Interaction between Polyelectrolyte and Oppositely Charged

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8976

J. Phys. Chem. B 2004, 108, 8976-8982

Interaction between Polyelectrolyte and Oppositely Charged Surfactant: Effect of Charge Density C. Wang and K. C. Tam* School of Mechanical and Production Engineering, Nanyang Technological UniVersity, 50 Nanyang AVenue, Singapore 639798 ReceiVed: January 26, 2004; In Final Form: March 22, 2004

The binding of dodecyltrimethylammonium bromide (DoTab) to poly(acrylic acid) (PAA) at different degrees of neutralization (R) was investigated using isothermal titration calorimetry (ITC) and laser light scattering (LLS) techniques. The surfactant binds to the polymer at all R; however, the mechanism varies. When R is lower than a critical value (RC), the hydrocarbon chains of DoTab cooperatively bind to the apolar segments of PAA driven by hydrophobic interaction at very low DoTab concentration (CDoTab e 0.2 mM). In this binding region, the ITC profile exhibits a significant exothermic peak and the mixture precipitates, which is attributed to the interchain complexation via hydrogen bonding induced by the binding. The precipitation is soon resolubilized with further addition of surfactant as more DoTab micelles are bound to the polymer backbones with their ionic headgroups extending outward. When R > RC, the hydrophobic binding ceases as the polymer is progressively ionized and DoTab binds to the charged polymer chains driven by electrostatic attraction. The condensed counterions on the charged polymer chains are released via the ion-exchange process, resulting in an endothermic maximum on the ITC profile. The value of RC determined from ITC is approximately 0.3, which is reasonably close to the theoretical value derived from the Manning’s counterion condensation theory (approximately 0.35).

Introduction Interactions between polymers and surfactants bearing opposite charges in aqueous solutions have attracted increasing attention because of their complex behaviors and potential applications in rheological control, detergency, and pharmaceutical formulations, etc. A number of research groups have devoted their attention to advancing the fundamental understanding on the physics governing these interactions.1-13 The strong electrostatic attraction between oppositely charged polyelectrolytes and surfactants is clearly observable and occurs at a concentration several orders of magnitude below the critical micelle concentration (CMC). The association between a polyelectrolyte and oppositely charged surfactant is generally accepted as an ion-exchange process, in which the electrostatic attraction is reinforced by a cooperative aggregation of the bound surfactant molecules.1,9,11-13 Most of the works focus on the interactions between strong polyelectrolytes and oppositely charged surfactants; however, interactions between weak polyelectrolytes and surfactants are less extensively studied. As a weak polyelectrolyte, polycarboxylic acids (e.g. poly(acrylic acid) (PAA), poly(methacrylic acid) (PMAA), and their derivatives) have surprisingly strong affinity with cationic surfactants. This system has potential applications such as control of chemical reactivity, drug delivery, and nonspecific binding of DNA with basic protein,14 and it may be used as a simplified model for elucidating the behavior of biological systems.14,15 Such a system is beginning to attract increasing attention because of its industrial and fundamental importance.1,2,14,16-21 The charge density, backbone flexibility, and hydrophobicity of polyacids can be easily modified by varying the degrees of * To whom correspondence should be addressed. Fax: 65-6791-1859. E-mail: [email protected].

neutralization (R). Several studies have focused on the interaction of cationic surfactants and polyacids at different Rs to obtain additional insights into polymer/surfactant binding. Kwak et al. studied the effect of charge density on the binding of cationic surfactants onto polyanions, such as PAA, poly(vinyl sulfate), and carboxymethyl cellulose. They pointed out that the binding is governed by several contributing factors, such as charge effect, chain flexibility, and the chemical nature of charged sites.1,2 Kosmella et al. studied the interaction of cationic dodecylpyridinum chloride (DopyCl) with the sodium salt of PAA using surfactant selective electrode. A highly cooperative binding driven by electrostatic forces commences at a DopyCl concentration two or three orders of magnitude lower than its CMC, and the saturation of binding deviates from the 1:1 stoichiometry.21 Pyrene-labeled PAA was used as fluorescence probe to investigate the interactions between PAA and dodecyltrimethylammonium (DoTab) at different pH and salt conditions.16 Addition of salt increases the critical aggregation concentration (CAC) because of the suppression of the electrostatic binding force. However, it is surprising to observe that the CAC increases with pH, which is at odds with the fact that an increase of pH enhances the extent of electrostatic binding and consequently a decrease of CAC would be expected. The authors believed that the main binding force between PAA and DoTab at low pH is still electrostatic force, and they attributed the increase of the CAC with pH to the different flexibility and conformation of PAA chains at various pHs.16 Several researchers have reported that forces other than electrostatic attraction are involved in the binding of cationic surfactant to poly(carboxylic acids) with low charge densities. Dubin and co-workers examined the effect of pH on the interactions between PAA and cationic/nonionic mixed micelles.14 The binding is electrostatically driven when pH is greater than

10.1021/jp049647m CCC: $27.50 © 2004 American Chemical Society Published on Web 05/28/2004

Binding of DoTab to PAA 4.2, but it is controlled by hydrogen bonding at low pH. At moderate pH, the contribution of hydrogen bonding to the binding is superimposed on the electrostatically controlled polymer/micelle interaction.14 The significance of hydrogen bonding in the polymer/surfactant complexation was also proposed by Anghel et al. based on their surface tension and viscometric studies on mixtures of PAA and nonionic surfactants. They found that polymer/surfactant complexes with stoichiometric composition with respect to proton donor and acceptor only exist within a narrow range of pH.22 Kogej et al. studied the interactions between alkylpyridinium surfactants with PAA and PMAA using X-ray scattering at various Rs, where the effect of charge density of polyanion on the binding and structure of polymer/surfactant complex were reported. The binding at low R corresponded to the polymer-induced micellization of surfactant driven by hydrophobic interactions. At higher R, Coulombic interaction between the polyanion and surfactant molecules was responsible for the binding, resulting in the formation of the highly organized cubic structure of the polymer/surfactant complex.18 The pH values of polyacid solutions change with the addition of surfactant, making quantitative analysis of the influence of pH on the binding interactions difficult.17,19 To circumvent this difficulty, several researchers determined the binding isotherms in buffer solutions, where the pH values remained constant over the entire course of binding. Recently, the binding isotherms of dodecylpyridinium chloride (C12PyCl) with partially neutralized PAA and PMAA in buffer solutions were determined by Katsuura et al. on the basis of a series of electromotive force (EMF) measurements. The systems exhibited multistep bindings that are dependent on pH. At pH < 3.2, the binding is considered to be the micellization of the hydrocarbon chains of C12Py on the apolar region of the compact form of the polyacids.20 In summary, we had reviewed some of the major studies on the binding of cationic surfactant to polyacids, where different perspectives were proposed. However, a complete understanding on the nature of the binding is still elusive, and the debate is ongoing. In this study, the binding of dodecyltrimethylammonium bromide (DoTab) to poly(acrylic acid) at various degrees of neutralization was examined using isothermal titration calorimetry (ITC) and laser light scattering (LLS) techniques. The critical concentrations and thermodynamic parameters were determined from the enthalpy profiles. The mechanism of polymer/surfactant complexation and the microstructure of the polymer/surfactant complex were elucidated using the light scattering technique. This study provides new insights into the effect of polymer charge density on the binding of cationic surfactant to polycarboxylic acid. Experimental Details Materials. PAA with MW ) 64 900 and a polydispersity index (PDI) of 1.02 was obtained from Chemical Source Co. A standard 1 M NaOH solution from Merck was used to adjust the degree of neutralization of PAA. The cationic surfactant, dodecyltrimethylammouium bromide (DoTab, >99%) was obtained from Fluka and used as received without further purification. Water was obtained from the Millipore Alpha-Q water purification system, which has a resistivity of 18.2 µΩ‚cm. Isothermal Titration Calorimetry. The microcalorimetry study was carried out using the Microcal isothermal titration calorimeter. This power compensation, differential instrument was previously described in detail by Wiseman et al.23,24 It has a reference cell and a sample cell of approximately 1.35 mL, and the cells are both insulated by an adiabatic shield. The

J. Phys. Chem. B, Vol. 108, No. 26, 2004 8977 titration was carried out at 25.00 ( 0.02 °C, by injecting 0.1 M DoTab solution from a 250 µL injection syringe into the sample cell filled with 5 mM (monomer concentration) PAA solution. The syringe is tailored-made such that the tip acts as a bladetype stirrer to ensure an optimum mixing efficiency at 400 rpm. An injection schedule was automatically carried out using interactive software after setting up the number of injections, the volume of each injection, and the time between each injection. Dynamic Laser Light Scattering. The dynamic laser light scattering experiments were conducted using the Brookhaven laser light scattering system. This system consists of a BI200SM goniometer, a BI-9000AT digital correlator, and other supporting data acquisition and analysis software and accessories. An argonion vertically polarized 488 nm laser was used as the light source. The time correlation function of the scattered intensity G2(t), which is defined as G2(t) ) I(t) I(t+∆t), where I(t) is the intensity at time t and ∆t is the lag time, was analyzed using the inverse Laplace transformation technique (REPES for our case) to produce the distribution function of decay times. Thus, the apparent hydrodynamic radius can be determined from the decay rate via the Stokes-Einstein equation:

Rh )

kTq2 6πηΓ

(1)

where k is the Boltzmann constant, q is the scattering vector (q ) [4πn sin(θ/2)]/λ), in which n is the refractive index of the solution, θ is the scattering angle, and λ is the wavelength of the incident laser light in a vacuum), η is the solvent viscosity, and Γ is the decay rate. Several measurements were performed at 90° for a sample to obtain an average hydrodynamic radius, and the variation in the Rh values was found to be small. Fourier Transform Infrared Spectroscopy. Fourier transform infrared (FT-IR) spectra were determined for the PAA/ DoTab mixture at different Rs to investigate the contribution of H-bonding in the formation of the PAA/DoTab complex. The precipitation of the PAA/DoTab complex was separated from the solution using a centrifuge and dried at 60 °C in a vacuum oven for 2 days to remove residual water prior to the measurements. The FT-IR spectra were measured using a Perkin-Elmer 1720 FT-IR spectrometer at a 4 cm-1 resolution. Results and Discussion Isothermal calorimetric titrations were conducted by stepwise injections of 0.1 M DoTab solution into the sample cell filled with 5 mM (monomer concentration) PAA in 0.1 M NaCl at different degrees of neutralization ranging from 0.02 to 1.0, where R is defined as,

R)

[BASE] + [H+] - [OH-] CCOOH

(2)

and [BASE], [H+], and [OH-] are the molarities of added base, free hydrogen ion, and hydroxide ion, respectively. CCOOH is the total concentration of carboxylic groups expressed in moles per liter. The thermogram demonstrating the cell feedback (CFB) of the titrations in the PAA solutions at R ) 1 is shown in Figure 1. Integration of the area of CFB corrected for the dilution heat obtained from blank titration (titrating DoTab into 0.1 M NaCl) gives the differential enthalpy curves. The differential enthalpy curves of titrations at R ) 0.02, 0.2, 0.25, 0.3, 0.4, 0.6, 0.8, and 1 were plotted in Figure 2a. The CMC and the ∆H

8978 J. Phys. Chem. B, Vol. 108, No. 26, 2004

Figure 1. Thermogram showing CFB versus time obtained from titrating 0.1 M DoTab into 5 mM PAA in 0.1 M NaCl solution at R ) 1.

Figure 2. (a) Differential enthalpy curves for titration of 0.1 M DoTab into 5 mM PAA at different R in 0.1 M NaCl solution: (0) R ) 0.02; (]) R ) 0.2; (4) R ) 0.25; (O) R ) 0.3; (b) R ) 0.4; (2) R ) 0.6; ([) R ) 0.8; (9) R ) 1.0. (b) Differential enthalpy curves for titration of 0.1 M DoTab into 5 mM PAA at different R in 0.1 M NaCl solution (CDoTab in logarithmic scale): (0) R ) 0.02; (]) R ) 0.2; (4) R ) 0.25; (O) R ) 0.3; (b) R ) 0.4; (2) R ) 0.6; ([) R ) 0.8; (9) R ) 1.0.

(micellization) for DoTab in 0.1 M NaCl are 9.3 mM and -1.83 kJ/mol, respectively. At R e 0.3, the enthalpy profile corresponding to the DoTab/ PAA binding exhibits an exothermic peak at a very low onset concentration (∼0.16 mM). With increasing R, the sharp exothermic peak at low DoTab concentration diminishes and it completely disappears at R > 0.3. On the other hand, an endothermic peak appears at higher DoTab concentration (∼1.5 mM) at R ) 0.25 and it becomes more significant with increasing R. The gradual transition of the enthalpy profile from

Wang and Tam exothermic to endothermic is shown more clearly in Figure 2b, where the enthalpy was plotted against the logarithm of the DoTab concentration. In our previous study, it was proven that the endothermic peak at R higher than 0.3 corresponds to the electrostatic binding of DoTa+ onto COO- groups along the polymer chains.25 This conclusion is further reinforced by the enthalpy profiles shown in Figure 2b, where the breadth of the endothermic peak at R g 0.3 is proportional to the R of PAA, indicating that more DoTa+ participate in the electrostatic interaction with PAA of more charged sites. We had demonstrated previously that the electrostatic binding is driven by the release of condensed counterions (sodium ions in our case) via ion-exchange interaction, where the Na+ regains their translational entropy.25,26 According to the Manning counterion condensation theory, the criteria for dilute polyelectrolyte solution to condense counterion is that the charge density parameter ξ ) 1. In other words, if polyelectrolyte charge density is too low (ξ < 1), counterions are not condensed on the polymer chains and consequently the electrostatic binding driven by the release of condensed counterion cannot take place. This explains why the endothermic peak representing the electrostatic binding is only significant when R is sufficiently high (R g 0.25). The charge density parameter is defined as ξ ) lBj/b, where lBj is the Bjerrum length defined as lBj ) e2/kBT. Applying the critical condition for counterion condensation, ξ ) 1, the critical linear charge spacing equals the Bjerrum length, which is 7.1 Å in aqueous solution. The linear charge spacing b of PAA at different R can be expressed as b(R) ) bR)1/R, where bR)1 is the linear charge spacing of fully neutralized PA, i.e., 2.53 Å.17,27 From Manning’s counterion condensation theory, the critical degree of neutralization for counterion condensation, designated as RC, was estimated to be ∼0.35. The critical degree of neutralization measured from ITC is approximately 0.3, which is reasonably close to the theoretical value. The reason the experimental value of RC is lower than the theoretical value is because the actual charge spacing may be smaller than the linear charge spacing derived from the space-filling model, where the polymer chain is treated as a straight line. This is not true for most polyelectrolytes that exist in coiled conformation, where the two neighboring charged sites are closer together, which allows the electrostatic binding to occur at a lower R. The critical concentration for the electrostatic binding determined from the onset of the endothermic peak, namely C1, is independent of the degree of neutralization. Thus, when the condition for the release of condensed counterion is satisfied (R > RC), electrostatic binding occurs at the same DoTab concentration (∼1.5 mM) regardless of the charge density of PAA. This agrees with the results reported by Kiefer et al. for the binding of TTab onto PAA at different Rs, where they observed that the binding isotherms for PAA at R ) 0.5 and R ) 1 are nearly identical.17 On the other hand, PAA with higher R exhibits a broader endothermic peak and the enthalpy profile merges with the dilution curve at higher DoTab concentration. This suggests that more surfactant molecules are electrostatically bound to polymer chains of higher charge density, which results in a higher saturation concentration, designated as C2. The binding fraction at saturation concentration C2 is given by the following expression:

φ)

(C2 - C1) CCOO- × R

(3)

It should be pointed out that φ was derived under the assumption that the free monomer concentration remains constant when C1

Binding of DoTab to PAA

J. Phys. Chem. B, Vol. 108, No. 26, 2004 8979

TABLE 1: C1, C2, and the Amount of Bound DoTa+ per -COO- for the Interaction between 0.1 M DoTab and 5 mM PAA at Different rs (r g 0.25) in 0.1 M NaCl Solution R

C1 (mM)

C2 (mM)

binding fraction (φ) at C2

0.3 0.4 0.6 0.8 1.0

1.51 1.51 1.51 1.51 1.51

3.11 4.31 5.21 6.21 6.90

0.096 1.41 1.23 1.12 1.08

is reached for the electrostatic binding. φ was found to be independent of R, and the value is close to unity, suggesting that the amount of electrostatically bound DoTa+ at saturation concentration is approximately equivalent to the amount of ionized carboxylate groups. The values of C1, C2, and binding fraction (φ) at different R (R g 0.3) determined from ITC are summarized in Table 1. Attempts to measure the binding isotherm using the electromotive force technique with a DoTab-selective electrode were not successful due to the formation of “sticky” precipitates that adsorbed on the surface of the polymeric membrane of the electrode, which interfered with the EMF measurements. Thus, this approach was not pursued further, and work is currently in progress to overcome this problem. The most striking feature of the enthalpy curves shown in Figure 2a,b is the distinct exothermic peak observed at low DoTab concentration when R e 0.25. At R ) 0, the exothermic maximum on the enthalpy profile has a value of -19.7 kJ/mol, and it occurs at a very low onset concentration of ∼0.16 mM. With further addition of DoTab into the PAA solution, the enthalpy diminishes rapidly and the binding profile merges with the dilution curve at CDoTab ∼ 1.28 mM. It then exhibits an endothermic peak at CDoTab ∼ 10 mM corresponding to the formation of free micelles.25,26 With increasing R, the exothermic maximum still persists over a similar DoTab concentration regime, but with a smaller magnitude. When R > 0.3, the exothermic peak no longer exists, and the enthalpy corresponding to the DoTab/PAA binding becomes endothermic, suggesting that the interaction is dominated by electrostatic interaction. It is expected that the interaction between PAA and DoTab is stronger at higher R because of stronger electrostatic attractive forces. However, this is experimentally observed only when R is greater than a threshold value (RC) of 0.3. At R lower than 0.3, DoTab interacts strongly with unneutralized and partially neutralized PAA, which is somewhat surprising since PAA is generally uncharged at 0 e R < 0.3 and electrostatic attraction between DoTa+ and carboxylate groups is weak. This observation contradicts the general understanding that cationic surfactant has negligible or weak interaction with neutral polymers.3 As shown in Figure 2b, the onset of binding takes place at DoTab concentration as low as ∼0.16 mM, which is even lower than the critical concentration for the electrostatic binding (C1 ∼ 1.5 mM). The binding only exists over a narrow range of DoTab concentration, and the interaction ceases with further addition of surfactant. Furthermore, the exothermic enthalpy peak progressively levels off with the increase of R, indicating that the binding is weakened by the enhancement of polymer charge density. From the features on the binding of DoTab to PAA at low R described above, we believed that the interaction is not electrostatically driven. This binding may be initiated by the hydrophobic interaction between DoTab and un-ionized PAA. The backbone of unneutralized or partially neutralized PAA is relatively hydrophobic, and several polymer chains may aggregate to form a compact aggregate.13,17,28 The formation of

interchain hydrophobic domains enhances the overall hydrophobicity of the system, which induces the micellization of DoTab on apolar PAA chains at R < RC (∼0.3). As shown in Figure 2a, the exothermic peak occurs at CDoTab ∼ 0.16 mM and rapidly reaches its maximum at CDoTab ∼ 0.6 mM, suggesting that the interaction is highly cooperative. This corroborates with the observation that moderately hydrophobic polymer induces cooperative micellization of surfactant on the hydrophobic domains.3 At R > RC (0.3), PAA chains uncoil and the hydrophobic domains are destroyed with ionization; thus, the polymer-induced micellization ceases, and the interaction is then dominated by electrostatic interaction. We observed from Figure 2a,b that the enthalpy profiles at R ) 0.25 and 0.3 display both exothermic and endothermic maxima, suggesting the coexistence of hydrophobic and electrostatic bindings. However, both bindings are insignificant because of weak hydrophobic and electrostatic interactions between PAA and DoTab at moderate R. In summary, we observed that the interaction between PAA and DoTab is purely hydrophobic at R < 0.25, and predominantly electrostatic at R > 0.3. A transition at R between 0.25 and 0.3 is evident, where DoTab binds to PAA via a combination of hydrophobic and electrostatic interactions. To differentiate the characterization of the hydrophobic and electrostatic binding interactions, the following designations are proposed: (a) Critical aggregation concentration (CAC) defines the onset for hydrophobic binding. (b) Saturation concentration (CS) represents the condition where the polymer chains are completely saturated with hydrophobically bound surfactant monomers. (c) C1 and C2 correspond to the onset and saturation concentrations for electrostatic binding, a terminology we had used previously.25,26 To verify the mechanism for the PAA/DoTab complexation derived from ITC studies, dynamic light scattering was performed to examine the interaction between DoTab and PAA and to elucidate the microstructure of the polymer/surfactant complex at different Rs. The concentration of PAA was kept at 5 mM (monomer AA concentration), and all the measurements were conducted in 0.1 M NaCl solutions. The dependence of scattering intensity of PAA at R ) 0.02, 0.6, 0.8, and 1 on DoTab concentration is shown in Figure 3a. The apparent hydrodynamic radii (Rhapp) calculated from the decay rate via the Stokes-Einstein equation at different Rs were plotted against DoTab concentrations in Figure 3b. As shown in Figure 3a,b, the dependencies of scattering intensity and particle size on DoTab concentrations changes according to the magnitude of R. For highly ionized PAA (R ) 0.6, 0.8, and 1), the scattering intensities at CDoTab ) 0 are identical with an average value of ∼270 kcps (kilo-counts per second). The hydrodynamic radius measured at CDoTab ) 0 corresponds to the size of random coiled PAA chains. The Rhapp increases from 53.9 to 62.5 nm as R increases from 0.6 to 1, characterizing the expansion of PAA coils due to electrostatic repulsion between carboxylate groups upon neutralization. When DoTab is added, the scattering intensity decreases slightly, and the Rhapp also decreases to approximately 25 nm. As discussed in our previous paper,25 such behaviors characterize the electrostatic binding of individual DoTab molecules onto carboxylate groups of PAA, resulting in the charge neutralization of PAA chains and the shrinkage of the polymer coils. When DoTab concentration reaches a critical value, the solution becomes opaque, the scattering intensity increases sharply by several orders of magnitude, and the particle size increases by approximately 200

8980 J. Phys. Chem. B, Vol. 108, No. 26, 2004

Figure 3. (a) Dependence of scattering intensity on DoTab concentration measured in 5 mM PAA (0.1 M NaCl solution) at different R: (0) R ) 0.02; (O) R ) 0.6; (4) R ) 0.8; (]) R ) 1. (b) Dependence of Rhapp on DoTab concentration in 5 mM PAA (0.1 M NaCl solution) at different R: (9) R ) 0.02; ([) R ) 0.2; (4) R ) 0.6; (]) R ) 0.8; (0) R ) 1.0.

nm. These features characterize the complexation of PAA/ DoTab upon the micellization of polymer-bound DoTab molecules at critical concentration C′, beyond which the complex precipitates and the mixture becomes heterogeneous. Moreover, C′ increases from ∼1.3 to ∼2.2 mM when R increases from 0.6 to 1.0, indicating that a lower charge density of polymer (relatively higher hydrophobicity) is favorable for the micellization of bound surfactant molecules. The above findings agree with our previous results obtained in the MAA-EA/DoTab system.25 For the PAA with low charge density (R ) 0.02 and 0.2), the dependencies of scattering intensity and Rhapp on the DoTab concentrations are summarized as follows. The intensity of PAA in the absence of DoTab is ∼280 kcps, and it remains essentially constant when the DoTab concentration is lower than 0.2 mM (open squares in Figure 3a). Beyond CDoTab ) 0.2 mM, the solution becomes opaque and the scattering intensity increases sharply and reaches a maximum of ∼7500 kcps at CDoTab ) 0.6 mM. With further addition of DoTab, the solution becomes transparent again and the intensity decreases to ∼33 kcps at CDoTab ) 0.8 mM and thereafter remains unchanged. As shown in Figure 3b, the Rhapp of the PAA coil at R ) 0.02 and 0.2 without addition of DoTab are 40.0 and 45.1 nm, respectively, and increase slightly upon the addition of DoTab until CDoTab ) 0.2 mM. Beyond this, Rhapp increase sharply and reach their maxima; i.e., ∼130 nm for R ) 0.02 and ∼180 nm for R ) 0.2 at CDoTab ∼ 0.5 mM. Thereafter Rhapp decreases rapidly with further addition of DoTab and reaches a minimum of ∼18 nm

Wang and Tam

Figure 4. Binding profiles obtained from ITC and light scattering for the PAA/DoTab pair in 0.1 M NaCl at R ) 0.02: (9) hydrodynamic radius (nm); ([) scattering intensity (kcps); (2) differential enthalpy (kJ/mol).

at CDoTab ∼ 0.8 mM (for PAA at R ) 0.02), beyond which the scattering intensity becomes very weak and reliable g2(t) correlation function could not be determined. Figure 4 shows the comparison of the binding profiles obtained from light scattering and ITC in the DoTab/PAA mixture at R ) 0.02. From these results, we proposed a physical mechanism for the interaction between DoTab and PAA of low charge density. The occurrence of the opaque region, the sharp increase in the particle size, and the onset of the exothermic enthalpy peak are in agreement at DoTab concentration of ∼0.2 mM. This DoTab concentration is designated as the critical aggregation concentration for PAA/DoTab mixture when R < RC, at which PAA-bound DoTab micelles start to form and consequently induce the precipitation of the polymer/surfactant complex. As more DoTab was added, the intensity and Rhapp decreases rapidly as the mixture becomes transparent, and the enthalpic binding profile merges with the dilution curve at CDoTab ∼ 1 mM. This concentration is designated as the saturation concentration (CS) for R < RC, and beyond this point the binding ceases and the polymer/surfactant complex resolubilizes. At R ) 0.02, the binding fraction φ defined by eq 3 at saturation concentration (CS) was calculated to be approximately 8, which confirms that the binding at low R is not the electrostatic interaction between DoTa+ and COO-. It was reported that the aggregation number of polymer-bound DoTab micelle is close to or lower than that of free micelle,13,29 which is approximately

Binding of DoTab to PAA

Figure 5. Enthalpy curves for titrating 0.1 M DoTab into 5 mM PAA at R ) 0: (9) in the absence of urea; (]) in the presence of 2 M urea.

60 according to literature.30 On the basis of the above reasoning and the amount of PAA-bound DoTab molecules determined from the binding profiles shown in Figure 4, we determined that each PAA chain (Mw ) 64 900; Mw/Mn ) 1.02) binds approximately three DoTab micelles. It is noted that the precipitation of the PAA/DoTab complex at R ) 0.02 is unusual because the solubility of PAA should increase upon the polymer-induced micellization as the DoTab molecules bind on the polymer chains with their hydrophilic headgroups extending outward. Moreover, the exothermic maximum on the enthalpy curve is also unexpected because it contradicts with the experimental observation that the polymerinduced micellization is normally endothermic.6,31-34 These unusual features in the binding profiles suggest that interactions other than polymer-induced micellization is involved. The observed interaction is enthalpic driven, and it leads to the interchain complexation resulting in precipitation. We observed that the enthalpy at R < 0.3 measured by ITC varies from -7 to -19 kJ/mol, which agrees with the value of enthalpic contribution from hydrogen bonding (H-bonding) (-5 to -20 kJ/mol).35 The carboxylic group has a hydrogen donor as well as an acceptor, and they can readily form H-bonding with each other as either heterocyclic dimer or open array. It has been observed that poly(carboxylic acid)s (e.g., PAA or PMAA) form an insoluble interpolymer complex with polymers consisting of proton acceptor via H-bonding.36,37 Moreover, it was reported

J. Phys. Chem. B, Vol. 108, No. 26, 2004 8981 that binding between cationic surfactant and neutral polymer is normally weak;3,6 however, a number of research publications suggested that cationic surfactants exhibited strong interaction with neutral polymers, and some important features of such binding were discussed. Interestingly, most of these neutral polymers are poly(carboxylic acids),4,13,17,18,20,38 which can form interchain complexes through H-bonding. Therefore, we postulated that the exothermic maximum on the enthalpy curve and the precipitation in the binding region is most likely due to H-bonding between the carboxylic groups on PAA chains. To verify this hypothesis, the effect of urea on the DoTab/ PAA interaction was studied using ITC. Urea is known as a “denaturant” and has been shown to disrupt existing H-bonding by forming H-bonding with all proton donors and acceptors. In Figure 5, the enthalpy curves obtained from titrating 0.1 M DoTab into 5 mM PAA at R ) 0.02 in the absence and presence of 2 M urea. The presence of 2 M urea would increase the pH of PAA solution from pH 3.39 to 4.35; thus, an appropriate amount of HCl was added into the solution to adjust the pH back to ∼3.39 and offset the influence of urea on the ionization of PAA. As shown in the figure, the exothermic maximum is still observable and the breadth of the peak remains essentially unchanged when 2 M urea was added. However, the onset of the peak is delayed from 0.16 to 0.58 mM, and the enthalpy maximum is significantly reduced from -19.7 to -12.0 kJ/ mol. This indicates that the extent of H-bonding between the carboxylic groups of PAA is weakened since most of the protondonating and -accepting sites are occupied by urea molecules. The FTIR spectra depicting the band of O-H stretching observed from ∼3670 to 3200 cm-1 for the PAA/DoTab complexes at R ) 0.02 and 0.3 are shown in Figure 6a. For R ) 0.02, the broadness of the peak and its bands observed at lower wavenumber (i.e. ∼3450 and 3400 cm-1) indicate the existence of H-bonds of various strength formed between hydroxyl groups in the PAA/DoTab complex. With increasing R to 0.3, the O-H stretching peak is still broad; however, the peak exhibits a single maximum at ∼3460 cm-1, and the bands at lower wavenumber are not observed, suggesting that some hydroxyl groups participate in H-bonds but the average strength of H-bonds decreases. Figure 6b shows the CdO stretching band observed from ∼1750 to ∼1550 cm-1 for the PAA/DoTab complexes at R ) 0.02 and 0.3. For R ) 0.02, two bands are observed at ∼1637 and 1617 cm-1, which are attributed respectively to the un-hydrogen-bonded carboxylic groups and

Figure 6. FT-IR spectra of the PAA/DoTab complex at R ) 0 and R ) 0.3: (a) OH stretch region; (b) CdO stretch region.

8982 J. Phys. Chem. B, Vol. 108, No. 26, 2004

Wang and Tam cooperatively bind to the nonpolar polymer based on hydrophobic interaction. In this binding region, the ITC profile exhibits a significant exothermic peak and the mixture precipitates, which is attributed to the interchain complexation via hydrogen bonding induced by the binding. When R > RC, the hydrophobic binding ceases as the polymer is progressively ionized and DoTab binds to the charged polymer chains driven by electrostatic attraction. Moreover, the value of RC determined from ITC is approximately 0.3, which is reasonably close to the theoretical value derived from the Manning theory of counterion condensation (approximately 0.35). References and Notes

Figure 7. Schematic binding and complexation mechanism of the PAA/ DoTab system at low R (