Enhanced aqueous solubility of polycyclic aromatic hydrocarbons by green diester-linked cationic gemini surfactants and their binary solutions

Article abstract of DOI:10.1016/j.molstruc.2016.02.083

Three homologues of a novel biodegradable diester-linked cationic gemini surfactant series, C_mH_2m+1 (CH₃)₂N⁺(CH₂COOCH₂)₂N⁺(CH₃)₂C_mH_2m+1.2Cl^- (m-E2-m; m = 12, 14, 16), were used for investigation of the solubilization of polycyclic aromatic hydrocarbons (PAHs) such as naphthalene, anthracene and pyrene in single as well as binary surfactant solutions. Physicochemical parameters of the pure/mixed systems were derived by conductivity and surface tension measurements. Dissolution capacity of the equimolar binary surfactant solutions towards the PAHs was studied from the molar solubilization ratio (MSR), micelle-water partition coefficient (K_m) and free energy of solubilization (ΔG_s⁰) of the solubilizates. Influence of hydrophobic chain length of the dimeric surfactants on solubilization was characterized. Aqueous solubility of the PAHs was enhanced linearly with concentration of the surfactant in all the pure and mixed gemini-gemini surfactant systems.

Full text of DOI:10.1016/j.molstruc.2016.02.083

Accepted Manuscript
Enhanced aqueous solubility of polycyclic aromatic hydrocarbons by green diester-
linked cationic gemini surfactants and their binary solutions
Manorama Panda, Nazish Fatma, Kabir-ud-Din
PII:
S0022-2860(16)30177-6
10.1016/j.molstruc.2016.02.083
MOLSTR 22292
DOI:
Reference:
To appear in: Journal of Molecular Structure
Received Date: 15 January 2016
Revised Date: 13 February 2016
Accepted Date: 23 February 2016
Please cite this article as: M. Panda, N. Fatma, Kabir-ud-Din, Enhanced aqueous solubility of polycyclic
aromatic hydrocarbons by green diester-linked cationic gemini surfactants and their binary solutions,
Journal of Molecular Structure (2016), doi: 10.1016/j.molstruc.2016.02.083.
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ACCEPTED MANUSCRIPT
Enhanced aqueous solubility of polycyclic aromatic hydrocarbons by green
diester-linked cationic gemini surfactants and their binary solutions
Manorama Panda*, Nazish Fatma and Kabir-ud-Din
Department of Chemistry, Aligarh Muslim University, Aligarh-202002, India
Abstract
Three homologues of a novel biodegradable diester-linked cationic gemini surfactant series,
+
+
−
C H
(CH ) N (CH COOCH ) N (CH ) C H
.2Cl (m-E2-m; m = 12, 14, 16), were
m
2m+1
3 2
2
2 2
3 2
m
2m+1
used for investigation of the solubilization of polycyclic aromatic hydrocarbons (PAHs) such
as naphthalene, anthracene and pyrene in single as well as binary surfactant solutions.
Physicochemical parameters of the pure/mixed systems were derived by using conductivity
and surface tension measurements. Dissolution capacity of the equimolar binary surfactant
solutions towards the PAHs was studied from the molar solubilization ratio (MSR), micelle –
0
s
water partition coefficient (K ) and free energy of solubilization (∆G ) of the solubilizates.
m
Influence of hydrophobic chain length of the dimeric surfactants on solubilization was
characterized. Aqueous solubility of the PAHs was enhanced linearly with concentration of
the surfactant in all the pure and mixed gemini-gemini surfactant systems.
Keywords:
Diester-linked gemini surfactant; Polycyclic aromatic hydrocarbon; Mixed micelle;
Synergistic interaction; Micellar solubilization
*
Corresponding author.
E-mail address: manoramapanda01@gmail.com (M. Panda)
1

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1
. Introduction
The presence of polycyclic aromatic hydrocarbons (PAHs) in nature is of great
concern as these are highly toxic, carcinogenic and mutagenic to microorganism as well as to
all living beings including humans [1]. PAHs are highly lipid-soluble, also get readily
adsorbed and distributed uniformly in the tissues of the body cell producing carcinogenic
effect. The main reason for the prolonged persistence of PAHs in the environment is their low
aqueous solubility owing to their hydrophobicity that enhances their sorption to soils and
sediments, accumulation in aquatic living organisms and transfer to humans through seafood
consumption [2,3]. The biological, physical and chemical methods used to remove these
pollutants have some limitations and side effects [4] and, therefore, the apparent water
solubility of the hydrophobic hydrocarbons could be improved by the addition of synthetic
surfactants [5]. An important property of the surfactants is that they associate in water and
form colloidal aggregates called micelles above a threshold concentration known as the
critical micelle concentration (cmc). Micellar solutions have the ability to increase the
aqueous solubility of hydrophobic compounds by incorporating them into the
hydrophilic/hydrophobic regions depending upon polarity of the solutes [6]. Solubilization
power of the mixed surfactant systems depends not only on the micellar and interface
properties of the component surfactant, but also on the hydrophilic/hydrophobic properties of
solutes. The amounts of organic compounds that can be solubilized in the micelles depend on
the surfactant structure, aggregation number, micelle geometry, ionic strength, temperature,
solubilizate chemistry and their size [7]. A good number of studies has been taken up in
single surfactant systems [8-24] mostly with relatively high surfactant concentrations that
would increase the remediation cost. To reduce the treatment cost, and also the consumption
of surfactants, a new class of amphiphiles, i.e., the gemini surfactants, possessing two
hydrophobic chains and two hydrophilic head groups joined together through a spacer − that
may be rigid or flexible, hydrophilic or hydrophobic − could preferably be used (instead of
the conventional surfactants). This is because of their lower cmc, higher dissolution
capability, good wetting property, and higher efficiency in reducing the surface and
interfacial tension of water [25-29]. As mixed micelles possess improved physicochemical
properties [30], last few years have witnessed a number of studies in the gemini-conventional
mixed surfactant systems toward the solubilization of sparingly soluble compounds [31-36].
2

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Despite the potential use of gemini surfactants for the solubilization of water-
insoluble compounds in micelles, solubilization of PAHs in gemini-gemini surfactant
mixtures are poorly studied. Structure of surfactant and composition of mixture influence the
solubilization capability of the mixed system. The goal of the present investigation is to
determine the extent of solubilization of PAHs in pure surfactant solutions as well as their
equimolar binary mixtures whereby influence of the hydrophobic chain lengths of the dimeric
surfactants on solubilization behavior is also seen. We have used, herein, the ester-linked
cationic gemini surfactants, because of their lower cmc and low toxicity than the other
cationic gemini surfactants [37,38]. Such fundamental and extensive investigations will help
to predict the properties of mixed micelles containing gemini surfactants of the same charge,
and also the mixing effect on solubilization capabilities that will extend the scope of
surfactant−enhanced applications.
2
. Experimental Section
2
.1. Materials
Molecular formula and properties of the solubilizates used in this study, i.e., the PAHs
−
naphthalene (≥ 99.0%, Fluka, Switzerland), anthracene (99.5%, Koch-Light Laboratories
Ltd., England) and pyrene (99%, Fluka, Switzerland) − are listed in Table 1. The cationic
gemini surfactants (m-E2-m, m = 12, 14, 16) were synthesized in the laboratory (Scheme 1),
using a known procedure and the details are reported elsewhere [38].
2
.2. Methods
Solutions of the gemini surfactants and their equimolar binary mixtures were prepared
in double distilled water by mixing pre-calculated volumes of the stock solutions.
2
.2.1. Surface tension measurement
The surface tension (
the Platinum ring detachment method with a Kruss11 Tensiometer (K11MK3, Germany).
The values were measured until consistency in was noticed (Fig. 1a) indicating that the
equilibrium was attained. Intersection of linear portions of the plots of
γ) measurement, used to determine the cmc values, was based on
γ
γ
against log C
(logarithm of the total surfactant concentration for the pure solution or the equimolar binary
mixture) was the cmc.
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2
.2.2. Conductance measurement
Conductometry was also used for the cmc determination. The specific conductivity
data were collected with a ELICO conductivity bridge model CM82T using a platinum
-1
electrode with cell constant of 1.026 cm . The data obtained for all the measurements for all
°
the solutions at 30 ± 0.1 C were perfectly reproducible. In each case, cmc was determined as
the concentration at intersection of the linear portions of the plots of specific conductance
versus concentration of surfactants (Fig. 1b).
2
.2.3. Solubility determination
Solubility of the PAHs was measured in pure gemini and mixed gemini-gemini
micellar solutions. Above cmc, surfactant solutions of different concentrations were prepared
in double distilled water. To ensure maximum solubilization, excess of the solute was added
to 3 mL of solvent in screw-capped sealed vial, and then the mixture was shaken on a
magnetic stirrer for 24 h at room temperature. The mixture was centrifuged at 12000 rpm to
remove the undissolved solutes and then aliquot of the sample was collected. Concentrations
of the dissolved solutes were determined by taking absorbance of the solutions at
wavelengths 276 nm, 253 nm and 272 nm, respectively, for naphthalene, anthracene and
pyrene, using the Shimadzu UV-Vis (UV-mini-1240) spectrophotometer. The surfactant
concentration was kept the same in both the reference and measurement cells to minimize the
error on UV absorbance. Absorbance of the clear solution (i.e., the supernatant liquid taken
after centrifugation) was determined at λmax of the PAH after suitable dilution with the
appropriate solvent. Solubility was calculated by multiplying the PAH concentration, so
obtained, by the appropriate dilution factor. From the absorbance data, concentration of the
PAH was calculated by using the Lambert-Beer law (molar extinction coefficients of
3
-1
-l
5
-1
-l
4
naphthalene, anthracene and pyrene are 5.2 × 10 M cm , 1.82 × 10 M cm and 4.71 × 10
-1
-l
M cm , respectively, as reported by Moroi et al. [13]).
. Results and Discussions
The present study mainly focuses on the micellar solubilization of PAHs by ester-
3
bonded pure and mixed gemini-gemini surfactant systems. Solubilization is directly related
to solution properties of the surfactant systems and it is important to understand the behavior
of micelles and mixed micelles in aqueous medium. Before the solubilization study, it is
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necessary to discuss the micellar and surface properties of the pure and mixed surfactant
systems.
3.1. Mutual interaction between the gemini surfactants in mixed surfactant systems
Pseudophase separation model, based on Clint’s equation [39], was used to calculate
the ideal cmc (cmcideal) which can then be compared with the experimental cmc (cmc12)
1
α1
α2
=
+
(1)
cmcideal cmc1 cmc2
where cmc , cmc are the cmcs and α , α are the stoichoimetric mole fractions of surfactant 1
1
2
1
2
and surfactant 2, respectively, in the mixed micelle solution.
cmc12 values of the binary mixtures are lower than the cmcs of the individual
components (Table 2). Negative deviation of cmc from the cmc values in all the mixtures
ideal
1
2
implies that the mixed micelles show synergism indicating nonideality of the systems. The
order of cmc values is 16-E2-16 + 14-E2-14 < 16-E2-16 + 12-E2-12 ≅ 14-E2-14 + 12-E2-
1
2
1
2.
A quantitative estimation of the nonideality and nature of interactions among both the
components can be made by the regular solution theory, based on Rubingh’s approach, which
explains nonideal mixing of the components in mixed systems [40]. Rubingh’s model was
used to find out the contribution of each surfactant in the form of mole fraction in micellar
m
m
state (
X
1
) and interaction parameter ( β ) by means of the equations (2) and (3).
m
2
m
(
m
X ) ln(cmc12α1 /cmc X )
1 1 1
=
1
(2)
(3)
2
m
(
1− X ) ln{cmc (1−α1)/cmc (1− X )}
1 12 2 1
m
1
ln(cmc12α1 /cmc X )
m
1
β
=
m
2
(
1− X )
1
m
The
β
values, listed in Table 2, give an idea about the type of interactions among the two
m
components in the mixed micelles, which lead to their deviation from ideality. Positive
β
values are related to the net repulsive interaction between the two components, while
m
negative
negative
β
β
are caused by an attractive interaction. In all the systems, we have obtained
m
values indicating synergistic interaction between both the geminis in the mixed
m
surfactant systems which again favors the mixed micellization process. Highest
β
value
5

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was obtained for the system containing both the components of higher hydrophobicity, i.e.,
the 16-E2-16 + 14-E2-14 system. The system 14-E2-14 + 12-E2-12, in which both the
components are of lower hydrophibicity, shows least synergism.
The ideality/non-ideality of mixed micelles and the presence of synergistic interaction
among the surfactants can be suggested by evaluating the activity coefficients of ith
m
component, i.e., f .
i
m
m
m 2
1
f1 = exp{
β
β
(1− X ) }
(4)
(5)
m
m
m 2
f2 = exp{
(X ) }
1
m
m
The results, f > f , for 14-E2-14 + 12-E2-12 and 16-E2-16 + 12-E2-12 implies that the
1
2
activity coefficient of 12-E2-12 is lower than the more hydrophobic gemini surfactant present
m
m
in the system, whereas for 16-E2-16 + 14-E2-14 system, f < f , and activity coefficient of
1
2
1
6-E2-16 is lower than that of 14-E2-14 (Table 2).
3
.2. Interfacial properties of the pure and mixed surfactant systems
Maximum surface excess (Γmax) generally represents the amount of surfactant adsorbed at
the air-water interface. It can be evaluated, for the dilute solution, by Gibbs equation
1

∂
γ 
Γmax




(6)
= −
2
.303nRT ∂logC


where γ, n, C, R and T are the interfacial tension, number of counter ions, concentration of
surfactant, universal gas constant and temperature of the surfactant solutions, respectively.
Here, the values of n are taken as 3 and 4, respectively, for pure and mixed gemini-gemini
surfactant systems. Γmax values for the mixed micelles of 16-E2-16 + 12-E2-12 are greater
than the other systems (Table 2) which indicates that greater is the difference in
hydrophobicity higher is the adsorbing tendency at the air-water interface.
The minimum surface area of surfactant molecule (A ) can be determined by Eq. (7)
min
1
Amin
=
(7)
N Γ
A
max
The values of Γmax and Amin for the single and binary solutions are listed in Table 2. Amin
values of the surfactant mixtures are higher than the pure surfactants because of the
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electrostatic repulsion which requires larger area per molecule. Among all the surfactant
mixtures, Amin is highest for the 14-E2-14 + 12-E2-12 system which may be due to relatively
shorter alkyl chains.
There is genuine interest in determining the thermodynamic parameters of mixed
o
m
micelles like the standard free energy of micellization (∆G ) and excess free energy of
m
micellization (∆Gex ) as these quantify the relative importance of the hydrophobic interactions
and head group repulsions (for ionic amphiphiles).
o
m
m
∆
∆
ꢀ
G
and ∆Gex were calculated using the following equations
o
m
G
=
RT ln Xcmc
(8)
(9)
m
m
m
1
m
m
2
G = RT [X
1
lnf + (1 −X
1
)lnf ]
ex
o
m
The values of free energies (∆G m
,
∆Gex ) were found to be negative suggesting the stability
and spontaneity of the formation of mixed micelles.
3
.3. Solubilization capacity of the single/binary surfactant systems towards PAHs
In this investigation, we have mainly focused on the solubilization of PAHs in the
micellar solution of cationic dimeric surfactants with variable hydrophobic chain lengths (m-
E2-m) and their equimolar binary mixtures. Micellar solubilization is associated with the
properties of surfactant solutions.
3
.3.1. Molar solubilization ratio and micelle phase/aqueous phase partitioning of PAHs
Plots of the variation of solubility of naphthalene, anthracene and pyrene, as a function
of total surfactant concentration of the single/binary surfactant systems, are shown in Figs. 2
and 3. Solubility of the PAHs was significantly enhanced by the single/binary surfactant
solutions
− the linear increase over the range of surfactant concentrations above cmc suggests
micellar solubilization of the solutes. These results demonstrate the potential capacity of the
pure and mixed micelles to enhance the solubility of PAHs efficiently due to adsorption at
micelle−water interface, in addition to solubilization inside the micelles.
In order to quantify these observations, the solubilization capacities were analyzed in
terms of molar solubilization ratio (MSR) [10]. MSR depends on the ratio of the number of
solubilizate molecules inside an aggregate and the number of surfactant molecules
7

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constituting the micelle. Evidently, mole fraction of the solutes in the micellar pseudophase
(Xm) and the MSR are related as Xm
= MSR / (1+ MSR).
MSR = (St−Scmc) / (C −Ccmc)
(10)
t
where St is total apparent solubility of the solute in a given surfactant solution at a specific
total surfactant concentration, Ct is concentration above cmc Scmc is the apparent solubility of
PAH at cmc MSR values of the single/mixed surfactant systems were obtained from the slope
,
.
of the straight line obtained from the plot of the concentration of solubilizate against the total
surfactant concentration above cmc (Figs. 2 and 3). The most interesting variable, which
influences the solubilization of the PAHs [8], is the cmc
.
To further measure the effectiveness of solubilization, the micelle-water partition
coefficients (Km), which depend on the molecular structure of the solubilizate [12-14,41],
were determined. Km is a thermodynamic parameter that represents the affinity of a given
solubilizate to micellar phase as compared to the aqueous phase, and is used to determine the
amount of PAH molecules solubilized by the micelles. For evaluation of Km, Eq. (11) was
used
Km
=
MSR / {[Scmc
]
Vm (1+ MSR)}
(11)
-1
Here Vm is the molar volume of water (= 0.01807 Lmol at 30 °C). The increase in Km
parallels the increase in hydrophobicity of the solubilizate indicating that the solubilization is
controlled mainly by the hydrophobic interactions.
In single surfactant systems, the order of MSR is 16-E2-16
naphthalene, 14-E2-14 12-E2-12 16-E2-16 for anthracene and 14-E2-14
2-E2-12 for pyrene (Table 3). With the increase of surfactant concentration above cmc, the
>
14-E2-14
>
12-E2-12 for
>
>
>
16-E2-16 >
1
number of micelles increases thereby enhancing the PAH solubility. There are different
possible localization sites in the micelles for availability of the solubilizate. Their structural
arrangement and polarity decide the location sites in the micellar systems. Slight polarity of
the aromatic hydrocarbons allows these compounds to be located both in outer shell and also
in the hydrophobic core of surfactant micelles [9,11,15,17]. Naphthalene is slightly polar and
participates in hydrogen bonding with water because of the presence of π-electrons in the
aromatic rings [17,42]. Surface of the micelles, space between the hydrophilic head groups,
i.e., palisade layer, and inner core of micelles are the possible localization sites of the solutes
affecting the extent of solubilization.
8

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In mixed micelles the order of solubilization for naphthalene is 16-E2-16 + 12-E2-12
>
16-E2-16 + 14-E2-14
>
14-E2-14 + 12-E2-12, but for anthracene and pyrene the order is
1
6-E2-16 + 12-E2-12
>
14-E2-14 + 12-E2-12 16-E2-16 + 14-E2-14. Higher MSR values
>
were obtained for the 16-E2-16 + 12-E2-12 system for all the three solubilizates which might
be due to the difference in chain length of both the component surfactants in the mixed
systems. In general, electrostatic interaction between the π-electrons of PAH and positive
charge on the quaternary ammonium head groups facilitate micellar solubilization. The outer
hydrophilic corona of the micelles has efficiency for solubilizing the slightly polar
naphthalene of smaller molecular volume. The presence of diester bonded spacer may also
play an important role in solubilization as compared to the polymethylene spacer. Longer
spacer provides hydrophobic environment by looping upon itself whereas ester-linked spacer
facilitates the solubilization of slightly polar naphthalene as there is a possibility of
interaction between the π-electrons of naphthalene and the carbonyl group, i.e., [−CO(O)−].
Naphthalene gets solubilized at the micelle/water interface and between the hydrophilic head
groups, i.e., the palisade layer.
Higher MSR and Km values for pyrene than anthracene in all the surfactant systems
indicates that anthracene is less soluble in micelles because of its larger lateral dimension
resulting in lesser intercalation in the micelles as compared to pyrene. The extent of
solubilization depends upon molecular architecture of the solute and space between the head
groups of the surfactants in micelles. In general, there is a positive correlation between the
micellar core size and the solubility enhancement, although the potential of expansion of
micelles due to addition of the solute (i.e., formation of swollen micelles) also plays an
important and additional role in solubilization [22,42]. The fused aromatic rings makes
pyrene more hydrophobic than naphthalene and is responsible for its deeper occupancy
resulting in the increase of more hydrophobic region, i.e., micellar core volume. This is
indicated by the appreciably larger Km values for the binary mixtures. Such difference in
solubilization probably suggests the location of the PAHs in micelles. The dicationic gemini-
gemini surfactant mixtures represent better systems for solubilization of PAHs than the other
studied surfactant systems [31-34]. For anthracene and pyrene, higher MSR and Km values
were observed in the mixed systems than the single surfactant systems. The synergism is
attributable to the reduced polarity of the mixed micelles which results the increase of MSR
(or Km).
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3
.3.2. Micelle-water partitioning of the PAHs
Each surfactant mixture is different from the other because it contains the gemini
surfactants with unequal length of hydrophobic tails. From thermodynamic point of view [43,
4], the partition coefficient in mixed micelle, Km12, can be expressed as a function of the
4
micellar composition. The partition coefficient of PAH between the aqueous and micellar
phases in the binary surfactant solutions can be evaluated by using Eq. (12)
m
m
m
m
ln Km12= X lnK +
(1- X1
)
lnK +B X (
1- X1
)
(12)
1
m1
m2
1
Km1
, Km2, Km12 are the micelle-water partition coefficients of the PAHs in micelles having
component 1 (gemini surfactant with higher chain length), component 2 (gemini surfactant
with lower chain length) and in mixed micelles, respectively (Table 4).
B is an empirical
m
parameter that considers both the surfactant-surfactant (as in β ) and surfactant-solute
interactions. When
aqueous and micellar phases [45].
except for naphthalene in 14-E2-14 + 12-E2-12 and 16-E2-16 + 14-E2-14 (Table 4). Negative
value of implies that the solubilization decreases upon mixing of surfactants.
Deviation ratio ( ) can be defined as the ratio of MSR (experimental) and MSRideal
ideal MSR), given as MSR MSRideal, where MSRideal ΣiMSRiαi. MSRi is the
th surfactant solution whose bulk mole
, from Table 4, show positive deviation
B
= 0, mixing has no effect on the partitioning of a solute between the
B
values are positive for all the binary surfactant mixtures
B
R
(
R
=
/
=
experimental MSR value of solubilizate in the pure
i
fraction in the mixture is α . Values of the parameter
R
i
from ideality for all the binary surfactant mixtures except for the solubilization of
naphthalene in 14-E2-14 + 12-E2-12 and 16-E2-16 + 14-E2-14. Positive value implies the
positive mixing effect of the amphiphiles on the PAHs, negative for the dissolution of
R
R
naphthalene in 14-E2-14 + 12-E2-12 and 16-E2-16 + 14-E2-14 may possibly be due to the
reduction in surface adsorption of naphthalene molecules in the mixed micelles. Such a
variation of deviation ratio depends upon the surfactant molecular microstructures (packing
of surfactant molecules at micelle–water interface as well as in the mixed micelle core), and
also on the surfactant-solute interaction in the mixed micelles. As compared to the other
cationic gemini-gemini mixed surfactant systems, diester-linked cationic gemini surfactant
mixtures show notably higher
solubilization [33,34].
R values, implying greater positive effect of mixing on
1
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0
S
The standard free energy of solubilization, ∆G , was obtained by using Eq. (13) [46]
0
∆GS
=
−RT lnKm
(13)
0
The ∆GS values come out to be negative for all the mixed systems (Table 4) indicating
spontaneous solubilization. Solubilization of the studied bi-component micelles can be
considered as a normal partitioning of the solubilizates between the micellar and aqueous
0
phases. ∆GS values of the mixed micelles for different solubilizates follow the order: pyrene
0
>
anthracene > naphthalene. Among all the surfactant mixtures the magnitude of ∆GS is
maximum in case of 16-E2-16 + 12-E2-12 for all the three PAHs.
4
. Conclusion
Physicochemical properties of pure and mixed (1:1) surfactant solutions of the
biodegradable ester-linked dicationic geminis of different chain length (m-E2-m: m = 12, 14,
6) were investigated by the conductivity and surface tension methods. Various surface and
1
micellar properties were evaluated in the light of several theoretical models suggested by
Clint and Rubingh. All the mixed surfactant solutions showed nonideality as indicated by the
m
cmc and β values. Effect of the solution compositions of the selected single/binary gemini-
gemini surfactant systems on solubilization of three PAHs
− naphthalene, anthracene and
pyrene were studied. The single/binary surfactant solutions increase the aqueous solubility
−
of PAHs which illustrates potential capacity of the used systems to facilitate the
solubilization in water. Anthracene and pyrene were solublized synergistically in all the
mixed systems. In the gemini-gemini mixed surfactant micelles the solution properties are
influenced not only by the constituents but also by electrostatic interaction of the head groups
of surfactants with the
π-electron ring of PAHs. The binary surfactant solutions have
relatively selective solubilization capacities towards the PAHs.
Acknowledgment
Manorama Panda acknowledges financial assistance under the Department of Science
and Technology, Government of India, Nazish Fatma is thankful to the University Grants
Commission (India) for fellowship and Kabir-ud-Din is thankful to UGC for awarding
Faculty Fellowship under its BSR Program.
1
1

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1
5

ACCEPTED MANUSCRIPT
Scheme 1. Synthesis route of the biodegradable cationic gemini surfactants (m-E2-m;
m = 12, 14, 16).
1
6

ACCEPTED MANUSCRIPT
(
a)
7
6
6
6
5
5
4
4
2
8
4
0
6
2
8
4
1
4-E2-14+12-E2-12
16-E2-16+14-E2-14
6-E2-16+12-E2-12
1
-7.6
-7.2
-6.8
-6.4
-6.0
-5.6
log[Surfactant]
0
0
0
0
0
0
0
0
0
.16
.14
.12
.10
.08
.06
.04
.02
.00
(
b)
1
1
1
4-E2-14+12-E2-12
6-E2-16+14-E2-14
6-E2-16+12-E2-12
0
.0000
0.0005
0.0010
0.0015
0.0020
0.0025
0.0030
[Surfactant] (mM)
Fig. 1. Plots of (a) surface tension vs. logarithm of surfactant concentration and (b) specific
conductivity vs. surfactant concentration.
1
7

ACCEPTED MANUSCRIPT
0
0
0
0
0
0
0
.48
.40
.32
.24
.16
.08
.00
12-E2-12
0.010
1
1
1
2-E2-12
4-E2-14
6-E2-16
1
1
4-E2-14
6-E2-16
0.008
0.006
0.004
0.002
0.000
0.0
0.2
0.4
0.6
0.8
1.0
0
1
2
3
4
5
6
[Surfactant] (mM)
[
Surfactant] (mM)
0.06
0.05
0.04
0.03
0.02
0.01
0.00
1
1
2-E2-12
4-E2-14
16-E2-16
0
1
2
3
4
[Surfactant] (mM)
Fig. 2. Variations of solubility of naphthalene, anthracene and pyrene in pure cationic gemini
surfactants (m-E2-m: m = 12, 14, 16).
18

ACCEPTED MANUSCRIPT
0
0
0
0
0
0
0
0
0
0
.50
.45
.40
.35
.30
.25
.20
.15
.10
.05
1
1
1
4-E2-14+12-E2-12
6-E2-16+14-E2-14
6-E2-16+12-E2-12
0.007
14-E2-14+12-E2-12
16-E2-16+14-E2-14
16-E2-16+12-E2-12
0
0
0
0
.006
.005
.004
.003
0.002
0
0
.001
.000
0
.0
0.2
0.4
0.6
0.8
1.0
1.2
0
.0
0.2
0.4
0.6
0.8
1.0
[
Surfactant] (mM)
[
Surfactant] (mM)
14-E2-14+12-E2-12
16-E2-16+14-E2-14
16-E2-16+12-E2-12
0.035
0.030
0.025
0.020
0.015
0.010
0.005
0.000
0.0
0.2
0.4
0.6
0.8
1.0
[Surfactant] (mM)
Fig. 3. Variation of solubility for naphthalene, anthracene and pyrene in different concentrations of
gemini surfactants in their equimolar binary mixtures.
19

ACCEPTED MANUSCRIPT
Table 1
Some important properties of the PAHs used for the study.
Property
Naphthalene
Anthracene
Pyrene
Molecular formula
Molecular weight
C
10
H
8
C
14
H
10
16 10
C H
128.2
178.2
202.3
-4
-7
-7
Aqueous Solubility
2.44×10
2.53×10
6.57×10
3
(
mol/dm )
a
log KOW
3.36
4.54
5.18
3
Molecular volume (Å )
126.90
157.60
161.90
a
KOW is octanol-water partition coefficient.
20

ACCEPTED MANUSCRIPT
Table 2
m
m
Experimental cmc (cmc ), ideal cmc (cmcideal), micellar mole fraction (
X
1
), interaction parameter ( β ),
1
2
m
m
activity coefficients ( f1
,
f2 ), maximum surface excess concentration (Γmax), minimum area per head group
m
ex
o
(
Amin), excess free energy of micellization ( ∆G ) and standard free energy of micellization (∆
G
) of the binary
m
surfactant (1:1) solutions at 30 °C.
7
m
m
m
m
m
ex
o
m
-1
Mole
Fraction,
α1
cmc,
cmc12
(mM)
cmcideal
(mM)
Γ
max × 10
A
min
(Å )
X
1
β
f1
f2
−
∆G
− ∆G
a
-2
2
(mol m )
-1
(
kJmol )
(kJmol )
1
4-E2-14+12-E2-12
8.41
b
0
0
1
.0
.5
.0
0.0016a
-
-
-
-
-
197.38
232.53
188.04
-
0.93
-
--
44.9
44.1
0.0010 0.0015 0.5234 −1.49 0.7130 0.6650
7.14
8.83
c
0.0014
-
-
-
-
1
-
-
6-E2-16+14-E2-14
8.83
c
0
0
1
.0
.5
.0
0.0014a
-
-
-
-
188.04
211.46
140.68
-
1.34
-
44.1
45.5
43.9
0.0008 0.0014 0.4891 −2.12 0.5741 0.6013
7.85
11.80
d
0.0013
-
-
-
-
1
-
-
6-E2-16+12-E2-12
8.41
b
0
0
1
.0
.5
.0
0.0016a
-
-
-
-
197.38
210.54
140.68
-
1.13
-
--
45.4
43.9
0.0010 0.0016 0.5093 −1.79 0.6501 0.6288
7.88
11.80
d
0.0013
-
-
-
-
-
a
Average of the cmc values obtained by conductivity and surface tension measurements.
b
c
d
Ref. [38a], Ref. [38d], Ref. [38c].
21

ACCEPTED MANUSCRIPT
Table 3
MSR MSRideal for naphthalene, anthracene, pyrene in the pure/mixed surfactant systems at 30 °C.
,
Naphthalene
Anthracene
Pyrene
System
MSR
MSRideal
MSR
MSRideal
MSR
MSRideal
1
1
1
1
1
1
2-E2-12
0.6070
0.8550
1.0820
0.0064
0.0076
0.0014
0.0183
0.0634
0.0408
4-E2-14
6-E2-16
4-E2-14+12-E2-12
6-E2-16+14-E2-14
6-E2-16+12-E2-12
0.5910 0.7312
0.7170 0.9687
1.0400 0.8445
0.0143 0.0070
0.0132 0.0045
0.0160 0.0039
0.0884 0.0409
0.0612 0.0521
0.0941 0.0296
22

ACCEPTED MANUSCRIPT
Table 4
Partition coefficient (ln Km), free energy of solubilization (∆Gs ), deviation
0
ratio (
R
), and experimental interaction parameter (
B
) for the binary surfactant
(1:1) systems at 30 °C.
0
System
ln Km
R
B
∆Gs
-1
(
kJmol )
Naphthalene
0.81
1
1
1
4-E2-14+12-E2-12
6-E2-16+14-E2-14
6-E2-16+12-E2-12
11.34
11.60
11.66
−0.48
−0.25
0.17
−28.58
−28.88
−29.38
0.74
1.23
Anthracene
2.11
1
1
1
4-E2-14+12-E2-12
6-E2-16+14-E2-14
6-E2-16+12-E2-12
14.94
14.86
15.05
3.01
5.73
6.60
−37.65
−37.46
−37.93
1.95
3.90
Pyrene
1
1
1
4-E2-14+12-E2-12
6-E2-16+14-E2-14
6-E2-16+12-E2-12
15.73
15.39
15.79
2.16
1.17
3.18
3.53
0.69
4.68
−39.66
−38.80
−39.82
23

ACCEPTED MANUSCRIPT
Highlights
•
•
Mixed micellization of diester-linked cationic gemini surfactants was studied.
PAHs (naphthalene, anthracene and pyrene) were effectively solublized in pure/mixed
systems.
•
•
Solubilization was influenced by hydrophohobicity of the PAHs.
Anthracene and pyrene were solublized synergistically in all the mixed systems.