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Coadsorption of Sodium Dodecyl Sulfate with Hydrophobically Modified Nonionic Cellulose Polymers. 2. Role of Surface Selectivity in Adsorption Hysteresis K. Derek Berglund,† Todd M. Przybycien,†,‡ and Robert D. Tilton*,† Departments of Chemical Engineering and Biomedical Engineering and Center for Complex Fluids Engineering, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213 Received August 19, 2002. In Final Form: December 16, 2002 We studied the reversibility of coadsorption from mixtures of the anionic surfactant sodium dodecyl sulfate with either hydroxypropyl cellulose (HPC) or hydrophobically modified hydroxyethyl cellulose (hmHEC) using optical reflectometry. The coadsorption to nonselective hydrophobic poly(dimethylsiloxane) (PDMS) surfaces was compared with coadsorption to negatively charged silica surfaces that were selective for polymer adsorption. The surface selectivity determines the reversibility of coadsorption with respect to changes in the solution sodium dodecyl sulfate (SDS) concentration. On the selective silica surface, an adsorbed layer becomes kinetically trapped in path-dependent states because SDS is electrostatically repelled from the negatively charged surface and is therefore unable to solubilize the polymer/surface contacts. On the nonselective PDMS surface, coadsorption in the HPC/SDS system is reversible, and although some irreversibility persists in the hmHEC/SDS system, the severity of the kinetic trapping effect is greatly reduced compared with that of the same system on silica. The decreased kinetic trapping effects are attributed to surfactant adsorption to the hydrophobic PDMS surface. Finally, a streaming current technique was used to measure the electrokinetic thickness of kinetically trapped polymer layers that were formed on silica by coadsorption with SDS, followed by rinsing with SDS-free polymer solution. The layer thickness was history-dependent: despite prolonged exposure to a constant concentration polymer solution, the adsorbed layer thickness depended on the SDS concentration that existed during the initial coadsorption stage.
Introduction Surfactants and cellulosic polymers are important components of many multiphase pharmaceutical, personal care product, and processed food formulations. Two phenomena that must be appreciated to systematically formulate such systems are complexation in bulk solution1-9 and coadsorption to interfaces.10-16 * To whom correspondence should be addressed. E-mail: tilton@ andrew.cmu.edu. † Department of Chemical Engineering and Center for Complex Fluids Engineering. ‡ Department of Biomedical Engineering. (1) Kamenka, N.; Burgaud, I.; Zana, R.; Lindman, B. J. Phys. Chem. 1994, 98, 6785-9. (2) Nilsson, S. Macromolecules 1995, 28, 7837-44. (3) Thuresson, K.; Nystroem, B.; Wang, G.; Lindman, B. Langmuir 1995, 11, 3730-6. (4) Thuresson, K.; Soederman, O.; Hansson, P.; Wang, G. J. Phys. Chem. 1996, 100, 4909-18. (5) Medeiros, G. M. M.; Costa, S. M. B. Colloids Surf., A 1996, 119, 141-148. (6) Evertsson, H.; Nilsson, S. Macromolecules 1997, 30, 2377-2385. (7) Singh, S. K.; Nilsson, S. J. Colloid Interface Sci. 1999, 213, 152159. (8) Singh, S. K.; Nilsson, S. J. Colloid Interface Sci. 1999, 213, 133151. (9) Ghoreishi, S. M.; Li, Y.; Bloor, D. M.; Warr, J.; Wyn-Jones, E. Langmuir 1999, 15, 4380-4387. (10) Claesson, P. M.; Malmsten, M.; Lindman, B. Langmuir 1991, 7, 1441-6. (11) Tanaka, R.; Williams, P. A.; Meadows, J.; Phillips, G. O. Colloids Surf. 1992, 66, 63-72. (12) Yamanaka, Y.; Esumi, K. Colloids Surf., A 1997, 122, 121-133. (13) Lauten, R. A.; Kjoniksen, A.-L.; Nystroem, B. Langmuir 2000, 16, 4478-4484. (14) Lauten, R. A.; Kjoniksen, A.-L.; Nystroem, B. Langmuir 2001, 17, 924-930. (15) Joabsson, F.; Thuresson, K.; Blomberg, E. Langmuir 2001, 17, 1506-1510. (16) Joabsson, F.; Thuresson, K.; Lindman, B. Langmuir 2001, 17, 1499-1505.
In part 1 of this series (preceding paper in this issue), we discussed the aqueous-phase complexation of anionic sodium dodecyl sulfate (SDS) surfactants with either hydroxypropyl cellulose (HPC) or hydrophobically modified hydroxyethyl cellulose (hmHEC) and used those binding observations to interpret the total extent of coadsorption on selective silica surfaces and on nonselective poly(dimethylsiloxane) (PDMS) surfaces. Above the critical aggregation concentration (cac), the total adsorbed amount decreased with increasing SDS concentration, for either polymer on either type of surface. On silica, with either polymer, all adsorption was suppressed to below the detection limit of the optical reflectometer (0.05 mg/ m2) at high SDS concentrations, but it was necessary to far exceed the aqueous-phase binding saturation concentration (csat). On the nonselective, hydrophobic PDMS surface, a polymer-free SDS monolayer adsorbed when the SDS concentration exceeded the ordinary critical micelle concentration (cmc). In this part 2, we emphasize reversibility issues, especially factors that determine the extent to which the adsorbed layers become kinetically trapped in persistent nonequilibrium, or hysteretic, states. The occurrence of hysteretic adsorbed states is likely responsible for the sensitivity that suspension formulations often display to changes in the order of component addition or mixing conditions. Experimental evidence has shown that adsorbed polymer layers often fail to equilibrate with the bulk solution on practical time scales. Even if several segments can detach from the surface, other segments remain adsorbed and constrain neighboring segments. This limits the ability of the polymer to undergo conformational changes in response to changes in solution condition. This is usually reported in the context of the “irreversibility” of adsorption when layers are rinsed by pure solvent, but there are also
10.1021/la026430f CCC: $25.00 © 2003 American Chemical Society Published on Web 02/12/2003
Surface Selectivity in Adsorption Hysteresis
cases where persistent departures from thermodynamic equilibrium are revealed through the path dependence of dynamic and/or structural characteristics of the layer.17-19 Moreover, strong nonequilibrium or order-of-addition effects have also been reported for coadsorbing polymer/ surfactant systems.16,20-28 In this paper we explore how the surface’s adsorption selectivity and the type of hydrophobic modification on the cellulose polymer influence the persistence of nonequilibrium states in the adsorbed layer. While observations of irreversible polymer adsorption are hardly novel, our interest in these coadsorption systems comes from the way in which the surfactant controls the adsorption reversibility. Moreover, we are also interested in the longlasting effect that transient surfactant exposure has on the structure of polymer layers after the surfactant has been removed from the system. The way in which the layer adapts to surfactant exposure depends on the hydrophobic modification of the cellulose polymer. For the case of the selective, negatively charged silica surface, we demonstrate that the thickness of an adsorbed cellulose polymer layer depends on its history of exposure to surfactant. Experimental Section Materials. We purified all water by reverse osmosis followed by treatment with the Milli-Q Plus system of ionic exchange and organic adsorption cartridges from Millipore Corp. (Bedford, MA). The pH of all solutions was unmodified from the air-saturated aqueous solution pH of 5.5-6.0. The cleaning and preparation of the adsorption substrates for the reflectometry experiments is described in part 1 of this series. We used soda-lime glass microscope slides from Fisher Scientific in the streaming current experiments. Following Rebar and Santore,29 we first treated these glass slides with concentrated sulfuric acid (36 N) for 16 h to extract dopants and leave a surface whose composition matches pure silica. Following the initial acid treatment, the glass slides were cleaned using the same protocol described in part 1 for oxidized silicon wafers. We stored the clean glass slides in water, and rinsed them profusely with pure water immediately before mounting them in the experimental apparatus. Once cleaned, the glass substrates were not allowed to dry. Optical Reflectometry. We used the scanning angle reflectometry protocols described in part 1 to measure the total extent of adsorption. The wall shear rate and temperature for all reflectometry experiments were 1.0 s-1 and 25 °C, respectively. After a clean wafer was mounted in the reflectometer flowcell, it was allowed to soak in a 1.0 mM NaCl solution for more than 15 min, to check for adsorption of surface-active impurities before polymer/surfactant solutions were introduced. For an adsorbed (17) Kelley, T. W.; Schorr, P. A.; Johnson, K. D.; Tirrell, M.; Frisbie, C. D. Macromolecules 1998, 31, 4297-4300. (18) Sukhishvili, S. A.; Dhinojwala, A.; Granick, S. Langmuir 1999, 15, 8474-8482. (19) Gobel, J. G.; Besseling, N. A. M.; Stuart, M. A. C.; Poncet, C. J. Colloid Interface Sci. 1999, 209, 129-135. (20) Arnold, G. B.; Breuer, M. M. Colloids Surf. 1985, 13, 103-12. (21) Kronberg, B.; Kuortti, J.; Stenius, P. Colloids Surf. 1986, 18, 411-25. (22) Furst, E. M.; Pagac, E. S.; Tilton, R. D. Ind. Eng. Chem. Res. 1996, 35, 1566-74. (23) Neivandt, D. J.; Gee, M. L.; Tripp, C. P.; Hair, M. L. Langmuir 1997, 13, 2519-2526. (24) Fan, A.; Somasundaran, P.; Turro, N. J. Colloids Surf., A 1999, 146, 397-403. (25) Sjoberg, M.; Bergstrom, L.; Larsson, A.; Sjostrom, E. Colloids Surf., A 1999, 159, 197-208. (26) Braem, A. D.; Prieve, D. C.; Tilton, R. D. Langmuir 2001, 17, 883-890. (27) Velegol, S. B.; Tilton, R. D. Langmuir 2001, 17, 219-227. (28) Sakai, K.; Yoshimura, T.; Esumi, K. Langmuir 2002, 18, 39933998. (29) Rebar, V. A.; Santore, M. M. J. Colloid Interface Sci. 1996, 178, 29.
Langmuir, Vol. 19, No. 7, 2003 2715 layer that contains more than one species, adsorption and desorption kinetics are monitored in terms of the effective optical thickness of the adsorbed layer, dav(nav - n0), by measuring the reflectivity at the Brewster angle, θb, as a function of time, t, and employing the approximation
dav(nav - n0) ) R[Rp(θb,t)1/2 - Rp(θb,0)1/2]
(1)
where dav is the average layer thickness, nav - n0 is the difference between the average refractive index of the layer and the refractive index of the bulk solution, and Rp(θb,0) is the p-polarized reflectivity recorded at the Brewster angle before adsorption. The proportionality constant, R, is generated numerically via a homogeneous two-layer optical model.30 For a single-component adsorbed layer, the effective optical thickness is directly proportional to the surface excess concentration. In the case of multicomponent adsorption, the effective optical thickness of the layer is related to the surface excess concentration of each species, Γi, by
dav(nav - n0) )
∑Γ (dn/dC ) i
i
(2)
i
where dn/dCi is the refractive index increment of the adsorbing species i at the wavelength of interest. In this paper, we report the effective optical thickness, the quantity that emerges directly from the reflectivity data, as well as the apparent polymer surface excess concentration. We calculate the latter from the effective optical thickness by assuming only polymer is present in the layer and using the polymer refractive index increment in eq 2. The average refractive index and average thickness of the adsorbed layer are highly coupled and model dependent; the effective optical thickness defined by eq 2 is model independent. Kinetic plots are obtained by the dynamic implementation of optical reflectometry described by eq 1, while adsorption “isotherms” contain data from scanning angle reflectometry measurements made at the steady state. Streaming Current. As mentioned, the average thickness of an adsorbed layer is not reliably decoupled from the average refractive index using optical reflectometry. Thus, we use an electrokinetic technique to measure the thickness of the polymer layer from its effect on the ζ potential, where the latter is defined as the electrostatic potential at the hydrodynamic plane of shear adjacent to a charged surface. When polymer adsorbs, the fluid within the polymer layer is immobilized with respect to shear and the hydrodynamic shear plane moves to a distance determined largely by the distance of the tail segments from the surface.31,32 Thicknesses measured in this way are referred to as “electrokinetic layer thicknesses” and closely match hydrodynamic layer thicknesses.31,33 In our apparatus we measure the steady-state electrical current, i.e., streaming current, that develops as a result of pressure-driven flow over a charged surface. The ζ potential dictates the proportionality between the streaming current, Is, and the pressure drop, ∆P, in the device. For a rectangular slit flowcell, the relationship among ζ, Is, and ∆P is given by
ζ)-
( )
µL Is 0wh ∆P
(3)
where µ is the viscosity of the solvent, 0 is the permittivity of free space, is the dielectric constant of the solvent, and L, w, and h are the length, width, and height of the flowcell.34,35 The instrument we use is described in detail elsewhere.34,35 (30) Tilton, R. D. In Scanning Angle Reflectometry and Its Application to Polymer Adsorption and Coadsorption with Surfactants; Farinato, R. S., Dubin, P. L., Eds.; John Wiley & Sons: New York, 1999; p 331. (31) Stuart, M. A. C.; Waajen, F. H. W. H.; Dukhin, S. S. Colloid Polym. Sci. 1984, 262, 423-6. (32) Fleer, G. J.; Cohen Stuart, M. A.; Scheutjens, J. M. H. M.; Cosgrove, T.; Vincent, B. Polymers at Interfaces; Chapman & Hall: London, 1993. (33) Pagac, E. S.; Prieve, D. C.; Solomentsev, Y.; Tilton, R. D. Langmuir 1997, 13, 2993-3001. (34) Braem, A. D.; Prieve, D. C.; Tilton, R. D. Langmuir 2003, in press.
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Berglund et al. in calculations. During a series of pressure drop variations, we would randomize the sequence of pressure drops to make sure that polymer was not desorbing during the experiment. As a consistency check on the electrokinetic layer thickness and the assumption that the double layer is not significantly altered by the polymer layer, we also made separate thickness determinations from two pairs of ζ0 and ζads that we measured at different ionic strengths (different κ-1 values).
Results and Discussion
Figure 1. Streaming current as a function of applied pressure drop on a bare silica surface (b) and after adsorption of 0.01 wt % hmHEC in 1.0 mM NaCl (O). Both sets of streaming current data are measured in 0.1 mM NaCl. For the neutral polymers considered here, the electrokinetic layer thickness is determined by measuring the ζ potential before and after adsorption.33-35 By shifting the plane of shear further from the solid surface, the adsorbed layer decreases the magnitude of the ζ potential relative to the bare surface potential. These measurements are conducted after surfactant is rinsed from the system to examine the structure of the kinetically trapped polymer layer that remains after “surfactant processing”. Figure 1 displays the streaming current as a function of pressure drop before and after adsorption of a 0.01 wt % hmHEC solution from a surfactant-free solution. The ζ potential, calculated from the linearly regressed slope of Is vs ∆P, is significantly smaller in magnitude after adsorption. Assuming the neutral polymer layer does not perturb the electrostatic double layer, the polymer layer thickness, d, is calculated from the measured ζ potentials before, ζ0, and after, ζads, adsorption by applying Gouy-Chapman theory to the decaying electrostatic potential:
{
( ) ( )
eζ0 exp(-κd) 2kT 4kT ζads ) ln e eζ0 1 - tanh exp(-κd) 4kT 1 + tanh
}
(4)
where κ is the Debye parameter, e is the elemental charge, T is the temperature, and k is Boltzmann’s constant. The Debye screening length was calculated from the known ionic strength of the solution. Instrumental and theoretical issues, particularly concerning the advantage of streaming current measurements over streaming potential measurements, are described elsewhere.34,35 Upon installing a pair of cleaned glass slides into the flowcell, we passed 500 mL of a salt solution (either 0.1 or 1.0 mM NaCl) through the cell and measured the streaming current over time as a check against surface-active impurities. The ζ potential of the bare surfaces was -129.5 ( 8.4 and -106.7 ( 8.7 mV in 0.1 and 1.0 mM NaCl, respectively. Polymer/surfactant solutions were introduced into the flowcell at a wall shear rate of approximately 110 s-1. We allowed adsorption to occur for more than 2 h before rinsing with NaCl solutions. From reflectometry experiments, we found that adsorption required 30 min to reach surface saturation at a wall shear rate of only 1.0 s-1. To remove any surfactant present in the adsorbed layer, we passed 500 mL of salt solution (more than 3330 flowcell volumes) through the cell before measuring the streaming current as a function of pressure drop after adsorption. In this way, only polymer is left at the interface and eq 4 can be applied. The streaming current experiments were conducted at room temperature, approximately 23 °C, with the temperature being recorded in all cases for use (35) Braem, A. D. Colloidal and Interfacial Phenomena in Polymer/ Surfactant Mixtures; Carnegie Mellon University: Pittsburgh, 2001; p 166.
Kinetic Trapping on the Selective Silica Surface. Kinetic trapping was most severe on the silica surface. We measured the adsorption of HPC and hmHEC from surfactant-free, 1.0 mM NaCl solutions onto silica surfaces. On silica, 0.05 wt % HPC and 0.01 wt % hmHEC solutions produced surface excess concentrations of 0.67 ( 0.05 and 0.91 ( 0.05 mg/m2, respectively. The coadsorption isotherms were reported for the HPC/SDS and hmHEC/SDS systems on silica in the first paper of this two-part series. Briefly, the total adsorbed amount for HPC/SDS experiences a maximum near the cac and decreases thereafter. A surfactant concentration greater than 10 mM prevents the adsorption of the HPC/SDS complex to the negatively charged silica surface, to within the detection limit of the reflectometer ((0.05 mg/m2). Since the cac is extremely small for the hmHEC/SDS system, we did not detect a maximum in surface concentration. The total adsorbed amount declined monotonically with increasing SDS concentration. An SDS concentration of 25 mM was required to prevent hmHEC adsorption to silica. Rinsing either of the adsorbed polymer layers on silica with a 1.0 mM NaCl solution for up to 3 h resulted in little (
