Environment-friendly ester bonded gemini surfactant: Mixed micellization of 14-E2-14 with ionic and nonionic conventional surfactants

Article abstract of DOI:10.1016/j.molliq.2015.06.064

Abstract This paper deals with a comprehensive study of the biodegradability, cleavability, hemolytic activity, and physicochemical properties of an ester-linked gemini surfactant, ethane-1,2-diyl bis(N,N-dimethyl-N-tetradecylammoniumacetoxy) dichloride (14-E2-14), along with the gemini-conventional mixed surfactant systems at different mole fractions of 14-E2-14. Some typical conventional surfactants of different polarities were used in the investigation at 303.15 K in aqueous medium by performing conductometric and tensiometric measurements. The data from both the techniques were used to obtain the critical micelle concentration (cmc) of mixed surfactant systems at different mole fractions. The decrease in cmc values indicates nonideality of the binary systems, and also occurrence of mixed micellization. Interaction parameters (β^m and β^σ) along with energetics of mixed micellization were evaluated by using theoretical models suggested by Clint, Rubingh, Rosen, Motomura and Maeda for mixed surfactant systems. Negative values of β indicate an overall attractive force in the mixed state. Also, the excess free energy of mixing has negative values for all the systems. The hydrophilic spacer of the gemini surfactant 14-E2-14 shows strong interaction with the conventional surfactants used as well as with the biological membranes such as human erythrocytes (RBC).

Full text of DOI:10.1016/j.molliq.2015.06.064

Journal of Molecular Liquids 211 (2015) 247–255
Contents lists available at ScienceDirect
Journal of Molecular Liquids
journal homepage: www.elsevier.com/locate/molliq
Environment-friendly ester bonded gemini surfactant: Mixed
micellization of 14-E2-14 with ionic and nonionic
conventional surfactants
Nazish Fatma, Manorama Panda ⁎, Wajid Husain Ansari ⁎, Kabir-ud-Din
Department of Chemistry, Aligarh Muslim University, Aligarh 202002, India
a r t i c l e i n f o
a b s t r a c t
Article history:
This paper deals with a comprehensive study of the biodegradability, cleavability, hemolytic activity, and
physicochemical properties of an ester-linked gemini surfactant, ethane-1,2-diyl bis(N,N-dimethyl-N-
tetradecylammoniumacetoxy) dichloride (14-E2-14), along with the gemini-conventional mixed surfactant
systems at different mole fractions of 14-E2-14. Some typical conventional surfactants of different polarities
were used in the investigation at 303.15 K in aqueous medium by performing conductometric and tensiometric
measurements. The data from both the techniques were used to obtain the critical micelle concentration (cmc) of
mixed surfactant systems at different mole fractions. The decrease in cmc values indicates nonideality of the
binary systems, and also occurrence of mixed micellization. Interaction parameters (β and β ) along with
energetics of mixed micellization were evaluated by using theoretical models suggested by Clint, Rubingh,
Rosen, Motomura and Maeda for mixed surfactant systems. Negative values of β indicate an overall attractive
force in the mixed state. Also, the excess free energy of mixing has negative values for all the systems. The
hydrophilic spacer of the gemini surfactant 14-E2-14 shows strong interaction with the conventional surfactants
used as well as with the biological membranes such as human erythrocytes (RBC).
Received 25 April 2015
Accepted 25 June 2015
Available online xxxx
Keywords:
Biodegradable gemini surfactant
Cleavability
Hemolytic activity
Mixed micelles
Synergism
m
σ
©
2015 Elsevier B.V. All rights reserved.
1
. Introduction
the spacer parts, respectively). Cleavable/biodegradable gemini surfac-
tants are the better alternatives. With this view point, a number of surfac-
tants with polar or labile bonds (amide, carbonate, ethoxylated,
fluorinated and ester containing) which are highly soluble, easily
hydrolysable, less stable and degradable, have, therefore, been synthe-
sized by many workers [17–21]. As the nature of spacer and the constit-
uent groups play a significant role in the micellization process of the
gemini surfactants, the abovementioned surfactants show interesting
properties. As none of them are available commercially, mixed micelliza-
tion studies are now gaining more attention because less amount of
surfactants are consumed without affecting the physicochemical
properties of the pure component surfactants. Several mixed micelliza-
tion studies have been done for the cationic m–s–m type geminis
containing basic polymethylene spacer with conventional surfactants
[22–25], but studies of mixed micellization on cleavable cationic geminis
with conventional surfactants are limited [26,27].
Gemini surfactants are made up of two monomeric units connected at
the level of head groups by a spacer that can be flexible or rigid,
hydrophilic or hydrophobic [1–3]. Owing to their unusual properties
(
such as high surface activity, much lower cmc, higher solubilization
capacity, low Krafft temperature, good wetting, detergency and emulsify-
ing properties), the geminis have been considered the next generation
surfactants [1–7]. Cationic geminis, in particular, have been shown to pos-
sess high antifungal, antibacterial and antimicrobial activities [8–11] and
have attracted considerable attention towards their interaction with bio-
logically important ligands [12–16]. Surfactants interact with the mem-
branes causing cell lysis. Cell lysis by the surfactants has fundamental
and practical importance mainly in drug targeting or in pharmacology
[
14–16]. Despite very encouraging results of the above interaction
studies, the gemini surfactants invariably been used are of m–s–m type
m and s are the number of carbon atoms in the hydrophobic tail and
(
Mixed micelles of gemini (dicationic) and conventional (cationic,
anionic, nonionic) surfactants were prepared in terms of mole fraction
of the gemini (α14-E2–14) in the binary solutions, the total concentrations
of all the solutions remained the same for all the mole fractions — for
each system cmc of the single and binary mixtures were determined.
This piece of work is expected to contribute for further development of
⁎
Corresponding authors at: Department of Chemistry, Aligarh Muslim University,
Aligarh 202002, India.
E-mail addresses: manoramapanda01@gmail.com (M. Panda),
wajidhusain.chem@gmail.com (W.H. Ansari).
http://dx.doi.org/10.1016/j.molliq.2015.06.064
0167-7322/© 2015 Elsevier B.V. All rights reserved.

248
N. Fatma et al. / Journal of Molecular Liquids 211 (2015) 247–255
new environmentally acceptable surfactant systems, containing multi-
functional groups, in both industrial and academic sectors.
of 14-E2-14 was checked by Silica gel thin layer plate chromatography.
Its structural characterization was done by FT-IR, H NMR and ESI-MS
1
(
+) spectroscopy (Figs. S1 and S2).
2
. Experimental
2.2.1. Structural data
FT-IR (KBr, ν/cm−1): 2922.47, 2850.90 (C–H); 1749.52 (C_O); 1471
2
.1. Materials
(
C–O); 1188.89 (C–N); 719.71.
The amphiphile SDS (sodium dodecyl sulfate) (~99%, Sigma) was
1H NMR (300 MHz, CDCl3, δ/ppm): 0.844–0.867 (t, 6H, –CH
alkyl chain); 1.237–1.326 (m, 44H, –(CH
1.759 (m, 4H, –N CH
3.768–3.777 (s, 4H, –CH
(s, 4H, –N CH COO × 2).
ESI-MS (+): m/z 662 (M–Cl ), 661(M –Cl ), 611(M–CH
6
× 2,
alkyl chain);
3
recrystallized twice before use. The anionic surfactant SDBS (sodium do-
decyl benzene sulfonate, TCI), cationic surfactants CPC (cetylpyridinium
chloride, Merck) and TTAC (tetradecyltrimethylammonium chloride)
99%, Sigma-Aldrich), nonionic surfactants Brij 58 (polyoxyethylene
20) cetyl ether, Merck) and TX-100 (polyoxyethylene octyl phenyl
ether, Fluka) were used as received. N,N-dimethyltetradecylamine
≥95%, Fluka), ethylene glycol (99%, Sigma-Aldrich) and chloroacetyl
chloride (98%, Loba chemie) were also used without further purification.
Chemical structures of the conventional surfactants used in this study are
shown in scheme S1 (Supplementary materials).
2
)
11 × 2
,
+
+
2
CH
2
× 2); 3.517 (s, 12H, –N (CH
3
)
2
× 2);
+
(
(
2
O × 2); 4.475 (s, 4H, –N CH
2
× 2); 5.36
+
2
−
+
−
−
3
Cl ),
(
−
+
33(M–C
2
+
H
5
Cl ), 298 (C12
H
25 (CH
3 2 2 2
) N CH COOCH_CH ), 130
(
(CH
.3. Biodegradability
Once utilized, surfactants, in general, are discharged into the
) N CH COOCH_CH ).
3 2 2 2
2
2
.2. Synthesis of gemini surfactant and structural characterization
environment where they remain for a long time — this is because of
their lower degradability. Thus, we need to develop biodegradable
surfactants for safer environment.
A standard test of biodegradability, the biological oxygen demand
(BOD) test by the oxygen consumption method, was used to determine
the biodegradation of the synthesized surfactant, 14-E2-14. The experi-
ment needed 5 days to complete [28]. 100 mg of the sample was added
to 100 ml of basic culture solution. The change in the BOD (mg) of the
The cationic ester-bonded gemini surfactant 14-E2-14 was synthe-
sized in two steps following a reported procedure [19]. The protocol of
reaction route for the synthesis of 14-E2-14 is given in scheme 1(a):
yield 31.72 g (75%), white solid, R
f 3 3
= 0.62 (CHCl :CH OH, 6:4, v/v).
The melting point, visually determined by the microscopic melting
point apparatus (Reichert Thermovar Jung, Austria), is 443 ± 0.1 K.
The molecular structure of 14-E2-14 is shown in scheme 1(b). Purity
Scheme 1. (a). Synthesis protocol of the cationic gemini surfactant ethane-1,2-diyl bis(N,N-dimethyl-N-tetradecylammoniumacetoxy) dichloride (14-E2-14). (b). Molecular structure of
the gemini surfactant.

N. Fatma et al. / Journal of Molecular Liquids 211 (2015) 247–255
249
system was monitored for 5 days. The biodegradability was evaluated
2.7. Tensiometric measurements
using the following equation:
Surface tension (γ) is probably the most common physical
parameter for the determination of cmc of surfactant systems. The cmc
measurements of single/binary surfactant solutions were performed
by surface tension measurements with a Kruss11 Tensiometer
Biodegradabilityð%Þ ¼ ½ðBOD−blankÞ=TODꢀ ꢁ 100
(
K11MK3, Germany) using platinum ring detachment method at a con-
(
TOD is the theoretical oxygen demand (mg) when the test compound
stant temperature of 303.15 K. The temperature was maintained by cir-
culating water from a ORBIT RS 10S thermostat around the sample
holder. The ring was cleaned by heating it in alcohol flame. Typical
decrease and constancy regions were found in each of the single and
binary surfactant solutions of different mole fractions with addition of
concentrated surfactant solution in pure water (Fig. 2(b) and S5). The
cmc values, obtained from such γ ~ logarithm of total surfactant concen-
tration isotherms, are given in Table 2. Each experiment was repeated
thrice to achieve good reproducibility. Surface tension values were
is completely oxidized).
2
.4. Cleavable properties
The gemini surfactant, owing to the presence of weak bonds, i.e.,
ester groups in the spacer, is cleavable in nature. Cationic 14-E2-14
was found to undergo chemical hydrolysis in alkaline condition and
thus was cleaved through chemical means by using phosphate-
buffered saline and sodium hydroxide/potassium hydrogen phosphate
−
1
accurate within ± 0.1 mN·m . Various other parameters, e.g., maxi-
mum surface excess concentration at the air/solution interface (Γmax),
minimum area per head group at the air/solution interface (Amin),
(
Ringer Buffer) [21].
The FT-IR spectral results at pHs 7.4 and 12 (Figs. S3(a) and
S3(b)) show that 14-E2-14 gets cleaved in presence of the buffer
solution in 8 h. Absorption band for the carbonyl functional group
of ester [–OC_O] at 1749.52 cm−1 is shifted to 1637.87 cm−1 and
excess molar Gibbs energy of mixing (GEm), standard Gibbs energy of
∘
m
micellization (ΔG ), standard Gibbs energy of adsorption at the inter-
face (ΔadsG), minimum energy of surface (Gmin), were calculated
using the surface tension data.
−
1
new absorption band for the –OH groups appears at 3532.55 cm
.
Thus, formation of the easily degradable compounds such as fatty
acid salt, respective diol or the compound with hydroxyl group
takes place implying the cleavability of 14-E2-14.
3. Results and discussion
As the gemini surfactant, 14-E2-14, showed good biodegradability
39% of the amphiphile biodegraded after five days), it was considered
2
.5. Hemolytic assessment
(
for the mixed micellization study with the conventional surfactants of
different polarities.
Hemolysis of erythrocytes by interaction of surfactants has a great
fundamental and practical importance mainly in pharmacology. The
hemolytic activity of 14-E2-14 against human erythrocytes can be
used as a measure for its cytotoxicity. For the hemolysis of 14-E2-14,
we have followed a reported procedure [12,26]. When surfactants are
added to erythrocyte suspension in aqueous medium, these distribute
between the erythrocyte membrane and surfactant solution by adsorp-
tion until equilibrium is attained. Interaction between the surfactant
and erythrocytes at sublytic concentration could be governed by affinity
of each surfactant for the aqueous medium or the membrane, i.e., closely
related to hydrophobicity of the amphiphiles. HC50 is the concentration
that induces hemolysis of 50% of erythrocyte cells and is quantified from
the plots of percentage hemolysis as a function of amphiphile concen-
tration. Our results (Fig. 1) show that 14-E2-14 is less hemolytic than
the conventional surfactant TTAC of equivalent chain length. It is report-
ed that hemolytic activity of gemini surfactants increases with aliphatic
alkyl chain lengths of the hydrophobic tail [29].
The average cmc values, obtained by the tensiometry and
conductometry methods, were used to calculate the cmc-derived
parameters. The cmc values of the pure surfactants decrease in the
order: SDS N TTAC N SDBS N CPC N TX-100 N Brij 58 N 14-E2-14. The
gemini surfactant, 14-E2-14, has noticeably very low cmc than the
corresponding conventional monomeric surfactant TTAC due to its
greater hydrophobicity owing to the double-tailed structure. The hydro-
philicity of the spacer also might be a reason for the micelle formation at
100
8
0
0
1
90.5 (
μ
g/L)
2
.6. Conductometric measurements
6
Conductivity method is a very useful technique for determination of
the cmc of ionic surfactants [22–25,27]. Specific conductance of pure/
mixed surfactant solutions were measured with an ELICO conductivity
bridge (model CM82T) equipped with dip-type platinized electrodes
40
20
0
−
1
of cell constant = 1.02 cm . The alternating current method was
used for electrical conductivity experiments. The conductivity runs
were carried out by adding concentrated surfactant stock solution
gradually to water (a Hamilton microsyringe was used for the addition).
Temperature of the system was maintained at 303.15 ± 0.1 K by circu-
lating water through jacketed container holding the solution under
study. The cmc values were obtained from the intersection of the two
straight lines drawn before and after the break in the specific conduc-
tance vs. total surfactant concentration plots (Fig. 2(a) and S4). Accuracy
0
50
100
150
200
250
300
−
1
of the measurements was within ± 0.0001 mS·cm . Slopes of the
[14-E2-14] / μg.L-1
specific conductance plots before and after the cmc, as reported in
Table 1, yield the apparent degree of counterion binding, g
1
.
Fig. 1. Hemolytic activity as a function of 14-E2-14 concentration.

250
N. Fatma et al. / Journal of Molecular Liquids 211 (2015) 247–255
a
(Table 1) than the rest of the surfactant mixtures (except for CPC).
Stronger counterion association reduces the cmc.
0
0
0
0
0
.20
.15
.10
.05
.00
3
.2. Mutual interaction between the surfactants in mixed micelles
The theories suggested by Clint, Rubingh, Motomura and Maeda
have been used to explain the physicochemical parameters obtained
from the experiments (see “Supplementary materials” for definition of
terms and the equations used to evaluate the parameters).
The nature of interactions between 14-E2-14 and the conventional
surfactant molecules, in their binary mixtures, can be explained in
terms of Clint equation which is based on the pseudophase thermody-
namic formulation (Eq. (S1)) [31]. The experimental cmc (cmc12) values
are lower than cmcideal which is attributable to the nonideal behavior
and the attractive interactions between the constituent surfactants of
binary mixtures. The order of cmc values of the binary mixtures with
1
4-E2-14 is as follows: TTAC N SDS N SDBS N CPC N TX-100 N Brij 58.
Rubingh model [32], based on regular solution approach (RSA), is
the first model used for the surfactant mixtures for treatment of nonide-
al mixing. It is mostly used even after development of more complex
models due to its simplicity. Comparison of the values of X
mole fraction of 14-E2-14) in the mixed micelle and X
corresponding micellar mole fraction in ideal state) (Fig. 3, Table 1S),
which were evaluated by using Eqs. (S2) and (S3), demonstrates that
the mixed micelles contain less surfactant molecules in comparison to
0.0000
0.0006
0.0012
0.0018
0.0024
0.0030
m
1
(micellar
ideal
1
(the
[
14-E2-14] / 10-3 mol kg-1
b
7
0
5
0
5
0
5
m
ideal
m
that in ideal state (X
1
b X
1
1
). Also, the values of X (Fig. 3) increase
with increase in the mole fraction of 14-E2-14 for most of the binary
mixtures which indicates that the mixed micelle formation is favored
in comparison to the formation of micelles by single components, and
the mixed micellar phase is enriched in gemini surfactant molecules.
Mutual interactions between the surfactants in mixed micelles are
due to the difference in structures of both the components, e.g., length
and type of the hydrophobic chains, and electrostatic interactions
among the hydrophilic parts. It is quantified in terms of interaction
6
6
5
5
4
m
1
2
1
2
parameter,β , which is ½W12−ð W11 þ W22Þꢀ=RT, W being the molec-
ular interaction energy between the indicated constituents (Eq. (S4)).
m
Negative β values indicate that the attractive interaction (synergism)
between the components in the mixed micelle is higher than the self-
attraction of the surfactants before mixing. Zero β value indicates
ideal mixing while positive values show lesser attraction after mixing
m
m
(
antagonism). Negative β values were obtained throughout the
study for all the mixed systems suggesting synergism in the mixed
micelle formation. Higher absolute values of βav for 14-E2-14 + anionic
m
(SDS and SDBS) systems are because of the electrostatic attractive
interactions between the positively charged ammonium groups of the
dicationic gemini and the negatively charged sulfate/sulfonate head
groups. Nature and position of the functional group (i.e., presence of
an ester linkage [–CO(O)–]) in spacer also affect the micellization
process which makes it more hydrophilic that prompts micelle forma-
tion at low concentration [33]. In 14-E2-14 + cationic (CPC and TTAC)
-
7.2
-6.8
-6.4
-6.0
-5.6
log [Surfactant]
Fig. 2. (a). Variation of specific conductivity as a function of surfactant concentration for
pure 14-E2-14 at 303.15 K. (b). Plot of surface tension vs. log [surfactant] for pure 14-
E2-14 at 303.15 K.
m
systems, the βav value is less than 14-E2-14 + anionic mixed systems
due to electrostatic repulsion between the same charged head groups.
Stronger interaction of 14-E2-14 with CPC as compared to TTAC is
caused by the higher hydrophobicity owing to the presence of
a much lower concentration. As expected, the nonionic surfactants have
lower cmc values as compared to the cationic/anionic surfactants.
2 2
pyridinium ring and additional –(CH –CH )– group in CPC. It is clear
from the data shown in Table 1 that least synergism is observed for
the mixed systems of 14-E2-14 + nonionic (Brij 58 and TX-100)
3
.1. Counterion binding
surfactants.
m
The layer just adjacent to the surface of the ionic micelles is known
Activity coefficients (f
i
) in the mixed micelles, according to the RSA,
m
m
as the Stern layer to which the counterions are bound strongly and mi-
grate with the micelles in an electrical field. Counterion association (g
of the pure/mixed micelles was evaluated from the degree of dissocia-
tion (g , obtained from the ratio of post- and pre-micellar slopes of
were calculated by Eqs. (S5) and (S6). Both f
showing nonideality of the systems. The mixed systems show higher f
1 2
and f are less than unity
m
1
)
1
m
value than f
tional surfactants.
Another suitable model, i.e., Motomura's approach [34], was used for
2
suggesting higher activity of 14-E2-14 than the conven-
2
the specific conductance (κ) vs. [surfactant] plots) [30]. Higher counter-
ion binding was observed for the 14-E2-14 + SDS/SDBS system
M
1
the evaluation of micellar composition (X ) and other physicochemical

N. Fatma et al. / Journal of Molecular Liquids 211 (2015) 247–255
251
Table 1
Values of the critical micelle concentration (cmc, cmc12), ideal cmc (cmcideal), counterion association (g
M
1
), micellar mole fraction calculated using Motomura's model (X
1 2
parameter (β ) and activity coefficients (f and f ) for pure and mixed gemini − conventional surfactant systems for different mole fractions at 303.15 K.
1
), interaction
m
m
m
a
M
m
m
m
Surfactant
Mole fraction of 14-E2-14, α14-E2–14
cmc or cmc12
cmcideal
g
1
X
1
β
f
1
f
2
−
3
−1
10−3mol kg−1
1
0
mol kg
1
SDS
4-E2-14
1.0
0.0
0.0014
8.02
0.0047
0.0025
0.0011
0.0011
0.64
0.66
0.73
0.74
0.82
0.72
0.2
0.4
0.6
0.8
0.0068
0.0034
0.0023
0.0017
0.6348
−8.11
−8.82
−12.76
−12.01
0.8231
0.8505
0.6053
0.7565
0.0030
0.0014
0.0003
0.0002
(
β
ma v = −10.43)b
SDBS
CPC
0.0
2.76
0.63
0.77
0.45
0.63
0.69
0.2
0.4
0.6
0.8
0.0038
0.0022
0.0020
0.0012
0.0068
0.0034
0.0023
0.0017
0.4370
0.8314
0.9748
-
−8.23
−8.54
−6.43
−9.73
0.7095
0.7801
0.9662
0.8415
0.0054
0.0028
0.0040
0.0007
(
β
ma v = −8.23)b
0.0
0.837
0.53
0.61
0.71
0.75
0.89
0
0
0
0
.2
.4
.6
.8
0.0030
0.0020
0.0017
0.0010
0.0068
0.0034
0.0023
0.0017
0.3830
0.6799
0.8917
−8.03
−7.94
−6.92
0.5969
0.7147
0.8757
0.7422
0.0114
0.0066
0.0059
0.0014
−9.66
m
b
(
β
av = −8.14)
TTAC
Brij 58
TX-100
0.0
4.1
0.64
0.39
0.47
0.48
0.21
0.2
0.4
0.6
0.8
0.0059
0.0028
0.0019
0.0013
0.0068
0.0034
0.0023
0.0017
0.4427
0.8667
−5.44
−6.98
−7.70
−9.87
0.9552
0.9301
0.9346
0.8685
0.0112
0.0036
0.0018
0.0005
(
β
ma v = −7.50)b
0.0
0.0046
0.0025
0.0013
0.0012
0.0010
0.2
0.4
0.6
0.8
0.0031
0.0024
0.0019
0.0016
0.61
0.15
0.40
0.63
0.4290
0.6671
0.8184
0.9232
−0.96
−2.71
−2.35
−3.19
0.7643
0.6277
0.7887
0.8085
0.8075
0.3943
0.3343
0.1721
(
β
ma v = −2.30)b
0.0
0.18
0.2
0.4
0.6
0.8
0.0032
0.0018
0.0014
0.0011
0.0066
0.0034
0.0023
0.0017
0.48
0.57
0.38
0.49
0.4293
0.6673
0.8186
0.9232
−5.63
−6.39
−6.50
−7.65
0.6531
0.6925
0.7717
0.7665
0.0518
0.0248
0.0155
0.0063
(
β
ma v = −6.54)b
Standard uncertainties for temperature are 0.1 K.
a
g1 values are obtained from conductometric plots.
Average values.
b
parameters. It is applicable to any kind of surfactant mixtures and is
independent of counterions of the amphiphiles. According to this
approach, mixed micelles are considered as macroscopic bulk phase
Minimum area per surfactant head group adsorbed at the interface
(Amin) provides the idea about close or loose packing of surfactant
molecules at the air–water interface. In the submicellar region, Γmax
and Amin can be calculated using the following set of equations [35]:
and the related energetics of such systems can be evaluated in terms
M
of excess thermodynamic quantities. Wherever possible, the X
1
values
M
ꢀ
ꢁ
were calculated using Eqs. (S7)–(S12). The X
the Motomura model (Table 1) also support the X
1
values quantified from
1
∂γ
logC
m
Γmax ¼ −
ð1Þ
ð2Þ
1
values.
2:303nRT
∂
1
018
3
.3. Interaction of surfactants in mixed monolayer
Amin
¼
NA Γmax
Interfacial composition (Xσ
) and interaction parameter (β ) at
σ
1
air–water interface can be evaluated by Rosen's model (Eq. (S13))
where n represents the number of ionic species whose concentrations at
the interface vary with surfactant concentration in solution. Many
researchers have considered n = 2 for ionic surfactants where surfactant
ion and counterion are univalent, n = 3 for dimeric surfactant made up of
divalent surfactant ion and two univalent counterions [36]. In the present
work the n values are taken as 1, 2, and 3 for nonionic, anionic/cationic,
and gemini surfactants, respectively. For gemini-nonionic, gemini-
cationic/anionic mixed micelles, n values are 4 and 5, respectively. C is
σ
m
[
2]. The X
1 1
values are lower than X indicating that the mixed mono-
layer contains less surfactant molecules than the mixed micelles.
X
1
values are further used to calculate βσ (repulsive or attractive)
σ
at the air–water interface with the help of Eq. (S14) (Table 2). Here
too the mixed entities are formed synergistically. The activity
σ
σ
1 2
coefficients, f and f , evaluated by using Eqs. (S15) and (S16), are
less than one (Table 2) showing nonideality of the mixed systems
at the interface.
the concentration of surfactant, dγ/dlogC slope of γ versus log C plot, N
the Avogadro's number, and R and T have their usual significance.
A
3
.4. Surface and interfacial properties
Obviously, Amin decreases when Γmax increases (Table 2). For pure
surfactants the order of decrease of Amin is 14-E2-14 N SDBS N
CPC N TTAC N SDS N TX-100 N Brij 58, and for surfactant mixtures of
14-E2-14 the order is TX-100 N TTAC N CPC N Brij 58 N SDS N SDBS.
Maximum surface excess (i.e., surface saturation, Γmax) is a measure
of the extent of adsorption of various components at the interface.

252
N. Fatma et al. / Journal of Molecular Liquids 211 (2015) 247–255
Fig. 3. Plots of Xm
(•) and X
ideal
(■) vs. the mole fraction of 14-E2-14 mixed with different conventional surfactants at 303.15 K.
1
1
Among all the binary solutions, the lowest Amin value was obtained for
the 14-E2-14 + SDBS system. Due to the electrostatic attraction in 14-
E2-14 + anionic surfactant mixtures, surfactant molecules are more
tightly packed at interface lowering the Amin. Amin values increase with
3.5. Thermodynamic properties of the gemini + conventional surfactant
systems
Several types of forces such as electrostatic, covalent, hydropho-
bic and hydrogen bonding of various species are involved in adsorp-
tion of surfactants at the interface. Influence of such forces on the
adsorption behavior is discussed here from a thermodynamic point
of view.
α14-E2–14 for 14-E2-14 + SDS/SDBS and 14-E2-14 + Brij 58, whereas
for 14-E2-14 + TX-100 system Γmax increases and thus Amin decreases
with the increase of α14-E2–14. No trend is observed for the systems
with cationic surfactants.

N. Fatma et al. / Journal of Molecular Liquids 211 (2015) 247–255
253
Table 2
σ
σ
σ
σ
Values of interfacial mole fraction (X
1
), interfacial interaction parameter (β ), activity coefficients (f
1
and f
2
), surface pressure at cmc (Πcmc), maximum surface excess concentration
(Γmax) and minimum area per head group (Amin) of the pure and mixed gemini − conventional surfactant systems at 303.15 K.
σ
βσ
σ
σ
cmc/mN·m−1
7
−2
A
min/nm2
System
4-E2-14
Mole fraction of 14-E2-14, α14-E2–14
X
1
f
1
f
2
Π
Γ
max 10 /mol·m
1
1.0
0.0
20.4
41.0
33.3
27.2
23.7
21.0
40.4
29.3
18.4
24.0
20.8
24.9
21.4
26.4
26.7
12.7
29.7
13.3
15.3
15.9
16.7
29.7
25.5
22.7
12.9
9.9
8.8
1.88
0.63
1.61
1.95
2.42
2.78
1.33
1.11
1.56
1.92
1.98
1.01
2.83
1.92
1.93
2.42
0.74
3.61
3.32
3.50
3.70
0.52
1.53
1.73
2.45
3.36
0.52
4.38
4.27
4.05
3.57
SDS
26.4
10.3
8.5
0.2
0.4
0.6
0.8
0.7037
0.7430
–
−3.07
−4.19
–
0.7637
0.7584
–
0.2185
0.0991
–
6.9
6.0
0.8459
−4.60
0.8964
0.0370
SDBS
CPC
0.0
12.5
15.0
10.6
8.6
0.2
0.4
0.6
0.8
0.6951
0.8826
0.8647
–
−5.53
−2.55
−4.01
–
0.5980
0.9655
0.9292
–
0.0691
0.1377
0.0498
–
8.4
0.0
16.5
5.9
0.2
0.4
0.6
0.8
0.7090
0.7462
0.7580
–
−7.12
−7.66
−8.75
–
0.5470
0.6106
0.5988
–
0.0278
0.0141
0.0065
–
8.6
8.6
6.9
TTAC
Brij58
TX-100
0.0
22.4
4.6
0
0
0
0
.2
.4
.6
.8
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
5.0
4.7
4.8
0.0
32.1
10.9
9.6
0.2
0.4
0.6
0.8
0.4052
0.4685
0.5366
0.6699
−5.19
−4.16
−3.77
−2.05
0.1592
0.3088
0.4443
0.7996
0.4263
0.4014
0.3370
0.3981
6.8
4.9
0.0
38.5
22.0
15.9
14.9
14.3
32.0
3.8
0
0
0
0
.2
.4
.6
.8
3.9
4.1
4.7
Standard uncertainties for temperature are 0.1 K.
Table 3
o
Values of excess molar Gibbs energy of mixing (GEm), standard Gibbs energy of micellization (ΔG
the pure and mixed gemini − conventional surfactant systems at 303.15 K.
m
), standard Gibbs energy of adsorption (ΔadsG) and minimum surface energy (Gmin) for
1
°
/kJ·mol−1
−ΔadsG/kJ·mol−1
G
min/kJ·mol−1
System
4-E2-14
Mole fraction of 14-E2-14, α14-E2–14
−GEm/kJ·mol−
− ΔG
1
1.0
0.0
44.1
22.3
40.3
41.9
44.7
44.4
24.9
41.5
42.8
43.1
43.8
28.4
42.2
43.2
43.7
44.4
24.0
40.2
42.0
43.3
44.4
41.1
43.1
44.2
44.3
44.9
31.9
41.5
43.5
44.3
44.7
46.4
24.8
43.5
45.1
48.2
47.9
28.2
43.4
44.6
45.9
46.3
30.5
45.8
46.2
46.8
46.2
25.7
43.1
45.1
46.6
48.2
42.0
45.4
46.6
46.2
46.9
43.9
49.9
48.0
48.0
47.8
55.6
40.5
35.8
49.6
68.1
82.1
22.5
27.1
47.8
53.6
58.9
37.0
82.4
50.4
50.4
84.0
14.5
123.8
109.4
114.9
119.5
12.2
41.4
49.9
84.6
121.1
9.9
SDS
0.2
0.4
0.6
0.8
2.7
2.6
5.1
3.9
SDBS
CPC
0.0
0.2
0.4
0.6
0.8
3.4
3.0
1.1
2.8
0.0
0.2
0.4
0.6
0.8
3.8
3.3
2.1
3.5
TTAC
Brij58
TX-100
0.0
0.2
0.4
0.6
0.8
1.7
1.6
1.7
2.6
0.0
0.2
0.4
0.6
0.8
0.6
1.7
1.3
1.5
0.0
0.2
0.4
0.6
0.8
2.8
2.9
2.6
2.9
182.0
152.8
135.3
119.3
Standard uncertainties for temperature are 0.1 K.

254
N. Fatma et al. / Journal of Molecular Liquids 211 (2015) 247–255
The excess molar Gibbs energy of mixing, GEm, can be calculated
Accordingly,
m
m
using the values of X
i
and f
i
as per Eq. (3)
n
o
ꢄ
ꢅ
ο
m
1
m
1
2
ΔG
¼ RT B0 þ B1X þ B2 X
:
ð7Þ
Maeda
ꢂ
ꢃ
GEm ¼ RT X1mlnf þ ð1−X1 Þlnf
m
m
m
2
:
ð3Þ
1
The independent term B
component as
0
is related to the cmc of the nonionic
Negative GEm (Table 3) shows the stability of the mixed micellar
systems. The GEm values indicate more spontaneous mixed micelliza-
tion of 14-E2-14 with SDS than with SDBS. It is also evident from the
B0 ¼ lncmc2:
ð8Þ
m
higher value of β .
B1 is a parameter related to the standard Gibbs energy change due to the
exchange of a nonionic constituent in the nonionic pure micelle with an
Standard Gibbs energy of micellization
m
ionic constituent. The last coefficient, B
2
= − β (the interaction
ꢂ
m
m
parameter β in negative sign). Further, the two parameters B
1 2
and B
ΔG ¼ RTlnXcmc
ð4Þ
are related to the pure components' cmc values
ꢀ
ꢁ
measures the tendency of surfactant to form micelles (Xcmc indicates cmc
cmc1
cmc2
B1 þ B2 ¼ ln
:
ð9Þ
1
was evaluated with the help of Eq. (9), which
of the mixture in the form of mole fraction). The values are found to be
°
negative for all the systems; the order of the average ΔG
m
for the mixed
systems in presence of the gemini surfactant is Brij 58 N TX-
m
Using cmc and β , B
are: 14-E2-14 + Brij 58, −2.17, −3.92, −3.57, −4.41; 14-E2-14 + TX-
00, −10.51, −11.28, −11.39 and −12.54. Higher B values indicate
more chain–chain interaction and higher stability of the mixed micelles.
The ΔGMaeda values, calculated from Eq. (7), are recorded in Table 4
°
1
m
00 N CPC N SDS N SDBS N TTAC. The negative ΔG values (Table 3)
again support the occurrence of the mixed micellization process to be
favorable.
1
1
The standard Gibbs energy of adsorption at air/solution interface
∘
°
(
ΔadsG) [37] is correlated to ΔG
m
as
which show comparable values.
Πcmc
Γmax
4. Conclusions
∘
ΔadsG ¼ ΔG −
ð5Þ
m
The surface and micellar properties of the gemini surfactant ethane-
1
,2-diyl bis(N,N-dimethyl-N-tetradecylammoniumacetoxy)dichloride
where Πcmc (= γ
are the surface tensions of pure water and surfactant solution at cmc. For
all the systems, higher ΔadsG than the ΔG
spontaneity to interfacial adsorption vis-à-vis micelle formation. Maxi-
mum value of ΔadsG was obtained for the gemini + TX-100 system
whereas the gemini + SDBS surfactant mixture had a minimum value.
w
− γcmc) is the surface pressure at cmc, γ
w
and γcmc
(
14-E2-14) and its binary mixtures with six conventional surfactants
°
were investigated. For the pure/mixed surfactant micelles, the cmc
values were measured employing conductivity and surface tension
methods. Various theoretical models have been used to obtain the
physicochemical parameters and explanations are provided for the
synergistic interactions between 14-E2-14 and the monomeric surfac-
tants in the mixed systems. For all the binary systems, the cmc values
for different mole fractions of the gemini were found to be less than
ideal cmc indicating synergistic interaction between the surfactants.
Low toxicity of the cleavable/biodegradable surfactant 14-E2-14
makes it particularly interesting for mixed micellization study. Due to
its low toxicity to membranes, it can be considered as a suitable surfac-
tant in drug formulations.
m
(Table 3) shows more
Gmin is the work required to relocate the surfactant molecules from
the bulk phase to the interface of the surfactant solution
Gmin ¼ γcmcAminNA:
ð6Þ
The lowest Gmin of the gemini–anionic surfactant mixtures indicates
that a thermodynamically stable surface, with synergistic interactions,
is formed which is evident also by their interaction parameter values.
Acknowledgments
3
.6. Maeda approach
Manorama Panda acknowledges financial assistance under the
Department of Science and Technology, Government of India (Project
No. SR/WOS-A/CS-46/2007), Nazish Fatma is thankful to the University
Grants Commission (India) for fellowship and Kabir-ud-Din thanks the
UGC for awarding Faculty Fellowship under its BSR Program.
In addition to the electrostatic interactions between the head
groups, steric repulsions due to different hydrophobic chain lengths of
surfactants can also be taken into account. According to Maeda [38]
and Ruiz and Aguiar [39], hydrophobic chain–chain and hydrophilic
head group–head group interactions of the surfactant molecules are
found in mixed micelles.
Appendix A. Supplementary materials
Supplementary materials associated with this article can be found in
the online version, at http://dx.doi.org/10.1016/j.molliq.2015.06.064.
Surfactant structures, various spectra of 14-E2-14, equations used for
the evaluation of physicochemical parameters of the mixed systems.
Conductivity and tensiometry plots of all the mixed systems.
Table 4
ΔG ∘M aeda and B
values for the 14-E2-14 + nonionic binary surfactant systems at 303.15 K.
1
System
Mole fraction of 14-E2-14, α14-E2–14 −ΔG M∘ aeda/kJ·mol−1
B
1
14-E2-14 1.0
–
–
Brij 58
0.2
15.6
17.0
16.9
17.4
16.3
16.6
16.8
17.3
−2.17
−3.92
−3.57
−4.41
−10.51
−11.28
−11.39
−12.54
0
0
0
.4
.6
.8
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