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Jan 22, 2010 - bromide (CTAB), was investigated by measuring their. UV–visible absorption spectra. Conductance measurements as a function of surfactant ...
J Surfact Deterg (2010) 13:529–537 DOI 10.1007/s11743-009-1177-8

ORIGINAL ARTICLE

Interaction of Azo Dye with Cationic Surfactant Under Different pH Conditions Muhammad Faizan Nazar • Syed Sakhawat Shah Muhammad Arshad Khosa



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

M. F. Nazar  S. S. Shah  M. A. Khosa (&) Department of Chemistry, Quaid-i-Azam University, Islamabad 45320, Pakistan e-mail: [email protected]

where the role of surfactants is very important [1]. Micelles are aggregates formed by amphiphilic molecules (hydrophobic 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 paramount interest in several industrial applications of surfactants. 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 contributing 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 adsorbent 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 hydrogen bonding makes dye molecules more hydrophobic; this is responsible for their incorporation into the micelle and

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J Surfact Deterg (2010) 13:529–537 O

O H O

N N

(1)

O2N

Dianionic form

Monoanionic forms O

OH

O O

O

O O

OH OH

H

H N

N N

N

H

N

H

N

O2N

O2N

O2N

pKa = 11.0

pKa = 5.0

(2) N Br

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

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 partition coefficient (Kx), standard free energy change of solubilization (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.

employed concentration range and the solution pH was adjusted using phosphate buffer.

Experimental

Conductivity Experiments

Materials

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 presence 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.

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

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Procedure UV–Visible Spectroscopy Spectrometric measurements were performed on a PerkinElmer 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.

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Results and Discussion

0.6

Absorption Spectra of Alizarin Yellow R Under Different pH Conditions

0.5

λ max= 492

(d) λ max =

absorbance

0.4

(a)

(b)

0.3

(c) 0.2

0.1

0.0 300

400

500

600

wavelength (nm) 0.4

8 7 6 5 4 3 2 1

(I)

0.3

absorbance

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.

373

0.2

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 solubilization of dye molecules within the micelle. A proposed 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

0.1

300

350

400

450

500

wavelength (nm) 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; 6 3.0 mM; 7 4.0 mM; 8 8.0 mM; Reference solution—water; the cuvette is 1 cm long

region, the AYR solubilization in quaternary ammonium solution takes place initially by absorption at the micellarwater 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

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The approximate number of dye molecules incorporated into a single micelle (n) is calculated by the following relations [11, 12]:

395 0.36 390

A

A λ max

0.32

380

375 0.30 370 0.000

0.001

0.002

0.003

0.004

CS (mol/dm3 ) Fig. 2 Relation between absorbance of (CTAB ? dye) and surfactant concentration

surrounding environment are intrinsically related to spectral characteristics, their changes are used for the determination of corresponding partition coefficients and the approximate number of dye molecules per micelle.

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

Cm M Cs  CMC M¼ N

n¼ λ max

385

0.34

ð1Þ ð2Þ

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 ¼

Ao  A eo  em

ð3Þ

where Ao is the absorbance of dye solution without surfactant, 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

(a)

N O

O

OH N

N

O2 N

(b)

N N HO O N

N O OH O

N

O N N

NN

N NO2

O2N

N

N

N N

N N

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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

0.2333

-5

14,400

4.25 9 10-6

4

23,018

-6

3

(a) 0.12

10.0 12.0

0.4655

1.75 9 10

-5

1.48 9 10

4.25 9 10

ΔA

0.06

0.00

-0.06

Differential Absorbance Differential absorption spectra of dye (AYR) in the presence of various concentrations of CTAB at pH 6.6 and 10.0 are shown in Fig. 4. Elevated values of DA with increasing surfactant concentration 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 1 1 ¼ þ mo DA Kc DA1 ðCa þ Cs Þ DA1

ð4Þ

Ca denotes dye concentration, Cmo s 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

-0.12 300

350

400

450

500

450

500

wavelength (nm)

(b) 0.15

0.10

ΔA

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 hydrophobicity 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 aggregated 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 aromatic parts of the azo dye is also important in the aggregation process besides electrostatic interactions.

0.05

0.00

-0.05 300

350

400

wavelength (nm) 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)

straight line by plotting 1/DA against 1/(Ca ? Cmo s ), 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

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(b)

(a) 10.5 pH 4.0 pH 6.6 pH 10.0 pH 12.0

pH 4.0 pH 6.6 pH 10.0 pH 12.0

12

(Ca)(Cs)x10-7 /Δ A

9.0

12

1/Δ A

7.5

6.0

10 8 6 4

4.5 2 3.0

0 0

2

4 mo -1

(Ca+Cs

(c)

8

3

) x10 dm mol

10

0

15

-1

30

45

60

-4

75

90

-3

Cs x 10 (mol dm )

30

(d) pH 4.0 pH 6.6 pH 10.0 pH 12.0

25

9.2 8.8

20

1/Δ A

St/So

6 2

15

8.4 8.0

10 7.6 5 7.2 0 0

2

4

6

8

-5

0

10

-3

M x 10 (mol dm )

[Cs

Fig. 5 a Plot of inverse of differential absorbance (1/DA) versus -1 for 1FuE and concentration of CTAB in different (Ca ? Cmo s ) media; b Relationship between (Ca 9 Cs)/DA for AYR and concentration of CTAB in various pH ranges; c Relationship between

Table 2 Values of Kc, Kx, Ks and DG0p of Alizarin Yellow R in micellar solution of cationic surfactant (CTAB) in various pH ranges

- kCa

4

6

-1

8 2

10

3

+ (1+k)Ca j] x10 dm mol-1

relative solubility of AYR and CTAB micellar concentration in 1 various pH ranges; d Relationship between 1/DA and ðCmo kCa þð1þkÞC a jÞ s for CTAB in the presence of AYR

pH

Kc (dm3 mol-1)

Kx

DG0p (kJ/mol)

Ks (dm3 mol-1)

4.0

1.55 9 104

8.6 9 105

-33.85

7,517

7,883

6.6

3

3.23 9 10

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

overcome the electrostatic interaction with the head group of CTAB [20]. This is clear from the high values of Kx and more negative DG0p for 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

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2

mo

De (dm3 mol-1 cm-1)

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 factors 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]:

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½AYRT ½CTAB ½CTAB 1 ¼ þ DA Del Ks Del

ð6Þ

where [AYR]T is the total dye concentration (Ca), [CTAB] is the molar surfactant concentration (Cs), DA is the difference 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 deprotonation 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= ¼ 1 þ Kx vM So

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 relationship 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

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

(a) Specific conductance (μ S/cm)

In order to calculate AYR–CTAB binding constant, a quantitative approach is provided by the following relation [21]:

coefficient values. In addition, either acidic or basic media also affect the relative solubility (Fig. 5c).

120

90

no dye

60

0.1mM dye 0.5mM dye 0.7mM dye 1.0mM dye

30

0 0

1

2

3 -3

-3

4

5

C s x 10 (mol dm )

(b) CMC x 10-4 (mol/dm3)

AYR + CTAB ion-pair complex

9.6

9.2

8.8

8.4

8.0

7.6 0.0

0.2

0.4 -3

Ca x 10

0.6

0.8

1.0

3

(mol/dm )

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)

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Table 3 Partition coefficient (Kx) and Standard free energy change of penetration (DG0p ) of dye (AYR) in aqueous micellar solution (using Eq. 8) k = dCMC/dCa j -0.1639

Kc (dm3 mol-1) Kx

0.78 3.28 9 103

DG0p (kJ/mol)

1.82 9 105 -30.00

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 1 1  þ ¼ DA Kc DA1 Csmo  kCa þ ð1 þ kÞCa j DA1

ð8Þ

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 Cam ffi Ca Thus, we can write: j ¼ DA=DA1

ð9Þ 1=½Csmo

 kCa þ ð1 þ kÞCa j, By plotting 1/DA against 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 determined by Eq. 4 at constant AYR (Ca) and those determined by Eq. 8 in a variable concentration (Ca) of AYR in presence 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 measurements and conductance data indicate an enhanced

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solubility of AYR dye in the micellar region. Intramolecular 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 significant pKa shifts of Alizarin Yellow R that can be rationalized 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 theoretical 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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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 completed 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 techniques and spectroscopic studies of dye-surfactant interactions.

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