Effect of Cosolvents DMSO and Glycerol on the Self-Assembly

Article pubs.acs.org/jced

Cite This: J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Effect of Cosolvents DMSO and Glycerol on the Self-Assembly Behavior of SDBS and CPC: An Experimental and Theoretical Approach Vivek Sharma,† Plinio Cantero-López,*,‡,§ Osvaldo Yañez-Osses,§ and Ashish Kumar*,† †

Department of Chemistry, Faculty of Technology and Sciences, Lovely Professional University, Phagwara, Punjab, India Center of Applied Nanoscience (CANS), Facultad de Ciencias Exactas, Universidad Andres Bello, Av. República 275, Santiago, Chile § Relativistic Molecular Physics (ReMoPh) Group, Ph.D. Program in Molecular Physical Chemistry, Universidad Andrés Bello, Av. República 275, Santiago, Chile J. Chem. Eng. Data Downloaded from pubs.acs.org by NAGOYA UNIV on 07/31/18. For personal use only.

‡

S Supporting Information *

ABSTRACT: The study of the effect of cosolvents on the micellization of ionic surfactants is relevant in different industrial applications. In this article, we studied the effects of binary aqueous mixtures of DMSO and glycerol on the micellization and thermodynamic behavior of sodium dodecyl benzenesulfonate (SDBS) and cetylpyridinium chloride (CPC) in the range of (293.15−308.15) K. The electrical conductivity method was employed to determine the parameters of micellization such as the critical micellar concentration (CMC) and degree of counterion dissociation (α). The temperature dependence of CMC values was used to calculate other thermodynamic parameters of micellization such as the standard free energy of micellization (ΔG0m), the standard enthalpy of micellization (ΔHm0), and the standard entropy of micellization (ΔS0m). Result analysis showed that the CMCs of both surfactants (SDBS and CPC) were directly proportional to the cosolvent concentration. The standard free energy of micellization (ΔG0m) was negative in all cases, and it increased with increases in the cosolvent concentration, thereby reducing the driving force for micellization. Quantum chemical calculations and molecular dynamic (MD) simulations were essential to understanding the phenomena and provide a well-supported molecular picture of the micellization process. In this work, a combined experimental and molecular modeling study on the effect of cosolvents such as DMSO and glycerol on the selfassembly behavior of SDBS and CPC was performed. It was found that the supramolecular assembly is inhibited by the presence of glycerol and DMSO because hydrophobic regions in the solvent media increased with increasing concentration of the nonaqueous solvents, imparting cosolvency and promoting the decrease in amphiphilic assembly. breaking ability of cosolvents.35 To study the effect of cosolvents on the micellization of ionic surfactants, two of the most common industrially important cosolvents, glycerol (polar protic) and DMSO (polar aprotic), have been selected. This selection enabled us to compare the effects of kosmotropic properties of glycerol and chaotropic properties of DMSO on the aggregation behavior of ionic surfactants.36 Similarly, to gain complete understanding of the hydrophobic effect, two oppositely charged ionic surfactants, sodium dodecyl benzenesulfonate (SDBS) and cetylpyridinium chloride (CPC), were selected for this work. Because both anionic (SDBS) and cationic (CPC) surfactants contain aromaticity in their headgroups, this enabled us to understand the role of hydrophobic headgroups in the micellization of

1. INTRODUCTION An attempt has been made to determine the dependence of the thermodynamic parameters of micellization on cosolvent concentration and to provide a comprehensive picture of cosolvent effects on the micellization of ionic surfactants. Applications of surfactants in various industries and household processes generally depend on their self-aggregating properties, which largely depend on the solvent medium in which they are used. Hence, different additives and cosolvents are added to surfactant formulations to improve their surface properties1−17 The formation of micelles largely depends on the hydrophobicity of surfactants as well as solvent medium.18−27 Therefore, it is important to study the role of solvent media to understand the process of micellization.28−30 Previous studies on the cosolvent effect on the micellization process have been reported in terms of intermolecular interactions between water and cosolvent,31−34 and the results of such studies have been interpreted in terms of the water structure-making or structure© XXXX American Chemical Society

Received: April 22, 2018 Accepted: July 18, 2018

A

DOI: 10.1021/acs.jced.8b00326 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

Article

Figure 1. Chemical structures of glycerol, cetylpyridinium chloride (CPC), and dimethyl sulfoxide (DMSO).

Table 1. Specification and Mass Fraction Purity of Chemical Samples chemical name

source

CAS registry number

purification method

mass fraction

sodium dodecyl benzenesulfonate (SDBS) cetylpyridinium chloride monohydrate (CPC) glycerol dimethyl sulfoxide (DMSO)

Sigma-Aldrich Loba Chemie Pvt. Ltd. Loba Chemie Pvt. Ltd. Finar Pvt. Ltd.

25155-30-0 6004-24-6 56-81-5 67-68-5

recrystallizationa none none none

>0.90b 0.99b 0.99b 0.99b

a

Reference 30. bDeclared by the supplier.

was used without further purification. Dimethyl sulfoxide of 99% purity was supplied by Finar Ltd. and was used without further purification. Glycerol of 99% purity (confirming the IP) was obtained from LOBA Chemie Pvt. Ltd. Aqueous solutions of surfactants (SDBS and CPC) of different molal concentrations in the range of (0.3 to 3.0) m·mol·kg−1 for SDBS (C) and (0.2 to 2.0) mmol·kg−1 for CPC (C*) were prepared by the addition of small aliquots of a concentrated solution of surfactant to 50 mL of water (solvent medium).The solutions obtained were gently stirred using a magnetic stirrer before being subjected to conductivity measurements. Similar experiments were repeated in the presence of 5% (m1), 10% (m2), 15% (m3), and 20% (m4) w/w glycerol and 5% (m5), 10% (m6), 15% (m7), and 20% (m8) w/w DMSO solutions at 293.15, 298.15, 303.15, and 308.15 K, respectively. All of the conductivity experiments were performed with a digital conductivity meter supplied by Labtronics Pvt. Ltd. (India). A digital water thermostat [Bombay Scientific Pvt. Ltd. (India)] was used to maintain a temperature accuracy of 0.05 K. The conductivity cell was calibrated with a 0.01 M KCl solution supplied by S. D. Fine Pvt. Ltd. (India), and the cell constant was determined to be 1 cm−1. The specifications and mass percentage purity of the chemicals used are provided in Table 1.

these ionic surfactants. A conductometric technique was employed to study the thermodynamics of micellization and the effect of different cosolvents on the micellization behavior of these ionic surfactants because of its simplicity and accuracy. Moreover, this technique does not require any additional treatment or addition of foreign species to stabilize the solution properties (i.e., pH and ionic strength) during measurements, which may introduce errors into the results.37−39 The structures of both cosolvents and ionic surfactants, which were used in this work, are shown in Figure 1. In this context, quantum chemical calculations and molecular dynamic (MD) simulations provide important information and represent new insight into the study of the micellization process, allowing us to identify significant intermolecular characteristics such as aggregation phenomena and the behavior of the hydrophilic heads and the hydrophobic tails of surfactants in a certain environment, such as organic solvents. This helps to establish a better molecular picture of the micellization process.

2. EXPERIMENTAL SECTION 2.1. Materials and Methods. Ordinary tap water with conductivity in the range of 3 × 10−6−5 × 10−6 S·cm−1 was distilled three time in the presence of alkaline KMnO4. Triply distilled water in the conductivity range is 0−2 × 10−7 S cm−1, and pH 6.8−7.0 was used in all of the experiments. SDBS obtained from Sigma-Aldrich was recrystallized several times with ethanol (99.9% pure) to get a pure sample of SDBS (>90 %) as suggested in the literature.40 CPC (confirming to USP) of 99% purity was obtained from LOBA Chemie Pvt. Ltd. and

3. COMPUTATIONAL DETAILS 3.1. Quantum Chemical Calculations. The final geometry for all species involved was found using molecular dynamics simulation. Subsequent full structural optimizations B



Article

ionic surfactants, it was necessary to determine the CMC values of these surfactants (SDBS and CPC) in an aqueous medium at different temperatures. CMC values of SDBS and CPC were determined by conductivity experiments employing William’s method.40 In this method, CMC values are determined from the inflection in the plots between the specific conductance (κ) and the surfactant concentration.52,53 This inflection/break point between two straight lines gives the value of the CMC. The values of the critical micellar concentration (CMC) have been converted to their mole fraction unit (XCMC) before determining other thermodynamic parameters of micellization. The CMC values and corresponding XCMC values for SDBS and CPC in aqueous solution have been reported in Table 2 and Figures 3 and 4, respectively.

and frequency calculations for the low-lying structures were carried out using the B3LYP functional and 6-31G** basis set. Noncovalent interactions (NCI) were found using the Multiwfn program.41 3.2. Noncovalent Interaction Index (NCI). To reveal possible noncovalent interactions, such as hydrogen bonds, steric repulsion, and van der Waals interactions, the noncovalent interaction index (NCI) was used. NCI is based on the reduced density gradient (s) in low-density regions (r). This analysis provides a graphical index that allows the characterization of the interactions mentioned before. In this framework, the reduced density gradient is given by eq 142,43 S=

∇ρ 1 2 1/3 4/3 2(3π ) ρ

(1)

Table 2. CMC and Corresponding Mole Fraction of CMC (XCMC) with Values in Aqueous Solutions of SDBS and CPC at Different Temperatures and Pressure p = 100 kPaa

The reduced density gradient in low-density regions verifies the presence of noncovalent interactions. To distinguish between attractive and repulsive interactions, one must consider the accumulation or depletion of density in the plane perpendicular to the interaction. This is mainly characterized by the second eigenvalue, λ, of the electrondensity Hessian (second-derivative) matrix. The values of λ give information about the type of binding force: attractive forces such as hydrogen bonds (λ < 0), weak interactions (λ = 0), and repulsive forces (λ > 0). 3.3. Molecular Dynamics. MD calculations were performed for binary aqueous mixtures of DMSO and glycerol in the presence of sodium dodecyl benzenesulfonate (SDBS) and cetylpyridinium chloride (CPC). The cosolvents and ionic surfactants were parametrized by analogy using the ParamChem service and implementing the CHARMM general force field for organic molecules.44−47 The simulations were carried out using the CHARMM force field in an explicit solvent with the TIP3P water model within the NAMD software.48 Starting configurations were generated in cubic boxes with lateral dimensions of 70 Å. The systems were prepared by randomly placing cosolvents, ionic surfactants, and water molecules in the simulation box using a packing molecule in defined regions of space called Packmol.49,50 First, each system was minimized (20 000 steps) and equilibrated (1000 ps). Then, 20-ns-long production MD simulations were performed on each system. During the MD simulations, the equations of motion were integrated with a 1 fs time step in the NPT ensemble at a pressure of 1 atm. The SHAKE algorithm was applied for all hydrogen atoms, and the van der Waals (VDW) cutoff was set to 12 Å. The temperature was maintained at 298.15 K by employing the Nosé−Hoover thermostat method with a relaxation time of 1 ps. The Nosé− Hoover Langevin piston was used to control the pressure at 1 atm. Long-range electrostatic interactions were considered by means of the particle mesh Ewald (PME) approach. Data were collected every 1 ps during the MD runs. Molecular visualization of the systems and MD trajectory analysis were carried out with the VMD software package.51 We have measured the radial distribution functions (RDFs) using VMD.

SDBS(C) T/K

103(mol·kg−1), CMC

293.15 298.15 303.15 308.15

1.26 1.29 1.35 1.41

CPC(C*) 105, XCMC

103(mol·kg−1), CMC

105, XCMC

2.268 2.322 2.43 2.538

0.90 0.96 1.00 1.04

1.62 1.728 1.8 1.872

a Standard uncertainties u are u(CSDBS) = 5 × 10−5 mol· kg−1, u(C*CPC) = 3.3 × 10−5 mol· kg−1, u(CMC) = 0.01 × 10−3, and u(XCMC) = 0.02 × 10−5 for SDBS; u(CMC) = 0.02 × 10−3 and u(XCMC) = 0.04 × 10−5 for CPC, u(T) = 0.05 K, and u(p) = 10 kPa (0.68 level of confidence).

4. RESULTS AND DISCUSSION 4.1. Critical Micellar Concentration and Temperature Dependence of XCMC. CMC is the most important parameter used to study the micellization behavior of amphiphiles, so to analyze the effect of cosolvents on the micellization behavior of

Figure 2. Conductivity vs concentration plots for aqueous solution of (a) SDBS and (b) CPC at T = 293.15 K, blue −●−; 298.15 K, red −●−; 303.15 K, green −●−; and 308.15 K, purple −●−. C



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and 6ϕC12), and it is very difficult to separate and identify these isomers from their mixture.54 Micellization properties of SDBS isomers are different from each other and are reported in Table 3.55−59 The SDBS obtained had a molecular formula of Table 3. Reported CMC Values for Isomeric SDBS isomeric forms SDBS

T/K

103(mol·kg−1) CMC

references

2ϕC12 3ϕC12 3ϕC12 4ϕC12 5ϕC12 6ϕC12

298.15 298.15 298.15 298.15 298.15 298.15

0.10 1.30, 1.50 1.29a 1.40, 1.70 2.20, 2.40 2.25, 2.40, 2.78

67 67 and 68 this work 67 and 68 67 and 68 59, 69, and 70

a Standard uncertainties u are u(CSDBS) = 5 × 10−5 mol·kg−1, u(CMC) = 0.01 × 10−3 for SDBS, u(T) = 0.05 K, and u(p) = 10 kPa (0.68 level of confidence).

C12H25ϕSO3− Na+, and its isomeric composition was not provided by the company. We checked the purity of SDBS by HPLC and found one major component with an indication of one minor component. HPLC of recrystallized SDBS has been reported in Figure S1 (Supporting Information).The CMCs of both surfactants (SDBS and CPC) in water were in excellent agreement with the literature values.37,60,61 The CMC values obtained for SDBS at 298 0.15 K was 1.29 × 10−3 mol·kg−1, which was approximately equal to the value of 3ϕC12 isomer as reported in the literature.54 Thus we consider that the SDBS studied in this research was essentially 3ϕC12 (major component found in HPLC analysis). The variation of κ (specific conductance) of the surfactants was quite linear before and after the inflection. The comparison of the monomeric and micellar regions of both surfactants (SDBS and CPC) was made by determining the pre- (S1) and postmicellization (S2) slopes. The slopes of premicellar regions were always found to be greater than those of postmicellar regions.62 In the case of SDBS, breakpoints in conductivity data were not as sharp as compared to the conductivity data of CPC, which may be due to stepwise micelle formation or insoluble salt formation in the case of SDBS upon addition of organic solvents.63,64 Figure 2 depicts the representative plots of κ versus surfactant concentration in pure water at T = (293.15, 298.15, 303.15, 308.15) K. The corresponding conductivity data for SDBS and CPC in pure water at all four temperatures have been summarized in Tables S1 and S2 (Supporting Information). We observed the inhibitory effect of temperature on the micellization of SDBS and CPC. The linear increase in CMCs for both surfactants with respect to temperature65,66 signified the increase in the thermal motion of surfactants and the solvent system. The increased thermal motions inhibited the formation of ordered micelles due to disruption in the structure of water. 4.2. Effect of Solvent System on Micellization Characteristics. The micellization behavior of ionic surfactants can be understood on the basis of differences in the properties of glycerol−water and DMSO− water solvent systems. The conductivity data for SDBS and CPC in the presence of cosolvents (glycerol and DMSO) have been summarized in Tables S3−S6 (Supporting Information). In this case, micellization is inhibited by the presence of cosolvents in the medium. Cosolvents such as glycerol and DMSO, when added to water, increase the CMCs of ionic surfactants by decreasing the polarity of water and increasing

Figure 3. Plots of XCMC vs temperature in aqueous solution for (a) SDBS and (b) CPC containing the following concentrations of glycerol: 0% w/w (pure water), blue −●−; 5% w/w (m1), red −●−; 10% w/w (m2), green −●−; 15% w/w (m3), purple −●−; and 20% w/w (m4), teal −●−.

Figure 4. Plots of XCMC vs temperature in aqueous solution for (a) SDBS and (b) CPC containing the following concentrations of DMSO: 0% w/w (pure water), blue −●−; 5% w/w (m1), red −●−; 10% w/w (m2), green −●−; 15% w/w (m3), purple −●−; and 20% w/w (m4), teal −●−.

The commercially available samples of SDBS may contain different positional isomers (i.e., 2ϕC12, 3ϕC12, 4ϕC12, 5ϕC12, D



Article 0 ΔGtrans = (ΔGm0)glycerol/DMSO + water − (ΔGm0)water

the solubility of hydrocarbon chains, leading to increases in the hydrophobic character of the solvent medium.16,71,72,75 Increases in the CMC/XCMC values of SDBS and CPC on addition of cosolvents may be due to different properties of glycerol and DMSO. In the case of glycerol, the increase in the XCMC of surfactants can be attributed to the decrease in the dielectric constant of the medium, resulting in better solubility of hydrocarbon chains of surfactants in the glycerol−water mixed solvent system.73 Glycerol is a kosmotropic cosolvent which is known for its water structure-making properties over the latter.74 DMSO is known for its chaotropic effect on water structure, and its hydrogen-bonding properties with water are responsible for lowering the CMC of ionic surfactants. However, in this case the former dominates (disruption of the water structure) and is considered to be a nonpenetrating cosolvent which breaks the micelles, hence CMC/XCMC values of surfactants are directly proportional to the concentration of DMSO in the medium.17,33 The CMC values of SDBS and CPC exhibiting a cosolvent effect are represented in Supporting Information Tables S7 and S8. 4.3. Thermodynamics of Micelle Formation. The thermodynamics of micellization can be interpreted by two widely accepted approaches: phase separation and mass action. Whereas the mass action model is best suited for ionic surfactants, the thermodynamic parameters of micellization for ionic surfactants were deduced from the temperature dependence of XCMC using the mass-action model.57 Thermodynamic parameters of micellization are important in understanding the effects of structural and environmental factors on the process of micellization. To predict the effects of these factors (i.e., cosolvents) on the micellization of SDBS and CPC, the thermodynamic parameters of micellization (ΔG0m, ΔH0m, and ΔS0m) were calculated in aqueous solution as well as in aqueous solution containing organic solvents (glycerol and DMSO). The standard enthalpy of micellization ΔH0m of the ionic surfactants has been deduced from eq 2 61 ΔHm0 = − RT 2(2 − ∝ )

d ln XCMC dT

The values of thermodynamic parameters of micellization for aqueous solutions of SDBS and CPC are presented in Table 4. Table 4. Standard Thermodynamic Parameters of Micellization (Free Energy (ΔG0m), Enthalpy (ΔH0m), Entropy (ΔS0m), and Degree of Counterion Dissociation (α)) for Aqueous Solution of SDBS and CPC at Different Temperatures and Pressure p = 100 kPaa

postmicellization region (S2) premicellization region (S1)

where S1 and S2 are the slopes of the pre- and postmicellar regions obtained by plotting the conductivity data against the surfactant concentration. The standard free energy of micellization ΔG0m and the standard entropy of micellization ΔS0m were calculated by using eqs 4 and 5, respectively.

ΔHm0 − ΔGm0 T

(5)

ΔSm0 =

293.15 298.15 303.15 308.15

−31.027 −31.296 −31.534 −31.897

293.15 298.15 303.15 308.15

−42.853 −42.686 −43.016 −43.501

ΔH0m/kJ· mol−1 SDBS −6.515 −6.698 −6.892 −7.115 CPC −10.81 −11.01 −11.33 −11.69

ΔS0m/J·K−1·mol−1

α

83.615 82.502 81.286 80.422

0.810 0.817 0.822 0.823

109.314 106.228 104.531 103.246

0.406 0.43 0.438 0.440

After analyzing the data, we found that the ΔH0m values for SDBS and CPC in aqueous solution were negative and largely dependent on temperature. This observation reflected the exothermal and spontaneous behavior of the micellization process. ΔH0m values of SDBS and CPC were much smaller as compared to ΔS0m values in an aqueous medium, which suggests that the micellization process was entropically favored. Upon comparing between the ΔH0m values of these surfactants, it was observed that ΔHv values of SDBS were less negative as compared to those of CPC, so the micellization process in the case of SDBS was more entropically driven than in CPC, in which it was enthalpically driven in pure water. Decreases in the values of ΔS0m with increasing temperature indicate that the self-assembly of SDBS and CPC become poorer at high temperatures as a result of the melting of iceberg clusters around the alkyl chains of surfactant molecules and enhanced thermal motions/randomness of alkyl chains toward the micellar core at high temperatures.4,78,79 Negative values of ΔGm0 suggest that both SDBS and CPC form micelles spontaneously in water as well as in the presence of a water + cosolvent (DMSO and glycerol) medium. The values of thermodynamic parameters of micellization (ΔG0m, ΔH0m, and ΔS0m) and the free energy of surfactant tail transfer, ΔG0trans, in the water + cosolvent medium are reported in Tables 5−8 and were found to be negative, negative, and positive, respectively, in all cases. ΔG0m values were more negative with respect to temperature and became less negative on increasing the cosolvent concentration in the medium. This type of result signified that the spontaneity of the system increased with the increase in temperature and was inversely proportional to the cosolvent concentration. ΔG0trans values were positive and showed a gradual increase with increases in the concentrations of glycerol and DMSO in the water at specific temperatures80,81 This type of result signified the interactions between the surfactant tail with cosolvents (glycerol and DMSO) and

(3)

(4)

ΔG0m/kJ· mol−1

Standard uncertainties u, in temperature, pressure, and thermodynamic measurements for SDBS and CPC are u(CSDBS) = 5 × 10−5 mol·kg1−, u(C*CPC) = 3.3 × 10−5 mol· kg−1, u(α) = 0.02, u(ΔG0m) = 0.65 kJ·mol−1, u(ΔH0m) = 0.3 kJ·mol−1, u(ΔS0m) = 1.5 J·K−1.mol−1, u(T) = 0.05 K, and u(p) = 10 kPa (0.68 level of confidence).

(2)

ΔGm0 = (2 − α)RT ln XCMC

T/K

a

where R is the gas constant, T is temperature in Kelvin, d ln XCMC dT is the slope of the straight line obtained by plotting ln XCMC against T, and α is the degree of counterion dissociation and was calculated using eq 3 71 α=

(6)

The effects of cosolvents (glycerol and DMSO) on the micellization process of SDBS and CPC can be calculated using the free energy of surfactant tail transfer, ΔG0trans, as given by eq 676,77 E



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Table 5. Mole Fraction of CMC (XCMC) Values, Standard Thermodynamic Parameters of Micellization (Free Energy (ΔG0m), Enthalpy (ΔH0m), Entropy (ΔS0m), Degree of Counter Ion Dissociation (α), and Free Energy of Transfer (ΔG0trans) for SDBS (C) in Aqueous Mixtures of Glycerol at Different Temperatures and at Pressure p = 100 kPa ΔG0m/kJ mol−1

T/K

XCMC × 105

α

293.15 298.15 303.15 308.15

2.286 2.376 2.448 2.556

0.815 0.827 0.828 0.836

293.15 298.15 303.15 308.15

2.34 2.394 2.466 2.592

0.828 0.837 0.841 0.845

293.15 298.15 303.15 308.15

2.43 2.592 2.628 2.646

0.832 0.84 0.846 0.849

293.15 298.15 303.15 308.15

2.538 2.61 2.646 2.682

0.833 0.846 0.849 0.85

ΔG0trans/kJ mol−1

SDBS(C) in 5% w/w Glycerol (m1) −30.871 0.155396 −30.961 0.335895 −31.374 0.159533 −31.536 0.360867 SDBS(C) in 10% w/w Glycerol (m2) −30.449 0.577741 −30.686 0.610341 −30.998 0.535249 −31.26 0.637313 SDBS(C) in 15% w/w Glycerol (m3) −30.239 0.787598 −30.367 0.9297 −30.684 0.849663 −31.09 0.807434 SDBS(C) in 20% w/w Glycerol (m4) −30.107 0.919574 −30.19 1.106452 −30.58 0.953518 −31.016 0.881262

ΔH0m/kJ mol−1

ΔS0m/J K−1 mol−1

−6.174 −6.32 −6.53 −6.699

84.249 82.645 81.953 80.6

−5.642 −5.796 −5.969 −6.148

84.622 83.484 82.563 81.493

−4.497 −4.621 −4.754 −4.899

87.813 86.352 85.536 84.993

−2.994 −3.062 −3.158 −3.26

92.488 90.988 90.459 90.074

Table 6. Mole Fraction of CMC (XCMC) Values, Standard Thermodynamic Parameters of Micellization (Free Energy (ΔG0m), Enthalpy (ΔH0m), Entropy (ΔS0m), Degree of Counterion Dissociation (α), and Free Energy of Transfer (ΔG0trans) for CPC(C*) in Aqueous Mixtures of Glycerol at Different Temperatures and at Pressure p = 100 kPaa,b T/K

ΔG0m

XCMC 10

α

293.15 298.15 303.15 308.15

1.656 1.764 1.836 1.89

0.414 0.432 0.438 0.442

293.15 298.15 303.15 308.15

1.764 1.8 1.944 1.98

0.421 0.438 0.439 0.443

293.15 298.15 303.15 308.15

1.944 2.052 2.088 2.196

0.446 0.455 0.460 0.471

293.15 298.15 303.15 308.15

2.016 2.124 2.16 2.232

0.448 0.456 0.462 0.479

5

ΔG0trans

−1

kJ mol

kJ mol

−1

CPC(C*) in 5% w/w Glycerol (m1) −42.555 0.29801 −42.553 0.132855 −42.932 0.084064 −43.403 0.09854 CPC(C*) in 10% w/w Glycerol (m2) −42.126 0.727319 −42.310 0.376218 −42.668 0.348449 −43.196 0.305345 CPC(C*) in 15% w/w Glycerol (m3) −41.094 1.759641 −41.340 1.346113 −41.819 1.196573 −42.030 1.470583 CPC(C*) in 20% w/w Glycerol (m4) −40.899 1.954623 −41.184 1.501703 −41.631 1.385245 −41.735 1.765875

ΔH0m kJ mol

ΔS0m

−1

JK

−1

mol−1

−9.893 −10.12 −10.42 −10.74

111.418 108.783 107.251 106.011

−9.556 −9.78 −10.1 −10.41

111.102 109.105 107.434 106.394

−8.506 −8.747 −9.011 −9.249

111.163 109.317 108.225 106.381

−7.154 −7.353 −7.569 −7.778

115.111 113.47 112.359 110.196

a

m1 is the molality of 5% w/w glycerol corresponding to a concentration of 0.542 mol·kg1−. m2 is the molality of 10% w/w glycerol corresponding to a concentration of 1.09 mol· kg1−. m3 is the molality of 15% w/w glycerol corresponding to a concentration of 1.63 mol· kg1−. m4 is the molality of 20% w/w glycerol corresponding to a concentration of 2.17 mol· kg−1. bStandard uncertainties u in temperature, pressure, and thermodynamic measurements for SDBS are u(CSDBS) = 5 × 10−5 mol·kg−1, u(XCMC) = 0.02 × 10−5 for SDBS, u(α) = 0.02, u(ΔG0m) = 0.65 kJ·mol−1, u(ΔH0m) = 0.3 kJ·mol−1, u(ΔS0m) = 1.5 J·K−1.mol−1, u(mglycerol) = 0.22 mol· kg−1, u(T) = 0.05 K, and u(p) = 10 kPa (0.68 level of confidence).

ionic headgroups with water. The positive values of ΔG0trans signified the reduction in solvophobic interactions caused by improved solvation of the hydrocarbon tail, which resulted in reducing the ability of surfactants (SDBS and CPC) to aggregate and promoting the delay in the micellization

process.82 The positive values of the standard entropy of micellization and negative values of the standard free energy of micellization reflected the dominance of London dispersion interactions.83 The presence of cosolvents inhibited micellization, and the entropic contribution increased with the increase F



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Table 7. Mole Fraction of CMC (XCMC) Values, Standard Thermodynamic Parameters of Micellization (Free Energy (ΔG0m), Enthalpy (ΔH0m), Entropy (ΔS0m), Degree of Counterion Dissociation (α), and Free Energy of Transfer (ΔG0trans) for SDBS(C) in Aqueous Mixtures of DMSO at Different Temperatures and at Pressure p = 100 kPa T/K

XCMC

α

ΔG0m

ΔG0trans

−1

5

10

kJ mol

293.15 298.15 303.15 308.15

2.322 2.358 2.448 2.574

0.814 0.817 0.822 0.834

293.15 298.15 303.15 308.15

2.43 2.484 2.592 2.664

0.825 0.828 0.833 0.837

293.15 298.15 303.15 308.15

2.484 2.592 2.628 2.7

0.828 0.832 0.839 0.842

293.15 298.15 303.15 308.15

2.592 2.646 2.664 2.736

0.832 0.837 0.843 0.848

kJ mol

−1

SDBS (C) in 5% w/w DMSO (m5) −30.833 0.193499 −31.244 0.052685 −31.436 0.097301 −31.571 0.326294 SDBS (C) in 10% w/w DMSO (m6) −30.432 0.594738 −30.814 0.482539 −31.061 0.47292 −31.393 0.504554 SDBS (C) in 15% w/w DMSO (m7) −30.289 0.737853 −30.577 0.71954 −30.86 0.673412 −31.215 0.681816 SDBS (C) in 20% w/w DMSO (m8) −30.067 0.960175 −30.373 0.923399 −30.716 0.817284 −31.006 0.890821

ΔH0m kJ mol

ΔS0m

−1

JK

−1

mol−1

−5.87 −6.059 −6.24 −6.38

85.154 84.471 83.446 81.749

−5.357 −5.528 −5.689 −5.859

85.537 84.809 83.695 82.86

−4.422 −4.558 −4.684 −4.828

88.239 87.268 86.349 85.632

−2.813 −2.895 −2.979 −3.065

92.969 92.16 91.496 90.676

Table 8. Mole Fraction of CMC (XCMC) Values, Standard Thermodynamic Parameters of Micellization (Free Energy (ΔG0m), Enthalpy (ΔH0m), Entropy (ΔS0m), Degree of Counterion Dissociation (α) and Free Energy of Transfer (ΔG0trans) for CPC(C*) in Aqueous Mixtures of DMSO at Different Temperatures and at Pressure p = 100 kPab T/K

ΔG0m

XCMC 10

α

293.15 298.15 303.15 308.15

1.854 1.944 2.052 2.088

0.425 0.428 0.433 0.437

293.15 298.15 303.15 308.15

1.98 2.124 2.196 2.232

0.429 0.439 0.441 0.448

293.15 298.15 303.15 308.15

2.16 2.268 2.34 2.412

0.463 0.474 0.479 0.480

293.15 298.15 303.15 308.15

2.232 2.34 2.394 2.448

0.486 0.491 0.500 0.509

5

ΔG0trans

−1

kJ mol

kJ mol

−1

CPC(C*) in 5% w/w DMSO (m5) −41.831 1.021967 −42.281 0.405036 −42.629 0.387159 −43.167 0.33376 CPC(C*) in 10% w/w DMSO (m6) −41.47 1.383014 −41.646 1.039294 −42.159 0.856722 −42.589 0.912036 CPC(C*) in 15% w/w DMSO (m7) −40.245 2.608083 −40.46 2.225835 −40.872 2.143787 −41.407 2.093844 CPC(C*) in 20% w/w DMSO (m8) −39.528 3.325538 −39.88 2.805951 −40.223 2.79294 −40.549 2.951736

ΔH0m kJ mol

ΔS0m

−1

JK

−1

mol−1

−9.24 −9.54 −9.829 −10.13

111.175 109.812 108.196 107.199

−8.812 −9.059 −9.353 −9.619

111.405 109.299 108.217 106.993

−7.962 −8.178 −8.425 −8.7

110.125 108.274 107.035 106.139

−6.492 −6.691 −6.876 −7.061

112.694 111.317 110.001 108.675

a

m5 is the molality of 5% w/w DMSO corresponding to a concentration of 0.64 mol·kg1−. m6 is the molality of 10% w/w DMSO corresponding to a concentration of 1.28 mol· kg1−. m7 is the molality of 15% w/w DMSO corresponding to a concentration of 1.92 mol· kg1−. m8 is the molality of 20% w/w DMSO corresponding to a concentration of 2.56 mol· kg−1. bStandard uncertainties u in temperature, pressure, and thermodynamic measurements for CPC are u(C*CPC) = 3.3 × 10−5 mol· kg1−, u(XCMC) = 0.04 × 10−5 for CPC, u(α) = 0.02, u(ΔG0m) = 0.65 kJ·mol−1, u(ΔH0m) = 0.3 kJ·mol−1, u(ΔS0m) = 1.5 J·K−1.mol−1, u(mDMSO) = 0.26 mol· kg1−, u(T) = 0.05 K, and u(p) = 10 kPa (0.68 level of confidence).

glycerol dominated that of DMSO as the values of ΔH0m in the presence of glycerol were more negative than in DMSO, and main reason behind this observation was the low polarity of glycerol (ϵ = 42.5) as compared to that of DMSO (ϵ =

in the concentration of cosolvents in the medium52 because the cosolvent addition decreases the cohesiveness of the solvent medium and reduces the solvophobic effect by solubilizing hydrocarbon tails.84 The inhibitory effect of G



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48.9).85 As a result, the solubility of hydrocarbon chains of surfactant molecules increases in the water + cosolvent mixed medium and reduces the aggregation tendency of surfactant monomers.71,86 The temperature dependence of the hydrophobic effect can be expressed in terms of the heat capacity of micellization (ΔmC0p), which is calculated from the slope of the ΔH0m vs temperature curve and is represented as follows: Δm C p0 =

∂ΔHm0 ∂T

where Xi and Vi are the mole fraction and molar volume of the ith component, respectively. The plots of Gordon parameters with ΔG0m have been represented in Figure S2. A curvilinear relationship between ΔG0m and G has been observed, and similar results were obtained by Bhattarai et al.87 A single solvent property might not be sufficient to explain the process of micellization of SDBS/CPC in the presence of cosolvents glycerol/DMSO, so we correlated the ΔG0m values with different solvent parameters (ET(30), D, G, and δ) of water + glycerol/DMSO systems. Correlation is always established with ΔG m0 because it is considered to be the best thermodynamic parameter as counterion binding influences it, especially in the case of ionic surfactants.87 The ET(30) and D values for the glycerol + water system87,89 and the DMSO + water system were taken from the literature. δ values were also determined using the relation in refs 90 and 91, and δ’s relationship with ΔG0m is represented in Figure S3

(7)

The calculated values of the heat capacity of micellization are represented in Table 9. Table 9. Values of ΔmC0p (J mol−1 K−1) for SDBS and CPC in Aqueous and Binary Aqueous Mixtures of Glycerol/ DMSO at Pressure p = 100 kPaa CPC

The calculated physiochemical parameters for glycerol + water and DMSO + water systems are reported in Tables 10 and 11, respectively. 4.5. Correlation of the Free Energy of Micellization (ΔG0m) with the Solvophobic Parameter (SP). Sp values for binary aqueous mixtures of DMSO were adopted from the literature,92 and Sp values for the glycerol−water system have not been reported due to unavailability in the literature.93 The concentrations investigated in this work are different from those reported in the literature, so Sp values for the present system were found with the help of the correlation method suggested by Wang et al.93 A linear relationship between the DMSO concentration (% w/w) and Sp has been observed for DMSO + water systems (Figure S4). The plot between (ΔG0m) and Sp also exhibits a linear relationship (Figure S5). 4.6. MD Simulation and Quantum Chemical Calculations. Experimental data confirms that the micellization process is affected by the use of different cosolvents such as glycerol and DMSO. From a theoretical point of view, we selected two types of systems, one undisturbed by the action of the cosolvent and the other in the presence of the cosolvent as shown in Figure 5. In the first two cases (Figure 5a,d), where only the surfactant is an aqueous medium, the formation of molecular aggregates that leads to the formation of micelles is clearly observed. In the following cases, when cosolvents such as glycerol and DMSO are added to water, the driving force required for the aggregation of surfactant monomer decreases. Similar behavior was observed in the experimental results. This suggests a modification of the water structure due to the interaction between glycerol and DMSO with the water and

SDBS

% cosolvent

glycerol

DMSO

glycerol

DMSO

0 5 10 15 20

−58.9467 −57.623 −56.5174 −49.8697 −41.7905

−39.8856 −58.3918 −54.3183 −49.2363 −37.8749

−35.7425 −33.8168 −26.7964 −17.8283

−34.201 −33.3683 −26.8928 −16.7947

a

Standard uncertainties u in temperature, pressure, and thermodynamic measurements for CPC are u(ΔH0m) = 0.3 kJ·mol−1, u(ΔmC0p) = 3.22 J·mol−1 K−1, u(T) = 0.05 K, and u(p) = 10 kPa (0.68 level of confidence).

All values of ΔmC0p were negative and show linear increases with increasing percentage of cosolvent (glycerol/DMSO). These types of results were also observed by Bhattarai et al.87 4.4. Correlation of the Free Energy of Micellization (ΔG0m) with Solvent Parameters. The self-assembly of ionic surfactants is generally controlled by the solvophobic effect and is related to solvent cohesiveness, which can be expressed in terms of the Gordon parameter (G). It was determined from eq 888 γ G = sol 3 V (8) m where γsol is the surface tension of the solution and Vm is the molar volume of the solution and is determined using relation 2

Vm =

(10)

δ = 0.45D + 18.5

ΔmCp0 (J mol−1 K−1)

∑ XiVi

(9)

i=1

Table 10. Various Physiochemical Parameters of Mixed Solvent (Glycerol + Water) at 298.15 K and Pressure p = 100 kPaa % glycerol

surface tension

molar volume

Gordon parameter

Reichardt parameter

w/w

dielectric constant

γ0/mN m−1

Vm/dm3, mol−1

G/J·m−3

ET/kJ·mol−1

δ

0 5 10 15 20 100

79.50 77.10 75.70 74.30 72.90 40.10

71.80 71.20 70.50 70.00 69.50 63.00

18.07 18.61 19.19 19.83 20.54 73.21

2.74 2.69 2.63 2.59 2.54 1.51

264.01 262.76 261.50 260.24 258.99 238.49

54.28 53.20 52.57 51.94 51.31 36.55

a

Standard uncertainties u in temperature, pressure, and thermodynamic measurements for CPC are u(T) = 0.05 K and u(p) = 10 kPa (0.68 level of confidence). H



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Table 11. Various Physiochemical Parameters of Mixed Solvent (DMSO + Water) at 298.15 K and Pressure p = 100 kPaa % DMSO

surface tension

molar volume

Gordon parameter

Reichardt parameter

w/w

dielectric constant

γ0/mN·m−1

Vm/dm3, mol−1

G/J·m−3

ET/kJ·mol−1

δ

0 5 10 15 20 100

79.50 79.30 79.00 78.90 78.90 48.40

71.80 71.65 71.54 70.00 68.45 42.80

18.07 20.73 23.39 26.05 28.71 71.3

2.74 2.61 2.50 2.36 2.24 1.03

264.01 260.22 256.44 252.65 248.86 188.28

54.28 54.19 54.05 54.01 54.00 40.28

a

Standard uncertainties u in temperature, pressure, and thermodynamic measurements for CPC are u(T) = 0.05 K and u(p) = 10 kPa (0.68 level of confidence).

Figure 5. Schematic representations at the end (20 ns) of their respective production runs for a bulk simulation with the different molecules being shown in different colors and (a−f) several mixtures of solution models: (a) a mixture of water and ionic surfactant CPC, (b) water and cosolvent glycerol with ionic surfactant CPC in the interface, (c) water and cosolvent DMSO with ionic surfactant CPC in the interface, (d) a mixture of water and ionic surfactant SDBS, (e) water and cosolvent glycerol with ionic surfactant SDBS in the interface, and (f) water and cosolvent DMSO with ionic surfactant SDBS in the interface. CPC and SDBS are in licorice representation. Water (white), glycerol (lime), and DMSO (pink) are in a quicksurf representation.

structure maker. In the other case, DMSO has the ability to disrupt the hydrogen-bonding network of water by the chaotropic effect through the strongest hydrogen bond interactions between the DMSO oxygen atom and a water hydrogen atom, being more stable than the hydrogen bond of the water. We also observe that oxygen atom of a water molecule is in close proximity to a hydrogen atom of the DMSO methyl group. In the same way, the DMSO methyl group has a weak interaction (van der Waals forces) with the hydrocarbon chain in the ionic surfactant (Figure 8). In both cases, the van der Waals forces are more intense when the cosolvent is added (Figure 6), and the intermolecular hydrogen bridging the structural water network is interrupted. Therefore, it is clear that the supramolecular assembly is

ionic surfactants. In the case of glycerol, a more pronounced breakdown of the micellar structure is observed (Figure 5). This behavior can be mainly attributed to (1) hydrogen bonds between hydroxyl groups belonging to glycerol and the water molecules that are in contact with the hydrophilic regions (Figure 6) and (2) van der Waals forces that occur as a result of a fluctuating electrical charge between the hydrocarbon chains of ionic surfactant and the hydrocarbon part of the glycerol (Figure 7). This induces a temporary charge in the neighboring carbon atoms by attracting or repelling the electrons associated with it. This allows better solubility of the hydrocarbon chains of surfactants in the glycerol−water mixed solvent system such as was mentioned before. Therefore, it demonstrates the action of glycerol as a water I



Article

Figure 6. Bottom of the NCIPLOT gradient isosurface (0.6 au) for four geometric structures. (A) Water with ionic surfactant CPC. (B) Water and cosolvent DMSO with ionic surfactant CPC in the interface. (C) Water and cosolvent glycerol with ionic surfactant CPC in the interface. (D) Water with ionic surfactant SDBS. (E) Water and cosolvent DMSO with ionic surfactant SDBS in the interface. (F) Water and cosolvent glycerol with ionic surfactant SDBS in the interface (kcal/mol−1). The surfaces are colored on a blue−green−red scale according to the strength and type (attractive or repulsive) of interaction. Blue indicates strong attractive interactions, green indicate weak VDW interactions, and red indicates a strong nonbonded overlap. These calculations were made at the B3LYP/6-31G** level.

Figure 7. NCIPLOT gradient isosurface (0.6 au) for water and cosolvent DMSO with ionic surfactant CPC. The surfaces are colored on a blue− green−red scale according to the strength and type (attractive or repulsive) of interaction. Blue indicates strong attractive interactions, green indicates weak VDW interactions, and red indicates a strong nonbonded overlap. These calculations were made at the B3LYP/6-31G** level.

inhibited by the presence glycerol and DMSO because hydrophobic regions in the solvent media increased with increasing concentration of the nonaqueous solvents, imparting cosolvency and promoting the decrease in the amphiphilic assembly. This analysis can be complemented through the radial distribution functions (RDFs) as shown in Figures 9 and 10, respectively, where the terminal carbon of each tail of the surfactant is taken as reference. These figures show an intense peak located at (3−4) Å, which corresponds to the first sphere of solvation. Here it is possible to observe that there is a lower probability of finding water molecules near the tail of the

surfactant in the micellar system (SDBS + water, CPC + water). A higher probability for the case of DMSO and glycerol was observed, being higher in the case of the glycerol. This is in agreement with the experimental results that were previously discussed.

5. CONCLUSIONS A combined experimental and molecular modeling study of the effect of cosolvents such as DMSO and glycerol on the selfassembly behavior of SDBS and CPC was performed. This study helped us to understand the micellization process of SDBS and CPC in mixed aquo-organic solvents. The J



Article

Figure 8. NCIPLOT gradient isosurface (0.6 au) for water and cosolvent glycerol with ionic surfactant SDBS. The surfaces are colored on a blue− green−red scale according to the strength and type (attractive or repulsive) of interaction. Blue indicates strong attractive interactions, green indicate weak VDW interactions, and red indicates strong nonbonded overlap. These calculations were made at the B3LYP/6-31G** level.

these surfactants. From a theoretical point of view, it was found that the balance in interaction between the solvent and the hydrophobic and hydrophilic segments of the surfactants (SDBS and CPC) determined their solubility in mixed aquoorganic solvents. Surfactant aggregation in solution depends on hydrophobic, hydrophilic, and counterion interactions. Furthermore, the comparison between the effect of kosmotropic (glycerol) and chaotropic (DMSO) cosolvents on the aggregation of SDBS and CPC manifested the role of such systems as agents for their potential use in many industrial and household processes. In future work, it will be interesting to study the micellization behavior of these surfactants in the presence of other agrochemical, pharmaceutical, and biochemical products in the presence of cosolvents for their enhanced applications in pharmaceutical, agrochemical, and other industries.

Figure 9. Radial distribution function between the terminal carbon of the tail of SDBS and the oxygen atom in the water molecule.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jced.8b00326. Experimental values of specific conductance and CMC values of SDBS and CPC in aqueous mixtures at different concentrations and temperatures and supporting figures (PDF)

■

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected].

Figure 10. Radial distribution function between the terminal carbon of the tail of CPC and the oxygen atom in the water molecule.

ORCID

micellization of both of the surfactants was inhibited by the cosolvents. Temperature also played an inhibitory role in the micellization of ionic surfactants (SDBS and CPC). On analyzing the thermodynamic parameters, it was found that the micellization of both of the surfactants was entropy-driven and spontaneous in nature. Comparison between the micellization behavior of SDBS and CPC exhibited the importance of aromatic headgroups in the self-assembly of

Plinio Cantero-López: 0000-0003-4090-9879 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful to Shree Ashok Mittal, Honourable Chancellor, Lovely Professional University, Punjab, India for providing the K



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facilities to carry out the aforementioned research. P.C.-L. acknowledges CONICYT for postdoctoral project FONDECYT/postdoctorado-2018 no. 3180449. Furthermore, O.Y.-O. acknowledge CONICYT-PCHA/Doctorado Nacional/201421140667 for a Ph.D. fellowship.

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