Thermodynamic Study of Mixed Surfactants of Polyoxyethylene tert

Article pubs.acs.org/jced

Thermodynamic Study of Mixed Surfactants of Polyoxyethylene tertOctyl Phenyl Ether and Dodecyltrimethylammonium Bromide Peizhu Zheng,†,# Xianshuo Zhang,†,# Jian Fang,† and Weiguo Shen*,†,‡ †

Department of Chemistry, Lanzhou University, Lanzhou, Gansu 730000, China School of Chemistry and Molecular Engineering, East China University of Science and Technology, Shanghai 200237, China

‡

S Supporting Information *

ABSTRACT: Mixtures of polyoxyethylene tert-octyl phenyl ether (TX100) and dodecyltrimethylammonium bromide (DTAB) were investigated using isothermal titration calorimetry. On the base of the pseudophase separation model and using the two-parameter Margules equation, the compositions of mixed micelles and activity coefficient of each surfactant in mixed micelles were obtained and further used to calculate the thermodynamic parameters of micellization and excess properties of mixed surfactants in the micelles. Through three titration methods, the tendency of surfactants to form mixed micelles and the interactions between TX100 and DTAB in the micelles were investigated.

1. INTRODUCTION Study of mixed surfactant solutions is currently an interesting subject because of their considerable practical importance. For example they are used as coatings to improve wettability of surfaces and as detergents to increase the washing activity. Surfactant mixtures can show a very different behavior in comparison to their components. It is well-known that mixed surfactants usually present much higher surface activity and lower critical micelle concentration (cmc) resulting from the synergistic (attractive) interaction between two surfactants, thus their solutions have superior properties to those containing only a single surfactant.1−5 Developing a fundamental understanding of the behavior of mixed micellar solutions will help to design solutions of surfactant mixtures for desirable properties. To tailor solutions of surfactant mixtures to a particular application, many properties such as (i) the tendency of surfactant mixtures to form mixed micelles, (ii) the distribution of surfactant species between mixed micelles and monomers, (iii) the composition of the mixed micelles, and (iv) the interaction between two surfactants in micelles are required to be investigated. The pseudophase separation approach is usually used to model the ideality of surfactant aqueous solutions. This model assumes that micelles are formed in a single process and predicts an abrupt change of the solution physical properties at the cmc, in which the micelle is treated as a separate, infinite phase in equilibrium with the monomer phase.6,7 The most common nonideal model is the regular solution theory established by Rubingh.8 In regular solution theory, the nonideality is represented by a term of the form βx(1 − x), where β is an interaction coefficient and x is the micelle composition. A negative or positive value of β indicates synergism or antagonism in mixed micelle formation. Recently, the two-parameter Margules equation has been proposed to describe the nonideality for the complicated binary surfactant © XXXX American Chemical Society

systems, by using which, the micellar compositions, the active coefficients of two surfactants, and excess Gibbs free energy of the mixed surfactants in the micelles may be deduced from the critical micelle concentrations measured for various total compositions of two surfactants.9,10 Polyoxyethylene tert-octyl phenyl ether (TX100) and nalkyltrimethylammonium bromide (CnTAB) are much investigated versatile surfactants, which have been widely used in many application fields.11−15 It is well-known that the interactions between different kinds of surfactants can lead to various synergism or antagonism in their aqueous solutions,16 thus the mixing behaviors of TX100 and CnTAB in aqueous solutions are interesting not only for the fundamental scientific research but also for extending their practical applications. Gharibi’s group used ion-selective electrode (ISE) technique17,18 and PFG-NMR spectroscopy19 to determine the monomer concentrations and the mixed micelle compositions of TX100/CnTAB mixtures. Ruiz et al. used fluorescent probe technique to study the system of TX100/CnTAB, and the result indicated that the CnTAB molecules would make the TX100/ CnTAB mixed surfactant more closely packed on the micelle interface, but less order than pure TX100 micelle.20 The isothermal titration calorimetry (ITC) is a sensitive technique to investigate the micellization process and intermolecular interactions from a thermodynamic aspect,21−24 and it is a direct way to evaluate the interaction strength of surfactants in the micelles. The calorimetric studies can also give the corresponding thermodynamic parameters such as changes of the enthalpy (ΔH), entropy (ΔS) and Gibbs energy (ΔG) of micellization which are crucial to understanding the micellization process. To the best of our knowledge, most of studies Received: September 25, 2015 Accepted: January 13, 2016

A

DOI: 10.1021/acs.jced.5b00824 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

Article

where a 40.29 mmol·kg−1 NaBr aqueous solution or surfactant/ 40.29 mmol·kg−1 NaBr solution was loaded in advance. The volume of each injection was 10 μL. In the study of the formation of mixed micelles, the titrations were carried out in three ways. “Type I” titrations correspond to the addition of TX100/DTAB mixed micelles with certain molar ratios of TX100 to DTAB into NaBr solution; in “type II” titrations, DTAB aqueous solution was added to NaBr solution containing different TX100 concentrations; in “type III” titrations, TX100 aqueous solution was added to NaBr solution containing different DTAB concentrations. The stirring speed in the sample cell was set at 60 rpm, and the experiment temperature was 25.00 °C with the standard uncertainty of 0.02 °C. The values of the observed differential enthalpy (ΔHobs) for various concentrations of surfactants were obtained by the integral of the areas under the calorimetric peaks, and normalized by the small amounts of injected surfactants.

have been focused on the cmc and excess free energy of mixed surfactant solutions; and the studies of excess enthalpy25−27 and entropy26,27 are scarce. However, excess enthalpy and excess entropy are important thermodynamic properties which reflect the differences of the interactions of molecules in the mixed micelles. In previous studies, we developed a method to deduce the excess enthalpy from the micellization enthalpies of a mixed micelle system at various compositions determined from ITC.26,27 In this paper, mixed surfactants of TX100/DTAB have been studied by ITC. On the basis of the pseudophase separation model and using the two-parameter Margules equation, the composition of mixed micelles and activity coefficient of each surfactant in mixed micelles are obtained, which are used to calculate the thermodynamic parameters of micellization and excess properties of the mixed surfactants in the micelles. Moreover, the tendency of surfactants to form mixed micelles and the interactions between TX100 and DTAB are discussed.

3. RESULTS AND DISCUSSION 3.1. Single Surfactant. The enthalpy curves of 71.76 mmol·kg−1 DTAB or 14.17 mmol·kg−1 TX100 titration into NaBr solution are shown in Figure 1a. Both the curves have the

2. EXPERIMENTAL DETAILS 2.1. Materials. Table 1 lists the suppliers, purities, and stored method of cationic surfactant dodecyltrimethylammoTable 1. Suppliers, Purities, and Stored Method chemical name DTAB TX100 NaBr

suppliers

purity

stored method

J&K chemical LTD Fluka Tianjin Sitong Chemical Company

mass fraction ≥ 99% n20 D = 1.490−1.494 mass fraction ≥ 99.0%

stored in a desiccator over P2O5

nium bromide (DTAB), nonionic surfactant polyoxyethylene tert-octyl phenyl ether (TX100), and sodium bromide (NaBr). All materials were used without further purification. Twice distilled water was used in preparations of the solutions with various surfactant compositions, which were characterized by the TX100 mole fractions in the total mixed surfactants (α1) and the TX100 mole fractions in the mixed micelles (x1). 2.2. Isothermal Titration Microcalorimetry (ITC). The isothermal titration data were collected by using the TAM 2277−201 microcalorimetric system (Thermometric AB, Järfäfla, Sweden), a schematic diagram of the ITC apparatus is shown in Scheme 1. This apparatus has 4 mL sample and reference cells and the standard uncertainty in measurement of the heat flow is about 7.5 nW. A titration was conducted by injection of a concentrated surfactant or mixed surfactant aqueous solution in a Hamilton syringe into the sample cell Scheme 1. Schematic Diagram of the ITC Apparatus Figure 1. (a) Observed differential enthalpies as a function of surfactant molalities for titrating 14.17 mmol·kg−1 TX100 or 71.76 mmol·kg−1 DTAB into 40.29 mmol·kg−1 NaBr aqueous solutions. (b) Determination of cmc and ΔHmic.

sigmoid shapes with abrupt changes at threshold concentrations corresponding to the micelle formations. The critical micelle concentration (cmc) may be determined by extrapolations of the initial linear portion and the linear rapidly change portion of the enthalpy curve (see Figure 1b).28,29 This was done by linear fittings of above two straight lines and calculation of their intersection. With the assumption that the monomers in the B



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Table 2. Values of cmc, Enthalpies of Micellization (ΔHmic), Gibbs Free Energies of Micellization (ΔGmic), Entropies of Micellization (ΔSmic) and Their Standard Uncertainties (u) for TX100 and DTAB in 40.29 mmol·kg−1 NaBr Aqueous Solutions at 298.15 K and Pressure p = 85 kPaa u(cmc)

cmc surfactant TX100 DTAB a

mmol·kg

−1

0.190 5.990

mmol·kg 0.033 0.049

−1

ΔHmic −1

u(ΔHmic) −1

kJ·mol

kJ·mol

8.02 −2.16

0.04 0.03

ΔGmic −1

kJ·mol

−21.25 −22.45

u(ΔGmic) kJ·mol 0.20 0.21

−1

ΔSmic J·(mol·K) 98.16 68.07

u(ΔSmic) −1

·J·(mol·K)−1 0.02 0.01

Standard uncertainties of temperature and pressure are u(T) = 0.02 K and u(p) = 3 kPa, respectively.

concentrated surfactant solutions in the syringe may be negligible, the molar enthalpy ΔHmic of micellization was determined from the difference of the ΔHobs between the two linear segments of the plot at the cmc, demonstrated also in Figure 1b. The molar standard Gibbs free energy of micellization (ΔGmic), which represents the transfer of one mole of surfactant from the aqueous phase to the micellar pseudophase, is calculated by the expression ΔGmic = (1 + f )RT ln cmc

(1)

where f stands for the fraction of charges of micellized ions neutralized by counterions, with a value of 0.77 for DTAB30 and 0 for nonionic surfactant TX100. The entropy of micellization (ΔSmic) can be obtained from ΔSmic =

ΔHmic − ΔGmic T

(2)

The values of cmc, ΔHmic, ΔGmic, and ΔSmic including their standard uncertainties are listed in Table 2. These standard uncertainties were estimated by the standard uncertainties of the slopes and intercepts from fitting the straight lines described above and their corresponding error propagations. The cmc of TX100 are much smaller than that of DTAB, which may be because the hydrophobicity of TX100 is much stronger than that of DTAB. For DTAB, the values of ΔHmic and ΔGmic are all negative, while the value of ΔSmic is positive, which are caused by disrupting the water structure during micellization. These results indicate that the micellization is driven by entropy and enthalpy simultaneously. However, for TX100, the values of both ΔHmic and ΔSmic are positive, which shows that the micellization is driven by entropy only. 3.2. Mixed Surfactants. 3.2.1. Titration of TX100/DTAB into NaBr Aqueous Solution. (1). Micellization of Mixed Surfactants. The enthalpy curves of titrating TX100/DTAB into NaBr solutions are shown in Figure 2. When α1 = 0.07 and 0.17, the observed enthalpy first increases then decreases with the addition of surfactant. As it may be seen in Table 2, the values of ΔHmic are positive for pure TX100 micellization but negative for pure DTAB micellization, the bell-shaped curves indicate that the pure TX100 micelles or TX100-rich mixed micelles form first since TX100 is more surface-active, then DTAB molecules incorporate into those micelles, which are consistent with those reported in literature.17−20 On further increase of the total surfactant concentration, DTAB molecules dissolve in the TX100 mixed micelles contiguously, then DTAB-rich mixed micelles may form. The amounts of DTAB molecules entering into the TX100 micelles will depend on the packing characteristics of the two surfactants.19 The abrupt change of ΔHobs occurs in the each of the initiate parts of the two titration curves with the values of α1 being 0.07 and 0.17, and the corresponding points are determined as the first critical

Figure 2. Observed differential enthalpies as a function of the total surfactant molalities mtotal for titrating TX100/DTAB mixed micelles into NaBr aqueous solutions. The concentrations of surfactants in the syringe are (a) 77.79 mmol·kg−1, (b) 42.78 mmol·kg−1, (c) 21.24 mmol·kg−1, (d) 15.45 mmol·kg−1, (e) 20.15 mmol·kg−1, (f) 16.28 mmol·kg−1, (g) 14.15 mmol·kg−1, (h) 6.34 mmol·kg−1, respectively; α1 is TX100 mole fraction in the total mixed surfactants.

micelle concentrations (cmcmix1) (see Figure 2a). In this part, the enthalpy of micellization is endothermic, as the micellization process of pure TX100. After certain total surfactant concentrations the enthalpies decrease, because more DTAB molecules participate in the mixed micelles. The surfactant concentrations at which the enthalpies start to decrease are defined as the second critical micelle concentrations (cmcmix2) (see Figure 2a). The second critical micelle concentrations were also reported by Du,26 Sharma,31 Moulik,32,33 and Hao10 for different mixed surfactant systems. Sharma31 and Moulik32,33 thought the formation of the second type of micelles beyond the first cmc is the manifestation of a structural change. By analyzing the systems of mixed surfactants that have two kinds of micelles, it was found that there was a common point: the difference of surface activities or the cmc values between two surfactants was relatively large. Hence, the surfactant of the lower cmc was absolutely dominant in the micelles just above cmcmix1; then more and more others entered the aggregates as the total surfactant concentration increased, resulting in variations of the composition in mixed micelles and possibly a structure change of the mixed micelles corresponding to a microphase titration. When α1 is larger than 0.17, as shown in Figure 2c−h, it seems that only TX100-rich mixed micelles form and no DTAB-rich micelles may be observed; thus the shape of all enthalpy curves is similar to that of pure TX100. C



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Table 3. Experimental cmc, cmc of Ideal Mixing (Cm), and their Standard Uncertainties (u) for TX100/DTAB Mixed Micelles in 40.29 mmol·kg−1 NaBr Aqueous Solutions with Various TX100 Mole Fractions in the Total Mixed Surfactants (α1) at 298.15 K and Pressure p = 85 kPaa u(cmcmix1)

cmcmix1 α1

mmol·kg

0.07 0.17 0.29 0.39 0.50 0.63 0.72 0.83 a

−1

mmol·kg

1.169 0.636 0.412 0.322 0.273 0.233 0.213 0.189

−1

0.043 0.032 0.036 0.040 0.032 0.033 0.023 0.024

u(cmcmix2)

cmcmix2 mmol·kg

−1

mmol·kg

4.215 2.729

Cm

−1

u(Cm)

mmol·kg

0.034 0.033

−1

mmol·kg−1

1.833 0.982 0.615 0.461 0.366 0.298 0.261 0.226

0.053 0.043 0.045 0.043 0.038 0.035 0.031 0.029


Table 4. TX100 Mole Fractions in the Mixed Micelles (x1), Enthalpies of Micellization (ΔHmic), Gibbs Free Energies of Micellization (ΔGmic), Entropies of Micellization (ΔSmic) and their Standard Uncertainties (u) for TX100/DTAB Mixed Micelles in 40.29 mmol·kg−1 NaBr Aqueous Solutions with Various TX100 Mole Fractions in the Total Mixed Surfactants (α1) at 298.15 K and Pressure p = 85 kPaa ΔHmic

a

α1

x1

u(x1)

kJ·mol

0.07 0.17 0.29 0.39 0.50 0.63 0.72 0.83

0.768 0.809 0.838 0.856 0.871 0.887 0.900 0.920

0.005 0.003 0.002 0.001 0.001 0.001 0.001 0.001

1.03 1.72 2.25 3.83 4.63 5.72 6.02 6.55

−1

ΔGmic

u(ΔHmic) −1

kJ·mol

kJ·mol

−1

−23.18 −23.16 −23.15 −23.12 −23.02 −22.95 −22.87 −22.79

0.05 0.12 0.18 0.20 0.21 0.21 0.23 0.23

ΔSmic

u(ΔGmic) kJ·mol

−1

J·(mol·K)

0.14 0.16 0.14 0.15 0.16 0.13 0.17 0.16

81.20 83.45 85.21 90.40 92.72 96.15 96.92 98.40

u(ΔSmic) −1

J·(mol·K)−1 0.18 0.15 0.17 0.20 0.17 0.21 0.21 0.18


The values of cmcmix1 and cmcmix2 for all α1 are listed in Table 3. From Table 3, it is known that the values of cmcmix1 and cmcmix2 decrease with increasing α1, suggesting that the addition of TX100 facilitates the formation of mixed micelles. The decrease of cmcmix1 may occur because the properties of the mixed micelles approach that of the TX100 micelle as α1 increases; while the decrease of cmcmix2 may be attributed to the decrease of the electrostatic repulsion between the surfactant headgroups by adding the nonionic surfactant into the charged micelles.31 The ideality behaviors of two-surfactant solutions can be expressed in terms of the Clint’s equation7 α1 1 − α1 1 = + Cm cmc1 cmc 2

where xi and cmci are the mole fractions in the mixed micelle and the cmc value of component i (i = 1 for TX100 and i = 2 for DTAB). The activity coefficients γi can be expressed as a function of xi and the parameters A and B by the two-parameter Margules equation: (5)

⎛ 3 ⎞ ln γ2 = ⎜A + B⎟x12 − Bx13 ⎝ 2 ⎠

(6)

The combination of eqs 4−6 yields α1cmc mix1 = exp(Ax 2 2 + Bx 2 3)x1cmc1

(7)

⎡⎛ 3 ⎞ ⎞ (1 − α1)cmc mix1 = exp⎢⎜A + B⎟x12⎟ − Bx13]x 2cmc 2 ⎣⎝ 2 ⎠ ⎠

(3)

where cmc1 and cmc2 are the critical micelle concentrations of TX100 and DTAB, respectively. The values of Cm for the mixed surfactants TX100/DTAB were calculated by eq 3 and are summarized in Table 3. As shown in Table 3, all values of cmcmix1 are smaller than the values of Cm calculated through the Clint’s equation, indicating negative deviations from the ideal mixing behavior in the mixed micelles. According to the pseudo-phase-separation model, for the formation of nonideal binary mixed micelles at the cmcmix1, there is a simple relation between cmcmix1 and cmc for each pure surfactant: αi cmc mix1 = γixi cmci

ln γ1 = Ax 22 + Bx 23

(8)

The cmcmix1 values for various α1 listed in Table 3 were used to fit eqs 7 and 8 to obtain the parameters A, B, and x1. The values of A and B are −13.80 and 18.29 with standard uncertainties of 0.61 and 1.00, respectively; and the optimized values of x1 with their standard uncertainties from fitting are given in Table 4. It is found that even at very low α1, the mixed micelle is TX100 dominant; however, the value of x1 increases with the increasing of α1, indicating that the more TX100 in the system there is, the more TX100 participates in mixed micelles. The experimental cmc values are compared with the ones predicted from the Margules model and Clint ideal model as shown in Figure 3. It is found that the two parameter Margules

(4) D



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model is well fitted with the experimental data, and the mixing behavior departs from the ideal mixing, showing a synergetic effect.

rich micelles exist in the system. The structure of TX100 micelles consists of a tightly packed deep core of hydrocarbon and a hydrated POE shell occupying the major part of the volume of the micelle.35 As the cationic component in the mixed micelles increases, the increasing charge density leads to the ion-dipole interaction between the headgroup of DTAB and the ethylene oxide group of TX100, which results in the release of the hydrogen-bonded or trapped water molecules from the PEO shell of TX100; thus, a more closed and dehydrated structure forms,20 and the release of the hydrogen-bonded or trapped water molecules causes an entropy increase, and hence, a decrease of Gibss free energy and increase of the stability of the mixed micelle. It is worth noting that both the values of ΔGmic for pure TX100 and DTAB (see Table 2) are significantly larger than that of mixed micelles near x = 0.76 shown in Figure 4, indicating a significant synergetic effect. Moreover, it may be predicted that there exists a critical composition in the lower x region at which ΔGmic/RT has a minimum value corresponding to the most stable micelle state. (2). Excess Properties of Mixed Surfactants. The excess molar enthalpy, excess Gibbs free energy, and excess entropy of mixed surfactants in the micelles can be calculated by eqs 10−12, respectively21,22

Figure 3. Comparison of the predicted and experimental cmcmix1 values of TX100/DTAB aqueous solutions.

Assuming that the mixed micelles are made up of N surfactant monomers, including x1N TX100 monomers, (1−x1) N DTAB monomers and f (1−x1)N counterions, namely bromine ions; the standard Gibbs free energy of such micellization may be calculated by ΔG = RT[x1 ln(α1cmc) + (1 − x1) ln((1 − α1)cmc) + f (1 − x1) ln(cmc + C NaBr)]

(9)

HE = ΔHmic − x1ΔHmic,1 − (1 − x1)ΔHmic,2

(10)

GE = RT[x1 ln γ1 + (1 − x1) ln γ2]

(11)

S E = (HE − GE)/T

(12)

where γ1 and γ2 can be calculated through eqs 5 and 6. All the values of γ1, γ2, HE, GE, and SE are summarized in Table 5. From Table 5, it is found that the values of γ1 increases and γ2 decreases with the increasing of α1. The activity coefficients reflect the intensity of intermolecular interactions. The values of HE and SE increase together with the increase of α1; the compensation between the excess enthalpy and the excess entropy results in a small negative GE for the mixed micelle in all compositions. The negative values of GE indicate that the Gibbs free energy of micelle formation of TX100/DTAB mixed micelles is more negative than that of the ideal mixing micelles. It suggests that the interaction between TX100 and DTAB enhances the stability of the mixed micelles, consistent with the result discussed above. 3.2.2. Titration of DTAB into TX100/NaBr Aqueous Solution. To further understand the formation mechanism of mixed micelles, “type II” titration, namely titrating 71.76 mmol· kg−1 DTAB into NaBr solutions with different TX100 concentrations was applied to study the intermolecular interaction of TX100 and DTAB. Figure 5 shows the titration of DTAB into NaBr solution with different initial TX100 concentrations (C0TX100). For C0TX100 = 0.05 mmol·kg−1, the TX100 concentration is less than its cmc, TX100 monomers only exist in the solution. The observed enthalpy curve is similar to that of TX100/DTAB titration into NaBr solution with α1 = 0.07. When the DTAB concentration reaches about 1.56 mmol·kg−1 (cmcmix1), the enthalpy increases abruptly, the TX100-rich micelles form; as DTAB concentration increases to about 6.36 mmol·kg−1 (cmcmix2), DTAB-rich micelles start to form. For C0TX100 = 5.64, 14.17, and 28.60 mmol·kg−1, the TX100 concentrations are all above its cmc, the TX100 monomers and micelles coexist in solutions. In this case, when DTAB is titrated into a TX100 solution, the DTAB micelles disassociate into monomers; and some monomers stay in bulk

where CNaBr is the concentration of NaBr. The values of ΔHmic are obtained through the method shown in Figure 1b; the values of the entropy of micellization ΔSmic were calculated by eq 2. All values of ΔGmic, ΔHmic, and ΔSmic together with their estimated standard uncertainties are summarized in Table 4. As known from Table 4, all the values of ΔGmic are negative and the values of ΔHmic and ΔSmic are all positive, indicating that the micellization of TX100/DTAB mixed surfactants is mainly driven by the entropy. The values of ΔHmic increase with the increasing of ΔSmic, revealing the compensation of enthalpy and entropy. According to the Maeda model,34 the stability of the mixed micelle can be described by ΔGmic/RT. The dependence of ΔGmic/RT on the micellar composition is shown in Figure 4 for the composition range we studied, from which it is found that the stability of the mixed micelles is enhanced as x1 decreases. In this composition range, as we described above, only TX100-

Figure 4. Plots of the stability of TX100/DTAB mixed micelles versus the TX100 mole fractions x1 in the mixed micelles. E



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Table 5. Values of Activity Coefficients (γ1) and (γ2), Excess Enthalpy (HE), Excess Gibbs Free Energy (GE), Excess Entropy (SE), and their Standard Uncertainties (u) for TX100/DTAB Mixed Micelles in 40.29 mmol·kg−1 NaBr Aqueous Solutions with Various TX100 Mole Fractions in the Total Mixed Surfactants (α1) at 298.15 K and Pressure p = 85 kPaa HE

a

u(HE) −1

−1

GE

u(GE) −1

−1

SE

u(SE) −1

α1

γ1

u(γ1)

γ2

u(γ2)

kJ·mol

kJ·mol

kJ·mol

kJ·mol

J·(mol·K)

0.07 0.17 0.29 0.39 0.50 0.63 0.72 0.83

0.60 0.69 0.75 0.79 0.83 0.86 0.89 0.92

0.05 0.04 0.05 0.06 0.04 0.05 0.05 0.06

0.79 0.47 0.31 0.23 0.18 0.13 0.10 0.07

0.04 0.03 0.04 0.03 0.02 0.02 0.02 0.01

−4.63 −4.35 −4.12 −2.72 −2.07 −1.15 −0.98 −0.66

0.34 0.28 0.31 0.24 0.22 0.14 0.11 0.08

−1.12 −1.11 −1.07 −1.02 −0.97 −0.90 −0.83 −0.71

0.08 0.08 0.09 0.07 0.11 0.06 0.07 0.04

−11.78 −10.87 −10.22 −5.69 −3.71 −0.85 −0.48 0.18

J·(mol·K)−1 0.31 0.33 0.27 0.15 0.12 0.01 0.01 0.01


Figure 5. Observed differential enthalpies as a function of DTAB molalities for titrating 71.76 mmol·kg−1 DTAB into 40.29 mmol·kg−1 NaBr aqueous solutions with different TX100 concentrations.

Figure 6. Observed differential enthalpies as a function of TX100 molalities for titrating 5.03 mmol·kg−1 TX100 into 40.29 mmol·kg−1 NaBr aqueous solutions with different DTAB concentrations.

solution, some participate in TX100 micelles to form TX100/ DTAB micelles. It is worth noting that the values of observed enthalpy are much larger than those of the DTAB titration into NaBr solution, with the initial TX100 concentration being 0 or 0.05 mmol·kg−1, and increase with the initial TX100 concentration. In other words, the more TX100 there is in solution, the more endothermic it is, indicating that DTAB participation in TX100 micelles needs to absorb energy, which may result from overcoming the steric hindrance of the hydrophobic chain when DTAB monomers enter into the TX100 micelles; thus, this process may be driven by entropy. The observed enthalpy decreases with the increase of DTAB, which may occur because the hydrophobic interaction between TX100 and DTAB molecules increases with the increase of DTAB dissolved in the TX100/DTAB mixed micelles. This may be thought of as the change of synergistic effect with the composition of mixed micelles. 3.2.3. Titration of TX100 into DTAB/NaBr Aqueous Solution. The third titration method is titrating 5.03 mmol· kg−1 TX100 into NaBr solution with different DTAB concentrations. The enthalpy curves of titrating TX100 into NaBr solution with different initial DTAB concentrations (C0DTAB) are given in Figure 6. For C0DTAB = 1.51, 4.52 mmol· kg−1, the DTAB concentrations in the sample cell are less than its cmc value; hence there are only DTAB monomers in the solutions. In each of the two cases, after TX100 is added into the DTAB aqueous solution the enthalpy changes slowly at first, then rapidly increases with the TX100 concentration after the total surfactant concentration and α1 reach 1.56 mmol·kg−1 and 0.064 for C0DTAB = 1.51 mmol·kg−1, and 4.50 mmol·kg−1

and 0.008 for C0DTAB = 4.52 mmol·kg−1; which indicate the formation of mixed micelles. Although the TX100 concentrations in the above two cases are much less than its cmc (0.190 mmol·kg−1), the strong surface activity of TX100 and the synergistic effect of TX100 and DTAB result in the formation of TX100-rich mixed micelles at very low concentration of TX100. For C0DTAB = 8.04, 12.80 mmol·kg−1, the DTAB concentrations in the sample cell are above its cmc; the DTAB monomers and DTAB micelles coexist in these solutions. For C0DTAB = 8.04 mmol·kg−1, the value of α1 at the first titration point is 0.003. The shape of the enthalpy curve is similar to that of TX100 titrating into NaBr solution, but the value of enthalpy is larger than those of C0DTAB = 1.51, 4.52 mmol·kg−1 at the same TX100 concentration, which may occur because some TX100 molecules form TX100-rich mixed micelles with DTAB monomers, and some TX100 molecules join in the DTAB micelles to form DTAB-rich mixed micelles, and participation of TX100 molecules in the DTAB-rich mixed micelles results in larger increase of enthalpy. For C0DTAB = 12.08 mmol·kg−1, at the first titration point, the value of α1 is 0.002, the mole fraction of TX100 is too low to form the TX100-rich mixed micelles, and TX100 molecules only dissolve in the DTAB-rich micelles. The enthalpy decreases with the increase of TX100 concentration, which may be because after more TX100 molecules join in the mixed micelles, the hydrophobic environment of the micelles becomes somewhat more accommodating to the additional TX100 molecules. A comparison of the enthalpy curves for C0DTAB = 8.04 mmol· F



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kg−1 and C0DTAB = 12.08 mmol·kg−1 shows that the enthalpy for C0DTAB = 12.08 mmol·kg−1 is larger than that for C0DTAB = 8.04 mmol·kg−1, which may be explained by that only part of added TX100 molecules enter into the DTAB-rich mixed micelles for C0DTAB = 8.04 mmol·kg−1. The participation of TX100 in the DTAB-rich mixed micelles causes a larger increase of enthalpy as compared with its participation in the TX100-rich micelles, because participation of TX100 in the DTAB-rich mixed micelles must overcome the steric hindrance of their hydrophobic chains.

Funding

CONCLUSION Isothermal titration calorimetry was used to investigate mixed surfactants of TX100 and DTAB. For TX100/DTAB titration into NaBr solution, it was observed that at low TX100 mole fraction, TX100-rich mixed micelles formed immediately when the surfactant concentration reached cmcmix1, and then more and more DTAB monomers dissolved in TX100 micelles to form DTAB-rich mixed micelles; at higher TX100 mole fraction, only TX100-rich mixed micelles formed. For DTAB titration into TX100/NaBr solution, when the initial TX100 concentration was lower than its cmc, TX100-rich mixed micelles formed first, and then transferred to DTAB-rich micelles as further DTAB was added; however, when the initial TX100 concentration was higher than its cmc, the DTAB monomers participated with TX100 micelles to form TX100rich mixed micelles. For TX100 titration into DTAB/NaBr solution, when initial DTAB concentration was low, only TX100-rich mixed micelles formed; however, when initial DTAB concentration was about twice its cmc, the TX100 monomers were dissolved into DTAB micelles to form DTABrich mixed micelles. The two-parameter Margules equation was successfully used to determine the compositions of mixed micelles, activity coefficient of each surfactant, and excess properties of mixed surfactants in the mixed micelles. It was found that the micellization of TX100/DTAB mixed surfactants was mainly driven by entropy, and ΔHmic increased with the increase of ΔSmic, revealing the compensation of enthalpy and entropy. The values of excess enthalpy HE and excess entropy SE increased with the increasing of α1, the compensation between HE and SE resulted in a small negative GE for the mixed micelles in all compositions, indicating that the interaction between TX100 and DTAB enhanced the stability of the mixed micelles. It is hoped that this study can provide a fundamental understanding of the mixing behaviors of ionic/nonionic surfactant aqueous solutions, and help to further design TX100/DTAB aqueous solutions for their desirable applications.

(1) Puvvada, S.; Blankschtein, D. Theoretical and Experimental Investigations of Micellar Properties of Aqueous Solutions Containing Binary Mixtures of Nonionic Surfactants. J. Phys. Chem. 1992, 96, 5579−5592. (2) Holland, P. M.; Rubingh, D. N., In Mixed Surfactant Systems. ACS Symposium Series; American Chemical Society: Washington, DC, 1992; Chapters 1, 3. (3) Fan, H.; Li, B.; Yan, Y.; Huang, J.; Kang, W. Phase Behavior and Microstructures in a Mixture of Anionic Gemini and Cationic Surfactants. Soft Matter 2014, 10, 4506−4512. (4) Das, S.; Maiti, S.; Ghosh, S. Synthesis of Two Biofriendly Anionic Surfactants (N-n-decanoyl-L-valine and N-n-decanoyl-L-leucine) and Their Mixed Micellization with Nonionic Surfactant Mega-10 in Trisbuffer Medium at pH 9. RSC Adv. 2014, 4, 12275−12286. (5) Wang, R.; Li, Y. Micellar and Interfacial Behavior in Mixtures of Dodecyldiethoxylamine Oxide with Ionic Surfactants. J. Chem. Eng. Data 2013, 58, 2240−2247. (6) Shinoda, K. The Critical Micelle Concentration of Soap Mixtures (Two-Component Mixture). J. Phys. Chem. 1954, 58, 541−548. (7) Clint, J. H. Micellization of Mixed Nonionic Surface Active Agents. J. Chem. Soc., Faraday Trans. 1 1975, 71, 1327−1334. (8) Rubingh, D. N. In Solution Chemistry of Surfactants; Mittal, K., Ed.; Plenum: New York, 1979; Vol. 1, p 337. (9) Hu, J.; Zhou, L.; Feng, J.; Liu, H.; Hu, Y. Nonideal Mixed Micelles of Gemini Surfactant Homologues and Their Application as Templates for Mesoporous Material MCM-48. J. Colloid Interface Sci. 2007, 315, 761−767. (10) Hao, L.-S.; Deng, Y.-T.; Zhou, L.-S.; Ye, H.; Nan, Y.-Q.; Hu, P. Mixed Micellization and the Dissociated Margules Model for Cationic/Anionic Surfactant Systems. J. Phys. Chem. B 2012, 116, 5213−5225. (11) Izumrudov, V. A.; Zhiryakova, M. V.; Goulko, A. A. Ethidium Bromide as a Promising Probe for Studying DNA Interaction with Cationic Amphiphiles and Stability of the Resulting Complexes. Langmuir 2002, 18, 10348−10356. (12) Matulis, D.; Rouzina, I.; Bloomfield, V. A. Thermodynamics of Cationic Lipid Binding to DNA and DNA Condensation: Roles of Electrostatics and Hydrophobicity. J. Am. Chem. Soc. 2002, 124, 7331− 7342. (13) Altschuler, M.; Heddens, D. K.; DiveleyJr, R. R.; Kresheck, G. C. Plasmid DNA isolation utilizing a novel nonionic detergent. Biotechniques 1994, 17, 434−436. (14) Harendra, S.; Vipulanandan, C. Solubilization and degradation of Perchloroethylene (PCE) in Cationic and Nonionic Surfactant Solutions. J. Environ. Sci. 2011, 23, 1240−1248. (15) Guha, S.; Jaffe, P. R.; Peters, C. A. Solubilization of PAH Mixtures by a Nonionic Surfactant. Environ. Sci. Technol. 1998, 32, 930−935. (16) Hoffmann, H.; Pössnecker, G. The Mixing Behavior of Surfactants. Langmuir 1994, 10, 381−389. (17) Gharibi, H.; Razavizadeh, B. M.; Hashemianzaheh, M. New Approach for the Studies of Physicochemical Parameters of Interaction of Triton X-100 with Cationic surfactants. Colloids Surf., A 2000, 174, 375−386. (18) Razavizadeh, B. M.; Mousavi-Khoshdel, M.; Gharibi, H.; Behjatmanesh-Ardakani, R.; Javadian, S.; Sohrabi, B. Thermodynamic Studies of Mixed Ionic/Nonionic Surfactant Systems. J. Colloid Interface Sci. 2004, 276, 197−207.

This work was supported by the National Natural Science Foundation of China (Projects 21173080, 21373085 and 21403098), the Fundamental Research Funds for the Central Universities (lzujbky-2014-181), and Natural Science Foundation of Gansu province (1506RJYA237). Notes

The authors declare no competing financial interest.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jced.5b00824. Tables of enthalpies and molalities for different titration methods (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions #

P.Z. and X.Z. contributed equally. G

REFERENCES



Article

(19) Gharibi, H.; Javadian, S.; Sohrabi, B.; Behjatmanesh, R. Investigation of Interaction Parameters in Mixed Micelle Using Pulsed Field Gradient NMR Spectroscopy. J. Colloid Interface Sci. 2005, 285, 351−359. (20) Ruiz, C. C.; Aguiar, J. Interaction, Stability, and Microenvironmental Properties of Mixed Micelles of Triton X100 and nAlkyltrimethylammonium Bromides: Influence of Alkyl Chain Length. Langmuir 2000, 16, 7946−7953. (21) Bai, G.; Wang, Y.; Wang, J.; Han, B.; Yan, H. Microcalorimetric Studies of the Interaction between DDAB and SDS and the Phase Behavior of the Mixture. Langmuir 2001, 17, 3522−3525. (22) Norvaišas, P.; Petrauskas, V.; Matulis, D. Thermodynamics of Cationic and Anionic Surfactant Interaction. J. Phys. Chem. B 2012, 116, 2138−2144. (23) Guillemet, F.; Piculell, L. Interactions in Aqueous Mixtures of Hydrophobically Modified Polyelectrolyte and Oppositely Charged Surfactant. Mixed Micelle Formation and Associative Phase Separation. J. Phys. Chem. 1995, 99, 9201−9209. (24) Liu, Z.; Fan, Y.; Tian, M.; Wang, R.; Han, Y.; Wang, Y. Surfactant Selection Principle for Reducing Critical Micelle Concentration in Mixtures of Oppositely Charged Gemini Surfactants. Langmuir 2014, 30, 7968−7976. (25) Meagher, R. J.; Hatton, T. A.; Bose, A. Enthalpy Measurements in Aqueous SDS/DTAB Solutions Using Isothermal Titration Microcalorimetry. Langmuir 1998, 14, 4081−4087. (26) Du, C.; Cai, D.; Qin, M.; Zheng, P.; Hao, Z.; Yin, T.; Zhao, J.; Shen, W. Thermodynamics of Mixed Surfactant Solutions of N,N′Bis(dimethyldodecyl)-1,2-ethanediammoniumdibromide with 1-Dodecyl-3-methylimidazolium Bromide. J. Phys. Chem. B 2014, 118, 1168−1179. (27) Cai, D.; Du, C.; Yin, T.; Zheng, P.; Shen, W. Thermodynamic and Microscopic Properties of AOT/Dynol-604 Mixed Micelles in Aqueous Solutions. Fluid Phase Equilib. 2015, 387, 50−58. (28) Van Os, N. M.; Daane, G. J.; Haandrikman, G. The effect of Chemical Structure upon the Thermodynamics of Micellization of Model Alkylarenesulfonates: III. Determination of the Critical Micelle Concentration and the Enthalpy of Demicellization by Means of Microcalorimetry and a Comparison with the Phase Separation Model. J. Colloid Interface Sci. 1991, 141, 199−217. (29) Bai, G. Y.; Wang, Y. J.; Yan, H. K.; Thomas, R. K.; Kwak, J. C. T. Thermodynamics of Interaction between Cationic Gemini Surfactants and Hydrophobically Modified Polymers in Aqueous Solutions. J. Phys. Chem. B 2002, 106, 2153−2159. (30) Majhi, P. R.; Moulik, S. P. Energetics of Micellization: Reassessment by a High-Sensitivity Titration Microcalorimeter. Langmuir 1998, 14, 3986−3990. (31) Sharma, K. S.; Patil, S. R.; Rakshit, A. K.; Glenn, K.; Doiron, M.; Palepu, R. M.; Hassan, P. A. Self-Aggregation of a Cationic Nonionic Surfactant Mixture in Aqueous Media: Tensiometric, Conductometric, Density, Light Scattering, Potentiometric, and Fluorometric Studies. J. Phys. Chem. B 2004, 108, 12804−12812. (32) Prasad, M.; Moulik, S. P. Self-Aggregation of Alkyl (C10-, C12-, C14-, and C16-) Triphenyl Phosphonium Bromides and Their 1:1 Molar Mixtures in Aqueous Medium: A Thermodynamic Study. J. Phys. Chem. B 2004, 108, 355−362. (33) Ray, G. B.; Chakraborty, I.; Ghosh, S.; Moulik, S. P.; Palepu, R. Self-Aggregation of Alkyltrimethylammonium Bromides (C10-, C12-, C14-, and C16TAB) and Their Binary Mixtures in Aqueous Medium: A Critical and Comprehensive Assessment of Interfacial Behavior and Bulk Properties with Reference to Two Types of Micelle Formation. Langmuir 2005, 21, 10958−10967. (34) Maeda, H. A simple Thermodynamic Analysis of the Stability of Ionic/Nonionic Mixed Micelles. J. Colloid Interface Sci. 1995, 172, 98− 105. (35) Robson, R. J.; Dennls, E. A. The Size, Shape, and Hydration of Nonionic Surfactant Micelles. Triton X-100. J. Phys. Chem. 1977, 81, 1075−1078.

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