ORIGINAL ARTICLE

Interaction of Azo Dye with Cationic Surfactant Under DifferentpH Conditions

Muhammad Faizan Nazar • Syed Sakhawat Shah •

Muhammad Arshad Khosa

Received: 27 October 2009 / Accepted: 4 December 2009 / Published online: 22 January 2010

� AOCS 2010

Abstract The aggregation induced by Alizarin Yellow R

(AYR) in the cationic surfactant, cetyltrimethylammonium

bromide (CTAB), was investigated by measuring their

UV–visible absorption spectra. Conductance measurements

as a function of surfactant concentration below and above

the critical micelle concentration (CMC) were studied.

CTAB aggregation takes place at the concentration far

below its normal CMC in the presence of AYR. Both

hydrophobic and electrostatic interactions affect the

aggregation process in aqueous solution. The dye effect on

the CMC of CTAB was noted by a specific conductivity

method as well. AYR–CTAB binding constant (Ks) and

water–micelle partition co-efficient (Kx) were quantified

with the help of mathematical models employed to deter-

mine the partitioning of organic additives in the micellar

phase. The number of dye molecules per micelle was

estimated at particular CTAB concentrations above CMC,

during this study.

Keywords Alizarin Yellow R � Cationic surfactant �Specific conductivity � Hydrophobicity �Partition coefficient � Binding constant

Introduction

Amphiphilic properties of surfactants have attracted

growing attention for their use in biological and chemical

research applications especially in the dyeing process

where the role of surfactants is very important [1]. Micelles

are aggregates formed by amphiphilic molecules (hydro-

phobic chain and hydrophilic head group) above their

critical micelle concentration (CMC). They are composed

of a hydrophilic surface and a hydrophobic core. This

specific micellar structure shows chemical interactions

with hydrophilic or lipophilic molecules [2] that can be

applied in analytical chemistry as well as pharmaceutical

industries. The structure of micellar aggregates is of par-

amount interest in several industrial applications of sur-

factants. One of the most fundamental properties of

aqueous micellar solutions is their ability to solubilize a

wide variety of organic solutes with quite distinct polarities

and degrees of hydrophobicity. Among various contribut-

ing factors, the favorable (hydrophobic) sites of organic

additives are supportive for their readily solubilization in

the micellar aggregate [3, 4]. Surfactants–dye associations

are significant in both dyeing processes and detergency [5].

This surfactant–dye interaction also customizes the uptake

of dye into substrate such as cellulose and keratin fiber [6].

In this study, the amphiphilic azo dye (Alizarin Yellow

R) was used as an organic additive made by the di-azo

coupling reaction. This azo dye is a pH indicator and its

ion-association complex of nickel in the presence of

polysorbate 80 had been successfully applied to the micro

determination of Ni(II) in pharmaceutical samples [7]. This

dye could be used for the determination of formaldehyde in

water samples [8]. It is also effective as a specific adsor-

bent for the removal of aluminum from both drinking and

dialysis water [9]. The intra-molecular hydrogen bonding

between the alcoholic (–OH) group at position-1 and acidic

oxygen produces a stable six-member ring system

(Scheme 1). Therefore, as a result, intra-molecular hydro-

gen bonding makes dye molecules more hydrophobic; this

is responsible for their incorporation into the micelle and

M. F. Nazar � S. S. Shah � M. A. Khosa (&)

Department of Chemistry, Quaid-i-Azam University,

Islamabad 45320, Pakistan

e-mail: khosa73pk@yahoo.com

123

J Surfact Deterg (2010) 13:529–537

DOI 10.1007/s11743-009-1177-8

for the increase in the number (n) of dye molecules

incorporated per micelle.

In this work, the conductivity measurements for critical

micelle concentration (CMC) values and UV–visible

spectral measurements for spectral changes are reported to

explain the CTAB–AYR interaction under different pH

conditions. The aggregation behavior of CTAB–AYR in

water was studied using simple spectroscopy, differential

spectroscopy, and conductivity. The micelle–water parti-

tion coefficient (Kx), standard free energy change of solu-

bilization (DG0p), AYR–CTAB binding constant (Ks) and

the number of dye molecules per micelle solution (n) were

calculated by employing the absorbance, differential

absorbance and conductivity data at 25 �C.

Experimental

Materials

CTAB was purchased from Sigma Chemical Co., Alizarin

Yellow R [5-(4-nitrophenylazo) salicylic acid] was

obtained from Fluka. A 10 mM solution of AYR was

prepared by weighing exactly 0.144 g of reagent and the

solution was diluted up to 50 cm3 with doubly distilled

water. Other solutions were prepared by dilution. All

experiments were carried out with analytical reagent grade

chemicals using both distilled and demineralized water.

The dye used in the present study obeys Beer’s law in the

employed concentration range and the solution pH was

adjusted using phosphate buffer.

Procedure

UV–Visible Spectroscopy

Spectrometric measurements were performed on a Perkin-

Elmer Lambda 20 ultraviolet–visible spectrophotometer

with 1.0-cm quartz cells at a temperature of 25.0 ± 0.1 �C.

Differential absorbance measurements were made in such a

way that the additive solution of a particular concentration

was kept on the reference side and the surfactant–dye

solution on the sample side in the spectrophotometer.

Conductivity Experiments

Critical aggregation concentrations were determined by

conductivity experiments. The specific conductance of

surfactant solutions with and without additive (AYR) was

measured on a Microprocessor Conductivity Meter (WTW

82362 Weilheim) fitted with an electrode (WTW

06140418). The CMC of CTAB in water and in the pres-

ence of additive was determined by plotting the specific

conductance against the surfactant concentration (Cs).

Solutions in the conductivity cell were stirred magnetically

while a thermostat was used to maintain the temperature at

25.0 ± 0.1 �C.

NN

OH

OHO

NN

O

OO

O2N

H

O2N

NN

O

OO

O2N

NN

OH

OO

O2N

Monoanionic forms

Dianionic form

H H

H

pKa = 5.0 pKa = 11.0

H

NBr

(1)

(2)

Scheme 1 1 Molecular structures of Alizarin Yellow R; 2 Cetyltrimethylammonium bromide

530 J Surfact Deterg (2010) 13:529–537

123

Results and Discussion

Absorption Spectra of Alizarin Yellow R Under

Different pH Conditions

Alizarin Yellow R is a poly-functional molecule with pKa

values 5.0 and 11.0 (Scheme 1, [7]). This azo dye is

slightly soluble in water in a strong acidic medium (pH 1 or

2) while the functional groups of carboxylic acid and

phenol are not ionized. AYR is in mono-ionic form at

pH \ 4.5, while it shows di-anionic behavior at pH [ 10.

The aqueous solubility increases due to ionization of the

carboxylic group (pKa = 5.0) at pH 4.4 and the phenate

form is dominant in an acidic medium. UV–visible spectra

of Alizarin Yellow R have been shown in Fig. 1. A strong

bathochromic effect (kmax = 373 to kmax = 492 nm) can

be seen at pH 12.0. In a sufficiently strong base (above pH

10.0), the di-anion is formed on account of ionization of the

carboxylic as well as the hydroxyl group (Scheme 1).

Alizarin Yellow R experiences a bathochromic shift from

373 to 493 nm due to extensive delocalization of negative

charges, and no change in absorption maxima from 4.5 to

10.0 pH range was observed because intra-molecular

H-bonding plays a role in keeping the kmax at 373 nm as in

an aqueous solution.

Aggregation Behavior of CTAB–Alizarin Yellow R

Absorption spectra of azo dye (AYR) were also recorded at

different concentrations of CTAB in aqueous solution

(Fig. 1). A particular type of dye–surfactant aggregation is

observed when the anionic component is a dye molecule in

combination with a cationic surfactant (CTAB). Low

concentrations of CTAB shift the band from 372 nm to a

new band at 388 nm and increase the intensity of the

shifted band with an increase in the concentration of

CTAB.

At very low CTAB concentration, the AYR band-I

intensity initially decreases and then increases with

increases in CTAB concentration. The process is shown in

Fig. 2.

The initial decrease in intensity of band-I is due to the

self-aggregation of dye molecules assisted by the surfactant

chain [10]. Dye–surfactant interaction below CMC allows

the dye to absorb light favorably; hence absorbance is

enhanced in the sub-micellar region. The leveling off the

curve above the surfactant CMC reveals a maximum sol-

ubilization of dye molecules within the micelle. A pro-

posed mechanism to explain CTAB–AYR interaction is

shown in Fig. 3.

The increase in absorbance is a result of the stabilization

of AYR by the positive charge of the monomers of CATB

as shown in the Fig. 3a. As it proceeds to the post-micellar

region, the AYR solubilization in quaternary ammonium

solution takes place initially by absorption at the micellar-

water interface replacing water molecules and thereafter

solubilization of additional dye occurs in the palisade layer

(Fig. 3b). Spectra of both di-anionic AYR (pH 12.0) and

neutral AYR (pH 4.0), experience significant bathochromic

shifts (kmax) of 40 and 20 nm, respectively, in the presence

of CTAB. Dye–micelle interaction is better explained by

quantifying its magnitude by determining the dye–micelle

partition coefficient (Kx), dye–surfactant binding constant

(Ks), standard free energy of solubilization (DG0p) of dye in

micelles and approximate numbers of dye molecules per

micelle. In the case where molecular interactions with its

300 400 500 6000.0

0.1

0.2

0.3

0.4

0.5

0.6

abso

rban

ce

wavelength (nm)

λmax= 492

(a)

λmax = 373

(d)

(c)

(b)

300 350 400 450 500

0.1

0.2

0.3

0.4

abso

rban

ce

wavelength (nm)

12

34

56

78(I)

Fig. 1 Absorption spectra of Alizarin Yellow R: a pH 4.0; b pH 6.6;

c pH 10.0; d pH 12.0 and Effect of CTAB on the absorption spectrum

of Alizarin Yellow R in aqueous solution at 25 �C; I without

surfactant; 1 0.7 mM; 2 0.8 mM; 3 0.9 mM; 4 1.0 mM; 5 2.0 mM; 63.0 mM; 7 4.0 mM; 8 8.0 mM; Reference solution—water; the

cuvette is 1 cm long

J Surfact Deterg (2010) 13:529–537 531

123

surrounding environment are intrinsically related to spec-

tral characteristics, their changes are used for the deter-

mination of corresponding partition coefficients and the

approximate number of dye molecules per micelle.

The approximate number of dye molecules incorporated

into a single micelle (n) is calculated by the following

relations [11, 12]:

n ¼ Cm

Mð1Þ

M ¼ Cs � CMC

Nð2Þ

where Cm is the concentration of dye solubilized in the

micelle, M is the micelle concentration, Cs is the total

surfactant concentration and N is the mean aggregation

number of micelles at CMC in water. The normal CMC of

the CTAB is 0.9 mM [13]; Cm is the concentration of

solubilized dye that is determined as [14]:

Cm ¼Ao � A

eo � em

ð3Þ

where Ao is the absorbance of dye solution without sur-

factant, A is the absorbance at any point in the presence of

surfactant above the CMC, eo is calculated from Ao, and em

is determined at higher surfactant concentration above the

CMC when absorbance of the dye–surfactant solution

0.000 0.001 0.002 0.003 0.004

0.30

0.32

0.34

0.36

A λλ

max

CS (mol/dm3)

A

370

375

380

385

390

395

λmax

Fig. 2 Relation between absorbance of (CTAB ? dye) and surfac-

tant concentration

N

NN

OH

OO

O2N

(a)

N

N

N

N

N

NN

N

N

N

N

N

NN

HOO

O

NO2

NN

OHOO

O2N

(b)

Fig. 3 Proposed mechanism of

action of AYR in different

concentration regions of CTAB.

a Interaction of AYR with

CTAB in its monomeric form

clearly indicate that an

attractive force is present.

b Interaction of AYR with

CTAB micelle

532 J Surfact Deterg (2010) 13:529–537

123

becomes almost constant. The micellar aggregation number

used for CTAB is 80 [15]. For a particular concentration of

CTAB (Cs), a higher value of ‘n’ shows greater hydro-

phobicity of Alizarin Yellow R in aqueous solutions. The

results are shown in Table 1.

Dye molecules can aggregate either in a parallel or in a

head-to-tail fashion. A blue shift in the absorption band is

observed in the case of parallel dimeric aggregation dye

and head to tail assemblage of the dimeric dye leads to a

red shift in the absorption band as compared to the

monomeric dye [16]. In our spectrometric study, a new

absorption band of surfactant–dye aggregate is red-shifted

with respect to the absorption band of the dye in aqueous

solution; and this indicates that dye molecules are aggre-

gated in a head-to-tail fashion. An AYR dye molecule that

binds to a cationic surfactant creates a more hydrophobic

binding site and facilitates the binding of another dye

molecule. This implies that hydrophobic stacking of aro-

matic parts of the azo dye is also important in the aggre-

gation process besides electrostatic interactions.

Differential Absorbance

Differential absorption spectra of dye (AYR) in the pres-

ence of various concentrations of CTAB at pH 6.6 and 10.0

are shown in Fig. 4.

Elevated values of DA with increasing surfactant con-

centration correspond with the enhanced solubilization of

AYR molecules in the micelles. Solubilized dye molecules

are distributed according to their polarity between the

highly non-polar central region and the relatively polar

interfacial region of the micelles [17, 18]. A useful physical

parameter to quantify ARY solubilization in different

micellar media is partition coefficient Kc (dm3 mol-1). It

can be calculated by the following equation [19]:

1

DA¼ 1

KcDA1ðCa þ Cmos Þþ 1

DA1ð4Þ

Ca denotes dye concentration, Csmo represents Cs - CMC0

(CMC0 is the CMC of surfactant in water), DA? is the

differential absorbance at the infinity of Cs and Kc is

obtained through intercept and slope values from the

straight line by plotting 1/DA against 1/(Ca ? Csmo), as

shown in Fig. 5a and the value of Kc has been shown in

Table 2. The dimensionless partition coefficient Kx is

related to Kc as Kx = Kcnw, where nw is the number of

moles of water per dm3, and is reported in Table 2. The

standard free energy change of the transfer of additive,

DG0p from bulk water to micelle can be calculated using the

following relation:

DG0p ¼ �RT ln Kx ð5Þ

Here T is absolute temperature and R is the gas constant.

The value of DG0p for the dye, using Kx is reported in

Table 2.

The high negative value of DG0p indicates the ease of

penetration of the dye into the micelles. A dye molecule

does not penetrate deeply enough into the micelle unless

the dye’s hydrophobicity is sufficiently strong enough to

Table 1 Number of AYR molecules incorporated per CTAB micelle

in various pH ranges, at 25.0 �C

pH A0 Cm

(mol/dm3)

em

(dm3/

mol cm)

M(mol/dm3)

n = Cm/M

4.0 0.3617 1.98 9 10-5 19,225 4.25 9 10-6 5

6.6 0.3567 2.24 9 10-5 17,389 4.25 9 10-6 5

10.0 0.2333 1.75 9 10-5 14,400 4.25 9 10-6 4

12.0 0.4655 1.48 9 10-5 23,018 4.25 9 10-6 3

-0.12

-0.06

0.00

0.06

0.12

ΔA

wavelength (nm)

(a)

300 350 400 450 500

-0.05

0.00

0.05

0.10

0.15

ΔA

wavelength (nm)

(b)

300 350 400 450 500

Fig. 4 Differential absorption spectra of Alizarin Yellow R–CTAB at

different pH; a pH 6.6, b pH 10.0 (Arrow indicates the wavelength

used for analysis)

J Surfact Deterg (2010) 13:529–537 533

123

overcome the electrostatic interaction with the head group

of CTAB [20]. This is clear from the high values of Kx and

more negative DG0pfor AYR, as shown in Table 2.

It is assumed that Alizarin Yellow R forms a complex

with CTAB in the bulk of the solution via electrostatic

interactions before it penetrates into micelles. At first,

adhesion of the dye–surfactant complex to the micelle

surface takes place and then dye molecules reorient

themselves into the inner hydrophobic portion of the

micelles and finally it make their way deeper into the

interior (core) of the micelle. The structure of the additive

molecule (AYR) and charge on the surfactant contribute

largely towards the phenomenon of solubilization. In

addition to the hydrophobic interactions, electrostatic fac-

tors play an important role in binding of AYR to the

micelle of CTAB. The formation of an ion-pair complex

between anionic Alizarin Yellow R and the positive head

group of CTAB micelles was confirmed by the initial

absorbance changes. The initial rapid reaction may be

represented as [21]:

0 2 4 6 8 103.0

4.5

6.0

7.5

9.0

10.5

1/Δ

A

(Ca+Csmo)-1x102dm3mol-1

pH 4.0 pH 6.6 pH 10.0 pH 12.0

(a)

0

2

4

6

8

10

12

12

(Ca)

(Cs)

x10-7

/ΔA

Cs x 10-4 (mol dm-3)

pH 4.0 pH 6.6 pH 10.0 pH 12.0

(b)

0

5

10

15

20

25

30

S t/S

o

M x 10-5 (mol dm-3)

pH 4.0 pH 6.6 pH 10.0 pH 12.0

(c)

0 15 30 45 60 75 90

0 2 4 6 8 10

7.2

7.6

8.0

8.4

8.8

9.2

1/Δ

A

[Csmo - kCa + (1+k)Ca j]-1 x102dm3mol-1

(d)

0 2 4 6 8 10

Fig. 5 a Plot of inverse of differential absorbance (1/DA) versus

(Ca ? Csmo)-1 for 1FuE and concentration of CTAB in different

media; b Relationship between (Ca 9 Cs)/DA for AYR and concen-

tration of CTAB in various pH ranges; c Relationship between

relative solubility of AYR and CTAB micellar concentration in

various pH ranges; d Relationship between 1/DA and 1ðCmo

s �kCaþð1þkÞCajÞfor CTAB in the presence of AYR

Table 2 Values of Kc, Kx, Ks

and DG0p of Alizarin Yellow R

in micellar solution of cationic

surfactant (CTAB) in various

pH ranges

pH Kc (dm3 mol-1) Kx DG0p (kJ/mol) Ks (dm3 mol-1) De (dm3 mol-1 cm-1)

4.0 1.55 9 104 8.6 9 105 -33.85 7,517 7,883

6.6 3.23 9 103 1.8 9 105 -29.98 2,020 7,110

10.0 1.95 9 103 1.1 9 105 -28.76 1,267 9,513

12.0 1.75 9 103 9.7 9 104 -28.45 1,256 17,039

534 J Surfact Deterg (2010) 13:529–537

123

AYR + CTAB� ion-pair complex

In order to calculate AYR–CTAB binding constant, a

quantitative approach is provided by the following relation

[21]:

AYR½ �T CTAB½ �DA

¼ CTAB½ �Del

þ 1

KsDelð6Þ

where [AYR]T is the total dye concentration (Ca), [CTAB]

is the molar surfactant concentration (Cs), DA is the dif-

ference in absorbance between the complex and AYR

obtained from the differential absorbance spectrum, De is

the difference in absorption coefficients and l is the path

length (1.0 cm). To test the validity of Eq. 6, the left-hand

side term in the equation was plotted against [CTAB] in

different media, which was found to be fairly linear

(Fig. 5b).

From the slope and intercept of the straight line, values

of the binding constant (Ks) and De were calculated and are

reported in Table 2. To identify the position or location of

solubilized dye molecules in the micelle, it is useful to

calculate the binding constant and partition coefficient of

AYR–CTAB in various pH ranges. The results (Table 2)

indicate that the partition of AYR into the micelle with its

hydroxyl and carboxylic moieties near the water–micelle

interface takes place in such a way that it leads to depro-

tonation and thus making it a charged molecule. This

phenomenon facilitates insertion of an azo ring into the

core of the micelle.

Decreased pH of the medium (acidic medium) causes

protonation of the ionizable carboxylic group of AYR,

leading to elevated hydrophobicity and finally aggregation

results. In this way more hydrophobic additives are buried

deeply inside the core of micelles at a lower pH value.

Conversely, additives with hydrophilic interactions are

oriented near the surface region of the micelle. Neutral

species, mono and di-anionic forms of AYR, bind to CTAB

showing a strong pH effect on the binding constant. Using

partition coefficient (Kx) values obtained from differential

absorbance method, the relative solubility (St/So) of AYR

in various pH ranges can be obtained by employing the

relationship [22].

St=So¼ 1þ KxvM ð7Þ

St and So are total and intrinsic water solubility values,

respectively, m is the partial molal volume of the micelle

that in case of CTAB is 0.3654 dm3 mol-1 [15], and M is

micellar concentration and is given by well-known rela-

tionship in Eq. 2. Relative solubility of Alizarin Yellow R

increases with the increase of micellar concentration and its

hydrophobic interactions within the micelles. This implies

that relative solubility depends upon the hydrophobicity of

the additive molecules which is also shown by partition

coefficient values. In addition, either acidic or basic media

also affect the relative solubility (Fig. 5c).

Conductivity Experiments

The critical micelle concentration of CTAB in aqueous

solution containing AYR was determined by plotting the

specific conductance against the surfactant concentration

(Cs), shown in Fig. 6a. Conductance experiments were

carried out at pH [ 10 to maximize the azo dye (dianionic

form) solubility in aqueous solution by using phosphate

buffer. Small amounts of organic additives may produce

marked changes in the CMC in aqueous media [17].

The critical micelle concentration of CTAB decreases

linearly with increases in concentration of AYR dye as

shown in Fig. 6b. This indicates that the CMC is a function

of additive concentration (Ca) and its depression by adding

solubilized material is due to a greater degree of interaction

between the hydrophobic group of the surfactant and the

hydrophobic chain of the additive used. In addition,a strong

0 1 2 3 4 5

0

30

60

90

120Sp

ecif

ic c

ondu

ctan

ce (μ

S/cm

)

Cs x 10-3(mol dm-3)

no dye

0.1mM dye

0.5mM dye

0.7mM dye

1.0mM dye

(a)

0.0 0.2 0.4 0.6 0.8 1.07.6

8.0

8.4

8.8

9.2

9.6

CM

C x

10-4

(m

ol/d

m3 )

Ca x 10-3 (mol/dm3)

(b)

Fig. 6 Effect of concentration of dye on the CMC of CTAB, at

25.0 �C; a Specific conductance versus concentration of CTAB,

b CMC of CTAB as a function of AYR concentration (Ca)

J Surfact Deterg (2010) 13:529–537 535

123

oppositely charged attractive force between the di-anionic

form of the azo dye and the cationic surfactant CTAB on

the stern layer of surfactant is responsible for a declining

CMC of CTAB. Discussing this more quantitatively, the

entropy of the mixing dye in micellar solution causes a

reduction in the free energy of micelles in addition to

hydrophobic interactions; hence the CMC is lowered [23].

The water–micelle partition coefficient Kc (dm3 mol-1) of

AYR is calculated by using an improved relationship given

in [19]; this gives a relatively precise approximation

introducing two new factors j and k into Eq. 4:

1

DA¼ 1

KcDA1 Cmos � kCa þ ð1þ kÞCaj

� �þ 1

DA1ð8Þ

In the above equation, k can be obtained by plotting the

CMC of CTAB against different dye concentrations as

shown in Fig. 6b. Slope of line through CMCo (dCMC/

dCa) provides k as given in Table 3, whereas j is the

fractional amount of solubilized organic additive of total

added organic additive in the solution. Factor j becomes

zero at a certain Ca in the premicellar region up to the

CMC and increases with increasing Cs above the CMC. As

Cs increases up to infinity, j approaches unity, since

virtually all the added organic additive has been solubilized

in micelles Cma ffi Ca Thus, we can write:

j ¼ DA=DA1 ð9ÞBy plotting 1/DA against 1=½Cmo

s � kCa þ ð1þ kÞCaj�,intercept and slope of the straight line give the value of Kc

as shown in Fig. 5d.

The partition coefficient obtained from Eq. 8 and the

standard free energy change is calculated from Eq. 5, as

reported in Table 3.

There seems to be agreement between Kc values deter-

mined by Eq. 4 at constant AYR (Ca) and those determined

by Eq. 8 in a variable concentration (Ca) of AYR in pres-

ence of a higher surfactant concentration region (Cs). The

standard free energy change (DG0p) is shown in Table 3. It

was found that Kc is independent of both Cs and Ca in such a

low Ca region whereas j and k are slightly dependent on Ca.

Conclusion

Different parameters obtained from spectroscopic mea-

surements and conductance data indicate an enhanced

solubility of AYR dye in the micellar region. Intra-

molecular hydrogen bonding within the dye molecule

effectively reduces intermolecular attraction, thereby

increasing solubility in non-polar solvents (micelles).

Medium effects on the position of the long wavelength

absorption band of the azo dye characterize it as a pH

chromic reporter molecule. A partitioning study of the

solubilized system provides useful insight into the process

of solubilization that is applicable to the general problem

of membrane solubilization properties and in drug delivery

to quantify the degree of drug-micelle interaction. The

partition coefficient value obtained is important in micellar

electro–kinetic capillary chromatography and high pressure

liquid chromatography (HPLC) for drug quality control.

Thus, interaction with micellar aggregates induces signifi-

cant pKa shifts of Alizarin Yellow R that can be rational-

ized in terms of the partitioning of species and electrostatic

contribution. Likewise, knowledge of the effects of organic

additives on the CMC of surfactants is used both for the-

oretical and practical purposes because some additives are

likely to be present as impurities or byproducts in the

manufacturing of surfactants and their presence may cause

significant differences in supposedly similar commercial

surfactants.

Acknowledgments The financial support of the Quaid-i-Azam

University and the Higher Education Commission of Pakistan is duly

acknowledged.

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-0.1639 0.78 3.28 9 103 1.82 9 105 -30.00

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Author Biographies

Muhammad Faizan Nazar obtained his masters degree in chemistry

from Quaid-i-Azam University Islamabad, Pakistan in 2005. He

obtained his M.Phil. in 2007 from the same institute. He is a Ph.D.

student of the Chemistry Department at Quaid-i-Azam University,

Islamabad. His research interests are synthesis and characterization of

microemulsions and their applications in drug delivery systems, as

well as electronic and hydrophobic interactions in dye-surfactant

aggregates.

Syed Sakhawat Shah obtained his masters degree (chemistry) in

1971 and M.Phil. (chemistry) degree in 1973 in Pakistan. He

specialized in colloids and surfactants and obtained his Ph.D. in

chemistry in Germany in 1978. He received the award of the

president’s pride of performance in 2003 and is now a professor of the

Chemistry Department at Quaid-i-Azam University, Islamabad,

Pakistan. His research interests include micellar drug delivery system,

colloidal interactions, separation and purification techniques using

surfactants.

Muhammad Arshad Khosa obtained his M.Sc. in chemistry from

Bahaudin Zakariya University, Multan Pakistan in 1994. He com-

pleted his M.Phil. degree in 2005. He is currently a Ph.D. student at

the chemistry department, Quaid-i-Azam University, Islamabad,

Pakistan. His research interests include the removal of pollutants

from aqueous solutions by micellar enhanced ultrafiltration tech-

niques and spectroscopic studies of dye-surfactant interactions.

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