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Phase Behavior of Cationic Hydroxyethyl Cellulose-Sodium Dodecyl Sulfate Mixtures: Effects of Molecular Weight and Ethylene Oxide Side Chain Length of Polymers Shuiqin Zhou,* Chang Xu, Jun Wang, Patricia Golas, and James Batteas Department of Chemistry, College of Staten Island and The Graduate Center, The City University of New York, 2800 Victory Boulevard, Staten Island, New York 10314
Lowell Kreeger Amerchol R&D, The Dow Chemical Company, 171 River Road, Piscataway, New Jersey 08854 Received April 2, 2004. In Final Form: July 28, 2004 Novel cationic hydroxyethyl cellulose (HEC) polymers with different molecular weights (1.1 × 105 to 1.7 × 106 g/mol) and ethylene oxide (EO) side chain lengths (1.5-2.9 EO units) were mixed with sodium dodecyl sulfate (SDS) in aqueous solutions. The phase diagrams of cationic HEC-SDS complexes were determined in the dilute polymer concentration regime (99%) was purchased from Acros and used as received. Deionized water from the Millipore purification equipment (18.2 MΩ‚cm) was used to prepare all the solutions involved. Sample Preparation and Phase Observation. The polymer-surfactant complexes were prepared by mixing the stock
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Table 1. Summary of Structural Parameters of Cationic HEC Polymers polymer name
Mwa (103 g/mol)
Mwb (103 g/mol)
Mw/Mnb
Rgb (nm)
charge density (mol %)
EO side chain length (number of EO units)
EO22Mw110 EO22Mw400 EO22Mw1700 EO15Mw400 EO29Mw400
110 400 1700 400 400
109 ( 5 290 ( 6 1770 ( 70 232 ( 4 327 ( 4
1.6 2.5 1.4 1.9 2.2
27 ( 2 65 ( 1 141 ( 4 53 ( 1 74 ( 1
43.2 42.3 44.1 41.3 44.3
2.2 2.2 2.2 1.5 2.9
a
Provided by The Dow Chemical Company. b Measured by using the online GPC-MALS (miniDAWN) instrument.
solutions of both cationic HEC polymers and SDS in a desired weight ratio. The mixtures were stirred for enough time to ensure the homogeneity. When stirring is stopped and the mixture solutions show that they are homogeneous under a laser beam, no further stirring is carried out. Otherwise, continuous stirring for the mixtures will be applied until the solutions become homogeneous. For samples with high viscosity, smooth heating at 40-50 °C with gentle shaking was used to get homogeneous mixing. The samples were equilibrated at room temperature for a few days to a few weeks, depending on the viscosity of the mixtures. They were then centrifuged at 3000 rpm for 10 hours. Transparent samples without macroscopic phase separation after the centrifugation were considered to be one phase. Samples with two neatly separated phases after centrifugation were considered to be in the two-phase region. Viscosity Measurements. The viscosity of the cationic HEC polymer-SDS complexes was measured using an ARES-RFS rheometer (TA Instruments) with parallel plates geometry (diameter 50 mm, typical distance 0.5 mm). The rheometer was equipped with a Peltier Plate temperature control, and the solution temperature was maintained at 25 °C throughout the measurement. DLS. A standard laser light scattering spectrometer (BI200SM) equipped with a BI-9000 AT digital time correlator (Brookhaven Instrument, Inc.) and a solid-state laser (DPSS, SUWTECH, 200 mW, 532 nm) with an adjustable output was used to perform DLS studies over a scattering angular range of 30-90°. The temperature was controlled at 25 ( 0.05 °C. The equilibrated cationic HEC-SDS complex solutions were passed through a 0.45-µm Millipore Millex-HN filter to remove dust before DLS measurements. The Laplace inversion of each measured intensity-intensity time correlation function in a dilute solution can result in a characteristic line width distribution G(Γ).40,41 For a purely diffusive relaxation, Γ is related to the translational diffusion coefficient D by (Γ/q2,)Cf0,qf0 ) D, where q is the scattering vector defined as q ) (4πn/λ) sin(θ/2) with n being the refractive index of the solvent, λ being the wavelength of the incident beam, and θ being the scattering angle, respectively. After knowing the D value, the hydrodynamic radius Rh can be obtained by the Stokes-Einstein equation: Rh ) kBT/ 6πηD, with T, kB, and η being the absolute temperature, the Boltzmann constant, and the solvent viscosity, respectively. For the cationic HEC (0.2 wt % EO22Mw110)-SDS systems examined in this study, the complex particles become diffusive at certain high concentrations of SDS (e.g., CSDS g 0.08cmc) with the relaxation rates Γ of complex particles being q2-dependent (see Supporting Information). Small-Angle X-ray Scattering (SAXS). SAXS measurements were carried out at the SUNY X3A2 beamline of the National Synchrotron Light Source (NSLS), Brookhaven National Laboratory (BNL). A laser-aided prealigned pinhole collimator was used to define the incident X-ray beam. The incident X-ray wavelength (λ) was tuned at 0.1542 nm. A two-dimensional imaging plate was used in conjunction with an image scanner as the detection system. The sample-to-detector distance for SAXS was 841.9 mm. The scattering vector q is expressed as q ) (4π/λ) sin(θ/2) with θ being the scattering angle between the incident and the scattered X-rays. The d spacing of the ordered structures can be calculated as d ) 2π/qmax.
Figure 2. Phase diagrams of solution mixtures of SDS with cationic HEC polymers in different molecular weights (a) and different EO side chain lengths (b). The SDS concentration is given as a fraction of the cmc.
Results and Discussion
chain lengths (b) in the dilute polymer solutions. The concentrations of SDS are normalized by its cmc of 8.1 × 10-3 M at 25 °C. Following the generic scenario of an aqueous mixture of polyelectrolyte and oppositely charged surfactant,1,26 three distinct phase regions were observed. At a given cationic HEC polymer concentration but a gradual increase in SDS concentration, the mixtures were first shown as one transparent phase at low SDS concentrations (region I), then turned to be hazy, and reached two distinct phases when the opposite charge ratios were close to unity (region II). Upon further addition of SDS, the sedimentary aggregates were gradually redissolved into another clear phase (region III). Beyond this generic phase characteristic, it is obvious that the molecular weight and the EO side chain length of cationic HEC polymers influence the onset SDS concentrations for phase separation and redissolution. In Figure 2a, the 40 mol % charged cationic HEC polymers of EO22Mw110, EO22Mw400, and EO22Mw1700 were each mixed with SDS. The increase in the polymer chain length only slightly increases the onset SDS concentrations for phase redissolution but lowers the onset SDS concentration for phase separation. The general trend is that the increase in the molecular weight of polymers broadens the two-phase region. In contrast to the contradiction of the phase diagrams in hyaluronan-CnTAB systems with the theoretical model,25 our results of the molecular weight effect on the phase diagrams fit well into the theoretically modeled phase diagrams for a system of two polymers (polyelectrolyte mixed with micelles) in
Phase Diagram. Figure 2 compares the phase diagrams of the mixtures of cationic HEC polymers with SDS at different molecular weights (a) and different EO side
(40) Chu, B. Laser Light Scattering, 2nd ed.; Academic Press: New York, 1991. (41) Provencher, S. W. Makromol. Chem. 1979, 180, 201.
Phase Behavior of Cationic HEC-SDS Mixtures
Figure 3. Newtonian viscosity evolution of cationic HECSDS complex systems in different molecular weights (a) and different EO side chain lengths (b) of polymers as a function of SDS concentration at 25 °C. In the two-phase region II, only the viscosities of the supernatants were measured (open symbols).
a common solvent;25 namely, the longer polymer chains contribute less to the entropy of the mixed system in comparison with the shorter polymer chains, and, thus, the mixing free energy will be favorable to the concentrated phase when the molecular weight of polyelectrolyte is increased. Figure 2b shows the effect of the EO side chain length of the cationic HEC polymers on the phase boundary of cationic HEC-SDS complexes. At a fixed molecular weight and charge density of polymers, the decrease in the average side EO chain length from 2.9 to 1.5 EO units increases the onset SDS concentration for phase redissolution but lowers the onset SDS concentration for phase separation. The shortening of EO side chains will make the cationic HEC polymers less hydrophilic. Assuming the same amount of SDS molecules were bound to the polymer chains, the complexes formed by short EO cationic HEC polymers would be more hydrophobic and more favorable to aggregate together to form large particles. On the other hand, the shortening of the EO side chain length may also change the structures of the cationic HEC-SDS complexes because the charge sites at the end of the EO side chains will be closer to the semirigid backbone and become less mobile and less hydrated. To understand how the molecular weight and the EO side chain length affect the phase behavior of the cationic HEC-SDS complexes, the viscosity and the microstructures of the cationic HECSDS complexes at each phase evolution stage were characterized. Phase Characterization. A. Viscosity. Figure 3 shows the Newtonian viscosity change of the cationic HEC-SDS complexes during the phase evolution process for the polymers of different molecular weights (a) and EO side chain lengths (b) at a 0.2 wt % concentration interacting with different amounts of SDS. Generally, the presence of a small amount of SDS (
