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Interactions of cationic surfactants with polyvinylpyrrolidone (PVP): Effects of counter ions and temperature
Journal Pre-proof Interactions of cationic surfactants with polyvinylpyrrolidone (PVP): Effects of counter ions and temperature
Murat Bali, Özgür Masalcı PII:
S0167-7322(19)35269-9
DOI:
https://doi.org/10.1016/j.molliq.2020.112576
Reference:
MOLLIQ 112576
To appear in:
Journal of Molecular Liquids
Received date:
20 September 2019
Revised date:
13 January 2020
Accepted date:
24 January 2020
Please cite this article as: M. Bali and Ö. Masalcı, Interactions of cationic surfactants with polyvinylpyrrolidone (PVP): Effects of counter ions and temperature, Journal of Molecular Liquids(2018), https://doi.org/10.1016/j.molliq.2020.112576
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© 2018 Published by Elsevier.
Journal Pre-proof
Interactions of Cationic Surfactants with Polyvinylpyrrolidone (PVP): Effects of Counter Ions and
of
Temperature
-p
ro
Murat Balia, Özgür Masalcıa* a
Jo ur
na
lP
re
Ege University, Faculty of Science, Department of Physics, 35100, Bornova, Izmir, TURKEY
* Corresponding author. Phone: +90-232-3115439; fax: +90-232-3112833 E-mail: [email protected]
1
Journal Pre-proof Interactions of Cationic Surfactants with Polyvinylpyrrolidone (PVP): Effects of Counter Ions and Temperature The interactions between the polymer polyvinylpyrrolidone (PVP) and conventional surfactants such as cetyltrimethylammonium chloride (CTACl)
and
cetyltrimethylammonium
bromide
(CTABr)
were
investigated using a conductometric method. The critical micelle concentrations (CMCs), the critical aggregation concentrations (CACs) and degrees of ionization (α) of aqueous solutions of the surfactants and
of
PVP were determined via electrical conductivity measurements at various temperatures. The thermodynamic parameters (Gibbs free energy,
ro
enthalpy and entropy) of the surfactants were calculated in the absence and
-p
presence of the polymer, and the results were compared to determine the effects of counter ion on the micellization of the surfactants and the
re
energetics of the process. The main goal of this work is to elucidate the various driving forces that lead to complexation between the surfactants
lP
and polymer. Electrostatic interactions play a major role in the binding of surfactants to polymers or surfaces in aqueous systems. The strength of
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ion type.
na
the interaction is found to depend on the nature of the surfactant counter
Keywords: Aggregation, Polymer-surfactant interaction, Gibbs free energy, Entropy
2
Journal Pre-proof Introduction The wide use of surfactant-polymer systems in industry has led to the further study and investigation of surfactant-polymer mixtures [1-2]. Applications of surfactantpolymer mixtures vary greatly from industrial products in almost all parts of daily life (detergents, cosmetics, paints, coatings, foods and pharmaceutical manufacturing) to biological systems (structure and function of membranes and transfer of lipids) [3-7]. A clear understanding of the nature of surfactant-polymer interactions in aqueous solutions has been attracting scientific interest. In view of the applications, comprehending these interactions is very important for estimating solution properties.
of
Because surfactant molecules have distinct properties such as hydrophilic head
ro
and hydrophobic tail groups, their solutions do not act as a simple solution. Their characteristics, which vary, based on their concentrations. As the concentration of the
-p
surfactant increases in the solution, these molecules tend to combine at the critical
re
micelle concentration (CMC) due to their self-assembly properties. The similarities between aggregations of surfactants and polymers solutions in water make it difficult to
lP
understand surfactant-polymer systems. The hydrophobic-hydrophilic structure of a neutral polymer affects its solubility. According to the Cabane and Duplessix model [8,
na
9], the polymer-surfactant interactions start at the critical aggregation concentration (CAC). This concentration is denoted the CAC because it is concentration at which
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surfactants begin to interact with a polymer, and it is usually less than the CMC. Surfactants that are subsequently added to the solution later bond to the saturated polymer, forming a micelle in the solution. The concentration at which typical micelles form is called C2 [10].
The dissolution behaviour in water is controlled by the intermolecular forces between water and water and/or water and the soluble material. The interactions of surfactants can be categorized into three groups based on the following: the hydrophobicity of the alkyl chain, the hydrophilicity of the head group structure and the electrostatic character of the head group in ionic surfactants [11]. The hydrophobic properties of surfactants are mostly related to the length or size of the alkyl chain. The hydrophilic properties of ionic surfactants are associated with the polarity, hydrogenbonding capacity and charge distribution of the head group structure. The electrostatic interactions of the ionic surfactant head group can account for particular transitions of the ionic surfactant to other phases and for the formation of ion pairs and ion bridges 3
Journal Pre-proof with other phases [12]. The resulting hydration sphere of the surfactant greatly affects its alignment in water [13]. All the physico-chemical characteristics influence the organization of water molecules in the vicinity of the surfactant and determine whether hydration is strong or weak, that is, kosmotropic or chaotropic, respectively. The interactions between the head groups of the surfactants and the ions/counter ions at the micelle surfaces can be examined using the Hofmeister ion series, and therefore, the ions can also be classified as kosmotropic and chaotropic; specifically Cl is a kosmotrope, and Br is a chaotrope [14, 15]. The counter ions of the charged head groups of ionic surfactants have been known to affect the organization or arrangement
of
of surfactant molecules in water [16]. The counter ion impacts the adsorption layer and micelle aggregation process, due to its different nature, charge and polarizability or
ro
hydration degree [17].
-p
Although numerous studies have explored the interactions between natural polymers and surfactants, the nature of neutral polymer-surfactant interactions is not yet
re
fully understood [18, 19]. Until recently, either variations in the length of the chain of the surfactant or the effects of different polymer masses were generally examined [20-
lP
24]. For example Majhi et al. (2001) [22] investigated the interactions of CTAB and TTAB surfactants with PVP at different PVP concentrations and a constant temperature.
na
CTAB and TTAB have the same head group but different chain lengths. Moreover, the study also examined to what degree the enthalpy of dilution of the surfactant and the
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salt solution is directly influenced by PVP to acquire data about the quality of its interaction with the the polymer. Sood et al. (2009) [23] studied the micellization behaviour of cationic surfactants with bulky head groups. In particular, they examined alkyldimethylbenzylammonium chlorides (C14, C16, and C18BCl) in water and in aqueous solutions of PVP, PEG-20, and PEG-06 at 298.15 K. The mixed micelle properties of these surfactants in the presence of PVP were also investigated and at various temperatures. The chosen surfactants and prepared mixed surfactants had increasing, chain lengths. Therefore, the effects of the chain length on the polymersurfactant interactions were investigated. Sardar et al. (2013) [24] systematically examined the interactions between cationic gemini surfactants and the neutral polymer polyvinylpyrrolidone (PVP K15 and K90), focusing on the impacts of the molecular weight on the neutral polymer-cationic surfactant interactions and the rheological characteristics a function of the polymer and surfactant concentrations. The study also explored the effects of the chain length on the polymer-gemini surfactant interactions. 4
Journal Pre-proof In
this
study,
cetyltrimethylammonium
chloride
(CTACl)
and
cetyltrimethylammonium bromide (CTABr), which have the same chain length but different counter ions, were used. The interactions between the cationic surfactants with different counter ions and the natural polymer polyvinylpyrrolidone (PVP) were investigated based on the thermodynamic parameters (Gibbs free energy, enthalpy and entropy) of aggregation. Materials and methods
of
Materials Cetyltrimethylammonium chloride (CTACl, cat no: 52366 Fluka) with a molecular
ro
weight of 320.00g/mole and cetyltrimethylammonium bromide (CTABr, cat no: 52369
-p
Fluka) with a molecular weight of 364.45 g/mole were supplied by Fluka and, used without any processing or modifications. Polyvinylpyrrolidone (PVP, cat no: 9003-39-
re
8) with an average molar mass of 10000 g/mole was obtained from Sigma-Aldrich. The purity of the purchased and used chemicals was more than 99% (purity>99%). The
lP
molecular structures of surfactants (CTACl and CTABr) and polymer (PVP) are shown in Figure 1. Deionized water was used as the solvent medium in the experiments. The
na
electrical conductivity of the deionized water was measured to be 8μS/cm at 25 oC (298
Jo ur
K).
Cetyltrimethylammonium Bromide (CTABr)
Cetyltrimethylammonium Bromide (CTABr)
Polyvinylpyrrolidone (PVP)
C6H9NO)n (a)
(b)
(c )
Figure 1. Structures and chemical formula of (a) CTACl, (b) CTABr and (c) PVP
5
Journal Pre-proof Experimental Procedure The quantities of the materials used in the experiments were measured using an AND trademark HR-120 model assay balance with a 10-4 sensitivity. Different amounts of the surfactants (0.5 mM-5 mM) were added to a constant volume of deionized water to prepare individual solution samples with different concentrations. The determined amounts of the surfactants (0.5 mM-5 mM ) were added to a PVP-water solution (%1 PVP (w/V)) with a constant volume to study the interactions between polymer and surfactant. The conductivities of all the samples were measured in the temperature range
of
of 293 K to 313 K. The conductivity of the deionized water was measured at all the temperatures
ro
used in this study. In addition, conductivity of the PVP-water solution without a surfactant was measured in the range of all the temperatures. The measurements of the
re
-p
conductivity and temperature had sensitivities ∓5% μS/cm and ∓0.2 oC, respectively.
Conductivity Measurements
lP
Methods
na
Electroconductivity measurements were performed using the WTW inoLab Cond Level 3 module. For convenience, TetraCon 325 probe was chosen for the measurements and, coupled to the cable-galvanized module. The probe measurement
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ranges for the temperature and electroconductivity were 5–100 °C (with an accuracy of 0.2 °C) and 1 μS/cm-2 S/cm (the accuracy, which is temperature dependent, is 5%), respectively.
Prior to the experiments, it is necessary to calibrate the probe; a 0.1 mol/l KCl calibration liquid was used to perform the procedure according to the operating manual. To measure the dependence of the electrical conductivities of the samples on the temperature, a PHYWE trademark thermostat and water bath were used. Using the temperature sensor on the probe, the temperature of the sample could be directly read from the measurement terminal. When the temperature reached equilibrium and the system was stabilized, the electrical conductivity of the sample was measured at the given temperature. The experiments were repeated at least 3 times.
6
Journal Pre-proof Results and Discussion Conductivity measurements Fig. 2 shows the specific conductivity (κ) profiles as a function of surfactant concentration. In Figures 2a and 2c, the intersection point of the two straight lines represents the usual CMC of surfactants CTACl and CTABr, respectively, at a constant temperature. The change in the conductivity values following the CMC is explained by the binding of the counter ion to the micelle. The degree of ionization (α1) of the micelle can be calculated from the ratio of the slopes the conductivity curves below (S1)
Jo ur
na
lP
re
-p
ro
of
and above (S2) the CMC [25].
Figure 2. Plot of the specific conductivity versus the concentration for a) CTACl in water, b) CTACl in the presence of water+1%PVP (w/V), c) CTABr in water, and d) CTABr in the presence of water+1% PVP (w/V) at 303K. 7
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Table 1. Changes in the critical concentrations (CMC, CAC and C2) and degrees of ionization (α1 and α2) with temperature CTACl α1
α2
CAC
C2
(mM)
(mM)
(mM)
293.15
2.16
0.72
1.67
0.41
298.15
2.15
0.79
1.69
303.15
2.06
0.83
308.15
2.20
313.15
2.46
α1
α2
1.34
0.35
0.19
0.84
1.38
0.32
0.20
0.87
1.40
0.31
0.22
0.85
1.39
0.36
0.21
0.91
1.47
0.39
0.23
CMC
CAC
C2
(mM)
(mM)
(mM)
0.22
0.93
0.78
0.43
0.24
1.07
1.71
0.41
0.26
0.95
0.82
1.71
0.40
0.22
1.37
0.85
1.76
0.48
0.27
1.40
of
CMC
ro
T (K)
CTABr
-p
As shown in Table 1 and, Figures 2a and 2c, the CMC value of CTACl is higher
re
than that of CTABr due to difference in the counter-ion radius. The ionic radius of bromine (rBr=0.196 nm) is larger than that of chlorine (rCl=0.181 nm), i.e., rBr>rCl [16].
lP
The strength of the electrostatic attraction decreases as the counter-ion radius increases. Therefore, the electrical repulsion decreases between the micelle head groups of the
na
CTABr molecules, which have Br counter ions enabling micelles to aggregate at lower concentration. It is a characteristic feature of micellization that CMC is to a first
Jo ur
approximation independent of temperature [10]. The CMC may decrease increase or indicate a conspicuous minimum with increasing temperature. Thus, the increase in the CMC values of CTACl and CTABr with temperature may be ascribed to disruption of structured solvent molecules surrounding the hydrophobic groups and desolvation of the ionic head groups [26].
The counter-ion influences the process of micelle aggregation. If the counter ion Br is more polarisable than Cl [27] and has a higher binding efficiency to the micelles, a strong dispersive interaction results, leading to a greater tendency to form cylindrical micelles. Br ions have a stronger ability to form micelles than Cl ions [27], which can be explained by the fact that Br ions are readily able to penetrate into the Stern-layer and facilitate the aggregation of micelles. The electrostatic interactions in neutral polymer/surfactant systems are much weaker than those in the absence of the polymer [28]; thus, the specific conductivity trend can change. The two breaks observed in the presence of the polymer indicate that 8
Journal Pre-proof two types of aggregation occur. The first is called CAC, at which the interaction of the surfactant with the polymer chain begins. As shown in Table 1, the CAC values are smaller than the CMC values. The second is defined as C2, at which the surfactant molecules completely saturate the polymer chains and form normal micelles as well [29, 30]. The degree of ionization of the surfactant (CTABr or CTACl)-polymer (PVP) mixture (α1) can be estimated from the slope and appears to be higher than the degree of ionization of the ordinary micelles in the polymer-containing solution (α2). This result indicates that the micelles at C2 are less ionized than those at the CAC, which is attributed to the decrease the interfacial polarity due to the structural transitions [8]. The
ro
of
ratios of S2/S1 and S3/S1 can be used to obtain the α1 and α2 values, respectively.
1% PVP (w/V)
-p
1.8
re
1.4 1.2 1
0.8 0.6 296
Jo ur
na
291
lP
C(mM)
1.6
CTACl
301
CTACl
306
311
316
T(K) CTABr
CTABr
Figure 3. Changes in the critical concentrations with temperature in the presence of PVP
The CAC and C2 values obtained in the presence of PVP are plotted in Figure 3 and shown in Table 1. The CAC and C2 values change slightly with temperature in the presence of the polymer. Although the CAC values of the two surfactants are close to each other, the C2 value of CTACl is higher than that of CTABr (a comparison of the CMC values of CTACl and CTABr shows that the CMC value of CTACl is higher than that of CTABr). The range of C2-CAC gives information about the bonding of the surfactant to the polymer [15, 31]. An increase in the CAC-C2 range indicates that the bonding is stronger. The CTACl, range for CTACl is wider than that for CTABr; therefore, the 9
Journal Pre-proof bonding of CTACl/PVP is stronger than that of CTABr/PVP, due to the difference in the radii of the counter ions. The decrease in the C2-CAC value with the increasing counter ion radius shows that the strength of the interaction decreases as the counter-ion size increases (rBr> rCl). That is, the strength of the interaction and the radius of counter ion are inversely proportional. These results are consistent with those of Dubin et al. [20] and Benrraou et al. [28].
1% PVP( w/V) 0.6
of
0.5
ro
α
0.4 0.3
-p
0.2 0.1 290
re
0 295
300
305
310
315
CTACl_α2
CTABr_α1
CTABr_α2
na
CTACl_α1
lP
T(K)
Figure 4. Variations in the degree of ionization (α) as a function of temperature in the
Jo ur
presence of PVP
The variations in the degree of ionization (α) are shown as a function of temperature in Figure 4 and Table1. The α1 and α2 values vary non-linearly, which can be explained by the micelle-polymer interactions in the mixture. The degree of ionization shows that an increase in the counter-ion kinetic energy leads to an increase in the counter-ion mobility. The α1 and α2 values when the counter ion changed from Cl to Br, due to the increase in the counter-ion dimension, which results in a reduction in the strength of hydration [16, 17]. Less hydrated ions can be collected more easily in the micellar surface, which decreases the charge repulsion between the polar groups and therefore makes micellization easier, whereas the charges of heavily hydrated ions are partly shielded by the surrounding polar water molecules, and thus, these counter ions are less effective at reducing the charge repulsion between the head groups [25].
10
Journal Pre-proof The degree of ionization of the surfactant (CTACl or CTABr)-PVP complex (α1) could be evaluated from the slopes, and it is higher than the degree of ionization of ordinary micelles in the polymer-containing solution (α2), indicating that the micelles at the CAC are more ionized than those at C2, which can be explained by the enhancement in the interfacial polarity due to the structural change [8, 20]. The higher value of α1 for the complex micelle reveals an increase in the degree of ionic dissociation as a consequence of the interactions between the surfactants and polymer [32]. The appearance of ordinary micelles at C2 shows that the α2 degree of ionization is less influenced by the temperature and nearly stable. Although the α1
of
degree of ionization hardly changes at low temperatures, it increases slightly at
ro
temperatures over 303.15 K.
-p
Thermodynamics parameters of aggregation
re
A pseudo-phase separation model [33, 34] was used to estimate the Gibbs free energy (∆𝐺), enthalpy (∆𝐻), and entropy (∆𝑆) of CTACl and CTABr in aqueous
lP
solutions. These parameters can be calculated by the equations given below. (1)
na
∆𝐺 = (2−∝)𝑅𝑇𝑙𝑛𝑋𝐶𝑀𝐶
where α is the degree of counter-ion dissociation from the micelle, R is the gas constant,
Jo ur
XCMC is the CMC expressed in mole fraction units and T is the temperature in Kelvin. The enthalpy of micellization can be obtained using the Gibbs–Helmholtz equation. ∆𝐻 = −(2−∝)𝑅𝑇 2 (
𝜕𝑙𝑛𝑋𝐶𝑀𝐶 𝜕𝑇
)
(2)
The entropy of micellization (∆𝑆 ) can be estimated from the given values of ∆𝐺 and ∆𝐻 using the following relation. ∆𝑆 =
∆𝐻−∆𝐺 𝑇
(3)
The Gibbs free energy (∆𝐺𝑝𝑠𝑖 ) of a polymer-surfactant system has been defined by Gilany and Wolfram [35]. 𝛥𝐺𝑝𝑠𝑖 = (2−∝)𝑅𝑇𝑙𝑛𝑋𝐶𝐴𝐶
(4)
11
Journal Pre-proof The values of the enthalpy (∆𝐻𝑝𝑠𝑖 ) and entropy (∆𝑆𝑝𝑠𝑖 ) of aggregation in the presence of polymer were calculated using the following equations. ∆𝐻𝑝𝑠𝑖 = −(2−∝)𝑅𝑇 2 (
𝜕𝑙𝑛𝑋𝐶𝐴𝐶 𝜕𝑇
)
(5)
and ∆𝑆𝑝𝑠𝑖 =
∆𝐻𝑝𝑠𝑖 −∆𝐺𝑝𝑠𝑖
(6)
𝑇
Thermodynamic parameters are important for understanding the process of
of
micellization. They must be determined to examine the effects of structural and
ro
environmental factors on the CMC and CAC values and to consider the effects of new structural and environmental changes on them in the presence of different additives,
-p
which contribute to the comprehension of the interactions between the surfactant and
re
polymer [36].
Table 2: Variations in the Gibbs free energy (∆G), free energy of transfer (ΔG_tr),
lP
enthalpy (∆H) and entropy (∆S) of CTACl with temperature in the absence and
T (K)
∆G
Jo ur
CTACl
na
presence of PVP in water
(J/mol)
∆Gpsi
∆G_tr
∆H
∆Hpsi
∆S (J/mol ∆Spsi (J/mol
(J/mol)
(J/mol)
(J/mol)
(J/mol)
K)
with PVP
with PVP
with PVP
K) PVP
293.15
-42674.6
-41871.1
-4110.3
-14485.6
22165.5
96.16
218.4
298.15
-42855.5
-42586.7
-4099.6
-24952.0
7948.9
60.05
169.5
303.15
-43071.1
-44242.2
-4105.3
-35887.6
-7431.2
23.70
121.4
308.15
-44319.2
-45285.5
-4633.7
-48514.8
-2412.9
-13.61
139.1
313.15
-45273.6
-43834.4
5307.4
-62379.9
-39743.3
-54.63
13.1
T=298K ∆G=-42.73 KJ/mol, Zhang et al. (2017) [ref 17] T=301.9 K ∆H=-42.98 KJ/mol, Michele et al. (2011) [25] 12
with
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Table 3: Variations in the Gibbs free energy (∆G), free energy of transfer (ΔG_tr), enthalpy (∆H) and entropy (∆S) of CTABr with temperature in the absence and presence of PVP in water CTABr T (K)
∆G (J/mol) ∆Gpsi
∆G_tr
∆H
∆Hpsi
∆S
∆Spsi
(J/mol) with (J/mol)
(J/mol)
(J/mol
(J/mol K)
with PVP
PVP
with PVP
K)
with PVP
293.15 -46614.4
-44411.2
62.9
-2994.6
15173.9
148.79
203.3
298.15 -48956.9
-45480.3
-123.64
-12651.2
25736.5
ro
of
(J/mol)
121.77
-14140a
113a
-47260b
-6420b
140b
-46494.7
100.05
-48390a
Jo ur
-47940b
-46221.2
313.15 -49870.2
-1838.98
na
308.15 -49175.4
lP
-48150a
-47560b
-45488.4
-48590a
-48240b
-22683.1
re
303.15 -50046.8
-p
-47940a
-3039.56
37060.3
90.26
-14510a
111a
-6590b
140b
-33435.3
47623.0
51.08
-14900a
109a
-6760b
130b
-45349.6 -15280a
-6930
b
58412.2
16.20
238.9
275.6
304.5
331.8
106a 130b
a: Chauhan et al. (2016) [ref 37] b: Ali et al. (2014) [ref 38]
13
Journal Pre-proof
ΔG of CTACl -41000 290
295
300
305
310
315 ΔG(J/mol)
-43000 -44000
-44000 -45000 290 -46000 -47000 -48000 -49000 -50000 -51000
300
T (K) 310
320
of
ΔG (J/mol)
-42000
ΔG of CTABr
T(K)
-45000
1%PVP(w/V)
-p
0% PVP
(b)
lP
ΔG_tr
295
300
1% PVP (w/V)
T (K) 305
310
315
Jo ur
na
1000 0 -1000 290 -2000 -3000 -4000 -5000 -6000
0% PVP
re
(a)
ΔG_tr (J/mol)
ro
-46000
ΔG_tr_CTABr
ΔG_tr_CTACl
(c )
Figure 5: Variations in the a) Gibbs free energy (∆G) of CTACl, b) Gibbs free energy (∆G) of CTABr, c) free energy of transfer (ΔG_tr) for the surfactant-polymer system with temperature
The Gibbs free energy of the micellization of each surfactant can be calculated using equations (1) and (4) for the pure and polymer-surfactant systems, respectively. Figures 5a and 5b show the variations in the Gibbs free energy with temperature, and 14
Journal Pre-proof the results are listed in tables 2 and 3. The values of ∆G are negative at all the investigated temperatures; thus, the micellization is thermodynamically spontaneous in the surfactant solution. Furthermore, and the decreasing trend in the ∆G values with increasing temperature indicates the desolvation of the hydrophilic group of the surfactant [38]. Therefore, the surfactant molecule is not as hydrated and requires a small amount of energy for the process of adsorption to occur at higher temperatures [39]. With the addition of the polymer (PVP) to the CTACl aqueous solution, the Gibbs free energy of the mixture becomes more complicated with increasing temperature. Additionally, when the polymer (PVP) is added to the CTABr aqueous solution, the
of
Gibbs free energy of the mixture becomes less negative with increasing temperature. It is also observed that the Gibbs free energy becomes less negative when the
ro
counter ion is changed from Br to Cl, or its size reduced. The lower the Gibbs energy is,
-p
the more thermodynamically stable it [33].
The free energy of transfer (∆𝐺_𝑡𝑟 ) associated with the binding interaction
re
between the surfactant and polymer is given by equation (7). Figure 5c shows the variations in the free energy of transfer of the surfactant-polymer system with
lP
temperature, and the results are listed in tables 2 and 3.The graphs show that the transfer
the complex [40]. 𝑋
na
free energy decreases with increasing temperature enabling surfactant to penetrate into
∆𝐺_𝑡𝑟 = (2−∝)𝑅𝑇𝑙𝑛 𝑋 𝐶𝐴𝐶
(7)
Jo ur
𝐶𝑀𝐶
15
Journal Pre-proof
ΔH of CTACl
ΔH of CTABr
40000
80000 60000
T(K)
0 -20000
290
295
300
305
310
315
40000
ΔH (J/mol)
ΔH (J/mol)
20000
0
-40000
-20000
-60000
-40000
290
295
300
305
310
315
-60000
1% PVP(w/V)
305
Jo ur
300
0% PVP
(b)
310
ΔS of CTABr ΔS (J/mol K)
na
lP
re
ΔS of CTACl
295
1% PVP (w/V)
-p
(a)
300 250 200 150 100 50 0 -50 290 -100
0% PVP
ro
0% PVP
of
-80000
ΔS (J/mol)
T (K)
20000
T (K) 315
1% PVP(w/V)
350 300 250 200 150 100 50 0 290
295
300
305
310
T (K)
0% PVP
(c)
1% PVP (w/V)
(d)
Figure 6: Variations in the a) enthalpy (∆H) of CTACl, b) enthalpy (ΔH) of CTABr, c) entropy (∆S) of CTACl, and d) entropy (ΔS) of CTABr with temperature
Figures 6a-b and tables 2 and 3 show the variations in the enthalpy (∆H) as a function of temperature for each surfactant. In pure water, the enthalpy (∆H) is negative 16
315
Journal Pre-proof (exothermic) for both CTABr and CTACl and decreases with increasing temperature. When PVP added to the mixture, the enthalpy becomes positive (endothermic). This change in enthalpy indicates that the process changes from exothermic to endothermic. The variations in the enthalpy are also different with increasing temperature. In the presence of PVP, the enthalpy changes from positive to negative for CTACl but it remains positive for CTABr with increasing temperature. Figures 6c-d and tables 2 and 3 show the change of entropy (∆S) as a function of temperature for each surfactant. The entropy value is positive for the micellization of pure CTACl at low temperatures (Figure 6c). When the temperature is increased, the
of
entropy decreases and becomes negative. The addition of PVP to water improves the
approaching zero, but it always remains positive.
ro
value of entropy. The positive entropy value decreases as a function of temperature,
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For the micellization of pure CTABr, the entropy (∆S) value is positive (Figure 6d). When the temperature is increased, the entropy decreases and becomes positive.
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The addition of PVP to water increases the value of the entropy. The value of the positive entropy increases with increasing temperature.
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The magnitudes and signs of both the enthalpy (ΔH) and entropy (ΔS) indicate the destruction of the hydrophobic hydration shell in the micelle formation process [41].
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For the micellization of pure CTACl in water, although the enthalpy is negative at all temperatures, the entropy is positive at low temperatures, but negative at higher
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temperatures. The micellization of CTACl in water is thus an entropy/enthalpy controlled process at lower temperatures and becomes enthalpy-controlled with increasing temperature. The value of the enthalpy (ΔH) might be negative when the hydration of the hydrophilic head groups becomes more significant than the destruction of the water structure around the hydrophobic alkyl chains of the surfactant monomer [42]. For the micellization of the CTACl surfactant in the presence of PVP the enthalpy (ΔH) decreases with increasing temperature and changes from positive to negative at high temperatures. The value of ΔS decreases and remains positive, even with increasing temperature. Whereas the micellization process becomes entropycontrolled at low temperatures, it is an entropy/enthalpy controlled process at high temperatures. Accordingly, the micellization process of the CTACl/PVP system depends on the simultaneous changes in the enthalpy and entropy [43]. 17
Journal Pre-proof For the micellization of pure CTABr in water, although the enthalpy is negative, the entropy is positive at all temperatures. The micellization of CTABr in water is thus an entropy/enthalpy controlled process. However, the addition of PVP to the mixture results in positive entropy (∆S) and enthalpy (ΔH) values; thus, the micellization becomes entropy-controlled. Their positive values indicate the importance of hydrophobic interactions [37]. A negative enthalpy value suggests that Londondispersion interactions are a significant attractive force in micellization, whereas a positive value implies the break-up of the water structure around the hydrophobic chains of the molecule [42]. The positive entropy value for the mixed surfactant
of
micelles might be caused by (1) variations in the hydrophobic section moving from the hydrated form in the aqueous environment to the non-polar micellar interior and (2) the
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increased freedom of the hydrophobic parts in the micellar interior compared with that
re
-p
in the aqueous medium [31, 42].
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Conclusions
An electroconductivity method was employed to study the interactions between a
na
neutral polymer and cationic surfactant. The surfactant-PVP interactions are observed to be inherently cooperative and dependent on the temperature. It is therefore assumed that
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for cationic surfactants, the polymer combines with the surfactant to form aggregates due to hydrophobic interactions between the polymer and aggregate. The counter ion of the surfactant affects the aggregation and thermodynamic properties. For both CTABr and CTACl interactions in pure water, the driving force was proposed to result from hydrophobic effects [44]. When PVP is added to the water and temperature is increased, the PVP-CTABr interaction is an entropy-controlled process, whereas the PVP-CTACl interaction is an entropy/enthalpy-controlled process, which can be explained by the fact that the Br counter ion is more chaotropic (disorder maker) than the Cl counter ion. Thus the Br counter ion forms more easily destabilized hydrophobic aggregates and increases the solubility of the hydrophobic chains. Accordingly, when the CTABr surfactant forms a micelle, the water structure around the hydrocarbon part is easily broken down, and an entropy change is the driving force in the presence of PVP. In addition, the Br counter ions have a much larger affinity to the interface than the Cl
18
Journal Pre-proof counter ions; hence, the repulsion of Cl from the interface is larger than that of Br due to the greater polarizability of Cl [27]. The polymer-surfactant interaction and the protein folding process resemble each other [25]. Accordingly, the present study is hopefully expected to contribute to comprehension of the process involved.
Funding details
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This work was supported by the Ege University under the grant Scientific Research Project (BAP), Project Number: 2016Fen033.
19
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Journal Pre-proof Tables
Table 1. Changes in the critical concentrations (CMC, CAC and C2) and degrees of ionization (α1 and α2) with temperature
T (K)
CTABr α1
α1
α2
1.34
0.35
0.19
0.84
1.38
0.32
0.20
0.95
0.87
1.40
0.31
0.22
-p
CTACl α2
1.37
0.85
1.39
0.36
0.21
1.40
0.91
1.47
0.39
0.23
CAC
C2
CMC
CAC
C2
(mM)
(mM)
(mM)
(mM)
(mM)
(mM)
293.15
2.16
0.72
1.67
0.41
0.22
0.93
0.78
298.15
2.15
0.79
1.69
0.43
0.24
1.07
303.15
2.06
0.83
1.71
0.41
0.26
308.15
2.20
0.82
1.71
0.40
0.22
313.15
2.46
0.85
1.76
0.48
0.27
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lP
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CMC
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Table 2: Variations in the Gibbs free energy (∆G), free energy of transfer (ΔG_tr), enthalpy (∆H) and entropy (∆S) of CTACl with temperature in the absence and
CTACl ∆Gpsi
∆G_tr
∆H
(J/mol)
(J/mol)
(J/mol)
(J/mol)
with PVP
with PVP
-42674.6
-41871.1
-4110.3
298.15
-42855.5
-42586.7
-4099.6
303.15
-43071.1
-44242.2
-4105.3
308.15
-44319.2
-45285.5
313.15
-45273.6
-43834.4
-14485.6
∆S (J/mol ∆Spsi (J/mol
(J/mol)
K)
with PVP
K) PVP
22165.5
96.16
218.4
-24952.0
7948.9
60.05
169.5
-35887.6
-7431.2
23.70
121.4
-4633.7
-48514.8
-2412.9
-13.61
139.1
5307.4
-62379.9
-39743.3
-54.63
13.1
na
lP
re
293.15
∆Hpsi
ro
∆G
-p
T (K)
of
presence of PVP in water
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T=298K ∆G=-42.73 KJ/mol, Zhang et al. (2017) [ref 17] T=301.9 K ∆H=-42.98 KJ/mol, Michele et al. (2011) [25]
27
with
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Table 3: Variations in the Gibbs free energy (∆G), free energy of transfer (ΔG_tr), enthalpy (∆H) and entropy (∆S) of CTABr with temperature in the absence and presence of PVP in water CTABr ∆G (J/mol) ∆Gpsi
∆G_tr
∆H
∆Hpsi
∆S
∆Spsi
(J/mol)
(J/mol
(J/mol K)
with PVP
K)
with PVP
-2994.6
15173.9
148.79
203.3
-12651.2
25736.5
of
T (K)
(J/mol) with (J/mol)
with PVP
PVP
293.15 -46614.4
-44411.2
62.9
298.15 -48956.9
-45480.3
-123.64
-48150a
Jo ur
-47560b
-46494.7
308.15 -49175.4
100.05
na
303.15 -50046.8
-p
lP
-47260b
-46221.2
-1838.98
121.77
-14140a
113a
-6420b
140b
re
-47940a
ro
(J/mol)
-22683.1
37060.3
-14510a
-6590
140b
47623.0
51.08
-48390a
-14900a
109a
-47940b
-6760b
130b
313.15 -49870.2
-45488.4
-3039.56
-45349.6
275.6
111a
b
-33435.3
90.26
238.9
58412.2
16.20
-48590a
-15280a
106a
-48240b
-6930b
130b
304.5
331.8
a: Chauhan et al. (2016) [ref 37] b: Ali et al. (2014) [ref 38]
28
Journal Pre-proof Figures
Cetyltrimethylammonium Bromide (CTABr)
Cetyltrimethylammonium Bromide (CTABr)
Polyvinylpyrrolidone (PVP)
of
C6H9NO)n (b)
(a)
(c )
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-p
ro
Figure 1. Structures and chemical formula of (a) CTACl, (b) CTABr and (c) PVP
29
lP
re
-p
ro
of
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water, at 303K.
na
Figure 2. Plot of the specific conductivity versus the concentration for a) CTACl in
30
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Figure 2. Plot of the specific conductivity versus the concentration for b) CTACl in the
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presence of water+1%PVP (w/V) at 303K.
31
lP
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water, at 303K.
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Figure 2. Plot of the specific conductivity versus the concentration for c) CTABr in
32
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Figure 2. Plot of the specific conductivity versus the concentration for d) CTABr in the
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na
presence of water+1% PVP (w/V) at 303K.
33
Journal Pre-proof
1% PVP(w/V)
1.8 1.6
C(mM)
1.4 1.2 1 0.8 0.6 291
293
295
297
299
301
303
305
307
309
311
313
315
CTACl
CTABr
CTABr
ro
CTACl
of
T(K)
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na
lP
re
-p
Figure 3. Changes in the critical concentrations with temperature in the presence of PVP
34
Journal Pre-proof
1% PVP( w/V) 0.6 0.5
α
0.4 0.3 0.2 0.1 0 295
300
305 T(K)
CTABr_α1
ro
CTACl_α2
310
315
CTABr_α2
-p
CTACl_α1
of
290
Figure 4. Variations in the degree of ionization (α) as a function of temperature in the
Jo ur
na
lP
re
presence of PVP
35
Journal Pre-proof
ΔG of CTACl -41000 290
295
300
305
310
315 ΔG(J/mol)
ΔG (J/mol)
-42000
ΔG of CTABr
T(K)
-43000 -44000
300
T (K) 310
320
of
-45000
-44000 -45000 290 -46000 -47000 -48000 -49000 -50000 -51000
1%PVP(w/V)
re lP
300
na
295
ΔG_tr_CTABr
1% PVP (w/V) (b)
ΔG_tr
Jo ur
ΔG_tr (J/mol)
(b)
1000 0 -1000 290 -2000 -3000 -4000 -5000 -6000
0% PVP
-p
0% PVP
ro
-46000
T (K)
305
310
315
ΔG_tr_CTACl
(c )
Figure 5: Variations in the a) Gibbs free energy (∆G) of CTACl, b) Gibbs free energy (∆G) of CTABr, c) free energy of transfer (ΔG_tr) for the surfactant-polymer system with temperature
36
Journal Pre-proof
ΔH of CTACl
ΔH of CTABr
40000
80000 60000
T(K)
0 -20000
290
295
300
305
310
ΔH (J/mol)
ΔH (J/mol)
20000
315
40000
0
-40000
-20000
-60000
-40000
290
295
300
305
310
315
-60000
1% PVP(w/V)
295
300
0% PVP
305
310
T (K)
315
1% PVP(w/V)
ΔS (J/mol K)
re
Jo ur
na
lP
ΔS of CTACl 300 250 200 150 100 50 0 -50 290 -100
0% PVP
1% PVP (w/V)
(b)
-p
(b)
ro
0% PVP
of
-80000
ΔS (J/mol)
T (K)
20000
ΔS of CTABr 350 300 250 200 150 100 50 0 290
295
300
305
310
T (K)
0% PVP
(c)
1% PVP (w/V)
(d)
Figure 6: Variations in the a) enthalpy (∆H) of CTACl, b) enthalpy (ΔH) of CTABr, c) entropy (∆S) of CTACl, and d) entropy (ΔS) of CTABr with temperature
Graphical abstract
37
315
Journal Pre-proof
Highlights
CMC values were determined for two surfactants with same length chain but different counter ions
After polymer(PVP) was added to aqueous solution of surfactant,
of
CAC and C2 values were determined
ro
Interactions between surfactant-polymer have been investigated
of
counter-ion
play
an
important
role
in
the
re
Properties
-p
using thermodynamic parameters of aggregation
Jo ur
na
lP
thermodynamic of aggregation
38
Murat Bali, Özgür Masalcı PII:
S0167-7322(19)35269-9
DOI:
https://doi.org/10.1016/j.molliq.2020.112576
Reference:
MOLLIQ 112576
To appear in:
Journal of Molecular Liquids
Received date:
20 September 2019
Revised date:
13 January 2020
Accepted date:
24 January 2020
Please cite this article as: M. Bali and Ö. Masalcı, Interactions of cationic surfactants with polyvinylpyrrolidone (PVP): Effects of counter ions and temperature, Journal of Molecular Liquids(2018), https://doi.org/10.1016/j.molliq.2020.112576
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© 2018 Published by Elsevier.
Journal Pre-proof
Interactions of Cationic Surfactants with Polyvinylpyrrolidone (PVP): Effects of Counter Ions and
of
Temperature
-p
ro
Murat Balia, Özgür Masalcıa* a
Jo ur
na
lP
re
Ege University, Faculty of Science, Department of Physics, 35100, Bornova, Izmir, TURKEY
* Corresponding author. Phone: +90-232-3115439; fax: +90-232-3112833 E-mail: [email protected]
1
Journal Pre-proof Interactions of Cationic Surfactants with Polyvinylpyrrolidone (PVP): Effects of Counter Ions and Temperature The interactions between the polymer polyvinylpyrrolidone (PVP) and conventional surfactants such as cetyltrimethylammonium chloride (CTACl)
and
cetyltrimethylammonium
bromide
(CTABr)
were
investigated using a conductometric method. The critical micelle concentrations (CMCs), the critical aggregation concentrations (CACs) and degrees of ionization (α) of aqueous solutions of the surfactants and
of
PVP were determined via electrical conductivity measurements at various temperatures. The thermodynamic parameters (Gibbs free energy,
ro
enthalpy and entropy) of the surfactants were calculated in the absence and
-p
presence of the polymer, and the results were compared to determine the effects of counter ion on the micellization of the surfactants and the
re
energetics of the process. The main goal of this work is to elucidate the various driving forces that lead to complexation between the surfactants
lP
and polymer. Electrostatic interactions play a major role in the binding of surfactants to polymers or surfaces in aqueous systems. The strength of
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ion type.
na
the interaction is found to depend on the nature of the surfactant counter
Keywords: Aggregation, Polymer-surfactant interaction, Gibbs free energy, Entropy
2
Journal Pre-proof Introduction The wide use of surfactant-polymer systems in industry has led to the further study and investigation of surfactant-polymer mixtures [1-2]. Applications of surfactantpolymer mixtures vary greatly from industrial products in almost all parts of daily life (detergents, cosmetics, paints, coatings, foods and pharmaceutical manufacturing) to biological systems (structure and function of membranes and transfer of lipids) [3-7]. A clear understanding of the nature of surfactant-polymer interactions in aqueous solutions has been attracting scientific interest. In view of the applications, comprehending these interactions is very important for estimating solution properties.
of
Because surfactant molecules have distinct properties such as hydrophilic head
ro
and hydrophobic tail groups, their solutions do not act as a simple solution. Their characteristics, which vary, based on their concentrations. As the concentration of the
-p
surfactant increases in the solution, these molecules tend to combine at the critical
re
micelle concentration (CMC) due to their self-assembly properties. The similarities between aggregations of surfactants and polymers solutions in water make it difficult to
lP
understand surfactant-polymer systems. The hydrophobic-hydrophilic structure of a neutral polymer affects its solubility. According to the Cabane and Duplessix model [8,
na
9], the polymer-surfactant interactions start at the critical aggregation concentration (CAC). This concentration is denoted the CAC because it is concentration at which
Jo ur
surfactants begin to interact with a polymer, and it is usually less than the CMC. Surfactants that are subsequently added to the solution later bond to the saturated polymer, forming a micelle in the solution. The concentration at which typical micelles form is called C2 [10].
The dissolution behaviour in water is controlled by the intermolecular forces between water and water and/or water and the soluble material. The interactions of surfactants can be categorized into three groups based on the following: the hydrophobicity of the alkyl chain, the hydrophilicity of the head group structure and the electrostatic character of the head group in ionic surfactants [11]. The hydrophobic properties of surfactants are mostly related to the length or size of the alkyl chain. The hydrophilic properties of ionic surfactants are associated with the polarity, hydrogenbonding capacity and charge distribution of the head group structure. The electrostatic interactions of the ionic surfactant head group can account for particular transitions of the ionic surfactant to other phases and for the formation of ion pairs and ion bridges 3
Journal Pre-proof with other phases [12]. The resulting hydration sphere of the surfactant greatly affects its alignment in water [13]. All the physico-chemical characteristics influence the organization of water molecules in the vicinity of the surfactant and determine whether hydration is strong or weak, that is, kosmotropic or chaotropic, respectively. The interactions between the head groups of the surfactants and the ions/counter ions at the micelle surfaces can be examined using the Hofmeister ion series, and therefore, the ions can also be classified as kosmotropic and chaotropic; specifically Cl is a kosmotrope, and Br is a chaotrope [14, 15]. The counter ions of the charged head groups of ionic surfactants have been known to affect the organization or arrangement
of
of surfactant molecules in water [16]. The counter ion impacts the adsorption layer and micelle aggregation process, due to its different nature, charge and polarizability or
ro
hydration degree [17].
-p
Although numerous studies have explored the interactions between natural polymers and surfactants, the nature of neutral polymer-surfactant interactions is not yet
re
fully understood [18, 19]. Until recently, either variations in the length of the chain of the surfactant or the effects of different polymer masses were generally examined [20-
lP
24]. For example Majhi et al. (2001) [22] investigated the interactions of CTAB and TTAB surfactants with PVP at different PVP concentrations and a constant temperature.
na
CTAB and TTAB have the same head group but different chain lengths. Moreover, the study also examined to what degree the enthalpy of dilution of the surfactant and the
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salt solution is directly influenced by PVP to acquire data about the quality of its interaction with the the polymer. Sood et al. (2009) [23] studied the micellization behaviour of cationic surfactants with bulky head groups. In particular, they examined alkyldimethylbenzylammonium chlorides (C14, C16, and C18BCl) in water and in aqueous solutions of PVP, PEG-20, and PEG-06 at 298.15 K. The mixed micelle properties of these surfactants in the presence of PVP were also investigated and at various temperatures. The chosen surfactants and prepared mixed surfactants had increasing, chain lengths. Therefore, the effects of the chain length on the polymersurfactant interactions were investigated. Sardar et al. (2013) [24] systematically examined the interactions between cationic gemini surfactants and the neutral polymer polyvinylpyrrolidone (PVP K15 and K90), focusing on the impacts of the molecular weight on the neutral polymer-cationic surfactant interactions and the rheological characteristics a function of the polymer and surfactant concentrations. The study also explored the effects of the chain length on the polymer-gemini surfactant interactions. 4
Journal Pre-proof In
this
study,
cetyltrimethylammonium
chloride
(CTACl)
and
cetyltrimethylammonium bromide (CTABr), which have the same chain length but different counter ions, were used. The interactions between the cationic surfactants with different counter ions and the natural polymer polyvinylpyrrolidone (PVP) were investigated based on the thermodynamic parameters (Gibbs free energy, enthalpy and entropy) of aggregation. Materials and methods
of
Materials Cetyltrimethylammonium chloride (CTACl, cat no: 52366 Fluka) with a molecular
ro
weight of 320.00g/mole and cetyltrimethylammonium bromide (CTABr, cat no: 52369
-p
Fluka) with a molecular weight of 364.45 g/mole were supplied by Fluka and, used without any processing or modifications. Polyvinylpyrrolidone (PVP, cat no: 9003-39-
re
8) with an average molar mass of 10000 g/mole was obtained from Sigma-Aldrich. The purity of the purchased and used chemicals was more than 99% (purity>99%). The
lP
molecular structures of surfactants (CTACl and CTABr) and polymer (PVP) are shown in Figure 1. Deionized water was used as the solvent medium in the experiments. The
na
electrical conductivity of the deionized water was measured to be 8μS/cm at 25 oC (298
Jo ur
K).
Cetyltrimethylammonium Bromide (CTABr)
Cetyltrimethylammonium Bromide (CTABr)
Polyvinylpyrrolidone (PVP)
C6H9NO)n (a)
(b)
(c )
Figure 1. Structures and chemical formula of (a) CTACl, (b) CTABr and (c) PVP
5
Journal Pre-proof Experimental Procedure The quantities of the materials used in the experiments were measured using an AND trademark HR-120 model assay balance with a 10-4 sensitivity. Different amounts of the surfactants (0.5 mM-5 mM) were added to a constant volume of deionized water to prepare individual solution samples with different concentrations. The determined amounts of the surfactants (0.5 mM-5 mM ) were added to a PVP-water solution (%1 PVP (w/V)) with a constant volume to study the interactions between polymer and surfactant. The conductivities of all the samples were measured in the temperature range
of
of 293 K to 313 K. The conductivity of the deionized water was measured at all the temperatures
ro
used in this study. In addition, conductivity of the PVP-water solution without a surfactant was measured in the range of all the temperatures. The measurements of the
re
-p
conductivity and temperature had sensitivities ∓5% μS/cm and ∓0.2 oC, respectively.
Conductivity Measurements
lP
Methods
na
Electroconductivity measurements were performed using the WTW inoLab Cond Level 3 module. For convenience, TetraCon 325 probe was chosen for the measurements and, coupled to the cable-galvanized module. The probe measurement
Jo ur
ranges for the temperature and electroconductivity were 5–100 °C (with an accuracy of 0.2 °C) and 1 μS/cm-2 S/cm (the accuracy, which is temperature dependent, is 5%), respectively.
Prior to the experiments, it is necessary to calibrate the probe; a 0.1 mol/l KCl calibration liquid was used to perform the procedure according to the operating manual. To measure the dependence of the electrical conductivities of the samples on the temperature, a PHYWE trademark thermostat and water bath were used. Using the temperature sensor on the probe, the temperature of the sample could be directly read from the measurement terminal. When the temperature reached equilibrium and the system was stabilized, the electrical conductivity of the sample was measured at the given temperature. The experiments were repeated at least 3 times.
6
Journal Pre-proof Results and Discussion Conductivity measurements Fig. 2 shows the specific conductivity (κ) profiles as a function of surfactant concentration. In Figures 2a and 2c, the intersection point of the two straight lines represents the usual CMC of surfactants CTACl and CTABr, respectively, at a constant temperature. The change in the conductivity values following the CMC is explained by the binding of the counter ion to the micelle. The degree of ionization (α1) of the micelle can be calculated from the ratio of the slopes the conductivity curves below (S1)
Jo ur
na
lP
re
-p
ro
of
and above (S2) the CMC [25].
Figure 2. Plot of the specific conductivity versus the concentration for a) CTACl in water, b) CTACl in the presence of water+1%PVP (w/V), c) CTABr in water, and d) CTABr in the presence of water+1% PVP (w/V) at 303K. 7
Journal Pre-proof
Table 1. Changes in the critical concentrations (CMC, CAC and C2) and degrees of ionization (α1 and α2) with temperature CTACl α1
α2
CAC
C2
(mM)
(mM)
(mM)
293.15
2.16
0.72
1.67
0.41
298.15
2.15
0.79
1.69
303.15
2.06
0.83
308.15
2.20
313.15
2.46
α1
α2
1.34
0.35
0.19
0.84
1.38
0.32
0.20
0.87
1.40
0.31
0.22
0.85
1.39
0.36
0.21
0.91
1.47
0.39
0.23
CMC
CAC
C2
(mM)
(mM)
(mM)
0.22
0.93
0.78
0.43
0.24
1.07
1.71
0.41
0.26
0.95
0.82
1.71
0.40
0.22
1.37
0.85
1.76
0.48
0.27
1.40
of
CMC
ro
T (K)
CTABr
-p
As shown in Table 1 and, Figures 2a and 2c, the CMC value of CTACl is higher
re
than that of CTABr due to difference in the counter-ion radius. The ionic radius of bromine (rBr=0.196 nm) is larger than that of chlorine (rCl=0.181 nm), i.e., rBr>rCl [16].
lP
The strength of the electrostatic attraction decreases as the counter-ion radius increases. Therefore, the electrical repulsion decreases between the micelle head groups of the
na
CTABr molecules, which have Br counter ions enabling micelles to aggregate at lower concentration. It is a characteristic feature of micellization that CMC is to a first
Jo ur
approximation independent of temperature [10]. The CMC may decrease increase or indicate a conspicuous minimum with increasing temperature. Thus, the increase in the CMC values of CTACl and CTABr with temperature may be ascribed to disruption of structured solvent molecules surrounding the hydrophobic groups and desolvation of the ionic head groups [26].
The counter-ion influences the process of micelle aggregation. If the counter ion Br is more polarisable than Cl [27] and has a higher binding efficiency to the micelles, a strong dispersive interaction results, leading to a greater tendency to form cylindrical micelles. Br ions have a stronger ability to form micelles than Cl ions [27], which can be explained by the fact that Br ions are readily able to penetrate into the Stern-layer and facilitate the aggregation of micelles. The electrostatic interactions in neutral polymer/surfactant systems are much weaker than those in the absence of the polymer [28]; thus, the specific conductivity trend can change. The two breaks observed in the presence of the polymer indicate that 8
Journal Pre-proof two types of aggregation occur. The first is called CAC, at which the interaction of the surfactant with the polymer chain begins. As shown in Table 1, the CAC values are smaller than the CMC values. The second is defined as C2, at which the surfactant molecules completely saturate the polymer chains and form normal micelles as well [29, 30]. The degree of ionization of the surfactant (CTABr or CTACl)-polymer (PVP) mixture (α1) can be estimated from the slope and appears to be higher than the degree of ionization of the ordinary micelles in the polymer-containing solution (α2). This result indicates that the micelles at C2 are less ionized than those at the CAC, which is attributed to the decrease the interfacial polarity due to the structural transitions [8]. The
ro
of
ratios of S2/S1 and S3/S1 can be used to obtain the α1 and α2 values, respectively.
1% PVP (w/V)
-p
1.8
re
1.4 1.2 1
0.8 0.6 296
Jo ur
na
291
lP
C(mM)
1.6
CTACl
301
CTACl
306
311
316
T(K) CTABr
CTABr
Figure 3. Changes in the critical concentrations with temperature in the presence of PVP
The CAC and C2 values obtained in the presence of PVP are plotted in Figure 3 and shown in Table 1. The CAC and C2 values change slightly with temperature in the presence of the polymer. Although the CAC values of the two surfactants are close to each other, the C2 value of CTACl is higher than that of CTABr (a comparison of the CMC values of CTACl and CTABr shows that the CMC value of CTACl is higher than that of CTABr). The range of C2-CAC gives information about the bonding of the surfactant to the polymer [15, 31]. An increase in the CAC-C2 range indicates that the bonding is stronger. The CTACl, range for CTACl is wider than that for CTABr; therefore, the 9
Journal Pre-proof bonding of CTACl/PVP is stronger than that of CTABr/PVP, due to the difference in the radii of the counter ions. The decrease in the C2-CAC value with the increasing counter ion radius shows that the strength of the interaction decreases as the counter-ion size increases (rBr> rCl). That is, the strength of the interaction and the radius of counter ion are inversely proportional. These results are consistent with those of Dubin et al. [20] and Benrraou et al. [28].
1% PVP( w/V) 0.6
of
0.5
ro
α
0.4 0.3
-p
0.2 0.1 290
re
0 295
300
305
310
315
CTACl_α2
CTABr_α1
CTABr_α2
na
CTACl_α1
lP
T(K)
Figure 4. Variations in the degree of ionization (α) as a function of temperature in the
Jo ur
presence of PVP
The variations in the degree of ionization (α) are shown as a function of temperature in Figure 4 and Table1. The α1 and α2 values vary non-linearly, which can be explained by the micelle-polymer interactions in the mixture. The degree of ionization shows that an increase in the counter-ion kinetic energy leads to an increase in the counter-ion mobility. The α1 and α2 values when the counter ion changed from Cl to Br, due to the increase in the counter-ion dimension, which results in a reduction in the strength of hydration [16, 17]. Less hydrated ions can be collected more easily in the micellar surface, which decreases the charge repulsion between the polar groups and therefore makes micellization easier, whereas the charges of heavily hydrated ions are partly shielded by the surrounding polar water molecules, and thus, these counter ions are less effective at reducing the charge repulsion between the head groups [25].
10
Journal Pre-proof The degree of ionization of the surfactant (CTACl or CTABr)-PVP complex (α1) could be evaluated from the slopes, and it is higher than the degree of ionization of ordinary micelles in the polymer-containing solution (α2), indicating that the micelles at the CAC are more ionized than those at C2, which can be explained by the enhancement in the interfacial polarity due to the structural change [8, 20]. The higher value of α1 for the complex micelle reveals an increase in the degree of ionic dissociation as a consequence of the interactions between the surfactants and polymer [32]. The appearance of ordinary micelles at C2 shows that the α2 degree of ionization is less influenced by the temperature and nearly stable. Although the α1
of
degree of ionization hardly changes at low temperatures, it increases slightly at
ro
temperatures over 303.15 K.
-p
Thermodynamics parameters of aggregation
re
A pseudo-phase separation model [33, 34] was used to estimate the Gibbs free energy (∆𝐺), enthalpy (∆𝐻), and entropy (∆𝑆) of CTACl and CTABr in aqueous
lP
solutions. These parameters can be calculated by the equations given below. (1)
na
∆𝐺 = (2−∝)𝑅𝑇𝑙𝑛𝑋𝐶𝑀𝐶
where α is the degree of counter-ion dissociation from the micelle, R is the gas constant,
Jo ur
XCMC is the CMC expressed in mole fraction units and T is the temperature in Kelvin. The enthalpy of micellization can be obtained using the Gibbs–Helmholtz equation. ∆𝐻 = −(2−∝)𝑅𝑇 2 (
𝜕𝑙𝑛𝑋𝐶𝑀𝐶 𝜕𝑇
)
(2)
The entropy of micellization (∆𝑆 ) can be estimated from the given values of ∆𝐺 and ∆𝐻 using the following relation. ∆𝑆 =
∆𝐻−∆𝐺 𝑇
(3)
The Gibbs free energy (∆𝐺𝑝𝑠𝑖 ) of a polymer-surfactant system has been defined by Gilany and Wolfram [35]. 𝛥𝐺𝑝𝑠𝑖 = (2−∝)𝑅𝑇𝑙𝑛𝑋𝐶𝐴𝐶
(4)
11
Journal Pre-proof The values of the enthalpy (∆𝐻𝑝𝑠𝑖 ) and entropy (∆𝑆𝑝𝑠𝑖 ) of aggregation in the presence of polymer were calculated using the following equations. ∆𝐻𝑝𝑠𝑖 = −(2−∝)𝑅𝑇 2 (
𝜕𝑙𝑛𝑋𝐶𝐴𝐶 𝜕𝑇
)
(5)
and ∆𝑆𝑝𝑠𝑖 =
∆𝐻𝑝𝑠𝑖 −∆𝐺𝑝𝑠𝑖
(6)
𝑇
Thermodynamic parameters are important for understanding the process of
of
micellization. They must be determined to examine the effects of structural and
ro
environmental factors on the CMC and CAC values and to consider the effects of new structural and environmental changes on them in the presence of different additives,
-p
which contribute to the comprehension of the interactions between the surfactant and
re
polymer [36].
Table 2: Variations in the Gibbs free energy (∆G), free energy of transfer (ΔG_tr),
lP
enthalpy (∆H) and entropy (∆S) of CTACl with temperature in the absence and
T (K)
∆G
Jo ur
CTACl
na
presence of PVP in water
(J/mol)
∆Gpsi
∆G_tr
∆H
∆Hpsi
∆S (J/mol ∆Spsi (J/mol
(J/mol)
(J/mol)
(J/mol)
(J/mol)
K)
with PVP
with PVP
with PVP
K) PVP
293.15
-42674.6
-41871.1
-4110.3
-14485.6
22165.5
96.16
218.4
298.15
-42855.5
-42586.7
-4099.6
-24952.0
7948.9
60.05
169.5
303.15
-43071.1
-44242.2
-4105.3
-35887.6
-7431.2
23.70
121.4
308.15
-44319.2
-45285.5
-4633.7
-48514.8
-2412.9
-13.61
139.1
313.15
-45273.6
-43834.4
5307.4
-62379.9
-39743.3
-54.63
13.1
T=298K ∆G=-42.73 KJ/mol, Zhang et al. (2017) [ref 17] T=301.9 K ∆H=-42.98 KJ/mol, Michele et al. (2011) [25] 12
with
Journal Pre-proof
Table 3: Variations in the Gibbs free energy (∆G), free energy of transfer (ΔG_tr), enthalpy (∆H) and entropy (∆S) of CTABr with temperature in the absence and presence of PVP in water CTABr T (K)
∆G (J/mol) ∆Gpsi
∆G_tr
∆H
∆Hpsi
∆S
∆Spsi
(J/mol) with (J/mol)
(J/mol)
(J/mol
(J/mol K)
with PVP
PVP
with PVP
K)
with PVP
293.15 -46614.4
-44411.2
62.9
-2994.6
15173.9
148.79
203.3
298.15 -48956.9
-45480.3
-123.64
-12651.2
25736.5
ro
of
(J/mol)
121.77
-14140a
113a
-47260b
-6420b
140b
-46494.7
100.05
-48390a
Jo ur
-47940b
-46221.2
313.15 -49870.2
-1838.98
na
308.15 -49175.4
lP
-48150a
-47560b
-45488.4
-48590a
-48240b
-22683.1
re
303.15 -50046.8
-p
-47940a
-3039.56
37060.3
90.26
-14510a
111a
-6590b
140b
-33435.3
47623.0
51.08
-14900a
109a
-6760b
130b
-45349.6 -15280a
-6930
b
58412.2
16.20
238.9
275.6
304.5
331.8
106a 130b
a: Chauhan et al. (2016) [ref 37] b: Ali et al. (2014) [ref 38]
13
Journal Pre-proof
ΔG of CTACl -41000 290
295
300
305
310
315 ΔG(J/mol)
-43000 -44000
-44000 -45000 290 -46000 -47000 -48000 -49000 -50000 -51000
300
T (K) 310
320
of
ΔG (J/mol)
-42000
ΔG of CTABr
T(K)
-45000
1%PVP(w/V)
-p
0% PVP
(b)
lP
ΔG_tr
295
300
1% PVP (w/V)
T (K) 305
310
315
Jo ur
na
1000 0 -1000 290 -2000 -3000 -4000 -5000 -6000
0% PVP
re
(a)
ΔG_tr (J/mol)
ro
-46000
ΔG_tr_CTABr
ΔG_tr_CTACl
(c )
Figure 5: Variations in the a) Gibbs free energy (∆G) of CTACl, b) Gibbs free energy (∆G) of CTABr, c) free energy of transfer (ΔG_tr) for the surfactant-polymer system with temperature
The Gibbs free energy of the micellization of each surfactant can be calculated using equations (1) and (4) for the pure and polymer-surfactant systems, respectively. Figures 5a and 5b show the variations in the Gibbs free energy with temperature, and 14
Journal Pre-proof the results are listed in tables 2 and 3. The values of ∆G are negative at all the investigated temperatures; thus, the micellization is thermodynamically spontaneous in the surfactant solution. Furthermore, and the decreasing trend in the ∆G values with increasing temperature indicates the desolvation of the hydrophilic group of the surfactant [38]. Therefore, the surfactant molecule is not as hydrated and requires a small amount of energy for the process of adsorption to occur at higher temperatures [39]. With the addition of the polymer (PVP) to the CTACl aqueous solution, the Gibbs free energy of the mixture becomes more complicated with increasing temperature. Additionally, when the polymer (PVP) is added to the CTABr aqueous solution, the
of
Gibbs free energy of the mixture becomes less negative with increasing temperature. It is also observed that the Gibbs free energy becomes less negative when the
ro
counter ion is changed from Br to Cl, or its size reduced. The lower the Gibbs energy is,
-p
the more thermodynamically stable it [33].
The free energy of transfer (∆𝐺_𝑡𝑟 ) associated with the binding interaction
re
between the surfactant and polymer is given by equation (7). Figure 5c shows the variations in the free energy of transfer of the surfactant-polymer system with
lP
temperature, and the results are listed in tables 2 and 3.The graphs show that the transfer
the complex [40]. 𝑋
na
free energy decreases with increasing temperature enabling surfactant to penetrate into
∆𝐺_𝑡𝑟 = (2−∝)𝑅𝑇𝑙𝑛 𝑋 𝐶𝐴𝐶
(7)
Jo ur
𝐶𝑀𝐶
15
Journal Pre-proof
ΔH of CTACl
ΔH of CTABr
40000
80000 60000
T(K)
0 -20000
290
295
300
305
310
315
40000
ΔH (J/mol)
ΔH (J/mol)
20000
0
-40000
-20000
-60000
-40000
290
295
300
305
310
315
-60000
1% PVP(w/V)
305
Jo ur
300
0% PVP
(b)
310
ΔS of CTABr ΔS (J/mol K)
na
lP
re
ΔS of CTACl
295
1% PVP (w/V)
-p
(a)
300 250 200 150 100 50 0 -50 290 -100
0% PVP
ro
0% PVP
of
-80000
ΔS (J/mol)
T (K)
20000
T (K) 315
1% PVP(w/V)
350 300 250 200 150 100 50 0 290
295
300
305
310
T (K)
0% PVP
(c)
1% PVP (w/V)
(d)
Figure 6: Variations in the a) enthalpy (∆H) of CTACl, b) enthalpy (ΔH) of CTABr, c) entropy (∆S) of CTACl, and d) entropy (ΔS) of CTABr with temperature
Figures 6a-b and tables 2 and 3 show the variations in the enthalpy (∆H) as a function of temperature for each surfactant. In pure water, the enthalpy (∆H) is negative 16
315
Journal Pre-proof (exothermic) for both CTABr and CTACl and decreases with increasing temperature. When PVP added to the mixture, the enthalpy becomes positive (endothermic). This change in enthalpy indicates that the process changes from exothermic to endothermic. The variations in the enthalpy are also different with increasing temperature. In the presence of PVP, the enthalpy changes from positive to negative for CTACl but it remains positive for CTABr with increasing temperature. Figures 6c-d and tables 2 and 3 show the change of entropy (∆S) as a function of temperature for each surfactant. The entropy value is positive for the micellization of pure CTACl at low temperatures (Figure 6c). When the temperature is increased, the
of
entropy decreases and becomes negative. The addition of PVP to water improves the
approaching zero, but it always remains positive.
ro
value of entropy. The positive entropy value decreases as a function of temperature,
-p
For the micellization of pure CTABr, the entropy (∆S) value is positive (Figure 6d). When the temperature is increased, the entropy decreases and becomes positive.
re
The addition of PVP to water increases the value of the entropy. The value of the positive entropy increases with increasing temperature.
lP
The magnitudes and signs of both the enthalpy (ΔH) and entropy (ΔS) indicate the destruction of the hydrophobic hydration shell in the micelle formation process [41].
na
For the micellization of pure CTACl in water, although the enthalpy is negative at all temperatures, the entropy is positive at low temperatures, but negative at higher
Jo ur
temperatures. The micellization of CTACl in water is thus an entropy/enthalpy controlled process at lower temperatures and becomes enthalpy-controlled with increasing temperature. The value of the enthalpy (ΔH) might be negative when the hydration of the hydrophilic head groups becomes more significant than the destruction of the water structure around the hydrophobic alkyl chains of the surfactant monomer [42]. For the micellization of the CTACl surfactant in the presence of PVP the enthalpy (ΔH) decreases with increasing temperature and changes from positive to negative at high temperatures. The value of ΔS decreases and remains positive, even with increasing temperature. Whereas the micellization process becomes entropycontrolled at low temperatures, it is an entropy/enthalpy controlled process at high temperatures. Accordingly, the micellization process of the CTACl/PVP system depends on the simultaneous changes in the enthalpy and entropy [43]. 17
Journal Pre-proof For the micellization of pure CTABr in water, although the enthalpy is negative, the entropy is positive at all temperatures. The micellization of CTABr in water is thus an entropy/enthalpy controlled process. However, the addition of PVP to the mixture results in positive entropy (∆S) and enthalpy (ΔH) values; thus, the micellization becomes entropy-controlled. Their positive values indicate the importance of hydrophobic interactions [37]. A negative enthalpy value suggests that Londondispersion interactions are a significant attractive force in micellization, whereas a positive value implies the break-up of the water structure around the hydrophobic chains of the molecule [42]. The positive entropy value for the mixed surfactant
of
micelles might be caused by (1) variations in the hydrophobic section moving from the hydrated form in the aqueous environment to the non-polar micellar interior and (2) the
ro
increased freedom of the hydrophobic parts in the micellar interior compared with that
re
-p
in the aqueous medium [31, 42].
lP
Conclusions
An electroconductivity method was employed to study the interactions between a
na
neutral polymer and cationic surfactant. The surfactant-PVP interactions are observed to be inherently cooperative and dependent on the temperature. It is therefore assumed that
Jo ur
for cationic surfactants, the polymer combines with the surfactant to form aggregates due to hydrophobic interactions between the polymer and aggregate. The counter ion of the surfactant affects the aggregation and thermodynamic properties. For both CTABr and CTACl interactions in pure water, the driving force was proposed to result from hydrophobic effects [44]. When PVP is added to the water and temperature is increased, the PVP-CTABr interaction is an entropy-controlled process, whereas the PVP-CTACl interaction is an entropy/enthalpy-controlled process, which can be explained by the fact that the Br counter ion is more chaotropic (disorder maker) than the Cl counter ion. Thus the Br counter ion forms more easily destabilized hydrophobic aggregates and increases the solubility of the hydrophobic chains. Accordingly, when the CTABr surfactant forms a micelle, the water structure around the hydrocarbon part is easily broken down, and an entropy change is the driving force in the presence of PVP. In addition, the Br counter ions have a much larger affinity to the interface than the Cl
18
Journal Pre-proof counter ions; hence, the repulsion of Cl from the interface is larger than that of Br due to the greater polarizability of Cl [27]. The polymer-surfactant interaction and the protein folding process resemble each other [25]. Accordingly, the present study is hopefully expected to contribute to comprehension of the process involved.
Funding details
Jo ur
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ro
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This work was supported by the Ege University under the grant Scientific Research Project (BAP), Project Number: 2016Fen033.
19
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25
Journal Pre-proof Tables
Table 1. Changes in the critical concentrations (CMC, CAC and C2) and degrees of ionization (α1 and α2) with temperature
T (K)
CTABr α1
α1
α2
1.34
0.35
0.19
0.84
1.38
0.32
0.20
0.95
0.87
1.40
0.31
0.22
-p
CTACl α2
1.37
0.85
1.39
0.36
0.21
1.40
0.91
1.47
0.39
0.23
CAC
C2
CMC
CAC
C2
(mM)
(mM)
(mM)
(mM)
(mM)
(mM)
293.15
2.16
0.72
1.67
0.41
0.22
0.93
0.78
298.15
2.15
0.79
1.69
0.43
0.24
1.07
303.15
2.06
0.83
1.71
0.41
0.26
308.15
2.20
0.82
1.71
0.40
0.22
313.15
2.46
0.85
1.76
0.48
0.27
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na
lP
re
ro
of
CMC
26
Journal Pre-proof
Table 2: Variations in the Gibbs free energy (∆G), free energy of transfer (ΔG_tr), enthalpy (∆H) and entropy (∆S) of CTACl with temperature in the absence and
CTACl ∆Gpsi
∆G_tr
∆H
(J/mol)
(J/mol)
(J/mol)
(J/mol)
with PVP
with PVP
-42674.6
-41871.1
-4110.3
298.15
-42855.5
-42586.7
-4099.6
303.15
-43071.1
-44242.2
-4105.3
308.15
-44319.2
-45285.5
313.15
-45273.6
-43834.4
-14485.6
∆S (J/mol ∆Spsi (J/mol
(J/mol)
K)
with PVP
K) PVP
22165.5
96.16
218.4
-24952.0
7948.9
60.05
169.5
-35887.6
-7431.2
23.70
121.4
-4633.7
-48514.8
-2412.9
-13.61
139.1
5307.4
-62379.9
-39743.3
-54.63
13.1
na
lP
re
293.15
∆Hpsi
ro
∆G
-p
T (K)
of
presence of PVP in water
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T=298K ∆G=-42.73 KJ/mol, Zhang et al. (2017) [ref 17] T=301.9 K ∆H=-42.98 KJ/mol, Michele et al. (2011) [25]
27
with
Journal Pre-proof
Table 3: Variations in the Gibbs free energy (∆G), free energy of transfer (ΔG_tr), enthalpy (∆H) and entropy (∆S) of CTABr with temperature in the absence and presence of PVP in water CTABr ∆G (J/mol) ∆Gpsi
∆G_tr
∆H
∆Hpsi
∆S
∆Spsi
(J/mol)
(J/mol
(J/mol K)
with PVP
K)
with PVP
-2994.6
15173.9
148.79
203.3
-12651.2
25736.5
of
T (K)
(J/mol) with (J/mol)
with PVP
PVP
293.15 -46614.4
-44411.2
62.9
298.15 -48956.9
-45480.3
-123.64
-48150a
Jo ur
-47560b
-46494.7
308.15 -49175.4
100.05
na
303.15 -50046.8
-p
lP
-47260b
-46221.2
-1838.98
121.77
-14140a
113a
-6420b
140b
re
-47940a
ro
(J/mol)
-22683.1
37060.3
-14510a
-6590
140b
47623.0
51.08
-48390a
-14900a
109a
-47940b
-6760b
130b
313.15 -49870.2
-45488.4
-3039.56
-45349.6
275.6
111a
b
-33435.3
90.26
238.9
58412.2
16.20
-48590a
-15280a
106a
-48240b
-6930b
130b
304.5
331.8
a: Chauhan et al. (2016) [ref 37] b: Ali et al. (2014) [ref 38]
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Journal Pre-proof Figures
Cetyltrimethylammonium Bromide (CTABr)
Cetyltrimethylammonium Bromide (CTABr)
Polyvinylpyrrolidone (PVP)
of
C6H9NO)n (b)
(a)
(c )
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na
lP
re
-p
ro
Figure 1. Structures and chemical formula of (a) CTACl, (b) CTABr and (c) PVP
29
lP
re
-p
ro
of
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water, at 303K.
na
Figure 2. Plot of the specific conductivity versus the concentration for a) CTACl in
30
lP
re
-p
ro
of
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na
Figure 2. Plot of the specific conductivity versus the concentration for b) CTACl in the
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presence of water+1%PVP (w/V) at 303K.
31
lP
re
-p
ro
of
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water, at 303K.
na
Figure 2. Plot of the specific conductivity versus the concentration for c) CTABr in
32
lP
re
-p
ro
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Figure 2. Plot of the specific conductivity versus the concentration for d) CTABr in the
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na
presence of water+1% PVP (w/V) at 303K.
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1% PVP(w/V)
1.8 1.6
C(mM)
1.4 1.2 1 0.8 0.6 291
293
295
297
299
301
303
305
307
309
311
313
315
CTACl
CTABr
CTABr
ro
CTACl
of
T(K)
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na
lP
re
-p
Figure 3. Changes in the critical concentrations with temperature in the presence of PVP
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1% PVP( w/V) 0.6 0.5
α
0.4 0.3 0.2 0.1 0 295
300
305 T(K)
CTABr_α1
ro
CTACl_α2
310
315
CTABr_α2
-p
CTACl_α1
of
290
Figure 4. Variations in the degree of ionization (α) as a function of temperature in the
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na
lP
re
presence of PVP
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ΔG of CTACl -41000 290
295
300
305
310
315 ΔG(J/mol)
ΔG (J/mol)
-42000
ΔG of CTABr
T(K)
-43000 -44000
300
T (K) 310
320
of
-45000
-44000 -45000 290 -46000 -47000 -48000 -49000 -50000 -51000
1%PVP(w/V)
re lP
300
na
295
ΔG_tr_CTABr
1% PVP (w/V) (b)
ΔG_tr
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ΔG_tr (J/mol)
(b)
1000 0 -1000 290 -2000 -3000 -4000 -5000 -6000
0% PVP
-p
0% PVP
ro
-46000
T (K)
305
310
315
ΔG_tr_CTACl
(c )
Figure 5: Variations in the a) Gibbs free energy (∆G) of CTACl, b) Gibbs free energy (∆G) of CTABr, c) free energy of transfer (ΔG_tr) for the surfactant-polymer system with temperature
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ΔH of CTACl
ΔH of CTABr
40000
80000 60000
T(K)
0 -20000
290
295
300
305
310
ΔH (J/mol)
ΔH (J/mol)
20000
315
40000
0
-40000
-20000
-60000
-40000
290
295
300
305
310
315
-60000
1% PVP(w/V)
295
300
0% PVP
305
310
T (K)
315
1% PVP(w/V)
ΔS (J/mol K)
re
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na
lP
ΔS of CTACl 300 250 200 150 100 50 0 -50 290 -100
0% PVP
1% PVP (w/V)
(b)
-p
(b)
ro
0% PVP
of
-80000
ΔS (J/mol)
T (K)
20000
ΔS of CTABr 350 300 250 200 150 100 50 0 290
295
300
305
310
T (K)
0% PVP
(c)
1% PVP (w/V)
(d)
Figure 6: Variations in the a) enthalpy (∆H) of CTACl, b) enthalpy (ΔH) of CTABr, c) entropy (∆S) of CTACl, and d) entropy (ΔS) of CTABr with temperature
Graphical abstract
37
315
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Highlights
CMC values were determined for two surfactants with same length chain but different counter ions
After polymer(PVP) was added to aqueous solution of surfactant,
of
CAC and C2 values were determined
ro
Interactions between surfactant-polymer have been investigated
of
counter-ion
play
an
important
role
in
the
re
Properties
-p
using thermodynamic parameters of aggregation
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na
lP
thermodynamic of aggregation
38