Unique role of ionic liquid bminBF4 during curcumin–surfactant association and micellization of cationic, anionic and non ionic surfactant solutions

Spectrochimica

Acta

Part

A

79 (2011) 1823–

1828
Contents

lists

available

at

ScienceDirect
Spectrochimica

Acta

Part

A:

Molecular

and

Biomolecular

Spectroscopy
j

ourna

l

ho

me

page:

www.elsevier.com/locate/saa
Unique

role

of

ionic

liquid

[bmin][BF
4
]

during

curcumin–surfactant

association
and

micellization

of

cationic,

anionic

and

non-ionic

surfactant

solutions
Digambara

Patra
∗
,

Christelle

Barakat
Department

of

Chemistry,

Faculty

of

Arts

and

Sciences,

American

University

of

Beirut,

P.O.

Box:

11-0236,

Riad

El

Solh,

Beirut,

1107-2020,

Lebanon
a

r

t

i

c

l

e

i

n

f

o
Article

history:
Received

30

September

2010
Received

in

revised

form

17

May

2011
Accepted

24

May

2011
Keywords:
Curcumin
Hydrophilic

ionic

liquid
Micelle
Surfactant
Spectroscopy
a

b

s

t

r

a

c

t
Hydrophilic

ionic

liquid,

1-butyl-3-methylimidazolium

tetrafluoroburate,

modified

the

properties

of
aqueous

surfactant

solutions

associated

with

curcumin.

Because

of

potential

pharmaceutical

applications
as

an

antioxidant,

anti-inflammatory

and

anti-carcinogenic

agent,

curcumin

has

received

ample

attention
as

potential

drug.

The

interaction

of

curcumin

with

various

charged

aqueous

surfactant

solutions

showed
it

exists

in

deprotonated

enol

form

in

surfactant

solutions.

The

nitro

and

hydroxyl

groups

of

o-nitrophenol
interact

with

the

carbonyl

and

hydroxyl

groups

of

the

enol

form

of

curcumin

by

forming

ground

state

complex

through

hydrogen

bonds

and

offered

interesting

information

about

the

nature

of

the

interac-
tions

between

the

aqueous

surfactant

solutions

and

curcumin

depending

on

charge

of

head

group

of

the
surfactant.

IL[bmin][BF
4
]

encouraged

early

formation

of

micelle

in

case

of

cationic

and

anionic

aqueous
surfactant

solutions,

but

slightly

prolonged

micelle

formation

in

the

case

of

neutral

aqueous

surfactant
solution.

However,

for

curcumin

IL

[bmin][BF
4
]

favored

strong

association

(7-fold

increase)

with

neutral
surfactant

solution,

marginally

supported

association

with

anionic

surfactant

solution

and

discouraged
(∼2-fold

decrease)

association

with

cationic

surfactant

solution.
© 2011 Elsevier B.V. All rights reserved.
1.

Introduction
Micellar

systems

of

aqueous

origin

have

immense

technological
applications

as

flow

field

regulators,

solubilizing

and

emulsify-
ing

agents,

membrane

mimetic

media,

nanoreactors

for

enzymatic
reaction

and

drug

delivery

system

[1–8].

It

is

anticipated

that
curcumin,

1,7-bis(4-hydroxy-3-methoxyphenyl)-1,6-heptadiene-
3,5-dione,

may

find

applications

as

a

novel

drug

in

the

near
future

to

control

various

diseases,

including

inflammatory

dis-
orders,

carcinogenesis

and

oxidative

stress-induced

pathogenesis
[9–12].

Curcumin

has

drawn

intense

interest

recently

due

to

its
potential

pharmaceutical

importance

[13–24].

However,

curcumin
is

very

poorly

soluble

in

water

by

reducing

its

effectiveness

as

a
drug.

Therefore,

various

methods

are

being

developed

to

make

cur-
cumin

better

soluble

and

enhance

effectiveness

of

the

drug

during
its

delivery

[16].
Physiochemical

properties

of

an

aqueous

surfactant

solution
depend

on

the

identity

of

the

surfactant.

The

aqueous

solution

of
a

surfactant

at

a

given

concentration

posses

more

or

less

fixed
physiochemical

properties

that

are

difficult

to

modulate.

Other
than

changing

temperature

and

pressure,

the

usual

way

to

mod-
ify

the

physiochemical

properties

of

a

given

surfactant

solution
is

to

use

external

additives,

such

as

cosolvents,

cosurfactants,
∗
Corresponding

author.

Tel.:

+961

1350

000x3985;

fax:

+961

1365217.
E-mail

address:



(D.

Patra).
electrolytes,

non-polar

organics,

polar

organics,

etc.

Ionic

liquids
(ILs)

are

solvents

composed

entirely

of

ions

and

composed

of
poorly

coordinating

ions

and

can

therefore

be

highly

polar

yet
non-coordinating

[25–27].

These

are

immiscible

with

a

number
of

organic

solvents

and

provide

non-aqueous

polar

alternatives
for

two

phase

systems.

They

are

of

particular

interest

because

of
their

environmentally

friendly

nature,

their

exciting

features

and
their

economical

convenience

[28–35].

The

unusual

properties

of
ILs

demonstrate

a

unique

role

in

altering

the

properties

of

aqueous
surfactant

solutions

such

as

aggregation

number

[3,4].

The

effec-
tiveness

of

this

modification

of

aqueous

surfactant

solutions

by

IL
may

largely

depend

on

the

kind

and

extent

of

interaction/s

between
cation/anion

of

the

IL

and

the

head

group

of

the

surfactant

[4].

How-
ever,

hydrophobic

effect

of

IL

with

surfactant

molecule

might

play
a

role.

In

addition

we

hypothesize

that

IL

may

drive

the

associa-
tion

of

the

drug

molecule

towards

better

solubilization

in

micellar
system

(which

is

very

important

during

drug

delivery)

as

per

the
head

group

of

the

surfactant

charge

and

physiochemical

properties
of

the

drug

molecule.
In

order

to

understand

the

better

insight

of

the

role

of

these
interactions

of

IL

during

solubilization

of

poorly

water

soluble
drug

such

as

curcumin

in

micellar

systems

and

micellization,

we
extend

the

study

of

interaction

of

IL

and

surfactant

solutions

[4]
further

to

systems

composed

of

various

(positive

and

negative)
charged

and

uncharged

surfactant

solutions,

curcumin

and

an
IL

(1-butyl-3-methylimidazolium

tetrafluoroburate,

[bmin][BF
4
]).
The

association

of

curcumin

with

various

charged

surfactant
1386-1425/$

–

see

front

matter ©

2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2011.05.064
1824 D.

Patra,

C.

Barakat

/

Spectrochimica

Acta

Part

A

79 (2011) 1823–

1828
solutions

and

fluorescence

quenching

of

curcumin

by

o-
nitrophenol

in

different

surfactant

solutions

may

explore

the

kind
of

interaction

between

curcumin

and

various

charged/uncharged
surfactant

solutions

without

IL.

Due

to

cation/anion

of

the

IL,

it

may
remarkably

alter

the

interaction

of

curcumin

and

surfactant

solu-
tions

based

on

the

charge

of

the

head

group

of

the

surfactant

and
deprotonated

form

of

curcumin,

therefore

impact

drug–surfactant
association.

Comparative

study

of

various

charged/uncharged

sur-
factant

molecules

may

conclude

importance

of

hydrophobic

effect
of

IL

during

micellization.
2.

Materials

and

methods
2.1.

Materials
The

surfactants

cetyl

trimethyl

ammonium

bromide

(CTAB),
sodium

dodecyl

sulfate

(SDS)

and

Triton

X-100

(TX100)

were
obtained

from

Acros

Organics

and

were

dissolved

in

different
volumes

of

double

distilled

water

for

the

preparation

of

several

con-
centrations

of

surfactant

solutions.

The

stock

solutions

consisted

of
10

mM

CTAB,

100

mM

SDS

and

10

mM

TX100.

Curcumin

was

also
obtained

from

Acros

Organics

and

was

used

without

further

purifi-
cation.

To

prepare

the

stock

solution,

curcumin

was

dissolved

in
spectroscopic

grade

acetonitrile

(Acros

Organics)

so

that

the

final
concentration

of

acetonitrile

in

the

surfactant

solutions

remained
less

than

1%

(v/v).

1-Butyl-3-methylimidazolium

tetrafluoroburate,

[bmin][BF4]

was

obtained

from

Fluka

and

o-nitrophenol

was

a
Merck

Schuchardt

product.

The

solvents

were

used

without

further
purification.
2.2.

Spectroscopic

measurements
The

absorption

spectra

in

various

solvents

and

in

cationic

CTAB,
anionic

SDS,

and

neutral

TX100

were

recorded

at

room

temperature
using

a

JASCO

V-570

UV–VIS–NIR

Spectrophotometer.

Fluores-
cence

measurements

were

done

on

a

JOBIN

YVON

Horiba

Fluorolog
3

spectrofluorometer.

The

excitation

source

was

a

100

W

Xenon
lamp.

The

detector

used

was

R-928

operating

at

a

voltage

of

950

V.
The

excitation

and

emission

slits

width

were

5

nm.

The

spectral
data

were

collected

using

Fluorescence

software

and

data

analysis
was

made

using

OrginPro

6.0

software.
3.

Results

and

discussion

3.1.

Curcumin–surfactant

interaction

in

absence

of

IL
Generally,

curcumin

showed

a

strong

and

intense

absorption
band

in

the

350–480

nm

wavelength

region

in

all

the

investi-
gated

surfactant

solutions.

Representative

absorption

spectra

of
curcumin

in

various

concentrations

of

TX100

solutions

are

depicted
in

Fig.

1.
The

interaction

between

curcumin

and

micelles

can

be
described

as:
C

+

S
K
b
CS
where

C

is

curcumin;

S

is

the

surfactant

(CTAB,

SDS

or

TX100);
CS

is

the

curcumin–surfactant

complex;

and

K
b
is

the

association
constant.
The

concentration

of

the

micellized

surfactant

is

given

by:
S
m
=

S
s
−

cmc
where

S
s
is

the

surfactant

concentration.
700600500400300
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
7-9
6
5
4
3
2
1
Curcu

min with [TX100]
Absorbance
Wavelength

(nm

)
(1)

0.0

2 mM
(2)

0.0

4 mM
(3)

0.1

mM
(4)

0.2

mM
(5)

0.4

mM
(6)

0.6

mM
(7)

1.0

mM
(8)

1.2

mM
(9)

1.4

mM
Fig.

1.

Absorption

spectra

of

curcumin

in

various

aqueous

TX100

concentrations.
Table

1
Association

rate

constants

of

curcumin

with

various

aqueous

surfactant

solutions
in

the

absence

and

presence

of

ionic

liquid.
Sample cmc

used

for

calculation

(mM)

K
b
SDS

7.3

6193

M
−1
CTAB

0.8

20,467

M
−1
TX100

0.2

11,555

M
−1
SDS

+

IL

(1%,

v/v)

0.95

6315

M
−1
CTAB

+

IL

(1%,

v/v) 0.1

10,227

M
−1
TX100

+

IL

(1%,

v/v)

0.4

82,737

M
−1
The

association

constants

can

be

determined

[6,36–39]

as:
C
T

S
m
l
A
=
S
m
ε
s
−

ε
0
+
1
K
gb
(ε
s
−

ε
0
)
where

l

is

the

optical

path

length,

ε
m
is

the

molar

excitation

coeffi-
cient

of

curcumin

fully

bound

to

micelles,

ε
0
is

the

molar

excitation
coefficient

of

curcumin

in

the

solvent,

C
T
is

the

total

curcumin

con-
centration

and

A

=

A

− A
0
where

A

is

the

absorbance

of

curcumin

in

the

presence

of

surfactant

solution

and

A
0
is

the

absorbance

of
curcumin

in

the

absence

of

micelle/surfactant.
Using

Scott’s

plots

[6,36–39],

the

association

constants

of

CTAB,
SDS

and

TX100

were

determined

to

be

20,467

M
−1
,

6193

M
−1
and
11,555

M
−1
(Table

1),

respectively.

It

should

be

noted

that

the

crtical
micellar

concentration

(cmc)

for

the

calculation

of

association

con-
stants

for

various

micelle

was

estimated

by

fluorescence

method
as

explained

later

on.

It

is

observed

that,

K

b

CTAB
>

K
b

TX100
>

K
b

SDS
.
These

results

implied

that

the

different

micelles

have

different
affinities

for

curcumin.

Cationic

CTAB

is

bound

to

curcumin

with
the

highest

affinity,

followed

by

neutral

TX100

and

then

anionic
SDS.

This

could

be

due

to

the

electrostatic

interactions

between

cur-
cumin

and

the

positive

charge

on

the

head

group

of

CTAB

present

in
the

Stern

layer

of

the

micelle,

thus

indicating

that

curcumin

at

the
given

conditions

is

mainly

found

in

its

deprotonated

anionic

forms
[40]

(see

Supplement

1).

In

the

case

of

SDS,

the

repulsion

between
deprotonated

enol

(anionic)

forms

of

curcumin

and

the

negative
charge

on

the

head

group

of

SDS

present

in

the

Stern

layer

of

the
micelle

make

a

weaker

interaction,

hence

decreasing

the

associa-
tion

rate

constant.

However,

given

that

the

head

group

of

TX100

is
nonionic,

the

value

of

the

association

rate

constant

for

TX100

was
in

between

that

of

CTAB

and

SDS.

3.2.

Critical

micellar

concentration

determination
Fluorescence

excitation

and

emission

spectra

of

curcumin

with
various

concentrations

of

surfactant

noted

that

the

fluorescence
D.

Patra,

C.

Barakat

/

Spectrochimica

Acta

Part

A

79 (2011) 1823–

1828 1825

300 35

040

045

0

500 55

060

065

070

0
0.0
4.0x10
6
8.0x10
6
1.2x10
7
1.6x10
7
2.0x10
7
10
8-9

Curc

umin

with [TX100

]
(1) No

TX10

0
(2) 0.02

mM
(3) 0.04

mM
(4) 0.06

mM
(5) 0.1 mM
(6) 0.2 mM
(7) 0.6 mM
(8) 0.8 mM
(9) 1.0 mM
(10

) 1.2

mM
(11

) 1.4

mM
(12

) 1.6

mM
Fluorescence Intensity (a.u)
Wavelength

(nm

)
0.0
4.0x10
6
8.0x10
6
1.2x10
7
1.6x10
7
2.0x10
7
2.4x10
7

2.8x10
7
11-12
11-12
10
8-9
7
7
6
6
1-5
1-5
Fig.

2.

Fluorescence

excitation

and

emission

spectra

of

curcumin

in

various

aqueous
TX100

concentrations.
intensity

of

the

emission

and

excitation

spectra

of

curcumin

in
TX100

(shown

in

Fig.

2)

and

SDS

(not

shown)

increased

as

the
concentration

of

the

surfactant

was

increased.

However,

the

flu-
orescence

spectra

of

CTAB

exhibited

a

different

behavior

(not
shown).
The

fluorescence

intensity

initially

decreased

until

it

reached
0.5

mM

of

CTAB

and

once

the

cmc

was

reached,

the

intensity

started
increasing

with

concentration.

A

red

shift

was

also

observed

after
the

cmc

for

CTAB.

The

Stokes’

shift

of

curcumin

in

various

concen-
trations

of

CTAB,

SDS

and

TX100

was

determined

as

the

difference
between

absorption

and

emission

maxima

obtained

from

the

cor-
rected

spectra

on

the

wavenumber

scale

[41,42].

The

plot

of

Stokes’
shift

versus

surfactant

concentration

offered

three

different

kinds
of

change,

respectively,

for

cationic

(CTAB),

anionic

(SDS)

and

neu-
tral

(TX100)

surfactant

solutions.

In

the

case

of

CTAB,

the

value

of
Stokes’

shift

rarely

changed

before

the

cmc.

A

big

jump

of

5000

cm
−1
was

observed

around

the

cmc

and

after

the

cmc

it

remained

more
or

less

unaltered.

The

cmc

of

CTAB

was

estimated

by

finding

the
midpoint

of

the

tangent

joining

the

two

lines,

as

shown

in

Fig.

3A.
For

SDS,

Stokes’

shift

of

curcumin

for

different

surfactant

con-
centrations

varied

differently,

it

initially

decreased

till

the

cmc

was
reached.

Above

the

cmc,

it

marginally

increased.

By

extrapolating
these

two

linear

equations,

before

and

after

the

cmc,

with

respec-
tive

negative

and

positive

slopes,

a

minimum

intersecting

point

was
obtained

to

calculate

the

cmc

(Fig.

3B).

Stokes’

shift

of

curucmin
increased

with

TX100

concentration

until

cmc

was

attained

and
then

it

decreased

dramatically.

In

this

case

the

maximum

value

of
Stokes’s

shift

was

used

to

estimate

cmc

as

marked

in

Fig.

3C.

The

cmc
values

estimated

using

Stokes’

shift

of

curcumin

is

summarized

in
Table

2,

the

values

obtained

without

IL

are

similar

to

the

reported
values

[4,5,43]

establishing

the

reliability

of

the

method.

The

differ-
Table

2
cmc

values

of

aqueous

CTAB,

SDS

and

TX100

solutions

in

the

presence

and

absence
of

ionic

liquid.
Sample

cmc

Curcumin


(cm
−1
)
Pyrene

I
I
/I
III
a
Reported
b
SDS 7.3

mM

7.0

mM

6.0–8.0

mM
CTAB

0.8

mM

–

0.26

mM
TX100

0.2

mM

0.25–0.5

mM

0.9

mM
SDS

+

IL

(1%,

v/v)

0.95

mM

1

mM

(2%,

v/v)

–
CTAB

+

IL

(1%,

v/v)

0.1

mM

–

–

TX100

+

IL

(1%,

v/v) 0.4

mM

0.5–1.0

mM

(2%,

v/v)

–
a
From

Refs.

[3,4].
b
From

Ref.

[43].
0.0000 0.000

5

0.0010 0.001

5

0.0020
5000
6000
7000
8000
9000
1000

0
1100

0
1200

0
A
Stokes' shift (cm
-1
)

[CTAB]
CTAB
0.0000

0.0005

0.0010

0.0015

0.0020
35000
40000
45000
50000
55000
60000
65000
70000
cmc of CTAB

+ IL
cmc of

CT

AB
CTAB

+ IL

0.000

0.005

0.010

0.015

0.020
4000
4200
4400
4600
4800
5000
5200
5400
B
cmc of SDS + IL
Stokes' shift (cm
-1
)
[SDS]
SDS
0.00

0

0.005

0.010

0.015

0.020
18000
18200
18400
18600
18800
19000
19200
19400
19600
19800
cmc of S

DS
SDS + IL
0.000

0

0.000

3

0.0006

0.000

9

0.00

12

0.001

5

0.00

18
3000
3500
4000
4500
5000
5500
6000
C
Stokes' shift (cm
-1
)
[TX10

0]
TX100
0.00

00

0.000

3

0.00

06

0.000

9

0.0012

0.001

5

0.0018
0
1000
2000
3000
4000
5000
cmc of Tx10

0 + I

L
cmc of TX100
TX100 + IL
Fig.

3.

Variation

of

Stokes’

shift

of

curcumin

in

different

concentrations

of

aqueous

CTAB

(A),

SDS

(B)

and

TX100

(C)

in

the

absence

and

presence

of

IL.
ent

trends

of

Stokes’s

shift

for

various

surfactants

could

be

due

to
the

various

kinds

of

interactions

between

the

charged/uncharged
head

groups

of

the

surfactants

and

the

deprotonated

forms

of

cur-
cumin.
3.3.

Quenching

study

by

o-nitrophenol
o-Nitrophenol

can

strongly

quench

the

fluorescence

of

cur-
cumin

by

forming

a

ground

state

complex

through

hydrogen
bonding

[24]

as

given

in

Scheme

1.
However,

the

extent

to

which

it

quenches

may

highly

depend

on
the

conditions

of

the

medium

in

which

curcumin

and

o-nitrophenol
1826 D.

Patra,

C.

Barakat

/

Spectrochimica

Acta

Part

A

79 (2011) 1823–

1828
HO
O
O
H
3
CO
OCH
3

OH
H
H
O
O
-
N
O
Formation

cyclic groun

d stat

e compl

ex of curc

umin with

o-nitroph

enol
Scheme

1.

Ground

state

complex

formation

of

curcumin

with

o-nitrophenol

causing

fluorescence

quenching

of

curcumin

by

o-nitrophenol.
can

interact

and

hence,

on

the

nature

of

the

surfactants.

The
position

of

the

functional

groups

in

o-nitrophenol

and

the

geom-
etry

of

the

molecule

predict

the

location

of

o-nitrophenol

in

the
micelle

[44].

The

benzene

ring

of

the

phenol

is

pushed

towards

the
hydrocarbon

core

and

the

polar

functional

groups

remain

in

the
hydrophilic

layer

of

the

micelle

[44].

Given

that

the

stoichiometric
ratio

of

o-nitrophenol

to

curcumin

is

1:1,

the

nitro

and

hydroxyl
groups

of

the

quencher

interact

with

the

carbonyl

and

hydroxyl
groups

of

the

enol

form

of

curcumin

by

means

of

strong

hydrogen
bonds

[24].

This

associated

complex,

which

is

formed

in

the

ground
state,

greatly

quenches

the

fluorescence

of

curcumin

through

the
following

process:
curcumin* + o-nitrophe

nol [curcumin- o-nitrophenol

]* [curcumin- o-nitrophenol

]
curcumin + o-nitrophenol
[curcumin- o-nitrophenol

] [curcumin- o-nitrophenol

]*
hν
a
hν
a
hν

fl
hν
fl
Using

the

Stern

Volmer

equation

[45]

the

quenching

rate
constant

K
sv
of

curcumin

and

the

quencher,

o-nitrophenol,

was
determined

as
I
0
f
I
f
=

1

+

K
sv
[oNP]
I
0
f
I
f
=

1

+

k
q

0
[oNP]
where

K
sv
is

the

Stern

Volmer

rate

constant,

I
0
f
is

the

fluorescence
intensity

without

the

quencher,

I
f
is

the

fluorescence

intensity

with
the

quencher,

k
q
is

the

quencher

rate

coefficient,


0
is

the

fluores-
cence

lifetime

of

curcumin

without

the

presence

of

the

quencher
and

[oNP]

is

the

concentration

of

o-nitrophenol.

Fig.

4

illustrates
the

fluorescence

spectra

of

curcumin

in

the

presence

of

SDS

with-
out

and

with

various

concentrations

of

o-nitrophenol.

The

insert

in
Fig.

4

presents

the

Stern

Volmer

plot

[45]

for

curcumin

in

presence
of

various

concentration

of

o-nitrophenol.
The

fluorescence

spectra

of

curcumin

in

water,

CTAB

and

TX100
without

and

with

various

concentrations

of

o-nitrophenol

along
with

their

respective

Stern

Volmer

plots

showed

similar

trends

(not
shown).

The

estimated

values

of

K
sv
and

k
q
for

fluorescence

quench-
ing

of

curcumin

by

o-nitrophenol

in

water

and

various

micellar
media

is

determined

as

per

the

Stern

Volmer

equation

[45]

and

given

in

Table

3.

The

quenching

rate

constant

of

curcumin

by

o-
nitrophenol

in

water

was

determined

to

be

449

M
−1
in

comparison
to

3973

M
−1
in

cationic

CTAB.

The

high

quenching

rate

of

CTAB

is
due

to

the

stabilizing

electrostatic

interactions

between

the

pos-
itively

charged

head

groups

of

the

micelles

and

the

negatively
charged

enolic

curcumin

(see

Supplement

1).

This

attractive

inter-
action

facilitates

the

penetration

of

curcumin

in

the

Stern

layer

of
the

micelle

and

hence

the

formation

of

the

complex

[CUR–NP].

In
the

case

of

anionic

SDS,

a

decrease

in

the

quenching

rate

constant
was

found

relative

to

that

of

water.

This

change

can

be

linked

to
Fig.

4.

Fluorescence

emission

spectra

of

curcumin

in

SDS

in

the

presence

of

various
concentration

of

o-nitrophenol.

The

fluorescence

intensity

decreases

with

increase
in

o-nitrophenol

concentration.

Insert

shows

Stern

Volmer

plot

for

the

determina-
tion

of

the

quenching

rate

constant

K
sv
.
D.

Patra,

C.

Barakat

/

Spectrochimica

Acta

Part

A

79 (2011) 1823–

1828 1827
Table

3
Quenching

rate

constants

of

curcumin

by

o-nitrophenol

in

water,

CTAB,

SDS

and
TX100

surfactant

solutions.
Sample K
sv
(M
−1
)

k
q
((
0av
=

2.366

ns)
Water

449

1.9

×

10
11
M
−1
s
−1
CTAB 3973

1.7

×

10
12
M
−1
s
−1
SDS

367

1.6

×

10
11
M
−1
s
−1
TX100

550

2.3

×

10
11
M
−1
s
−1
the

repulsion

between

the

negatively

charged

head

groups

of

the
micelle

and

the

negative

charge

on

the

deprotonated

curcumin,
thus

destabilizing

the

complex

[CUR–NP].

In

the

case

of

neutral
TX100,

a

slight

increase

in

the

quenching

rate

was

observed

relative
to

that

of

water.

The

neutrality

of

this

surfactant

does

not

change
the

physical

properties

of

the

solvent

but

helps

in

bringing

together
o-nitrophenol

and

curcumin

due

to

hydrophobic

interactions.
3.4.

Effect

of

ionic

liquid

[bmin][BF
4
]

on

drug–surfactant
association
The

properties

of

various

aqueous

surfactant

solutions

were
modified

by

a

common

and

popular

hydrophilic

1-butyl-3-
methylimidilazolium

tetrafluoroborate,

[bmin][BF4].

For

modify-
ing

properties

of

aqueous

surfactant

solution,

the

IL

concentration
1%

(v/v)

was

chosen

from

the

literature

[4,5].

The

absorption

(see
Fig.

5)

and

fluorescence

excitation

and

emission

(see

Fig.

5)

spec-
tra

of

curcumin

in

various

surfactant

concentrations

in

presence
of

IL

showed

the

absorbance

or

fluorescence

intensity

of

curcumin
700600500400300
0.0
0.5
1.0
1.5
2.0
2.5
3.0
7-8
6
5
4
3
2
Absorbance
Wavelength (nm)
(1

) NO TX100
(2

) 0.02

mM
(3

) 0.06

mM
(4

) 0.2 mM
(5

) 0.4 mM
(6

) 0.6 mM
(7

) 0.8 mM
(8

) 1.0 mM
Curcum

in plus

IL

with

[T

X10

0]
1
700650600550500450400350300
0.0
4.0x10
6
8.0x10
6
1.2x10
7
1.6x10
7
2.0x10
7
8
8
7
7
6
6
4-5
Fluorescence Intensity (a.u)
Wave

leng

th (nm

)
0
1x10
6
2x10
6
3x10
6
4x10
6
5x10
6
6x10
6
7x10
6
8x10
6
9x10
6
Curcumin plus IL with TX100
10
10
9
9
4-5
1-3
1-3

(1) No TX100
(2) 0.02 mM
(3) 0.04 mM
(4) 0.06 mM
(5) 0.1 mM
(6) 0.2 mM
(7) 0.4 mM
(8) 0.8 mM
(9) 1.0 mM
(10

) 1.6 mM
Fig.

5.

Absorption

and

fluorescence

(excitation

and

emission)

spectra

of

curcumin
in

various

aqueous

TX100

concentrations

in

the

presence

of

IL.
in

CTAB,

SDS

and

TX100,

increased

with

surfactant

concentration.
The

association

constants

for

the

three

surfactants

with

curcumin
in

the

presence

of

IL

were

determined

as

explained

earlier

and
given

in

Table

1.

The

association

constant

of

CTAB

in

the

presence
of

IL

decreased

significantly

relative

to

CTAB

without

IL.

Though

the
short

hydrophobic

effect

of

the

tail

may

encourage

the

IL

to

locate
around

the

Stern

layer

of

the

micelle,

the

positive

charged

head
group

would

repulse

with

the

similar

charged

head

groups

of

CTAB.
Finally

both

CTAB

and

IL

will

compete

to

bind

with

deprotonated
form

of

curcumin.
This

competition

could

account

for

the

decrease

in

the

associa-
tion

constant

of

curcumin

with

CTAB.

However,

in

the

case

of

SDS

in
the

presence

of

IL,

an

increase

of

the

association

rate

constant

was
observed

compared

to

SDS

without

IL.

In

the

absence

of

IL,

there
is

repulsion

between

the

negative

charge

of

the

head

group

(sul-
fate

ion)

of

SDS

and

the

negative

charge

of

the

deprotonated

form
of

curcumin.

When

IL

is

added,

its

positive

charge

head

group

will
act

as

a

stabilizer

between

negatively

charged

SDS

and

negatively
charged

curcumin

(deprotonated

form),

thus

facilitating

the

asso-
ciation

of

curcumin

with

SDS.

On

the

other

hand,

the

association
rate

constant

of

curcumin

with

TX100

increased

significantly

in

the
presence

of

IL.

A

possible

explanation

would

be

the

induction

of
hydrogen

bonding

and

dipole–dipole

forces

by

the

positive

charge
of

the

head

group

of

the

IL

with

TX100

[4],

assisting

interaction

or
strong

association

of

curcumin

with

neutral

surfactant

solution.
3.5.

Effect

of

ionic

liquid

[bmin][BF
4
]

on

micellization
As

discussed

earlier,

the

cmc

of

various

aqueous

surfactant

solu-
tions

was

evaluated

based

on

the

change

in

Stokes’

shift

(see

Fig.

3)
of

curcumin

in

the

presence

of

1%

(v/v)

IL.

Variation

of

Stokes’

shift
with

surfactant

concentration

for

CTAB

with

and

without

IL

showed
similar

trends.

It

could

therefore

be

implied

that

there

is

no

new
kind

of

favorable

interaction

between

the

IL

and

CTAB.

However,
similar

plots

for

SDS

with

and

without

IL

gave

two

different

trends
indicating

that

the

interaction

of

curcumin

with

SDS

in

the

pres-
ence

and

absence

of

IL

are

not

similar.

As

shown

earlier,

in

the
absence

of

IL,

the

Stokes’

shift

of

curcumin

increased

with

increase
in

SDS

concentrations

until

cmc

was

reached.

However,

when

IL
was

present,

Stokes’

shift

continued

to

decrease,

but

at

a

much
smaller

rate,

with

increasing

SDS

concentration.

This

trend

could
imply

that

in

the

case

of

SDS,

there

could

be

a

favorable

interac-

tion

that

stabilizes

the

micelles

in

the

presence

of

IL.

For

TX100,
variation

of

Stokes’

shift

with

surfactant

concentration

showed

dif-
ferent

trends

in

the

presence

and

absence

of

IL.

Without

IL,

there
was

a

big

increase

in

Stokes’

shift

of

curcumin

after

the

cmc

was
reached

whereas

in

the

presence

of

IL,

there

was

a

notable

decrease
of

Stokes’

shift

after

the

cmc.

This

implies

that

the

interactions

of
TX100

solutions

in

the

presence

and

absence

of

IL

are

of

different
nature.

It

was

found

that

cmc

of

CTAB

decreased

when

1%

(v/v)

IL

was

added

(Table

2).

This

decrease

indicates

that

in

the

pres-
ence

of

the

hydrophilic

IL,

the

formation

of

micelles

is

favored

at
relatively

lower

concentrations.

A

possible

reason

for

this

observa-
tion

would

be

the

favorable

hydrophobic

interaction

of

the

carbon
chains

of

both

CTAB

and

[bmin][BF4]

as

well

as

the

cumulative
electrostatic

interaction

among

CTAB,

curcumin

and

[bmin][BF4].
Thus,

both

the

electrostatic

interaction

and

the

tendency

of

the
hydrophobic

chains

to

come

together

further

encourage

the

for-
mation

of

micelles

and

hence

lowers

the

cmc.

Similarly,

the

cmc
of

SDS

decreased

significantly

in

the

presence

of

IL

(Table

3).

The
lowering

of

the

cmc

of

SDS

in

the

presence

of

IL

was

also

reported
earlier

[3]

and

this

could

be

attributed

to

both

the

hydrophobic
effect

and

the

attraction

between

the

anionic

SDS

and

the

positively
charged

IL.

The

cmc

of

TX100

in

IL

increases

from

0.2

mM

to

0.4

mM
by

Stokes’

shift

measurement.

Along

with

an

aryl

and

an

eight

car-
bon

hydrophobic

chain

(C
8
H
17
),

TX100

has

100

monomoric

units
1828 D.

Patra,

C.

Barakat

/

Spectrochimica

Acta

Part

A

79 (2011) 1823–

1828
containing

an

oxygen

atom

(ether

group).

The

head

of

TX100

con-
tains

a

–OH

group

that

interacts

directly

with

the

head

group

of
IL

via

hydrogen

bonding

and

dipole–dipole

interactions

[4].

If

the
micellar

formation

of

TX100

had

to

be

favorable

in

the

presence
of

IL,

then

the

immediately

available

etheric

monomeric

group

of
TX100

(after

the

–OH

group)

must

interact

with

the

immediately
available

hydrophobic

tail

of

IL

(after

the

polar

head

group).

How-
ever,

the

short

hydrophobic

tail

of

IL

and

the

polar

monomeric

chain
of

TX100

make

this

interaction

unfavorable

at

low

concentrations.
Thus,

to

form

micelles,

the

etheric

chains

of

TX100

must

overcome
the

hydrophobic

effect

induced

by

the

tail

of

the

IL.

This

causes

the
cmc

of

TX100

to

increase

in

the

presence

of

IL.
4.

Conclusion
The

association

of

dye/drug

molecule

with

surfactant

solutions
depends

on

the

charge

of

the

head

group

of

the

surfactant

and
physiochemical

properties

of

the

dye

[36–39].

The

present

binding
study

of

curcumin

with

various

surfactant

solutions

and

quenching
of

curcumin

by

o-nitrophenol

clearly

predict

electrostatic

inter-
action

of

head

group

of

surfactant

molecule

and

deprotonated
form

of

curcumin,

while

curcumin

having

greatest

affinity

for
cationic

than

non-ionic

and

finally

anionic

surfactant

solution.
The

observation

that

the

changes

of

association

of

drug

like

cur-
cumin

with

surfactant

solutions

are

dramatic

in

the

presence

of

IL
[bmin][BF
4
]

compared

to

without

IL

[bmin][BF
4
]

presents

clear

evi-
dence

the

importance

of

IL

[bmin][BF
4
]

in

modulating

association
of

curcumin

with

surfactant

solutions.

The

interaction

involving
non-ionic

TX100

surfactant

appear

to

have

more

dramatic

effect
on

the

association

of

curcumin-surfactant

solutions

compared

to
that

involving

cationic

CTAB

and

then

anionic

SDS

surfactant

due
to

interactions

of

IL

[bmin][BF
4
],

curcumin

and

head

group

of

the
surfactant.

Though

the

major

reason

for

alternation

of

aggregation
number

by

IL

[bmin][BF
4
]

[3,4]

is

due

to

electrostatic

interactions
between

head

group

of

the

surfactant

and

anion

[46]

or

cation

[47]
of

the

IL

[bmin][BF
4
],

our

results

showing

early

formation

of

micelle
irrespective

of

cationic

or

anionic

aqueous

surfactant

solutions

and
delay

in

micelle

formation

in

the

case

of

neutral

aqueous

surfactant
solution

suggest

hydrophobic

interaction

of

IL

[bmin][BF
4
]

do

play
a

crucial

role.

These

findings

will

further

enhance

potential

appli-
cation

of

IL

as

a

modulator

in

solubilization

in

the

micellar

system,
association

of

drug–surfactant

during

drug

delivery,

micellization
and

chemistry.
Acknowledgements
Financial

support

provided

by

Lebanese

National

Council

for
Scientific

Research

(LNCSR)

and

American

University

of

Beirut,
Lebanon

through

the

University

Research

Board

(URB)

and

Long-
term

Faculty

Development

grant

to

carry

out

this

work

is

greatly
acknowledged.
Appendix

A.

Supplementary

data
Supplementary

data

associated

with

this

article

can

be

found,

in
the

online

version,

at

doi:10.1016/j.saa.2011.05.064.
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