Surface Activity and Micellization Parameters of ... - ACS Publications

Article pubs.acs.org/jced

Surface Activity and Micellization Parameters of Quaternary Ammonium Surfactants Containing a Hydroxyethyl Group Zhenzhong Fan,† Wei Tong,‡ Qing Zheng,‡ Qunfang Lei,‡ and Wenjun Fang*,‡ †

Department of Petroleum Engineering, Northeast Petroleum University, Daqing 163318, China Department of Chemistry, Zhejiang University, Hangzhou 310027, China

‡

ABSTRACT: Based on the measurements of conductivity and surface tension and our previously reported results of isothermal titration calorimetry (ITC), the surface activity and micellization parameters, such as the critical micellar concentration (CMC), the degree of counterion dissociation (β), and thermodynamic functions of micellization, for six quaternary ammonium surfactants, dodecyl-(2-hydroxyethyl)-dimethylazanium bromide, tetradecyl-(2-hydroxyethyl)-dimethylazanium bromide, hexadecyl-(2-hydroxyethyl)-dimethylazanium bromide, dodecyl-di(2hydroxyethyl)-methylazanium bromide, tetradecyl-di(2-hydroxyethyl)methylazanium bromide, and hexadecyl-di(2-hydroxyethyl)-methylazanium bromide in aqueous solutions have been investigated. The values of CMC determined from three different methods are compared, and they show reasonable agreement. From the CMC values obtained from the conductivity measurements in this work and the previously reported values of calorimetric enthalpy of micellization, the Gibbs free energy (ΔGmic) and entropy (ΔSmic) of micellization are calculated through the mass-action model. The influences on the micellization parameters of the temperature, the length of alkane chain, and the number of hydroxyethyl substituents on the surfactant headgroup are discussed.

■

INTRODUCTION Surfactants always receive considerable attention because of their wide applications in the fields of cleaning, paint, medicine, and biological systems. Particularly, the amphiphilic structure of surfactant molecules can lead to the formation of micelles in aqueous solution with the hydrophilic headgroups locating at the surface and the hydrophobic segments shielded from water. The critical micelle concentration (CMC), above which the micelles form, is recognized as the most important parameter for studies on the surface activity and micellization of surfactants. For ionic surfactants in the aqueous solution, monomers are considered to ionize completely below CMC. After the micellar formation takes place, however, counterions are bound to the core of micelle, and only part of them can be regarded as free ions, which is indicated by another important parameter, the degree of counterion dissociation (β). It is well-known that the headgroup structure and the length of the alkane chain can take significant influences on the physicochemical aspects of surfactants, such as CMC, β, and the aggregation number of micelles.1−4 In addition, the difference in the size of headgroups between two similar surfactants can also lead to several different micellization behaviors.5−7 Thermodynamics of the micellization process and the micellization parameters of surfactants in aqueous solutions have been widely investigated with many different techniques, such as surface tension measurements,8,9 isothermal titration calorimetry (ITC),10,11 and conductometry.12,13 As for the surface tension measurement, aside from obtaining the CMC value, it also detects the purity of surfactants through checking whether there is a minimum in plots of surface tension versus concentration. © XXXX American Chemical Society

Because of the capability for direct determination of the enthalpy of micellization (ΔHmic), the ITC measurement is more and more applied to the thermodynamic study of surfactant systems. Moreover, the conductivity measurement is usually simple and accurate enough to determine the CMC and β for aqueous solutions of cationic or anionic surfactants. In this work, on the basis of surface tension and electrical conductivity measurements along with our previously reported results of isothermal titration calorimetry (ITC),14 the micellar properties of six quaternary ammonium surfactants, dodecyl-(2hydroxyethyl)-dimethylazanium bromide (C12HDAB), tetradecyl-(2-hydroxyethyl)-dimethylazanium bromide (C14HDAB), hexadecyl-(2-hydroxyethyl)- dimethylazanium bromide (C16HDAB), dodecyl-di(2-hydroxyethyl)-methylazanium bromide (C12DHAB), tetradecyl-di(2-hydroxyethyl)-methylazanium bromide (C14DHAB), and hexadecyl-di(2-hydroxyethyl)-methylazanium bromide (C16DHAB) have been investigated. A series of micellization parameters in these aqueous surfactant solutions are evaluated and discussed from the influences of the temperature, the length of alkane chain, and the number of hydroxyethyl substituents on the surfactant headgroups.

■

EXPERIMENTAL SECTION Materials. 1-Bromododecane (CAS No. 143-15-7), 1-bromotetradecane (CAS No. 112-71-0), and 1-bromohexadecane

Received: August 19, 2012 Accepted: December 6, 2012

A

dx.doi.org/10.1021/je300873x | J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

Article

Table 1. Molecular Structure, Relative Molecular Mass (M), and Chemical Name and Abbreviation (N & A) of the Quaternary Ammonium Surfactants Studied in This Work

a

R is the alkyl group in each surfactant molecule.

Table 2. Specification of Chemical Samples final mass fraction purity

analysis method

recrystallization

0.99

NMR,a EAb

synthesis

recrystallization

0.99

NMR, EA

synthesis

recrystallization

0.99

NMR, EA

synthesis

recrystallization

0.99

NMR, EA

synthesis

recrystallization

0.99

NMR, EA

synthesis

recrystallization

0.99

NMR, EA

chemical name 1-bromododecane 1-bromotetradecane 1-bromohexadecane potassium chloride 2-(dimethylamino)ethanol 2-[2-hydroxyethyl(methyl)amino]ethanol dodecyl-(2-hydroxyethyl)-dimethylazanium bromide tetradecyl-(2-hydroxyethyl)-dimethylazanium bromide hexadecyl-(2-hydroxyethyl)-dimethylazanium bromide dodecyl-di(2-hydroxyethyl)-methylazanium bromide tetradecyl-di(2-hydroxyethyl)-methylazanium bromide hexadecyl-di(2-hydroxyethyl)-methylazanium bromide a

initial mass fraction purity

source Shanghai Zhuorui Chemical Company Shanghai Zhuorui Chemical Company Shanghai Zhuorui Chemical Company Sinopharm Chemical Reagent Company Sinopharm Chemical Reagent Company Sinopharm Chemical Reagent Company synthesis

purification method

0.985

none

0.985

none

0.99

none

0.99

none

0.99

none

0.99

none

Nuclear magnetic resonance. bElemental analysis.

(CAS No. 112-82-3) with mass fraction purities of 0.99 were purchased from Shanghai Zhuorui Chemical Company, China. Potassium chloride (CAS No. 7447-40-7), 2-(dimethylamino)ethanol (CAS No. 108-01-0), and 2-[2-hydroxyethyl(methyl)amino]ethanol (CAS No. 105-59-9) with mass fraction purities of 0.99 were purchased from Sinopharm Chemical Reagent Company, China. The above agents were used without further purification. The surfactants, CnHDAB (n = 12, 14, and 16) and CnDHAB (n = 12, 14, and 16), with the chemical structures shown in Table 1, were prepared from the reactions of 1-bromoalkane (1-bromododecane, 1-bromotetradecane, and 1-bromohexadecane) with 2-(dimethylamino)ethanol and 2-[2hydroxyethyl(methyl)amino]ethanol, respectively, in propan-2one (CAS No. 67-64-1) at about 333.15 K. They were purified by recrystallization in the mixed solvents of propan-2-one and methanol (CAS No. 67-56-1). The detailed characterizations of these surfactants, examined by NMR (Bruker Advance 2B/400 Hz), IR spectrum (NEXES 470), and elemental analysis (Carlabo EA1110), were described in our previous work.14 No minima were

Figure 1. Plots of surface tension (γ) versus surfactant concentration (m) for aqueous solutions of CnHDAB and CnDHAB (n = 12, 14, and 16) at temperature T = 298.15 K and pressure p = 0.1 MPa: □, C12HDAB; ○, C12DHAB; △, C14HDAB; ▽, C14DHAB; ☆, C16HDAB; +, C16DHAB. B



Article

Figure 2. Plots of conductivity (κ) versus surfactant concentration (m) for aqueous solutions of (a) C12HDAB, (b) C12DHAB, (c) C14HDAB, (d) C14DHAB, (e) C16HDAB, and (f) C16DHAB 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.

and the average value was obtained with an uncertainty within ± 5·10−5 S·m−1. The surface tension measurement was performed using a digital tensiometer (DropMeter A-100ρ) by means of the pendant-drop method, which is calibrated by pure water. The temperature of the samples was kept constant within ± 0.01 K. The drop of the surfactant solution formed at the tip of needle by supplying the solution through peristaltic pump. Sets of observations to obtain equilibrium surface tension are taken until the change of surface tension is 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 uncertainty is ± 1·10−5 N·m−1.

observed in plots of surface tension versus concentration for all of these surfactants, which indicates that there were no surface active impurities. Some information of the samples used in this work is listed in Table 2. All of the surfactant solutions for surface tension and electrical conductivity measurements were prepared with ultrapure water with the resistivity above 1.82·105 Ω·m at 298.15 K produced by the Millipore Q3 system. Methods. The specific conductivity for the surfactant solutions was measured as a function of surfactant concentration with a digital conductivity meter (DDS-112AT) at the temperature range from (293.15 to 323.15) K. The conductivity cell with a sample was immersed in a thermostat bath (DF-02) with the temperature fluctuation within ± 0.01 K. The cell constants were calibrated by using KCl solutions. At each concentration, the conductivity measurement was repeated three times,

■

RESULTS AND DISCUSSION Experimental Data and Calculation Models. The surface tension at 298.15 K and the conductivity at different C



Article

Table 3. Estimated Values of Parameters, a1, a2, Δm, and m0, in eq 1 Fitted to Conductivity Data for Aqueous Surfactant Solutions of CnHDAB and CnDHAB (n = 12, 14, and 16) at Different Temperatures (T) (Pressure p = 0.1 MPa)a T

a1 −1

surfactant

K

S·m ·mol ·kg

C12HDAB

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

8.093 8.759 9.381 10.52 11.48 12.63 8.102 8.674 9.615 10.60 11.47 12.88 8.352 9.278 10.18 11.22 11.99 13.10 8.701 9.470 10.24 11.28 12.32 13.29 10.05 10.53 11.60 13.26 12.93 14.03 10.30 11.09 11.63 13.00 14.52 15.53

C12DHAB

C14HDAB

C14DHAB

C16HDAB

C16DHAB

103·Δm

a2 −1

−1

S·m ·mol·kg

−1

2.323 2.650 2.922 3.408 3.827 4.257 2.426 2.697 3.171 3.590 3.985 4.626 2.129 2.537 2.881 3.255 3.535 4.036 2.247 2.626 2.944 3.372 3.783 4.156 2.312 2.590 3.072 3.576 3.597 4.122 2.451 2.871 3.212 3.754 4.318 4.791

mol·kg 0.48 0.54 0.49 0.61 0.71 0.92 0.52 0.52 0.59 0.59 0.70 0.91 0.15 0.15 0.14 0.21 0.20 0.23 0.17 0.11 0.13 0.15 0.16 0.15 0.08 0.10 0.08 0.06 0.15 0.13 0.08 0.12 0.09 0.09 0.06 0.14

−1

103·m0 mol·kg−1 13.87 14.23 14.57 14.91 15.29 15.92 12.92 13.21 13.43 13.79 14.22 14.67 3.48 3.53 3.67 3.82 4.03 4.20 3.20 3.29 3.39 3.55 3.72 3.93 0.80 0.84 0.87 0.92 0.98 1.07 0.75 0.79 0.82 0.86 0.90 0.96

The standard uncertainties u are u(T) = 0.01 K and u(p) = 20 Pa. The combined expanded uncertainties Uc are Uc(a1) = 0.005 S·m−1·mol−1·kg, Uc(a2) = 0.005 S·m−1·mol−1·kg, Uc(Δm) = 2·10−5 mol·kg−1, and Uc(m0) = 1·10−5 mol·kg−1 (0.95 level of confidence).

a

sponds to the CMC of the surfactant, and the degree of counterion dissociation (β) is calculated as β = a2/a1. The parameters, a1, a2, Δm, and m0, for these surfactants are given in Table 3. Based on new-found CMC and β values from conductivity measurements, the standard Gibbs free energy of micellization, ΔGmic, at each temperature is determined in accordance with the mass-action model17,18

temperatures from (293.15 to 323.15) K of aqueous solutions for surfactants, CnHDAB and CnDHAB (n = 12, 14, and 16), are presented in Figures 1 and 2, respectively. From surface tension measurements, the CMC values correspond to the break points in the plots of the surface tension versus the surfactant concentration. As shown in Figure 1, each surface tension plot shows a shape of two lines intersecting at the break point, which indicates the process of micellization. From conductivity measurements, the following method has been recently considered to be quite efficient for the determination of CMC values, which is based on the fit of the specific conductivity (κ) as a function of surfactant concentration (m) to the integral of the Boltzmann sigmoidal equation15,16 ⎛ 1 + e(m − m0)/ Δm ⎞ ⎟⎟ κ = κ0 + a1m + Δm(a 2 − a1)ln⎜⎜ ⎝ 1 + e−m0 / Δm ⎠

ΔGmic = (2 − β)RT ln(xCMC)

(2)

where xCMC is the critical micelle concentration (CMC) expressed as mole fraction, T is the absolute temperature, and R is the gas constant. With the calorimetric enthalpy of micellization, ΔHmic, taken from our previous publication,14 the entropy of micellization, ΔSmic, can be obtained from the following equation:

(1)

ΔSmic =

where κ0 is the conductivity of the pure water, a1 and a2 are the slopes in the pre- and postmicellar regions, and Δm is the width of the transition. The central point of the transition region (m0) corre-

ΔHmic − ΔGmic T

(3)

The determined values of CMC, β, ΔGmic, ΔSmic, and ΔHmic for the surfactants at different temperatures are summarized in D



Article

Table 4. Values of Critical Micelle Concentration (CMC) Determined from Different Measurements, Degree of Counterion Dissociation (β), Gibbs Free Energy (ΔGmic), Enthalpy (ΔHmic), and Entropy (ΔSmic) of Micellization for Aqueous Surfactant Solutions of CnHDAB and CnDHAB (n = 12, 14, and 16) at Different Temperatures (T) (Pressure p = 0.1 MPa)a 103·CMC/mol·kg−1

T b

surfactant

K

A

C12HDAB

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

13.87 14.23 14.57 14.91 15.29 15.92 12.92 13.21 13.43 13.79 14.22 14.67 3.48 3.53 3.67 3.82 4.03 4.20 3.20 3.29 3.39 3.55 3.72 3.93 0.80 0.84 0.87 0.92 0.98 1.07 0.75 0.79 0.82 0.86 0.90 0.96

C12DHAB

C14HDAB

C14DHAB

C16HDAB

C16DHAB

B

ΔGmic

c

13.07

12.89

3.17

2.92

0.78

0.74

C

d

13.17 13.38 13.84 14.12 14.69 15.25 12.43 12.57 13.02 13.26 13.76 14.39 3.309 3.342 3.484 3.608 3.706 3.951 3.109 3.152 3.250 3.350 3.546 3.687 0.795 0.835 0.874 0.918 0.965 1.042 0.736 0.783 0.794 0.832 0.900 0.962

β 0.287 0.303 0.311 0.324 0.333 0.337 0.299 0.311 0.330 0.339 0.347 0.359 0.255 0.273 0.283 0.290 0.295 0.308 0.258 0.277 0.288 0.299 0.307 0.313 0.230 0.246 0.265 0.270 0.285 0.294 0.238 0.259 0.276 0.289 0.297 0.309

−1

ΔHmice −1

kJ·mol

kJ·mol

−35.21 −35.36 −35.64 −35.85 −36.10 −36.39 −35.26 −35.50 −35.61 −35.87 −36.11 −36.27 −41.85 −42.02 −42.31 −42.63 −42.94 −43.07 −42.13 −42.25 −42.55 −42.74 −42.98 −43.26 −48.89 −49.06 −49.15 −49.54 −49.63 −49.72 −48.94 −49.00 −49.14 −49.28 −49.60 −49.80

−2.723 −4.195 −5.698 −6.726 −7.507 −7.955 −3.343 −4.934 −6.315 −7.507 −8.221 −8.935 −5.967 −8.645 −11.19 −13.58 −15.51 −17.22 −6.686 −9.366 −11.89 −14.04 −15.86 −17.57 −9.678 −12.44 −15.52 −18.01 −20.89 −22.88 −9.999 −12.91 −16.40 −19.29 −21.63 −23.77

ΔSmic J·mol−1·K−1 109.0 102.8 97.2 93.0 89.9 88.0 107.0 100.8 95.1 90.6 87.7 84.6 120.3 110.1 101.0 92.8 86.2 80.0 118.9 108.5 99.5 91.6 85.3 79.5 131.5 120.8 109.1 100.7 90.3 83.1 130.6 119.1 106.3 95.8 87.9 80.6

The standard uncertainties u are u(T) = 0.01 K and u(p) = 20 Pa. The combined expanded uncertainties Uc are Uc(CMC) = 1·10−5 mol·kg−1, Uc(β) = 0.002, Uc(ΔGmic) = 0.02 kJ·mol−1, and Uc(ΔSmic) = 0.1 J·mol−1·K−1 (0.95 level of confidence). bA for CMC values determined from the conductivity method. cB for CMC values determined from surface tension method. dC for CMC values from ref 14 determined from the isothermal titration calorimetry (ITC) method. eEnthalpy of micellization from ref 14. a

the former effect with increasing temperature becomes more important than the latter one in the investigated temperature region. In addition, with the increase of the length of the alkyl chain in surfactant molecules, the CMC value at a given temperature decreases obviously. The contribution of per −CH2− group to the change of CMC values comes near to that for a series of dodecyl(trimethyl)azanium bromide, tetradecyl(trimethyl)azanium bromide, and hexadecyl(trimethyl)azanium bromide.4 The hydrophobic carbon chain has strong effect on the micellization of the surfactants, which is also similar to the usual results previously reported.19,20 Over the temperature range investigated, an increase of the number of hydroxyethyl substituents on the quaternary ammonium headgroup brings a slight decrease of the CMC. The larger headgroup, because of the methyl group substituted with the hydroxyethyl one, makes a stronger steric effect. As a result, the surfactant molecules with hydroxyethyl

Table 4. The comparison of the CMC values at 298.15 K from three different methods is shown in Figure 3. Changes of CMC and β Values. From Table 4 and Figure 3, reasonable agreements of the CMC values at 298.15 K resulting from the measurements on conductivity, surface tension, and ITC14 are observed. At several temperatures, the results from conductivity and ITC measurements also show agreement. It is noted that the rise of the temperature causes a slight increase of the CMC values. As known, the increase of temperature can bring two opposing effects. On one hand, the rising temperature destroys the structure of water, which is to the disadvantage of hydrophobic interactions and induces an increment of the CMC; on the other hand, the increase of temperature weakens the hydration of the ionic headgroup, which induces the growth of hydrophobicity of the surfactant and thus a decrease of the CMC. Hence, as seen from the results in Table 4, E



Article

Figure 3. Comparison of critical micelle concentration (CMC) values determined from three different methods for CnHDAB and CnDHAB (n = 12, 14, and 16) at temperature T = 298.15 K and pressure p = 0.1 MPa: □, conductivity method; ○, surface tension method; △, isothermal titration calorimetry (ITC) method.

Figure 6. Plots of Gibbs free energy (ΔGmic) of micellization versus temperature (T) for the surfactants (pressure p = 0.1 MPa): □, C12HDAB; ○, C12DHAB; △, C14HDAB; ▽, C14DHAB; ☆, C16HDAB; +, C16DHAB.

linear dependence. Raising the temperature can accelerate the motion of ions and molecules in the solution surroundings, which reduces the number of counterions in the aggregates. Consequently, the degree of dissociation increases with the rise of temperature. The values of β versus carbon number (Nc) in the alkyl chain for surfactants with the same headgroup are given in Figure 5. It is obvious that the β value decreases with the increasing number of carbon atoms in the alkyl chain, which is ascribed to the decrease in surface charge density of the micelles.21 Surfactants with longer alkyl chain are favorable for aggregation in bulky structures where the surface-to-volume ratio is smaller. This means that the polar head-groups are packed more closely and are bound by a larger fraction of counterions. Besides, the β value increases slightly with the increase in the number of hydroxyethyl substituents at the headgroups. This is also attributed to the decrease in the charge density of the micelle surface as the headgroup size of the surfactant increases. Thermodynamic Functions of Micellization. The plots of ΔGmic, calculated from the CMC values and β based on the conductivity measurements, are shown in Figure 6. Approximately, the ΔGmic values decrease linearly with increasing the temperature. For all of the surfactants considered, the ΔGmic values are negative in the whole regions of the studied temperature, which means that the formation of micelles is a spontaneous process.

Figure 4. Plots of the degree of counterion dissociation (β) of the surfactant versus temperature (T) for the surfactants (pressure p = 0.1 MPa): □, C12HDAB; ○, C12DHAB; △, C14HDAB; ▽, C14DHAB; ☆, C16HDAB; +, C16DHAB.

substituents tend to aggregate at a relatively low surfactant concentration. The curves of the degree of counterion dissociation (β) versus temperature are shown in Figure 4, exhibiting approximately

Figure 5. Plots of the degree of counterion dissociation (β) versus carbon number (Nc) of alkyl chain for (a) CnHDAB (n = 12, 14, and 16) and (b) CnDHAB (n = 12, 14, and 16) 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. F



Article

Figure 7. Plots of Gibbs free energy (ΔGmic) of micellization versus carbon number (Nc) of alkyl chain for (a) CnHDAB (n = 12, 14, 16) and (b) CnDHAB (n = 12, 14, 16) 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.

Figure 8. Plots of micellization parameters, Gibbs free energy (ΔGmic), enthalpy (ΔHmic), entropy (ΔSmic), versus temperature (T) at pressure p = 0.1 MPa for (a) C12HDAB, (b) C12DHAB, (c) C14HDAB, (d) C14DHAB, (e) C16HDAB, and (f) C16DHAB: □, ΔGmic; ○, ΔHmic; △, TΔSmic. G



■

From Figure 6, it is also found that the curves of ΔGmic versus temperature for surfactants with the same chain length are close and tangled. This indicates that one or more hydroxyethyl substituents at the headgroup cannot affect the Gibbs free energy obviously. However, as discussed above, introducing more hydroxyethyl substituents induces the decrease in the CMC and leads to the increment of the degree of counterion dissociation. In other words, although the CMC decreases with increasing the number of substituents, it might be recovered by the increase of β. Hence, the final results should be attributed to the different influences on CMC and β of the substituents at the headgroups. The variation of ΔGmic with the carbon number of the alkyl chain is given in Figure 7. It is seen that the value of ΔGmic decreases obviously with the increase of the hydrophobic tail length and becomes more negative by approximate 3 kJ·mol−1 for each additional −CH2− group, which is close to the reported data for dodecyl(trimethyl)azanium bromide, tetradecyl(trimethyl)azanium bromide, and hexadecyl(trimethyl)azanium bromide.4 Besides, the slopes with no significant numerical disparity for CnHDAB and CnDHAB suggest that the difference in the headgroups does not significantly influence the ΔGmic per −CH2− group. As the ΔGmic is considered as the free energy of transferring surfactant from the aqueous phase to the micellar pseudophase,21 each slope in Figure 7 stands for the change of the free energy when 1 mol of methylene group transfers from the aqueous phase to the micellar pseudophase, which is irrespective of the property of the headgroups. As shown in Figure 8, the plots of ΔHmic and TΔSmic versus temperature are almost linear for all of these surfactants. Meanwhile, the enthalpy−entropy compensation can be observed. The ΔHmic value is observed to decrease with the growth of temperature. The more negative value of ΔHmic at the higher temperature resulted from the decrease in the amount of water molecules bound to the surfactant headgroups, which makes the dehydration heat decrease when the micelles form. The positive value of ΔSmic in all regions of temperature studied in this work is caused by the destruction of the iceberg structure of ordered water molecules surrounding the hydrophobic segments. Moreover, the temperature-dependent trend results from the decrease in the number of ordered water molecules with the temperature rise, and the destruction of the iceberg structure releases less disordered water molecules when the process of micellization takes place.

Article

AUTHOR INFORMATION

Corresponding Author

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

This work was financially supported by the National Natural Science Foundation of China under No. 21073164 and the Natural Science Foundation of Zhejiang Province, China under No. Y4080201. Notes

The authors declare no competing financial interest.

■

REFERENCES

(1) Silva, B. F. B.; Marques, E. F. Thermotropic Behavior of Asymmetric Chain Length Cationic Surfactants: The Influence of the Polar Head Group. J. Colloid Interface Sci. 2005, 290, 67−274. (2) Boyd, B. J.; Drummond, C. J.; Krodkiewska, I.; Grieser, F. How Chain Length, Headgroup Polymerization, and Anomeric Configuration Govern the Thermotropic and Lyotropic Liquid Crystalline Phase Behavior and the Air−Water Interfacial Adsorption of GlucoseBased Surfactants. Langmuir 2000, 16, 7359−7367. (3) Trone, M. D.; Khaledi, M. G. Characterization of Chemical Selectivity in Micellar Electrokinetic Chromatography. 4. Effect of Surfactant Headgroup. Anal. Chem. 1999, 71, 1270−1277. (4) Stodghill, S. P.; Smith, A. E.; O’Haver, J. H. Thermodynamics of Micellization and Adsorption of Three Alkyltrimethylammonium Bromides Using Isothermal Titration Calorimetry. Langmuir 2004, 20, 11387−11392. (5) Xing, H.; Yan, P.; Zhao, K.; Xiao, J. Effect of Headgroup Size on the Thermodynamic Properties of Micellization of Dodecyltrialkylammonium Bromides. J. Chem. Eng. Data 2010, 56, 865−873. (6) Buckingham, S. A.; Garvey, C. J.; Warr, G. G. Effect of Headgroup Size on Micellization and Phase Behavior in Quaternary Ammonium Surfactant Systems. J. Phys. Chem. 1993, 97, 10236− 10244. (7) Silvestre, M. P. C.; Chaiyasit, W.; Brannan, R. G.; McClements, D. J.; Decker, E. A. Ability of Surfactant Headgroup Size To Alter Lipid and Antioxidant Oxidation in Oil-in-Water Emulsions. J. Agric. Food Chem. 2000, 48, 2057−2061. (8) Li, N.; Zhang, S.; Zheng, L.; Wu, J.; Li, X.; Yu, L. Aggregation Behavior of a Fluorinated Surfactant in 1-Butyl-3-methylimidazolium Ionic Liquids. J. Phys. Chem. B 2008, 112, 12453−12460. (9) Fan, Y.; Hou, Y.; Xiang, J.; Yu, D.; Wu, C.; Tian, M.; Han, Y.; Wang, Y. Synthesis and Aggregation Behavior of a Hexameric Quaternary Ammonium Surfactant. Langmuir 2011, 27, 10570−10579. (10) Hou, Y.; Han, Y.; Deng, M.; Xiang, J.; Wang, Y. Aggregation Behavior of a Tetrameric Cationic Surfactant in Aqueous Solution. Langmuir 2009, 26, 28−33. (11) Tsamaloukas, A. D.; Beck, A.; Heerklotz, H. Modeling the Micellization Behavior of Mixed and Pure n-Alkyl-Maltosides. Langmuir 2009, 25, 4393−4401. (12) Martín, V. I.; Rodríguez, A.; Graciani, M. a. d. M.; Robina, I.; Moyá, M. a. L. Study of the Micellization and Micellar Growth in Pure Alkanediyl-α-ω-Bis(dodecyldimethylammonium) Bromide and MEGA10 Surfactant Solutions and Their Mixtures. Influence of the Spacer on the Enthalpy Change Accompanying Sphere-to-Rod Transitions. J. Phys. Chem. B 2010, 114, 7817−7829. (13) Haldar, J.; Bhattacharya, S. Microcalorimetric and Conductivity Studies with Micelles Prepared from Multi-Headed Pyridinium Surfactants. Langmuir 2005, 21, 5747−5751. (14) Tong, W.; Zheng, Q.; Shao, S.; Lei, Q.; Fang, W. Critical Micellar Concentrations of Quaternary Ammonium Surfactants with Hydroxyethyl Substituents on Headgroups Determined by Isothermal Titration Calorimetry. J. Chem. Eng. Data 2010, 55, 3766−3771. (15) Carpena, P.; Aguiar, J.; Bernaola-Galván, P.; Carnero Ruiz, C. Problems Associated with the Treatment of Conductivity-Concen-

■

CONCLUSIONS Surface activity and micellization parameters of six quaternary ammonium surfactants, CnHDAB and CnDHAB (n = 12, 14, and 16), were studied by measurements of conductivity, surface tension, and isothermal titration calorimetry (ITC). The values of the critical micellar concentration (CMC), the degree of counterion dissociation (β), and the thermodynamic parameters of micellization, ΔGmic, ΔHmic, and ΔSmic, are obtained and discussed. The increase of temperature results in the increments for both CMC and β. Meanwhile, ΔGmic, ΔHmic, and ΔSmic are all in decline when the temperature rises. As the alkyl chain lengthens, the values of CMC, β, and ΔGmic decrease obviously. The increase in the number of hydroxyethyl substituents on the quaternary ammonium headgroup induces the decline of CMC value and the growth of the degree of counterion association. No obvious effect on ΔGmic is observed with introduction of hydroxyethyl substituents. H



Article

tration Data in Surfactant Solutions: Simulations and Experiments. Langmuir 2002, 18, 6054−6058. (16) Kabirud, D.; Ajmal Koya, P. Effects of Solvent Media and Temperature on the Self-Aggregation of Cationic Dimeric Surfactant 14-6-14, 2Br− Studied by Conductometric and Fluorescence Techniques. Langmuir 2010, 26, 7905−7914. (17) 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. (18) Yoshikiyo, M. Mass Action Model of Micelle Formation: Its Application to Sodium Dodecyl Sulfate Solution. J. Colloid Interface Sci. 1988, 122, 308−314. (19) Li, G.; Gao, Y.; Li, X.; Liu, J.; Zheng, L.; Xing, H.; Xiao, J. Aggregation Behavior of N-alkyl Perfluorooctanesulfonamides in Dimethyl Sulfoxide Solution. J. Colloid Interface Sci. 2010, 342, 372− 381. (20) Dai, S.; Tam, K. C. Isothermal Titration Calorimetric Studies of Alkyl Phenol Ethoxylate Surfactants in Aqueous Solutions. Colloids Surf., A 2003, 229, 157−168. (21) Zana, R. Ionization of Cationic Micelles: Effect of the Detergent Structure. J. Colloid Interface Sci. 1980, 78, 330−337. (22) Zana, R. Critical Micellization Concentration of Surfactants in Aqueous Solution and Free Energy of Micellization. Langmuir 1996, 12, 1208−1211.

I


Surface Activity and Micellization Parameters of ... - ACS Publications

Recommend Documents