Binding of Dodecyltrimethylammonium Bromide to pH-Responsive

pH-Responsive Nanocolloids Containing Cross-Linked. Methacrylic Acid-Ethyl Acrylate .... influenced by the flexibility and conformation of PAA at vari...
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Langmuir 2004, 20, 7933-7939

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Binding of Dodecyltrimethylammonium Bromide to pH-Responsive Nanocolloids Containing Cross-Linked Methacrylic Acid-Ethyl Acrylate Copolymers C. Wang and K. C. Tam* School of Mechanical and Production Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798

C. B. Tan Asia-Pacific Technical Center, The Dow Chemical Company, 16 Science Park Drive, Singapore 118227, Republic of Singapore Received February 25, 2004. In Final Form: June 21, 2004 The binding of dodecyltrimethylammonium bromide (DoTab) to cross-linked methacrylic acid-ethyl acrylate (MAA-EA) copolymers with various MAA/EA molar ratios at different degrees of neutralization (R) was quantitatively studied using isothermal titration calorimetry, dynamic light scattering, surfactant selective electrode, and electrophoresis techniques. The surfactant binds to the polymers at all degrees of neutralization, but via different mechanisms. When R is sufficiently high, the binding is primarily electrostatic interaction between the surfactant and ionized polymer chains, which is reinforced by the micellization of electrostatically bound surfactant molecules. The saturation takes place at charge ratio ([DoTa+]/[∼COO-]) close to 1, indicating that the binding is a one-to-one charge neutralization between the cationic surfactant headgroups and anionic carboxylate sites of the polymers. When R is low, the binding of DoTab to the unneutralized polymers is driven by the hydrophobic interaction. The onset of hydrophobic binding takes place at DoTab concentration as low as 0.01 mM in 0.05 wt % polymer solution, where the saturation occurs at CDoTab ∼ 0.19 mM and the amount of bound surfactant is approximately 0.09 mmol of DoTab/(g of polymer) at saturation concentration. The binding results in the formation of the polymer-surfactant complex. For the polymer with low MAA/EA molar ratio, the complex coagulates at a higher DoTab concentration that leads to phase separation; however, for polymers with high MAA/EA molar ratio, the complex remains dispersed and the mixture is stable even at high DoTab concentration.

Introduction Interactions between oppositely charged polyelectrolytes and surfactants in aqueous solutions have attracted significant interest because of complex behaviors and widespread applications in rheological control, detergency, and pharmaceuticals, etc. Important information and understanding of the mechanisms, thermodynamics, and phase behavior of the interactions and the structure of the polymer-surfactant complex have been obtained.1-14 * To whom correspondence should be addressed. Fax: 65-7911859. E-mail: [email protected]. (1) Hayakawa, K.; Kwak, J. C. J. Phys. Chem. 1982, 86, 3866. (2) Hayakawa, K.; Santerre, J. P.; Kwak, J. C. Macromolecules 1983, 16, 1642. (3) Goddard, E. D. In Interactions of Surfactants with Polymers and Proteins; Goddard, E. D., Ananthapadmanabhan, K. P., Eds.; CRC Press: Boca Raton, FL, 1993; p 171. (4) Anthony, O.; Zana, R. Langmuir 1996, 12, 1967. (5) Khokhlov, A. R.; Dormidontova, E. E. Phys.-Usp. 1997, 40(2), 109. (6) Ghoreishi, S. M.; Fox, G. A.; Bloor, D. M.; Holzwarth, J. F.; WynJones, E. Langmuir 1999, 15, 5474. (7) Li, Y.; Ghoreishi, S. M.; Warr, J.; Bloor, D. M.; Holzwarth, J. F.; Wyn-Jones, E. Langmuir 1999, 15, 6326. (8) Merta, J.; Stenius, P. Colloids Surf., A 1999, 149, 367. (9) Konop, A. J.; Colly, R. H. Langmuir 1999, 15, 58. (10) Ghoreishi, S. M.; Li, Y.; Holzwarth, J. F.; Khoshdel, E.; Warr, J.; Bloor, D. M.; Wyn-Jones, E. Langmuir 1999, 15, 1938. (11) Ehtezazi, T.; Govender, T.; Stolink, S. Pharm. Res. 2000, 17, 871. (12) Matulis, D.; Rouzina, L.; Bloomfield, V. A. J. Mol. Biol. 2000, 296, 1053. (13) Fundin, J.; Hansson, P.; Brown, W.; Lidegran, I. Macromolecules 1997, 30, 1118. (14) Hansson, P. Langmuir 2001, 17, 4167.

The interactions between strong polyelectrolytes with oppositely charged surfactants have been extensively studied. The binding is generally considered an ion-exchange interaction via electrostatic binding, where condensed counterions on the polymer chains are replaced by bound surfactant molecules.1-5,8,11-13 Typically, the electrostatic interaction is reinforced by either the micellization of polymer-bound surfactant or the cooperative aggregation between alkyl chains of bound surfactant and the hydrophobic segment of polymer chains.1-5,8,11-13 On the other hand, the complexation of cationic surfactant with weak polyelectrolytes, such as partially neutralized or unneutralized polycarboxylic acids, is less extensively studied and is beginning to attract considerable attention because of their industrial applications.1,2,15-21 This system has potential utility in the control of chemical reactivity, drug delivery, and nonspecific binding of DNA with basic protein,15 and it may be used as a simplified model for understanding biological processes.22 (15) Yoshida, K.; Dubin, P. L. Colloids Surf., A 1999, 147, 161. (16) Chandar, P.; Somasundaran, P.; Turro, N. J. Macromolecules 1988, 21, 950. (17) Kiefer, J. J.; Somasundaran, P.; Ananthapadmanabhan, K. P. Langmuir 1993, 9, 1187. (18) Kogej, K.; Theunissen, E.; Reynaers, H. Langmuir 2002, 18, 8799. (19) Shimizu, T. Colloids Surf., A 1994, 84, 239. (20) Katsuura, H.; Kawamura, H.; Manabe, M.; Kawasaki, H.; Maeda, H. Colloid Polym. Sci. 2002, 280, 30. (21) Kosmella, S.; Ko¨tz, J.; Shirahama, K.; Liu, J. J. Phys. Chem. B 1998, 102, 6459. (22) Cavasino, F. P.; Hoffmann, H.; Sbriziolo, C.; Turco Liveri, M. L. Colloids Surf., A 2001, 183, 689.

10.1021/la049509o CCC: $27.50 © 2004 American Chemical Society Published on Web 07/30/2004

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The charge density, backbone flexibility, and hydrophobicity of polyacids can be easily modified by varying pH. Thus, a number of studies have focused on the interaction of cationic surfactants with polyacids at different degrees of neutralization to obtain insights into the polymer-surfactant binding. The effect of charge density on the binding of cationic surfactants to polyanions such as poly(acrylic acid) (PAA), poly(vinyl sulfate), and carboxymethyl cellulose was studied by Kwak and coworkers, and they found that the binding is governed by several factors, such as charge effect, chain flexibility, and chemical nature of charged sites.1,2 Fluorescence spectroscopic study on the binding of dodecyltrimethylammonium (DoTab) to pyrene-labeled PAA suggests that the primary binding force is electrostatic regardless of pH, and the binding is strongly influenced by the flexibility and conformation of PAA at various pH.16 Besides electrostatic interaction, other forces such as hydrophobic interaction and hydrogen bonding are also involved in the binding of surfactant to weakly charged polyacids.15 Kogej and co-workers reported that the binding of alkylpyridinium surfactants to unneutralized PAA and poly(methacrylic acid) (PMAA) is considered as a polymerinduced micellization of surfactant driven by hydrophobic interaction.18 Katsuura et al. observed from electromotive force studies20 that hydrophobic interaction is responsible for the binding of dodecylpyridinium chloride (C12PyCl) to PAA and PMAA at low pH (99%), was obtained from Fluka and was 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. Sample Preparation. The polymer latex at low pH (ca. 3-4) was dialyzed in distilled-deionized water using regenerated cellulose tubular membrane. The dialysis process was carried out over a 1-month period where most of the impurities and unreacted chemicals can be removed. A 3 wt % stock latex was prepared and stored at 4 °C prior to use. From the stock latex, 0.05 wt % polymer lattices adjusted to the desired degree of neutralization (using 1 M standard NaOH solution from Merck) were prepared for the measurements. Isothermal Titration Calorimetry. The microcalorimetric study was carried out using the Microcal isothermal titration calorimeter. This power compensation, differential instrument was previously described in detail by Wiseman et al.43,44 It has a reference and a sample cell of approximately 1.35 mL, and the cells are both insulated by an adiabatic shield. The titration was carried out at 25.0 ( 0.02 °C, by injecting 0.1 M DoTab solution (34) Fox, G. J.; Bloor, D. M.; Holzwarth, J. F.; Wyn-Jones, E. Langmuir 1998, 14, 1026. (35) Li, Y.; Bloor, D. M.; Wyn-Jones, E. Langmuir 1996, 12, 4476. (36) Li, Y.; Xu, R.; Couderc, S.; Bloor, D. M.; Holzwarth, J. F.; WynJones, E. Langmuir 2001, 17, 5742. (37) Xu, R.; Bloor, D. M. Langmuir 2000, 16, 9555. (38) Thurn, T.; Couderc, S.; Sidhu, J.; Bloor, D. M.; Penfold, J.; Holzwarth, J. F.; Wyn-Jones, E. Langmuir 2002, 18, 9267. (39) Wang, C.; Tam, K. C.; Jenkins, R. D.; Bassett, D. R. Phys. Chem. Chem. Phys. 2000, 2(9), 1967. (40) Islam, M. F.; Jenkins, R. D.; Bassett, D. R.; Lau, W.; Ou-Yang, H. D. Macromolecules 2000, 33, 2480. (41) Dai, S.; Tam, K. C.; Jenkins, R. D. Eur. Polym. J. 2000, 36, 2671. (42) Wang, C. Ph.D. Thesis, Nanyang Technological University, Singapore, 2004. (43) Wiseman, T.; Williston, S.; Brandts, J. F.; Lin, L. Anal. Biochem. 1989, 179, 131. (44) Jelesarov, I.; Bosshard, H. R. J. Mol. Recognit. 1999, 12, 3.

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from a 250 µL injection syringe into the sample cell filled with 0.05 wt % polymer latex. The syringe is tailor-made such that the tip acts as a blade-type stirrer to ensure an optimum mixing efficiency at 400 rpm. The heat evolved or absorbed by each injection in the course of titration is directly measured by the ITC unit, producing the raw heat signal, also known as cell feedback (CFB). Integration of each CFB gives the differential enthalpy curve for the titration. Surfactant Ion Selective Electrode. A coated-wire (CW) surfactant membrane electrode selective to dodecyltrimethylammouium cation (DoTa+) constructed at Salford University was used to determine the DoTab monomer concentrations by measuring their EMF relative to a commercial Ag/AgCl electrode.37 The novel CW surfactant ISE was prepared by coating the tip of a partially insulated silver wire (its tip is exposed) with a positively charged PVC conditioned to the DoTab surfactant. The detailed conditioning and construction steps of the CW ISE were reported by Xu and Bloor.37 The EMF was measured and recorded by the Radiometer ABU93 Tri-burette titration system with a built-in microvoltmeter. The EMF measurements were performed at 25 °C. A 0.1 M DoTab solution was added dropwise to a rapidly stirred 50 mL polymer latex of desired concentration (0.05 wt %) every 5 min, followed by the determination of EMF when the equilibrium was reached. The calibration curve (dependence of EMF on log(CDoTab)) obtained from titrating 0.1 M DoTab solution into 10-4 M NaBr solution at 298 K is shown in Figure 2. The calibration curve exhibits an inflection point at CDoTab ∼13.8 mM, characterizing the CMC of DoTab. Below the CMC, the plot is linear with a slope of 57.4 mV/decade that corresponds to Nernstian behavior. Dynamic Laser Light Scattering. The dynamic laser light scattering experiments were conducted using the Brookhaven MAS (Multiangle Sizing Option) dynamic light scattering system. A 5-15 mW solid-state laser of wavelength λ0 ) 671 nm 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 Stokes-Einstein equation

Rh )

kTq2 6πηΓ

(1)

where k is the Boltzmann constant, q is the scattering vector (q ) 4πn sin(θ/2)/λ, where n is the refractive index of the solvent, 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. Electrophoresis Study. The electrophoresis study was carried out using the Brookhaven Zeta PALS (phase analyzer light scattering). The Zeta PALS is an extension of laser electrophoretic light scattering (ELS), which measures the velocity of moving particles that scatter laser light. The ζ potential was calculated via the Smoluchowski model fitting of the mobility data, and thus the stability of the complex in the course of binding was determined.

Results and Discussion Overview of the Binding of DoTab to Cross-Linked MAA-EA Copolymers. The differential enthalpy curves for titrating 0.1 M DoTab into lattices of fully neutralized 0.05 wt % cross-linked MAA-EA copolymers (cross-linked by 4 wt % DAP) in the presence of 0.1 M NaCl were plotted in Figure 1. The enthalpy profiles shown in the figure are generally consistent with those corresponding to the binding of DoTab to random MAA-EA copolymers and PAA reported previously,45,46 suggesting that the binding mechanisms of cross-linked and random MAA-EA co-

Figure 1. Differential enthalpy curves for titrating 0.1 M DoTab into 0.05 wt % solutions of cross-linked MAA-EA copolymers with various MAA/EA molar ratios: (0) MA20EA80-4; (() MA30-EA70-4; (4) MA40-EA60-4; (b) MA50-EA504; (O) dilution curve for DoTab in water.

polymers with DoTab are identical. Three endothermic maximums designated as “A”, “B”, and “C” are marked on the enthalpy curves. Peak A, whose breadth is proportional to the molar percentage of MAA content (i.e. the amount of charged sites), characterizes the electrostatic binding of individual DoTa+ cations to carboxylate groups on the polymer. Peak B represents the hydrophobic association of polymer-bound surfactant molecules with EA segments, resulting in the formation of mixed micelles on the EA segments. Peak C observed at higher DoTab concentration corresponds to the formation of free micelles in the polymer solution. The characterization of the enthalpy profiles has been discussed in our previous papers and will not be repeated here.45,46 It was also observed that the enthalpy profiles for the binding of DoTab to MAA-EA copolymers with different cross-linked densities (ranging from 0.5 to 4 wt %) are identical, suggesting that the electrostatic binding and the micellization of electrostatically bound surfactant are mainly determined by the amount of charged sites on the polymer chains and are independent of cross-linked densities. Effect of Degree of Neutralization (r) on the Binding: EMF Study. The EMF measurements in mixtures of DoTab and cross-linked MAA-EA copolymer (MA50-EA50-4) at different R were carried out to elucidate the effect of charge density on the binding behavior. Free monomeric surfactant concentration was determined potentiometrically using a coated-wire surfactant membrane electrode selective to DoTa+ against a commercial Ag/AgCl electrode as a reference. The plots of EMF against DoTab concentration in 0.05 wt % MA50-EA50-4 at different R in 10-4 M NaBr solutions together with the calibration curve are given in Figure 2. It is shown that the EMF corresponding to the concentration of free DoTab monomers obtained in the presence of polymer in the range of R from 0.02 to 1 diverges from the calibration curve, characterizing the binding of DoTab onto the polymer with different charge densities. Our previous study has confirmed that the binding of DoTab to poly(acrylic acid) at low R is induced by hydrophobic interaction;47 thus, we may deduce the binding of DoTab to cross-linked MAA-EA copolymers at R ) 0.02 is also initiated by hydrophobic interaction. At (45) Wang, C.; Tam, K. C. Langmuir 2002, 18, 6484. (46) Wang, C.; Tam, K. C.; Jenkins, R. D.; Tan, C. B. J. Phys. Chem. B 2003, 107, 4667. (47) Wang, C.; Tam, K. C. J. Phys. Chem. B 2004, 108, 8976.

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Wang et al. Table 1. Summary on the Binding of DoTab to 0.05 wt % MA50-EA50-4 Lattices of Different r

Ra unneutralized (0.02) 0.15 0.5 0.8 1 a

Figure 2. EMF determined using the DoTab ion selective electrode for the binding of DoTab to MA50-EA50-4 at different R in 10-4 M NaBr: (b) calibration curve; (0) R ) 0.02; (9) R ) 0.15; ()) R ) 0.5; (() R ) 0.8; (4) R ) 1.

R ) 0.02, the carboxylic groups are nearly unneutralized and the polymer is un-ionized, thus, the binding of DoTab to the unneutralized latex particles is driven by hydrophobic interaction. As shown in Figure 2, the EMF in the presence of polymer at R ) 0.02 deviates from Nernstian behavior with the first addition of DoTab (CDoTab ∼ 0.01 mM), characterizing the binding of DoTab onto unneutralized polymer at extremely low DoTab concentration. With further increase of the DoTab concentration, the EMF curve becomes Nernstian and merges with the calibration curve at CDoTab ∼ 0.19 mM, indicating the saturation of the polymer with bound surfactant. Thereafter the binding curve exhibits an inflection point at CDoTab ∼ 14 mM, indicating the formation of free micelles in the presence of polymer. To determine the critical aggregation concentration (CAC) for the binding, the EMF corresponding to the binding was measured by titrating using a low initial DoTab concentration of 10-3 mM, and the result is documented in the Supporting Information. It is apparent that there is no CAC for the binding and DoTab binds to the polymer at extremely low DoTab concentration. The absence of CAC may be caused by the high hydrophobicity of the polymer, which exists as latex particles formed by chemically cross-linked polymer chains and contains a large proportion of hydrophobic domains comprised of EA segments. The high hydrophobicity of the polymer allows DoTab molecules to bind on the polymer uncooperatively, forming dimers, trimers, and small aggregates on the latex particles. Similar phenomenon was also reported for the binding of ionic surfactant to hydrophobically modified polymers.3,36 At higher R (R ) 0.15, 0.3, 0.5, and 1), the EMF data obtained in the presence of the polymer lattices exhibit an identical trend as shown in Figure 2. The EMF curves diverge from the calibration curve without exhibiting a CAC, indicating that electrostatic binding between DoTa+ and ∼COO- takes place immediately when the oppositely charged surfactant and polymer are mixed. This agrees with the ITC result shown in Figure 1 that the endothermic peak characterizing the electrostatic binding appears at the lowest DoTab concentration. Moreover, the polymer with higher R exhibits a lower EMF value in the binding region, simply reflecting the fact that polymers with higher charge density bind more surfactant molecules (as indicated in Table 1). With further addition of DoTab, the EMF increases steeply with DoTab concentration and approaches the calibration curve, indicating the saturation of the polymer with bound surfactant molecules.

C2 (mM)

[DoTab]/ [polymer] at C2 (mmol/g)

[DoTa+]/ [COO-] at C2

nature of binding

0.19

0.09

N/A

hydrophobic

0.42 1.40 3.10 3.78

0.35 1.17 2.48 4.99

1.1 1.1 1.3 1.5

electrostatic electrostatic electrostatic electrostatic

R ) degree of neutralization.

The concentration of free surfactant molecules in the presence of polymer can be determined by comparing the EMF data of the binding curve with that of the calibration curve, and the offset in surfactant concentration refers to the concentration of bound surfactant (Cb). An example is shown in Figure 2. Binding isotherms can be constructed, and they show the dependence of binding fraction on the concentration of total surfactant added. The binding fraction β is defined as

β)

Cb Ct

(2)

where Cb is the concentration of the polymer bound surfactant and Ct is the total surfactant concentration. The values of saturation concentrations (C2), the amount of bound surfactant per gram of polymer at C2, the natures of binding, and other important features revealed by EMF data are summarized in Table 1. The binding isotherms of DoTab to MA50-EA50-4 at R ) 0.02, 0.15, 0.5, and 1 in 10-4 M NaBr are shown in Figure 3a. At R ) 0.02, approximately 93% of added surfactant is bound on the polymer on the first addition of DoTab (CDoTab ∼ 0.01 mM), which is driven by hydrophobic interaction between DoTab and unneutralized polymeric latex particles. Thereafter, the polymer becomes saturated with additional surfactant molecules, and the binding fraction decreases steadily with DoTab concentration. The binding fraction reaches its minimum of approximately 23%, and the binding isotherm starts to level off at CDoTab ∼ 0.19 mM, which corresponds to the saturation of the polymer with bound DoTab. When CDoTab > C2, the decrease in the binding fraction is caused by the continuous increase of total surfactant concentration Ct. Referring to Table 1, the amount of bound surfactant at C2 is estimated to be ∼0.09 mmol of DoTab molecules/(g of polymer). In contrast to the binding isotherm at R ) 0.02, the binding fraction obtained at higher R (0.15, 0.5, 0.8, and 1) exhibits a plateau over a wider range of DoTab concentrations. The value of the plateau remains close to unity, and its breadth is proportional to the charge density of the polymer; thus, the plateau region corresponds to the binding of DoTab ions to negatively charged carboxylate groups, where nearly all the added DoTab molecules are bound to the polymer via electrostatic interaction. The binding fraction decreases sharply and exhibits a sigmoidal shape at a critical DoTab concentration, suggesting the charged carboxylate groups of the polymer are completely occupied by DoTa+ ions, that is, the saturation of the polymer with bound surfactant molecules at C2. In Figure 3b, the binding isotherms obtained at R ) 0.15, 0.5, 0.8, and 1 are represented as a function of charge ratio, defined as [DoTa+]/[∼COO-], where the binding

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Figure 4. Binding fraction (Cb/Ct) and scattering intensity as a function of DoTab concentration in 0.05 wt % unneutralized polymer solutions: ()) Cb/Ct of MA20-EA80-4; (() intensity of MA20-EA80-4; (0) Cb/Ct of MA50-EA50-4; (9) intensity of MA50EA50-4.

Figure 3. (a) Binding isotherms of DoTab to 0.05 wt % MA50EA50-4 in 10-4 M NaBr at different R: (0) R ) 0.02; (9) R ) 0.15; ()) R ) 0.5; (() R ) 0.8; (4) R ) 1.0. (b) Binding fraction (Cb/Ct) as a function of charge ratio (CDoTa+/CCOO-) for DoTab to 0.05 wt % MA50-EA50-4 in 10-4 M NaBr at different R: (9) R ) 0.15; ()) R ) 0.5; (() R ) 0.8; (4) R ) 1.

isotherms fall on a common curve and the saturation takes place at the charge ratio close to 1, indicating that the electrostatic binding is actually a one-to-one charge neutralization interaction between the polymer carboxylate groups and DoTa+ ions. However, the binding isotherm at R ) 1 deviates from the common curve, which may be caused by the precipitation of the polymer-surfactant complex at high DoTab concentration that affects the performance of the electrode. Moreover, the EMF and the binding isotherms at higher R suggest that the polymer is saturated before the formation of free micelles (C2 < Cm), which is consistent with our ITC results.45-47 It has been reported that in the neutral polymer/ionic surfactant system, saturation generally occurs after the formation of free micelles (Cm < C2).6,7,10,36,38 However, for the polyelectrolyte-oppositely charged surfactant system, it is possible for the saturation to occur prior to the formation of free micelle, as the polymer-surfactant interaction is driven by strong electrostatic attraction. Interaction between DoTab and Unneutralized Cross-Linked MAA-EA Copolymers: Dynamic Light Scattering, Electrophoresis, and EMF Studies. The interactions between DoTab and unneutralized crosslinked MAA-EA copolymers with different MAA/EA molar ratios were studied by dynamic light scattering, electrophoresis and EMF techniques. The results for the EMF data for titrating DoTab to 0.05 wt % unneutralized MA20EA80-4 and MA50-EA50-4 lattices are documented in the Supporting Information. In Figure 4, the binding isotherms derived from EMF were plotted together with the scattering intensity

measured by dynamic light scattering. In the binding region defined by the EMF data, the scattering intensity increases progressively upon the addition of DoTab, signaling the gradual complexation between DoTab and the polymers. With further addition of DoTab, the complex precipitates at CDoTab ∼ 0.1 mM, resulting in the sharp increase in the scattering intensity as shown in Figure 4. Moreover, the saturation concentration C2 (∼0.2 mM) and the amount of bound surfactant per gram of polymer (approximately 0.14 mmol) were found to be identical for MA20-EA80-4-DoTab and MA50-EA50-4-DoTab mixtures. This suggests that the unneutralized cross-linked MAA-EA copolymers are almost equally hydrophobic and identical amounts of surfactant are bound to the polymer regardless of the MAA/EA molar ratio. This is totally unlike the binding between DoTab to fully neutralized cross-linked MAA-EA copolymers (driven by electrostatic interaction), where the polymer with higher MAA/EA molar ratio binds more surfactant molecules. The relaxation time distribution functions of 0.05 wt % unneutralized MA20-EA80-4 and MA50-EA50-4 with added DoTab (from 0 to 0.3 mM) measured at 90° are shown in Figure 5. The relaxation time distribution functions for the polymer-DoTab mixture were also measured at different scattering angles (from 50 to 90° at 10° intervals) in order to determine the dependence of decay rate Γ on q2. It was found that Γ exhibits a good linear relationship with q2, confirming that the distribution functions are caused by the translational diffusion of polymer or polymer-surfactant complex. In the absence of surfactant, the polymers exist as insoluble latex particles and the distribution function is narrow. The relaxation time for MA20-EA80-4 remains unchanged upon the addition of DoTab until the DoTab concentration reaches 0.09 mM, beyond which the distribution function changes from monomodal to bimodal. The fast mode is believed to correspond to individual latex particles with bound DoTab, whereas the slow mode characterizes the coagulated latex particles due to the polymer-surfactant complexation. For MA50-EA50-4, the distribution function is monomodal over the entire course of binding. The relaxation time progressively shifts to higher values, corresponding to gradual formation of the polymer-DoTab complex with larger particle size. The hydrodynamic radius (Rhapp) calculated from the relaxation time through the Stokes-Einstein equation

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Figure 5. Relaxation time distribution functions of DoTab in unneutralized 0.05 wt % MA20-EA80-4 and MA50-EA50-4 solutions measured at 298 K and 90°.

Figure 6. (a) Dependence of hydrodynamic radius on DoTab concentration in 0.05 wt % unneutralized MA20-EA80-4 and MA50-EA50-4 solutions: (0) MA20-EA80-4, fast mode; (9) MA20-EA80-4, slow mode; (() MA50-EA50-4. (b) Dependence of ζ potential on DoTab concentration in 0.05 wt % unneutralized MA20-EA80-4 and MA50-EA50-4 solutions: (9) MA20-EA804; (() MA50-EA50-4.

was plotted against DoTab concentration for MA20EA80-4 and MA50-EA50-4 in Figure 6a. For MA20-EA804, addition of DoTab has no significant effect on the particle size until the DoTab concentration reaches 0.04 mM. Thereafter the particle size increases gradually from ∼70 to 101 nm with an increase of DoTab concentration from 0.04 to 0.09 mM. This is attributed to the expansion of

latex particles driven by the electrostatic repulsion of bound DoTab molecules. When the DoTab concentration is higher than 0.09 mM, the slow mode appears with a corresponding Rhapp of 197 nm, representing the coagulation of several colloidal latex particles to produce a larger aggregate. To further elucidate the effect of surfactant on the phase behavior of cross-linked MAA-EA copolymers, electrophoresis measurement was performed and results are shown in Figure 6b, in which the ζ potentials of the polymer-DoTab mixtures were plotted against DoTab concentration. The ζ potential of MA20-EA80-4 alone is negative, indicating the surface of the latex particles is negatively charged due to the polarity and spontaneous dissociation of carboxylic groups. The value of the ζ potential is sufficiently negative (-22.8 mV) to yield a stable latex. When DoTab was added, the binding of cationic DoTab to the polymer introduces positive charges to the particle surface, which decreases the value of the ζ potential to ∼-14 mV and consequently reduces the repulsive force that stabilizes the latex particles. When the DoTab concentration exceeds 0.09 mM, the reduction in the net surface charges promotes the coagulation of latex particles, resulting in the sharp decrease in the ζ potential at CDoTab ∼ 0.1 mM, as well as the increase in scattering intensity and particle size shown in Figures 4 and 6a, respectively. For MA50-EA50-4, Rhapp remains constant at approximately 140 nm when the DoTab concentration is lower than 0.09 mM. Afterward the particle size increases steadily with an increase of DoTab concentration, corresponding to the expansion of the polymer particles caused by the electrostatic repulsion introduce by bound DoTab. When the DoTab concentration exceeds 0.2 mM, the particle size reaches its maximum of approximately 245 nm and the scattering intensity increases to 1100 kcps, indicating the formation of insoluble polymer-surfactant complexes. Unlike MA20-EA80-4 that exhibits a sudden increase in particle size at CDoTab ∼ 0.09 mM, Rhapp of MA50EA50-4 increases gradually with addition of DoTab, suggesting that the particles of MA50-EA50-4 do not coagulate upon complexation. This can be interpreted by the electrophoresis study illustrated in Figure 6b. The ζ potential of MA50-EA50-4 is also negative with a value of -42.4 mV, which is higher than that of MA20-EA80-4

Binding of DoTab to pH-Responsive Nanocolloids

(-22.8 mV). The high negative ζ potential of MA50-EA50-4 is attributed to its higher MAA portion, and it produces more stable latex compared to MA20-EA80-4. The binding of DoTab to the polymer driven by the hydrophobic interaction with its cationic headgroups pointing outward decreases the effective charge of the latex particles. However, the ζ potential value only decreases marginally, i.e., from -42.4 to -31.2 mV as the DoTab concentration increases from 0 to 0.3 mM, and it is still sufficiently strong to disperse the particles and prevent coagulation. Conclusions The interaction between DoTab and cross-linked MAA-EA copolymers at different R was quantitatively studied using ITC, surfactant ion selective electrode, light scattering, and electrophoresis techniques. The mechanism of binding is found to be different depending on the R values of the polymers. When R is sufficiently high (g0.15), the binding is primarily an electrostatic interaction between the surfactant and ionized polymer chains, which is reinforced by the micellization of electrostatically bound surfactant molecules. The saturation takes place at charge ratio ([DoTa+]/[∼COO-]) close to 1, indicating that the binding is actually a one-to-one charge neutralization between the cationic surfactant headgroups and anionic carboxylate sites of the polymers. For the unneutralized polymer, the electrostatic attraction between polymer and surfactant is weak and the binding is hydrophobically driven. The onset of hydro-

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phobic binding takes place at DoTab concentration as low as 0.01 mM in 0.05 wt % MA50-EA50-4 solution, the saturation occurs at CDoTab ∼ 0.19 mM, and the amount of bound surfactant is approximately 0.09 mmol of DoTab/(g of polymer) at saturation concentration. The binding results in the polymer-surfactant complexation that precipitates when CDoTab > 0.19 mM. It is found that the phase behavior of the polymer-surfactant complex is dependent on the MAA/EA molar ratio of the polymer. For the polymer with low MAA composition (MA20-EA80-4), the complex coagulates at higher DoTab concentration that leads to phase separation, whereas the complex of polymer with high MAA composition (MA50-EA50-4) remains stable (without phase separation) even at high DoTab concentration. Acknowledgment. We acknowledge Professor WynJones, Dr. Bloor, and Mr. Jagraj Sidhu of the University of Salford for providing us the surfactant ion-selective electrodes and for helpful discussions on the DoTab surfactant selective electrode. C.W. acknowledges the financial support provided by Nanyang Technological University. Supporting Information Available: Additional EMF data for titration from low DTAB concentration and DTAB to two different cross-linked systems. This information is available free of charge via the Internet at http://pubs.acs.org. LA049509O