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Available online at www.sciencedirect.com

Colloids and Surfaces A: Physicochem. Eng. Aspects 317 (2008) 585–591

Interaction of 1-anthracene sulphonate with cationic micelles of alkyltrimethyl ammonium bromides (CnTAB): A spectroscopic study

Arindam Sarkar, Smritimoy Pramanik, Paltu Banerjee, S.C. Bhattacharya ∗Department of Chemistry, Jadavpur University, Kolkata 700032, India

Received 31 July 2007; received in revised form 19 November 2007; accepted 27 November 2007Available online 4 December 2007

bstract

The emission properties of 1-anthracene sulphonate (1-AS) have been studied in cationic micellar media of alkyl trimethyl ammonium bromidesCnTAB) of varying chain lengths. The changes in the emission characteristics as a function of surfactant concentration have been utilized toetermine the binding constant of the probe with the micelles and location of the probe within these micelles. The interactions of 1-AS with the

ationic micelles depend on the hydrophobic chain length of the cationic surfactants. The binding constants of 1-AS with CnTAB micelles decreaseith decreasing n. The fluorescence of 1-AS is quenched by cetyl pyridinium chloride (CpCl) at the surface of the cationic micelles. The rate ofuorescence quenching decreases with decrease in alkyl chain length of the cationic surfactants. 2007 Elsevier B.V. All rights reserved.

abrodsscweaAaslihs

eywords: Fluorescence; Micelle; Distribution; Quenching; Lifetime

. Introduction

Micelles, vesicles and liposomes are often used as membraneiomimetic agents. The importance of membrane in biologicalystems lies in their capacity to provide a matrix for arranging theeaction sequentially for efficient interaction, i.e., they help inompartmentalization of the reactants dynamically [1–10]. Dueo the importance of micelles as model systems mimicking bio-

embranes in biological processes, attention has been drawno the effect of micelles on the nature and rates of reactions.urfactant entrapped water molecules provide unique microen-ironments for interactions and reactions. Water molecules,hich are tightly bound to the surfactant head groups of micelles,

esemble the hydrophilic pockets of enzymes and have highiscosities, low mobilities and polarities [11–12].

Anthracene is a highly fluorescent molecule. Anthracene andts derivatives are used as fluorescent probes for the study ofynamics of biological molecules. Furthermore because of their

ood quantam yield efficiency for triplet state generation, theyave provided some information on triplet state photophysics ofrganic molecules leading to better knowledge of their structure

∗ Corresponding author. Tel.: +91 33 2414 6223; fax: +91 33 24146584.E-mail address: sbjuchem@yahoo.com (S.C. Bhattacharya).

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927-7757/$ – see front matter © 2007 Elsevier B.V. All rights reserved.oi:10.1016/j.colsurfa.2007.11.038

nd reactivity. On sulphonation, the monosulphonate formedecomes water soluble and produces interesting fluorescent cha-acteristics. The sulphonate group affects the overall symmetryf the anthracene moiety and due to its charge it also intro-uces negatively charged centers in the molecule in aqueousolution [13,14]. The emission properties of 1-anthracene mono-ulphonate (1-AS) are sensitive to solvent [15]. 1-AS gives aompletely structureless emission spectrum in aqueous solutionhich developes vibrational features on addition of acetonitrile,

thanol, ethylene glycol, etc. Since anthracene sulphonates aremphiphilic in nature, two types of water matrix around the 1-S molecules are considered (1) a hydrogen bonded structure

round the hydrophilic SO3− groups and (2) an ice-like water

tructure around the hydrophobic region of the anthracene ske-eton. The hydrophobic water clusters are more tightly boundnternally than the hydrogen bonded water structure around theydrophilic region. The hydrophobic hydration in the excitedtate has an important bearing on the study of biological mole-ules such as proteins, nucleic acids and their aggregates. Thehotophysics and photochemistry of anthracene sulphonatesave been studied in our laboratory [13–16]. As an extension

f our study the role of chain lengths of cationic surfactantsf a homologous series on the photophysics of 1-AS has beennvestigated. Such investigation using this probe is rare in lite-ature. Fluorescence spectral studies in micellar medium have

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2

rfrzTvcorflmTos0nf

UosmmositsTada

tv

3

3

(tgpaaiccfbctvsftmlrstfasmrange, the micelles may be considered as individual compart-ment and multiple equilibria model [20,21] may be consideredfor micellization of the solute. For a given concentration of thesolute and the micelle, the solute will be distributed between the

86 A. Sarkar et al. / Colloids and Surfaces A: P

stablished that 1-AS can be used as a probe for water poolsn membranes and micelles [16]. Fluorescence quenching byeutral or charged species in micelles have been extensivelytudied to determine the characteristics of organized assem-lies and location of probe molecule into the micelle. Quencherith properties similar to that of the surfactant offers important

dvantages of ideal mixing in micelles over the conventionaluencher [17]. For this reason cetyl pyridinium chloride (CpCl)as been used as quencher. The choice of fluorescent probeolecule is of course important because its residence depth in

he interface of micelles is used to estimate the polarity andature of the region in which it is located. It has been foundhat the reactivity of the micelles of CnTAB series dependsn the length of non-polar tail of the surfactants [18]. In thiseport, the fluorescence characteristics of 1-AS in micellar solu-ions of cetyltrimethylammonium bromide (CTAB, C16TAB),etradecyltrimethylammonium bromide (TTAB, C14TAB) andodecyltrimethylammonium bromide (DTAB, C12TAB) and itsuenching by CpCl were studied to investigate the role of hydro-hobic chain length of the cationic surfactants of a homologouseries on the photophysics of 1-AS.

. Experimental details

1-Anthracene sulphonate was prepared by reducing the cor-esponding anthraquinone with Zn dust and 20% NaOH solutionor 4–6 h. The products were treated with active charcoal toemove traces of anthraquinone and other impurities, crystalli-ed four times from water and obtained as sodium salt. CTAB,TAB, DTAB, CpCl were of Aldrich products and used as recei-ed. Prior to their use, it was checked that the surfactants do notontribute to either absorption or fluorescence in the region ofur interest. The emission spectrum of 1-AS was recorded in theange 375–500 nm by exciting 1-AS molecules at 365 nm in auorescence spectrophotometer (Fluorolog F II A Spectrofluori-eter, Spex Inc., New Jersey, USA) with a slit width of 1.25 nm.he concentration of 1-AS used in micellar solutions was of therder of 10−3 mmol dm−3. In the fluorescence quenching mea-urements,the concentration of CpCl quencher was varied fromto 0.5 mmol dm−3. The concentration of CpCl was maintai-

ed below its cmc value (0.8 mM), so that no micelle of CpCl isormed.

Absorption spectra were recorded using a PharmaspecV–vis 1700 spectrophotometer, Shimadzu, with a matched pairf silica cuvettes (path length 1 cm). Absorbance and emissionpectra of the probe in varying composition of dioxan–waterixture were also recorded to determine the polarity of theicroenvironment surrounding the probe molecules. The effect

f urea on probe within micellar environment has also beentudied. Fluorescence lifetimes measurements for 1-AS, bothn aqueous media and in micellar media were determined usingime correlated single photon counting method and using a nano-econd diode laser at 370 nm (IBH, nanoLED-7) as light source.

he response time of the instrument is 1.1 ns. The decays werenalyzed using IBH DAS-6 decay analysis software. The recor-ed lifetime value of 1-AS in aqueous media is 6.86 ns. Onddition of surfactant, from pre-micellar to post-micellar range,

F((

ochem. Eng. Aspects 317 (2008) 585–591

he decay becomes faster with a gradual decrease in the lifetimealue of 1-AS.

. Results and discussion

.1. Absorbance and fluorescence

The absorbance and wavelength of maximum absorption365 nm) of 1-AS remain unchanged in presence of the surfac-ants, indicating no interaction of the surfactants with 1-AS in theround state. The fluorescence intensity of 1-AS decreases in there-micellar region and then increases with vibrational featuresfter cmc of the surfactant solution and then becomes maximumt higher micellar concentration. The decrease of fluorescencentensity upon addition of the cationic surfactants in the premi-ellar range has been ascribed to the formation of an associationomplex between the excited anionic probe and cationic sur-actants due to electrostatic attraction. Similar cases have alsoeen obtained with anionic probe pyranine with CTAB [19]. Thehanges observed in the fluorescence spectra of 1-AS as a func-ion of CnTAB concentration is presented in Fig. 1. Considerableariation in vibrational features as well as in fluorescence inten-ity has been observed. Fluorescence intensity was plotted as aunction of concentration of the surfactants. From the break ofhe two curves the cmc values of the surfactants have been deter-

ined and are presented in Table 1. The values are close to theiterature values [31] and also to the values obtained from fluo-escence decay measurements, discussed in Section 3.4. Thesepectral data were used to calculate the binding constant K, inhe excited state for the substrate micelle interaction. The inter-ace of the cationic micelles being positively charged helps thenionic 1-AS molecules orient towards the interface by electro-tatic attraction. Since the fluorescence lifetimes of the substrateolecules (discussed in the next section) are in the nanosecond

ig. 1. Fluorescence spectra of 1-AS in DTAB, [DTAB]: (1) 0 mM, (2) 3 mM,3) 6 mM, (4) 9 mM, (5) 12 mM, (6) 15 mM, (7) 18 mM, (8) 21 mM, (9) 24 mM,10) 27 mM, (11) 30 mM.

A. Sarkar et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 317 (2008) 585–591 587

Table 1Micellar characteristics and binding constants of 1-AS with micelles

Surfactant Binding constants(K × 10−4/dm3 mol−1)

cmc (from FI plot)(mmol dm−3)

cmc (from 1/τ plot)(mmol dm−3)

cmca (lit)(mmol dm−3)

Aggregationnumber (n)a

Micellarradiia (A)

E30T (kcal mol−1)

CTAB 2.24 0.8 0.7 0.8 92 35 52TD

at

S

K

wiiKitpb

b

(

w1mm

dvdttC

3

mTm

P

wmataohtcc(mAmoaoeoc

TD

S

C

T

D

[

TAB 1.60 3.1 3.2TAB 0.90 13.5 13.7

a Reference [31].

queous phase (Sw) and the micellar phase (Sm) according tohe model

w + M � Sm (1)

The binding constant (K) for solute–micelle interaction is

= [Sm]

[Sw][M]or K[M] = [Sm]

[Sw]

here [Sm] and [Sw] represent the concentration of the solutesn the micellar phase and the aqueous phase, respectively, [M]s the effective micellar concentration. For a given value of [M],

[M] assumes the role of partition coefficient for the soluten two phases. Assuming that the observed fluorescence (F) inhe solution is the sum of the fractional fluorescence in aqueoushase (Fw) and the micellar phase (Fm), the expression (2) cane deduced considering equilibrium (1).

The binding constant K of 1-AS with the micelles [22] haseen determined using the relation (2) given below:

Fw − F )−1 = (Fw − Fm)−1(

1+1

K[M]

)(2)

here F, Fw and Fm represent the fluorescence intensities of-AS in micellar solution, water and micellar solution showingaximum fluorescence intensity, respectively. [M] is effectiveicellar concentration.M = ([surfactant] − cmc)/aggregation number. K values were

etermined from the slope of the plot of (Fw − Fm)(Fw − F)−1

s. 1/[M] and the values are given in Table 1. The bin-

ing constant values increase with increase in chain length ofhe hydrophobic segments of the surfactants. In other words,he probe is fairly bound with the micelles in the orderTAB > TTAB > DTAB.

aofF

able 2istribution of solute molecules according to Poisson statistics and [Sm]/[Sw] values

urfactant (S) [S] (mM) P0 P1

TAB 5 0.86 12.6 ×8 0.92 7.3 ×

TAB 10 0.94 6.2 ×20 0.98 1.9 ×

TAB 20 0.95 4.7 ×30 0.97 2.1 ×

1-AS] = 6.8 × 10−6 M.

3.5 68 28 5515.0 54 24 59

.2. Distribution of solute molecules in the micelles

It has been established that statistical distribution of soluteolecules in the micellar cages follows Poisson statistics [23].he probability P(n) that n solute molecules will reside in a givenicellar cage is given by

(n) =( 〈S〉n

n!

)e−〈S〉

here 〈S〉 is the average number of solubilized molecules pericelle and n is the occupancy number of solute molecules. Forconcentration of [1-AS] = 10−6 M and a fixed value of [M],

he percentage of micelles with 0, 1, 2 and 3 solute moleculesre calculated and presented in Table 2. The gross distributionf solute molecules between the micellar and the aqueous phaseave been calculated from the expression [Sm]/[Sw] = K[M] forhe three micellar solutions at higher concentrations than theirorresponding cmc’s and are given in Table 2. The data indi-ate that for the solutions at higher concentrations of surfactantshigher than their cmc), the total concentration of solute in theicellar phase is more than that in the aqueous phase. The 1-S molecules occupy a fraction of the micelles and most of theicellar cages are empty. It has been seen from the data that most

f the 1-AS molecules are within the micelle following the massction law given by Eq. (1). Therefore the emission spectrumbtained at a higher concentration of micelles, represents themission of 1-AS totally bound to micelles. The spectral changesbserved with increasing micellar concentration are alike thehanges induced when increasing proportions of dioxan were

dded to an aqueous solution of 1-AS. From this observationne can say that 1-AS molecules are moving towards the inter-ace of the micelles. It may be assumed that spectrum (11) inig. 1 corresponds to 1-AS in completely micellized condition.

P2 P3 [Sm]/[Sw]

10−2 1.2 × 10−2 4.5 × 10−4 1.0210−2 2.9 × 10−3 7.8 × 10−5 1.75

10−2 2.0 × 10−3 4.5 × 10−5 1.6210−2 1.9 × 10−4 1.3 × 10−6 3.97

10−2 1.2 × 10−3 1.9 × 10−5 1.0810−2 2.3 × 10−4 1.7 × 10−6 2.75

588 A. Sarkar et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 317 (2008) 585–591

F(8

3

bwfltoovdhs(rpmtii

FA

F(

sttdtpcTisocTtpt

ig. 2. Fluorescence spectra of 1-AS in water–dioxan mixture. Dioxan (v/v):1) 0%, (2) 10%, (3) 20%, (4) 30%, (5) 40%, (6) 50%, (7) 60%, (8) 70%, (9)0%, (10) 90%, (11) 100%.

.3. Location of probe molecules in the micellar phase

The location of the probe molecules in the micellar phase haseen determined by comparing the observed spectral changesith those obtained for solvents of known polarity, since theuorescence of 1-AS is solvent sensitive. The fluorescence spec-

ra of 1-AS in dioxan–water mixture (Fig. 2) are alike the spectraf 1-AS in the micellar media (Fig. 1). It is evident that the ratiof the peak heights of 1–0 and 0–0 vibrational bands of 1-ASary in different environments like the variation in intensity ofifferent peaks of pyrene in different solvents [24]. The peakeight ratio (F1/F2) of 1-AS in different dioxan–water compo-ition has been plotted against the ET(30) values of the mediumFig. 3). A linear relation is obtained. Assuming that the sameelation is also obeyed in the micellar medium and comparing theeak height ratio in the micellar media, the ET(30) value of the

icellar region surrounding the probe has been determined and

he values are given in Table 1. From this observation one cannfer that 1-AS is located at the interfacial region and its motions restricted. The most likely situation is that negatively charged

ig. 3. Plot of F1/F2 of 1-AS vs. ET(30) of dioxan–water mixture, [1-S] = 6.8 × 10−6 M.

c

3

mivmae

I

wpsil

lp

ig. 4. Fluorescence decay curve of 1-AS: (a) prompt, in (b) 23 mM DTAB andc) water.

ulphonate groups reside near the positively charged surface ofhe hydrophilic side of the micelles and the hydrocarbon ringowards the inner side of the micelles. Before cmc, the initialecrease in fluorescence intensity may be due to the electrosta-ic attraction of the negatively charged probe molecules with theositively charged surfactant monomers and the resultant spe-ies formed has low fluorescence intensity than the probe itself.he gradual increase in fluorescence intensity after cmc, is the

ntensity of the probe molecules in the completely micellizedtates. When urea (1–8 M) was added in the micellar solutionsf 1-AS, the nature of the fluorescence spectra of 1-AS did nothange and the fluorescence intensities were almost identical.his indicates that urea was unable to loosen the strong elec-

rostatic binding between the negatively charged probe and theositively charged micellar surface. The strong binding betweenhe probe and the micelles is also evident from the high bindingonstant values of 1-AS in different micellar media (Table 1).

.4. Flourescence decays

The fluorescence decay times of 1-AS were determined inicellar solutions of CTAB, TTAB and DTAB. Improvement

n the value of χ2 and the distribution of residuals was obser-ed when going from a single to a biexponential fit in theicellar solutions, suggesting a specific solute solvent inter-

ction (Fig. 4). The lifetime values were calculated using thexpression,

(t) =n∑

i=1

Ai e−t/τ i (3)

here I(t) is the intensity of the fluorescence at time t, Ai is there-exponential factor for the fraction of the fluorescence inten-ity, τi is the fluorescence lifetime of the emitting species and ns the total number of emitting species. The mean fluorescence

ifetimes were calculated using the relation τ = a1τ1 + a2τ2.

With increase in surfactant concentration, population in theong time component decreases and that in the short time com-onent increases with faster decay with an average lifetime

A. Sarkar et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 317 (2008) 585–591 589

ntrati

dTobntnmdftTcp

psoomtmtbwtvs

3

dS

S

waociiSmicelles as CTAB > TTAB > DTAB. This follows the order ofarea of the micellar interface where quenching occurs. Assu-ming that the excess quencher ion is present in the interfacialregion of the micelles, local concentration of cetyl pyridinium

Fig. 5. Plot of inverse of lifetime (1/τ) of 1-AS vs. surfactant conce

ecreasing from 6.86 ns to 1.54 ns, 1.55 ns and 1.56 ns in CTAB,TAB and DTAB micellar media, respectively. The rate of decayf fluorescence of 1-AS in different micellar media (1/τ) haseen plotted against surfactant concentration. From the obtai-ed plots, the cmc of the micelles were determined (Fig. 5) andhe values (Table 1) are almost identical with the values obtai-ed from the fluorescence intensity plot. From the plot of 1/τ vs.icellar concentration, it is found that the variation of rate of

ecay of 1-AS with micellar concentration in different micellesollow the order CTAB > TTAB > DTAB. This is expected sincehe surface area of the micelles decreases in the same order.he variation of decay rate follows the order of the hydrophobichain length of the surfactants and the binding constant of therobe with the micelles.

Chakrabarty et al. [25] reported an increase in lifetime of aositively charged probe (norharmane), when bounded by oppo-itely charged anionic micelles of SnDS. In our case the lifetimef the anionic probe (1-AS) decreases when it is bounded byppositely charged cationic micelles of CnTAB. The lifetimeeasurements indicate that the fluorescence lifetime of 1-AS in

he three micelles became almost constant (1.5 ns) after completeicellization of the probe. This demonstrates the fact that nega-

ively charged probe 1-AS, in cationic surfactants of CnTAB, iseing protected from the environment by associating themselvesith the positive boundary of the micelles, through electrosta-

ic attraction, and forming an exciplex having a lower lifetimealue than the probe itself. This exciplex formation has also beenupported from steady state fluorescence measurement.

.5. Fluorescence quenching

The fluorescence intensity of 1-AS in different micellar mediaecreases in the presence of quencher (CpCl) (Fig. 6). Thetern–Volmer constants (Ksv) have been calculated using the

FA((

on: (1) CTAB, inset (2) TTAB, (3) DTAB [1-AS] = 6.8 × 10−6 M.

tern–Volmer equation

F0

F= 1 + Ksv[Q] (4)

here F0 and F are the fluorescence intensities in absencend presence of quencher and [Q] is the bulk concentrationf quencher. The quenching process has been studied at fixedoncentrations of the micelles. The values of Ksv are givenn Table 3. The linearity of the Stern–Volmer plot (Fig. 7)ndicates that only one type of quenching occurs [26]. Thetern–Volmer quenching constant values follow the order of

ig. 6. Fluorescence quenching of 1-AS by CpCl in 2 mM CTAB. [1-S] = 6.8 × 10−6 M. [CpCl]: (1) 0 mM, (2) 0.05 mM, (3) 0.1 mM, (4) 0.15 mM,

5) 0.2 mM, (6) 0.25 mM, (7) 0.3 mM, (8) 0.35 mM, (9) 0.4 mM, (10) 0.45 mM,11) 0.5 mM.

590 A. Sarkar et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 317 (2008) 585–591

Table 3Stern–Volmer constant (Ksv), association constant of the quencher with the micelles (Kqm), partition coefficient of quencher between micellar and aqueous phase(Kp) and molar volume of surfactants (Vm)

Surfactant Concentration(mmol dm−3)

Ksv × 10−2 (dm−3 mol−1) Q Kqm × 10−3

(dm3 mol−1)Kp (partition coefficient)×10−2 Micellar molar volume

(Vm) (dm3 mol−1)

CTAB

2.0 51.4

8.5 215 20.0 108.12.5 28.73.5 21.75.0 11.6

TTAB

10.5 14.5

2.8 15.5 2.8 55.320.0 7.530.7 7.140.0 5.2

DTAB

20.0 12.5

1.4 2.8 0.8 34.825.0 8.035.0 4.1

K

iAoTotcstdbm

3

mb

F2A

tc

K

Qr

[

w

v

50.0 3.1

sv in water = 1.3 × 105 mol−1 dm3.

on was calculated using those parameters given in Table 1. 1-S resides at the micellar interface and since aqueous solutionf quencher is used, they also interact at the micellar interface.he quencher ions have no effect on the absorption spectrumf 1-AS in aqueous as well as in micellar medium. Hencehe quenching mechanism is same in all the media. It is theollisional quenching that plays the major role in this photophy-ical process. This is supported by the decreased Ksv values inhe micellar media. The quenching efficiency of the quencherecreases with increase in micellar concentration, which maye due to decrease in effective concentration of quenchers in theicelles.

.6. Association of the quencher with the micelles

The nature of association of the quencher ions with theicelles has been determined with the help of the model descri-

ed by Blatt et al. [27] and Encinas and Lissi [28]. According to

ig. 7. Plot of Stern–Volmer equation for quenching of 1-AS by CpCl inmM micellar solution of CTAB, 10 mM TTAB and 35 mM DTAB. [1-S] = 6.8 × 10−6 M.

fiptrl[

cm

K

tugTbtT

oqT

he model, the association constant (Kqm) of the quencher mole-ule with the micelle is represented by the following equation:

qm = [Q]mic

[Q]water[M](5)

If the average concentration of the quencher per micelle is¯ and the total concentration of the quencher is [Q]T, then theelation (6) holds

Q]T = Q

Kqm+ Q[M] (6)

here [M] is the effective micellar concentration.From the Stern–Volmer plot of F0/F vs. [Q] at different [M],

alues of [Q]T were found. The [Q]T value required to attain axed value of F0/F at different micellar concentrations, whenlotted against the micellar concentration [M], should yieldhe values of both Q and Kqm from the slope and intercept,espectively. The concentration of the quencher in the micel-ar phase is [Q]m = Q[M] and the total quencher concentrationQ]T = [Q]m + [Q]w.

The values of Q and [Q]w were determined. The partitionoefficient (Kp) can be related to Kqm and molar volume oficelles (Vm) as

p = Kqm

Vm= Q

QwVm

The molar volume (Vm) of the micelles were calculated usinghe method adopted by Robson and Dennis [29] and others [30]sing Stoke’s equation (4/3)�R3 = Vm/N0 where N0 is the Avo-adro number. The radii of the micelles [31] are given in Table 1.he obtained values of Kp increase with increase in hydropho-ic chain length of the micelles, i.e., with increase in size ofhe micelles. The Kqm, Kp, Ksv, Vm, and Q values are given inable 3.

The Stern–Volmer plot, i.e., F0/F vs. local concentrationf CpCl is also linear in each micelle. Local concentration ofuencher was calculated using the partition coefficient values.he quenching rate constants in the micellar media are found

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broivf

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ra

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A. Sarkar et al. / Colloids and Surfaces A

o increase with increase in hydrophobic chain length of theicelles, i.e., with the increasing area of the micellar inter-

ace.Since 1-AS and CpCl were distributed between the aqueous

ulk and the micellar interface, the quenching process was theesultant of the effect in these two regions. The dependencef the quenching phenomenon on the degree of fluidity of thenterfacial environment of the micelles may be due to the micro-iscosity of the micellar interface, which will be taken up inuture works.

. Conclusions

Spectroscopic investigations reveal the sensitivity of fluo-escence of 1-anthracene sulphonate (1-AS) in micellar media,lthough the absorption remains unchanged.

Fluorescence of 1-anthracene sulphonate, a negatively char-ed probe, decreases in cationic surfactant solution of alkylrimethyl ammonium bromides but the fluorescence intensitys enhanced in the micellar solution of the same surfactantsfter their cmc and the spectra is very similar to that of 1-ASn varying composition of dioxan–water mixture, clearly indica-ing the fluorescence sensitivity of the probe towards the polarityf the microheterogeneous environment surrounding the probe.teady-state fluorescence measurements demonstrate that therobe molecules occupy a small fraction of micelles. The loca-ion of the probe in different micelles has also been ascertained.ifetime measurements indicate the formation of an exciplexetween the anionic probe 1-AS and the cationic micelles, thexciplex having a lower lifetime value than the probe itself andhe dependence of the decay rate constant on the hydrophobichain length of the micelles. The longer is the chain length,he faster is the rate of decay. In cationic micellar medium theuenching of fluorescence of 1-AS by CpCl depends on the asso-iation of the probe with the micelles which follows the orderTAB > TTAB > DTAB. The quenching process is supposed toccur predominantly at the micellar interface and the quenchingonstant (Ksv) values increases with increase in the hydrophobichain length of the surfactants.

cknowledgements

One of the authors A.S. acknowledges UGC for providing aRF and S.P. acknowledges CSIR for providing a SRF.

[[[

[

sicochem. Eng. Aspects 317 (2008) 585–591 591

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