Micellization Parameters of Six Gemini Quaternary Ammonium

Article pubs.acs.org/jced

Micellization Parameters of Six Gemini Quaternary Ammonium Surfactants from Measurements of Conductivity and Surface Tension Shanshan Zhang, Jing Yu, Jianzhou Wu, Wei Tong, Qunfang Lei,* and Wenjun Fang* Department of Chemistry, Zhejiang University, Hangzhou 310027, China S Supporting Information *

ABSTRACT: The micellization of six Gemini quaternary ammonium surfactants aqueous solutions has been investigated from measurements on specific conductivity as a function of surfactant concentration at different temperatures from (298.15 to 323.15) K. The micellization parameters such as the critical micellar concentration (CMC) and the degree of counterion dissociation (β), Gibbs free energy (ΔGmic), enthalpy (ΔHmic), and entropy (ΔSmic) of micellization are then obtained. It is shown that the conductometry measurements provide agreement of the CMC values at 298.15 K with the surface tension studies. With the rise of temperature, the values of CMC and β increase, while ΔGmic changes little. The linear plots of TΔSmic versus ΔHmic show the effects of enthalpy−entropy compensation. The length of alkyl chain and the spacer group of the Gemini surfactant have significant influences on micellization parameters.

1. INTRODUCTION Gemini surfactants, which possess two hydrocarbon tails and two ionic groups covalently linked by a spacer, have received considerable attention during the past decade.1 As a novel class of amphiphilic molecules, their surface activities are usually superior to conventional single-chain surfactants.2−4 For example, they generally possess much lower values of critical micelle concentration (CMC) and Kraff point. They are more efficient in lowering the surface tension of water, and have better wetting properties.5,6 Some Gemini surfactants have also shown antimicrobial or antibacterial activities.7−10 Because of these mentioned advantages, intensive investigations on Gemini surfactants should be worth doing for both fundamental study and application prospect. It is widely recognized that researches on micellization processes are valuable to understand the theoretical evaluations and the practical applications of Gemini surfactants.11 Critical micelle concentration (CMC) and the degree of counterion dissociation (β) are considered the most important parameters in studies dealing with micellization of surfactants. Thermodynamic information on the micellization process can be obtained by analyzing the CMC and β values of the surfactant in a temperature range using a determined model of micelle formation.12 Thermodynamics of micellization process and the corresponding parameters for different types of Gemini surfactants in aqueous solutions have been investigated with various techniques such as surface tension measurements,13,14 isothermal titration calorimetry (ITC),15 conductometry,16,17 dynamic light scattering (DLS),18 and steady-state fluorescence.19 The surface tension measurements are typically used to determine the CMC value and detect the purity of surfactants by checking whether there is a minimum plot in surface tension versus concentration.20 The ITC method can present the CMC value and the enthalpy (ΔHmic) of micellization directly, and then the change of Gibbs © 2014 American Chemical Society

free energy (ΔGmic) can be obtained by using an appropriate model.21,22 Hydrodynamic diameters of the aggregates and the aggregation number (Nagg) of the surfactant micelles can be obtained from dynamic light scattering and steady-state fluorescence measurements, respectively.6,23 Carpena et al.1,12 have proposed an efficient method to analyze the conductivity-concentration curves of ionic surfactant solutions to determine the micellization parameters such as CMC and β values. Zana et al.24 have proposed the relationship of CMC, β, and the Gibbs free energy (ΔGmic) of micellization for Gemini surfactants with monovalent counterions. The other parameters like enthalpy (ΔH mic) and entropy (ΔSmic ) of micellization are then obtained by using related formulas. The conductometry is accurate and convenient enough to obtain the micellization parameters for aqueous solutions of cationic or anionic surfactants. Herein, the conductivity measurements are performed to investigate the micellization properties of six Gemini quaternary ammonium surfactants. The micellization parameters in aqueous surfactant solutions are evaluated and discussed from the influences of temperature, the length of alkane chain, and the spacer groups of these Gemini surfactants.

2. EXPERIMENTAL SECTION 2.1. Materials. Six Gemini quaternary ammonium surfactants, 12−n−12 (n = 3, 4, and 6) and 14−n−14 (n = 3, 4, and 6), with their chemical structures, names, and abbreviations summarized in Table 1, were synthesized respectively with the same procedure as described in the previous work.25 They were prepared from the reactions of alkyl dibromide, 1,3-dibromopropane Received: June 7, 2014 Accepted: August 4, 2014 Published: August 12, 2014 2891

dx.doi.org/10.1021/je500513b | J. Chem. Eng. Data 2014, 59, 2891−2900

Journal of Chemical & Engineering Data

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Table 1. Molecular Structure, Relative Molecular Mass (M), and Chemical Name and Abbreviation (N & A) of Gemini Surfactants

a

n is the carbon number of the spacer group in each of the Gemini surfactants.

Table 2. Specification of Chemical Samples

a

chemical name

source

purification method

final mass fraction purity

analysis method

1,3-dibromopropane 1,4-dibromobutane 1,6-dibromohexane N,N-dimethyl dodecyl amine N,N-dimethyl tetradecyl amine isopropyl alcohol acetone methanol 12−3−12 12−4−12 12−6−12 14−3−14 14−4−14 14−6−14

J&K Scientific Ltd. J&K Scientific Ltd. J&K Scientific Ltd. Jiangsu Feixiang Chemical company Jiangsu Feixiang Chemical company J&K Scientific Ltd. J&K Scientific Ltd. J&K Scientific Ltd. synthesis synthesis synthesis synthesis synthesis synthesis

0.99 0.99 0.98 0.97 0.97 0.995 0.998 0.998 recrystallization recrystallization recrystallization recrystallization recrystallization recrystallization

none none none none none none none none 0.99 0.99 0.99 0.99 0.99 0.99

NMRa, IRb,EAc NMR, IR, EA NMR, IR, EA NMR, IR, EA NMR, IR, EA NMR, IR, EA

Nuclear magnetic resonance. bInfrared spectroscopy. cElemental analysis.

required solutions were obtained by successive dilutions. Ultrapure water produced by the Millipore Q3 system with a resistivity above 1.82 × 105 Ω·m at 298.15 K was used. 2.2. Methods. Conductivity measurements for the aqueous surfactant solutions were carried out at the temperature range from (298.15 to 323.15) K by a digital conductivity meter (Seven Compact S230, Mettler Toledo, Swizerland) with an electrode (inLab 710, Mettler Toledo, Swizerland). The conductivity cell with a sample was kept in a thermostat bath (PolyScience, USA) with temperature fluctuation within ± 0.01 K. The picture of the conductivity meter is shown in Supporting Information, Figure S1. It gives the value with three significant figures, and the uncertainty of which is 0.5 %. The instrument was calibrated by several KCl solutions with known concentrations. Each reported value of the conductivity is the average of three measurements. A digital tensiometer (DropMeter Standed A-100, MAIST Vision Inspection & Measurement Co., Ltd.), calibrated by pure water, was used to determine the surface tensions by means of the

(CAS No. 109-64-8), 1,4-dibromobutane (CAS No. 110-52-1), and 1,6-dibromohexane (CAS No. 629-03-8) with N,N-dimethyl dodecyl amine (CAS No. 112-18-5) and N,N-dimethyl tetradecyl amine (CAS No. 112-75-4) in iso-propanol (CAS No. 67-63-0) under reflux for 12 h, respectively. The products were purified by recrystallization in the mixed solvents of acetone (CAS No. 67-64-1) and methanol (CAS No. 67-56-1). The characterizations of these synthesized surfactants were carried out with NMR (Bruker Advance 2B/400 Hz), IR spectrum (NEXES 470), and elemental analysis (CarlaboEA1110). The surfactants were also checked by the surface tension measurements. No minima were observed in the plots of surface tension versus concentration for all of the aqueous surfactant solutions, indicating that there were no surface active impurities. Basic information on the samples used in this work is found in Table 2. All of the aqueous surfactant solutions used for measurements of electrical conductivity and surface tension were initially prepared by mass using an electronic balance (AB265-S, Mettler Toledo) with an uncertainty of 1 × 10−4 g. The other 2892



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surfactant concentration, the CMC values at 298.15 K corresponding to the break points have been obtained and listed in Table 4. The CMC values from conductivity measurements are obtained by fitting the specific conductivity (k) as a function of surfactant concentration (m) to the integral of Boltzmann sigmoidal equation1,12

pendant-drop method. The picture of the device is given in Supporting Information, Figure S2. Schematic diagrams of the facilities and pendant-drop method for surface tension measurement are shown in Figure 1. The theory of the pendant-drop

⎛ 1 + e(m − m0)/ Δm ⎞ ⎟⎟ k = k 0 + a1m + Δm(a 2 − a1) ln⎜⎜ ⎝ 1 + e−m0 / Δm ⎠

(1)

where k0 is the conductivity of the solution at zero concentration of the surfactant, that is, the conductivity of water, S·m−1; a1 is the premicellar slope; a2 is the postmicellar slope; and Δm is the width of the transition, mol·kg−1. The central point (m0) on the width of the transition corresponds to the CMC. The degree of counterion dissociation (β) is determined from the ratio of postmicellar slope to premicellar slope as β = a2/a1. With origin 8.0 software, data fitting was performed by employing initial guess values of a1, a2, Δm, and m0 in eq 1 to calculate an approximate value of conductivity, κiapprox, corresponding to each concentration of the surfactant. Chi-square, χ2, the sum of the squares of the deviations of κiapprox from the experimental values, defined as

Figure 1. Schematic diagrams of the facilities and pendant-drop method for measurement of surface tension.

n

x2 =

∑ (κi − κiapprox)2 i=1

method has been reported in the literature.26 The temperature of the samples was set to 298.15 K and kept constant within ± 0.01 K controlled by a digital temperature controller (PolyScience, USA). It was considered to reach equilibrium until the change of surface tension was less than 5 × 10−5 N·m−1 every 120 s. Each reported value of surface tension is the average of three measurements, and the standard uncertainty in the measurements is 0.1 %. The accuracy of the tension meter was confirmed to be acceptable by determining the pure water at 298.15 K with the value of 71.98 × 10−3 N·m−1, in agreement with the literature value of 72.01 × 10−3 N·m−1 and 71.98 × 10−3 N·m−1, respectively.27,28

(2)

where n is the number of data points, κi is the experimental conductivity, and κiapprox is the approximate conductivity. χ2 is minimized with respect to these parameters. The values corresponding to the minimum χ2 were then used as the new set of guess values in an iterative procedure until χ2 stopped decreasing. Finally the ultimate values of a1, a2, Δm, and m0 were obtained and considered as the best fitted parameters. The correlated parameters, a1, a2, Δm, and m0, for these Gemini surfactants are listed in Table 3. According to the mass action model for micelle formation, the micellization can be seen as an association−dissociation process, during which micelles and surfactant molecules (or ions) coexist. They separate and aggregate constantly and achieve dynamic equilibrium ultimately.29 The micellization of Gemini surfactant can be written as follow

3. RESULTS AND DISCUSSION 3.1. Experimental Data and Calculation Models. The experimental data of surface tension at 298.15 K and conductivity at different temperatures from (298.15 to 323.15) K for aqueous solutions of six Gemini quaternary ammonium surfactants, 12−n−12 and 14−n−14 (n = 3, 4, and 6), are shown in Figures 2 and 3, respectively. From the plots of surface tension versus

nG2 + + 2αn Br − ⇌ M2n(1 − α) + 2+

−

(3)

2n(1‑α)+

where G , Br , and M represent the gemini cation, the counterion, and the aggregate monomer, respectively; n is the micelle aggregation number; α is the degree of counterion binding,

Figure 2. Plots of surface tension (γ) versus surfactant concentration (m) for aqueous solutions of 12−n−12 and 14−n−14 (n = 3, 4, and 6) at temperature T = 298.15 K and pressure p = 0.1 MPa. (a) □, 12−3−12; ○, 12−4−12; △, 12−6−12; (b) □, 14−3−14; ○, 14−4−14; △, 14−6−14. 2893



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Figure 3. Plots of conductivity (k) versus surfactant concentration (m) for aqueous solutions of (a) 12−3−12, (b) 12−4−12, (c) 12−6−12, (d) 14−3− 14, (e) 14−4−14, and (f) 14−6−14 at pressure p = 0.1 MPa and different temperatures (T): □, 298.15 K; ○, 303.15 K; △, 308.15 K; ▽, 313.15 K; ☆, 318.15 K; +, 323.15 K.

energy of micellization, ΔGmic, for each Gemini surfactant at a certain temperature can be written as24,30,31

and α + β = 1. The Gibbs free energy of micelle formation per mole of the Gemini surfactant, ΔGmic, is given by ⎡ 1 ⎤ ΔGmic = RT ⎢ − ln a M2n(1−α)+ + ln aG2+ + 2αn ln aBr−⎥ ⎣ n ⎦

ΔGmic = (3 − 2β)RT ln xCMC

According to the Gibbs−Helmholtz equation,

(4)

where R is the gas constant; T is the temperature, K; a is the activity of the ion. For a micelle formed with a large number of monomer units and the value of n is very large, the first term in the parentheses would be small and can be neglected. Besides, the activities of the ions have the following relationship: aG2+ ≈

1 a Br− ≈ CMC 2

(6)

⎡ ΔGmic ⎢∂ T ⎢ ∂T ⎢⎣

(

) ⎤⎥

ΔHmic ⎥ =− T2 ⎥⎦ p

(7)

The enthalpy change of micellization, ΔHmic, is calculated by the following equation

(5)

⎛ ∂ ln xCMC ⎞ ⎟ ΔHmic = −(3 − 2β)RT 2⎜ ⎝ ∂T ⎠ p

Activities of the ions can be replaced by xCMC, which is the CMC expressed in mole mass fraction of the surfactant in the aqueous solution with very low concentration. For β = 1 − a, the Gibbs free 2894

(8)



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Table 3. Evaluation Values of Parameters, a1, a2, Δm, and m0 in eq 1 Fitted to Conductivity Data for Gemini Surfactant Solutions of 12−n−12 and 14−n−14 (n = 3, 4, and 6) at Different Temperatures (T) and Pressure p = 0.1 MPaa T

103·Δm

a2 −1

−1

−1

surfactant

K

S·m ·mol ·kg

S·m ·mol ·kg

12−3−12

298.15 303.15 308.15 313.15 318.15 323.15 298.15 303.15 308.15 313.15 318.15 323.15 298.15 303.15 308.15 313.15 318.15 323.15 298.15 303.15 308.15 313.15 318.15 323.15 298.15 303.15 308.15 313.15 318.15 323.15 298.15 303.15 308.15 313.15 318.15 323.15

20.93 (±0.10) 22.87 (±0.08) 25.34 (±0.08) 27.81 (±0.15) 28.09 (±0.27) 32.58 (±0.06) 20.62 (±0.05) 23.29 (±0.09) 25.45 (±0.09) 27.28 (±0.05) 29.77 (±0.06) 32.60 (±0.12) 20.80 (±0.12) 21.91 (±0.17) 24.19 (±0.41) 27.18 (±0.40) 28.67 (±0.16) 31.99 (±0.35) 22.27 (±0.25) 25.82 (±0.36) 25.45 (±0.54) 26.17 (±0.10) 27.48 (±0.20) 31.09 (±0.32) 24.03 (±0.31) 24.91 (±0.42) 28.89 (±2.44) 29.77 (±0.29) 29.80 (±1.75) 30.94 (±0.32) 21.36 (±0.14) 23.20 (±0.13) 25.03 (±1.48) 28.69 (±0.23) 29.29 (±0.50) 31.81 (±0.39)

4.83 (±0.02) 5.51 (±0.01) 6.64 (±0.02) 7.56 (±0.04) 8.03 (±0.08) 9.55 (±0.02) 5.05 (±0.01) 5.89 (±0.02) 6.82 (±0.02) 7.58 (±0.01) 8.57 (±0.02) 9.57 (±0.04) 7.49 (±0.04) 8.22 (±0.06) 9.14 (±0.15) 10.49 (±0.15) 11.53 (±0.06) 13.18 (±0.15) 5.88 (±0.07) 7.18 (±0.10) 7.30 (±0.15) 7.67 (±0.32) 8.57 (±0.06) 10.29 (±0.10) 7.28 (±0.09) 7.82 (±0.13) 9.30 (±0.79) 10.21 (±0.10) 11.00 (±0.65) 11.94 (±0.12) 7.82 (±0.05) 8.84 (±0.05) 9.66 (±0.57) 11.28 (±0.09) 11.92 (±0.20) 13.46 (±0.16)

12−4−12

12−6−12

14−3−14

14−4−14

14−6−14

a

a1 −1

mol·kg

103·m0

−1

0.090 (±0.003) 0.073 (±0.003) 0.102 (±0.004) 0.101 (±0.006) 0.142 (±0.007) 0.104 (±0.005) 0.152 (±0.003) 0.162 (±0.006) 0.157 (±0.006) 0.156 (±0.004) 0.163 (±0.005) 0.186 (±0.010) 0.087 (±0.010) 0.126 (±0.003) 0.152 (±0.007) 0.143 (±0.006) 0.158 (±0.011) 0.110 (±0.006) 0.015 (±0.001) 0.019 (±0.001) 0.023 (±0.001) 0.026 (±0.003) 0.014 (±0.002) 0.013 (±0.001) 0.016 (±0.001) 0.019 (±0.001) 0.028 (±0.003) 0.019 (±0.001) 0.026 (±0.003) 0.015 (±0.001) 0.051 (±0.002) 0.038 (±0.002) 0.057 (±0.008) 0.042 (±0.004) 0.031 (±0.006) 0.032 (±0.002)

mol·kg−1

R2

0.829 (±0.001) 0.870 (±0.002) 0.897 (±0.004) 0.986 (±0.007) 1.066 (±0.006) 1.127 (±0.004) 1.039 (±0.003) 1.045 (±0.006) 1.094 (±0.006) 1.181 (±0.003) 1.240 (±0.004) 1.309 (±0.007) 0.930 (±0.010) 0.966 (±0.008) 1.007 (±0.017) 1.058 (±0.016) 1.112 (±0.010) 1.184 (±0.012) 0.132 (±0.001) 0.140 (±0.001) 0.159 (±0.003) 0.182 (±0.006) 0.202 (±0.002) 0.211 (±0.001) 0.149 (±0.001) 0.158 (±0.002) 0.171 (±0.012) 0.189 (±0.001) 0.196 (±0.010) 0.232 (±0.002) 0.160 (±0.008) 0.180 (±0.001) 0.194 (±0.017) 0.200 (±0.002) 0.220 (±0.005) 0.250 (±0.002)

0.9993 0.9994 0.9997 0.9987 0.9979 0.9998 0.9999 0.9997 0.9997 0.9999 0.9999 0.9997 0.9989 0.9999 0.9998 0.9998 0.9993 0.9998 0.9999 0.9999 0.9998 0.9993 0.9998 0.9997 0.9997 0.9997 0.9999 0.9999 0.9997 0.9997 0.9997 0.9995 0.9995 0.9988 0.9998 0.9999

Standard uncertainties u are u(T) = 0.01 K and u(p) = 20 Pa. Standard deviations of the parameters are given in parentheses.

Table 4. Coefficients A, B, and C in eq 9 for Aqueous Surfactant Solutions of 12−n−12 and 14−n−14 (n = 3, 4, and 6) from the Temperatures Range (298.15 to 323.15) K at Pressure p = 0.1 MPaa

a

surfactant

104·A

B

C

R2

12−3−12 12−4−12 12−6−12 14−3−14 14−4−14 14−6−14

1.656 (±0.012) 1.849 (±0.011) 1.117 (±0.113) 2.885 (±0.001) 2.744 (±0.019) 0.979 (±0.021)

−0.090 (±0.001) −0.105 (±0.001) −0.059 (±0.007) −0.155 (±0.001) −0.153 (±0.001) −0.044 (±0.001)

0.913 (±0.112) 3.835 (±0.107) −3.210 (±0.001) 7.665 (±0.177) 8.465 (±0.181) −8.293 (±0.205)

0.9903 0.9799 0.9950 0.9724 0.9800 0.9759

Standard uncertainties u are u(T) = 0.01 K and u(p) = 20 Pa. Standard deviations of the parameters are given in parentheses.

Accordingly, the entropy change, ΔSmic, can be acquired by the expression

It has been reported that the variation of ln xCMC with temperature can be empirically estimated by a second degree polynomial:32 ln xCMC(T ) = AT 2 + BT + C

ΔSmic =

(9)

ΔHmic − ΔGmic T

(10)

The obtained values of CMC, β, ΔGmic, ΔHmic, and ΔSmic of the Gemini surfactants at different temperatures are summarized in Table 5.

The coefficients of A, B, and C, which can be acquired by fitting ln xCMC as a function of the corresponding temperature (T), are listed in Table 4. Then the value of ΔHmic is obtained. 2895



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Table 5. Micellization and Thermodynamic Parameters (CMC, β, ΔGmic, ΔHmic, and ΔSmic) for Aqueous Surfactant Solutions of 12−n−12 and 14−n−14 (n = 3, 4, and 6) at Different Temperatures (T) and Pressure p = 0.1 MPaa 103·CMC/mol·kg−1

T surfactant 12−3−12

12−4−12

12−6−12

14−3−14

14−4−14

14−6−14

K 298.15 303.15 308.15 313.15 318.15 323.15 298.15 303.15 308.15 313.15 318.15 323.15 298.15 303.15 308.15 313.15 318.15 323.15 298.15 303.15 308.15 313.15 318.15 323.15 298.15 303.15 308.15 313.15 318.15 323.15 298.15 303.15 308.15 313.15 318.15 323.15

A

b

0.829 0.870 0.897 0.986 1.066 1.127 1.039 1.045 1.094 1.181 1.240 1.309 0.929 0.966 1.007 1.058 1.112 1.184 0.132 0.140 0.159 0.182 0.202 0.211 0.149 0.158 0.171 0.189 0.196 0.232 0.160 0.180 0.194 0.200 0.220 0.250

c

B

ΔGmic d

10 ·xCMC

β

e

14.97 (±0.02) 15.73 (±0.02) 16.24 (±0.02) 17.89 (±0.02) 19.39 (±0.02) 20.53 (±0.02) 18.76 (±0.02) 18.89 (±0.02) 19.81 (±0.02) 21.42 (±0.02) 22.55 (±0.02) 23.85 (±0.02) 16.77 (±0.02) 17.46 (±0.02) 18.23 (±0.02) 19.19 (±0.02) 20.22 (±0.02) 21.57 (±0.02) 2.38 (±0.02) 2.53 (±0.02) 2.90 (±0.02) 3.30 (±0.02) 3.67 (±0.02) 3.84 (±0.02) 2.70 (±0.02) 2.86 (±0.02) 3.10 (±0.02) 3.43 (±0.02) 3.56 (±0.02) 4.23 (±0.02) 2.89 (±0.02) 3.25 (±0.02) 3.51 (±0.02) 3.63 (±0.02) 4.38 (±0.02) 4.55 (±0.02)

0.231 (±0.001) 0.241 (±0.001) 0.262 (±0.001) 0.272 (±0.002) 0.286 (±0.004) 0.293 (±0.001) 0.245 (±0.001) 0.253 (±0.001) 0.268 (±0.001) 0.278 (±0.001) 0.288 (±0.001) 0.298 (±0.002) 0.360 (±0.003) 0.375 (±0.004) 0.378 (±0.009) 0.386 (±0.008) 0.402 (±0.003) 0.412 (±0.007) 0.264 (±0.004) 0.278 (±0.005) 0.287 (±0.008) 0.293 (±0.012) 0.312 (±0.003) 0.331 (±0.005) 0.303 (±0.005) 0.314 (±0.007) 0.322 (±0.039) 0.343 (±0.005) 0.369 (±0.031) 0.386 (±0.006) 0.366 (±0.003) 0.381 (±0.003) 0.386 (±0.032) 0.393 (±0.004) 0.407 (±0.010) 0.423 (±0.007)

C

0.77 (±0.03)

0.87

1.16 (±0.04)

1.09e

1.02 (±0.02)

1.01e

0.10 (±0.05)

0.137f

0.12 (±0.03)

0.146g 0.151g 0.173g

0.14 (±0.02)

0.158h 0.183h 0.217h 0.255h

6

−1

kJ·mol

−69.89 (±0.06) −70.19 (±0.06) −69.95 (±0.06) −69.90 (±0.06) −69.69 (±0.06) −70.00 (±0.06) −67.72 (±0.06) −68.37 (±0.06) −68.36 (±0.06) −68.41 (±0.06) −68.60 (±0.06) −68.75 (±0.06) −62.15 (±0.06) −62.13 (±0.06) −62.73 (±0.06) −63.00 (±0.06) −62.78 (±0.06) −62.81 (±0.06) −79.34 (±0.06) −79.38 (±0.07) −79.29 (±0.07) −79.32 (±0.07) −78.65 (±0.07) −78.32 (±0.07) −76.11 (±0.06) −76.32 (±0.06) −76.57 (±0.07) −75.81 (±0.07) −75.06 (±0.07) −74.07 (±0.07) −71.71 (±0.06) −71.27 (±0.06) −71.69 (±0.06) −72.21 (±0.07) −71.34 (±0.07) −71.18 (±0.07)

ΔHmic −1

kJ·mol

−16.94 (±0.02) −20.56 (±0.02) −24.12 (±0.02) −28.03 (±0.02) −31.98 (±0.03) −36.28 (±0.03) −10.62 (±0.01) −14.43 (±0.01) −18.33 (±0.02) −22.46 (±0.02) −26.76 (±0.02) −31.24 (±0.03) −12.14 (±0.01) −14.31 (±0.01) −16.73 (±0.02) −19.18 (±0.02) −21.58 (±0.02) −24.17 (±0.02) −30.80 (±0.03) −36.87 (±0.03) −43.34 (±0.04) −50.22 (±0.04) −56.79 (±0.05) −63.51 (±0.06) −18.44 (±0.02) −23.86 (±0.02) −29.60 (±0.03) −35.20 (±0.03) −40.74 (±0.04) −46.71 (±0.04) −23.96 (±0.02) −26.11 (±0.02) −28.58 (±0.03) −31.10 (±0.03) −33.50 (±0.03) −35.88 (±0.03)

TΔSmic kJ·mol−1 52.96 (±0.07) 49.63 (±0.07) 45.83 (±0.08) 41.87 (±0.08) 37.71 (±0.09) 33.73 (±0.09) 57.10 (±0.07) 53.94 (±0.07) 50.04 (±0.07) 45.95 (±0.08) 41.84 (±0.08) 37.51 (±0.08) 50.00 (±0.07) 47.82 (±0.07) 46.01 (±0.07) 43.82 (±0.07) 41.21 (±0.08) 38.64 (±0.08) 48.53 (±0.09) 42.51 (±0.09) 35.95 (±0.10) 29.10 (±0.10) 21.86 (±0.12) 14.82 (±0.12) 57.67 (±0.08) 52.45 (±0.09) 46.97 (±0.09) 40.61 (±0.10) 34.32 (±0.10) 27.36 (±0.11) 47.75 (±0.08) 45.16 (±0.09) 43.10 (±0.09) 41.11 (±0.09) 38.37 (±0.10) 35.29 (±0.10)

a

Standard uncertainties u are u(T) = 0.01 K and u(p) = 20 Pa. Standard deviations of the parameters are given in parentheses. bA for CMC values determined from the conductivity method. cB for CMC values determined from surface tension method. dC for CMC values determined from conductometry measurement. eReference 35. fReference 36. gReference 37. hReference 2.

Figure 4. Plots of CMC versus carbon number (Nc) of spacer group for (a) 12−n−12 (n = 3, 4, and 6), and (b) 14−n−14 (n = 3, 4, and 6) at pressure p = 0.1 MPa and different temperatures (T): □, 298.15 K; ○, 303.15 K; △, 308.15 K; ▽, 313.15 K; ☆, 318.15 K; +, 323.15 K.

3.2. Changes of CMC and β Values. As seen from Table 5, the values of CMC determined from the measurements of

conductivity and surface tension at 298.15 K show reasonable agreement and are consistent with the values reported in the 2896



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thermal motion, which leads to the prolonged spatial distance and the weakened interactions among the hydrophobic chains. Thus, more surfactants are required to form micelles. Furthermore, the CMC value at a given temperature decreases obviously with the increase of the alkyl chain length of Gemini surfactants. It indicates that the hydrophobic carbon chain has a strong effect on the micellization of the Gemini surfactants, which is similar to the usual results reported previously.33,34 The carbon number of the spacer group also has significant influence on the CMC value. For Gemini surfactants with C12 alkyl chains, the CMC value of 12−4−12 is larger than that of 12−3−12 or 12−6−12, which is shown in Figure 4. Similar phenomenon has been found in the literature:35 For surfactants of 12−n−12 type, the CMC value increases with the growth of the spacer group length when n < 4, and that shows approximately linear decrease when n > 4, which is independent of the anion species. For Gemini surfactants with C14 alkyl chains, the CMC value is observed to increase with the growth of the carbon number of the spacer group.

Figure 5. Plots of the degree of counterion dissociation (β) versus temperature (T) at pressure p = 0.1 MPa for the Gemini surfactants. □, 12−3−12; ○, 12−4−12; △, 12−6−12; ▽, 14−3−14; ☆, 14−4−14; +, 14−6−14.

literature. The CMC values increase slightly with the rise of temperature. This can be attributed to the enhanced molecular

Figure 6. Plots of Gibbs energy (ΔGmic), enthalpy (ΔHmic), and entropy (ΔSmic) of micellization versus temperature (T) at pressure p = 0.1 MPa for the Gemini surfactants. (a) 12−3−12, (b) 12−4−12, (c) 12−6−12, (d) 14−3−14, (e) 14−4−14, and (f) 14−6−14: □, ΔGmic; ○, ΔHmic; △, TΔSmic. 2897



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Figure 7. Plots of TΔSmic versus ΔHmic at pressure p = 0.1 MPa for the Gemini surfactants. (a) 12−n−12, and (b) 14−n−14: □, n = 3; ○, n = 4; △, n = 6.

micellization, and the system becomes more disordered. Besides, the effect of the iceberg structure destruction is stronger than that of the ordering of the randomly oriented Gemini cations from the solvated form to the micelles. With the rising of the temperature, the number of the ordered water molecules decreases, and the destruction of the iceberg structure releases less free water molecules when the micellization process takes place at higher temperatures.20 Furthermore, as shown in Figure 7, the plots of TΔSmic versus ΔHmic are almost linear for all of the Gemini surfactants, and the enthalpy−entropy compensation can be observed. It is apparent that TΔSmic > −ΔHmic, showing that the micellization of each Gemini surfactant in aqueous solution should be an entropy-driven process.

The increase of temperature can accelerate the motion of ions and molecules in the solution, which makes it easier for the counterions to dissociate from the electrostatic bondage of the micelles. As a result, the degree of dissociation (β) increases with the rise of temperature as shown in Figure 5, which is similar to that for the corresponding single-chain surfactants.20 From Table 5, it is also observed that the β values for the Gemini surfactants with C12 alkyl chains are slightly less than those with C14 alkyl chains at the same spacer group length. Besides, the β value decreases with the shortening of the spacer group length. These results should be mainly ascribed to the change of surface charge density of the micelles. Two cationic head groups of the Gemini surfactant are coupled together rigidly by the spacer group through a covalent bond and are packed more closely with the spacer length becoming shorter, which induces the charge density to increase. Consequently, the electrostatic interactions between surfactant ions and counterions are enhanced, and the degree of dissociation (β) is reduced significantly. 3.3. Thermodynamics of Micellization. As the process of micellization is sensitive to temperature, various thermodynamic parameters have been calculated from the temperature and CMC values. Plots of Gibbs free energy (ΔGmic), enthalpy (ΔHmic), and entropy (ΔSmic) of micellization versus temperature are shown in Figure 6. The values of ΔGmic at different temperatures are negative for all of the considered surfactants, which means that the micellization is a spontaneous process. Meanwhile, the ΔGmic values become negative with the shortening of the spacer group length at a specific temperature. This manifests that the micellization is favored with the decrease of spacer length, which is also in consistent with the changes of the CMC values. It is found that the ΔHmic values are negative at different temperatures, indicating that the micellization is an exothermic process. With the same spacer carbon number, the ΔHmic value decreases with the rise of temperature. For the ionic surfactants, the temperature affects the ΔHmic value mainly through two aspects. On one hand, increasing temperature can break the structure of water molecular aggregates around the alkyl chain, which decreases ΔHmic greatly. On the other hand, the condensation of alkyl chains into micelle is negatively associated with temperature rise, which induces ΔHmic to increase negatively. Obviously, the former is the dominant factor as concluded from the experimental results. Therefore, for each Gemini surfactant, the ΔHmic value becomes more negative with the increase of temperature. The ΔSmic values are positive and decrease with the increase of temperature. This indicates that, at a given temperature, the iceberg structure formed by ordered water molecules which are surrounding the hydrophobic groups was destructed by the

4. CONCLUSION The micellization parameters and surface activity of six Gemini quaternary ammonium surfactants, 12−n−12 and 14−n−14 (n = 3, 4, and 6), have been determined by measurements on conductivity. The values of the critical micellar concentration (CMC), the degree of counterion dissociation (β), Gibbs free energy (ΔGmic), enthalpy (ΔHmic), and entropy (ΔSmic) of micellization have been obtained and discussed. The rise of temperature results in the increments of the CMC and β values, but it leads to little change of ΔGmic. Meanwhile, ΔHmic and TΔSmic both decline as the temperature increases. At a certain temperature, the CMC values decline and the β values increase as the alkyl chain lengthens; the CMC of 12−n−12 (n = 3, 4, and 6) passes through a weak maximum with increasing spacer length, and the CMC of 14−n−14 (n = 3, 4, and 6) increases slightly with progressive spacer length.

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ASSOCIATED CONTENT

S Supporting Information *

Experimental data of surface tension (γ) and conductivity (k) for aqueous solutions of Gemini surfactants with various concentrations (m) at pressure p = 0.1 MPa and different temperatures (T) (Tables S1,S2). Pictures of the electrical conductivity meter and the surface tension meter (Figure S1,S2). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +86 571 88981416. Fax: +86-571-88981416. 2898



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Funding

(17) Shahir, A. A.; Javadian, S.; Razavizadeh, B. B. M.; Gharibi, H. Comprehensive study of tartrazine/cationic surfactant interaction. J. Phys. Chem. B 2011, 115, 14435−14444. (18) Ghosh, S.; Ghatak, C.; Banerjee, C.; Mandal, S.; Kuchlyan, J.; Sarkar, N. Spontaneous transition of micelle−vesicle−micelle in a mixture of cationic surfactant and anionic surfactant-like ionic liquid: A pure nonlipid small unilamellar vesicular template used for solvent and rotational relaxation study. Langmuir 2013, 29, 10066−10076. (19) Chauhan, S.; Sharma, K. Effect of temperature and additives on the critical micelle concentration and thermodynamics of micelle formation of sodium dodecyl benzene sulfonate and dodecyltrimethylammonium bromide in aqueous solution: A conductometric study. J. Chem. Thermodyn. 2014, 71, 205−211. (20) Fan, Z. Z.; Tong, W.; Zheng, Q.; Lei, Q. F.; Fang, W. J. Surface activity and micellization parameters of quaternary ammonium surfactants containing a hydroxyethyl group. J. Chem. Eng. Data 2013, 58, 334−342. (21) Desnoyers, J. E.; Caron, G.; Delisi, R.; Roberts, D.; Roux, A.; Perron, G. Thermodynamic properties of alkyldimethylamine oxides in water: Application of a mass-action model for micellization. J. Phys. Chem. 1983, 87, 1397−1406. (22) Kroflic, A.; Sarac, B.; Bester-Rogac, M. Thermodynamic characterization of 3-(3-cholamidopropyl)-dimethylammonium-1-propanesulfonate (CHAPS) micellization using isothermal titration calorimetry: Temperature, salt, and pH dependence. Langmuir 2012, 28, 10363−10371. (23) Zhang, Q.; Zhao, Y. X.; Sun, D. Z.; Wei, X. L.; Liu, J. Study on micelle formation of three homologous surfactants in aqueous solution by surface tension, conductivity, and fluorescence measurements. J. Dispersion Sci. Technol. 2013, 34, 1725−1732. (24) Zana, R. Critical micellization concentration of surfactants in aqueous solution and free energy of micellization. Langmuir 1996, 12, 1208−1211. (25) Zhang, S. S.; Lu, X. X.; Wu, J. Z.; Tong, W.; Lei, Q. F.; Fang, W. J. Interfacial tensions for system of n-heptane + water with quaternary ammonium surfactants and additives of NaCl or C2−C4 alcohols. J. Chem. Eng. Data 2014, 59, 860−868. (26) Manish, P. S.; Satish, K. M.; Yogendra, L. V.; Abhishek, K. G.; Rajendra, K. S.; Suresh, C. Viscoelastic, surface, and volumetric properties of ionic liquids [BMIM][OcSO4], [BMIM][PF6], and [EMIM][MeSO3]. J. Chem. Eng. Data 2014, DOI: 10.1021/je5000617. (27) Alvarez, E.; Gomez-Diaz, D.; La Rubia, M. D.; Navazalt, J. M. Surface tension of aqueous binary mixtures of 2-(methylamino)ethanol and 2-(ethylamino)ethanol and aqueous ternary mixtures of these amines with triethanolamine or N-methyldiethanolamine from (293.15 to 323.15) K. J. Chem. Eng. Data 2008, 53, 318−321. (28) Lee, J. W.; Park, S. B.; Lee, H. Densities, surface tensions, and refractive indices of the water + 1,3-propanediol system. J. Chem. Eng. Data 2000, 45, 166−168. (29) Garidel, P.; Hildebrand, A.; Neubert, R.; Blume, A. Thermodynamic characterization of bile salt aggregation as a function of temperature and ionic strength using isothermal titration calorimetry. Langmuir 2000, 16, 5267−5275. (30) Rodriguez, A.; Graciani, M. D.; Cordobes, F.; Moya, M. L. Water−ethylene glycol cationic dimeric micellar solutions: Aggregation, micellar growth, and characteristics as reaction media. J. Phys. Chem. B 2009, 113, 7767−7779. (31) Zana, R. Dimeric and oligomeric surfactants. Behavior at interfaces and in aqueous solution: a review. Adv. Colloid Interface Sci. 2002, 97, 205−253. (32) Paula, S.; Sus, W.; Tuchtenhagen, J.; Blume, A. Thermodynamics of micelle formation as a function of temperature: A high sensitivity titration calorimetry study. J. Phys. Chem. 1995, 99, 11742−11751. (33) Gao, B.; Sharma, M. M. A family of alkyl sulfate gemini surfactants. 1. Characterization of surface properties. J. Colloid Interface Sci. 2013, 404, 80−84. (34) Aiad, I.; El-Sukkary, M. M.; El-Deeb, A.; El-Awady, M. Y.; Shaban, S. M. Surface properties, thermodynamic aspects and antimicrobial

The authors are grateful for the financial supports from the National Natural Science Foundation of China (No.21073164, J1210042). Notes

The authors declare no competing financial interest.

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Micellization Parameters of Six Gemini Quaternary Ammonium

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