S1

Electronic Supporting Information

A quantitative appraisal of the binding interactions between anionic dye, Alizarin Red S

and alkyloxypyridinium surfactants: a detailed micellization, spectroscopic and

electrochemical study

Renu Sharma, Ajar Kamal and Rakesh Kumar Mahajan*

*Department of Chemistry, UGC-Centre for Advanced Studies-I, Guru Nanak Dev University,

Amritsar-143005, India

*To whom correspondence should be addressed:

e-mail: rakesh_chem@yahoo.com; Fax: +91 183 2258820

Electronic Supplementary Material (ESI) for Soft Matter.This journal is © The Royal Society of Chemistry 2015

S2

Annexure SI

1. Methods (Detailed Description)

1.1 Conductivity measurements. Conductivity was measured using a digital Systronics

conductivity meter model 306 with a dip-type conductivity cell having a cell constant of

1.01 cm-1

at 298.15 K. The specific conductivity of deionised double distilled water used in

present study was measured to be 2.0-3.0 µS cm-1

.

1.2 Surface tension measurements. Surface tension measurements have been performed

using Kruss (Hamburg, Germany) Easy dyne tensiometer using ring detachment method with an

accuracy of ± 0.15 mN m-1

at room temperature of 298.15 K. A mixing time of two minutes

followed by an equilibration time of 5 minutes was adopted before each measurement

1.3 UV-visible measurements. The absorption spectra were recorded on a UV-1800

Shimadzu spectrophotometer with a quartz cuvette having a path length of 1 cm. The titrations

were performed at 298.15 K by successive additions of small aliquots of stock solutions of

alkyloxypyridinium surfactants directly into the quartz cuvette containing 2.0 mL of 0.02 mmol

dm-3

ARS solution. After every addition, the solution was equilibrated for 5 minutes to reach

thermal equilibrium. The spectra were recorded in the range 350-700 nm.

1.4 Voltammetric measurements. Voltammetric measurements were performed on a PC

controlled CHI660D (Austin, USA) electrochemical workstation equipped with a conventional

three electrode system comprising of a working glassy electrode (3.0 mm in diameter), a counter

Pt wire and a reference Ag/AgCl electrode. All the solutions were deoxygenated with N2 and

working electrodes were polished with slurry of alumina powder. The voltammetric

measurements were carried out in the presence of 0.1 M phosphate buffer having pH of 7.4 as a

supporting electrolyte.

1.5 Potentiometric measurements. The potentiometric measurements were carried out by

using an Equiptronics digital potentiometer, Model EQ-602 employing the following

electrochemical cell assembly:

Ag/AgCl 3M KCl Test Solution PVC

membrane

Internal Reference

Solution

3M KCl Ag/AgCl

S3

The neutral ion-pair complexes of alkyloxypyridinium surfactants and sodium

dodecylsulfate as [HECnOPy] +DS ̄ were prepared by the procedure earlier reported by our

laboratory [1, 2]. Equimolar aqueous solutions of [HECnOPyBr], n = 14, 16 and sodium

dodecylsulfate (SDS) were mixed and after continuous stirring for a considerable time, the white

precipitates of HECnOPy+DS ̄ were obtained . The precipitates so obtained were washed

repeatedly with water to remove NaCl and recrystallized thrice from acetone. The PVC (176

mg), ion-pair (5 mg) and plasticizer (550 mg) were mixed and dissolved in minimum quantity of

THF. The resulting mixture was poured in 50-mm petri dish after removing the air-bubbles. The

solvent THF was allowed to evaporate at room temperature. The resulting membrane was cut to

required size and attached to PVC tubes with PVC glue and equilibrated in 1 mmol dm-3

of

respective surfactant solution. The internal reference solution was 1mM of these surfactants in

1 mmol dm-3

NaCl. The given composition of the components used for membrane formation

represents the best system in terms of slope values, correlation coefficient, linear range and

detection limit. The EMF measurements were carried out by titration method at 298.15K in the

presence of 1 mmol dm-3

NaCl solution.

1.6 Dynamic light scattering measurements. Dynamic light scattering measurements were

performed using a Malvern Nano-ZS Zetasizer instrument employing a He-Ne laser (λ = 632

nm) at a scattering angle of 173˚ using quartz cuvette having path length of 1 cm. The

temperature of the measurements was controlled to an accuracy of ±0.1°C using a built-in

temperature controller.

1.7 1H NMR measurements.

1H NMR experiments were performed on a 500 MHz JEOL-FT

NMR-AL spectrometer using CD3CN as solvent. The NMR titration experiments were

performed by titrating one equivalent of ARS prepared in CD3CN with increasing equivalents of

alkyloxypyridinium surfactants. Chemical shifts were given on the δ scale.

S4

Annexure SII.

The maximum surface excess concentration (Гmax) and minimum area per molecule [3, 4] at the

air-solution interface can be calculated using equation (1) and (2) as follows:

(1)

Amin =1020

/ (NA. Гmax) (2)

The Gibbs free energy of adsorption (ΔG˚ads) [5] is calculated using the equation (3)

ΔG˚ads= ΔG˚mic - Пcmc/ Гmax (3)

maxln

nRTCd

d

T

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System C1(mmol dm-3

) C2 = cmc(mmol dm-3

)

ARS - -

[HEC14OPyBr] - 0.83

ARS+[HEC14OPyBr] 0.05 0.78

[HEC16OPyBr] - 0.16

ARS+[HEC16OPyBr] 0.02 0.13

ARS + [HEC14OPyBr]

xARS 0.0 0.1 0.3 0.5 0.7 0.9

Dh / nm 1.2 4.50 11.7 25.8 18.6 10.4

ARS + [HEC16OPyBr]

xARS 0.0 0.1 0.3 0.5 0.7 0.9

Dh / nm 1.5 6.20 13.5 28.2 21.3 12.8

Table S2. Hydrodynamic diameter (Dh) for ARS-alkoxypyridinium surfactants, [HEC14OPyBr]

and [HEC16OPyBr] mixed systems at varying mole fractions of ARS (xARS).

Table S1. The concentration corresponding to ion-pair formation (C1), critical micelle

concentration (cmc) of alkyloxypyridinium surfactants, [HEC14OPyBr] and [HEC16OPyBr]

surfactants in the absence and presence of ARS determined from potentiometric measurements.

S6

ARS-[HEC14OPyBr]

system

1H-NMR

chemical shift

range in δ

(ppm) for

aromatic

protons of

ARS

Observations

after addition

of surfactant

solution

1H-NMR

chemical shift

range in δ (ppm)

for aromatic and

aliphatic protons

of surfactants

Observations after

addition of

surfactant solution

ARS 11.52 (s, H1)

11.79 (s, H2)

3.99 (m, Hy),4.23

(t, Hn), 4.65 (m,

Hx), 8.38 (d, Hq),

8.53 (s, Hr)

1 equivalent ARS+

0.25 equivalent of

[HEC14OPyBr]

11.51 (s, H1)

12.79 (s, H1)

Upfield shift

for H1 proton.

3.96 (m, Hy),4.22

(t, Hn), 4.64 (m,

Hx), 8.37 (d, Hq),

8.51 (s, Hr)

Upfield shift for

all the protons.

1 equivalent ARS+

0.5 equivalent of

[HEC14OPyBr]

11.49 (s, H1)

12.79 (s, H1)

Upfield shift

for H1 proton.

3.98 (m, Hy),4.23

(t, Hn), 4.67 (m,

Hx), 8.40 (d, Hq),

8.57 (s, Hr)

Downfield shift

for all the protons.

1 equivalent ARS+

1.0 equivalent of

[HEC14OPyBr]

11.48 (s, H1)

12.79 (s, H1)

Upfield shift

for H1 proton.

3.98 (m, Hy),4.25

(t, Hn), 4.69 (m,

Hx), 8.42 (d, Hq),

8.60 (s, Hr)

Downfield shift

for all the protons.

1 equivalent ARS+

1.5 equivalent of

[HEC14OPyBr]

11.47 (s, H1)

12.79 (s, H1)

Upfield shift

for H1 proton.

3.99 (m, Hy),4.23

(t, Hn), 4.70 (m,

Hx), 8.43 (d, Hq),

8.61 (s, Hr)

Downfield shift

for all the protons.

Table S3. Chemical shifts and observations determined by 1H-NMR titrations of alizarin red S

(ARS) with increasing equivalents of alkyloxypyridinium surfactant, [HEC14OPyBr].

S7

ARS-[HEC16OPyBr]

system

1H-NMR

chemical shift

range in δ

(ppm) for

aromatic

protons of

ARS

Observations

after addition

of surfactant

solution

1H-NMR

chemical shift

range in δ (ppm)

for aromatic and

aliphatic protons

of surfactants

Observations after

addition of

surfactant solution

ARS 11.52 (s, H1)

11.79 (s, H2)

3.99 (m, Hy),4.23

(t, Hn), 4.65 (m,

Hx), 8.38 (d, Hq),

8.53 (s, Hr)

1 equivalent ARS+

0.50 equivalent of

[HEC16OPyBr]

11.53 (s, H1)

12.79 (s, H1)

Upfield shift

for H1 proton.

3.97 (m, Hy),4.21

(t, Hn), 4.57 (m,

Hx), 8.39(s, Hq)

Upfield shift for

all the protons.

1.0 equivalent ARS+

1.0 equivalent of

[HEC16OPyBr]

11.49 (s, H1)

12.79 (s, H1)

Upfield shift

for H1 proton.

3.98 (s, Hy),4.23

(t, Hn), 4.64 (s,

Hx), 8.35 (d, Hq),

8.52 (s, Hr)

Downfield shift

for all the protons.

1.0 equivalent ARS+

1.5 equivalent of

[HEC16OPyBr]

11.48 (s, H1)

12.79 (s, H1)

Upfield shift

for H1 proton.

3.98 (s, Hy),4.23

(s, Hn), 4.66 (s,

Hx), 8.39 (s, Hq),

8.55 (s, Hr)

Downfield and

upfield shift for

protons.

Table S4. Chemical shifts and observations determined by 1H-NMR titrations of alizarin red S

(ARS) with increasing equivalents of alkyloxypyridinium surfactant, [HEC16OPyBr].

S8

400 500 600 700

0.0

0.2

0.4

0.6

0.8

1.0

[HEC16OPyBr](A)

Ab

sorb

an

ce

Wavelength / nm

0.000mmol dm-3

0.005mmol dm-3

0.009mmol dm-3

0.015mmol dm-3

0.020mmol dm-3

0.039mmol dm-3

0.077mmol dm-3

0.095mmol dm-3

0.113mmol dm-3

0.165mmol dm-3

0.182mmol dm-3

0.214mmol dm-3

0.276mmol dm-3

0.330mmol dm-3

10-5

10-4

0.25

0.30

0.35

0.40

0.45

0.50

0.55

0.60

0.65

0.70

(B)

Ab

sorb

an

ce

C / mol dm-3

420

440

460

480

500

520

540

560[HEC16OPyBr]

Wa

vele

ng

th (

ma

x) / n

m

-0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1

-0.40

-0.35

-0.30

-0.25

-0.20

-0.15

-0.10

-0.05

0.00

0.05

[HEC16OPyBr]

A

Xsurf

(C)

-6.5 -6.0 -5.5 -5.0 -4.5 -4.0 -3.5

35

40

45

50

55

60

65

[HEC16OPyBr]

/

mN

m-1

log C / mol dm-3

0.00 mmol dm-3ARS

0.02 mmol dm-3ARS

(B)

0.0000 0.0001 0.0002 0.0003

0

10

20

30

40

50

60[HEC16OPyBr]

0.000 mmol dm-3 ARS

0.020 mmol dm-3 ARS

0.050 mmol dm-3 ARS

0.075 mmol dm-3 ARS

0.100 mmol dm-3 ARS

(A)

/

S

cm

-1

C / mol dm-3

Fig.S1 (A) Variation of specific conductivity (κ) with molar concentration of [HEC16OPyBr] in the

absence and presence of varying amounts of ARS. (B) Variation of surface tension (γ) as a function of

logarithm of molar concentration of alkyloxypyridinium surfactant, [HEC16OPyBr] in the absence and

presence of 0.02 mmol dm-3

ARS.

Fig.S2 (A) UV-visible spectrum of 0.02 mmol dm-3

ARS in the presence of increasing concentrations

of alkyloxypyridinium surfactant, [HEC16OPyBr] (B) Plot of Absorbance and λmax vs increasing

concentration of [HEC16OPyBr] (C) Job’s plot depicting 1:1 stoichiometry of ARS-[HEC16OPyBr]

mixed system.

S9

Fig.S4 (A) Linear sweep voltammograms (LSV) of 0.05 mmol dm-3

alkyloxypyridinium surfactant,

[HEC14OPyBr] in ARS at various scan rates of potential (B) LSV of 0.25 mmol dm-3

[HEC14OPyBr]

in ARS at various scan rates of potential (C) LSV of 1.0 mmol dm-3

[HEC14OPyBr] in ARS at various

scan rates of potential

-1.5 -1.0 -0.5 0.0 0.5

-1.5x10-5

-1.0x10-5

-5.0x10-6

0.0

5.0x10-6 (A) [HEC16OPyBr]

0.000mmol dm-3

0.007mmol dm-3

0.013mmol dm-3

0.020mmol dm-3

0.026mmol dm-3

0.033mmol dm-3

0.039mmol dm-3

0.046mmol dm-3

I p /

A

Potential / V

-1.5 -1.0 -0.5 0.0 0.5 1.0

-1.5x10-5

-1.0x10-5

-5.0x10-6

0.0

5.0x10-6

-0.2 0.0 0.2 0.4 0.6 0.8

2.0x10-6

4.0x10-6

6.0x10-6

I p /

A

Potential / V

[HEC16OPyBr]

I p /

A

Potential / V

(B)

Fig.S3 (A) Cyclic voltammograms of ARS in the presence of increasing concentrations of

alkyloxypyridinium surfactant, [HEC16OPyBr] (B) Linear sweep voltammograms of ARS in the

presence of increasing concentrations of [HEC16OPyBr]

-1.5 -1.0 -0.5 0.0 0.5

-1.2x10-5

-9.0x10-6

-6.0x10-6

-3.0x10-6

0.0

3.0x10-6

0.05 mmol dm-3

[HEC14OPyBr] in ARS

Ip

/ A

Potential / V

0.01 V

0.02 V

0.03 V

0.04 V

0.05 V

(A)

-1.5 -1.0 -0.5 0.0 0.5

-1.0x10-5

-5.0x10-6

0.0

5.0x10-6

I p /

A

Potential / V

0.25 mmol dm-3

[HEC14OPyBr] in ARS

0.01 V

0.02 V

0.03 V

0.04 V

0.05 V

(B)

-1.5 -1.0 -0.5 0.0 0.5 1.0

-1.5x10-5

-1.0x10-5

-5.0x10-6

0.0

5.0x10-6

I p /

A

Potential / V

0.01 V

0.02 V

0.03 V

0.04 V

0.05 V

1.0 mmol dm-3

[HEC14OPyBr] in ARS

(C)

S10

-1.5 -1.0 -0.5 0.0 0.5

-1.0x10-5

-5.0x10-6

0.0

5.0x10-6

I p /

A

Potential / V

0.01 V

0.02 V

0.03 V

0.04 V

0.05 V

0.01 mmol dm-3

[HEC16OPyBr] in ARS

(A)

-1.5 -1.0 -0.5 0.0 0.5

-1.5x10-5

-1.2x10-5

-9.0x10-6

-6.0x10-6

-3.0x10-6

0.0

3.0x10-6

6.0x10-6

0.01 V

0.02 V

0.03 V

0.04 V

0.05 V

0.05 mmol dm-3

[HEC16OPyBr] in ARS

I p /

A

Potential / V

(B)

-1.5 -1.0 -0.5 0.0 0.5

-1.5x10-5

-1.0x10-5

-5.0x10-6

0.0

5.0x10-6

I p /

A

Potential / V

0.01 V

0.02 V

0.03 V

0.04 V

0.05 V

0.35 mmol dm-3

[HEC16OPyBr] in ARS

(C)

0.10 0.15 0.20 0.25

1.0x10-6

2.0x10-6

3.0x10-6

4.0x10-6

I p /

A

v1/2

[HEC16OPyBr]

0.00 mmol dm-3

0.01 mmol dm-3

0.10 mmol dm-3

0.35 mmol dm-3

(D)

Fig.S5 (A) LSV of 0.01 mmol dm-3

[HEC16OPyBr] in ARS at various scan rates of potential (B)

LSV of 0.05 mmol dm-3

[HEC16OPyBr] in ARS at various scan rates of potential (C) LSV of 0.35

mmol dm-3

[HEC16OPyBr] in ARS at various scan rates of potential (D) Variation of Ip (A) vs v1/2

for ARS in the absence and presence of increasing concentrations of [HEC16OPyBr].

S11

Fig.S6 Binding constant determination for the ARS-[HECnOPyBr] mixed systems using (A) changes

in UV-visible spectra of ARS (B) changes in the peak current of ARS.

4.0x103

6.0x103

8.0x103

5.0x104

1.0x105

2x105

3x105

4x105

5x105

6x105

7x105

8x105

9x105

1/

p

1/C

[HEC14OPyBr]

[HEC16OPyBr]

(B)

-5.5 -5.0 -4.5 -4.0 -3.5

-0.15

-0.10

-0.05

0.00

0.05

0.10

EM

F /

V

log C / mol dm-3

[HEC16OPyBr]

0.00mmol dm-3

0.02mmol dm-3

(A)

-5.5 -5.0 -4.5 -4.0 -3.5

1.0x109

1.2x109

1.4x109

1.6x109

1.8x109

(B)

log C / mol dm-3

[HEC16OPyBr]

v

Fig.S7 (A) EMF as a function of logarithm of molar concentration of alkyloxypyridinium surfactant,

[HEC16OPyBr] in the absence and presence of 0.02 mmol dm-3

ARS (B) Binding isotherms of

binding parameter (v) versus logarithm of molar concentration of [HEC16OPyBr] in the presence of

0.02 mmol dm-3

ARS.

0 1x104

2x104

3x104

4x104

0

10

20

30

40

50

60

70

1/C

[HEC14OPyBr]

[HEC16OPyBr]

1/

(A)

S12

N+

O

HO

Br-

q

p

or

n l

k

j

i g

f d b

e c a

x

y

m

h

s

Hy

Ha

Hr

Hn

Hx

Hm

Hl

Hq

Ho

Hp

Hb-Hk

(A)

SO3Na

O

O

OH

OH

HH

H

H

H

1

2

3

4

5

6

7

H3-H4

H7

H2

H1

H6

H5

(B)

Fig.S8 (A) 1H-NMR spectra of pure alkyloxypyridinium surfactant, [HEC14OPyBr] (B)

1H-

NMR spectra of pure ARS

S13

1.0 eq.of ARS+0.50 eq.of [HEC16OPyBr]

1.0 eq.of ARS+1.0 eq.of [HEC16OPyBr]

1.0 eq.of ARS+1.5 eq.of [HEC16OPyBr]

[HEC16OPyBr]

ARS

Fig.S9 1H-NMR titrations of ARS with increasing equivalents of alkyloxypyridinium surfactant,

[HEC16OPyBr]

S14

References

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Sci. 2008, 326, 191-195.

3. D. K. Chattoraj, K. S. Birdi, Adsorption and Gibbs Surface Excess, Plenum, New York,

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