micellar and salt effects on the interaction of [cu(ii)-gly-gly]+ with ninhydrin

Micellar and Salt Effectson the Interactionof [Cu(II)-Gly-Gly]+with NinhydrinMOHD. AKRAM, NEELAM HAZOOR ZAIDI, KABIR-UD-DIN

Department of Chemistry, Aligarh Muslim University, Aligarh 202 002, India

Received 18 December 2006; revised 5 April 2007; accepted 11 April 2007

DOI 10.1002/kin.20268Published online in Wiley InterScience (www.interscience.wiley.com).

ABSTRACT: The effect of cationic micelles of cetyltrimethylammonium bromide (CTAB) on thekinetics of interaction of copper dipeptide complex [Cu(II)-Gly-Gly]+ with ninhydrin has beenstudied spectrophotometrically at 70◦C and pH 5.0. The reaction follows first- and fractional-order kinetics, respectively, in complex and ninhydrin. The reaction is catalyzed by CTABmicelles, and the maximum rate enhancement is about twofold. The results obtained in themicellar medium are treated quantitatively in terms of the kinetic pseudophase and Piszkiewiczmodels. The rate constants (kobs or k� ), micellar-binding constants (K S for [Cu(II)-Gly-Gly]+,K N for ninhydrin), and index of cooperativity (n) have been evaluated. A mechanism is proposedin accordance with the experimental results. The influence of different inorganic (NaCl, NaBr,Na2SO4) and organic (NaBenz, NaSal) salts on the reaction rate has also been seen, and itis found that tightly bound/incorporated counterions are the most effective. C© 2007 WileyPeriodicals, Inc. Int J Chem Kinet 39: 556–564, 2007

INTRODUCTION

Micelles are known to provide different microenviron-ments as there is a nonpolar, hydrophobic interior thatcan provide binding force for similar functionalitieson the reactant and a polar, usually charged, palisadelayer that can interact with reactants’ polar parts [1].Catalysis by micelles involves at least three main steps:(i) binding of the substrate to the micelle, (ii) the actualchemical change in or at the surface of the micelle, and(iii) release of product(s). However, the actual micellarrate effect is caused by a composite of noncovalent in-teractions between the micelle on the one hand and the

Correspondence to: Kabir-ud-din; e-mail: [email protected]© 2007 Wiley Periodicals, Inc.

reactant and activated complex on the other hand. Thisis extremely complicated problem because a number ofdifferent interactions are involved including those as-sociated with the head group of the surfactant, differentsegments of the alkyl chain, and the counterions.

Ninhydrin reactions using manual and automatedtechniques as well as ninhydrin spray reagents arewidely used to analyze and characterize amino acids,peptides, and proteins as well as numerous otherninhydrin positive compounds in biomedical, clin-ical, food, forensic, histochemical, microbiological,and nutritional and plant studies. Ninhydrin (N) inaqueous solution reacts with α-amino acids to givea compound known as Ruhemann’s purple (diketo-hydrindylidenediketohydrindamine, DYDA) [2]. Asthe color of Ruhemann’s purple becomes faint at roomtemperature, several attempts were made to stabilize it.

MICELLAR AND SALT EFFECTS ON THE INTERACTION OF [Cu(II)-Gly-Gly] 557

Effects of metal ions on this reaction were also carriedout with this viewpoint. The kinetics and mechanism ofninhydrin–amino acid [3–6], ninhydrin–metal aminoacid complexes [7–10], and ninhydrin–dipeptide [11]reactions have been studied in aqueous as well as indifferent surfactant micelles. The present work is thefirst attempt, wherein the kinetics of a metal dipep-tide complex, [Cu(II)-Gly-Gly]+ and ninhydrin hasbeen described in aqueous and cationic micelles ofcetyltrimethylammonium bromide (CTAB).

EXPERIMENTAL

Materials

Gly–Gly (LOBA Chemie, Mumbai, India; 99%), nin-hydrin (Merck, Mumbai, India; 99%), CuSO4 (Merck;99%), CTAB (BDH, Poole, England; 99%), sodiumbenzoate (NaBenz, Merck, Mumbai, India; 99.5%),sodium salicylate (NaSal, CDH, Mumbai, India;99.5%), sodium bromide (LOBA Chemie, Mumbai,India; 99%), sodium chloride (BDH, Mumbai, India;99.9%), sodium sulfate (Qualigens, Mumbai, India;99%), sodium acetate (Merck, Mumbai, India; 99%)and acetic acid (Merck, Mumbai, India; 99.9%) wereused as received. Stock solutions of Gly–Gly, ninhy-drin, CuSO4, and CTAB were prepared in acetic acid–sodium acetate buffer. Double-distilled and deionizedwater (specific conductance (1–2) × 10−6 �−1 cm−1)was used throughout. A digital ELICO LI-122 pH me-ter was used for pH measurements.

Determination of Composition by Job’sMethod of Continuous Variations

To find out the composition of reaction productbetween ninhydrin and [Cu(II)-Gly-Gly]+complex,the Job’s method of continuous variations was em-ployed in the absence and presence of CTAB mi-celles (=20.0 × 10−3 mol dm−3). It was found that1 mol of ninhydrin reacts with 1 mol of [Cu(II)-Gly-Gly]+complex to give the product (Fig. 1).

Kinetic Measurements

In each set of experiments, the [Cu(II)-Gly-Gly]+ com-plex was prepared in situ by taking 1:1 solutions ofcopper sulfate and Gly–Gly [12] in a three-necked re-action vessel fitted with a double surface water con-denser (to prevent evaporation). The reaction vesselwas immersed in a thermostated oil-bath at the de-sired temperature (±0.1◦C). The resulting solution wasleft to stand for 30 min to complete the complexa-

Figure 1 Plot of �A370 versus mole fraction of ninhy-drin for determination of composition of the product formedby the interaction of [Cu(II)-Gly-Gly]+ complex with nin-hydrin: (A) in the presence of [CTAB] = 20.0 × 10−3 moldm−3 and (B) in the absence of CTAB.

tion. The reaction was then started by adding requisitevolume of thermally equilibrated ninhydrin solution.Zero time was recorded when half of the ninhydrinhad been added. A slow stream of pure N2-gas (freefrom O2 and CO2) was bubbled through the reactionmixture for stirring (this ensured an inert atmosphere aswell). Progress of the reaction was followed by mea-suring the absorbance of the reaction product at 310nm (λmax) at definite time intervals using a Bausch& Lomb (Belgium) Spectronic-20 spectrophotometer.During the kinetic experiments, pseudo-first-order con-ditions were maintained by keeping [ninhydrin]T in ex-cess. The first-order rate constants (kobs and k� , s−1)were calculated upto completion of three half-lives byusing rate constant = (2.303/t) log (A∞– Ao)/(A∞–At ) with the help of computer program. Other detailsregarding pH measurements and kinetic methodologywere the same as described elsewhere [7–10].

Determination of cmc by ConductivityMeasurements

Critical micellar concentration (cmc) values of CTABwere determined conductometrically at 70◦C usingPhilips conductivity meter (model PR 9500) with pla-tinized electrodes. The cmc values of surfactant in thepresence and absence of reactants were obtained fromthe break points of nearly two straight line portions ofthe specific conductivity versus surfactant concentra-tion plots [13]. Experiments were made under different

International Journal of Chemical Kinetics DOI 10.1002/kin

558 AKRAM, ZAIDI, AND KABIR-UD-DIN

Table I Values of cmc of CTAB under DifferentExperimental Conditions Determined by ConductivityMeasurements

cmc × 104 (mol dm−3)

Solutionsa 30◦C 70◦C

Water 9.50 (9.80)b 14.2 (15.0)c

Water + [Cu(II)-Gly-Gly]+ 1.85 5.9Water + [Cu(II)-Gly-Gly]+ 1.80 4.2

+ Ninhydrin

a [Cu(II)-Gly-Gly+] = 2.0 × 10−4 mol dm−3; [ninhydrin] =6.0 × 10−3 mol dm−3.

b Literature value at 25◦C [13].c Literature value at 70◦C [13].

conditions, that is, solvent being water, water + Cu(II)-Gly-Gly, or water + Cu(II)-Gly-Gly + ninhydrin, andthe respective cmc values are summarized in Table I.

Viscosity Measurements

To test possible micellar growth in our systems, espe-cially in the presence of salts, solution viscosities (ηr)were determined by using a modified Ubbelohde vis-cometer thermostated at 70 ± 0.1◦C [14]. The wholerange of concentrations of surfactant and salts couldnot be examined due to a large increase in viscosity,especially with organic salts. The solution viscositiesshow only the conditions inside the reaction vesselwhere the actual reaction is taking place.

RESULTS AND DISCUSSION

Spectra of the Product

The UV–visible spectra of the product formed bythe reaction between [Cu(II)-Gly-Gly]+ complex(2.0 × 10−4 mol dm−3) and ninhydrin (6.0 × 10−3 moldm−3) in buffer solution have been recorded underdifferent conditions and in the absence and presenceof CTAB micelles (Fig. 2). It can be seen that themaximum absorbance (λmax = 370 nm) of the productremains unchanged in the presence of micelles; this in-dicates that the reaction product of [Cu(II)-Gly-Gly]+

with ninhydrin is the same as in aqueous medium. Itis to be noted that the absorbance of the product ishigher in the presence of CTAB micelles, which maybe due to strong association between the end productand cationic CTAB micelles.

Effect of [Cu(II)-Gly-Gly+]on the Reaction Rate

The effect of [Cu(II)-Gly-Gly]+ concentration wasseen by carrying out the kinetic experiments at differ-

Figure 2 Absorption spectra of the reaction product of[Cu(II)-Gly-Gly]+ (2.0 × 10−4 mol dm−3) and ninhydrin(6.0 × 10−3 mol dm3) in the absence and the presence ofCTAB (20.0 × 10−3 mol dm−3) at pH 5.0: (A) immediatelyafter mixing the reactants; (B) after heating the solution (A)at 70◦C for 2 h; (C) same as solution (A) in the presence ofCTAB; and (D) after heating the solution (C) at 70◦C for 2 h.

ent concentrations from 1.0 × 10−4 to 3.5 × 10−4moldm−3 at constant values of [ninhydrin] (6.0 × 10−3 moldm−3), temperature (70◦C ), and pH (5.0) in the ab-sence and presence of CTAB micelles (20.0 × 10−3

mol dm−3) (Table II). It has already been establishedthat the optimum pH of the ninhydrin reaction withamino acids and peptides is 5.0 [2,15,16]. The valuesof rate constants (kobs or k�) were found to be indepen-dent of the initial concentration of [Cu(II)-Gly-Gly]+;thus, indicating the order of reaction with respect to[Cu(II)-Gly-Gly]+ to be unity in both the media. Therate law would then be

rate = d[Product]

dt= (kobs or kψ)

×[Cu(II)-Gly-Gly+]T (1)

Effect of [Ninhydrin] on the Reaction Rate

To find the order with respect to [ninhydrin], the rateconstants were determined at different initial ninhydrinconcentrations ranging from 6.0 × 10−3 to 40.0 × 10−3

mol dm−3 keeping [Cu(II)-Gly-Gly]+(2.0 × 10−4 moldm−3), temperature (70◦C), and pH (5.0) constant inboth aqueous and micellar media. The results are sum-marized in Table II. The plots of rate constants versus[ninhydrin] are nonlinear (Fig. 3), whereas log (rate



Table II Dependence of Pseudo-First-Order Rate Constants (kobs or k� ) on [Cu(II)-Gly-Gly+], [Ninhydrin],and Temperature at pH 5.0

[Cu(II)-Gly-Gly+] × 104 [Ninhydrin] × 103 Temperature kaobs × 105 kb

� × 105

(mol dm−3) (mol dm−3) (◦C) (S−1) (S

−1)

1.0 6 70 5.2 6.61.5 5.2 6.72.0 5.2 6.72.5 5.2 6.73.0 5.2 6.63.5 5.2 6.61.5 6 70 5.2 6.7

10 8.6 11.815 14.2 17.520 16.6 21.225 19.2 30.230 20.5 32.135 23.0 34.540 25.4 35.0

1.5 6 60 2.4 3.965 3.3 5.370 5.2 6.775 7.2 9.680 9.1 12.5

a In the absence of surfactant.b In the presence of [CTAB] = 20.0 × 10−3 mol dm−3.

constants) versus log [ninhydrin] plots are linear withslope 0.74 (in aqueous medium) and 0.78 (in micellarmedium); this indicates fractional-order kinetics withrespect to [ninhydrin] in both media.

Figure 3 Effect of [ninhydrin] on the reaction rate of nin-hydrin with [Cu(II)-Gly-Gly]+ in the presence (A) and theabsence of surfactant (B). Reaction conditions: [Cu(II)-Gly-Gly+] = 2.0 × 10−4 mol dm−3, [CTAB] = 20 × 10−3 moldm−3, pH 5.0, and temperature = 70◦C.

Effect of Temperature on the Reaction Rate

Activation parameters are believed to provide use-ful information regarding the environment in whichchemical reactions take place. To learn more about themicroenvironments of submicroscopic assemblies, aseries of kinetic runs were carried out at different tem-peratures, with fixed reactant concentrations both inthe absence and presence of CTAB micelles. The linearleast-squares regression technique was used to calcu-late activation parameters by using the Arrhenius andEyring equations.

Effect of Inorganic (NaCl, NaBr, Na2SO4)and Organic (NaBenz, NaSal) Saltson the Reaction Rate

The effects of added salts on the reaction rate werealso explored because salts may modify the substrate–surfactant interactions [1]. The salt effect on themicellar–catalyzed ninhydrin–[Cu(II)-Gly-Gly]+ re-action was studied in the presence of CTAB micelles(20.0 × 10−3 mol dm−3) at 70◦C, keeping other vari-ables constant. The results are depicted graphically inFigs. 4 and 5.



0.0 0.2 0.4 0.6 0.8 1.00

4

8

12

16

20

24

B

A

C

105

k ψ(s

− 1)

[Salt] (mol dm−3)

×

Figure 4 Effect of [inorganic salts] on the reaction ratefor the interaction of ninhydrin with [Cu(II)-Gly-Gly]+inthe presence of surfactant. Reaction conditions: [Cu(II)-Gly-Gly+] = 2.0 × 10−4 mol dm−3, [ninhydrin] = 6.0 × 10−3

mol dm−3, [CTAB] = 20 × 10−3 mol dm−3, pH 5.0, andtemperature = 70◦C. NaBr (A), NaCl (B), and Na2SO4 (C).

Reaction in the Absence of Surfactant

The reaction has been found to proceed through for-mation of a ternary labile complex in which Gly–Glyand ninhydrin are coordinated to the same copper(II)ion. The formation of product is due to the reaction

Figure 5 Effect of [organic salts] on the reaction rate (Aand B) and solution viscosity (C and D) for the interaction ofninhydrin with [Cu(II)-Gly-Gly]+ in the presence of surfac-tant. Reaction conditions: same as in Fig. 4. NaBenz (A andC) and NaSal (B and D).

O

O

O

+

+

+

OH2

k

K

+

OH2

NH

CH2

O

Cu

H2O O

N O

CH2C

O

C

O

P

O

O

Cu

H2O O

NH2

O

NH

NH

CH2CH2C

C

O

O

O

Cu

H2O OH2

NH2

CH2CH2C

C

O

O

A

B

N

Scheme 1

between the coordinated group of Gly–Gly and a coor-dinated carbonyl group of ninhydrin through the con-densation reaction. Such type of condensation reactionis also known as combination-of-ligands-attached-to-the-same-metal-ion (CLAM). The presence of metalion brings the reactive groups together and provides abetter chance for their combination within its coordi-nated sphere. Thus, the reaction proceeds through thekinetic template mechanism in which the condensationsphere of the metal ion induces the ligand molecule toorient in a manner that is suitable for the coordinationof the complex formation. On the basis of the above,a probable mechanism has been proposed (Scheme 1),in which the reaction occurs in two kinetically distin-guishable steps: the first, a fairly rapid ternary labile



complex formation between ninhydrin and [Cu(II)-Gly-Gly]+; the second, a slower condensation of anamino group to a carbonyl group.

On the basis of the observed rate law, Eq. (1), andthe proposed mechanism (Scheme1), the following rateequation is derived:

d[Product]

dt= kK[N]T[Cu(II)-Gly-Gly+]T

1 + K[N]T(2)

kobs = Kk[N]T

1 + K[N]T(3)

where [N]T is the total concentration of ninhydrin. Re-arrangement of Eq. (3) gives

1

kobs= 1

k+ 1

kK[N]T(4)

Thus, a plot of 1/kobs versus 1/[N]T should give astraight line with a positive slope (1/kK) and an in-tercept (1/k), which is found to be the case and thusconfirms the validity of the proposed mechanism. Thevalues of k and K were found to be 2.0 × 10−3 s−1 and4.3 mol−1 dm3, respectively.

Reaction in the Presence of CTAB Micelles

As no change in the position of maximum absorbance(λmax) was observed in the presence of CTAB micelles(Fig. 2), it is inferred that the same product is formedin both the aqueous and aqueous micellar media. Inthe discussion that follows, quantitative kinetic analy-ses for the title reaction in CTAB micellar system arepresented.

To confirm the proposed mechanism (Scheme1), the effect of [Cu(II)-Gly-Gly+], [ninhydrin], andtemperature was also studied in the presence of20.0 × 10−3 mol dm−3 CTAB. These results are sum-marized in Table II. It was found that the reactionfollows first-order and fractional-order kinetics withrespect to [Cu(II)-Gly-Gly]+and [ninhydrin], respec-tively. Thus, we can conclude that the reaction mech-anism remains the same in the presence of CTAB mi-celles as that in the aqueous medium with all possibleintermediary situations. In the presence of 20.0 × 10−3

mol dm−3 CTAB, the values of k and K were foundto be 2.5 × 10−3 s−1 and 5.2 mol−1 dm3, respectively,which suggest a catalytic role for the CTAB micelles.

To investigate the effect of CTAB micelles onthe reaction rate, the kinetic experiments were per-formed in the presence of varying [CTAB] at constant[Cu(II)-Gly-Gly]+ (2.0 – 10−4 mol dm−3), [ninhydrin](6.0 × 10−3 mol dm−3), and pH (5.0) at 70◦C. The ob-

Table III Effect of [CTAB] on the Pseudo-First-OrderRate Constants (k� ) for the Reaction of Ninhydrin with[Cu(II)-Gly-Gly]+ at pH 5.0

[CTAB] × 103 k�a × 105

(mol dm−3) (S−1)

0 5.22 5.24 5.26 5.48 5.910 5.915 6.220 6.730 8.040 9.150 9.560 10.270 10.980 9.090 7.5100 6.4

a [Cu(II)-Gly-Gly+] = 2.0 × 10−4 mol dm−3, [ninhydrin] =6.0 × 10−3 mol dm−3, temperature = 70◦C.

served pseudo-first-order rate constants (k�) increasedfrom 5.2 × 10−3 to 10.9 × 10−3 s−1 (about twofold)with the increase in [CTAB] from 0 to 70.0 × 10−3 moldm−3 (Table III). A plot of k�versus [CTAB] showsa rate maximum at [CTAB] = 70.0 × 10−3 mol dm−3

(Fig. 6), a very common characteristic of bimolecularreactions catalyzed by micelles [17,18]. A further in-crease in [CTAB] (≥70.0 × 10−3 mol dm−3) results ina decrease in the reaction rate.

In the presence of CTAB micelles, the reactantsmay be considered to be distributed in aqueous andmicellar pseudophases. The observed enhancement inthe reaction rate is quantitatively treated on the basisof the pseudophase model (Scheme 2), proposed byMenger and Portnoy [19] and developed by Bunton[20] and Romsted [21].

In Scheme 2, Dn represents the micellized surfactant(i.e., [Dn] = [CTAB]T − cmc]). The pseudo-first-orderrate constants k′

w and k′m stand for aqueous and micellar

pseudophases. The observed rate law equation (1) and

KS

+

Nw + Dn NDn

Products

+KN

k'mk'w

(Cu(II)-Gly-Gly)w + Dn (Cu(II)-Gly-Gly)m

Scheme 2



Figure 6 Effect of [CTAB] on the reaction rate for the inter-action of ninhydrin with [Cu(II)-Gly-Gly]+. Reaction con-ditions: [Cu(II)-Gly-Gly+] = 2.0 × 10−4 mol dm−3, [ninhy-drin] = 6.0 × 10−3 mol dm−3, pH 5.0, and temperature =70◦C.

Scheme 2 lead to Eq. (5)

kψ=k′w + k′

mKS[Dn]

1 + KS[Dn](5)

Equation (5) can be modified as Eq. (6)

kψ

kw[Nw] + (KSkm − kw)MSN[Dn]

1 + kS[Dn](6)

where kw = k′w/[Nw] and km = k′

m/MSN (kw and km are

second-order rate constants). KS is the binding con-stant of the [Cu(II)-Gly-Gly]+ complex to the CTABmicelles, and MS

N ([NDn]/[Dn]) is the mole ratio ofbound ninhydrin to the micellar head group whose val-ues of MS

N were estimated by considering the equilib-rium and mass balance to the ninhydrin concentration.

The cmc value (4.2 × 10−4 mol dm−3; Table I) wasdetermined conductimetrically under the kinetic condi-tions. Using this value of cmc (where the micelle shapewould be spherical), km and KS were calculated fromEq. (6) using the nonlinear least-squares techniquebased on the computer program (Table IV). The rateconstants were again calculated by substituting km, KS,and MS

N in Eq. (6). The calculated values of rate con-stants are in close agreement with the correspondingobserved values, which confirm the validity of Eq. (6).

The conversion of km (s−1) into the second-orderrate constant (km

2 , mol−1 dm3 s−1) requires the exact

Table IV Thermodynamic Parameters, Rate, andBinding Constant Values for the Reaction of[Cu(II)-Gly-Gly]+ and Ninhydrin at pH 5.0

Parameters and Constants Aqueous CTAB

Ea (kJ mol−1) 74.6 60.3�H �= (kJ mol−1) 71.8 57.4–�S �= (J K−1 mol−1) 133.6 156.0km × 104 (s−1)a – 3.0km

2 × 105(mol−1 dm3 s−1) – 4.2kw × 105 (mol−1 dm3 s−1)a – 5.2KS (mol−1 dm3)a – 4.0KN (mol−1 dm3)a – 65.3

a at 70◦C.

value of volume of the micellar pseudo-phase (V m).The value of V m = 0.14 dm3 mol−1 has been widelyused [22–24]. Therefore, km

2 was calculated from therelationship km

2 = V mkm. The second-order-rate con-stants km

2 and kw are similar in magnitude (Table IV).Generally, kw > km

2 for many bimolecular reactionsin aqueous and micellar pseudophases [25,26]. How-ever, there are many examples in which km

2 is similarin magnitude with kw [20].

According to the Piszkiewicz [27] model, whichwas proposed in analogy to the Hill model applied forthe enzyme-catalyzed reactions, the rate constant (k�)is given by

kψ = km[D]n + kwKD

KD + [D]n(7)

where n describes the stoichiometry of the reaction orindex of cooperativity and KD is the dissociation con-stant of micellized surfactant back to its components.

Equation (7) rearranges to give Eq. (8)

log

(kψ − kw

km − kψ

)= nlog[D] − logKD (8)

From the data, log (k� − kw/km − k�) versus log [D]plots have been drawn. Fairly linear correlation is ob-served. KD and n are 8.2 × 10−2 and 1.10, respectively.Value of n greater than unity indicates positive cooper-ativity, that is, the binding of the first molecule of thesubstrate makes it easier for subsequent molecules tobind.

The decrease in k� beyond [CTAB] > 70.0 × 10−3

mol dm−3 could be explained as follows: At [CTAB]> 70.0 × 10−3 mol dm−3, practically all the substratehas been incorporated into the micellar phase. Whenbulk of the substrate is incorporated into the micelle,addition of more CTAB generates more cationic mi-celles, which simply take up the ninhydrin moleculeinto the Stern layer, and thereby deactivate them,



because a ninhydrin molecule in one micelle shouldnot react with the complex in another [28]. Anotherreason of decrease in k� could be as a result of coun-terion inhibition.

Activation Parameters

The difference of activation parameters in cationicCTAB micelles as compared to water (Table IV) is asexpected, because incorporation of the reactants intothe cationic micelles reduces the activation enthalpy.This decrease in �H �= with CTAB may be related tothe low solubility of [Cu(II)-Gly-Gly]+ complex in themicellar phases relative to that in the aqueous phase.The large decrease in �S �= shows that the transitionstate is well structured in the micellar phase. However,more useful explanation of �H �= and �S �= is not pos-sible because the k� is a complex function of true rateand binding constants.

Probable Reaction Site

Because of the different properties of the micellar pseu-dophase, it is not possible to precisely locate the siteof the reaction but, at least, localization of the reac-tants can be considered. It is well known that most ofthe micellar reactions involving an ionic and a neutralreactant as well as ionic micelles are believed to takeplace either inside the Stern layer or at the interfacebetween the micellar surface and bulk water solvent[29,30]. The interfacial water is known to be less polarand more structured than the bulk water. The observedcatalytic effect of CTAB on the [Cu(II)-Gly-Gly]+ andninhydrin reaction can be easily explained as follows:Simply based on electrostatic considerations, ninhy-drin (due to the presence of an electron cloud on it [2])comes closer to the cationic CTAB micellar surfacethat increases the local molarities in the Stern layer. Asregards the complex, the removal of water moleculesfrom the inner solvation shell of Cu(II) by the coordi-nated Gly–Gly gives the complex some hydrophobiccharacter (despite of bearing a positive charge). Be-cause of this hydrophobic nature, the complex getsincorporated into the micelles. The micelles thus helpin bringing the ninhydrin and the [Cu(II)-Gly-Gly]+

complex close together into a small volume, that is,the Stern layer, wherein it may now orient in a mannersuitable for the condensation (see Scheme 1).

Salt Effect on Micellar Catalysis

The salt effect on micellar catalysis should be con-sidered in the light of competition with the substratemolecule that may interact with the micelles electro-

statically and hydrophobically and structural changeswhich occur on salt addition [31]. The addition of unre-active organic salts to solutions of cationic surfactantshas been found to increase micellar size [32], affectthe amount of residual charge in the micellar surface[33,34], and decrease the cmc [35].

The effect of inorganic salts on the reaction ratedoes not show any systematic pattern (Fig. 4). On theother hand, the hydrophobic salts, sodium benzoate(NaBenz), and sodium salicylate (NaSal) give markedrate enhancement at low salt concentrations, passingthrough a maximum as the [salt] is increased (Fig. 5).With such hydrophobic salts, penetration of the ben-zene ring into the micellar palisade layer (a few carbonatoms deep toward core) takes place with the carboxy-late group remaining in the outermost region of themicelle (a case of intercalation) [36]. Therefore, inaddition to neutralization of micellar surface charge,they restrict interior solubilization of reactants causingan increase in concentration of the latter in the Sternlayer; the reaction is thus catalyzed. As we increasethe [salt], the above site will be saturated. Once thissite is fully occupied, additional salt will try to get ad-sorbed at the micellar surface (a case of adsorption)and will thus compete for a site with reactants (a caseof benzoate and salicylate ions’ association in the formof adsorption). Consequently, [reactant] is decreasedat the reaction site by the latter effect (exclusion ofsubstrate). The progressive withdrawal of the substratefrom the reaction site would slow the rate, as was in-deed observed. Possibly, the above two effects worktogether on salt addition and the resultant effect is afunction of [salt]. Another factor that could inhibit therate, is the possible micellar growth at higher [salt] asreflected by the viscosity data (Fig. 5). In the presentcase, the change in morphology from spheroidal mi-celles to rod shaped (as inferred by viscosity increase[37]) would have certain changes on the characteristicsof the micelles. In the rod-shaped micelles, the coun-terion adsorption is not identical at every place in themicelles, for example, preferential adsorption of coun-terions to the central cylindrical part of the micellestake place. At the cylindrical part of the micelle, ionicinteraction with the reactants and, therefore, the reac-tion rate, is expected to be suppressed; this is indeedobserved at higher concentrations of organic counteri-ons where rod-shaped micelles are the bulk entity.

CONCLUSION

In conclusion, we observed that the cationic CTAB mi-celles catalyze the reaction of [Cu(II)-Gly-Gly]+ com-plex with ninhydrin by a factor of nearly two. Although



the presence of CTAB micelles does not bring out anydrastic change in the reaction rate, the present studiesmay stimulate and open up a new approach for study-ing the ninhydrin reaction for sensitivity. The effectof salts on the micellar catalysis seems to depend onthe nature of salt, which could accelerate/inhibit thereaction.

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