Surfactant Selection Principle for Reducing Critical Micelle

Jun 16, 2014 - Weiwei Zhao , Kai Song , Yao Chen , Hua Wang , Zhang Liu , Qiang Shi , Jianbin Huang , and Yilin Wang. Langmuir 2017 33 (44), 12719- ...
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Surfactant Selection Principle for Reducing Critical Micelle Concentration in Mixtures of Oppositely Charged Gemini Surfactants Zhang Liu, Yaxun Fan, Maozhang Tian, Ruijuan Wang, Yuchun Han, and Yilin Wang* Key Laboratory of Colloid and Interface Science, Beijing National Laboratory for Molecular Sciences (BNLMS), Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, P.R. China S Supporting Information *

ABSTRACT: Cationic quaternary ammonium gemini surfactants CnH2n+1(CH3)2N+CH2CHCHCH2(CH3)2N+CnH2n+12Br− (CnC4Cn, n = 12, 8, 6) with alkyl spacers, CnH2n+1(CH3)2N+CH2CHOHCHOHCH2(CH3)2N+CnH2n+12Br− (CnC4(OH)2Cn, n = 12, 8, 6, 4) with two hydroxyl groups in alkyl spacers, and cationic ammonium single-chain surfactants CnH2n+1(CH3)2N+Br− (CnTAB, n = 12, 8, 6) have been chosen to fabricate oppositely charged surfactant mixtures with anionic sulfonate gemini surfactant C12H25N(CH2CH2CH2SO3−)CH2CH2CH2(CH3)2N(CH2CH2CH2SO3−)C12H252Na (C12C3C12(SO3)2). Surface tension, electrical conductivity, and isothermal titration microcalorimetry (ITC) were used to study their surface properties, aggregation behaviors, and intermolecular interactions. The mixtures of C12C3C12(SO3)2/CnC4(OH)2Cn (n = 12, 8) and C12C3C12(SO3)2/C12C4C12 show anomalous larger critical micelle concentration (CMC) than C12C3C12(SO3)2, while the mixtures of C12C3C12(SO3)2/CnC4(OH)2Cn (n = 6, 4), C12C3C12(SO3)2/CnC4(OH)2Cn (n = 6, 4), and C12C3C12(SO3)2/ CnTAB (n = 12, 8, 6) exhibit much lower CMC than C12C3C12(SO3)2. The results indicate that strong hydrophobic interactions between the alkyl chains assisted by strong electrostatic attractions between the headgroups and hydrogen bonds between the spacers lead to the formation of less surface active premicellar aggregates in bulk solution, resulting in the increase of CMC. If these interactions are weakened or inhibited, less surface active premicellar aggregates are no longer formed in the mixtures, and thus the CMC values are reduced. The work reveals that the combination of two surfactants with great self-assembling ability separately may have strong intermolecular binding interactions; however, their mixtures do not always generate superior synergism properties. Only moderate intermolecular interaction can generate the strongest synergism in CMC reduction.

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INTRODUCTION Surfactant mixtures usually show synergistic behaviors and thus bring about more fascinating properties than their individual components, such as much higher surface activity and lower critical micelle concentration (CMC).1,2 CMC represents the concentration where the air−water interface is “saturated” with adsorbed surfactant molecules and micelles start to form in bulk solution. Great efforts have been made to construct mixed surfactants showing great synergism and low CMC.3,4 Studies on mixtures of two single-chain surfactants have found that the synergism decreases in the order of anionic/cationic > ionic/ zwitterionic > ionic/nonionic. Normally, cationic/anionic mixed surfactants can more closely pack at the air−water interface and in micelles, and thus exhibit stronger synergism because of electrostatic attraction between their oppositely charged headgroups besides hydrophobic interaction of their alkyl chains.4 With the developments of surfactants, gemini surfactants have been introduced to mixtures of surfactants in recent years. The CMC of gemini surfactants is almost 100-fold lower than their corresponding monomeric surfactants, and surface activity can be a 1000-fold improved.5,6 So the mixtures of cationic/ anionic surfactants containing gemini surfactants are believed to © XXXX American Chemical Society

be more efficient in CMC reduction than the mixtures of oppositely charged monomeric surfactants. The gemini/monomeric surfactant mixture, trimethylene-1,3-bis(dodecyldimethylammonium bromide) (12-3-12) and dodecylsulfonate (AS), has shown stronger synergism than alkyltrimethylammonium bromide (CnTAB) and AS, being proved by a larger negative molecular interaction parameter (βm) of 12-3-12/AS.7,8 However, not all of the gemini surfactant mixtures show superiority over singlechain surfactant mixtures. For example, the mixture of gemini surfactant 12-2-12 with sodium n-octyl sulfate (SOS) has larger CMC than the C12TAB/SOS mixture.9 In comparison with the mixture of C12TAB and sodium dodecyl sulfate (SDS), 12-212/SDS and 12-6-12/SDS mixtures display larger CMC and weaker synergism.10 Moreover, Rosen et al.8 found that the mixture of single-chain surfactant AS with cationic gemini surfactant C12C4(OH)2C12 having two hydroxyl groups in its spacer leads to an increase in CMC. So far, the relationship of CMC with surfactant molecule structures is too complicated Received: April 29, 2014 Revised: June 2, 2014

A

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to be predicted in mixtures of oppositely charged gemini surfactants. The present work is aimed at understanding the relationship of surfactant structures with CMC reduction in mixtures of oppositely charged gemini surfactants from the aspect of intermolecular interactions. Cationic gemini surfactants with or without hydroxyls in spacer CnC4(OH)2Cn (n = 12, 8, 6, 4), CnC4Cn (n = 12, 8, 6), and cationic monomeric surfactants CnTAB (n = 12, 8, 6) have been chosen to construct surfactant mixtures with anionic sulfonate gemini surfactant C12C3C12(SO3)2 at pH 9.5. The corresponding molecular structures are shown in Scheme 1. At pH 9.5, the sulfonate

Article

EXPERIMENTAL SECTION

Materials. Anionic gemini surfactant 1,3-bis(N-dodecyl-Npropanesulfonate sodium)-propane (C12C3C12(SO3)2),11 cationic ammonium gemini surfactants (CnC4(OH)2Cn, n = 12, 8, 6, 4),8,12,13 and CnC4Cn (n = 12, 8, 6)14,15 were synthesized and purified according to the corresponding literature. Cationic ammonium single-chain surfactants (CnTAB, n = 12, 8, 6) were purchased from the TCI company with a purity higher than 99% and recrystallized twice before use. Water used in all experiments was Milli-Q water of 18.2 MΩ·cm. All of the measurements were carried out at pH 9.5, which was adjusted by NaOH solution. Surface Tension Measurements. The surface tension of the mixed surfactants at different mixed molar fractions of C12C3C12(SO3)2 (Xa) was measured using the drop volume method.16 The value of γ was determined from at least five consistent measured values. Each surface tension curve was repeated at least twice. The measurement temperature was controlled at 25.00 ± 0.05 °C using a thermostat. Electrical Conductivity Measurements. The conductivity of the mixed cationic/anionic surfactant systems at different Xa values was measured as a function of surfactant concentration using a JENWAY model 4320 conductivity meter. The measurements were performed in a double-walled glass container with water circulation to control the temperature at 25.0 ± 0.1 °C. Turbidity Measurements. Turbidity was employed to characterize the phase behavior of C12C3C12(SO3)2/C12C4(OH)2C12 mixtures. The turbidity of titrating 2 mM C12C3C12(SO3)2 solution into 2 mM C12C4(OH)2C12 solution, reported as 100-%T, was measured at 450 nm using a Brinkman PC920 probe colorimeter equipped with a thermostated water-circulator to control the temperature at 25.0 ± 0.1 °C. The turbidity values were only recorded after the values became stable (about 2−4 min). Isothermal Titration Microcalorimetry (ITC). Calorimetric measurements were conducted on a TAM 2277-201 microcalorimetric system (Thermometric AB, Järfȧlla, Sweden) with a stainless-steel sample cell of 1 mL. The cell was initially loaded with 0.6 mL of cationic gemini surfactant solution. Two millimolar C12C3C12(SO3)2 solution was injected consecutively into the stirred sample cell in portions of 10 μL via a 500 μL Hamilton syringe controlled by a 612 Thermometric Lund pump until the desired range of concentration had been covered. The system was stirred at 60 rpm with a gold propeller. All of the measurements were conducted at 25.00 ± 0.01 °C. Each ITC curve was repeated at least twice with deviation within ±4%. The average of the final dilution enthalpy values after interaction saturation was subtracted from the original titrating line before the thermodynamic fitting process.17 Ideal Mixing Model. The pseudophase separation model is applied to study how the binary surfactant mixtures deviate from the ideal mixing model.18 For a mixture of two surfactants, a simple relationship exists between CMC12 of the mixture and the CMCi of each pure surfactant,

Scheme 1. Chemical Structures and Abbreviations of C12C3C12(SO3)2, CnC4(OH)2Cn, CnC4Cn, and CnTAB

groups of C12C3C12(SO3)2 are completely deprotonated, and thus, each molecule carries two negative charges.11 Surface tension, electrical conductivity, and ITC experiments have been used to study the micellization, surface activity, and intermolecular interaction of the surfactant mixtures. It has been found that the strong intermolecular interactions in the mixtures of C12C3C12(SO3)2/C12C4(OH)2C12, C 12 C 3 C 12 (SO 3 ) 2 /C 8 C 4 (OH) 2 C 8 , and C 12 C 3 C 12 (SO 3 ) 2 / C12C4C12 have led to the formation of less surface active premiceller aggregates, which results in abnormal increase in CMC in these mixtures. Strong hydrophobic interaction is the key factor that allows the surfactants to form less surface active premicellar aggregates, and strong electrostatic attraction between the oppositely charged headgroups and hydrogen bonds between the spacers assist premicellization. If substituting C12C4(OH)2C12, C8C4(OH)2C8, and C12C4C12 with CnC4(OH)2Cn (n = 6, 4) having shorter alkyl chains, CnC4Cn (n = 8, 6) without hydroxyl groups, or single-chain surfactants CnTAB (n = 12, 8, 6), hydrophobic interaction between alkyl chains, electrostatic attraction between the headgroups, or hydrogen bonds between the spacers are weakened or inhibited, respectively. Then, the less surface active premicellar aggregates are no longer formed in the mixture of these cationic surfactants with the anionic gemini surfactant, and thus, the CMC values are reduced in the mixtures.

αiCMC12m = xi fi CMCim (i = 1, 2)

(1)

where αi and xi are the mole fractions of surfactant i in the surfactant mixtures and in the mixed micelles, respectively. f i is the activity coefficient of surfactant i in mixed micelles. In an ideal mixing model, f i = 1 and α1 = x1, α2 = x 2 , x1 + x 2 = 1

(2)

The combination of eqs 1 and 2 gives a relationship between CMCm12 and CMCmi :

x1 x2 1 = + CMC12m CMC1m CMC 2m

(3)

Then, the ideal critical micelle concentration for mixed surfactants can be calculated from eq 3. The CMC values determined from experiments will be compared with the ideal values to evaluate the synergism between two oppositely charged surfactants. B

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RESULTS AND DISCUSSION Anomalously Large CMC in C12C3C12(SO3)2/C12C4(OH)2C12 Mixtures. Surface tension measurements of the mixed C 12C 3C 12(SO 3) 2/C 12C 4(OH) 2C 12 solutions were carried out as a function of the total surfactant concentration (CT) at the molar fraction of C12C 3C 12(SO 3) 2 (Xa) from 0 to 1.00. The surface tension curves at Xa = 0.50 and 0.60 are excluded because precipitation takes place (see Figure S1, Supporting Information) at these molar fractions where the positive/negative charge ratio is close to 1:1. The curves are classified into two groups according to the number of inflections: the surface tension curves for Xa = 0, 1.00, 0.90, 0.80, and 0.70 show one inflection point (Figure 1a), while

C12C4(OH)2C12 and C12C3C12(SO3)2 are 1.00 and 0.043 mM, respectively. The CMC values obtained from the surface tension curves at 0.70 ≤ Xa < 1.00 are between 0.043 and 1.00 mM. Instead of great reduction in CMC as most of cationic/anionic surfactant mixtures do, the C12C3C12(SO3)2/C12C4(OH)2C12 mixture shows no synergism in CMC reduction at all. As to the surface tension curves of the C12C3C12(SO3)2/C12C4(OH)2C12 mixture at 0 < Xa ≤ 0.40 shown in Figure 1b, with increasing CT, the curves decrease moderately to the first inflection point and speed up to the second inflection point, and stay unchanged thereafter. With increasing Xa from 0.10 to 0.40, the first inflection point decreases from 0.85 to 0.043 mM, while the second inflection points decrease from 1.54 to 0.82 mM. To define the two inflection points, electrical conductivity experiments were performed. In the conductivity curves shown in Figure 2, the CMC values are the concentrations at the intersection point of the two straight lines with different slopes. The CMC values from the conductivity curves are consistent with the values from the second inflection points in the surface tension curves (Figure 1b). The existence of a maximum in the molar conductivity Λ-CT0.5 curves (Figure 2) indicates that the first inflection point in the surface tension curves in Figure 1b is caused by premicellization.19,20 Figure 1c summarizes the CMC values of the C12C3C12(SO3)2/ C12C4(OH)2C12 mixtures obtained from the surface tension curves and calculated from the ideal mixing model. For most of the oppositely charged surfactant mixtures, the CMC values experimentally obtained are much lower than those predicted from the ideal mixing model, indicating strong synergism.7,19 However, the CMC values of the C12C3C12(SO3)2/ C12C4(OH)2C12 mixture are larger than those of C12C4(OH)2C12 and C12C3C12(SO3)2, and increase slightly when Xa increases from 0.10 to 0.20, and decrease rapidly and approach that of C12C3C12(SO3)2 when Xa increases from 0.30 to 0.90. The CMC curve of the C12C3C12(SO3)2/C12C4(OH)2C12 mixture is located high above the curve calculated from the ideal mixing model. This means that this mixture with oppositely charged gemini surfactants shows no synergism in CMC reduction. Normally, electrostatic attractions and the resulting charge neutralization between the headgroups of oppositely charged surfactants should be in favor of lowering CMC. However, the anomalous increase in CMC of the C12C3C12(SO3)2/C12C4(OH)2C12 mixture is exempt from this expectation. Relationship of Anomalously High CMC with Surfactant Structures. In order to avoid premicellization and thus reduce the CMC of surfactant mixtures, cationic surfactant C 12 C4 (OH)2C 12 in the C 12C 3 C 12(SO 3 ) 2/C12 C 4(OH)2 C12 mixtures is replaced in the following ways: (1) by CnC4(OH)2Cn (n = 8, 6, 4) with shorter alkyl chains to weaken hydrophobic interaction between alkyl chains; (2) by CnC4Cn (n = 12, 8, 6) without hydroxyl groups in the spacer to eliminate hydrogen bonds between the spacers; and (3) by monomeric surfactants CnTAB (n = 12, 8, 6) to reduce both electrostatic attractions between the headgroups and hydrophobic interaction between the alkyl chains. The surface tension curves of the C12C3C12(SO3)2/ CnC4(OH)2Cn, C12C3C12(SO3)2/CnC4Cn, and C12C3C12(SO3)2/ CnTAB mixtures at Xa = 0.7 and 0.3 are obtained, and the curves at Xa = 0.7 as representatives are plotted against the total surfactant concentration CT in Figure 3a, b, and c, respectively. The CMC values determined from the surface tension curves are summarized in Figure 3d and Table 1. Unusually, the CMC values of the C12C3C12(SO3)2/CnC4(OH)2Cn

Figure 1. Surface tension curves of the mixed C12C3C12(SO3)2/ C12C4(OH)2C12 solutions against the total surfactant concentration CT at different molar fractions of C12C3C12(SO3)2 (Xa): (a) Xa = 0, 0.70, 0.80, 0.90, and 1.00, and (b) Xa = 0.10, 0.20, 0.30, and 0.40. (c) The CMC values of the mixed C12C3C12(SO3)2/C12C4(OH)2C12 solutions against Xa. The solid data points represent experimental values, and the dashed line represents values calculated from the ideal mixing model.

those for Xa = 0.10, 0.20, 0.30, and 0.40 show two inflection points (Figure 1b). As shown in Figure 1a, the surface tension curves of C12C3C12(SO3)2/C12C4(OH)2C12 at 0.70 ≤ Xa ≤ 1.00 and Xa = 0 decrease rapidly with increasing CT at lower concentration, then remains almost unchanged beyond a inflection point. The concentration at the inflection point is the CMC. The curves at Xa = 0 and Xa = 1.00 show that the CMC values of C

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Figure 2. Variations of specific conductivity K against the total surfactant concentration CT and variations of molar conductivity Λ with CT0.5 for the C12C3C12(SO3)2/C12C4(OH)2C12 mixtures at Xa = 0.10, 0.20, 0.30, and 0.40.

do not always generate the mixtures with much stronger selfassembling ability. Previously, Rosen and Liu8 found that the mixture of singlechain surfactant AS with cationic gemini surfactant C12C4(OH)2C12 shows anomalous increase in CMC. They thought that strong hydrophobic interaction arising from the long alkyl chains may result in the formation of less surface active premicellar aggregate. The premicellization reduces the monomer concentrations of the individual surfactants that contribute to the reduction of the surface tension, thus leading to the increase of CMC. The results above already indicate that the C12C3C12(SO3)2/C12C4(OH)2C12 mixtures form premicelles. Therefore, the abnormal increase in the CMC of other mixtures of the gemini surfactants may be also caused by the formation of less surface active premicellar aggregates. Each gemini surfactant molecule has two charged headgroups and two hydrophobic chains, and thus, strong electrostatic binding between their oppositely charged headgroups and strong hydrophobic interaction between the alkyl chains can greatly promote premicellization. The formation of premicellar aggregates will be further studied in the following section. Premicellar Aggregates. Electrical conductivity is a widely used method to study the aggregate states at concentrations below CMC.19,20 The shape of Λ vs C0.5 curves can reveal the premicellar aggregation of surfactants: the formation of premicelles are signified by a maximum, while the formation of ion pairs shows a positive curvature toward the concentration axis.20 The electrical conductivity of the mixtures of C12C3C12(SO3)2/ CnC4(OH)2Cn (n = 12, 8, 6, 4), C12C3C12(SO3)2/CnC4Cn (n = 12, 8, 6), and C12C3C12(SO3)2/CnTAB (n = 12, 8, 6) at Xa = 0.70 has been studied to testify the existence of premicellar aggregates

mixtures decrease as the alkyl chain becomes shorter and undergo a minimum when the carbon number of the alkyl chain is 6. As to the C12C3C12(SO3)2/CnC4Cn mixtures, the CMC value sharply decreases from 0.10 mM to 0.028 mM as the alkyl chain n is shortened from 12 to 8 but almost does not change when n further decreases to 6. Moreover, the CMC values of C12C3C12(SO3)2/C12C4C12 and C12C3C12(SO3)2/ C8C4C8 are much smaller than those of the corresponding C 12 C 3 C 12 (SO 3 ) 2 /C 12 C 4 (OH) 2 C 12 and C 12 C 3 C 12 (SO 3 ) 2 / C8C4(OH)2C8 with the same length of alkyl chains, although the binding interaction of the headgroup areas in C12C3C12(SO3)2/ C12C4C12 and C12C3C12(SO3)2/C8C4C8 is weaker than that in C 12 C 3 C 12 (SO 3 ) 2 /C 12 C 4 (OH) 2 C 12 and C 12 C 3 C 12 (SO 3 ) 2 / C8C4(OH)2C8 because of the deletion of hydroxyl groups from the spacer of the cationic gemini surfactants. Even more surprisingly, upon replacing the cationic gemini surfactants by monomeric surfactant CnTAB, the CMC values of the C12C3C12(SO3)2/CnTAB mixtures are much smaller than the mixtures of gemini surfactants when the alkyl chain lengths are 12 and 8, and the CMC of the C12C3C12(SO3)2/C6TAB mixture is only slightly larger than the corresponding gemini surfactant mixtures (C 12C3C12(SO3)2/C6C4(OH)2C6 and C12C3C12(SO3)2/C6C4C6) with the same alkyl chain length. In brief, only the surfactants with moderate attraction between their headgroups and moderate hydrophobic interaction between their hydrophobic chains can fabricate surfactant mixtures with greatly lowered CMC. The CMC reduction can be achieved by changing C12C4(OH)2C12 to CnC4Cn, CnC4(OH)2Cn (n = 8, 6, 4), or CnTAB, where hydrogen bonds are inhibited, hydrophobic interactions are weakened, or both of the interactions are reduced, respectively. It is revealed that two surfactants with strong self-assembling ability by themselves D

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obvious curvature toward the concentration axis, which indicates the formation of ion pairs between cationic and anionic surfactants before CMC as explained in the literature.20,21 In the mixtures forming ion pairs, the CMC values of the mixed surfactants are smaller than those of C12C3C12(SO3)2 and the cationic surfactants themselves. An exception is the C12C3C12(SO3)2/ C4C4(OH)2C4 mixture, where the electrical conductivity curve indicates the formation of ion pairs, but the surface tension curve shows a larger CMC than C12C3C12(SO3)2. The reason should be that the ion pairs formed in the C12C3C12(SO3)2/ C4C4(OH)2C4 mixture are less surface active than pure C12C3C12(SO3)2 because of the very short alkyl chains of C4C4(OH)2C4. The C12C3C12(SO3)2 monomers are more favored to be adsorbed at the surface than the ion pairs of C12C3C12(SO3)2/C4C4(OH)2C4, thus leading to the increase of CMC. Table 1 summarizes the aggregation behaviors before CMC for all of the mixed surfactants studied. It is concluded that premicellar aggregates lead to the anomalous increase in CMC, while the formation of ion pairs normally reduces the CMC. For the premicelles, the headgroups of surfactants may form an immature hydrophilic shell, while the alkyl chains may entangle with each other loosely and form a weak hydrophobic domain. The counterion release dehydration of the surfactants results in great increase in entropy. So the micellization is favored from a thermodynamic aspect. However, the unsuitable geometry and structure of the premicellar aggregates hinder their further aggregation and thus hinder micellization. In order to understand the formation mechanism of premicellar aggregates and ion pairs, ITC has been applied to study the intermolecular interaction of anionic surfactant C12C3C12(SO3)2 with the cationic surfactants in the following section. Interaction between Oppositely Charged Surfactants. Normally, interaction parameter β is used to characterize the nature and strength of the interactions between different surfactants.8,22 However, the β calculation needs the CMC values of the surfactant mixtures and assumes that greater interactions between surfactants generate smaller CMCs of their mixtures. In the present work, greater interactions between oppositely charged gemini surfactants result in the formation of premicellar aggregates with poor surface activity than individual surfactants and lead to larger CMC. Therefore, the β parameter is no longer a proper parameter to estimate the interactions between these oppositely charged gemini surfactants. ITC is a powerful tool to investigate intermolecular interactions in physical and chemical processes from a thermodynamic aspect.23−28 Different from the β parameter method, ITC is a direct way to evaluate the interaction strength of surfactants in bulk solution. Figure 5 presents the observed enthalpy changes ΔHobs against Xa by titrating 2.0 mM C12C3C12(SO3)2 into 0.5 mM CnC4(OH)2Cn (n = 12, 8, 6, 4), CnC4Cn (n = 12, 8, 6), and CnTAB (n = 12, 8, 6). The ITC curve of titrating 2.0 mM C12C3C12(SO3)2 into water is also included for comparison. Because the CMC of C12C3C12(SO3)2 is 0.043 mM, 2.0 mM C12C3C12(SO3)2 exists as micelles before titrating, while 0.5 mM cationic surfactants in the vessel exist as monomers. Because the ΔHobs values of the ITC curve for titrating 2.0 mM C12C3C12(SO3)2 into water are very small, the effect of demicellization of C12C3C12(SO3)2 during the titration on the ITC curves can be neglected. For the C12C3C12(SO3)2/CnC4(OH)2Cn mixtures (Figure 5a), when the chain length of CnC4(OH)2Cn increases from 4 to 12, the ITC curves show three different patterns. When n is 4, the

Figure 3. Surface tension curves of the mixed solutions of C12C3C12(SO3)2 with (a) CnC4(OH)2Cn, (b) CnC4Cn, and (c) CnTAB against the total surfactant concentration CT at Xa = 0.70. (d) The derived CMC values of the mixtures against the carbon numbers (n) per hydrophobic alkyl chain. The black dotted line represents the CMC of pure C12C3C12(SO3)2.11

before CMC. Figure 4 shows the variations of the specific conductivity K against the total surfactant concentration CT and the molar conductivity Λ against CT0.5. Λ = (K − K0)/CT, and K0 is the specific conductivity of water at pH 9.5. The K − CT curves keep increasing with increasing CT over the whole CT range. However, the Λ − CT0.5 curves of C12C3C12(SO3)2/C12C4(OH)2C12, C12C3C12(SO3)2/C8C4(OH)2C8, and C12C3C12(SO3)2/C12C4C12 undergo a maximum upon increasing CT, suggesting that premicellization takes place in these three systems, which are just the systems whose CMC values show anomalous increase upon mixing. Differently, the Λ − CT0.5 curves of C12C3C12(SO3)2/ C n C 4 (OH) 2 C n (n = 6, 4), C 12 C 3 C 12 (SO 3 ) 2 /C n C 4 C n (n = 8, 6), and C12C3C12(SO3)2/CnTAB (n = 12, 8, 6) show E

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Table 1. CMC Values of the Mixed Solutions of C12C3C12(SO3)2/CnC4(OH)2Cn, C12C3C12(SO3)2/CnC4Cn, and C12C3C12(SO3)2/ CnTAB at Xa = 0, 0.30, and 0.70, and Their Aggregate States before CMC Derived from Electrical Conductivity Curves CnC4(OH)2Cn CMC (mM)

CnC4Cn

CnTAB

n

12

8

6

4

12

8

6

12

8

6

Xa = 0 Xa = 0.30 Xa = 0.70 premicelles ion pairs

1.02 1.18 0.23 yes no

18.21 2.07 0.10 yes no

28.43 0.027 0.023 no yes

39.44  0.091 no yes

1.29 1.72 0.10 yes no

21.96 0.37 0.028 no yes

32.16 0.045 0.027 no yes

12.97 0.16 0.039 no yes

252 0.068 0.031 no yes

552 0.041 0.035 no yes

Figure 4. Variations of specific conductivity K against the total surfactant concentration CT (black cube) and the molar conductivity Λ with CT0.5 (red circle) for the mixed surfactants at Xa = 0.70.

ΔHobs value is very small and almost does not change with the addition of C12C3C12(SO3)2, similar to the situation of titrating C12C3C12(SO3)2 into water, which means that the interaction

between C12C3C12(SO3)2 and C4C4(OH)2C4 is very weak. When n increases to 6, the ITC curve is approximately sigmoidal in shape and indicates that the interaction of C12C3C12(SO3)2 with F

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For the C12C3C12(SO3)2/CnC4Cn mixtures (Figure 5b) and the C12C3C12(SO3)2/CnTAB mixtures (Figure 5c), the ITC curves, that is the interaction processes, also change with the length of the alkyl chains (n) of the cationic surfactants. The ITC curves of the C12C3C12(SO3)2/CnC4Cn (n = 8, 6) and C12C3C12(SO3)2/CnTAB (n = 8, 6) show sigmoidal shape and reflect the interaction processes of the surfactants from endothermic interaction to interaction saturation. This is similar to the system of C12C3C12(SO3)2/C6C4(OH)2C6. When n increases to 12, the ITC curves of C12C3C12(SO3)2/C12C4C12 and C12C3C12(SO3)2/C12TAB undergo three stages: the exothermic binding of C12C3C12(SO3)2 with C12C4C12 or C12TAB, precipitation, and interaction completion. The situation is similar to those of C12C3C12(SO3)2/C8C4(OH)2C8 and C12C3C12(SO3)2/ C12C4(OH)2C12. The ITC curves are analyzed with the thermodynamic method described in Supporting Information, and the derived interaction thermodynamic parameters are listed in Table 2. In Figure 5, the dashed lines are just the lines connecting the experimental data, while the solid thick lines are fitting lines by the thermodynamic model. Because the negative valleys when precipitation occurs are mainly caused by the phase separation rather than the surfactant binding, the solid lines do not follow the data when precipitation occurs. It is assumed that the binding enthalpies of the surfactants become zero when the precipitation finishes and that the binding enthalpies during precipitation approximately use the values just before the precipitation. The binding constant (Kb) of C12C3C12(SO3)2 with CnC4(OH)2Cn, CnC4Cn, and CnTAB, the number of binding sites (N) of each C12C3C12(SO3)2 with cationic surfactants, and the binding enthalpy (ΔHb) are obtained directly from the fitting progress. Gibbs free energy (ΔGb) is calculated from ΔGb = −RT ln Kb, and the entropy change (TΔSb) is calculated from TΔSb = ΔHb − ΔGb. As shown in Table 2, the ΔGb values are negative, and the TΔSb values are large and positive for all of the mixed surfactant systems, which means that all of the binding processes of C12C3C12(SO3)2 with the cationic surfactants take place spontaneously and are entropy-driven. The ΔHb values are exothermic when the carbon number of alkyl chains of the cationic surfactants (n) is 12. That is to say, the ΔH b values of C 12 C 3 C 12 (SO 3 ) 2 with C 12 C 4 (OH) 2 C 12 , C12C4C12, and C12TAB are exothermic. When n decreases to 8 and 6, the ΔHb values of C12C3C12(SO3)2 with C8C4(OH)2C8, C6C4(OH)2C6, C8C4C8, C6C4C6, C8TAB, and C6TAB are all endothermic. The interactions between the oppositely charged surfactants mainly include electrostatic binding between the headgroups, hydrogen bonds between the spacer, the hydrophobic association between the alkyl chains, and the accompanying dehydration and counterion release of the headgroups. It has been shown that interaction between oppositely charged surfactants with long alkyl tails usually yields large exothermic enthalpy changes23,24 and that the dehydration and counterion release lead to endothermic enthalpy. Bai and her coworkers29 thought that exothermic enthalpy mainly results from electrostatic interaction between oppositely charged headgroups. However, Povilas Norvaiăs et al.28 proved that large exothermic enthalpy primarily arises from hydrophobic association of alkyl chains rather than electrostatic forces. Herein, for each series of CnC4(OH)2Cn, CnC4Cn, and CnTAB, the ammonium headgroups have the same structures, so the electrostatic interaction between the headgroups or the hydrogen bonding between the spacers is the same. Thus, the difference in the ΔHb values is mainly caused by variation of hydrophobic interactions

Figure 5. Observed enthalpy changes ΔHobs against Xa for titrating 2.0 mM C12C3C12(SO3)2 into 0.5 mM cationic surfactants: (a) CnC4(OH)2Cn (n = 12, 8, 6, 4), (b) CnC4Cn (n = 12, 8, 6), and (c) CnTAB (n = 12, 8, 6). ΔHobs values are expressed in kJ/mol of C12C3C12(SO3)2.

C6C4(OH)2C6 changes from endothermic to zero, reaching the saturation point of interaction. When n increases to 8 and 12, both the systems of C12C3C12(SO3)2/C8C4(OH)2C8 and C12C3C12(SO3)2/C12C4(OH)2C12 experience a three-step interaction process: (1) a platform where ΔHobs values are exothermic or endothermic, but remain invariant, which corresponds to the gradual binding of C12C3C12(SO3)2 with C12C4(OH)2C12 or C8C4(OH)2C8. (2) The negative valleys correspond to the strong exothermic precipitation process, which is caused by the strong hydrophobic interaction in the surfactant mixtures where the oppositely charged surfactants have been electrostaticlly bound. (3) Finally, the ΔHobs values change to zero, implying that the interaction processes finish. G

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Table 2. Thermodynamic Parameters of the Mixtures of C12C3C12(SO3)2/CnC4(OH)2Cn, C12C3C12(SO3)2/CnC4Cn, and C12C3C12(SO3)2/CnTAB Derived from ITC Curves C12C3C12(SO3)2/CnC4(OH)2Cn chain length (n)

12

8

6

ΔHb (kJ/mol) ΔGb(kJ/mol) TΔS (kJ/mol) Kb (M−1) ×104 N

−18.59 −36.17 17.59 162.87 1.12

12.52 −32.53 45.03 62.64 0.71

9.45 −31.22 40.67 34.96 0.76

C12C3C12(SO3)2/CnC4Cn 4

C12C3C12(SO3)2/CnTAB

12

8

6

12

8

6

−10.87 −34.95 24.08 110.34 0.61

13.45 −31.71 45.16 45.13 0.82

9.74 −25.56 35.30 3.41 0.61

−3.15 −32.46 29.31 46.52 0.64

3.65 −24.92 28.57 2.75 0.89

1.39 −23.84 24.10 1.51 1.24

CnC4(OH)2Cn (n = 12, 8, 6, 4), and single-chain surfactants CnTAB (n = 12, 8, 6), have been chosen to constitute oppositely charged surfactant mixtures with anionic sulfonate gemini surfactant C12C3C12(SO3)2. C12C3C12(SO3)2/C12C4(OH)2C12, C 12 C 3 C 12 (SO 3 ) 2 /C 8 C 4 (OH) 2 C 8 , and C 12 C 3 C 12 (SO 3 ) 2 / C12C4C12 show anomalous increase in CMC because strong electrostatic attractions between the headgroups, strong hydrophobic interactions between the alkyl chains, and hydrogen bonds between the spacers lead to the formation of less surface active premicellar aggregates in bulk solution. If CnC4(OH)2Cn (n = 6, 4) having shorter alkyl chains, CnC4Cn (n = 8, 6) without hydroxyl groups, or single-chain surfactants CnTAB (n = 12, 8, 6) are used instead, hydrophobic interaction between alkyl chains, electrostatic attraction between the headgroups, or hydrogen bonds between the spacers are weakened or inhibited, respectively, then ion pairs of surfactants are formed, and thus, the CMC values are lowered. Premicellization is mainly driven by the strong hydrophobic interaction between the alkyl chains, while electrostatic attraction between the headgroups and hydrogen bonds between the spacers work as assistant factors. The work reveals that the combination of two surfactants with strong self-assembling ability separately does not always generate superior properties. Too strong intermolecular binding of two oppositely charged surfactants can lead to an anomalous increase in CMC. Only moderate intermolecular binding between the surfactants can generate the greatest synergism in CMC reduction. This work provides some guidance on how to choose surfactants to construct efficient surfactant mixtures.

between the alkyl chains. When the carbon numbers of alkyl chains of the cationic surfactants (n) are 8 and 6, the ΔHb values are endothermic because the exothermic hydrophobic interaction decreases and is no longer able to counteract the endothermic effect caused by dehydration and counterion release of the headgroups in electrostatic binding. The binding constant (Kb) is a comprehensive result of all the intermolecular interactions and reflect the interaction strength of two surfactants. As presented in Table 2, the binding constant (Kb) of C12C3C12(SO3)2 with CnC4(OH)2Cn, CnC4Cn, and CnTAB increases with increasing the carbon number (n) of the alkyl chains of the cationic surfactants. For the same carbon number, the Kb value increases in the order of CnC4(OH)2Cn > CnC4Cn > CnTAB. For the binding of C12C3C12(SO3)2 with C4C4(OH)2C4, the binding strength is too small so that the interaction parameters cannot be obtained by fitting its ITC titration curve. In summary, Kb undergoes a monotonic reduction with the decrease of the alkyl chain length for each series of mixed surfactants, while the corresponding CMC experiences a minimum. This suggests that the largest Kb does not show the best effect in lowering CMC and that there is an optimum chain length for the surfactants to achieve moderate Kb and the smallest CMC. That is to say, hydrophobic interactions between two surfactants should not be too strong or too weak to achieve the greatest synergism and smallest CMC. Combining with the results in Table 1, we found that the cationic surfactants with long alkyl chains make the binding interactions with C12C3C12(SO3)2 strong enough to endow the surfactants with very large Kb and that then the mixed surfactants prefer to form less surface active premicellar aggregates and thus lead to larger CMC. The strong hydrophobic interaction is the key factor in this process. Besides, it is noteworthy that premicellar aggregates form in C12C3C12(SO3)2/C8C4(OH)2C8 but not in C12C3C12(SO3)2/C8C4C8, which means the hydroxyl groups in the spacer do help in the formation of premicellar aggregates through hydrogen bonds. However, the situation changes when the interactions above become weak. When the carbon number of each alkyl chain of the cationic surfactants is 6, the short chain leads to weak hydrophobic interaction with C12C3C12(SO3)2, and the Kb values decrease in the order of C12C3C12(SO3)2/C6C4(OH)2C6 > C12C3C12(SO3)2/C6C4C6 > C12C3C12(SO3)2/C6TAB. Reversely, the CMC increases in the same order, indicating that larger Kb results in lower CMC. In other words, if the hydrophobic interaction between the alkyl chains is not strong enough, the binding between the surfactants only leads to the formation ion pairs. In the cases where a premicellar aggregate cannot form, larger Kb generates greater synergism in CMC reduction.

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ASSOCIATED CONTENT

S Supporting Information *

Turbidity of C12C3C12(SO3)2/C12C4(OH)2C12 mixed solutions and the ITC analysis process. 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]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Chinese Academy of Sciences and National Natural Science Foundation of China (21025313, 21021003).

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CONCLUSIONS In this work, three series of cationic quaternary ammonium surfactants, including gemini surfactants with alkyl spacers CnC4Cn (n = 12, 8, 6), gemini surfactants with hydroxyl spacers

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