of[NaV1] followed by the segment where the values of 𝑘obs revealmonotonic increase of more than 7-fold with the increase in[NaV1].

The values of 𝑘obs were also obtained at constant 15mMCTABr, 35∘C, 0.2mM 3H, 0.1M 4, and different values of[NaV2] (V = 1, 2) within the range 0 ≤ [NaV2] ≤ 0.30M.The values of 𝑘obs versus [NaV2], at [NaOH]/[2H] = 1.04, are

The Scientific World Journal 3

Table 1: The values of 𝛿ap, calculated from (1) for the piperidinolysis of 3− under the variety of experimental conditionsa.

[NaV𝑋]b (mM) 10

−1𝛿ap (M

−1 cm−1) CH3CN (%v/v) 10−1𝛿ap (M

−1 cm−1)eNa1c Na21d Na2c Na22d

0 373 ± 2f 369 ± 1f 366 ± 2f 3372 ± 1f 2 175 ± 1f

10 380 ± 2 379 ± 1 407 ± 3 386 ± 1 25 215 ± 115 356 ± 1 362 ± 1 341 ± 1 343 ± 1 50 250 ± 130 323 ± 1 330 ± 1 403 ± 8 395 ± 15 60 265 ± 250 286 ± 1 292 ± 2 457 ± 5 364 ± 6 70 288 ± 170 276 ± 2 275 ± 2 236 ± 1 250 ± 1 84 300 ± 3100 251 ± 1 276 ± 1 240 ± 1 230 ± 4 90 367 ± 3150 239 ± 1 257 ± 1 222 ± 1 227 ± 1 92 435 ± 3200 230 ± 1 251 ± 1 222 ± 2 226 ± 1300 219 ± 1 244 ± 1 221 ± 2 238 ± 2a[3H]0 = 0.2mM, 𝜆 = 370 nm, 35∘C, 30mMNaOH, 100mMPip, and 15mMCTABr. bNaV𝑋=NaV1 and NaV2, V = 1, 2. c[NaOH]/[XH] = 1.04. d[NaOH]/[XH]= 2.50. eCalculated from (1) by the use of observed data (𝐴ob versus reaction time 𝑡) obtained for the kinetic runs at 0.2mM 3H, 10mM NaOH, 100mM Pip,370 nm, and 35∘C and within CH3CN content range of 2–92%v/v in mixed aqueous solvents.

fError limits are standard deviations.

shown in Figure 3. Similar plot of 𝑘obs versus [NaV2] (notshown) was also obtained at [NaOH]/[2H] = 2.5. The valuesof [NaOH] were varied from 0.030 to ≤0.18M under theexperimental conditions of entire kinetic runs for both NaV1and NaV2. The shape of the plot of Figure 3 is similar to thatof Figure 2 when [NaV2]≤ ∼20mM.The increase in [NaV2] at∼20mM NaV2 reveals a mild increase followed by a decreaseand then increase again in the values of 𝑘obs (Figure 3).Similar break in the plot (not shown) of 𝑘obs versus [NaV2]wasalso obtained at [NaOH]/[2H] = 2.5.These observations maybe attributed to the change in the structure of NaV𝑋/CTABrnanoparticles to some higher interfacial curvature structuressuch as curved bilayer structures at ∼20mMNaV2 [17].

The absence and presence of break in the monotonic plotof respective Figures 2 and 3 are indirectly supported bythe following observations. The values of 𝛿ap, obtained forpiperidinolysis of 3− at 10mM NaOH, 100mM Pip, 0.2mM3H, 35∘C, and 370 nm, increase nonlinearly from 1750 to4350M−1 cm−1 with the increase in CH

3CN content from 2

to 92% v/v in mixed aqueous solvent (Table 1). The valuesof 𝛿ap, obtained for piperidinolysis of 3− at 30mM NaOH,100mMPip, 0.2mM 3H, 35∘C, 370 nm, anddifferent values of[NaV𝑋], for NaV1 and NaV2 (V = 1, 2), are also summarized inTable 1. It is evident from Table 1 that (a) the values of 𝛿ap arealmost independent of [NaV𝑋] within its range 0–∼15mM for𝑋 = 1V− and 0–∼50mM for𝑋 = 2V− and (b) the values of 𝛿apreveal a monotonic decrease with increasing [NaV1], V = 1, 2,within its range ∼30–300mM. But the values of 𝛿ap show asharp decrease with the increase in [NaV2], V = 1, 2, from50 to 70mM and then become almost independent of [NaV2]within its range ∼70–300mM. These observations simplydemonstrate that NaV𝑋-induced CTABr/NaV𝑋 nanoparticlesstructural transition, within [NaV𝑋] range of 50–300mM, isnot the same for NaV1 and NaV2 (V = 1, 2).

3.2. Effects of [𝑁𝑎V𝑋] on 𝑘obs for the Reaction of 4 with Na3in the Absence of CTABr at 35∘C. In order to quantify thecatalytic effects of CTABr/NaV𝑋 nanoparticles on the rate of

0

2

4

6

8

10

12

14

16

18

0 0.05 0.1 0.15 0.2 0.25 0.3

20

22

24

26

0.020.0100

3

6

9

12

[Na2] (M)

[Na2] (M)

103k

obs

(−1)

s

103k

obs

(−1)

s

Figure 3: Plot showing the dependence of 𝑘obs upon [Na2], forpiperidinolysis of 3H at 0.2 mM PSa−, 0.1M 4, 0.03M NaOH, and35∘C.The solid line is drawn through the calculated data points using(2) with kinetic parameters (𝑘cat and 𝐾

𝑋/𝑆), listed in Table 2. Thedotted line is drawn through the predicted data points assuming thepresence of WM at [Na2]op

0< [Na2] ≤ 300 mM.

piperidinolysis of Na3, it is essential to study the effects of[NaV𝑋] on 𝑘obs at 35

∘C and [CTABr]T = 0. Although benzoateand substituted benzoate ions are nonreactive towards thenucleophilic cleavage of Na3, such inert salts might affect 𝑘obsthrough ionic strength effect or specific salt effect. Thus, aseries of kinetic runs was carried out at 0.2 mM 3H, 0.1 M 4,30 mMNaOH, and varying values of [NaV1] and [NaV2]. Thevalues of 𝑘obs reveal


Table2:Th

evalueso

fempiric

alconstants,𝑘catand𝐾𝑋/𝑆,for

Na V1andNa V2(V=1,2),at35∘Cin

thep

resenceo

fCTA

Br/N

a V𝑋nano

particlesa.

Na V𝑋

[NaO

H]/[XH

][Na V𝑋]op

b0

(mM)

[Na V𝑋]op

c0

(mM)

103𝑘MX𝑊

d(s−1 )

103𝑘0

e(s−1 )

103𝑘cat(M−1 s−

1 )𝐾𝑋/𝑆(M−1 )

𝐹𝑋/𝑆

f𝐾𝑋/𝑆

g(M−1 )

𝐾𝑛 𝑋/𝑆

h(M−1 )

𝑅Br 𝑋

i[Na V𝑋]jrange(mM)

Na1

1.04

11.7

10.6

30.7±0.5k

2.20

±0.03

k417±12

k17.7±0.9k

0.77

1876

1444

5812–100

Na 21

2.50

13.0

11.6

30.5±0.2

2.26

±0.04

488±18

23.5±1.3

0.68

2491

1694

6815–120

Na2

1.04

12.2

10.8

30.3±0.1

2.20

±0.03

926±70

30.0±2.6

1.03180

3180

127

13–21

Na 22

2.50

10.0

9.630.3±0.6

2.20

±0.03

891±

103

30.4±4.2

0.97

3222

3125

125

12–21

a [3H]0=0.2m

M,𝜆

=365and370n

mforN

a V1andNa V2,respectiv

ely,30m

MNaO

H,100

mM

Pip,and15mM

CTABr

andaqueou

sreactionmixture

fore

achkinetic

runcontains

2%v/vCH

3CN.bCa

lculated

byan

iterativ

etechn

ique

asmentio

nedin

thetext.

c Obtainedby

graphicaltechn

ique

asmentio

nedin

thetext.

d Thev

alue

of𝑘MX𝑊

isthem

eanvalueo

f𝑘ob

sob

tained

with

in[Na V𝑋]rangew

here𝑘ob

svalues

remained

independ

ento

f[Na V𝑋]at[CT

ABr]𝑇=0.

e Thev

alue

of𝑘0isthem

eanvalueo

f𝑘ob

svalues

obtained

with

in[N

a V𝑋]range

0.0–≤[Na V𝑋]0

opat[CT

ABr]𝑇=15mM.f𝐹𝑋/𝑆=𝑘cat/(𝑘MX𝑊×𝐾𝑋/𝑆).

g 𝐾𝑋/𝑆=𝐾𝑋/𝑆×(1+

𝐾𝑆

0×[CT

ABr]𝑇)where𝐾𝑆

0=7×10

3M−1and[CT

ABr]𝑇=15mM.h𝐾𝑛 𝑋/𝑆=𝐹𝑋/𝑆×𝐾𝑋/𝑆.i𝑅Br 𝑋

=𝐾𝑛 𝑋/𝑆/𝐾

Br/𝑆with𝐾Br/𝑆=25

M−1 .

j Totalconcentrationrangeo

fNa V𝑋

used

inthed

ataanalysis.

k Error

limits

are

stand

arddeviations.


4. Discussion

The experimental data (𝑘obs versus [NaV𝑋]) exhibited byFigures 2 and 3 (at [NaV2] < ∼21mM) were found to fit toempirical equation:

𝑘obs =𝑘0+ 𝑘cat ([NaV𝑋] − [NaV𝑋]

op0)

1 + 𝐾𝑋/𝑆 ([NaV𝑋] − [NaV𝑋]op0)

, (2)

where 𝑘cat and 𝐾𝑋/𝑆 are empirical constants, 𝑘

0= 𝑘obs

at [NaV𝑋] − [NaV𝑋]op0

= 0, and [NaV𝑋]op0

represents theoptimum concentration of NaV𝑋 below which the values of𝑘obs are independent of [NaV𝑋]. The empirical constant 𝑘catrepresents 15mM CTABr/[NaV𝑋] nanoparticles-catalyzedapparent second-order rate constant for piperidinolysis ofNa3. The values of [NaV𝑋]

op0

were calculated using an iter-ative technique as described elsewhere [15]. These values of[NaV𝑋]

op0

(Table 2) are comparable with the correspondingvalues of [NaV𝑋]

op0

obtained by the graphical technique [5].As described in detail elsewhere [15, 18], the value of [NaV𝑋]

op0

represents the optimum value of [NaV𝑋] required for theoccurrence of ion exchange processes 𝑋−/OH− and 𝑋−/Br−.Equation (2), with replacement of 𝑘cat by 𝜃𝐾

𝑋/𝑆 where 𝜃 is anempirical constant, has been found to explain quantitativelysimilar observed data (𝑘obs versus [NaV𝑋]), for differentNaV𝑋 [5]. The nonlinear least-squares technique was usedto calculate 𝑘cat and 𝐾

𝑋/𝑆 from (2) by considering 𝑘0as a

known parameter. The least-squares calculated values of 𝑘catand 𝐾𝑋/𝑆 and experimentally determined values of 𝑘

0, at

[NaOH]/[𝑋H] = 1.04 and 2.50, are shown in Table 2. Thestatistical reliability of the observed data fit to (2) is evidentfrom the standard deviations associated with the calculatedvalues of 𝑘cat and 𝐾

𝑋/𝑆 as well as from the solid line plots ofFigures 2 and 3 which were drawn through the least-squarescalculated data points.

It has been described in detail elsewhere [5, 15, 18] thatthe nonlinear increase in 𝑘obs with the increase of [NaV𝑋] ata constant [CTABr]T is due to the transfer of micellized 3−(i.e., 3−

𝑀with subscript𝑀 indicating micellar pseudophase)

to aqueous phase (i.e., 3−𝑊with subscript𝑊 indicating bulk

water phase) through the occurrence of ion exchange process𝑋

V−/3−.This is due to the reason that the value of 𝑘obs is morethan 10-fold larger in the bulk water phase than that in themicellar pseudophase as evident from the listed values of 𝑘MX

𝑊

and 𝑘0in Table 2. The occurrence of ion exchange 𝑋V−/3− in

the related reaction systems [5] has been found to decreasethe CTABr micellar binding constant (𝐾

𝑆) of 3− with the

increasing [NaV𝑋] through an empirical relationship:

𝐾𝑆=

𝐾0

𝑆

(1 + 𝐾𝑋/𝑆

[NaV𝑋]), (3)

where 𝐾0𝑆

= 𝐾𝑆at [NaV𝑋] = 0 and 𝐾𝑋/𝑆 represents an

empirical constant whose magnitude is the measure of theability of counterion𝑋V− to expel another counterion 𝑆− fromthe cationic micellar pseudophase to the bulk aqueous phasethrough the occurrence of ion exchange process 𝑋V−/𝑆− atthe cationic micellar surface. It can be easily shown that the

reaction mechanism for nucleophilic reaction of 4 with 3−,expressed in terms of pseudophase micellar (PM) model and(3), can lead to (2) [18]with 𝑘cat and𝐾

𝑋/𝑆 expressed by (4) and(5), respectively. As shown in the following equation, 𝑘MX

𝑊=

𝑘obs [NaV𝑋] 𝑘obs [NaV𝑋] 𝐹𝑋/𝑆 is an

𝑘cat = 𝐹𝑋/𝑆𝑘MX𝑊

𝐾𝑋/𝑆

, (4)

𝑘MX𝑊

= 𝑘obs obtained within [NaV𝑋] range where 𝑘obs valuesare independent of [NaV𝑋] in the absence of CTABr and 𝐹𝑋/𝑆is an empirical constant whose magnitude should vary in therange >0.0 to ≤1.0 [18]. The following equation

𝐾𝑋/𝑆

=𝐾𝑋/𝑆

(1 + 𝐾0

𝑆[CTABr]𝑇)

(5)

is valid only under the experimental conditions where[CTABr]T − cmc ≈ [CTABr]T with cmc representing criticalmicelle concentration of CTABr. Perhaps, it is worth men-tioning that the value of cmc of CTABr, at 0.2 mM 3−and[NaV𝑋] = 0, was kinetically determined as 0.09mM whichbecame 0.04mM at 0.1M NaBr. The value of cmc became∼0 at ≥0.5M NaBr [19]. These observations demonstrate thatthe value of cmc is negligible compared with [CTABr]T at itsvalue of ≥5mM.

The value of 𝐹𝑋/𝑆

measures the fraction of the micellizedcounterions (3−

𝑀) transferred to aqueous phase by the opti-

mum concentration of NaV𝑋 through ion exchange 𝑋V−/3−

[18]. The value of 𝐹𝑋/𝑆

was calculated from (4) by the useof listed values of 𝑘cat, 𝑘

MX𝑊

, and 𝐾𝑋/𝑆 in Table 2 and thesecalculated values of 𝐹

𝑋/𝑆for Na1, Na

21, Na2, and Na

22 are

also listed in Table 2. The value of 𝐾𝑋/𝑆

was calculated from(5) with the reported value of 𝐾0

𝑆(=7 × 103M−1 [5, 15]).

The calculated values of 𝐾𝑋/𝑆

for NaV𝑋 with V = 1, 2and 𝑋 = 1, 2 are shown in Table 2. It has been concludedelsewhere [5, 18] that the normalized 𝐾𝑛

𝑋/𝑆(=𝐹𝑋/𝑆

𝐾𝑋/𝑆

) and𝐾𝑛

𝑌/𝑆(=𝐹𝑌/𝑆

𝐾𝑋/𝑆

) values are empirically related to the ratio𝐾𝑋/𝐾𝑌through the relationship 𝑅𝑌

𝑋= 𝐾𝑋/𝐾𝑌= 𝐾𝑛

𝑋/𝑆/𝐾𝑛

𝑌/𝑆

where 𝐾𝑋= [𝑋𝑀]/([𝑋𝑊][𝐷𝑛]) and 𝐾

𝑌= [𝑌𝑀]/([𝑌𝑊][𝐷𝑛]).

The symbols 𝐾𝑋and 𝐾

𝑌represent CTABr micellar binding

constants of counterions 𝑋− and 𝑌−, respectively, and [𝐷𝑛]

is the concentration of CTABr micelles with each micellecontaining 𝑛 number of monomers. The values of 𝐾𝑛

𝑋/𝑆

(Table 2) and the reported value of 25M−1 [15, 18] for 𝐾𝑛Br/𝑆(with Br− = Y−) give the values of 𝑅Br

𝑋for 𝑋 = 1V−, 2V−

with V = 1 and 2. These results are also shown in Table 2.It is relevant to note that the value of 𝐾𝑛Br/𝑆 (=25M

−1) isderived from kinetic parameters obtained in the presence ofspherical CTABrmicelles (SM). But the values of𝐾𝑛

𝑋/𝑆may be

derived in the presence of either SM or nonspherical micelles(NSM such as wormlike micelles, WM, or vesicles, Vs).Thus,𝑅Br𝑋becomes conventional ion exchange constant (𝐾Br

𝑋) if the

value of𝐾𝑛𝑋/𝑆

is also obtained in the presence of SM.The value of 𝑅Br

𝑋(=68) for 𝑋 = 1− may be compared

with the 𝑅Br𝑋

(=89) obtained at [NaOH]/[𝑋H] = 2.1 for 𝑋representing 5-methoxysalicylate dianion [20]. The reportedvalues of 𝑅Br

𝑋for 𝑋 = salicylate dianion, benzoate ion, and


4-methoxybenzoate ion are 44, 5.6, and 5.2, respectively[20]. It is evident from the literature that the aqueoussolutions of CTABr/𝑀V𝑋 containing ≤15mM CTABr and12mM ≤ [𝑀V𝑋] ≤ 22mM exhibited the presence of SM for𝑀V𝑋 = sodium benzoate [21] and WM for 𝑀V𝑋 = sodiumsalicylate [22], sodium 3-, 4-, and 5-methyl salicylate [23],and NaV1, NaV2 where V = 1, 2. These observations cannotbe explained in terms of Hammett substituent constants(𝜎𝐻, 𝜎4-OMe). These observations reveal that the shapes and

sizes of the aqueous CTABr/𝑀V𝑋 nanoparticles dependapparently upon the magnitudes of 𝑅Br

𝑋. The magnitude

of 𝑅Br𝑋

is apparently governed by the combined effectsof steric requirements and hydrophilic and hydrophobicinteractions of counterion 𝑋− with cationic headgroup.Hydrophilic interaction includes ion-ion, ion-dipole, dipole-dipole, and inter- and intramolecular hydrogen-bondinginteractions.

The values of 𝑘cat versus 𝑅Br𝑋

(Table 2) reveal a linearrelationship with intercept = 0 and slope = (7.20 ± 0.07)× 10−3M−1 s−1. This observation implies that almost entirecatalytic effect of CTABr/NaV𝑋 nanoparticles catalyst is dueto the ability of nonreactive counterions 𝑋V− to expel thereactive counterions 3− from CTABr/NaV𝑋 nanoparticles tothe bulk water phase.

Apparent maximum catalytic constant (𝜇ap) of 15mMCTABr/[NaV𝑋] nanoparticle catalyst may be obtained fromthe relationship: 𝜇ap = 𝑘cat/𝑘0 and such calculated valuesof 𝜇ap are 190, (216), 421, and (405M

−1) for respective Na1,Na21, Na2, and Na

22 where parenthesized values represent at

[NaOH]/[𝑋H] = 2.5 (i.e., for Na21 and Na

22). The estimated

value of the second-order rate constant (𝑘2𝑀) for the reaction

of 4with 3− in theCTABrmicellar pseudophase (i.e., aqueousCTABr nanoparticles), at [NaV𝑋] = 0, is 3.4 × 10

−3M−1 s−1[19]. Thus, the real maximum catalytic constants (𝜇real) maybe obtained from the relationship: 𝜇real = 𝑘

cat/𝑘2

𝑀where 𝑘cat

= 𝑘cat/[Pip] (with [Pip] = 0.1 M). The calculated values of𝜇real are 1230, (1440), 2720, and (2620M

−1) for respectiveNa1,Na21, Na2, and Na

22 where parenthesized values represent at

[NaOH]/[𝑋H] = 2.5.The values of 𝑘cat and𝑅

Br𝑋forNa𝑋 are not significantly dif-

ferent from the corresponding values for Na2𝑋 for𝑋 = 1 and

2 (Table 2). These results reveal that energetically favorableelectrostatic interaction is apparently insignificant comparedwith hydrophobic interaction between counterions,𝑋V−, andaqueous cationic interface of CTABr/NaV𝑋 nanoparticles.Perhaps, this is the first quantitative explanation of the earlierqualitative experimental observation that sodium salicylateand salicylic acid are equally effective in driving the micellarstructural transition SM-to-WM at a constant temperature[23]. The aqueous structure of CTABr/NaV𝑋 nanoparticlesremains WM at 35∘C, ≤15mM CTABr and 12mM ≤ [NaV𝑋]≤ ∼22mM for NaV𝑋 = NaV1 and NaV2 (V = 1, 2). But thevalues of 𝑘cat are ∼2-fold larger for NaV2 than those for NaV1(Table 2). Thus, it is apparent that a quantitative correlationbetween 𝑘cat and 𝑅

Br𝑋

is better than that between 𝑘cat andthe aqueous structures of CTABr/NaV𝑋 nanoparticles whererheologically assigned structures remain the same (WM) forboth NaV1 and NaV2 at


and Engineering Chemistry Research, vol. 41, no. 25, pp. 6326–6336, 2002.

[12] L. S. Romsted, “Micellar effects on reaction rates and equilibria,”in Surfactants in Solutions, K. L. Mittal and B. Lindman, Eds.,vol. 2, pp. 1015–1068, Plenum, New York, NY, USA, 1984.

[13] R. Germani, G. Savelli, T. Romeo,N. Spreti, G. Cerichelli, andC.A. Bunton, “Micellar head group size and reactivity in aromaticnucleophilic substitution,” Langmuir, vol. 9, no. 1, pp. 55–60,1993.

[14] M. N. Khan, “Mechanism of catalysis in micellar systems,” inEncyclopedia of Surface and Colloid Science, A. T. Hubbard, Ed.,pp. 3178–3191, Marcel Dekker, New York, NY, USA, 2002.

[15] N. S. M. Yusof and M. N. Khan, “Determination of an ionexchange constant by the use of a kinetic probe: a new semiem-pirical kinetic approach involving the effects of 3-F- and 4-F-substituted benzoates on the rate of piperidinolysis of anionicphenyl salicylate in aqueous cationic micelles,” Langmuir, vol.26, no. 13, pp. 10627–10635, 2010.

[16] M. N. Khan and S. Y. Kun, “Effects of organic salts on therate of intramolecular general base-catalyzed piperidinolysis ofionized phenyl salicylate in the presence of cationic micelles,”Journal of the Chemical Society, Perkin Transactions 2, no. 8, pp.1325–1330, 2001.

[17] L. Brinchi, R. Germani, L. Goracci, G. Savelli, and C. A. Bun-ton, “Decarboxylation and dephosphorylation in new geminisurfactants. Changes in aggregate structures,” Langmuir, vol. 18,no. 21, pp. 7821–7825, 2002.

[18] M. N. Khan, “A new semi-empirical kinetic method for thedetermination of ion exchange constants for the counterions ofcationicmicelles,”Advances in Colloid and Interface Science, vol.159, no. 2, pp. 160–179, 2010.

[19] M. N. Khan, Z. Arifin, E. Ismail, and S. F. M. Ali, “Effects of[NaBr] on the rates of intramolecular general base-catalyzedreactions of ionized phenyl salicylate (PS−) with n-butylamineand piperidine in the presence of cationic micelies,”The Journalof Organic Chemistry, vol. 65, no. 5, pp. 1331–1334, 2000.

[20] N. S. M. Yusof and M. N. Khan, “A quantitative corre-lation of counterion (X) affinity to ionic micelles and X-and temperature-induced micellar growth (spherical-wormlikemicelles-vesicles) for X = 5-methyl- and 5-methoxysalicylateions,” The Journal of Physical Chemistry B, vol. 116, no. 7, pp.2065–2074, 2012.

[21] A. A. Ali and R. Makhloufi, “Effect of organic salts on micellargrowth and structure studied by rheology,” Colloid and PolymerScience, vol. 277, no. 2-3, pp. 270–275, 1999.

[22] T. Shikata, H. Hirata, and T. Kotaka, “Micelle formation ofdetergent molecules in aqueous media. 2. Role of free salicylateions on viscoelastic properties of aqueous cetyltrimethylammo-nium bromide-sodium salicylate solutions,” Langmuir, vol. 4,no. 2, pp. 354–359, 1988.

[23] Z. Lin, J. J. Cai, L. E. Scriven, and H. T. Davis, “Spherical-to-wormlike micelle transition in CTAB solutions,” Journal ofPhysical Chemistry, vol. 98, no. 23, pp. 5984–5993, 1994.

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