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Sep 22, 2009 - Morphologically Tunable Coassembly of Double Hydrophilic Block ... Dashuang Fan, Handi Yin, Dongxing Cai, Zhencun Cui, and Weiguo ...
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J. Phys. Chem. B 2009, 113, 13566–13575

Interactions and Aggregations in Aqueous and Brine Solutions of Poly(diallydimethylammonium chloride)/Sodium Bis(2-ethylhexyl) Sulfosuccinate Peizhu Zheng,† Xueqin An,‡,§ Xuhong Peng,† and Weiguo Shen*,†,‡ Department of Chemistry, Lanzhou UniVersity, Lanzhou, Gansu, 730000, China, Department of Chemistry, East China UniVersity of Science and Technology, Shanghai, 200237, China, and Jiangsu Key Laboratory of Biofunctional Materals, Nanjing Normal UniVersity, Nanjing 210097, China ReceiVed: March 20, 2009; ReVised Manuscript ReceiVed: September 2, 2009

The interactions between the anionic surfactant sodium bis(2-ethylhexyl) sulfosuccinate (AOT) and the polycation poly(diallydimethylammonium chloride) (PDDAC), the aggregations of AOT and PDDAC-bound AOT in PDDAC/AOT aqueous solutions, and the influence of salt on the interactions and aggregations have been studied by isothermal titration calorimetry (ITC), dynamic light scattering (DLS), and negative staining transmission electron microscopy (TEM). The adsorptions of AOT onto PDDAC and the formations of PDDACbound AOT micelles, free AOT micelles, and AOT vesicles were examined, and the corresponding critical concentrations were determined. Combining calculations of thermodynamic parameters with the above three experimental techniques, it was shown that the micellization of free AOT is driven by entropy gain, while the adsorption of AOT onto PDDAC and the micellization of PDDAC-bound AOT are driven by both enthalpy and entropy. It was also found that addition of salt enhances the binding of AOT onto PDDAC through the ion exchange and favors the formations of PDDAC/AOT micelles, free AOT micelles, and free AOT vesicles but prevents the transition of PDDAC/AOT micelles to the vesicles. Thermodynamic analysis suggested that the adsorption of AOT onto PDDAC and the micellization of PDDAC/AOT in PDDAC/AOT brine solutions are different in mechanism compared with that in corresponding aqueous solutions. 1. Introduction Aqueous solutions containing polymers and surfactants are of importance, because they have wide various applications in industrial, cosmetic, pharmaceutical, agricultural, and domestic products.1-10 A large number of researchers have devoted their attention to advancing the fundamental understanding on the interactions between polymers and surfactants and the aggregations and phase behaviors of aqueous polymer/surfactant systems using different techniques, such as electromotive force (EMF) measurement,2,4,11 surface tension measurement,1,7 fluorescence quenching,3,9 light scattering,3,5 and microcalorimetry.4,7-12 Among these techniques, the isothermal titration calorimetry (ITC) is so powerful that not only the phase behaviors of the polymer/surfactant systems but also the interactions between polymer and surfactant and the aggregations of free surfactant and polymer-bound surfactant can be investigated. The calorimetric studies can also give the corresponding thermodynamic parameters such as enthalpy (∆H), entropy (∆S), Gibbs energy (∆G), and various critical concentrations. It is generally accepted that the main interaction between the nonionic polymers and surfactants is hydrophobic13,14 while the interactions between polyelectrolytes and oppositely charged surfactants are both hydrophobic and electrostatic.6,10,12,15,16 Recently, some studies showed that in an aqueous polyelectrolyte/opposite-charged-surfactant systems, the surfactant monomers bind to the polymer chains through the ion-exchange process in which condensed counterions on the polymer chains * Corresponding author. Phone: +86-21-64250047. Fax: +86-2164252510. E-mail: [email protected]. † Lanzhou University. ‡ East China University of Science and Technology. § Nanjing Normal University.

are replaced by the surfactant and the abrupt positive enthalpy changes occur; thus, the electrostatic binding was believed to be mainly driven by the translational entropy regained by the released condensed counterions.6,10,12,16 However, the interactions of polyelectrolyte and oppositely charged surfactant in the aqueous solutions are complex and they have been found to be dependent on a variety of factors, such as the polyion molecular weight,17 chain flexibility of polyelectrolyte backbone,18 charge density,17-19 the size and structure of the head and tail groups of the surfactant,20 and the ion strength.12,21,22 The balance of those interactions determines the binding mechanism of surfactant monomers onto the polymers. It has been observed that the initial bindings of surfactant monomers to the polymer chains are driven by the hydrophobic interaction even in some polyelectrolyte/opposite-charged-surfactant system such as poly(acrylic acid)/dodecyltrimethylammonium bromide solutions with a lower degree of neutralization and the hydrophobic binding energies are usually negative.23 It was also found that the critical concentrations of the binding driven by the hydrophobic interactions are lower than that by the electrostatic ones.23 Binding of surfactant onto a polyelectrolyte chain elevates the local concentration around the polymer and leads to formation of polymer-bound surfactant aggregates. Studies of hydrophobic and electrostatic effects on polymer/surfactant interactions and on surfactant aggregations in the presence of polymer will help to design rational polymer/surfactant systems for various applications. Several theoretical models have been proposed for the interaction between polymer and surfactant and the aggregation of polymer-bound surfactant. On the basis of the Gibbs equation and the Langmuir isotherm, Bell1 developed a model to describe the variations in surface tension with the concentration of bulk surfactant for strongly interacting polymer-surfactant systems.

10.1021/jp902536t CCC: $40.75  2009 American Chemical Society Published on Web 09/22/2009

Interaction between PDDAC and AOT Zimm-Bragg theory24 treated the bound surfactant as an adsorbed one-dimensional lattice gas. Wang and Tam10,12 applied Manning’s theory to discuss the influence of the free counterion concentration of polymer on the equilibrium of ion-exchange interaction, and applied a pseudophase separation model to obtain the thermodynamic parameters of micellization of polymer-bound surfactant. The aqueous solution of poly(diallydimethylammonium chloride) (PDDAC)/sodium dodecyl sulfate (SDS) is a typical one of polyelectrolyte/opposite-charged-surfactant systems and has been investigated widely.15-19,21,25 Lee and Moroi15 found that the critical aggregation concentration of PDDAC/SDS in their aqueous solutions is very small (about 1.9 × 10-3 mM) because PDDAC has a high charge density. Carnali and Shah16 reported that the predominant feature in the phase map of PDDAC/SDS aqueous solutions is a broad, two-phase region which lies asymmetrically around the 1:1 stoichiometry of surfactant charge groups to polymer charge units. The interaction between the polymer and the surfactant and the phase behaviors can be modified by the variation of the molar ratio of SDS to polyoxyethylene tert-octyl phenyl ether (TX100) in their mixed surfactants.17-19,21 The anionic surfactant sodium bis(2-ethylhexyl) sulfosuccinate (AOT) is a versatile surfactant, which has been widely used as a component of microemulsions without cosurfactant. Comparing with the single chain surfactant SDS which has a negative enthalpy of micellization, the double chain surfactant AOT has a smaller critical micelle concentration (cmc) but a positive enthalpy of micellization.26 The double tails can be regarded as a small hydrophobic pool to accommodate the hydrophobic solutes and chains,27 and possibly more favor to contact with the polymer backbone as compared with the single tail of SDS. This characteristic of AOT may result in a different mechanism from SDS in the interaction of the surfactant with polyelectrolyte and the polymer-bound surfactant aggregation. To the best of our knowledge, the interaction between AOT and oppositely charged polymer, the aggregation of polyelectrolyte-bound AOT, and the phase behavior of PDDAC/AOT aqueous and brine solutions have not yet been investigated. In this paper, we study the interaction between AOT and PDDAC, the aggregation of the PDDAC-bound AOT, and the influence of sodium sulfate (Na2SO4) on the interactions and aggregations in their aqueous solution using isothermal titration calorimetry, dynamic light scattering (DLS), and negative staining transmission electron microscopy (TEM). The ITC results are analyzed to give the thermodynamic parameters of the free AOT micellization, the interaction between AOT and PDDAC, the aggregation of the PDDAC-bound AOT, and the vesicle formation. By combining ITC with DLS and TEM techniques, detailed information on the energetic and structural changes is detected to probe into the mechanism of formation of polymer/surfactant complexation. 2. Experimental Details 2.1. Material. Poly(diallydimethylammonium chloride) (PDDAC, 20% aqueous solution) was provided by Aldrich Chemical Co. which has an average molecular weight of approximately 3.2 × 105 g/mol determined by static light scattering measurements in our laboratory. Sodium bis(2ethylhexyl) sulfosuccinate (AOT, g99%) and sodium sulfate (Na2SO4, g99%) were purchased from Sigma and Guangfu Chemical Co, respectively. The chemical structures of PDDAC and AOT are shown schematically in Figure 1. Twice distilled water was used in all experiments.

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Figure 1. Chemical structures of PDDAC and AOT.

2.2. Isothermal Titration Microcalorimetry. The isothermal titration data were obtained using the TAM 2277-201 microcalorimetric system (Thermometric AB, Ja¨rfa¨fla, Sweden), which has 4 mL sample and reference cells. The titration was carried out at 25.00 ( 0.02 °C by injecting concentrated surfactant solution from a 1000 µL syringe into the sample cell filled with 2.2 mL of water or Na2SO4 aqueous solution, PDDAC aqueous solution, or PDDAC/Na2SO4 aqueous solution. The volume of each injection was 10-15 µL, and the total titration volume was 0.8-0.9 mL. The concentration of AOT in the syringe was 0.04 M except for the experiments with 50 mM Na2SO4 where the AOT concentration was 0.02 M. The injection schedule was automatically carried out using Digitam 4.1 software after setting up the number of injections, volume of each injection, and time between each injection. All experiments were carried out at least twice. 2.3. Dynamic Light Scattering. The Brookhaven laser light scattering system, which consists of a BI200SM goniometer and a BI9000AT digital correlator, was used to determine the intensity of scattering light (I) and the hydrodynamic radius (Rh). The light source is an argon-ion laser with a wavelength of 488 nm. All measurements were carried out at a scattering angle of 90° and at 25 ( 0.1 °C. 2.4. Transmission Electron Microscopy. The samples were observed by using the negative staining method, and 4 wt % phosphotungstic acid was used as the staining agent. The solution was placed onto a copper grid, and the excess liquid was sucked away by filter paper. After drying, the samples were imaged under a JEM - 1200EX transmission electron microscope. 3. Results and Discussion 3.1. Micellization of AOT in the Brine. First, we studied the micellization of AOT in the brine with the Na2SO4 concentrations (CNa2SO4) being 0, 5, 10, 20, and 50 mM, respectively, by using isothermal titration calorimetry. The values of the observed differential enthalpy (∆Hobs) for various concentrations of AOT were obtained from the integral of the areas under the calorimetric peaks, and normalized by the small amounts of injected surfactants. The plots of ∆Hobs versus concentrations of AOT, which are named as enthalpy curves throughout this paper, are shown in Figure 2a. All of the enthalpy curves in Figure 2a are sigmoidal shapes. In the submicellar regions, the added micelles disassociate into monomers and the monomers are further diluted; therefore, ∆Hobs represents the total contributions of the disassociation and the dilution. When the AOT concentrations are above the critical micelle concentrations, the added micelles are only diluted without dissociation; thus, ∆Hobs represents the heat of the dilution of the micelles. At the critical micelle concentration, the monomers start to aggregate and there are abrupt changes of enthalpy curves corresponding to the micelle formations. The

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Zheng et al. creases and the hydrophobic interaction increases sufficiently by addition of the salt, the enthalpy becomes exothermic. The Gibbs free energy of micellization (∆G°mic), which represents the transfer of one mole of surfactant from the aqueous phase to the micellar pseudophase, is calculated by the expression30 o ∆Gmic ) (1 + f)RT ln(cmc)

(1)

where f stands for the fraction of charges of micellized ions neutralized by counterions, with a value of 0.1 for AOT.31 The entropy of micellization ∆S°mic can be obtained from

∆S°mic )

Figure 2. (a) Enthalpy curves of titrating AOT into Na2SO4 aqueous solutions at different CNa2SO4: (9) 0 mM; (O) 5 mM; (2) 10 mM; (3) 20 mM; ([) 50 mM. (b) Determinations of the cmc and the enthalpy of micellization from the enthalpy curve: (9) 0 mM.

TABLE 1: Thermodynamic Parameters of AOT at Different Na2SO4 Concentrations CNa2SO4 (mM)

cmc (mM)

∆H°mic (kJ/mol)

∆G°mic (kJ/mol)

∆S°mic (J/(mol K))

cvc (mM)

0 5 10 20 50

2.18 1.16 1.01 0.57 0.29

4.39 3.29 1.95 -3.30 -3.47

-16.71 -18.43 -18.81 -20.72 -22.21

70.77 72.85 69.62 57.25 62.86

7.72 5.91 3.66

critical micelle concentrations can be determined by extrapolations of the initial linear portions and the linear rapidly changing portions of the curves. The enthalpy of micellization (∆H°mic) for each Na2SO4 concentration can be obtained from the difference between the observed enthalpies of the two linear segments of the plots at the critical micelle concentration,28,29 as shown in Figure 2b. The values of cmc and ∆H°mic at different Na2SO4 concentrations are given in Table 1. From Table 1, it is found that the value of cmc decreases from 2.18 to 0.29 mM when the Na2SO4 concentration increases from 0 to 50 mM. This Na2SO4 concentration dependence may be attributed to the fact that the additional salt screens the electrostatic repulsion between the surfactant headgroups and increases the hydrophobicity of surfactant tails, thereby increasing the stability of the micelles. It is worth noting that the values of ∆H°mic are positive for CNa2SO4 e 10 mM but all negative for the rest of the concentrations. The interaction between AOT molecules is controlled by two contrary effects: the hydrophobic interaction of alkyl chains tends to make ∆H°mic negative, while the electrostatic repulsion between the surfactant headgroups and steric hindrance among the ethyl branches result in the endothermic effect and are unfavorable to aggregation. As the electrostatic repulsion between the surfactant headgroups de-

∆H°mic - ∆G°mic T

(2)

The Gibbs free energy and entropy of micellization are also summarized in Table 1. All of the values of ∆G°mic are negative, and the absolute value of ∆G°mic increases with CNa2SO4, indicating that addition of the salt favors the formation of AOT micelles. All of the values of ∆S°mic are positive, which are caused by disrupting the water structure during micellization. When CNa2SO4 e 10 mM, the micellization is entropy driven because ∆H°mic is positive and T∆S°mic g ∆H°mic, while, for CNa2SO4 ) 20 or 50 mM, the micellization is driven by entropy and enthalpy simultaneously. There is a turning point at the concentration of AOT being about 7.7 mM for the system without Na2SO4 (see Figure 2b), which indicates the existence of the critical vesicle concentration (cvc), corresponding to the transition of AOT micelles to the vesicles. This value is consistent with that reported in refs 13 and 32. The transitions of AOT micelles to the vesicles for the systems with Na2SO4 being 5 and 10 mM have also been detected, and the values of cvc are also listed in Table 1. The value of cvc decreases with an increase of Na2SO4 concentration, indicating that addition of Na2SO4 favors the formation of the AOT vesicles. The enthalpy changes of vesicle formation for the above three solutions are negative and the absolute values decrease with an increase of the concentration of Na2SO4. However, we were unable to detect the transition for the systems with the concentrations of Na2SO4 being 20 and 50 mM, possibly due to the small values of enthalpy changes of vesicle formation and large fluctuations in measurements. 3.2. Adsorption and Micellization of AOT in PDDAC Solutions. The enthalpy curves titrating AOT into PDDAC solutions with the concentrations of PDDAC (CPDDAC) being 0.112, 0.218, 0.400, and 1.088 g/L, respectively, are plotted in Figure 3a. All curves are also sigmoidal shapes with two exothermic peaks (A) and (B), and the sigmoidal profile is similar to that observed from the micellization of AOT in its aqueous solutions. As shown in Figure 3b, the critical concentrations, such as the onset (C′) of peak A, the concentration (Cmin) at the minimum of peak A, the onset (Cm) of the sigmoidal profile, the onset (C2) where the enthalpy curve merges with the dilution curve of the AOT micelle aqueous solution, and the onset (cvc) of peak B can be clearly identified from the plots of enthalpy curves. The locations of C′, Cm, and cvc can be determined as the intersections of two extrapolations of the linear portions on the left and right sides of the turning points. Table 2 lists all of the critical concentrations at different PDDAC concentrations. The higher polymer concentration, the larger values of C′, Cmin, and the area of peak A are, corresponding to

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Figure 4. Profiles of adsorption, micellization, and formation of vesicles obtained from light scattering and ITC for 0.218 g/L PDDAC solution.

Figure 3. (a) Enthalpy curves of titrating AOT into PDDAC aqueous solutions at different CPDDAC: (O) 0.112 g/L; (2) 0.218 g/L; (3) 0.400 g/L; ([)1.088 g/L. (b) Determinations of the critical concentrations and the enthalpy of micellization from the enthalpy curves at different CPDDAC: (9) 0 g/L; (3) 0.400 g/L.

TABLE 2: Critical Concentrations at Different PDDAC Concentrations CPDDAC (g/L) C′ (mM) Cmix (mM) Cm (mM) C2 (mM) cvc (mM) 0.112 0.218 0.400 1.088

0.56 1.32 2.13 5.17

1.07 1.83 2.80 5.64

2.84 3.58 4.51 7.37

4.40 5.36 7.47 10.01

10.13 10.52 10.47 10.36

the more surfactant molecules being adsorbed to the polymer chains and further transiting to polymer/surfactant micelles. The intensity of scattering light (I) at the scattering angle of 90° and the autocorrelation function of the diffusing particles were determined for a series of ternary aqueous solutions with various concentrations of AOT and a fixed concentration of PDDAC being 0.218 g/L. The hydrodynamic radii Rh of the particles then were calculated from the Stokes-Einstein expression:

Rh )

kTq2 6πηΓ

(3)

with the scattering vector q ) 4πn sin(θ/2)/λ, where n is the refractive index of the solution, θ is the scattering angle, λ is the wavelength of incident laser light, k is the Boltzmann constant, T is the absolute temperature, η is the viscosity of solvent water, and Γ is the decay rate. The hydrodynamic radius and scattering intensity together with the observed enthalpy titrating AOT micelle solution into 0.218 g/L PDDAC aqueous solution are plotted against the total concentration of AOT in Figure 4. As shown in Figure 4, the critical concentrations detected by light scattering are close to that from ITC. When a small amount of AOT micelle solution is added into a PDDAC aqueous solution, the AOT micelles demicellize into

a few monomers and the solution is transparent. The scattering intensity is about 390 kcps, and the hydrodynamic radius of the particles is about 160 nm, which is identical to the radius of gyration of the polymer molecules in their aqueous solutions. The observed enthalpy is negative and coincides with that of initially titrating AOT into water (see Figure 3b), and no interaction between AOT and PDDAC is revealed. With CAOT increasing, the solution gradually becomes opaque; the scattering intensity increases to about 1000 kcps, while the hydrodynamic radius decreases to about 110 nm, as demonstrated in region 1 of Figure 4. In this region, the AOT micelles demicellize into monomers and the monomers are adsorbed to the PDDAC chains. It weakens the electrostatic repulsion between the polymer chains and strengthens the hydrophobicity of the polymer which shrinks the polymer chains, leading to a slight decrease of Rh. In addition, the adsorbed surfactant molecules increase the molar mass of polymer and the scattering intensity.12 Slight deviation of the observed enthalpy from the dilution curve of AOT aqueous solution is found at the concentration of AOT being about 0.55 mM, and it continuously decreases slowly with CAOT increasing. The enthalpy curves shown in Figure 3 are significantly different from that of binding of surfactant onto polymer through the ion-exchange process, which usually shows a sharp positive change of the enthalpy curve and is known as entropy driving.10,12,33 However, they are similar to the binding enthalpy curves in aqueous dodecyltrimethylammonium bromide/poly(acrylic acid) solution with a lower degree of neutralization which shows negative binding energy. It suggests that the adsorptions of AOT molecules to PDDAC should be dominated by the hydrophobic interaction among the long alkyl tails of the surfactant molecules and the hydrophobic polymer backbones in our experimental concentration range. Although the adsorptions could possibly be initially driven by the electrostatic interaction between opposite charges and ion exchange at very low AOT concentration, once the surfactant molecules are in the polymer, the hydrophobic interaction could result in the formation of order structure in the polymer/surfactant complex.34 Possibly the double tail AOT more favors contact with the polymer backbone as compared with SDS; thus, only a few headgroups of AOT get close enough to the condensed counterions on the polymer chain and replace them to complete the ion-exchange process, while most of the tails of the AOT molecules penetrate to the polymer backbone and keep the headgroups toward the bulk water.23,35 The continuous decrease of observed enthalpy with increase of the AOT concentration implies that AOT molecules adsorbed on the polymer chains change the hydrophobic environment and favor the further adsorption of AOT.

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This slow decrease of ∆Hobs continues until the concentration reaches C′, where a remarkable exothermic effect (∆H°mp) starts to be detectible. This possibly indicates the formation of polymer/surfactant micelles and precipitation of the cross-link clusters; the process enters region 2 denoted in Figure 4. In this region, the added AOT micelles still experience demicellization, dilution of monomers, and adsorption of monomers to PDDAC chains; meanwhile, some neighborly adsorbed surfactant molecules start to aggregate through hydrophobic interaction and form the micelles bound by different polymer chains, which results in the cross-links of the polymer chains, thereby leading to a decrease in ∆Hobs and an increase in hydrodynamic radius. The maximum of the hydrodynamic radius is about 850 nm and located at a concentration between C′ and Cmin. The polymer solutions become more and more viscous, and the precipitation becomes more visible and adheres on the bottom of the sample cell as the concentration of AOT continually increases. On the other hand, the upper layer is getting more transparent, and the scattering intensity in this layer decreases sharply to about 33 kcps in this process. The formation of polymer/surfactant micelles may be regarded as a pseudophase transition, and the critical micelle concentration is Cmin. As more AOT is added, ∆Hobs and I increase, while Rh sharply decreases to 110 nm and then the decrease slows down; simultaneously, the precipitation is redissolved and finally invisible. It suggests that the added AOT molecules participate in restructuring the polymer/surfactant micelles and unfold the cross-links of the different polymer chains to yield a larger amount but smaller sized particles. Further added AOT molecules mainly increase the concentration of free AOT monomers, although a few of them possibly adsorb to the polymer chains or join into the PDDAC/AOT micelles. The increase of ∆Hobs slows down to the slope similar to that of a dilution curve of free AOT. When the concentration of AOT reaches Cm, further addition of AOT micelles causes ∆Hobs and I to increase sharply. This stage has an identical profile of ∆Hobs as the micellization of AOT in its aqueous solution and indicates the formation of free AOT micelles. The process enters region 3 denoted in Figure 4. After the enthalpy curve merges with the dilution curve at C2, designated as the saturation concentration of AOT where the polymer chains are fully saturated with surfactant molecules, the three curves level off and the average radius is about 70 nm. When the concentration of AOT is in the region between Cm and C2, except for keeping formation of the free micelles, a few of the added AOT molecules possibly also participate in the formation of the polymer/surfactant micelles, or adsorb to the polymer chains. However, when the concentration of AOT is higher than C2, the added AOT micelles only concentrate the free micelles, behaving as that in the binary aqueous solution without the polymer. It was found that the values of Cm and C2 increased as the concentration of the polymer, which resulted from the fact that more polymer molecules adsorbed more AOT molecules. The process of concentrating the free micelles is disturbed at cvc, which is believed to represent the transition point of the micelles to the vesicles. The curves enter region 4 denoted in Figure 4; the value of Rh increases slightly to 80 nm, whereas I decreases from 1500 to 1200 kcps. It was found that the values of cvc were nearly independent of PDDAC concentration, which was consistent with the results reported by Jose´ I Briz.32 If only the free micelles are involved in the transition from micelles to vesicles, the values of cvc corresponding to the total concentra-

Zheng et al. tion of AOT would vary with the concentration of PDDAC because the amount of AOT adsorbed onto the polymer should be proportional to the concentration of the polymer which in turn proportionally reduces the amount of nonadsorbed free AOT. Therefore, the transition from the micelles to the vesicles is possibly not only for the free micelles but also for the part of AOT molecules adsorbed on the polymer and the polymer/ surfactant micelles. The integral enthalpy changes of vesicle formation for the above four solutions are all negative, and their absolute values are significantly larger than that in the AOT aqueous solutions without polymer, indicating once again that the AOT molecules adsorbed on the polymer and the polymer/ surfactant micelles are involved in the formation of the vesicles. 3.3. Thermodynamic Description of Adsorption and Micellization in PDDAC/AOT Aqueous Solutions. Unlike an ionexchange process, the adsorption of AOT molecules onto PDDAC in the PDDAC/AOT aqueous solutions is nonchargedsaturated and their micellization occurs on polymer chains. On the basis of these characteristics, a thermodynamic model has been developed to describe the adsorption and micellization in the PDDAC/AOT aqueous solutions. The equilibrium of adsorption of the surfactant AOT on the polymer chains may be characterized by s p µAOT ) µAOT

(4)

s p and µAOT are the chemical potentials of AOT in the where µAOT solution and on the polymer chains, respectively, which are defined by

s s° µAOT ) µAOT + RT ln

y-x c1

(5)

x zc2

(6)

p po µAOT ) µAOT + RT ln

where y is the total concentration of AOT in the system (mmol/ L), x is the concentration of AOT adsorbed from the solution onto the polymer (mmol/L), z is the concentration of polymer (g/L) in the system, and y - x (mmol/L) and x/z (mmol/g) represent the concentrations of the free AOT monomers in the solution and the adsorbed AOT on the polymer chains. The units c1 and c2 of concentrations, equaling 1 mmol/L and 1 mmol/g, respectively, are used to normalize the concentration variables s° p° and µAOT are the chemical potentials at of y - x and x/z, µAOT the concentrations of y - x and x/z being 1 mmol/L and 1 mmol/ g. Equations 5 and 6 also assume that the departures from the ideal dilute solutions may be negligible. Equations 4-6 give

p° -(µAOT

-

s° µAOT )

) RT ln

( xz )c

1

(y - x)c2

(7)

The Gibbs energy ∆G°ads of adsorption may be expressed by p° s° ∆Goads ) µAOT - µAOT

(8)

which is defined as the change of the molar Gibbs free energy in adsorbing 1 mmol of AOT onto 1 g of polymer chains. Equation 7 becomes

Interaction between PDDAC and AOT

x c z 1 -∆G°ads ) RT ln ) RT ln Kads (y - x)c2

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∆Hobs )

(9)

where Kads is the equilibrium constant of adsorption. Rearrangement of eq 9 gives

x)

Kadszc2 y c1 + Kadszc2

(10)

Taking the formation of polymer/surfactant micelles on the polymer chains as a pseudophase transition, the Gibbs energy ∆G°mp of polymer-bound micellization is expressed by

ln

(

)

o ∆Gmp Kadsz y x ) ln )zc2 c1 + Kadszc2 z RT

(11)

∂∆H′ j ∂x ) ∆H ¯ o′ + ∆H ¯ o′ + ) ∆H ∂y ∂y Kadszc2 j ∆H c1 + Kadszc2

The experimentally observed enthalpy curves are almost linear with the total concentration of AOT (y); thus, we fitted the data of ∆Hobs for various y smaller than C′ by

∆Hobs ) a + by

Kadsz o y ) e-∆Gmp/RT ≡ g c1 + Kadszc2 z Kads z 1 ) y g c1 + Kadsc2 z

(12)

(13)

The values of Cmin, which is chosen as the critical concentration of PDDAC/AOT micellization, at different concentrations of PDDAC were used to fit eq 13 to obtain Kads and g, which were 10.6 and 5.7, respectively. The experimental dependence of z/y on c1/z and the calculated values from eq 13 are shown as dots and the line, respectively, in Figure 5. With the values of Kads and g, ∆G°ads and ∆G°mp can be calculated through eqs 9 and 11. The Gibbs energy changes ∆G°m of micellization of the free AOT in polymer solution were also calculated for the different concentrations of PDDAC by the equation o ∆Gm ) (1 + f)RT ln(y - x)

(14)

(17)

and obtained the values of a and b for various PDDAC concentrations which are listed in Table 3. As shown in the table, the values of a are nearly constant, which is 2.55 on average, and b decreases with an increase of the PDDAC j′ ) concentration. Comparing eq 16 with eq 17, we have ∆H ° a, and

j ) ∆H which yields

(16)

by(c1 + Kadszc2) Kadszc2

(18)

j of To examine the dependence of the molar enthalpy ∆H adsorption on the concentrations of AOT and PDDAC, we j through eq 18 and plotted it versus x/z for various calculated ∆H concentrations of PDDAC, which is shown in Figure 6. Almost all experimental points are in a single line, indicating that the j of adsorption is proportional to the molar enthalpy ∆H j with an inconcentration of x/z. The linear decrease of ∆H crease of x/z supports the mechanism of adsorption dominated by the hydrophobic interactions; the larger AOT concentration on the polymer chains, the larger hydrophobic interactions among them.12,23,36 A linear least-squares fit gives the depenj on x/z by dence of ∆H

j ) -0.006 - 0.090 x ∆H z

(19)

j x/z)1, defined as the value of which was used to calculate ∆H j at x/z ) 1. The enthalpy of adsorption ∆H°ads is defined as ∆H the change of the molar enthalpy in adsorbing 1 mmol of free j x/z)1. The AOT onto 1 g of polymer chains; thus, ∆H°ads ) ∆H enthalpies of micellization ∆H°mp, which characterize the formations of PDDAC/AOT micelles, were directly determined from the differences of enthalpies between the points C′ and the

As mentioned above, when the concentration of AOT is less than the value at C′, the enthalpy (∆Hobs, J/mol) curve characters the adsorption of AOT onto the polymer chains. The observed extensive enthalpy change ∆H′ (J) during the titration may be expressed as

jx ∆H′ ) ∆H′ + ∆H °

(15)

j (J/mol) is the molar enthalpy of adsorption of AOT where ∆H onto the polymer chains and ∆H′ (J) is the extensive enthalpy ° change contributed from the sources other than the adsorption. Partial differentiation of eq 15 with respect to y and substitution of x by eq 10 give the observed enthalpy ∆Hobs:

Figure 5. Plot of z/y versus c1/z. The dots are the experimental results, and the line represents values calculated by eq 13.

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j versus x/z at different PDDAC concentrations: Figure 6. Plots of ∆H (O) 0.112 g/L; (2) 0.218 g/L; (3) 0.400 g/L; ([)1.088 g/L.

TABLE 3: Optimal Coefficients of a and b in eq 17 for Different PDDAC Concentrations CPDDAC (g/L)

a

b

0.112 0.218 0.400 1.088

-2.52 -2.55 -2.52 -2.59

-0.24 -0.20 -0.15 -0.08

minimums of peaks A. The enthalpies of micellization of free AOT ∆H°m were determined by the procedures described for the AOT aqueous solution. The values of ∆S for various processes were calculated by

∆S )

∆H - ∆G T

(20)

The values of the thermodynamic quantities at different PDDAC concentrations for the adsorption (∆H°ads, ∆G°ads, ∆S°ads), the formation of PDDAC/AOT micelles (∆H°mp, ∆G°mp, ∆S°mp), and the formation of free micelles (∆H°m, ∆G°m, ∆S°m) are summarized in Table 4. In all cases, ∆S g 0, which may be attributed to expelling water molecules from disrupting solvent cages due to the adsorption and micellization.10,12 ∆H°ads and ∆H°mp are negative, which suggests that the processes of the adsorption and PDDAC/AOT micellization are driven by both enthalpy and entropy. Both ∆S°mp and ∆H°mp decrease with increase of the concentration of PDDAC and the compensation of entropy and enthalpy keeps ∆G°mp constant. The enthalpies of the micellization of free AOT monomers are all positive; however, T∆S°m > ∆H°m, which indicates that this process is driven by entropy gain. As the concentration of PDDAC increases, ∆H°m keeps almost unvaried as compared with its uncertainty; thus, increase of ∆S°m and decrease of ∆G°m with increase of the concentration of PDDAC are simultaneous, which evidence that a higher concentration of PDDAC favors the micellization of free AOT monomers. 3.4. Influence of Salt on the Characteristics of PDDAC/ AOT Aqueous Solutions. Addition of salt to PDDAC/AOT solutions may alter the electrostatic interactions among the polymer and surfactant molecules, which then impacts the behavior of the adsorption of AOT onto polymer chains and the micellizations of PDDCA/AOT. In this part, we study the effects of the concentration of Na2SO4 on the characteristics of PDDAC/AOT aqueous solutions. The enthalpy curves for titration of AOT into PDDAC/Na2SO4 aqueous solutions with the PDDAC concentration being 0.218 g/L and various Na2SO4 concentrations were depicted in Figure 7a. The shapes of the

enthalpy curves are like stairs. The first stair is similar to that of the binding curve driven by the electrostatic interaction reported by Bai et al.37 The enthalpy curves significantly depart from the AOT dilution curve, even the concentration of AOT at 0.014 mM, and have larger negative values of ∆Hobs as compared with that of polymer solutions in the absence of salt and with that of salt solutions in the absence of polymer. The existence of salt may reduce the hydrophilicity of AOT, the electrostatic repulsion among the headgroups and the counterions on polymer chains, and favors the bindings driven by both electrostatic and hydrophobic interactions. Therefore, it releases much more heat compared with adsorption of AOT onto the polymer chains in polymer solutions without the salt. In contradiction to that in the nonsalt solution, the enthalpy increases in the initial region with AOT concentration. It may be explained by the fact that more surfactant headgroups binding onto the polymer chains enhances the repulsion, which evidences that the binding is driven by the electrostatic interaction and ion exchange. The enthalpy curves start to increase sharply at C′, which corresponds to the critical concentration of PDDAC/ AOT micellization. Then, the curves level off until they reach Cm, where the free AOT micelles start to form and the profiles of the curves are very much similar to that in the brines with the same salt concentrations. Unlike the PDDAC/AOT aqueous solutions, the enthalpy curves determined for PDDAC/AOT brine solutions do not merge with the dilution curves of AOT micelle brine solutions. It implies that the saturation of AOT adsorbed on the polymer never reaches the concentration range we studied. We were unable to detect vesicle formation for all four concentrations of Na2SO4. It may be caused by the fact that the polymer-bound AOT headgroups do not properly function in vesicle formation, and only a small amount of vesicles, possibly the free vesicles without polymer participation were formed; thus, no enthalpy change was detectable. The determinations of the critical concentrations and the enthalpies of micellizations at different salt concentrations through the enthalpy curves of the solutions are illustrated in Figure 7b, and the values of the critical concentrations and the enthalpies of micellizations are summarized in Table 5. As shown in Table 5, the higher salt concentrations, the lower values of C′ and Cm are, because the salt reduces the repulsions among the polymer chains and the surfactant molecules, and increases the hydrophobicity of AOT, hence stabilizing the surfactant aggregates.22,38 The values of the enthalpy ∆H°mp of PDDAC/AOT micellization are all positive in the brine, while they are negative in the aqueous solution, which may be attributed to the different aggregation processes from the different states of AOT on polymer chains for the solutions with and without the salt. The values of ∆H°mp and the enthalpy ∆H°m of free micellization decrease from 1.47 to 0.82 kJ/mol and from 4.58 to -1.13 kJ/mol, respectively, as the Na2SO4 concentration increases from 5 to 50 mM, which also indicates that the presence of salt is favorable to both micellizations. We also used the dynamic light scattering technique to probe the process of binding, micellization, and vesicle formation of PDDAC/AOT/Na2SO4 aqueous solution with the concentrations of PDDAC and Na2SO4 being 0.218 g/L and 10 mM, respectively, which is depicted in Figure 8. The shapes of curves Rh and I are similar to the curves without salt but lacking region 4 characterizing the formation of vesicles. It once again indicates that a small amount of micelles is capable of forming the vesicles; thus, no significant changes of Rh and I were detectable. 3.5. Transmission Electron Microscopy Images. To identify the morphology of aggregates of AOT and PDDAC/AOT,

Interaction between PDDAC and AOT

J. Phys. Chem. B, Vol. 113, No. 41, 2009 13573

TABLE 4: Thermodynamic Properties of Adsorption and Micellization of AOT on PDDAC Chains for Different Concentrations of PDDAC CPDDAC (g/L) 0.112 0.218 0.400 1.088

∆H°ads (kJ/mol) -0.10

∆G°ads (kJ/mol) -5.86

∆S°ads (J/(mol K))

∆H°mp (kJ/mol)

0.02

-0.43 -0.95 -1.05 -1.52

TABLE 5: Critical Concentrations and Thermodynamic Parameters of Binding of AOT onto PDDAC for Different Na2SO4 Concentrations CNa2SO4 (mM) C′ (mM) ∆H°mp (kJ/mol) Cm (mM) ∆H°m (kJ/mol) 5 10 20 50

1.31 1.30 1.28 1.22

1.47 1.46 1.38 0.82

2.99 2.91 2.88 1.70

4.58 4.01 -0.56 -1.13

we performed the TEM measurements. Figure 9a shows a typical morphology of the vesicles in the AOT aqueous solution with the concentration of AOT being 11.4 mM. The radius of the vesicles varies from 40 to 200 nm. An image of the aqueous solution containing 5 mM Na2SO4 and 7.55 mM AOT is shown in Figure 9b, where the vesicles have an average radius of about 120 nm. No vesicles but spherical particles with much larger sizes were observed in the sample containing 1.088 g/L PDDAC and 7.55 mM AOT, characterizing the formation of precipitated polymer/surfactant complexes, which is shown in Figure 9c and consistent with the ITC study. Vesicles were observed again in the sample containing 1.088 g/L PDDAC and 11.4 mM AOT (Figure 9d), and it seems that two kinds of vesicles are

∆G°mp (kJ/mol)

∆S°mp (J/(mol K))

∆H°m (kJ/mol)

∆G°m (kJ/mol)

∆S°m (J/(mol K))

-4.32

13.02 11.28 10.95 9.37

4.24 4.53 4.40 4.64

-18.01 -18.45 -18.99 -19.78

74.63 77.08 78.45 81.93

coexisting; the radii of the larger vesicles are about 500 nm, significantly larger than that in AOT aqueous and brine solutions, while the smaller ones are 20-100 nm, close to that observed in AOT aqueous and brine solutions without the polymer. It may be attributed to the fact that not only the free AOT micelles but also the polymer-bound AOT micelles participate in the construction of the vesicles. Figure 9e shows the vesicles formed in the aqueous solution of 1.088 g/L PDDAC, 5 mM Na2SO4, and 11.4 mM AOT, which have a radius range from 40 to 100 nm. This size is close to that observed in Figure 9b, which supports the fact that the vesicles only formed from the free AOT micelles without polymer participation. The micrographs presented in Figures 9a, b, c, d, and e correspond to the solutions prepared 2 days before the observations. We are also interested in the influences of time and temperature on the vesicle formation; therefore, we performed the TEM measurement for sample d after it was kept at 4 °C for 3 weeks, which is shown in Figure 9f. The morphology changes with time may be attributed to the metastable properties of vesicles. However, we could not observe any vesicles after the solution was kept at normal temperature for 3 weeks, possibly resulting from the fact that the hydrolyzation of AOT becomes more significant at higher temperature. 4. Conclusion We have studied the interactions between PDDAC and AOT, the aggregations of AOT and PDDAC-bound AOT in PDDAC/ AOT aqueous solutions, and the influence of salt on the interactions and aggregations by using isothermal titration calorimetry, dynamic light scattering, and negative staining transmission electron microscopy. In the ITC investigation of AOT/Na2SO4 aqueous solutions, we determine the critical micelle concentrations and the critical vesicle concentrations of AOT at different salt concentrations and find that these critical concentrations decrease as the salt concentration increases, indicating that salt enhances the micellization of AOT and favors

Figure 7. (a) Enthalpy curves of titrating AOT into PDDAC/Na2SO4 solutions with CPDDAC ) 0.218 g/L and different CNa2SO4: (9) 5 mM; (2) 10 mM; (b) 20 mM; (1) 50 mM, plotted together with the dilution curves given by open symbols. (b) Determinations of the critical concentrations and the enthalpy of micellization from the enthalpy curves with CPDDAC ) 0.218 g/L and CNa2SO4 ) 5 mM (9), CPDDAC ) 0 g/L, and CNa2SO4 ) 5 mM (0).

Figure 8. Profiles of binding and micellization obtained from light scattering and ITC for the solutions with CPDDAC ) 0.218 g/L andCNa2SO4 ) 10 mM.

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Figure 9. Transmission electron micrograph of AOT and PDDAC/AOT aggregates: (a) 11.4 mM AOT aqueous solution; (b) aqueous solution containing 5 mM Na2SO4 and 7.55 mM AOT; (c) aqueous solution containing 1.088 g/L PDDAC and 7.55 mM AOT; (d) aqueous solution containing 1.088 g/L PDDAC and 11.4 mM AOT; (e) aqueous solution containing 1.088 g/L PDDAC, 5 mM Na2SO4, and 11.4 mM AOT; (f) sample d kept at 4 °C, measured after 3 weeks.

the formation of free vesicles. By combining ITC with DLS and TEM techniques, detailed information on energetic and structural changes for PDDAC/AOT systems are detected. The investigations of PDDAC/AOT aqueous solutions show that the adsorption of AOT onto PDDAC chains in our experimental concentration range is dominated by hydrophobic interaction, and as the concentration of AOT increases, some neighborly adsorbed surfactant molecules start to aggregate to form the micelles bound by different polymer chains and further yield the precipitation. With continuously increasing AOT concentration, the precipitation is redissolved, which possibly results from the fact that the added AOT molecules participate in restructuring the PDDAC/AOT micelles and unfold the cross-links of the different polymer chains. Further addition of AOT results in the formation of free micelles and then the free and polymerbound vesicles. Addition of salt into the above solutions enhances the binding of AOT onto the polymer chains and favors formation of the PDDAC/AOT micelles, the free AOT micelles, and the free AOT vesicles but prevents the transition of PDDAC/AOT micelles to the vesicles. Binding of AOT to polymer chains in brine possibly is mainly driven by the electrostatic interaction and ion exchange. Various thermodynamic parameters extracted from ITC investigations show that the micellization of free AOT is driven by entropy gain, while

adsorption of AOT onto PDDAC and micellization of PDDACbound AOT are driven by both enthalpy and entropy. The thermodynamic analysis also suggests that the adsorption of AOT onto PDDAC and the aggregations in PDDAC/AOT aqueous solutions are different in mechanism compared with that in corresponding brine solutions. Acknowledgment. This work is supported by the National Natural Science Foundation of China (Projects 20473035, 20603014, and 20673059), the Chinese Ministry of Education (Key project 105074), and Committee of Science and Technology of Shanghai (Projects 0652nm010 and 08jc14081). References and Notes (1) Bell, C. G.; Breward, C. J. W.; Howell, P. D.; Penfold, J.; Thomas, R. K. Langmuir 2007, 23, 6042–6052. (2) Anthony, O.; Zana, R. Langmuir 1996, 12, 1967–1975. (3) Fundin, J.; Hansson, P.; Brown, W.; Lidegran, I. Macromolecules 1997, 30, 1118–1126. (4) Ghoreishi, S. M.; Fox, G. A.; Bloor, D. M.; Holzwarth, J. F.; WynJones, E. Langmuir 1999, 15, 5474–5479. (5) Khokhlov, A. R.; Dormidontova, E. E. Phys.-Usp. 1997, 40, 109– 124. (6) Konop, A. J.; Colly, R. H. Langmuir 1999, 15, 58–65. (7) Charkraborty, T.; Charkraborty, I.; Moulik, S. P.; Ghosh, S. J. Phys. Chem. B 2007, 111, 2736–2746.

Interaction between PDDAC and AOT (8) Gianni, P.; Barghihi, A.; Bernazzani, L.; Mollica, V. Langmuir 2006, 22, 8001–8009. (9) Haldar, B.; Chakrabary, A.; Mallick, A.; Mandal, M. C.; Das, P.; Chattopadhyay, N. Langmuir 2006, 22, 3514–3520. (10) Wang, C.; Tam, K. C. Langmuir 2002, 18, 6484–6490. (11) Li, Y.; Ghoreishi, S. M.; Warr, J.; Bloor, D. M.; Holzwarth, J. F.; Wyn-Jones, E. Langmuir 1999, 15, 6326–6332. (12) Wang, C.; Tam, K. C. J. Phys. Chem. B 2003, 107, 4667–4675. (13) Fan, Y. R.; Li, Y. J.; Yuan, G. C.; Wang, Y. L.; Wang, J. B.; Han, C. C.; Yan, H. K. Langmuir 2005, 21, 3814–3820. (14) Qiu, X. M.; Wei, X. L.; Yin, B. L.; Sun, D. Z. J. Polym. Sci., Part B: Polym. Phys. 2005, 44, 275–283. (15) Lee, J.; Moroi, Y. Langmuir 2004, 20, 4376–4379. (16) Carnali, J. O.; Shah, P. J. Phys. Chem. B 2008, 112, 7171–7182. (17) Li, Y. J.; Xia, J. L.; Dubin, P. L. Macromolecules 1994, 27, 7049– 7055. (18) Li, Y. J.; Dubin, P. L. Macromolecules 1995, 28, 6795–6798. (19) Dubin, P. L.; Oteri, R. J. Colloid Interface Sci. 1983, 95, 453–461. (20) Malovikova, A.; Hayakawa, K.; Kwak, J. C. T. J. Phys. Chem. 1984, 88, 1930–1933. (21) Dubin, P. L.; Vea, M. E. Y.; Fallon, M. A.; The, S. S.; Rigsbee, D. R.; Gan, L. M. Langmuir 1990, 6, 1422–1427. (22) Wang, X. Y.; Li, Y. J.; Li, J. X.; Wang, J. B.; Wang, Y. L.; Guo, Z. X.; Yan, H. K. J. Phys. Chem. B 2005, 109, 10807–10812. (23) Wang, C.; Tam, K. C. J. Phys. Chem. B 2004, 108, 8976–8982. (24) Zimm, B. H.; Bragg, J. K. J. Chem. Phys. 1959, 31, 526–535.

J. Phys. Chem. B, Vol. 113, No. 41, 2009 13575 (25) Zhang, H.; Li, Y.; Dubin, P.; Kato, T. J. Colloid Interface Sci. 1996, 183, 546–551. (26) Pinaki, R. M.; Satya, P. M. Langmuir 1998, 14, 3986–3990. (27) Lin, J. M.; Nakagawa, M.; Uchiyama, K.; Hobo, T. Electrophoresis 2002, 23, 421–425. (28) Bai, G. Y.; Wang, Y. J.; Yan, H. K.; Thomas, R. K.; Kwak, J. C. T. J. Phys. Chem. B 2002, 106, 2153–2159. (29) Van Os, N. M.; Daane, G. J.; Haandrikman, G. J. Colloid Interface Sci. 1991, 141, 199–217. (30) Zana, R. Langmuir 1996, 12, 1208–1211. (31) Mukherjee, K.; Mukherjee, D. C.; Moulik, S. P. J. Phys. Chem. 1994, 98, 4713–4718. (32) Briz, J. I.; Vela´zquez, M. M. J. Colloid Interface Sci. 2002, 247, 437–446. (33) Wang, H.; Wang, Y. L.; Yan, H. K. Langmuir 2006, 22, 1526– 1533. (34) Yeh, F. J.; Sokolov, E. L.; Khokhlov, A. R.; Chu, B. J. Am. Chem. Soc. 1996, 118, 6615–6618. (35) Bai, G. Y.; Catia, J. A. M.; Nichifor, M.; Bastos, M. J. Phys. Chem. B 2007, 111, 11453–11462. (36) Chandar, P.; Somasundaran, P.; Turro, N. J. Macromolecules 1988, 21, 950–953. (37) Bai, G. Y.; Santos, L. M. N. B. F.; Nichifor, M.; Lopes, A.; Bastos, M. J. Phys. Chem. B 2004, 108, 405–413. (38) Fox, G. J.; Bloor, D. M.; Holzwarth, J. F.; Wyn-Hones, E. Langmuir 1998, 14, 1026–1030.

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