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Oct 18, 2016 - Department of Chemistry, H. P. University, Shimla,171005, India. ‡. Department of Chemistry, Maharaja Agrasen University, Baddi, Sola...
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Volumetric Analysis of Structural Changes of Cationic Micelles in the Presence of Quaternary Ammonium Salts Suvarcha Chauhan,*,† Maninder Kaur,† Dilbag Singh Rana,‡ and Mohinder Singh Chauhan† †

Department of Chemistry, H. P. University, Shimla,171005, India Department of Chemistry, Maharaja Agrasen University, Baddi, Solan, India

‡

S Supporting Information *

ABSTRACT: Density (ρ) and speed of sound (u) of a cationic surfactant, DTAB (dodecyltrimethylammonium bromide) in aqueous solutions of tetraalkylammonium bromide, R4NBr (R= propyl, butyl, and pentyl) have been measured in the temperature range (293.15−318.15) K at an interval of 5 K. The temperature dependence of speed of sound has been used to compute the critical micelle concentration (CMC) and second critical micelle concentration (2nd CMC). Various volumetric and compressibility parameters have been evaluated in micellar and postmicellar regions to investigate the behavior of elongated micelles in comparison to the spherical ones. The trends of all these parameters have been examined in terms of a competing pattern of various intermolecular interactions existing in the ternary (tetraalkylammonium bromide + water + surfactant) system. The hydrophobic−hydrophobic interactions show the predominance in the studied system, which is enhanced with the increase in the alkyl chain length of the bromide salt.

1. INTRODUCTION The study of structural transitions in ionic micelles plays an important role for the determination and optimization of various characteristic properties of surfactants for their use in many pharmaceutical, biotechnological, and chemical processes.1−5 In aqueous solution, the surfactant monomers form thermodynamically stable organized spherical assemblies of aggregates known as micelles above a particular minimum concentration referred to as critical micellar concentration (CMC). When the concentration of ionic surfactant further increases in the micellar region, the structure changes from spherical micelles to rodlike or ellipsoidal, or in some cases long flexible micelles may take place.6,7 These changes occur at a particular concentration of the surfactant known as second critical micellar concentration (2nd CMC) or micellar structure transition concentration (mstc). Various conventional and spectroscopic techniques have been usually employed to determine the structural transitions in the micelles; the basis of all these methods is an abrupt change in the related physical properties in the micellar and postmicellar region. In past few decades, ultrasonic studies have been emerged as one of the fundamental transport methods which throw light on the intermolecular/interionic interactions prevailing in electrolytic solutions.8−12 The study of densities of solutions provides information about solute−solvent and solvent−solvent interactions, whereas speed of sound is a parameter of great significance in relation to other fluid properties such as compressibility and heat capacity.13 Perez et al.14 have estimated the 2nd CMC for some cationic surfactants by conductivity, refractive index, density, and speed of sound © XXXX American Chemical Society

measurements and have shown that the ratio 2nd CMC/CMC lies in the range 2−10 in the case of 1:1 ionic surfactants. They have also obtained an inverted U-shaped trend for the sphereto-rod transition as a function of temperature.15,16 Lee et al.6 have advocated a distinctly narrow range of 2nd CMC/CMC ratio equal to almost 2−2.5 for homologous alkyltrimethylammonium bromides and alkylpyridinium chlorides. Recently, Savaroglu et al.17 have observed the effect of the structural transition in micelles on various physical and chemical properties such as speed of sound, density, conductivity of aqueous solutions of surfactants. On the other hand, quaternary ammonium salts (QAS/ QUATS) are important cationic compounds due to their wide array of applications such as phase transfer catalysts,18−20 disinfectants,21,22 and synthetic reagents.23,24 Certain QAS, especially those containing long alkyl chains, for example, tetraethylammonium bromide, didecyldimethylammonium chloride, cetylpyridinium chloride etc., show antimicrobial activity. So they also find a number of applications as hygienic adjuncts against bacterial growth in various industrial and clinical formulations.25,26 Among QAS, tetraalkylammonim salts afford to be an interesting class of organic electrolytes due to their symmetric structure containing four nonpolar hydrocarbon chains in one structure. Such a bulky structure has a tendency to orient water molecules around them depending on length of their alkyl chain, which results into the formation Received: April 23, 2016 Accepted: October 10, 2016

A

DOI: 10.1021/acs.jced.6b00332 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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Table 1. Specification and Mass Fraction Purity of Chemical Samples

a

chemical name

source

CAS no.

purification method

mass fraction puritya

tetrapropylammonium bromide, (C3H7)4NBr tetrabutylammonium bromide, (C4H9)4NBr tetrapentylammonium bromide, (C5H11)4NBr dodecyltrimethylammonium bromide, C12H25N(CH3)3Br

Fluka SRL Acros Organics S.D. Fine

631-40-3 1643-19-2 866-97-7 1119-94-4

none none none recrystallization

0.98 0.99 0.99 0.98

Provided by supplier.

Figure 1. Structures of chemicals used in the present study.

of more ordered and rigid structures of water surrounding the solute molecules.27 So these salts can give better insight into the effect of solute−solute, solute−solvent, and solvent−solvent interactions on the stability of systems of both industrial and biological importance. The fundamental knowledge of interactions of QAS with surface active agents is crucial in industrial processes for getting an idea about the interplay of hydrophobic and hydrophilic hydration that dominates the interactions of surfactants with water and hence affects their properties and functioning. Patiest et al.28 have reported the effect of tetraalkylammonium chlorides (TCnAC for n = 1, 2, 3 and 4) on the self-aggregation of SDS solutions in relation to its dynamic interfacial properties, surface viscosity and surface tension and concluded that the micellar stability increases with increase in concentration of TCnAC, probably due to the ionic interactions between the oppositely charged TCnAC and the SDS headgroup. Some researchers have investigated the effect of tetraalkylammonium bromides on the micellar behavior of ionic as well as nonionic surfactants and interpreted the results in terms of charge and hydrophobic character of tetraalkylammonium salts.29−31 Addition of these bromide salts has caused a sphere-to-rod transition for sodium dodecyl sulfate micelles.30 In our previous study,32 we have also explored the effect of organic electrolytes and temperature on the micellization of cationic surfactant DTAB by employing conductivity as well as fluorescence studies and observed the importance of reinforced hydrophobic interactions between the hydrophobic parts of surfactant and electrolyte in the micellization of surfactant. However, there is scarcely any study in the literature showing the effect of QAS on the volumetric parameters of surfactants in micellar and post micellar regions. Density and speed of sound data enable us to characterize the volumetric properties of such systems, which further give an idea about the structure-making/ breaking nature of the solute.33,34

Thus, the present work mainly focuses on the acoustical behavior of cationic surfactant, DTAB (dodecyltrimethylammonium bromide) in aqueous solutions of tetraalkylammonium bromide, R4NBr (R = propyl, butyl, and pentyl). The speed of sound data have been employed to calculate the CMC and 2nd CMC of surfactant in the presence of additives. Various volumetric and compressibility parameters have also been estimated in micellar and postmicellar regions at different temperatures using the pseudophase separation model.

2. EXPERIMENTAL SECTION 2.1. Materials Used. The deionized distilled water with a conductivity of 2 to 3·10−6 S·cm−1 and pH of 6.8 to 7.0 (at 298.15 K) was obtained from a Millipore−Elix system and was used for all the experiments. Dodecyltrimethylammonium bromide (DTAB) of A.R. grade was obtained from SD. Fine−Chem Ltd. (India) and was used after purification as mentioned in the literature.35 Tetrapropylammonium bromide, (C3H7)4NBr from Fluka (Switzerland), tetrabutylammonium bromide, (C4H9)4NBr from SRL (India) and tetrapentylammonium bromide, (C5H11)4NBr from Acros Organics (Belgium), all were of A.R. grade and were dried in vacuum oven at 333.15 K for 24 h before use. A summary of provenance and purity of the samples used has also been provided in Table 1 and their structures are given in Figure 1. 2.2. Methods Used. Density and speed of sound measurements were performed with a high-precision digital density and sound velocity analyzer-5000 (DSA-5000) supplied by Anton Paar GmbH, Austria. The instrument was calibrated periodically with two fluids, that is, dry air and distilled water over a temperature range (293.15−318.15) K. This two-in-one instrument has been equipped with a density cell and a speed of sound cell; both the cells are thermally controlled by an intrinsic Peltier thermostat. The sample is injected into a Ushaped glass tube that is electronically excited to vibrate at its B

DOI: 10.1021/acs.jced.6b00332 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

C

0 3 6 9 12 15 18 21 24 27 30 33

998.57 998.70 998.74 998.79 998.85 998.89 998.94 998.99 999.03

998.62 998.75 998.84 998.88 998.92 998.97 999.01 999.04 999.10 999.15 999.18 999.20

3 6 9 12 15 18 21 24 27 30 33 36 39 42 45

0 3 6 9 12 15 18 21 24

(997.18)c (997.06)d 997.20 997.26 997.32 997.37 997.43 997.49 997.53 997.58 997.63 997.67 997.71 997.75 997.79 997.82 997.85

(998.22)c (998.22)d 998.31 998.37 998.43 998.49 998.54 998.60 998.65 998.70 998.75 998.81 998.87 998.92 998.95 999.00 999.05

0

997.35 997.51 997.55 997.60 997.66 997.70 997.75 997.79 997.83

997.49 997.57 997.64 997.70 997.74 997.78 997.82 997.85 997.90 997.95 998.00 998.01

T/K = 298.15

T/K = 293.15

m·103

995.88 996.09 996.14 996.18 996.22 996.27 996.31 996.34 996.39

996.00 996.16 996.23 996.27 996.31 996.35 996.39 996.42 996.46 996.51 996.58 996.60

(995.63)c (995.65)d 995.71 995.76 995.82 995.87 995.92 995.97 996.02 996.07 996.11 996.16 996.21 996.24 996.29 996.32 996.36

T/K = 303.15

994.29 994.45 994.49 994.53 994.58 994.62 994.66 994.70 994.74

994.41 994.53 994.59 994.63 994.67 994.71 994.74 994.77 994.81 994.86 994.90 994.93

(994.19)c (994.03)d 994.26 994.31 994.36 994.41 994.46 994.51 994.55 994.59 994.63 994.67 994.72 994.77 994.79 994.82 994.84

T/K = 308.15

ρ/(kg·m−3) T/K = 318.15

T/K = 293.15

Pure Water (992.22)c 990.22 (1483.6)c (992.24)d (990.26)d (1483.0)d 992.29 990.28 1483.3 992.33 990.33 1484.2 992.38 990.37 1484.9 992.42 990.39 1485.6 992.47 990.42 1486.3 992.51 990.45 1486.7 992.55 990.48 1486.8 992.59 990.52 1486.9 992.63 990.55 1487.0 992.67 990.57 1487.0 992.71 990.61 1487.1 992.77 990.64 1487.1 992.81 990.67 1487.1 992.87 990.70 1487.1 992.90 990.72 1487.0 b [(C3H7)4NBr] = 0.01 mol·kg−1 992.61 990.53 1484.5 992.70 990.68 1485.4 992.75 990.74 1486.0 992.8 990.79 1486.6 992.83 990.81 1487.0 992.87 990.84 1487.5 992.90 990.87 1487.6 992.92 990.89 1487.6 992.96 990.93 1487.7 993.01 990.97 1487.7 992.98 990.98 1487.6 992.99 991.02 1487.4 b [(C4H9)4NBr] = 0.01 mol·kg−1 992.33 990.43 1485.3 992.58 990.57 1485.8 992.64 990.62 1486.7 992.69 990.66 1487.2 992.74 990.71 1487.8 992.79 990.75 1488.1 992.82 990.79 1488.2 992.86 990.83 1488.3 992.89 990.86 1488.5

T/K = 313.15

1499.2 1499.7 1500.1 1500.8 1501.3 1501.6 1501.7 1501.9 1501.8

1498.5 1499.1 1499.7 1500.2 1500.8 1501 1501.1 1501.1 1501.2 1501.2 1501.1 1500.9

(1497.5)c (1497.1)d 1497.2 1497.9 1498.6 1499.2 1499.9 1500.2 1500.3 1500.3 1500.4 1500.4 1500.5 1500.4 1500.5 1500.4 1500.5

T/K = 298.15

1511.4 1511.9 1512.4 1512.9 1513.4 1513.7 1513.7 1513.8 1513.8

1510.9 1511.4 1511.8 1512.5 1513.1 1513.2 1513.2 1513.3 1513.1 1513.3 1513.1 1513.0

(1510.0)c (1509.4)d 1509.7 1510.3 1511.0 1511.6 1512.0 1512.2 1512.2 1512.2 1512.2 1512.2 1512.2 1512.2 1512.3 1512.2 1512.3

T/K = 303.15

1521.9 1522.5 1522.9 1523.2 1523.6 1523.8 1523.9 1524.0 1524.1

1521.4 1522 1522.6 1523.1 1523.5 1523.6 1523.6 1523.6 1523.4 1523.4 1523.5 1523.5

(1520.6)c (1520.1)d 1520.3 1520.9 1521.5 1522.0 1522.6 1522.6 1522.6 1522.6 1522.6 1522.5 1522.5 1522.5 1522.5 1522.6 1522.5

T/K = 308.15

u/(m·s−1)

1531.0 1531.7 1531.8 1532.2 1532.5 1532.7 1532.7 1532.8 1532.8

1530.4 1530.9 1531.2 1531.6 1532 1532.2 1532.2 1532.3 1532.2 1532.2 1532.2 1532.3

(1529.7)c (1529.2)d 1529.4 1529.9 1530.4 1530.9 1531.4 1531.5 1531.4 1531.4 1531.3 1531.3 1531.2 1531.3 1531.3 1531.3 1531.2

T/K = 313.15

1538.6 1539.0 1539.2 1539.5 1539.8 1540.1 1540.0 1540.1 1540.0

1538.0 1538.4 1538.8 1539.0 1539.5 1539.7 1539.6 1539.6 1539.4 1539.4 1539.5 1539.5

1535.7 (1536.5)d 1536.3 1536.7 1537.2 1537.8 1538.2 1538.4 1538.6 1538.7 1538.7 1538.7 1538.9 1538.9 1539.0 1539.1 1539.1

T/K = 318.15

Table 2. Molality, m/ (mol·kg−1) of DTAB and Corresponding Density, ρ and Speed of Sound, u in the Absence and Presence of Aqueous Tetraalkylammonium Bromide Solutions at Different Temperatures T = 293.15-318.15 K and Pressure, p = 100 kPaa

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DOI: 10.1021/acs.jced.6b00332 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

998.34 998.54 998.59 998.63 998.68 998.74 998.78 998.83 998.87 998.90 998.96 998.99

0 3 6 9 12 15 18 21 24 27 30 33

997.00 997.34 997.39 997.44 997.49 997.54 997.59 997.62 997.67 997.72 997.76 997.79

997.89 997.93 997.98

T/K = 298.15

995.70 995.89 995.96 996.01 996.10 996.11 996.15 996.18 996.23 996.27 996.31 996.38

996.44 996.49 996.52

T/K = 303.15

994.05 994.28 994.32 994.37 994.41 994.46 994.49 994.53 994.57 994.61 994.65 994.68

994.79 994.83 994.87

T/K = 308.15

ρ/(kg·m−3)

991.93 992.30 992.37 992.42 992.50 992.56 992.6 992.67 992.71 992.75 992.80 992.86

992.94 992.96 993.00

b

T/K = 318.15

T/K = 293.15

[(C4H9)4NBr] = 0.01 mol·kg−1 990.89 1488.4 990.92 1488.4 990.97 1488.4 b [(C5H11)4NBr] = 0.01 mol·kg−1 990.16 1485.3 990.39 1486.2 990.44 1487.0 990.49 1487.8 990.54 1488.4 990.58 1488.6 990.61 1488.4 990.64 1488.3 990.67 1488.6 990.71 1488.7 990.74 1488.9 990.78 1488.9

T/K = 313.15

1499.1 1500.0 1500.6 1501.3 1501.9 1502.1 1501.9 1501.9 1502.0 1502.2 1502.2 1502.3

1501.9 1501.8 1501.7

T/K = 298.15

1511.2 1512.1 1512.6 1513.2 1513.7 1513.9 1513.8 1513.7 1513.8 1513.9 1514.0 1514.1

1513.8 1513.8 1513.7

T/K = 303.15

1521.6 1522.4 1522.9 1523.4 1523.8 1524.2 1524.1 1524.0 1524.0 1524.0 1524.1 1524.1

1524.0 1523.9 1523.9

T/K = 308.15

u/(m·s−1)

1530.5 1531.2 1531.7 1532.1 1532.6 1532.8 1532.7 1532.6 1532.6 1532.6 1532.7 1532.7

1532.6 1532.6 1532.6

T/K = 313.15

1537.9 1538.5 1539.0 1539.3 1539.7 1540.0 1539.9 1539.8 1539.7 1539.8 1539.8 1539.8

1539.8 1539.8 1539.8

T/K = 318.15

a

Standard uncertainties, u are u(T) = 0.01 K, u(m) = 0.2 mmol·kg−1, u(ρ) = 0.15 kg·m−3, u(u) = 0.5 m·s−1. u(p) = 10 kPa (level of confidence = 0.68). m is the molality of DTAB in the solution. bThe solvent system here refers to aqueous solution of R4NBr with standard uncertainty in molality of R4NBr equal to 0.003 mol·kg−1. cReference 38. dReference 39.

999.08 999.14 999.19

T/K = 293.15

27 30 33

m·10

3

Table 2. continued

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3. RESULTS AND DISCUSSION The density and speed of sound data for cationic surfactant, DTAB in the absence and presence of 0.01 mol·kg−1 aqueous solutions of tetraalkylammonium bromide salts, R4NBr, at different temperatures are given in Table 2. The density of DTAB in pure water decreases with rise in temperature. The ρ values of aqueous solutions of surfactant also show a remarkable decrease when we go from tetrapropylammonium bromide to tetrapentylammonium bromide salt. Enhancement in the length of alkyl chain in the salt may lead to an increase in nonpolar regions containing hydrocarbon chains which occupy more space, thereby decreasing the overall density.36 On the other hand, the speed of sound (u) values of DTAB in aqueous solutions increase with increase in temperature as well as size of additive. The addition of solute increases the electrostatic interactions between water and solute molecules which results in the shrinkage of volume of solvent caused by ionic solute in comparison to volume of solvent in water. Thus, speed of sound increases due to the cohesion brought about by hydration.37 3.1. Temperature Dependence of Critical Micellar Concentration (CMC) and Second Critical Micellar Concentration (2nd CMC). The isotherms of speed of sound of DTAB in pure water and 0.01 mol·kg−1 aqueous solution of R4NBr as a function of [DTAB] at 298.15 K have been reconstructed in Figure 2. Three linear segments with two

breaks have been observed in each case. The first break corresponds to the formation of self-associated spherical aggregates of the surfactant monomers and has been referred to as critical micelle concentration (CMC). The second break in the isotherm corresponds to the second critical micelle concentration (2nd CMC) or micellar structure transition concentration (mstc). It is defined as that concentration of surfactant above which structural changes of the micelles take place. Usually the 2nd CMC represents the transition of spherical micelles to rodlike aggregates. These structural changes have been explained by Lee and Woo.6 They have proposed that an increase in degree of counterion association results in a decrease in reduction of surface charge density of the micelles, thereby promoting the structural transitions from spherical to ellipsoidal micelles. The existence of the 2nd CMC is an outcome of increasing hydrophobic stabilities of the micelles due to the destruction of water structure surrounding the hydrocarbon chains. Linear fitting to the three linear segments of the isotherm allows us to evaluate the values of CMC and 2nd CMC (Table S1 of Supporting Information). It is evident from the table that the CMC and 2nd CMC of aqueous DTAB decrease on the addition of salts. The alteration of electrical atmosphere of surfactant molecules on the addition of salts with counterions of the same charge increases the effective charge on the head groups of the surfactant thereby increasing the electrostatic repulsions between polar head groups and inhibits the micelle formation. However, in case of quaternary ammonium salts, the penetration of some of the alkyl chains of the quaternary salts into the micellar core of the surfactant due to the hydrophobic interactions facilitates the micellization by acting as spacers between the surfactant’s head groups.31 The marked decrease in CMC and 2nd CMC as the size of alkyl group in the QAS increases also supports the above view. Figure 3 represents the variation of CMC and 2nd CMC of DTAB in pure water as a function of temperature. The CMC of DTAB in water at different temperatures from present work has been compared with those reported in literature. The temperature dependence of CMC shows that the CMC first decreases and then increases with the minimum at around 298.15 K. This is the typical behavior of ionic surfactants

Figure 2. Plot of speed of sound, u as a function of [DTAB] in pure water (red ■) and aqueous solution containing 0.01 mol·kg−1 (a) (C3H7)4NBr (green ●), (b) (C4H9)4NBr (blue ▲) and (c) (C5H11)4NBr (orange ▼) at 298.15 K.

Figure 3. Temperature dependence of CMC of DTAB in pure water from present work (■) and literature [ref 32] (red ●) and 2nd CMC of DTAB in pure water from present work (blue ▲).

characteristic frequency. This characteristic frequency depends upon the density of the sample. The working frequency for the measurement of speed of sound is ∼3 MHz. 34 The uncertainties in the density measurements were found to lie well within 0.15 kg·m−3 while the uncertainties in speed of sound data were found to be better than 0.5 m·s−1. The precision in temperature of the DSA-5000 was ±0.001 K. The density (ρ) and speed of sound (u) of DTAB in the concentration range (3.0 to 45.0) mmol·kg−1 in the absence and presence of 0.01 mol·kg −1 aqueous solutions of tetraalkylammonium bromide, R4NBr (R = propyl, butyl and pentyl) have been measured in the temperature range (293.15− 318.15) K at an interval of 5 K.

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observed in the literature also.40−42 This temperature dependence of CMC can be described by two opposing processes (i) the dehydration of headgroup resulting in an increase in the hydrophobic nature of molecules which favors micellization and (ii) an increase in thermal solubility of surfactant molecules delaying surfactant aggregation. As the temperature increases, the hydrophilicity of surfactant decreases due to the dehydration of the monomers, which favors micellization. However, the rise in temperature also causes the disruption of water structure around the hydrophobic groups, increasing the solubilization of surfactant monomers and hence inhibits the micelle formation.43 In the case of DTAB, the gradual decrease in the CMC at lower temperatures and then gradual increase at higher temperatures may be due to the dominance of first and second factors, respectively. Also as the temperature rises, the thermal motions of surfactant and solvent molecules are enhanced so that the formation of ordered micelle structures becomes difficult. The increase of temperature causes an increase in the kinetic energies and destroys the ordered micellar structures leading to an increase in the CMC value. Therefore, the higher is the temperature, the greater is the disaggregation degree of micelles, and consequently, the higher is the CMC. However, for the 2nd CMC, the plot shows a gradual increase and then decrease with rise in temperature, displaying a maximum in the 298.15−303.15 K range. At low temperatures, the thermal solubility of molecules dominates over the dehydration effect, which gives rise to an increase in the 2nd CMC. At higher temperatures, that is, above 303.15 K, the decrease in the 2nd CMC values may probably be due to the predominance of hydrophilic dehydration which outweighs the thermal solubility of molecules, causing the convenient formation of micelles in the bulk.15 The observed behavior of CMC and 2nd CMC can be attributed to the denser packing of nonspherical micelles in the postmicellar region, as a consequence of the smaller headgroup volume. This leads to less hydration of nonspherical aggregates than the spherical ones, so the water molecules from the hydration shell escapes easily with nonspherical micelles at high temperatures than with spherical micelles at lower temperatures. It means that at low temperatures, the sphere to rod transition occurs at low concentrations. The thermal agitation reduces the aggregation of the micelles and hence a critical aggregation number is required for the structural transitions.16 3.2. Volumetric and Compressibility Parameters. The structural transitions in the micelles lead to changes in various physicochemical properties such as NMR parameters, 6 volumetric properties14−16 of the aqueous solutions of the surfactants. Thus, the volumetric and compressibility parameters of the surfactant in the monomeric form and micellar form can be used to extract information regarding two types of micelles: spherical and rodlike. The apparent molar volume (Vφ), isentropic compressibility (κS), and apparent molar adiabatic compression (κS,φ) values of aqueous surfactant, DTAB below CMC have been calculated using the following relations 1, 2, and 3, respectively, and are recorded in Table S2 and Table S3 of the Supporting Information:

Vφ =

[ρ − ρ] M + o ρ mρρo

κs = 1/u2 ρ

κS , φ = Vφκs +

[κs − κo] mρo

(3)

where m is the molality of the solution which has been calculated as amount of substance (DTAB) per unit mass of solvent, in which the solvent was either pure water or 0.01 mol· kg−1 aqueous solutions of tetraalkylammonium bromide, M denotes the relative molar mass of surfactant, and ρ and ρo are the densities of the solution and pure solvent, respectively. The compressibility of micellar solutions is influenced by two major factors: (1) The compressibility of the hydrocarbon core and (2) the interactions between the head groups of the surfactant. It also shows some dependence on the variation of counterion binding as well as hydrophilicity of the headgroup of the surfactant. The κS values decrease with increase in temperature as well as concentration of the surfactant for all studied cases. The representative plot showing the variation of isentropic compressibility with concentration at 293.15 K is constructed in Figure 4. The κS values show similar trends in all cases at all temperatures studied.

Figure 4. Variation of isentropic compressibility, κS as a function of [DTAB] in pure water (■) and an aqueous solution containing 0.01 mol·kg−1 (a) (C3H7)4NBr (red ●), (b) (C4H9)4NBr (blue ▲) and (c) (C5H11)4NBr (green ▼) at 293.15 K.

The isentropic compressibility is a measure of internal pressure due to solute−solvent interactions which forms a high compact environment. In pure solvent, there is only one kind of interacting molecules, but the addition of surfactant leads to the development of ion−solvent interactions thereby increasing the compactness of the system.44,45 It is evident from Figure 4 that the value of isentropic compressibility decreases with increase in the alkyl chain length of tetraalkylammonium bromide salt. As we move from (C3H7)4N+ cation to (C5H11)4N+ cation, the interactions between the hydrophobic alkyl chains of tetraalkylammonium bromide and nonpolar tails of surfactant increase which further results into the development of increased electrostrictive compression of the system thereby decreasing the values of κS.46 The change in isentropic compressibility (ΔκS) and relative change in isentropic compressibility (ΔκS/κo) have also been calculated using relation 4, and the values are given in Table S4 of Supporting Information:

(1)

Δκs = κs − κo

(2) F

(4) DOI: 10.1021/acs.jced.6b00332 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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Both the parameters have been found to be negative at all temperatures studied. The negative values show the presence of strong attractive interactions between surfactant and dipolar water due to the solvation of ions in the present system.36,45 The negative contributions of ΔκS also indicate the contraction in the interacting species due to ionic strength and the tendency to make an ion hydration sphere. This implies that the solvent molecules around the solute are less compressible than those in bulk solutions. The volumetric properties such as molar volumes and compressibilities quantify solute−solute/solute−solvent/solvent−solvent interactions existing in a solution. The changes in these properties after micellization are mainly due to the change in water structure involved in hydrophobic dehydration and the changes in the electrostatic forces between polar head groups and counterions. To estimate the volumetric parameters in the postmicellar region, the pseudophase transition model for 1:1 electrolytes16 has been taken into account, and the trends of apparent molar volume (Vφ), and apparent molar adiabatic compression (κS,φ) above the CMC can be calculated as Xφ = Xφ , m − (CMC ΔXφ , m)/m

Figure 5. Variation of apparent molar volume, Vφ as a function of [DTAB] in pure water (■) and aqueous solution containing 0.01 mol· kg−1 (a) (C3H7)4NBr (red ●), (b) (C4H9)4NBr (blue ▲) and (c) (C5H11)4NBr (green ▼) at 293.15 K.

((C5H11)44NBr) results in an increase in hydrophobic interactions as well as a decrease in Vφ values. The graphs showing the variation of κS,φ values with surfactant concentration are given in Figure 6. It has been

(5)

where Xφ stands for the volumetric parameter apparent molar volume (Vφ) or apparent molar adiabatic compression (κS,φ). The Xφ,m and ΔXφ,m represent the properties in the micellar phase and the changes in the properties upon micellization, respectively. These parameters can be evaluated by applying linear least-squares fit methods to the plots of Xφ,m vs CMC/m. The corresponding volumetric property at the CMC has been calculated using the above equation and recorded in Table S1 of the Supporting Information. The trends of Vφ or Vφ,cmc values can be due to hydration behavior of a surfactant that comprises the following interactions in the present ternary system: 1. hydrophobic−hydrophobic interactions between the nonpolar tail of surfactant and hydrophobic alkyl chains of tetraalkylammonium bromide salt 2. ion−ion interactions between Br− ion of R4NBr salt and ionic headgroup of surfactant, DTAB 3. ion−ion interactions between R4N+ group of R4NBr salt and Br− ion of surfactant, DTAB According to cosphere overlap model,27 interactions of type 1 lead to an increase in electrostriction due to the insertion of an additional alkyl group which enhances the affinity of hydrophobic−hydrophobic and hydrophilic−hydrophobic groups to interact. This results in the destruction of the water structure, ending up with decreased value of apparent molar volume. On the other hand, interactions of type 2 and 3 weaken the electrostatic interactions and hence the overall water structure is enhanced. This leads to an increase in apparent molar volume. In the present study, the values of Vφ are found to be positive indicating the dominance of ion−ion interactions in all the cases studied. Vφ values slightly depend upon concentration of surfactant up to a certain concentration and then become almost linear as surfactant concentration is increased (Figure 5). The Vφ values for DTAB show a significant dependence on the nature of additives. The introduction of an additional (−CH2−) group at each step when we go from tetrapropylammonium bromide ((C3H7)4NBr) to tetrabutylammonium bromide ((C4H9)4NBr) and tetrapentylammonium bromide

Figure 6. Representative plot for variation of apparent molar adiabatic compression, κS,φ as a function of [DTAB] pure water (■) and aqueous solution containing 0.01 mol·kg−1 (a) (C3H7)4NBr (red●), (b) (C4H9)4NBr (blue ▲) and (c) (C5H11)4NBr (green ▼) at 293.15 K.

observed that the values of κS,φ are negative at lower surfactant concentration and become positive as the concentration of surfactant is increased. At the low surfactant concentration, the hydrophilic hydration of DTAB-aqueous tetraalkylammonium bromide complex may take place which results in strong attractive interactions, rendering κS,φ < 0. However, the negative values tend to increase with [DTAB]. This is likely due to the self-association of surfactant which results in the release of some water molecules from the counterion upon the binding to the micelles making the system more compressible.47 Also the rise in κS,φ values with increment in temperature as well as alkyl chain length of bromide salts may be due to the loss of hydrophobic hydration, showing the importance of solute− solvent interactions in the present system. G

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Notes

The various thermodynamic parameters upon micellization are given in Table S1 of the Supporting Information. The value of Vφ,cmc for DTAB in pure water at 298.15 K is 2.88 × 104 m3· mol−1, which is same as the value reported in literature.48 However, the values for apparent molar volume in micellar as well as postmicellar regions is lesser for DTAB than the value for dodecyldimethylethylammonium bromide (DEDAB) by an amount of about 0.15 × 104 m3·mol−1.49 This difference is probably due to the addition of a methylene (−CH2−) group in an alkyl chain of DTAB leading to a decrease in apparent molar volume. It is noteworthy that apparent molar adiabatic compression and the changes in volumes and compressibility are the same for DTAB and DEDAB, signifying that the insertion of the methylene group to the polar headgroup does not affect the hydration shell of molecules either in monomeric or micellar form and hence the compressibility does not change.

The authors declare no competing financial interest.

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4. CONCLUSION The main outcome of the study is that both CMC and second CMC for aqueous solution of DTAB are dependent on temperature as well as alkyl chain length of tetraalkylammonium bromide salt. A remarkable decrease in the CMC and second CMC of surfactant in the presence of QAS may be due to the penetration of some of alkyl chains of bromide salt into the micellar core. The trends of various volumetric and compressibility parameters reveal that the micellization of surfactant has been enhanced in the presence of these QAS. This is probably due to the reinforced hydrophobic interactions between the alkyl chains of bromide salt and nonpolar hydrocarbon tails of surfactant resulting into the compression of the system. The positive values of apparent molar volume Vφ may be probably an outcome of the dehydration of counterions attached to the micelles and the release of structured water molecules in the hydration shell of monomers that occurs on the micellization. The results show the importance of solute− solvent interactions in the ternary (quaternary ammonium salt + ionic surfactant + water) system.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jced.6b00332. Changes in thermodynamic properties upon micellization, isentropic compressibility, apparent molar volume, apparent molar isentropic compressibility, change in isentropic compressibility, and relative change in isentropic compressibility for DTAB in the absence and presence of aqueous tetraalkylammonium bromide solutions at different temperatures (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +91 177 2830803. Fax: +91 177 2830775. Funding

S. Chauhan and Maninder Kaur thank UGC, New Delhi, for financial assistance under the project (F. No. 42-249/2013/SR) and award of Junior Research Fellowship (No. F.17-40/ 2008(SA-1) dated 31.07.2014), respectively. Financial support from UGC-SAP (DRS-I) (No. F.540/3/DRS/2010 (SAP-1)) to Department of Chemistry, HPU is also acknowledged. H

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