Binding Characteristics of Hydrophobic Ethoxylated Urethane (HEUR

Oct 3, 2001 - The binding behaviors of an anionic surfactant, sodium dodecyl sulfate (SDS), to an associative polymer, hydrophobic ethoxylated urethan...
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J. Phys. Chem. B 2001, 105, 10189-10196

10189

Binding Characteristics of Hydrophobic Ethoxylated Urethane (HEUR) and an Anionic Surfactant: Microcalorimetry and Laser Light Scattering Studies S. Dai,† K. C. Tam,*,† and R. D. Jenkins‡ School of Mechanical and Production Engineering, Nanyang Technological UniVersity, Nanyang AVenue, Singapore 639798, Republic of Singapore, and The Dow Chemical Company, Asia-Pacific Technical Center, 16 Science Park DriVe, Singapore 118227, Republic of Singapore ReceiVed: February 21, 2001; In Final Form: June 1, 2001

The binding behaviors of an anionic surfactant, sodium dodecyl sulfate (SDS), to an associative polymer, hydrophobic ethoxylated urethane (HEUR) were studied by isothermal titration calorimetric (ITC) and laser light scattering (LLS) techniques. Static and dynamic light scattering results suggest that HEUR polymers form flowerlike micelles with an average aggregation number of about 20. Isothermal titration calorimetric studies of SDS and HEUR solutions were conducted by varying both the concentrations of SDS and HEUR. For the titration of SDS into HEUR solutions, the thermograms display an endothermic peak after the critical aggregation concentration (cac), followed by an exothermic peak that eventually merges with the SDS dilution curve. The endothermic and exothermic peaks are interpreted by different binding mechanisms at different surfactant concentrations. Although the values of cac are independent of HEUR concentrations, the saturation concentration C2 is strongly dependent on HEUR concentrations. With increasing HEUR concentrations, C2 increases proportionately. Dynamic light scattering results corroborate with the findings from ITC studies. The apparent hydrodynamic radii increase with increasing SDS concentrations for cac < CSDS< C2, and remained constant at CSDS > C2. However, the thermograms for titration of HEUR into SDS micellar solutions differ from those of titrating SDS into HEUR, suggesting that different binding mechanism must be in operation. The binding enthalpy changes increase with SDS concentration and decrease with HEUR concentration when titrating HEUR into SDS solutions.

Introduction Associative polymer is one of the important ingredients used in many water-borne coating formulations. It acts as a thickening agent to impart the correct viscosity profile needed for the application of water-borne coatings to the substrate.1 Hydrophobic ethoxylated urethane (HEUR) is one common associative polymer used in many water-borne coating formulations.2 In the mid-1980s, Glass and co-workers3,4 and Sperry and Schaller2,5 were among the first to report on such telechelic associating polymers. Since then, research on telechelic associative polymers has been actively pursued, and numerous publications have appeared in the literature. Various techniques such as rheology6-8 fluorescence spectroscopy,9,10 pulse gradient NMR,11,12 as well as the laser light scattering13,14 were employed in these studies. As a result, the association mechanism and the network structure of these polymers are well understood. For HEUR with C16 hydrophobic end-caps and a molecular weight greater than 10000, HEUR polymer segments self-associate into discrete micelles or rosettes consisting of a hydrophobic core that is surrounded by a corona of PEO chains looping back into the core. With increasing HEUR concentration, the micelles are connected by bridging chains yielding a network structure that exhibits interesting rheological behavior.15 In many water-borne coating formulations, polymer and surfactants are frequently employed together, particularly to * To whom correspondence should be addressed. Fax: (65) 791-1859. E-mail: [email protected]. † Nanyang Technological University. ‡ The Dow Chemical Company.

achieve colloidal stability, structuring, and rheological control. A detailed understanding on the interactions between HEUR and surfactant is necessary to enable formulators to develop superior water-borne coating formulations. Surfactant interacts strongly with HEUR and such interactions will influence the network structure of the HEUR. The presence of small amounts of SDS enhances the viscosity of HEUR solution, but excessive amounts of SDS causes the viscosity to decrease. Annable and co-workers attributed the viscosity increase to the formation of larger number of bridging junctions formed by hydrophobic stickers displaced by SDS monomers.16 At high surfactant concentration, the hydrophobic micellar junctions are solubilized by surfactant micelles and the network structure disintegrates. Studies on the rheological behavior of HEUR/surfactant system have been conducted by several research groups.17,18 However, not many studies have examined the thermodynamic and binding mechanism of this system in the dilute solution region. The interactions between surfactants and polymers can be described by two key critical concentration regimes.19 The first is the critical aggregation concentration (cac), which corresponds to the onset for the interaction between surfactant and polymer. The second critical concentration is defined by C2, which is the point when the polymer becomes saturated with surfactant molecules. In addition, another critical concentration defined as Cm is commonly used to describe the concentration when free surfactant micelles begin to form in the polymer solution. The interaction between polymers and surfactants can be understood by either the thermodynamic characterization using isothermal titration calorimetry or the structural characterization by laser light scattering.

10.1021/jp010672r CCC: $20.00 © 2001 American Chemical Society Published on Web 10/03/2001

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In this paper, the binding behavior between HEUR and SDS was examined systematically using the isothermal titration calorimetric (ITC) and laser light scattering (LLS) techniques. By combining these two techniques, detailed information on the energetics and the structural changes could be determined. The polymer and SDS concentrations were varied to examine the concentration dependence of the critical aggregation concentration (cac) and the saturation concentration (C2). Both the isothermal titration of SDS into HEUR and HEUR into SDS micellar solution were carried out, and the thermograms were compared. From the static and dynamic light scattering results, the polymer chain conformations and the hydrodynamic properties were discussed with respect to the calorimetric data. Experimental Section Materials and Solution Preparation. The HEUR sample (46RCHX22-3) was obtained from Dow Chemicals (formerly Union Carbide) with the chemical structure as follows:7

C16H33-O-(DI-PEG)6-DI-O-C16H33 where DI is an isophorone diisocyanate group and PEG is a poly(ethylene glycol) or poly(ethylene oxide) (PEO) segment of the nominal molecular weight of 8200 Da. It has a numberaveraged molecular weight of 51000 and Mw/Mn of 1.7. The chemical characterization of this polymer using 1H NMR was previously reported by Winnik and co-workers. They reported that one polymer chain contains approximately 1.7 hydrophobic groups.20 Sodium dodecyl sulfate (SDS) was from BDH. The deionized water was from Alpha-Q Millipore water purifying system. A 2 wt % HEUR polymer aqueous solution and 0.2 M SDS aqueous solution were prepared and used as stock solutions. The test samples were diluted from these stock solutions using fresh and filtered (0.22 µm filter) deionized water. For ITC experiments, solutions in both cell and syringe were degassed to remove dissolved gases. (1) Isothermal Titration. The enthalpy changes of HEURSDS interactions were measured using a Microcal isothermal titration calorimeter. A detailed description of this power compensated instrument could be found elsewhere.21,22 All ITC measurements were performed at a constant temperature of 25.0 ( 0.02 °C. (2) Laser Light Scattering. A Brookhaven BI200 goniometer and BI9000 digital correlator were used to perform static and dynamic light scattering measurements. For static light scattering (SLS), the refractive index increment was measured by a BIDNDC differential refractometer. For dynamic light scattering (DLS), the inverse Laplace transform of REPES supplied with the GENDIST software package was used to analyze the time correlation function and the ratio of reject was set to 0.5. Results and Discussion (1) HEUR Conformation by Static and Dynamic Light Scattering. Static Light Scattering. For static light scattering, the weight-averaged molecular weight (Mw), the second virial coefficient (A2), and the z-averaged radius of gyration (Rg) of polymers can be obtained from eq 1.23

(

)(

)

q2Rg2 KC 1 ) + 2A2C 1 + Rθ Mw 3

(1)

where K ()4π2n02(dn/dC)2/NAλ4) is an optical constant with NA, n0, and λ being Avogadro’s number, the solvent refractive

Figure 1. Zimm plot of dilute HEUR (46RCHX22-3) in aqueous solutions at 25 °C. (Concentrations: 2-10 mg/mL. Measurement angles: 45-135°).

index, and the wavelength of the light, respectively. C is the polymer concentration, Rθ is the excess Rayleigh ratio at scattering angle θ, and q () 4πn sin(θ/2)/λ) is the scattering vector. The refractive index increment of HEUR polymer solution at 25 °C, (dn/dC), was determined from the differential refractometer and found to be 0.160 mL/g. Figure 1 shows the Zimm plot of dilute HEUR solution at 25 °C. The apparent weight-averaged molecular weight and radius of gyration of HEUR were found to be 930000 Da and 51 nm, respectively. Winnik and co-workers had previously reported that the cmc for this polymer is about 0.01 wt %.15 The molecular weight obtained is much higher than that of HEUR unimers, reinforcing that the polymer chains are in the form of self-associated micelles. The number of HEUR unimer chains per micelles can be calculated from the expression

Nw )

Mw(micelles) Mw(unimers)

(2)

where Nw was determined to be approximately 11. Since on average each HEUR chain contains ∼1.7 C16 alkyl chains, the aggregation number of each micelle was determined to be ∼19 (i.e., Nagg ) 1.7Nw). This value is identical to that reported by Winnik and co-workers using fluorescence spectroscopy.24,25 Since 70% of the HEUR chains are fully capped at both ends (refers to as ABA system) and 30% are capped at only one end (refers to as AB system), one should be concerned with the types of microstructure that are present in solution. From surfactant theory, the cmc of the AB system should be significantly higher than the ABA system; hence, we do not expect the AB system to form micelles on its own. More likely than not, the flower micelles proposed by Winnik and coworkers contain a mixture of 70% ABA and 30% of AB systems. This was indirectly confirmed by the stability of these micelles, which do not phase separate at all concentrations due to the steric stabilization of the dangling PEO segments. On the contrary, the fully capped system examined by Russel and co-workers26 phase separated at concentration ranging from 0.1 to 1.5 wt % because steric stabilization was absent. Dynamic Light Scattering. The relaxation time distribution functions of HEUR at different polymer concentrations are shown in Figure 2. Only one peak is evident from the distribution functions. The linear dependence of the decay rates on q2 suggests that the relaxation mode is a translational diffusion mode and the slope corresponds to the translational diffusion coefficient of HEUR micelles. The concentration dependence of the diffusion coefficients is shown in Figure 3.

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Figure 2. The relaxation time distributions of different concentrations of HEUR in aqueous solutions at 25 °C. (Measurement angle of 90°).

Figure 3. The relationship between diffusion coefficients and concentrations of HEUR at 25 °C.

The diffusion coefficients decrease with increasing polymer concentrations. The diffusion coefficients determined at finite concentration are characterized by the diffusional second virial coefficient kD, described by the equation below;27

D ) D0(1+kDC + ... )

(3)

where D0 is the translational diffusion coefficient at infinitely dilute solution. It possesses a value of 7.1 × 10-12m2/s, which is identical to the self-diffusion coefficient for HEUR micelles as determined from pulse gradient NMR experiments.15 From the Stokes-Einstein expression, the hydrodynamic radius Rh can be determined from

Rh )

kT 6πη0D0

(4)

where k is the Boltzmann constant, T the absolute temperature in Kelvin, and η0 the solvent viscosity. The hydrodynamic radius of HEUR micelle was determined to be ∼34.5 nm. (2) The Binding Behaviors between SDS and HEUR. Titration of SDS into HEUR. The binding interactions between HEUR and SDS are more complicated than the interactions between HEUR and other surfactant (such as cationic surfactants and nonionic surfactants) systems. The SDS hydrophobic tails not only bind to the hydrophobic end groups of HEUR, but the headgroups of SDS micelles could also bind to the PEO segments.15,28,29 Previous rheological studies mainly focused on the hydrophobic interactions between SDS and the semidilute HEUR solutions. It was found that SDS monomers could bind

Figure 4. Calorimetric titration curves for titration of 0.2 M SDS into water (open circle) and 0.1 wt % HEUR (filled circle) at 25 °C and 1 atm. The insert figure is the difference curves.

to the core of the HEUR end groups and strengthen the rosette micelle of the HEUR at low SDS concentrations. With increasing SDS concentrations, SDS monomers would substitute some of the HEUR end groups in the HEUR micellar core. The substituted HEUR hydrophobic end groups are released to form bridges with other HEUR micelles, yielding a larger proportion of hydrophobic junctions, which enhances the solution viscosity. A further increase in the SDS concentration saturates the HEUR hydrophobic end groups, resulting in the destruction of the flowerlike structure thereby lowering the solution viscosity. When all the HEUR end-capped hydrophobes are fully saturated by SDS micelles, the interactions between SDS and HEUR are identical to those between SDS and PEO of similar molecular weight. The detailed descriptions of HEUR and SDS interaction were summarized by Zhang et al.15 and Binana-Limbele et al.29 Figure 4 (filled circle) shows the isothermal titration curve of 0.2 M SDS in 0.1 wt % HEUR solution together with the dilution curve of 0.2 M SDS in water shown by open circles. A large deviation between these two titration thermograms is evident. The difference is attributed to the interactions between SDS and HEUR. There is only one weak transition at 8.3 mM in the SDS dilution curve and this corresponds to the cmc of SDS, which is close to the literature value.30,31 At low SDS concentration, the titration curve for SDS/HEUR system begins to deviate from the SDS/water curve, but the enthalpy change is not significant. The slight increase in ∆H may be due to the uncooperative hydrophobic interaction between HEUR end groups and SDS hydrophobic tails. However, the enthalpy changes also include those arising from the change in the solvent environment caused by the presence of HEUR chains. When the SDS concentration reaches 2.7 mM, ∆H increases sharply and reaches a maximum at 5.4 mM and then decreases. This endothermic peak correlates to the formation of SDS mixed micelles on the HEUR chains (or HEUR/SDS aggregation complex) and the solubilization of both the HEUR end groups and the dehydated PEO segments from water phase into the hydrophobic core of mixed SDS micelles. The onset point for the sharp increase in ∆H is characterized by the critical aggregation concentration (cac). For the PEO/SDS system, the aggregation number of SDS inside the aggregation complex at cac is lower than that of free SDS micelles in water. With increasing SDS concentration, the aggregation number of SDS in the PEO/SDS complex continues to increase.32 However, as the aggregation number of the bound SDS molecules increases, the binding rate decreases due to electrostatic repulsions of these

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TABLE 1: Thermodynamic Parameters for SDS Titration HEUR (46RCHX22-3) at 25˚C and 1 atm cac (mM)a

∆H (kJ/mol)b

0.20 M 0.15 M 0.10 M 0.08 M 0.05 M

2.7 2.7 2.7 2.7 2.7

2.2 2.1 2.0 2.2 2.1

0.20% 0.15% 0.10% 0.08% 0.05% 0.03% 0.01%

2.7 2.7 2.7 2.7 2.7 2.7 3.2

2.3 2.3 2.2 2.2 2.2 2.2 2.1

a

∆H at ∆Hmax (kJ/mol)b

C2 (mM)a

Different Concentrations of SDS into 0.1 wt % HEUR -27.1 29.3 5.4 -27.1 29.2 5.4 -27.1 29.1 5.4 -27.1 29.3 5.4 -27.1 29.2 5.4

4.2 3.8 3.3 3.3 3.3

16.7 16.7 16.7

0.2 M SDS into Different Concentrations of HEUR -27.1 29.4 5.4 -27.1 29.4 5.4 -27.1 29.3 5.4 -27.1 29.3 5.4 -27.1 29.3 5.4 -27.1 29.3 5.4 -26.3 28.4 5.4

5.2 4.8 4.2 3.8 3.4 3.1 2.3

23.9 20.6 16.7 15.2 12.5 10.4 9.1

∆G (kJ/mol)

T∆S (kJ/mol)

C at ∆Hmax (mM)a

Estimated error (0.05 mM. b Estimated error (0.1 kJ/mol.

SDS headgroups, which results in a decrease in ∆H beyond the maximum value. The titration curve then intersects with the SDS/water curve, and continues to decrease to a minimum. The minimum exothermic peak is attributed to structural reorganization of the HEUR/SDS complex. With increasing SDS concentration, the aggregation number of SDS increases and the PEO segments in the SDS micellar core are expelled from the hydrophobic core into water due to their amphiphilic character. After rehydration, these PEO segments wrap around the charged surface of SDS micelles to form the necklace-like SDS/polymer complex. The driving force is the ion-dipole association between the dipoles of the hydrophilic PEO segments and the ionic headgroups of the surfactant. The above structure results in the screening of the electrostatic interactions between SDS hydrophilic heads. For the necklace-like complex, the wrapping of PEO segments of the HEUR chains also decreases the contact between the “exposed” hydrophobic segments of SDS micelles and the water phase. With further addition of SDS, the degree of the rehydration becomes more rapid. It slows down after the minimum in the ∆H value and approaches another critical concentration designated as C2, where the HEUR chains become saturated by SDS molecules. Beyond C2, no further bindings between HEUR and SDS molecules can be detected and the titration curve merges with the dilution curve of SDS in water at Cm, where free SDS micelles are present and coexist with HEUR/SDS complex. In this system, Cm is slightly larger than C2. However, Bloor and co-workers found that, in some systems, Cm was smaller than C2.33-35 Detailed information on Cm and C2 could be derived from the evaluation of the different binding constants.36 From the literature, it is evident that the definition and the determination of cac and C2 can be ambigious. Wang et al.19 determined these characteristic concentrations from a plot of the incremental enthalpy change, [∆H(k) - ∆H(k - 1)]/∆m, against SDS concentration, where ∆H(k) is the observed enthalpy change in the kth injection and ∆m is the change in molarity. The cac can be determined from the first peak in the difference enthalpy plot, whereas C2 is defined when the difference curve becomes zero. The difference curve for titrating 0.2 M SDS into 0.1 wt % HEUR is shown by the insert in Figure 4. The cac and C2 were determined to be 2.7 mM and 16.7 mM, respectively. The cac of HEUR/SDS system is smaller than that for PEO/SDS system (∼ 4.2 mM),19 which indicates that the hydrophobic modification of PEO chains with a C16 alkyl chains enhances the polymer/SDS interaction. The Gibbs energy can be described by the expressions below,36

∆G ) (1 + K)RT ln(cac)

(5)

∆G ) ∆H - T∆S

(6)

The factor of (1 + K) accounts for the electrostatic interactions observed for ionic surfactants, where K (micellar charge fraction) equals 0.85 for SDS. The thermodynamic parameters for the binding of SDS to the HEUR polymer at cac were determined and are summarized in Table 1. By comparing the thermodynamic parameters, it is evident that the formation of HEUR/ SDS complex at cac is driven by a gain in the entropy. The titration curves of different concentrations of SDS into 0.1 wt % HEUR are shown in Figure 5. It is obvious that the cac, C2, and the titration curves are not affected by titrant (SDS) concentrations. The slight fluctuations observed in the titration curves maybe due to slight variations in the ∆H for different SDS concentrations. Since the cac corresponds to the onset of HEUR/SDS complex formation, it should only be sensitive to the hydrophobicity of the solubilized polymer segments, the HLB of the surfactant, and the total free SDS monomer concentrations in the titration cell, but independent of the titrant (SDS) concentrations. At a constant HEUR concentration, the total number of binding sites remains constant. The saturation concentration C2 only depends on the concentration of SDS in the titration cell, but not the titrant (SDS) concentration. The thermodynamic properties for titrating different concentrations of SDS into 0.1 wt % HEUR solution are summarized in Table 1. The titration of 0.2 M SDS into different concentrations of HEUR was performed to examine the effects of HEUR concentrations on the binding characteristics between SDS and HEUR. From Figure 6, it is evident that the cac, the ∆H at cac, and the SDS concentration at the maximum of the endothermic peak are independent of HEUR concentrations, while the area for the endothermic peak increases with increasing HEUR concentrations. The cac is not sensitive to the total HEUR concentrations. However, with increasing HEUR concentrations, the binding reactions increase proportionately. This leads to an increase in the binding enthalpy changes as indicated by the increase in the area of the endothermic curve. The related thermodynamic parameters are summarized in Table 1. It is evident from Figure 6 that the exothermic peak and C2 shift to higher SDS concentrations with increasing HEUR concentrations. The difference between C2 and cac, i.e. (C2 - cac), represents the amount of SDS needed to saturate the HEUR chains. The area under the endothermic and above the exothermic curves becomes larger with increasing HEUR concentrations. At higher HEUR concentration, the total number of HEUR chains in the titration cell is larger and greater amounts of SDS

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Figure 5. The isothermal titration curves for titrating different concentrations of SDS into 0.1 wt % HEUR at 25 °C and 1 atm.

Figure 7. The relaxation time distribution functions of 0.1 wt % HEUR in different concentrations of SDS at 25 °C. (From bottom to top, the SDS concentrations are 0, 0.5, 1, 1.5, 2, 3.0, 5, 7.1, 9.9, 14.9, 20.1, 25.1, 29.9, 34.8, 39.7, 44.9, 49.7, and 54.3 mM, respectively.)

Figure 6. The isothermal titration curves for titrating 0.2 M SDS into different concentrations of HEUR at 25 °C and 1atm.

are needed to saturate the HEUR chains. Hence, C2 increases with increasing amounts of HEUR chains. Dynamic Light Scattering. The interactions between HEUR and SDS at different SDS concentrations were examined by DLS. The relaxation time distributions of 0.1 wt % HEUR and different concentrations of SDS are shown in Figure 7. Only one peak in the relaxation time distribution is evident when the SDS concentration is less than 2 mM. The peak corresponds to the translational diffusion of HEUR micelles. With increasing SDS concentrations, the relaxation time distributions exhibit two distinct peaks. The relaxation times of the fast and the slow peak increase with SDS concentration until the SDS concentration reaches 20 mM. After that, a very fast mode appears. In this concentration range, the relaxation time of the fast and the slow peaks are independent of SDS concentrations. The fast and the slow peak in the relaxation time distributions are attributed to the translational diffusions of the SDS bound HEUR unimers and the HEUR/SDS micellar aggregation complexes, respectively. The very fast peak at higher SDS concentration corresponds to the hydrodynamic radius of 1.5 nm, which is identical to the size of free SDS micelles. The relationship of the apparent hydrodynamic radii of the slow modes, which represent the HEUR/SDS complex, and the SDS concentration is shown in Figure 8. From the light scattering data, a microstructure for HEUR/SDS complex based on the aggregation mechanism as depicted in Figure 9 is proposed.37 It is found that the apparent hydrodynamic radii (∼ 40.7 nm) remain unchanged with increasing SDS concentration up to 2 mM. Beyond 2 mM, the hydrodynamic radii

Figure 8. The relationship of the apparent hydrodynamic radii and SDS concentration for 0.1 wt % HEUR and SDS (filled square). The open and filled circles are the thermograms for titrating 0.2 MSDS into water and 0.1 wt % HEUR at 25 °C and 1 atm, respectively.

increase with SDS concentration, reaching an asymptote of ∼220 nm at the SDS concentration of ∼20 mM. In the absence of SDS, the 0.1 wt % HEURs are in the form of micelles as shown in Figure 9a. At SDS concentration lower than the cac, SDS monomers bind un-cooperatively to the core of HEUR micelles (Figure 9b). The relaxation times and the hydrodynamic radii remain constant. When the SDS concentration exceeds ∼2mM, SDS monomers displace the C16 alkyl chains in the micellar core, which produces small amounts of HEUR unimers and HEUR micelles with one ends exposed to the water phase (Figure 9c). When the SDS concentration exceeds the cac of 2.7 mM, HEUR unimers are dehydrated from the water phase and solubilized into the SDS micellar core to form SDS bound HEUR unimers. This is represented by the fast relaxation mode with an apparent hydrodynamic radius of 9 nm, which is fairly close to the hydrodynamic radius of ∼9.5 nm for PEO chain of similar molecular weight (The Rh of the PEO chain can be determined from Rh ) 0.0145Mw0.571).13 For HEUR micelles, these exposed hydrophobic groups and the dehydrated PEO segments are solubilized by the polymer

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Figure 9. The schematic diagram describing the binding interactions between SDS and HEUR at different concentrations of SDS.

induced SDS mixed micelles, which form aggregates that grow in size, yielding clusters of a few HEUR micelles as depicted in Figure 9d. The HEUR/SDS aggregation complex corresponds to the slow relaxation peak in the relaxation time distribution. Since the HEUR is in the dilute solution regime and the electrostatic repulsion from SDS micelles is strong, free HEUR end groups are not able to form bridges with other HEUR micelles. Thus, the formation of the aggregation complex must be driven by the solubilization of several “exposed” HEUR hydrophobic groups by SDS micelles. The cac value determined from light scattering agrees with that determined from ITC technique. With increasing SDS concentrations, the SDS aggregation number will increase, giving rise to the increase in the relaxation times and the hydrodynamic radii for both fast and slow modes. When the SDS concentration exceeds 8 mM, the solubilized PEO segments rehydrate and bind to the SDS micelles via ion-dipole association, and the structure reorganizes into a necklace-like conformation where the PEO segments wrap around the headgroups of SDS micelles as indicated in Figure 9e. This reorganization results in the continuous increase in the cluster size from 150 to 220 nm when the SDS concentration is increased from 8 to 15 mM. At SDS concentration of 20 mM, all the HEURs are saturated by SDS micelles and free SDS micelles appear in solution. This concentration is very close to the value of Cm as determined from ITC measurements. Titration of HEUR into SDS Micellar Solutions. As described above, the binding interactions between SDS and HEUR strongly depend on the SDS concentrations. At low SDS concentration, the aggregation complex is produced by the binding of SDS monomers to the polymer backbone. However, at high SDS concentration, PEO segments of the HEUR backbones bind to the surface of SDS micelles through the iondipole associations to form the necklace-like aggregation complex. To verify this, the titration of HEUR into micellar SDS solution (CSDS . cmc) was carried out. In the presence of excess SDS, hydrophobic end groups of HEUR molecules are initially solubilized by SDS micellar cores. The binding interactions between SDS and HEUR chains are similar to that reported for SDS and PEO molecules of similar molecular weight.19,36,38-41 Before performing the titration of HEUR into SDS micellar solutions, the dilution behavior of 0.1 wt % HEUR in aqueous solution and the titration behavior of 0.1 wt % HEUR into 7.5 mM SDS solution (CSDS < cmc) were examined. Since the sensitivity of the Microcal ITC is 0.2 µCal, and the measured heats for these two titrations are lower than 0.2 µCal, reliable

Figure 10. The isothermal titration curves of titrating 0.1 wt % HEUR into different concentrations of SDS at 25 °C and 1 atm.

data cannot be obtained. The main factor for the extremely low heat is attributed to the high molecular weight of HEUR. However, on the basis of the upper limit of 0.2 µCal, the maximum apparent ∆H of demicellization for 0.1 wt % HEUR in aqueous solution can be estimated, which should be less than 5 kJ/mol. The thermograms for titrating 0.1 wt % HEUR into different concentrations of SDS solutions are shown in Figure 10. The titration curves differ from those for the titration of SDS into HEUR. The apparent ∆H increases with increasing SDS concentrations. The apparent binding enthalpy changes vary from 890 kJ/mol to 9405 kJ/mol, which is much larger than 5 kJ/mol for the maxinum enthalpy changes of the demicellization of HEUR and the binding of SDS monomers to the HEUR hydrophobic end groups. It is evident that the ∆H observed in the above titrations are mainly related to the binding of SDS micelles to the PEO segments of HEUR through the ion-dipole association. The HEUR used in this study possesses a reasonably high molecular weight, and thus more than one SDS micelle can bind to one HEUR unimer chain. Supposing that one HEUR unimer chain contains n binding sites for SDS micelles, then the binding reaction between HEUR unimers and SDS micelles can be expressed by the equation below:

{(1/n)HEUR} + (SDS micelle) / {(1/n)HEUR}/(SDS micelle) + ∆Hm where {(1/n)HEUR} represents one binding site in a HEUR unimer and the concentration of the binding site is n times of

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Figure 11. The isothermal titration curves of titrating different concentrations of HEUR into 0.2 M SDS at 25 °C and 1 atm.

the HEUR concentration. The heat recorded by the ITC for HEUR titrating into SDS micellar solution is the apparent ∆H for 1 mol of HEUR injected into the SDS micellar solution. If the fraction of the sites bound to SDS micelles is assumed to be θ, then the apparent ∆H per mole of HEUR is given by

∆H ) 〈Q/CV〉 ) nθ∆Hm

(7)

where Q is the raw heat, C the titrant concentration, and V the injection volume. For the binding reaction between SDS micelles and the binding sites, the binding constant K and the molar binding enthalpy changes, ∆Hm remained constant, but the fraction of sites bound to SDS micelles θ is strongly dependent on the concentrations of both SDS and HEUR. At high SDS concentration, larger amounts of SDS micelles are present in the titration cell. For a given HEUR concentration, the fraction of sites bound to SDS micelles, θ, increases with increasing SDS concentrations, which give rise to the larger ∆H. Titrations of eight different concentrations of HEUR into 0.2 M SDS were also performed and the titration thermograms are shown in Figure 11. The titration curves are similar to those for titrating 0.1 wt % HEUR into different concentrations of SDS, but the apparent binding ∆H decreases with increasing HEUR concentrations. The concentration of binding sites increases with increasing HEUR concentration, resulting in a corresponding reduction in the fraction of sites bound to SDS micelles, θ. The decrease in the fraction of binding sites gives rise to the decrease in the apparent binding enthalpy changes. Combining the above two series of titration results, the relationship between the apparent enthalpy changes and the ratio of titrate and titrant concentrations [SDS]/[HEUR] is obtained and shown in Figure 12, where a simple relationship can be observed:

∆Η ) 0.94

(

)

[SDS] - 503 [HEUR]

(8)

Using eq 8, the apparent enthalpy changes per mole of HEUR injected into SDS micellar solution at any given ratio of [SDS]/ [HEUR] can be determined. When ∆H f 0, the SDS concentration is estimated to be ∼10.3 mM, which is slightly larger than the cmc of SDS in aqueous solution. The critical concentration of 10.3 mM corresponds to onset point for the ion-dipole association between SDS micelles and the PEO segments of HEUR chains. This also reinforces that the binding interaction between SDS micelles and HEUR chains at high SDS concentration is different from that of SDS monomers and HEUR chains at low SDS concentration. In the former (high

Figure 12. The relationship between the binding enthalpy changes and [SDS]/[HEUR] for (i) titration of different concentrations of HEUR into 0.2 M SDS (filled circle) and (ii) titrating 0.1 wt % HEUR into different concentrations of SDS (open circle) at 25 °C and 1atm.

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