Spectroscopic, computational and electrochemical studies on 2-(4-nitrophenyl)-1H-benzo[d]imidazole and its interaction with cationic surfactant cetyltrimethylammonium bromide

Article abstract of DOI:10.1016/j.molliq.2016.04.049

2-(4-Nitrophenyl)-1H-benzo[d]imidazole (4-NBI) was prepared and characterized by experimental and computational methods. Electrochemical reduction of the compound was studied in dimethyl sulphoxide media which showed that the nitro group of the molecule undergoes quasireversible three-step reduction producing -NH₂ while the imidazole nitrogen bonded to hydrogen undergoes an irreversible one-electron reduction. The electronic spectra were studied by TDDFT computational method and compared with the experimental results which corroborated each other excellently. The interaction of 4-NBI with the cationic surfactant cetyltrimethylammonium bromide (CTAB) was investigated in aqueous solution at physiological pH (7.4) by UV-Vis spectroscopy. By using different nonlinear fitting methods binding parameters were evaluated for 4-NBI - CTAB micelles interaction. The results showed that the electrostatic interaction plays a major role over hydrophobic interaction in the binding of 4-NBI to CTAB micelles. The electrostatic interaction has also an important role in the distribution of 4-NBI between CTAB micelle-water phases. Gibbs free energy for the binding and distribution of 4-NBI between the bulk aqueous medium and surfactant micelles was calculated.

Full text of DOI:10.1016/j.molliq.2016.04.049

Journal of Molecular Liquids 219 (2016) 1058–1064
Contents lists available at ScienceDirect
Journal of Molecular Liquids
journal homepage: www.elsevier.com/locate/molliq
Spectroscopic, computational and electrochemical studies on 2-(4-
nitrophenyl)-1H-benzo[d]imidazole and its interaction with cationic
surfactant cetyltrimethylammonium bromide
a
a
b
a,
Arup Datta , Sanjay Roy , Palash Mondal , Partha Sarathi Guin ⁎
a
Department of Chemistry, Shibpur Dinobundhoo Institution (College), 412/1 G. T. Road (South), Howrah 711 102, India
Department of Chemistry (UG & PG), Vivekananda Mahavidyalaya, Burdwan 713 103, India
b
a r t i c l e i n f o
a b s t r a c t
Article history:
2-(4-Nitrophenyl)-1H-benzo[d]imidazole (4-NBI) was prepared and characterized by experimental and compu-
Received 16 March 2016
Accepted 13 April 2016
Available online xxxx
tational methods. Electrochemical reduction of the compound was studied in dimethyl sulphoxide media which
showed that the nitro group of the molecule undergoes quasireversible three-step reduction producing –NH
2
while the imidazole nitrogen bonded to hydrogen undergoes an irreversible one-electron reduction. The elec-
tronic spectra were studied by TDDFT computational method and compared with the experimental results
which corroborated each other excellently. The interaction of 4-NBI with the cationic surfactant
cetyltrimethylammonium bromide (CTAB) was investigated in aqueous solution at physiological pH (7.4) by
UV–Vis spectroscopy. By using different nonlinear fitting methods binding parameters were evaluated for 4-
NBI – CTAB micelles interaction. The results showed that the electrostatic interaction plays a major role over hy-
drophobic interaction in the binding of 4-NBI to CTAB micelles. The electrostatic interaction has also an important
role in the distribution of 4-NBI between CTAB micelle–water phases. Gibbs free energy for the binding and dis-
tribution of 4-NBI between the bulk aqueous medium and surfactant micelles was calculated.
Keywords:
4-NBI
Computational method
UV–vis
CTAB micelles
Binding parameters
©
2016 Elsevier B.V. All rights reserved.
1
. Introduction
of this class of molecules bind a limit of their use [22–24]. Previous stud-
ies have shown that the formation of radicals and/or anions being re-
sponsible for toxic side effects as well as for drug action is related with
the electrochemical behavior of these molecules [25]. The electrochem-
ical property of a molecule is further related to the electronic behavior
as well chemical structure of the molecule. This makes the structural,
spectroscopic and electrochemical studies on such molecules to be
highly relevant to determine their structure–activity relationship.
Several studies explored that the mechanism of action of a biologi-
cally active molecule is related to its interaction with biological tissues
through its binding to membranes at the molecular level. Several bio-
logical processes have been observed to happen at the ionizable surface
of the biomembranes or along their hydrophobic area, leading to a com-
parative study on the interaction of such molecule with cationic,
zwiterionic, anionic and neutral surfactants to be important [26–28].
This provides helpful information on the nature of drug–membrane in-
teraction. This is why the studies on drug–surfactant interactions have
been carried out by several researchers using various models and tech-
niques owing to the extensive application of surfactants in the field
pharmaceutical research. Earlier studies established that micellar sys-
tems have the ability to solubilize hydrophobic drugs [29–32] which in-
creases their bioavailability and can be used as a model system for
biomembrane, as well as drug carriers in various drug delivery and
drug targeting systems [33–35]. The physicochemical interactions of a
Benzimidazole, its analogues and derivatives are a class of important
nitrogen-containing heterocyclic compounds which has been used in
biological and pharmaceutical chemistry for quite a long time. Imidazole
nucleus plays many vital roles in human physiology in the form amino
acid like histidine, vit-B12, nucleic acid bases, caffeine, biotin, etc.
Several studies showed that these molecules are effective as
antimicrobial, antihelminthic, antibacterial, anticonvulsant, anti-
inflammatory, antiarrythmic, antioxidant, androgen receptor antago-
nist, antiprotozoal, antitumour, antiviral, antihypertensives, antihista-
minics, antifolate, antifungal agents [1–18]. Their activity against
several viruses such as HIV, herpes (HSV-1), influenza and human
cytomegalovirus (HCMV) [19–21] draw an enormous attention of
researchers to work in this field. Some established medicines such as
astemizole, esomeprazole, albendazole, cimetidine, azomycin, metroni-
dazole, etc., also consist of imidazole ring as the centre of drug action.
It is important to note that although some nitroimidazoles have been
used successfully in treating various infections, some distinct toxicities
Abbreviations:
cetyltrimethylammonium bromide.
Corresponding author.
4-NBI,
2-(4-nitrophenyl)-1H-benzo[d]imidazole;
CTAB,
⁎
E-mail address: parthasg@gmail.com (P.S. Guin).
http://dx.doi.org/10.1016/j.molliq.2016.04.049
0167-7322/© 2016 Elsevier B.V. All rights reserved.

A. Datta et al. / Journal of Molecular Liquids 219 (2016) 1058–1064
1059
Table 1
Structural parameters for the optimized molecular structure of 4-NBI at the B3LYP level.
(a) Bond length (Å)
Type of bond
Bond length (Å)
Type of bond
Bond length (Å)
N25-O27
N25-O26
C14-N12
C14-N11
C18-C25
1.231
1.231
1.384
1.318
1.468
C3-N11
C4-N12
N12-H13
C14-C15
C19-H23
1.380
1.380
1.006
1.464
1.082
Scheme 1. Chemical structure of 2-(4-nitrophenyl)-1H-benzo[d]imidazole (4-NBI).
(
b) Bond angle (o)
biologically active molecule with surfactant micelles may be envisaged
as an estimate for their interactions with biomembranes. This provides
an insight into complex biological processes such as passage of drugs
through cell membranes. However, there are several studies on
molecule–surfactant interactions were studied extensively with various
types of drugs for quite a long time, there is a lack of research involving
the interactions of benzimidazoles with surfactant micelles.
Type of bond
Bond angle (°)
Type of bond
Bond angle (°)
O27-N25-O26
C19-C18-25 N
C17-C18-N25
C15-C14-N12
124.65
118.96
119.23
123.61
C15-C14-N11
C4-N12-C14
C3-N11-C14
C4-N12-H13
124.15
107.33
105.56
125.92
In the present study the structure, electronic properties and electro-
chemical behaviors of 2-(4-nitrophenyl)-1H-benzo[d]imidazole (4-
NBI) (Scheme 1) were analyzed by theoretical and experimental tech-
niques in order to enrich our knowledge in the area of physicochemical
aspects of benzimidazole research. UV–Vis spectroscopy was employed
to see whether 4-NBI has ability to penetrate in a biological membrane
(Spectrochem, India). Acetonitrile (ACN) (99.8%, GR, Merck) was puri-
fied by refluxing with KOH (Merck) for several hours followed by frac-
tional distillation and then again refluxing with CaH (Merck) for
2
several hours followed by fractional distillation [38]. Only the middle
fraction was collected from each distillation, ensuring removal of all am-
monia evolved during the alkali treatment. The solvent was stored as
described above. CTAB (AR grade), purchased from Spectrochem, India
was used without further purification. NaOH (AR grade), E-Merck,
India was used in spectrometric titration. Phosphate buffer of ionic
strength 0.05 M (pH 7.4) was used to maintain the pH in studying the
interaction of the compound with the surfactant. In studying 4-NBI–
CTAB interaction, 10% DMSO solutions of 4-NBI were used since the sol-
ubility of it is small in water. Tetrabutylammoniumbromide (TBAB) (AR,
Spectrochem, India) was used as supporting electrolyte in the electro-
chemical measurements. All other reagents used were of AR grade.
Aqueous solutions were prepared in triple distilled water.
by using
a model study with help of cationic surfactant,
cetyltrimethylammoniumbromide (CTAB) (Scheme 1) as it is the most
accepted model system for studying different aspects of molecule–
membrane interactions. By using different non-linear fitting models
binding constant, partition coefficient and binding stoichiometry were
evaluated. The interaction of 4-NBI with CTAB micelles is of great inter-
est from the standpoint of both drug development and biological point
of view since it may hint about the biochemical activity of benzimid-
azole and this kind of study would definitely become a valuable addition
in the area of benzimidazole research.
2
. Experimental
2
.1. Materials
2.2. Theoretical studies
2
-(4-Nitrophenyl)-1H-benzo[d]imidazole (4-NBI) was prepared
and characterized by the method described earlier [36]. Anhydrous
dimethylsulfoxide (DMSO) was prepared from 99.0% DMSO
The GAUSSIAN 03 [39] was used in the present study for all theoret-
ical calculations. The structures of 4-NBI was optimized by using the
Perdew–Wang 1991 exchange functional, as modified by Adamo and
Barone and Perdew and Wang's 1991 gradient-corrected correlation
functional [40] and 6-31 g (d,p) basis set [41–43]. The electronic absorp-
tion bands of the compound were evaluated by time dependent (TD-
DFT) calculations.
(
Spectrochem, India) by drying over fused CaCl
2
for 3–4 days, followed
by decantation and distillation under reduced pressure [37]. The dis-
tilled DMSO was stored in a well stopper Jena bottle in desiccators and
redistilled before use. A stock solution of 4-NBI of strength 1 mM was
prepared by weighing an exact amount of it in anhydrous DMSO
Fig. 1. The optimized geometry of 4-NBI and the atom numbering was used in the calculation.

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A. Datta et al. / Journal of Molecular Liquids 219 (2016) 1058–1064
Fig. 2. Isodensity plot of HOMO, LUMO, HOMO −1 and LUMO +1 of 4-NBI.
2
.3. Instrumentation
dissolved oxygen, all experimental solutions were degassed for 45 min
with highly pure argon gas, before any cyclic voltammetry measure-
ment. Spectrophotometric studies were carried out using UV–Vis spec-
trophotometer, Model: MECASYS OPTIZEN POP, South Korea. Linear and
nonlinear fitting of the experimental data was performed by using
Grafit 3.0 software.
Electrochemical studies were carried out by using three-electrode
system with Digi-Ivy Potentiostat (Model DY2312). During this mea-
surement temperature was maintained at 298.15 K by a circulating
water bath. A glassy carbon electrode having surface area of
2
0
.07065 cm was used as the working electrode, a platinum wire was
used as the counter electrode and Ag/AgCl, saturated KCl was used as
the reference electrode. The glassy carbon electrode surface was cleaned
by following two successive steps such as abrasion with emery paper
and polishing with alumina-water suspensions with particle size
3. Results and discussion
3.1. Computational studies
0
.5 μm. The electrode was washed with water carefully in between
and after these steps. To make sure about the removal of adsorbed spe-
cies from the electrode surface, the electrode was then washed with
chloroform. Finally the activation of the glassy carbon electrode surface
was made when the potential was cycled at intermediate scan rates
The optimized molecular geometry of 4-NBI was calculated at the
B3LYP level which is shown in Fig. 1 and the structural parameters are
summarized in Table 1. From the structure it is interesting to note that
C3\N11 and C4\N12 bond lengths are equal (1.380 Å) which means that
the lone pair electron of N12 and N11 are delocalized over N12\C4 and
N11\C3 bonds respectively.
−
1
(
typically 0.1 V s ) in between a moderately negative potential and
more positive potential (−0.5 and +1.50 V). To avoid the effects of
The highest occupied molecular orbital (HOMO) and lowest unoccu-
pied molecular orbital (LUMO) constitute the frontier molecular orbitals
(
FMO) of a molecule. A difference in energy of the FMO has been
Fig. 3. Cyclic voltammogram of 4-NBI in anhydrous DMSO media. [4-NBI] = 1 mM,
Fig. 4. A plot of cathodic peak current versus square root of the scan rate fitted according to
Eq. (1) [Reduced chi squared = 0.5752 (●), =0.0055 (□), 0.2138 (■), 0.0377 (○)].
TBAB] = 0.1 M, scan rate 0.100 V s−
1
.
[

A. Datta et al. / Journal of Molecular Liquids 219 (2016) 1058–1064
1061
Fig. 6. Spectrophotometric titration of 4-NBI, shown by the variation of absorbance at
368 nm. [4-NBI] = 100 μM, [NaCl] = 10 mM, 25 °C.
2.106 V at the scan rate 0.100 V s− in which first three (I–III) are
1
−
quasireversible whereas the fourth (IV) is irreversible. The formal po-
tential for these reductions were found as −0.858, −1.073, −1.357
and −2.106 V, respectively. A plot of cathodic peak current versus
square root of the scan rate (Fig. 4) following Eq. (1) [44] for all these re-
ductions showed a linear relationship which suggests that the reduc-
tions are totally diffusion controlled and there is no contribution of
adsorption on the electrode surface during the reduction process. Com-
paring the present results with earlier [45] it may be said that the reduc-
tions I and II are due to the generation of a radical anion from the
reduction of nitro group of 4-NBI which is then reduced to dianion
and finally to nitroso group. This reduction requires a source of proton.
Earlier studies [46,47] suggested that though DMSO is a polar aprotic
solvent but it behaves as a weak acid. Thus in this case DMSO may act
as proton source. In addition here there is another possibility that imid-
azole nitrogen of 4-NBI may act as a proton source in the experimental
medium. The reductions III and IV involve the reduction of nitroso group
2
Scheme 2. Electrochemical reduction of aromatic \\N O of 4-NBI in DMSO media.
observed to associate with the chemical reactivity of the molecule since
a HOMO acts as an electron donor whereas a LUMO acts as an electron
acceptor. In case of the present molecule the energy gap between the
HOMO and LUMO was calculated as 0.13042 a.u. while the energy gap
between HOMO −1 and LUMO +1 was found as 0.19896 a.u. An
isodensity plot of the HOMO, LUMO, HOMO −1 and LUMO +1 is
shown in Fig. 2. It is clear from Fig. 2 that HOMO and HOMO −1 are
mainly delocalized over the benzoimidazole moiety which implied the
oxidation processes, whereas LUMO, LUMO −1 are delocalized on the
aromatic moiety implied the reduction processes, which is fully sup-
ported the experimental result (Section 3.2, Electrochemical Studies).
to\NHOH and finally\NH
2
as shown in Scheme 2.
ꢀ
ꢁ n3=2
5
DO1=2 A C v1=2
Ipc ¼ 2:69 ꢀ 10
ð1Þ
3
.2. Electrochemical studies
where, Ipc = cathodic peak current (A), n = number of electron in-
The electrochemical behavior of 4-NBI was studied in anhydrous
2
−1
O
volved in the reduction, D = diffusion coefficient (cm s ), A = area
DMSO media in the presence of TBAB as supporting electrolyte by cyclic
voltammetry. The cyclic voltammogram of the compound (Fig. 3)
showed four reduction peaks (I–IV) at −0.915, −1.132, −1.420, and
2
−3
of the electrode (cm ), C = concentration (mol·cm ) and v = scan
rate (V·s ).
−
1
3.3. Determination of pKs of 4-NBI
In order to find the role of\NH of 4-NBI and its effect during the inter-
action of 4-NBI with CTAB, it is necessary to determine its pK which was
−
4
evaluated by spectrometric titration. At first 1 × 10 M aqueous solu-
tion of 4-NBI was acidified to pH 2.15 and it was then titrated slowly
with 0.01 M NaOH solution keeping the concentration of 4-NBI constant
and absorption spectra at various pH values were monitored. In such ti-
tration it was found that after pH 3.03 the peak at 312 nm starts to shift
to 333 nm and it finally shifts to 368 nm at after pH (Fig. 5). Change in
the absorbance at such wavelengths indicates the release of the\NH pro-
ton in the pH range 3.03–12.24. The absorbance Aobs at 368 nm was
fitted according to Eq. (2) [48] against pH of the solution (Fig. 6) and
the pKs were determined as 3.86 and 9.61, respectively. At pH less
than 3.86 the protonated N(2) of 4-NBI releases one proton while
N(1)–H releases a proton at pH 9.61.
A1
A2
Aobs
¼
þ
þ 10ðpH−pK1Þ þ 10ðpH−pK2Þ 1 þ 10
ðpk −pHÞ
þ 10ðpH−pK2Þ
1
1
A3
þ
ð2Þ
1 þ 10ðpk1−pHÞ þ 10ðpk2−pHÞ
Fig. 5. UV–Vis spectra of 4-NBI at various pH. [4-NBI] = 100 μM, [NaCl] = 10 mM, 25 °C.

1062
A. Datta et al. / Journal of Molecular Liquids 219 (2016) 1058–1064
Fig. 7. (a) UV–Vis spectra of 4-NBI (10 μM) in the absence and presence of increasing CTAB concentrations: 0.02 mM to 0.584 mM; (b) Variation of the absorbance at 340 nm with CTAB
concentration.
where, Aobs is the overall absorbance of the solution at 368 nm at differ-
that below the CMC, the solution absorbance falls rapidly with the in-
crease in surfactant concentration while above CMC, the absorbance of
the solution falls very slowly and reaches to saturation level. This pro-
+
ent pH values while A
1 2 3
, A and A refer to the absorbance of 4-NBIH , 4-
−
NBI and 4-NBI , respectively.
−
4
duced the CMC value in the present study as 1.77 × 10
M. The ob-
3
.4. Interaction of 4-NBI with CTAB
Interaction of 4-NBI with the cationic surfactant CTAB was studied
served CMC value is smaller than the CMC of CTAB in pure water
−
4
−4
(
9.10 × 10 M) and in 0.1 M phosphate buffer (8.00 × 10 M) [51,
52]. The lowering in CMC value is definitely due to the presence of dif-
ferent ions and molecules in the current study [53].
by UV–Vis spectroscopy in the premicellar and micellar range of con-
centration at pH 7.4 in 0.05 M phosphate buffer. In the absence of sur-
factant, absorption spectrum of 4-NBI shows an absorption peak at
The variation of absorbance at 320 nm as a function of surfactant
concentration is shown in Fig. 7b which shows that the absorbance de-
creases with the increase in CTAB concentration and reaches a satura-
tion level after a certain concentration of CTAB. This may be assigned
as the incorporation of 4-NBI molecules into CTAB micelles. The binding
isotherm was analyzed by nonlinear fit by assuming 1:1 interaction be-
tween the 4-NBI and CTAB, with the help of Eq. (1) (Fig. 8). The binding
3
40 nm (Fig. 7a). The binding parameters for the interaction of 4-NBI
with CTAB surfactant micelles was determined by monitoring the ab-
sorption peak at 340 nm for a series of solutions containing a constant
concentration of 4-NBI and increasing concentrations of CTAB. The ex-
perimental measurements were made after 1 to 2 min from the mixing
of CTAB with 4-NBI. The UV–Vis spectra of 4-NBI in the absence and in
the presence of different concentrations of CTAB are shown in Fig. 5a.
The critical micelle concentration (CMC) of CTAB, an indication about
the beginning of micelle formation [50], in the presence of 4-NBI was
determined by monitoring the change in absorption spectra of 4-NBI
3
−1
constants was obtained as K = (9.74 ± 0.05) × 10 M (Table 2) [50,
53,54].
A þ A K½CTABꢁ
0
∞
A ¼
ð3Þ
1 þ K½CTABꢁ
−
4
and it was evaluated as 1.77 × 10 M. Further measuring the absor-
bance of 4NBI - surfactant mixture as a function of surfactant concentra-
tion allows one to determine the CMC of the solution. It was observed
where, A and A
0
are the measured absorbances at 340 nm of 4-NBI in the
∞
absence and presence of surfactant whereas A is the absorbance of 4-
NBI bound to surfactant.
The Gibbs free energy for the binding of 4-NBI to the CTAB surfactant
micelles was then evaluated by applying Eq. (4) [49,55].
0
ΔG ¼ −RT lnK
ð4Þ
where R is the molar gas constant and T is the absolute temperature
298.15 K). The Gibbs free energy was found as −22.77 kJ/mol for inter-
(
action of the compound (4-NBI) to the surfactant (Table 2).
Table 2
Binding constant (K
ergy of binding (ΔG
transfer of 4-NBI from aqueous phase to micellar phase (ΔG ) for
X
b
), partition coefficient (K
X
), the Gibbs free en-
0
b
) and the standard free energy change for the
0
the interaction of 4-NBI with CTAB micelles.
K
ΔG
K
ΔG
(9.74 ± 0.05) × 103
0
b
, kJ/mol
−22.77
(4.26 ± 0.06) × 106
−37.84
X
Fig. 8. Binding isotherm for the interaction of 4-NBI with CTAB micelles and the
corresponding non-linear fit using Eq. (3) (reduced Chi squared = 1.61 × 10− ).
0
, kJ/mol
5
X

A. Datta et al. / Journal of Molecular Liquids 219 (2016) 1058–1064
1063
Table 3
Dielectric constant of the solvent and wavelength maximum (λmax) of the absorption
spectra of 4-NBI.
Solvent
Dielectric constant
Wavelength maximum (λmax), nm
Acetonitrile
37.50
39.28
46.70
335
340
355
90% ethanol
DMSO
X
partition coefficient (K ) which is a thermodynamic parameter
representing the affinity of a given compound to penetrate into the mi-
cellar phase from the aqueous phase. This is an important parameter
elucidating the mechanism of solubilization of a drug molecule and def-
initely helps in understanding the biological phenomena like interac-
tion between drugs and biological membranes. By applying the
pseudo-phase model, [50,55–57] the partition coefficient was deter-
mined by using Eq. (5):
Fig. 9. Variation of the absorption maximum (λmax) of 4-NBI with CTAB concentrations.
From the absorption spectra of 4-NBI in the absence and presence of
CTAB, it is evident that in addition to hypochromic effect of the spectra
with the increasing concentration of CTAB, a bathochroimic shift of
about 7 nm (Fig. 7a) was observed which may provide an information
about the mode of interaction of the molecule with surfactant micelles.
The variation of the absorption maximum (λmax) with CTAB concentra-
tion is shown in Fig. 9. In order to characterize such a mode, the absorp-
tion spectra of the compound were measured in three different solvents
such as anhydrous acetonitrile, 90% ethanol and anhydrous
dimethylsulphoxide (Fig. 10) having different dielectric constants. The
observed values of λmax and dielectric constants of the media are sum-
marized in Table 3. The results clearly showed that the absorption spec-
trum of 4-NBI undergoes a bathochromic shift upon increase in the
dielectric constant of the media. In other words it may be said that
with the increase in polarity of the solvent, bathochromic effect in-
creases. Considering this observation and comparing it with the results
of 4-NBI - CTAB micelles interaction, it may be concluded that the elec-
trostatic contribution plays a major role than hydrophobic contribution
during the interaction of 4-NBI with CTAB micelles. Considering the pK
values of 4-NBI one can say that at pH 7.4, only 0.61% of 4-NBI is ionized
due to deprotonation of\N(1)\H. However, in all 4-NBI molecules at
pH 7.4 oxygen of nitro group carries a negative charge which possibly
interacts with the cationic CTAB.
1
1
nw
¼
þ
ð5Þ
ΔA ΔA∞ KXΔA∞ð½CTABꢁ þ CT−CMCÞ
where ΔA = (A − A ); ΔA = (A − A
larity of water. The value of K
of 1/ΔA vs. 1/([CTAB] + C
0.06) × 10 . It is important to mention here that this linear relation
holds good in a very high surfactant concentration region below
which the curve tends to bend upwards with decreasing surfactant con-
centration. This deviation from the linearity was due to the approxima-
tion made in the evaluation of Eq. (5) [58].
0
∞
b
0
) and n
w
= 55.5 M is the mo-
was obtained from the slope of the plot
− CMC) (Fig. 11) and it is (4.26 ±
X
T
6
The standard free energy change for the transfer of 4-NBI from the
bulk aqueous phase to micellar phase was obtained as −37.84 kJ/mol
by putting the value of K
X
in Eq. (6) [48] and it is summarized in Table 2.
0
ΔGx ¼ −RT lnKX
ð6Þ
X
A comparison of the partition coefficient (K ) and corresponding
free energy values of the present study with the values of other opposite
charged molecule–surfactant interactions [26,50,58] clearly showed
that the current results are significantly greater in magnitude which
means that the interaction between 4-NBI and CTAB is predominantly
electrostatic in nature. As the surface of biological membranes fre-
quently presents a net charge, the binding properties of charged mole-
cules such as drugs, as well as their membrane location are very
important. Therefore the binding parameters determined for the 4-
NBI–CTAB system in micellar and submicellar domain are also
Along with the measurement of binding constant and stoichiometry,
the compound – surfactant interaction was further characterized by
Fig. 10. Absorption spectra of 4-NBI in (a) anhydrous DMSO, (b) 90% ethanol (EtOH), and
Fig. 11. A plot of 1/ΔA vs. 1/([CTAB] + C
T
− CMC) following Eq. (5) for 4-NBI in CTAB
(c) anhydrous ACN.
micelles at pH 7.4 (reduced Chi squared = 0.4351).

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A. Datta et al. / Journal of Molecular Liquids 219 (2016) 1058–1064
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4
. Conclusions
[
4
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2
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