J. Phys. Chem. B 2010, 114, 10409–10416
10409
Studies on Interaction of Poly(sodium acrylate) and Poly(sodium styrenesulfonate) with Cationic Surfactants: Effects of Polyelectrolyte Molar Mass, Chain Flexibility, and Surfactant Architecture Hao Wang†,‡ and Yilin Wang*,† Key Laboratory of Colloid and Interface Science, Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, People’s Republic of China, and State Key Laboratory of Coal ConVersion, Institute of Coal Chemistry, Chinese Academy of Science, Taiyuan 030001, People’s Republic of China ReceiVed: October 26, 2009; ReVised Manuscript ReceiVed: June 22, 2010
Isothermal titration microcalorimetry, turbidity, and steady-state fluorescence measurements have been used to study interactions of cationic ammonium gemini surfactant (C12C6C12Br2) and single-chain surfactant dodecyltrimethylammonium bromide (DTAB) with anionic polyelectrolytes poly(sodium styrenesulfonates) (NaPSS) and poly(sodium acrylates) (NaPAA) with different molar masses. Without any surfactants, NaPSS with lower molar mass has already self-aggregated into aggregates, whereas NaPAA has no aggregation at any molar mass. All of the polyelectrolytes show a remarkable interaction with the cationic surfactants. Compared with DTAB, C12C6C12Br2 can bind to NaPSS and NaPAA at a very low concentration and has stronger interactions with NaPSS and NaPAA. The flexible NaPAA shows moderately endothermic enthalpies while interacting with the surfactants, but the interaction of the stiff NaPSS with the surfactants exhibits highly exothermic enthalpies. Moreover, the interaction of the stiff NaPSS with the surfactants strongly depends on the polyelectrolyte molar mass, but the polyelectrolyte molar mass almost does not affect the interaction of the flexible NaPAA with the surfactants. Especially, the effect of the polyelectrolyte molar mass becomes more significant when the polyelectrolytes interact with gemini surfactant than with single-chain surfactant. It is revealed that the effects of polyelectrolyte molar mass, chain flexibility, and surfactant architecture on surfactant/polyelectrolyte interactions confine each other. Introduction Interactions between polymers and surfactants in aqueous solutions have attracted significant interest because of their widespread applications and relatively complex behaviors.1-3 Strong electrostatic attraction and hydrophobic interaction between oppositely charged polyelectrolytes and surfactants occur at surfactant concentrations several orders of magnitude lower than the critical micelle concentrations of the surfactants. The mixtures of polyelectrolytes and surfactants with opposite charges often show a very strong tendency to form insoluble complexes and lead to phase separation,4,5 where monomeric units of polyelectrolyte form salt bonds with the surfactant headgroups and the hydrophobic segments of surfactants provide additional stabilization.6 These systems are complicated because the binding behavior is affected by so many variables related to polyelectrolyte structure, surfactant composition, and the solvent medium, including (i) polyelectrolyte composition, charge density, chain length, and chain flexibility; (ii) micelle surface charge density, micelle size, and shape; (iii) pH and ionic strength of the medium,7 and so on. All the above factors mutually control the interaction between polyelectrolyte and surfactant. We focus here on polymer molar mass, chain flexibility, and micelle structure. With regard to polyelectrolyte molar mass, Voorn and Overbeek8 predicted an increase in coacervation as the molar mass of biopolymers increases. Lindman and co-workers9 studied the effect of * Towhomcorrespondenceshouldbeaddressed.E-mail:
[email protected]. † Institute of Chemistry. ‡ Institute of Coal Chemistry.
polyelectrolyte molar mass on the phase behavior of a mixture of alkyltrimethylammonium bromide (CnTAB) and anionic polysaccharide hyaluronic acid. The tendency toward phase separation increased slightly with increasing polyelectrolyte molar mass, while the size of the two-phase region was little affected. Wang et al.10 studied the effects of micelle charge density, polymer molar mass, and polymer-to-surfactant ratio on coacervation. The coacervation region increased with micelle surface charge density and polymer molar mass, and an increase in molar mass reduced the micelle charge required for coacervation and also increased the coacervate volume fraction. Dubin et al.11 indicated that the molar mass of poly(dimethyldiallylammonium chloride) (PDMDAAC) did not exhibit a significant effect on PDMDAAC/TX100-SDS complex formation, but a critical molar ratio of SDS to TX100 decreased slightly with increasing molar mass of poly(sodium styrenesulfonate) (NaPSS) in the presence of TX100/DTAB mixed micelles. Choi and Kim12 studied the interaction between poly(acrylic acid) (PAA) and CnTAB. Their fluorescence measurements indicated that the critical aggregation concentration (cac) is lower for higher molar mass of PAA. Another essential parameter is the chain flexibility of polymer, which also plays an important role on the polyelectrolyte/surfactant interaction. Wallin and Linse13 considered the effect of polyelectrolyte chain flexibility on the interaction of charged micelles with oppositely charged polyelectrolyte. Simulations showed a very large accumulation of the polyelectrolyte charges close to the micelle for a flexible polyelectrolyte, which decreased strongly for some more rigid polymer chains. Skepo and Linse14 investigated the effect of salt concentration
10.1021/jp9102405 2010 American Chemical Society Published on Web 07/26/2010
10410
J. Phys. Chem. B, Vol. 114, No. 32, 2010
on the complexation of several macroions with one polyelectrolyte at varying flexibility. According to their simulation results, as the salt concentration increased, the number of bound polyelectrolyte segments near a charged macroion was found to decrease substantially for the flexible chain while it remained constant and smaller for the stiff one. Dubin and co-workers15 investigated the effect of polyelectrolyte chain flexibility on the adsorption of polyelectrolytes on oppositely charged colloidal particles. Their experimental and modeling results led to the conclusion that chain stiffness did influence the binding of polyelectrolytes with oppositely charged colloids, but the persistence length was not necessarily an appropriate measure of chain flexibility on short length scales. Hansson and Almgren16 investigated the aggregate structure and the phase behavior in NaPAA/DTAB and NaPAA/CnTAC comparing with the NaPSS/DTAB systems. They found that an associative twophase region existed in the NaPSS/DTAB systems, but the behavior was different from the NaPAA-DTAB system where no phase separation was observed as long as the polyelectrolyte was in excess. Macdonald and Tang17 studied the binding of NaPAA and NaPSS with cationic micelles, and the resistance of complexes to redissolution was attributed to more efficient ion pairing for more flexible polyelectrolytes. Among so many different polyelectrolytes, NaPAA and NaPSS are the representative polyelectrolytes with flexible chain and stiff chain, respectively. Although numerous studies on the interaction of NaPAA and NaPSS with surfactants have appeared,4,5,18-20 the combinational effects of polymer molar mass and chain flexibility have not yet been well understood. Gemini surfactants are composed of two hydrophobic chains linked to two ionic or polar groups with a spacer moiety. This novel class of surfactants has many unique properties that are superior to those of conventional single-chain surfactants, such as remarkably low critical micelle concentration (cmc), much higher surface activity, and unusual aggregation morphologies. The micellization of gemini surfactants has been systematically studied.21-24 Due to two connected charge groups and strong hydrophobic interaction among the chains of gemini surfactants, it can be predicted that the interaction of gemini surfactants with polyelectrolytes should have a remarkable difference from the interaction of single chain surfactants with polyelectrolytes, and gemini surfactants may show more significant sensitivity to the flexibility of the polyelectrolyte chain. However, although there have been reports on interactions between gemini surfactants and polyelectrolytes,23,25,26 the effect of polyelectrolyte chain flexibility and polyelectrolyte molar mass on the polyelectrolyte/gemini surfactant interaction has not yet reported. Thus, in the present work, the interactions of cationic gemini surfactant hexamethylene-1,6-bis(dodecyldimethylammonium bromide) [C12H25(CH3)2N(CH2)6N(CH3)2C12H25]Br2 (C12C6C12Br2) and single chain surfactant dodecyltrimethylammonium bromide (DTAB) with NaPSS and NaPAA have been systematically studied. Each of these two polymers has three different molar masses, and these polymers have comparable degrees of polymerization. We expect to gain deep insight into the effects of polyelectrolyte molar mass, chain flexibility, and surfactant architecture on the interaction of surfactants with polyelectrolytes. Experimental Section Materials. Three poly(sodium acrylate) (NaPAA) samples were designated as NaPAA-1, NaPAA-2, and NaPAA-3. NaPAA-1 and NaPAA-2 (40 wt % solution in water) were purchased from Aldrich, and their molar masses Mw are 2100 for NaPAA-1 and 30 000 for NaPAA-2 with different degrees
Wang and Wang of polymerization (DPn) of about 22 and 319, respectively. NaPAA-3 was prepared from PAA (Aldrich, Mw ) 450 000, DPn ) 6250) by adjusting the pH to 7.5 using NaOH. Poly(sodium styrenesulfonate) (NaPSS) samples also have three different molecular molar masses. NaPSS-1 with Mw ) 4 300 was purchased from Fluka, and NaPSS-2 with Mw ) 70 000 and NaPSS-3 with Mw ) 1 000 000 were obtained from Aldrich. Their DPn values are around 21, 339, and 4850, respectively. All DPn values of these polymers are presented in Table 2. Gemini surfactant C12C6C12Br2 was synthesized and purified according to the method of Menger and Littau.27 DTAB was from Aldrich and recrystallized before use. Steady-State Fluorescence Measurement. Pyrene (1 × 10-6 M) was used as the probe to investigate the micropolarity sensed in its solubilization site from measurement of pyrene polarity index I1/I3, which is the ratio of the intensities of the first and third vibronic peaks in the fluorescence spectrum.28,29 The fluorescence intensities were measured with a Hitachi F-4500 spectrofluorometer equipped with a thermostatted water-circulating bath. Pyrene was excited at 335 nm, and the emission spectra were scanned from 350 to 500 nm. All of the measurements were conducted at 303.15 ( 0.05 K. Isothermal Titration Microcalorimetry (ITC). The calorimeter used was a TAM 2277-201 microcalorimeter (Thermometric AB, Ja¨rfa¨lla, Sweden) with a 1 mL stainless steel sample cell. The cell was initially loaded with 0.5 mL of water or polymer solution, and the concentrated surfactant solution was injected into the stirred sample cell in portions of 10 µL using a 500 µL Hamilton syringe controlled by a Thermometric 612 Lund Pump. The system was stirred at 60 rpm with a gold propeller. The interval between two injections was 12 min, which was sufficiently long for the signal to return to the baseline. The accuracy of the calorimeter was periodically calibrated electrically and verified by measuring the dilution enthalpies of concentrated sucrose solution.30 All experiments were repeated at least twice at 303.15 ( 0.01 K, and the reproducibility was within (2%. The observed enthalpy (∆Hobs) was obtained by integration over the peak for each injection in the plot of heat flow P against time t. Turbidimetric Titration. The turbidity of the polymer/ surfactant solutions, reported as 100 - %T, was measured at 450 nm using a Brinkman PC920 probe colorimeter equipped with a thermostatted water-circulating bath at 303.15 ( 0.05 K. The final turbidity titration curves were only recorded after the values became stable (about 2-4 min) and were corrected by subtracting the turbidity curve from a polymer-free titration. Results and Discussion Self-Aggregation of NaPSS and NaPAA in the Absence of Surfactant. Figure 1 shows the variation of the pyrene polarity ratio I1/I3 for NaPSS and NaPAA with the polymer concentration C without any surfactants. The I1/I3 curves for NaPSS-1 (Mw ) 4300) and NaPSS-2 (Mw ) 70 000) are found to exhibit a distinct difference from that for NaPSS-3 (Mw ) 1 000 000). For NaPSS-1 and NaPSS-2, as C increases, a marked decrease of I1/I3 is observed and the I1/I3 values at high C are almost constants and close to the value of nonionic surfactant micelles.28 The I1/I3 values at high C for NaPSS-1 and NaPSS-2 show only a slight difference. However, through the investigated polymer concentration (C), I1/I3 for NaPSS-3 remains nearly constant at a high value. Moreover, all of the I1/I3 curves for NaPAA with three molar masses are very similar to the curve of NaPSS-3 and keep nearly constant at the total polymer concentration region.
Anionic Polyelectrolytes and Cationic Surfactants
Figure 1. Variations of I1/I3 for (a) NaPSS-1, NaPSS-2, and NaPSS-3 and (b) NaPAA-1, NaPAA-2, and NaPAA-3 with the polymer concentration C.
The decrease of I1/I3 values for NaPSS-1 and NaPSS-2 reveals the formation of aggregates in these two NaPSS solutions. When the NaPSS molecules are dissolved in aqueous solution, the polymer alkyl chains may tend to associate with each other, forming micelle-like aggregates to minimize their exposure to water. Compared with NaPSS-2, a lower value of I1/I3 at high C suggests that NaPSS-1 forms a little more hydrophobic aggregates. It is noted that the association of NaPSS is mainly governed by the molar mass of the polymers. When the molar mass of NaPSS decreases, the intermolecular interaction among a large number of polymer molecules makes the polymer aggregation possible, although the stiffness of the polymer chain has the tendency to prohibit the aggregation. The smaller the molar mass of NaPSS, the larger the amount of polymer molecules, and then the stronger the intermolecular aggregation of the polymer. Thus, NaPSS-3 cannot form aggregates due to its high molar mass. Unlike NaPSS, all three NaPAA solutions
J. Phys. Chem. B, Vol. 114, No. 32, 2010 10411 cannot form micelle-like aggregates, since NaPAA owns weaker amphiphilic features, and the distributed charge groups and very weak hydrophobic groups make the micelle-like aggregation impossible. Interaction of NaPSS with Cationic Surfactants. Microcalorimetric curves for the dilution of DTAB and C12C6C12Br2 into water are shown in Figure 2a and c. Both of the ITC curves have a sigmoid shape with an abrupt decrease at a threshold concentration corresponding to the micelle formation, allowing identification of the cmc by an extrapolation of the initial portion of the curve and of the rapidly decreasing portion of the curve.31,32 Meanwhile, the enthalpy of micellization (∆Hmic) can be determined from the difference at the cmc between ∆Hobs of the two linear segments of the plots.33 It should be indicated that the concentration of bulk DTAB is 3.4 times the cmc which means that the DTAB solution added to the polymer solution could consist of about 25% monomers and 75% in the form of micelles. Thus, the value of ∆Hmic obtained from ITC curves does not refer to 100% micelle formation. The correct value for 100% micelle formation is about -4.06 kJ/mol.46 The values of the derived cmc and ∆Hmic per mole of surfactant at 303.15K are presented in Table 1. The Gibbs free energy of micellization (∆Gmic) can be calculated using the following expressions for DTAB and C12C6C12Br2, respectively.34
∆Gmic ) RT(1 + β) ln(cmc)
(1)
∆Gmic ) RT(1 + 2β) ln 2(cmc) - RT ln 2
(2)
where β is the effective micellar charge fraction, which can be obtained by independent conductivity measurements. In the present work, we used literature values26,35,36 for the pure surfactants. All of the thermodynamic parameters obtained are listed in Table 1. The ITC and turbidity titration curves for the DTAB and C12C6C12Br2 solutions being titrated into 0.05 wt % NaPSS solution as well as into water are summarized in Figure 2. As shown in Figure 2, the titration curves of the surfactants into
Figure 2. (a) ITC curves for titrating 50 mM DTAB into 0.05% NaPSS solution and pure water. (b) Turbidity curves for titrating 50 mM DTAB into 0.05% NaPSS solution. (c) ITC curves for titrating 5 mM C12C6C12Br2 into 0.05% NaPSS solution and pure water. (d) Turbidity curves for titrating 5 mM C12C6C12Br2 into 0.05% NaPSS solution. (O) H2O; (b) NaPSS-1; (2) NaPSS-2; (0) NaPSS-3.
10412
J. Phys. Chem. B, Vol. 114, No. 32, 2010
Wang and Wang
TABLE 1: Values of Critical Micelle Concentration (cmc) and Thermodynamic Parameters for DTAB and C12C6C12Br2 at 303.15 ( 0.01 K surfactant
cmc (mM)
β
∆Hmic (kJ/mol)
∆Gmic (kJ/mol)
DTAB C12C6C12Br2
14.52 0.91
0.77 0.58
-4.06 -5.45
-18.88 -36.09
TABLE 2: Values of CP, CR, and CM for DTAB and C12C6C12Br2 in the Presence of NaPSS and NaPAA at 303.15 K CP (mM)
a
CR (mM)
CM (mM)
cac (mM)
polymer
DPn
Aa
Ba
A
B
A
B
A
B
NaPSS-1 NaPSS-2 NaPSS-3 NaPAA-1 NaPAA-2 NaPAA-3
21 339 4850 22 319 6250
∼2.0 ∼2.0 ∼2.1 ∼1.7 ∼2.5 ∼0.50
∼0.16 ∼0.19 ∼0.23 ∼0.38 ∼0.10 ∼0.05
2.2 2.4 2.7 5.8 6.1 4.9
0.74 0.79 0.77 1.28 1.53 1.35
14.9 15.5 16.1 16.8 18.5 18.0
1.2 1.2 1.3 ∼1.5 ∼1.6 ∼1.7
0.13 0.088 0.10 0.39 0.27 0.26
0.016 0.012 0.011 0.025 0.028 0.021
A and B indicate DTAB and C12C6C12Br2, respectively.
Figure 3. Variations of I1/I3 for (a) DTAB with 0.005 wt % NaPSS and (b) C12C6C12Br2 with 0.005 wt % NaPSS: (b) NaPSS-1; (2) NaPSS-2; (0) NaPSS-3.
NaPSS with different molar masses have similar shape and tendency. We repeated the experiments many times and found that here the precipitation starts to take place when the 100 %T value increases to about 20 for the colorimeter used. Thus, the Cp values, where precipitation starts to form, are estimated as the surfactant concentrations where the 100 - %T values become 20 in the turbidity curves. The first additions of the surfactants into NaPSS solutions result in exothermic ∆Hobs values and gradual increases of turbidity. Then, the exothermic ∆Hobs values decrease and the turbidity increases, accompanied with the precipitation. When more surfactant molecules are added, the turbidity increases sharply and reaches a maximum, and ∆Hobs becomes endothermic, leading to a pronounced endothermic peak followed by a sharp decrease from CR, where the precipitate starts to be redissolved. Above CR, the enthalpy decreases and then the curves merge with the dilution curve of the surfactants into water, while the turbidity steeply decreases and then keeps constant. The solutions become transparent again, and the precipitate is completely redissolved. Then, the ITC curves coincide with the dilution curve of the surfactant solutions into water. Finally, free micelles begin to form beyond CM for DTAB/NaPSS systems. For both the DTAB/NaPSS systems and the C12C6C12Br2/NaPSS systems, the CM values are slightly larger than the cmc of DTAB and C12C6C12Br2. The values of CP, CR, and CM are estimated on the basis of both calorimetric and turbidity curves and summarized in Table 2. In order to know the aggregation situation of the NaPSS with DTAB and C12C6C12Br2 in the monomer state, steady-state fluorescence measurement is used; the NaPSS concentration is selected as 0.005%, and the surfactant concentrations are lower
than the cmc. Figure 3 shows the dependence of the I1/I3 values on the surfactant concentration for the mixtures of DTAB or C12C6C12Br2 with NaPSS-1, NaPSS-2, and NaPSS-3. The plot shapes of the C12C6C12Br2/NaPSS systems are very similar to those of the DTAB/NaPSS systems. When the surfactant monomers are added into the polymers, the values of I1/I3 decrease markedly toward a plateau region and then remain constant. The critical aggregation concentrations (cac), where the polymer-surfactant aggregates start to form, are determined as the intercepts between the linear extrapolations of the rapidly varying portion of the curves and of the almost-horizontal portions at high concentration. The cac values are summarized in Table 2. Beyond the cac, the surfactant molecules start to form micelle-like aggregates with the NaPSS chains. It is noted that the cac values are almost 100 times lower than the cmc values of DTAB and C12C6C12Br2 in these systems. The very low cac values reflect the strong interaction between the polymers and DTAB. In addition, the cac values decrease with a decrease in polymer concentration.16 Thus, here, the lower cac values also can be attributed to the lower polymer concentration. For the DTAB/NaPSS systems (Figure 3a), both the cac values and the I1/I3 values beyond the cac are very similar for different molar masses of NaPSS. However, for the C12C6C12Br2/NaPSS systems (Figure 3b), the cac values decrease and the I1/I3 values beyond the cac increase obviously as the NaPSS molar mass increases. This tendency is consistent with the self-aggregation features of NaPSS without any surfactants. Compared with NaPSS-2 and NaPSS-3, NaPSS-1 with low molar mass leads to a more compact and hydrophobic micelle structure with the gemini surfactant.
Anionic Polyelectrolytes and Cationic Surfactants On the basis of the above results, we try to understand the interaction process of the surfactants with NaPSS reflected in Figure 2. It should be mentioned that, in the calorimetric experiments, 50 mM DTAB was used as the titrant solution. This concentration is only 3.4 times the cmc, which means that a fraction of DTAB molecules exist as monomers in the titrant solution. However, all of the monomers bound on the polymer will aggregate into the micelle-like aggregates in the presence of the polymer beyond the cac. In the ITC experiments, the surfactant concentrations are already beyond the cac upon the first titration. Therefore, the coexistence of monomers and micelles in the titrant solution only affects the values of the observed enthalpy changes. Without adding surfactants, the NaPSS polymers except NaPSS-3 exist in the polymer aggregate form in aqueous solution at the used concentration of 0.05%. For the surfactant/NaPSS systems, the observed enthalpies are quite exothermic at the low surfactant concentration. The interaction enthalpy change may include the following main contributions: electrostatic interaction of the surfactant micellelike aggregates with the polymers, hydrophobic interaction between the surfactant molecules and the hydrophobic moieties of the polymers, the morphology change caused by the interaction of the surfactant aggregates with the polymers, and the phase separation caused by the binding of the surfactant aggregates with the polymers. The present large exothermic enthalpy may be mainly contributed by electrostatic attraction between the cationic head groups of the surfactants and the anionic groups of NaPSS, the permeation of the benzene rings of NaPSS into the headgroup region of the surfactant aggregates, as well as hydrophobic attraction between the hydrocarbon tails of the surfactants and the uncharged hydrophobic segments of NaPSS.37 Above CP, with an increase of the surfactant concentration, the polymer aggregates may start to associate with each other with the help of the bound surfactant micelles on the polymer molecules, leading to the precipitation, where the features associated with the DTAB/NaPSS interaction are similar to those of uncharged polymer/ionic surfactant systems.38 Normally, phase separation is a characteristic feature of the mixtures of strong polyelectrolytes and ionic surfactants when the degree of ion pairing is high. Yashida and Dubin39 studied the PAA/CTAC/C12E8 system and attributed phase separation to a 1:1 electrostatic charge neutralization. Similarly here, due to the formation of the PSS--DTA+ insoluble complex, the precipitation occurred around the charge neutralization point. The molar concentration of the NaPSS charge unit at 0.05 wt % is 2.44 mM. It is noted that the Cp values for the DTAB/ NaPSS systems are very close to the point of 1:1 electrostatic charge neutralization. However, for the C12C6C12Br2/NaPSS and C12C6C12Br2/NaPAA systems, the precipitation starts far below the 1:1 charge ratio. The possible reason is that the gemini surfactant may bind with the polymers much more tightly and then the density of the polymer/gemini surfactant complexes may become very large even far below the 1:1 surfactant/ polymer charge ratio. Beyond CR, the turbidity starts to decrease accompanied with less endothermic ∆Hobs; i.e., the precipitation starts to be redissolved. This means that the enthalpy change for the dissolution of the precipitate is small. The redissolution may be caused by electrostatic repulsion between positively charged micelles attached to the polymer molecules, which may result in the disruption of intermolecular aggregation of the polymer molecules. Then, the ITC curves coincide with the dilution curve of the surfactant solutions into water, indicating that the interaction of the NaPSS with the surfactants is saturated. The further added surfactant micelles will dissociate
J. Phys. Chem. B, Vol. 114, No. 32, 2010 10413 into monomers, and finally, beyond CM, free micelles begin to exist, i.e., the free micelles start to coexist with the polymer/ micelle aggregates. Beyond CM, the added micelles are just diluted. As for the C12C6C12Br2/NaPSS systems, the ITC and turbidity curves show obvious differences from the DTAB/NaPSS systems. Much higher exothermic ∆Hobs at the early stage of C12C6C12Br2 binding with NaPSS can be attributed to much stronger electrostatic interaction between the gemini micellelike aggregates and the charge units of NaPSS.40 Moreover, precipitation starts to take place far below the 1:1 charge ratio because the binding between C12C6C12Br2 and NaPSS should be much stronger than that between DTAB and NaPSS. Thus, the observed enthalpy changes may be contributed from the interaction between the surfactant and the polymers as well as the formation of precipitation. It can also be observed from Figure 2c and d that the ∆Hobs values for NaPSS-2 and NaPSS-3 are much more negative than that for NaPSS-1. Therefore, it can be concluded that the interaction between NaPSS-1 and C12C6C12Br2 is weaker than the cases of NaPSS-2 and NaPSS3. For NaPSS-1 and NaPSS-2, addition of C12C6C12Br2 beyond CR may change the insoluble aggregates from neutral to positive; therefore, the precipitation starts to be redissolved. However, for NaPSS-3 with high molar mass, the precipitate cannot be redissolved over the whole investigated concentration range of C12C6C12Br2. There are two possible factors to explain this point. On one hand, the flexibility of the polymer chain increases with increasing polymer molar mass. On the other hand, C12C6C12Br2 has a higher micellar surface charge density than single-chain surfactant DTAB. These may lead to tight binding of C12C6C12Br2 molecules with NaPSS-3 and an increase in the effective ionization equilibrium constant (pKa) of the negatively charged groups on the NaPSS-3 when they are bound to C12C6C12Br2.41,42 Obviously, the interaction of the polyelectrolyte with the gemini surfactant is much stronger than that with the singlechain surfactant, and the effect of the polymer molar mass becomes more significant when the polyelectrolyte interacts with the gemini surfactant. Interaction of NaPAA with Cationic Surfactants. Figure 4 shows the dependence of the I1/I3 values for the mixtures of DTAB and C12C6C12Br2 with NaPAA, respectively. All of the curves present a similar shape. As the surfactant monomers are added into the polymer solutions, the I1/I3 values decrease markedly toward a plateau region and then remain constant. These variations indicate that the NaPAA and surfactant molecules form mixed aggregates. The estimated cac values are also listed in Table 2. The cac values are almost the same for each surfactant, and the cac values for C12C6C12Br2 are 1 order lower than those for DTAB. There is a very small difference among the I1/I3 values of these three NaPAA samples, indicating that the I1/I3 value depends only weakly on the NaPAA molar mass. Figure 5 shows the ITC and turbidity curves for the concentrated DTAB and C12C6C12Br2 solutions being titrated into 0.05 wt % NaPAA solutions against the final concentration of the surfactants, together with the curves of the corresponding surfactants being titrated into water for comparison. The titration curves for the surfactant solutions into the NaPAA solution become increasingly different from the characteristic sigmoid shape for the surfactant into water without NaPAA. The difference can be attributed to the interactions between NaPAA and the surfactants. Since the final surfactant concentrations in the polymer solution are all larger than the cac values, the
10414
J. Phys. Chem. B, Vol. 114, No. 32, 2010
Wang and Wang
Figure 4. Variations of I1/I3 for (a) DTAB with 0.05 wt % NaPAA solution and (b) C12C6C12Br2 with 0.05 wt % NaPAA solution: (b) NaPAA-1; (2) NaPAA-2; (0) NaPAA-3.
Figure 5. (a) ITC curves for titrating 50 mM DTAB into 0.05 wt % NaPAA and pure water solution. (b) Turbidity curves for titrating 50 mM DTAB into 0.05 wt % NaPAA solution. (c) ITC curves for titrating 5 mM C12C6C12Br2 into 0.05 wt % NaPAA and pure water solution. (d) Turbidity curves for titrating 5 mM C12C6C12Br2 into 0.05 wt % NaPAA solution. (O) H2O; (b) NaPAA-1; (2) NaPAA-2; (0) NaPAA-3.
surfactant micelles bind with NaPAA in the aggregate form upon the addition. Different from the NaPSS systems, the initial additions of the surfactants into NaPAA solutions result in moderately endothermic enthalpy values. The possible reason is that the oppositely charged NaPAA and the surfactants bind with each other through electrostatic interaction, accompanied with dehydration from NaPAA and from the charged groups of the surfactants, which may result in the present endothermic enthalpy changes. The following situations are different between the DTAB/NaPAA systems and the C12C6C12Br2/NaPAA systems. As for the DTAB/NaPAA systems, with more surfactant micelles added, the ∆Hobs becomes more endothermic, leading to a pronounced endothermic peak, accompanied with the rapid increase of the turbidity, which can be attributed to the formation of DTAB/NaPAA aggregates and the resultant precipitation (CP). Above the endothermic peak, ∆Hobs decreases sharply and becomes less endothermic until CR. After CR, the turbidity sharply decreases due to the redissolution of the precipitation and finally reaches a small constant value. The shape of the ITC curves becomes similar to the ITC curve of DTAB without polymers. As the CM is approached, free micelles start to appear and the further addition of DTAB only leads to the dilution of the micelle solution. It is noted that the molar mass of NaPAA does not affect the DTAB/NaPAA interaction obviously.
However, the ITC curves for the additions of C12C6C12Br2 into NaPAA exhibit obvious differences. For NaPAA-1 and NaPAA-2, after an endothermic plateau region, several fluctuating exothermic and endothermic peaks are present within the 0.3-1.7 mM C12C6C12Br2 concentration range. These peaks are believed to characterize the occurrence of phase separation in the solutions. Correspondingly, the turbidity increases steeply with the C12C6C12Br2 concentration and the precipitation appears. Finally, the ITC curves are close to the dilution curve of C12C6C12Br2 into water. The ITC curve of the C12C6C12Br2/ NaPAA-3 system exhibits a pronounced endothermic “plateau” peak and then ∆Hobs decreases rapidly followed by a steep increase. Unlike the NaPAA-1 and NaPAA-2 systems, NaPAA-3 displays only an exothermic peak. The sharp exothermic peak may be attributed to the precipitation, supported by the turbidity titration curve. Obviously, the precipitation starts to take place far below the 1:1 C12C6C12Br2/NaPAA charge ratio, and the precipitate can only be redissolved when the gemini concentration is higher than its cmc. This means that the gemini surfactant binds with the NaPAA much more strongly than DTAB. On the basis of the above discussion, it is obvious that both the NaPAA/DTAB and NaPAA/C12C6C12Br2 systems show a similar shape and feature in the turbidity curves but a remarkable difference in the ITC curves. These results show that the
Anionic Polyelectrolytes and Cationic Surfactants interaction of the surfactants with the polyelectrolytes is much stronger for a gemini surfactant in comparison with a singlechain surfactant, and the interaction of the polyelectrolytes with the surfactants is strongly dependent on the polyelectrolyte molar mass and the surfactant architecture. Comparison of the Interactions of NaPAA and NaPSS with Surfactants. Both NaPAA and NaPSS are strongly charged polyelectrolytes and have a significant interaction with cationic surfactants. However, we have seen that, although both NaPSS and NaPAA are vinyl-based polyelectrolytes with the same linear charge separation, they display great differences while interacting with surfactants. The interaction at lower surfactant concentration is a highly exothermic process for the NaPSS with the surfactants, whereas it turns into an endothermic process for the NaPAA with the surfactants. The interaction differences between the surfactant/NaPAA and surfactant/NaPSS systems can be attributed to their different architectures. The major contributing factors to the enthalpies of interaction may be (1) electrostatic attractions between the negative units of polymer and the positive segments of surfactant; (2) hydrophobic interaction between the polymer and the surfactant alkyl chains, the magnitude of which depends on the hydrophobicity and flexibility of the polymer; (3) hydrophobic interaction among the surfactant alkyl chains. As reported by Kwak and co-workers,43,44 for the DTAB/NaPSS systems, the first two methylene groups next to the surfactant headgroup are close to the benzene ring of the styrene groups of the NaPSS, suggesting that units of the PSS are incorporated in the micelle surface. As an effect of this inherent surface activity of the PSS, the charges on polyelectrolyte and micelle are in close contact, thus giving the complexes a low electrostatic internal energy. As a consequence, this binding is a highly exothermic process. However, for the DTAB/NaPAA systems, the interaction process of NaPAA with the surfactants is different. Due to the absence of the energetically favorable aggregates, the electrostatic interaction between NaPAA and DTAB is weaker than that between NaPSS and DTAB. In addition, NaPAA is much more flexible than NaPSS at a similar degree of polymerization, as demonstrated by their respective persistence lengths.45 Simulation of polyelectrolyte/surfactant interactions suggests that polymer chain flexibility plays a major role in determining the nature of the interaction.13 In the DTAB/NaPAA systems, the dehydration from NaPAA and from the charged groups of the surfactants may be accompanied with the electrostatic binding between oppositely charged NaPAA and the surfactants. Hence, the DTAB/NaPAA binding is endothermic. A similar difference from the ITC curves of the C12C6C12Br2/NaPSS and C12C6C12Br2/ NaPAA systems is also observed. On the basis of the above ITC, turbidity, and fluorescence results, it can be concluded that the interaction of a flexible polyelectrolyte NaPAA with surfactants depends only weakly on the polyelectrolyte molar mass, whereas the interaction of a stiff polyelectrolyte NaPSS with surfactants strongly depends upon the polyelectrolyte molar mass. Conclusion ITC, turbidity, and fluorescence measurements have been used to investigate the influence of the polyelectrolyte molar mass, chain flexibility, and surfactant architecture on the interaction of cationic surfactants with NaPSS and NaPAA. Without any surfactants, low molar mass NaPSS can form hydrophobic aggregates in the solution, whereas high molar mass NaPSS and all three NaPAA with different molar masses do not have this aggregation behavior. The interaction of the polyelectrolyte
J. Phys. Chem. B, Vol. 114, No. 32, 2010 10415 with gemini surfactant C12C6C12Br2 is much stronger than that with single-chain surfactant DTAB. Moreover, the interaction of the stiff NaPSS with the surfactants strongly depends on the polyelectrolyte molar mass, whereas the polyelectrolyte molar mass almost does not affect the interaction of the flexible NaPAA with the surfactants. Especially, the effect of the polyelectrolyte molar mass becomes more significant while the polyelectrolyte interacting with gemini surfactant than with single-chain surfactant. This study may shed new light on understanding the relationship among the effects of polyelectrolyte molar mass, polyelectrolyte flexibility, and surfactant structures on polyelectrolyte-surfactant interaction. Acknowledgment. We are grateful for the financial support from Chinese Academy of Sciences, National Natural Science Foundation of China, National Basic Research Program, and National High Technology Research and Development Program of China (grants 20633010, 20873158, 2005CB221300, and 2007AA090701-2). References and Notes (1) Evans, D. F.; Wennerstro¨m, H. The colloidal Domain where Physics, Chemistry, Biology, and Technology Meet; VCH Publisher: New York, 1994. (2) Jo¨nsson, B.; Lindman, B.; Holmberg, K.; Kronberg, B. Surfactants and Polymers in Aqueous Solution; John Wiley & Sons Ltd: West Sussex, U.K., 1998. (3) Iliopoulos, I. Curr. Opin. Colloid Interface Sci. 1998, 3, 493–498. (4) Ilekti, P.; Piculell, L.; Torunilhac, F.; Cabane, B. J. Phys. Chem. B 1998, 102, 344–351. (5) Ilekti, P.; Piculell, L.; Torunilhac, F.; Cabane, B. J. Phys. Chem. B 1999, 103, 9831–9840. (6) Goddard, E. D. J. Am. Oil Chem. Soc. 1994, 71, 1–16. (7) Kayitmazer, A. B.; Seyrek, E.; Dubin, P. L.; Staggemeier, B. A. J. Phys. Chem. B 2003, 107, 8158–8165. (8) Overbeek, J. T. G.; Voorn, M. J. J. Cell. Comput. Physiol. 1957, 49, 7–22. (9) Thalberg, K.; Lindman, B.; Karlstro¨m, G. J. Phys. Chem. 1991, 95, 3370–3376. (10) Wang, Y.; Kimura, K.; Dubin, P. L. Macromolecules 2000, 33, 3324–3331. (11) Dubin, P. L.; Chew, C. H.; Gan, L. M. J. Colloid Interface Sci. 1989, 128, 566–576. (12) Choi, L.; Kim, O. Langmuir 1994, 10, 57–60. (13) Wallin, T.; Linse, P. Langmuir 1996, 12, 305–314. (14) Skepo, M.; Linse, P. Phys. ReV. E 2002, 66, 51807–51814. (15) Kayitmazer, A. B.; Shaw, D.; Dubin, P. L. Macromolecules 2005, 38, 5198–5204. (16) Hansson, P.; Almgren, M. Langmuir 1994, 10, 2115–2124. (17) Macdonald, P. M.; Tang, A., Jr. Langmuir 1997, 13, 2259–2265. (18) Kiefer, J. J.; Somasundaran, P.; Ananthapadmanabhan, K. P. Langmuir 1993, 9, 1187–1192. (19) Lim, P. F. C.; Chee, L. Y.; Chen, S. B.; Chen, B. J. Phys. Chem. B 2003, 107, 6491–6496. (20) Taylor, D. J. F.; Thomas, R. K.; Li, P. X. Langmuir 2003, 19, 3712– 3719. (21) Zana, R. AdV. Colloid Interface Sci. 2002, 97, 205–253. (22) Menger, F. M.; Keiper, J. S. Angew. Chem., Int. Ed. 2000, 39, 1906– 1920. (23) Wang, X.; Li, Y.; Wang, J.; Wang, Y.; Ye, J.; Yan, H.; Zhang, J.; Thomas, R. K. J. Phys. Chem. B 2005, 109, 12850–12855. (24) Wang, X.; Wang, J.; Wang, Y.; Yan, H.; Li, P.; Thomas, R. K. Langmuir 2004, 20, 53–56. (25) Bai, G.; Wang, Y.; Yan, H.; Thomas, R. K.; Kwak, J. C. T. J. Phys. Chem. B 2002, 106, 2153–2159. (26) Jiang, N.; Li, P. X.; Wang, Y. L.; Wang, J. B.; Yan, H. K.; Thomas, R. K. J. Phys. Chem. B 2004, 108, 15385–15391. (27) Menger, F. M.; Littau, C. A. J. Am. Chem. Soc. 1993, 115, 10083– 10090. (28) Kalyanasundaram, K.; Thomas, J. R. J. Am. Chem. Soc. 1977, 99, 2039–2044. (29) Zana, R. In Surfactant Solutions. New Methods of InVestigation; Zana, R., Ed.; Marcel Dekker: New York, 1987; p 241. (30) Gucker, F. T., Jr.; Pickard, H. B.; Planck, R. W. J. Am. Chem. Soc. 1939, 61, 459–470. (31) Kreshech, G. C.; Haragraves, W. A. J. Colloid Interface Sci. 1974, 48, 481–493.
10416
J. Phys. Chem. B, Vol. 114, No. 32, 2010
(32) Anderson, B.; Olofsson, G. J. Chem. Soc., Faraday Trans. 1 1988, 84, 4087–4095. (33) Van Os, N. M.; Daane, G. J.; Haandrikman, G. J. Colloid Interface Sci. 1991, 141, 199–217. (34) Zana, R. Langmuir 1996, 12, 1208–1211. (35) Linse, P.; Piculell, L.; Hansson, P. In Polymer-Surfactant Systems; Kwak, J. C. T., Ed.; Marcel Dekker: New York, 1998; p 193. (36) Wang, Y.; Han, B.; Yan, H.; Kwak, J. C. T. Langmuir 1997, 13, 3119–3123. (37) Kelley, D.; McClements, D. J. Food Hydrocolloids 2003, 17, 73– 85. (38) Wang, G.; Olofsson, G. J. Phys. Chem. B 1998, 102, 9276–9283. (39) Yashida, K.; Dubin, P. L. Colloids Surf., A 1999, 147, 161–167.
Wang and Wang (40) Li, Y.; Ghoreishi, S. M.; Warr, J.; Bloor, D. M.; Holzwarth, J. F.; Wyn-Jones, E. Langmuir 2000, 16, 3093–3100. (41) Thongngam, M.; McClements, D. J. J. Agric. Food Chem. 2004, 52, 987–991. (42) Wang, Y.; Kimura, K.; Huang, Q.; Dubin, P. L.; Jaeger, W. Macromolecules 1999, 32, 7128–7134. (43) Gao, Z.; Kwak, J. C. T.; Wasylishen, R. E. J. Colloid Interface Sci. 1988, 126, 371–376. (44) Gao, Z.; Wasylishen, R. E.; Kwak, J. C. T. J. Phys. Chem. 1990, 94, 773–776. (45) Tricot, M. Macromolecules 1984, 17, 1698–1704. (46) Bashford, M. T.; Woolley, E. M. J. Phys. Chem. 1985, 89, 3173–3179.
JP9102405
