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Langmuir 1998, 14, 264-270
Interactions between Colloidal Poly(tetrafluoroethylene) Latex and Sodium Poly(styrenesulfonate) Tahir Jamil and Paul S. Russo* Department of Chemistry and Macromolecular Studies Group, Louisiana State University, Baton Rouge, Louisiana 70803-1804 Received August 26, 1996. In Final Form: October 22, 1997X The translational and rotational diffusion of poly(tetrafluoroethylene) latex in aqueous solutions of unbound sodium poly(styrenesulfonate) were measured by dynamic light scattering. Both quantities were affected by the viscosity of the poly(styrenesulfonate), which was itself a function of added salt. Adjusted for viscosity, the rotational diffusion decreased with added salt but did not depend on the polymer concentration in the range studied. This suggests that rotational diffusion of a probe particle can be used to sense the local ionic environment. Visual observations and static light scattering showed that saltinduced aggregation of the latex can be prevented or reversed by small amounts (several milligrams per milliliter) of sodium poly(styrenesulfonate).
Introduction The interaction between polymers and colloids is of interest to at least two communities. Colloid science is deeply concerned with stability, which depends sensitively on the presence of additives, including polymers. The situation is complex. Large particles are attracted to each other by dispersion forces, provided their dielectric constant differs from that of the solvent.1,2 Permanent charge on the particles helps to stabilize a suspension against such attractions but may be defeated when salt is added to screen the repulsions. A weak secondary minimum3 can appear in the interparticle potential energy function, causing reversible flocculation. Added polymers that bind to the colloidal particles may prevent strong aggregation by offering a steric barrier to close approach, but they can also form bridges between the particles, causing flocculation. Unbound polymers can destabilize colloidal suspensions by the depletion flocculation mechanism, first explained by Asakura and Oosawa4,5 as an imbalance of osmotic forces. The opposite effect, depletion stabilization, has also been postulated,6 and there have been recent reports of stabilization induced by unbound polymer at remarkably low concentrations.7,8 Given such variety, it is not surprising that the thermodynamic stability of colloid-polymer mixtures has attracted so much attention.1,7,9-14 The polymer community is interested in colloid* To whom correspondence should be addressed. X Abstract published in Advance ACS Abstracts, December 15, 1997. (1) Russel, W. B.; Saville, D. A.; Schowalter, W. R.Colloidal Dispersions; Cambridge: Cambridge, 1989. (2) Hunter, R. J. Foundations of Colloid Science; Clarendon: Oxford, 1986. (3) Ottewill, R. H. J. Colloid Interface Sci. 1977, 58, 357-373. (4) Asakura, S.; Oosawa, F. J. Chem. Phys. 1954, 22, 1255-1256. (5) Asakura, S.; Oosawa, F. J. Polym. Sci. 1958, 33, 183-192. (6) Feigin, R. I.; Napper, D. H. J. Colloid Interface Sci. 1980, 74, 567-571. (7) Seebergh, J. E.; Berg, J. C. Langmuir 1994, 10, 454-463. (8) Ogden, A. L.; Lewis, J. A. Langmuir 1996, 12, 3413-3424. (9) Napper, D. H. Polymeric Stabilization of Colloidal Dispersions; Academic: New York, 1983. (10) Pusey, P. N.; Pirie, A. D.; Poon, W. C. K. Phys. A 1993, 201, 322-331. (11) Ilett, S. M.; Orrock, A.; Poon, W. C. K.; Pusey, P. N. Phys. Rev. E 1995, 51, 1344-1352. (12) Walz, J. Y. J. Colloid Interface Sci. 1996, 178, 505-513.
polymer mixtures, too, but places a greater emphasis on macromolecular dynamics. Colloidal particles have been used as probes of viscoelastic solutions. A typical study might involve the diffusion of very dilute colloidal probes in semidilute polymer solutions to determine the polymer concentration at which the probes cease to obey the Stokes-Einstein relation. Such “probe diffusion” experiments15-24 help to define the limits of continuum behavior. It is usually hoped that the probe and polymer do not interact. Even in the absence of specific interactions, there has long been concern for the effect of the probes on the local structure of the polymer solution.17 Only recently have these been voiced in terms, such as “depletion zone”, familiar to the colloid community.25,26 In most studies, the colloidal probes have been polystyrene latex or other geometrically and optically isotropic particles that do not depolarize light in the single-scattering limit. Such particles are not useful for dynamic light-scattering (DLS) measurements of the rotational diffusion, which might provide the most “local” viewpoint possible, since rotation of a spherical object requires no restructuring of the surrounding polymeric matrix. Colloidal poly(tetrafluoroethylene) (PTFE) latex is composed of optically anisotropic, slightly elongated (13) Smith, N. J.; Williams, P. A. J. Chem. Soc., Faraday Trans. 1995, 91, 1483-1489. (14) Liang, W.; Tadros, T. F.; Luckham, P. F. Langmuir 1994, 10, 441-446. (15) Shenoy, V.; Rosenblatt, J. Macromolecules 1995, 28, 8751-8758. (16) Suzuki, Y.; Nishio, I. Phys. Rev. B 1992, 45, 4614-4619. (17) Won, J.; Onyenemezu, C.; Miller, W. G.; Lodge, T. P. Macromolecules 1994, 27, 7389-7396. (18) Reina, J. C.; Bansil, R.; Konak, C. Polymer 1990, 31, 10381044. (19) Phillies, G. D. J.; Richardson, C.; Quinlan, C. A.; Ren, S. Z. Macromolecules 1993, 26, 6849-6858. (20) DeSmedt, S. C.; Lauwers, A.; Demeester, J.; Engelborghs, Y.; DeMey, G.; Du, M. Macromolecules 1994, 27, 141-146. (21) Park, I. H.; Johnson, C. S., Jr.; Gabriel, D. A. Macromolecules 1990, 23, 1548-1553. (22) Johansson, L.; Skantze, U.; Lofroth, J. E. Macromolecules 1991, 24, 6019-6023. (23) Onyenemezu, C. N.; Gold, D.; Roman, M.; Miller, W. G. Macromolecules 1993, 26, 3833-3837. (24) Streletzky, K.; Phillies, G. D. J. Langmuir 1995, 11, 42-47. (25) Gold, D.; Onyenemezu, C.; Miller, W. G. Macromolecules 1996, 29, 5710-5716. (26) Gold, D.; Onyenemezu, C.; Miller, W. G. Macromolecules 1996, 29, 5700-5709.
S0743-7463(96)00841-4 CCC: $15.00 © 1998 American Chemical Society Published on Web 01/20/1998
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Langmuir, Vol. 14, No. 2, 1998 265
particles that exhibit strong depolarized scattering due to their semicrystalline internal structure.27-33 Kratohvil and Matijevic34 studied the stability of PTFE latex in the presence of surfactants, electrolytes, and macromolecules, while Piazza and co-workers31,33,35,36 reported the static and dynamic properties of PTFE and related latex particles in dilute and concentrated suspensions. The polarized and depolarized scattering from PTFE latex easily exceeds that from most polymers at any reasonable polymer concentration. This enables the determination of both translational and rotational diffusion of colloidal PTFE latex in complex media by DLS. However, since the particles are elongated, with a length/width of about 2-3, rotation and diffusion may be coupled to each other. To some degree, that compromises the local perspective. Nevertheless, the rotational diffusion of probe particles does provide new insights. The present study concerns the rotational and translational diffusion of PTFE latex through aqueous salt solutions of sodium poly(styrenesulfonate) (NaPSS), as determined by dynamic light scattering. Consideration of the stability of the colloid-polymer system is a necessary precursor to such a study, and unusual behavior is found even at low polymer concentrations. Binding of the polymer to the colloid is not expected, since PTFE latex and NaPSS are both negatively charged. To confirm this point, the diffusion of fluorescein-labeled NaPSS in the presence of the colloidal PTFE is monitored by fluorescence photobleaching recovery. Finally, consideration of the effects of salt and polymer concentration on translational and rotational diffusion, and on solution viscosity, enables a rare glimpse of the ionic environment shared by colloidal particle and polyelectrolyte.
the incident light.38 The intensity autocorrelation function, G(2)(t), of the scattered light in homodyne mode is related to the electric field correlation function, g(1)(t), by38
G(2)(t) ) B[1 + f|g(1)(t)|2]
Here, t is the lag time, B is the baseline, and f is an instrumental parameter (0 < f < 1) that depends mostly on the number of coherence areas detected and also on the scattering power of the solvent relative to the solution. For a distribution of decay times, g(1)(t) can be fitted with the cumulant expansion:39
h t + µ2t2 + higher terms ln g(1)(t) ) -Γ
The radius of gyration, Rg, of a particle can be determined from the Guinier relationship37 for the angulardependent polarized scattered intensity, I(q):
I(q) ≈ I(0) exp(-q2Rg2/3)
ΓVν ) q2Dt
(27) Russo, P. S.; Saunders, M. J.; DeLong, L. M.; Kuehl, S. K.; Langley, K. H.; Detenbeck, R. W. Anal. Chim. Acta 1986, 189, 69-87. (28) Okubo, T.; Shimizu, T. J. Colloid Interface Sci. 1990, 135, 300303. (29) Angel, M.; Hoffmann, H.; Huber, G.; Rehage, H. Ber. Bunsenges. Phys. Chem. 1988, 92, 10-16. (30) Chanzy, H. D.; Smith, P.; Revol, J.-F. J. Polym. Sci., Lett. Ed. 1986, 24, 557-563. (31) Piazza, R.; Stavans, J.; Bellini, T.; Degiorgio, V. Opt. Commun. 1989, 73, 263-267. (32) Camins, B. C.; Russo, P. S. Langmuir 1994, 10, 4053-4059. (33) Vailati, A.; Asnaghi, D.; Giglio, M.; Piazza, R. Phys. Rev. E 1994, 48, R2358-R2361. (34) Kratohvil, S.; Matijevic, E. J. Colloid Interface Sci. 1976, 57, 104-114. (35) Degiorgio, V.; Piazza, R.; Corti, M.; Stavans, J. J. Chem. Soc., Faraday Trans. 1991, 87, 431-434. (36) Piazza, R.; Degiorgio, V. Phys. A 1992, 182, 576-592. (37) Guinier, A.; Fournet, G. Small-Angle Scattering of X-rays; John Wiley and Sons: New York, 1955.
(4)
where Dt is the translational diffusion coefficient. For polydisperse scatterers and scatterers that show internal relaxation modes, ΓVν often increases a little faster than q2. In such a case, Dt is more accurately obtained from the zero q limit of the apparent diffusion coefficient:
Dt ) lim (ΓVν/q2) ) lim Dapp qf0
(4a)
Dt is related to the hydrodynamic radius, Rh, of the particle through the Stokes-Einstein equation:
Dt )
(1)
Here, q is the scattering wave vector magnitude (q ) 4πn sin(θ/2)/λ0, where λ0 is the wavelength in vacuo, n is the solution refractive index, and θ is the scattering angle). The expression is valid, in the limit of infinite dilution, for freely rotating particles of any shape, provided that q is sufficiently small. Dynamic light-scattering studies were conducted in the Vν (polarized) and Hν (depolarized) geometries. The first letter, V for vertical or H for horizontal, refers to the polarization sense of the detected light. The second letter, always ν in this paper, refers to the polarization sense of
(3)
where Γ h is the average decay rate. The third cumulant fit includes terms up to t3 in the analysis. The degree to which the correlation function obeys a single-exponential h 2, which decay is characterized by the unitless ratio, µ2/Γ is zero for a perfectly single-exponential decay and may approach unity for severely nonexponential decays. For uniform, spherical, and optically isotropic particles, (1) gVν (t) is an exponential with a decay rate ΓVν:
qf0
Light Scattering Background
(2)
kT 6πηRh
(5)
where k is Boltzmann’s constant, T is the absolute temperature, and η is the solvent viscosity. For optically anisotropic particles, the electric field (1) (t), is an correlation function in the Hν geometry, gHν exponential with a decay rate, ΓHν, related to the translational and rotational diffusive coefficients:
ΓHν ) q2Dt + 6Dr
(6)
where Dr is the rotational diffusion coefficient, related to the cube of the hydrodynamic radius of the particle through
Dr )
kT 8πηRh3
(7)
Experimental Section Materials. PTFE latex was generously supplied by Ausimont under the name ALGOFLON (D60V, Lot No. BR-6). This grade is stabilized by nonionic surfactant, ∼3%, and contains no ammonia. According to the manufacturer, the ζ potential of similar preparations is -50 mV ( 20%, independent of pH between 3.5 and 10. The hydrodynamic particle radius determined by Vν dynamic light scattering was 0.095 µm. The concentration of the supplied latex solution was determined by slow evaporation of a known weight of the solution followed by (38) Berne, B.; Pecora, R. Dynamic Light Scattering; Wiley: New York, 1976. (39) Koppel, D. E. J. Chem. Phys. 1972, 57, 4814-4820.
266 Langmuir, Vol. 14, No. 2, 1998 vacuum drying. A PTFE stock solution, made by adding a known volume of latex to dust-free water, was diluted in the lightscattering cells to give a concentration of about 70 µg/mL in all measurements. NaPSS with advertised molecular weight 70 000 was purchased from Scientific Polymer Products, Inc. Gel permeation chromatography with light-scattering detection showed that the NaPSS had a polydispersity ratio (weight to number-average mass) of less than 1.6. Elemental analysis showed that the NaPSS was 100% sulfonated. Prior to use, the NaPSS was dedusted by dissolving it in dust-free water and centrifuging at 5000g for 24 h. The supernatant solution was collected and dried in a vacuum oven. The NaPSS concentration studied was from 0 to 5 mg/mL. The electrolyte was reagentgrade NaCl, and its concentration varied from 0 to 0.1 M. Dust was removed from the NaCl solution by filtering through a 0.02µm Anotop filter and centrifuging at 5000g for several hours. The binary and ternary solutions were prepared directly in clean light-scattering cells (Pyrex 13- × 75-mm test tube) by mixing known volumes of the solutions. The cells were sealed with PTFEfaced screwtop lids. Light-Scattering Measurements. The light-scattering spectrometer is similar to the one described in a previous study of the size distribution of a similar PTFE latex.27 A Lexel Model 95-2 argon ion laser producing up to 1.4 W of vertically polarized light at λ0 ) 5145 Å was used. Static and dynamic measurements were made on the same samples, in the same cells, and with the same instrument. Five repeat runs of several seconds duration were used to obtain the intensity and corresponding uncertainty at each angle. The scattering volume at each angle θ obeyed a simple (sin θ)-1 relationship. All of the correlation functions were measured using an EMI-9863A/100 photomultiplier tube, Pacific Precision Model 126 photometer, and 272-channel Langley-Ford Model 1096 correlator, operated in linear mode. The channel (sampling) time of the correlator was set to ensure complete decay of the measured correlation function; typically 63% of the decay occurred within the first 32 channels. Viscosity Measurements. The macroscopic viscosities of PTFE/NaPSS/water solutions were measured on a Brookfield LVTDCP cone and plate viscometer. An extrapolation to zero shear rate was made, although shear thinning was very minor for the relatively dilute solutions studied in the available shear rate range (75-450 s-1). Fluorescence Photobleaching Recovery. The possibility of binding of NaPSS to the PTFE latex was studied by fluorescence photobleaching recovery (FPR), a tracer method40,41 that monitors the self-diffusion of fluorescently labeled molecules, in this case NaPSS. The attachment of a fluorescein-based dye to NaPSS is described elsewhere,42 and the labeled material is referred to as LNaPSS. About 0.24% of the monomer subunits were labeled, as estimated from visible absorption spectroscopy. The FPR instrument43 is conceptually similar to that first described by Lanni and Ware.40 In brief, a striped pattern of period L is bleached into the fluorescent sample by intense laser illumination of a coarse diffraction grating (Ronchi ruling) placed in the rear image plane of an objective mounted on a standard epifluorescence microscope. Then the Ronchi ruling is translated at a constant speed and projected into the sample by a less intense beam. Initially, the fluorescence intensity, detected by a photomultiplier tube, approximates a triangle wave. The ac modulation at the fundamental frequency is extracted with a tuned amplifier and a peak detection circuit. As bleached and unbleached molecules interchange by diffusion, the ac signal decays exponentially at a rate Γ ) DK2, where D is the tracer diffusion coefficient of the labeled molecules and K ) 2π/L. The ac and dc light levels are digitized and recorded by a personal computer equipped with a multifunction data acquisition and timing board. The decay rate is extracted by a nonlinear leastsquares algorithm using a floating base line. All experiments with LNaPSS in PTFE solutions were performed at one K value after it had been established that Γ varied linearly with K2, with a zero intercept, for this LNaPSS sample. Samples were drawn (40) Lanni, F.; Ware, B. R. Rev. Sci. Instrum. 1982, 53 (6), 905-908. (41) Ware, B. R. Am. Lab. 1984, 16, 16-28. (42) Sohn, D.; Russo, P. S.; Davila, A.; Poche, D. S.; McLaughlin, M. L. J. Colloid Interface Sci. 1996, 177, 31-44. (43) Bu, Z.; Russo, P. S. Macromolecules 1994, 27, 1187-1194.
Jamil and Russo
Figure 1. Guinier plots for PTFE/water and PTFE/NaPSS/ water without added NaCl. The scattering intensity units are arbitrary. into rectangular capillary cells (Vitrodynamics) of path length 200 µm and flame sealed. All measurements were made at 25.0 ( 0.1 oC.
Results and Discussion Visual Observation. PTFE latex particles remain suspended for several days in water. The addition of 0.01 M NaCl to binary PTFE/water solutions causes aggregation of the PTFE particles. These large aggregates are visible to the naked eye and precipitate quickly. Similar behavior of colloidal PTFE in the presence of added electrolytes has been observed previously.34 The saltinduced aggregation is explained by the DLVO theory of charged particle interactions and the extent of the diffuse double layer.1,2 PTFE latex is negatively charged, and its stability in water is governed by competition between the van der Waals attraction and electrostatic repulsion between the particles, modified by added surfactant. An increase in the ionic strength diminishes the thickness of the double layer and reduces the electrostatic repulsion. The van der Waals attraction then dominates and aggregation ensues. Therefore, light-scattering studies were not possible in the presence of added simple salt. However, PTFE particles in NaPSS/salt solutions remain suspended for the duration of the experiments and show no sign of aggregation. Moreover, the salt-induced aggregates of PTFE latex can be resuspended by the addition of NaPSS, even at low concentrations, followed by gentle agitation. Kratohvil and Matijevic34 found that neutral and negative polymers resulted in no flocculation of similar PTFE latex. The ability of a negatively charged polymer at low concentrations to reverse flocculation of colloidal PTFE appears to be a new observation. Static Light Scattering. Static light-scattering studies were performed on PTFE latex solutions with and without NaPSS and NaCl. Figure 1 shows a Guinier plot for the PTFE/water system without NaCl. A similar measurement on a ternary PTFE/NaPSS/water system with added NaCl also appears. Rg values for all measurable solutions appear in Table 1. The size does not change appreciably with polymer concentration or with salt, as long as polymer is present. It is not clear why the measured size in pure water (Rg ) 906 Å) exceeds that in NaPSS solutions (801-850 Å). The difference is experimentally significant, but only just so. Changes in the sizes of the semicrystalline PTFE particles are not expected. Perhaps the assumption of infinite dilution is at fault, since long-range forces exist in these ionic solutions. In any case, it is the lack of a trend with salt or polymer concentration that matters; this confirms the
Interactions between PTFE and NaPSS
Langmuir, Vol. 14, No. 2, 1998 267
Table 1. Radius of Gyration, Rg (Å), of PTFE Latex in Solutionsa NaPSS, mg/mL
0
0 1 2 3 4 5
906 848 845 851 838 825
[NaCl], 10-2 M 1 5
10
b 838 839 846 835 829
b 838 839 846 835 829
b 831 828 810 805 820
a Uncertainty is estimated at (3%. b These measurements were not performed due to PTFE latex aggregation and sedimentation.
Table 2. Self-Diffusion Coefficient, Ds, of LNaPSS in the Presence of PTFE Latex from FPR Measurements cPTFE, g/mL
Ds, 10-7 cm2 s-1
APTFE , cm2 mL-1
APTFE/ANaPSS
0 7.1 × 10-5 7.1 × 10-4 7.1 × 10-2
2.12 ( 0.04 1.95 ( 0.07 2.23 ( 0.04 2.23 ( 0.24
0 10 100 10000
0 0.02 0.2 20
a The concentration of LNaPSS was 4.0 × 10-5 g mL-1. A PTFE is the total surface area of the PTFE latex determined by assuming a spherical shape for the latex so that APTFE ) 4πRh2 with Rh(PTFE) ) 90.6 nm. ANaPSS is the projected area of a NaPSS coil, assumed to be πRh2 with Rh(NaPSS) ) 7 nm. In computing the number concentration of PTFE, the density used is FPTFE ) 2.14 g cm3. In computing the number concentration and total projected area of NaPSS, the molecular weight of NaPSS is taken as 70 000.
absence of PTFE aggregation, when polymer is present, and the restabilization phenomena observed visually. Fluorescence Photobleaching Recovery. Stabilization of latex particles by the addition of polymer is often a steric effect arising from the unwillingness of bound polymer to undergo compression as two latex particles approach.9 PTFE and NaPSS are both negatively charged, so their mutually repulsive interaction should minimize adsorption. Still, binding could take place at hydrophobic patches along the NaPSS chain or on the PTFE particles. This possibility was investigated by FPR using fluorescently labeled NaPSS (LNaPSS). Binding of LNaPSS to PTFE latices could occur in the rapid exchange limit or be permanent on the time scale of the fluorescence recovery (15 s or more, typically). Rapid exchange would lead to a single recovery mode and give an average diffusion coefficient: Davg ) xbDb + xfDf, where xb and xf are the bound and free fractions of LNaPSS and Db and Df are the diffusion coefficients of polymer bound to PTFE particles and of free polymer, respectively. It is assumed that absorption, fluorescence, and photobleachability are not changed on binding. With these same assumptions, permanent binding would lead to a bimodal recovery and the amplitudes of the two modes could provide the percent of dye bound to the particle. The concentration of LNaPSS must be kept in a very dilute range and the PTFE concentration varied in order for any possible adsorption of the polymer onto the latex surface to be visible. Table 2 shows the total PTFE particle surface area (assuming round, not elongated, particlessan approximation of little consequence) in relation to the area projected by all LNaPSS molecules, estimated from the hydrodynamic radius. At the lowest PTFE concentrations, the PTFE surface area would accommodate only about 2% of the NaPSS. Thus, even if the PTFE particles were fully coated by LNaPSS, 98% of the FPR recovery signal would come from the unbound LNaPSS polymers. Not surprisingly, the measured diffusion coefficient of LNaPSS at the lowest PTFE concentration did not differ from that in pure water. But at the highest PTFE concentration, the PTFE particles
could have accommodated 20× more LNaPSS molecules than actually were present. Under these conditions, and given the 5-10% precision of the FPR measurements of diffusion coefficients, binding of more than about 10% of the LNaPSS polymers to PTFE would be detectable. Yet a monomodal recovery pattern was measured, yielding a diffusion coefficient for the LNaPSS which differed imperceptibly from the value in pure water. Evidently, the stability behavior must be explained by mechanisms involving unbound polymer, even though flocculation is the more common result in this case.44-49 Discussion of Stability. The possibility that unbound polymer could actually stabilize polymer solutions was considered by Feigin and Napper6,50 and by Vincent and co-workers.44,51 The origin of the repulsion is that polymers lying between colloidal particles must be “accepted” by the bulk solution if the particles move closer together to squeeze them out. In thermodynamically good solvents, addition of new polymers to the bulk phase raises the polymer chemical potential and is therefore resisted. Nevertheless, once an exclusion volume between the spheres is formed, the same polymer-polymer repulsion will drive the colloidal particles closer together to maximize the space available to the polymers--the original depletion flocculation of Asakura and Oosawa. Subsequent developments are nicely reviewed in a paper by Walz and Sharma,52 who found that the nonideality of interactions among the polymers produced a repulsive barrier between colloidal particles. Long-range interactions enhanced the barrier. Similar behavior has been found in other studies,11,53 and it is expected that extended polymers (providing long-range interaction) may be more effective than random chains at effecting depletion stabilization.54 Although there is little doubt about the existence of repulsive energy barriers between colloidal spheres in the presence of unbound polymers, these alone cannot explain the present observations. In the first place, the concentration at which restabilization occurs in our system is very low. The restabilization phenomenon discussed by Vincent for uncharged polymers occurs near the upper bound of the semidilute regime (i.e., the semidiluteconcentrated crossover). Published intrinsic viscosity data55 suggest that the lower bound of the semidilute regime (the dilute-semidilute crossover) occurs at 1728 mg/mL for NaPSS of M ) 70 000, depending on the salt. The observed restabilization at several milligrams per milliliter is clearly a dilute regime effect. Long-range interactions due to the charged nature of our polymer (44) Clarke, J.; Vincent, B. J. Colloid Interface Sci. 1981, 82, 208216. (45) Vincent, B.; Luckham, P. F.; Waite, F. A. J. Colloid Interface Sci. 1980, 73, 508-521. (46) Gast, A. P.; Hall, C. K.; Russel, W. B. J. Colloid Interface Sci. 1983, 96, 251-267. (47) Sperry, P. R.; Hopfenberg, H. B.; Thomas, N. L. J. Colloid Interface Sci. 1981, 82, 62-76. (48) Sperry, P. R. J. Colloid Interface Sci. 1984, 99, 97-108. (49) van Oss, C. J.; Arnold, K.; Coakley, W. T. Cell Biophys. 1990, 17, 1-10. (50) Feigin, R. I.; Napper, D. H. J. Colloid Interface Sci. 1980, 75, 525-541. (51) Lambe, R.; Tadros, T. F.; Vincent, B. J. Colloid Interface Sci. 1978, 66, 77-84. (52) Walz, J. Y.; Sharma, A. J. Colloid Interface Sci. 1994, 168, 485496. (53) Warren, P. B.; Ilett, S. M.; Poon, W. C. K. Phys. Rev. E 1995, 52, 5205-5213. (54) Mao, Y.; Cates, M. E.; Lekkerkerker, H. N. W. Phys. Rev. Lett. 1995, 75, 4548-4551. (55) Takahashi, A.; Kato, T.; Nagasawa, M. J. Phys. Chem. 1967, 71, 2001-2010. We have assumed that the Mark-Houwink relationship in this reference holds at our molecular weight, which is slightly outside the range of molecular weights studied by these authors.
268 Langmuir, Vol. 14, No. 2, 1998
Jamil and Russo
Figure 2. Typical Vν correlation function in semi-log form, demonstrating nearly single-exponential behavior.
Figure 3. Decay rates plotted against squared scattering vector magnitude for PTFE/water Vν and Hν geometries.
might cause repulsive barriers to appear at low concentrations,52 but a fundamental problem still remains: barriers work both ways. Adding unbound polymer to an already flocculated system should prevent redispersion, contrary to observation. A possible explanation56 is that the colloidal PTFE particles may be only weakly flocculated. Shallow secondary minima in the interaction potential of charged colloids are known to exist.3,57 The nonionic surfactant shipped with the colloidal PTFE will also help to prevent very close, strong attraction. The gentle stirring that followed addition of the NaPSS to the salt-flocculated suspensions could disturb the floc structure enough to allow the polyelectrolyte to surround the colloidal particles. Even without agitation, the polymer could diffuse into the regions between colloidal particles held a distance away from each other in a weak, secondary minimum. Subsequently, the colloidal particles could enjoy a degree of kinetic stabilization due to a repulsive depletion barrier. This interpretation may apply to other cases where dilute polymer solutions restabilize flocculated colloids.7,8 Whatever the underlying mechanism, the stability of the NaPSS-PTFE latex system enables us to proceed with the probe diffusion study. Dynamic Light Scattering. PTFE Dispersions. Diffusion measurements on PTFE latex in water without added salt or polyelectrolyte were performed at a concentration of cPTFE ) 70 µg/mL. Figure 2 shows a typical correlation function in the Vν geometry. The correlation function decays nearly as a single exponential with a rate h 2, from third-order cumuΓ h . The reduced variance, µ2/Γ 39 was less than 0.3, in agreement with lants analysis, previous findings of modest polydispersity for this32 and h vs q2 plots of the PTFE/ similar27 latices. Figure 3 shows Γ water system for experiments performed in both the Hν and Vν geometries. If solution nonidealities are present, they are not strong enough to establish interparticle correlations that would be reflected in the angular h vs q2 distribution of the decay rate.58 The slopes of the Γ lines are similar for both Hν and Vν geometries and can yield the diffusion coefficient in the limit of low q. For the Vν data, the low-q limit was actually established by making a linear least-squares fit of Γ h Vν/q2 vs q2 to give the translational diffusion coefficient DtVν from the intercept according to eq 4a. In the Hν geometry, translational and rotational diffusion coefficients, DtHν and DrHν, were
Table 3. Hydrodynamic Radii, Rh (Å), of PTFE Latex from Polarized and Depolarized Scattering
(56) We are grateful to John Walz of Yale University for a valuable communication on this point. (57) Chu, X.; Wasan, D. T. J. Colloid Interface Sci. 1996, 184, 268278. (58) Jones, R. B.; Pusey, P. N. Annu. Rev. Phys. Chem. 1991, 42, 137-169.
NaPSS, mg/mL
exptl geometry
0
0
(Vν)t (Hν)t (Hν)r (Vν)t (Hν)t (Hν)r (Vν)t (Hν)t (Hν)r (Vν)t (Hν)t (Hν)r (Vν)t (Hν)t (Hν)r (Vν)t (Hν)t (Hν)r
947 978 1140 1110 1110 1200 1240 1200 1210 1300 1232 1240 1475 1330 1270 1550 1390 1260
1 2 3 4 5
[NaCl], 10-2 M 1 5 a a a 1055 1050 1190 1100 1100 1230 1230 1185 1260 1230 1330 1260 1390 1290 1300
a a a 1090 1040 1220 1130 1130 1220 1175 1140 1270 1200 1200 1270 1190 1265 1290
10 a a a 1050 1070 1240 1080 1090 1235 1150 1150 1250 1190 1180 1270 1160 1210 1280
a These measurements were not performed due to PTFE latex aggregation and sedimentation. The viscosity of the solvent is used in the calculation of Rh, which should therefore be interpreted as an apparent value. Actual diffusion coefficients can be obtained through eqs 5 or 7 which, for the conditions of these experiments, read Dt ) 2.45 × 10-5/Rh (Å) and Dr ) 1.84 × 1011/[Rh (Å)]3. Estimated uncertainty is (5%.
determined from the slope and intercept, respectively, of the Γ h vs q2 line using eq 6. Hydrodynamic radii determined from the Vν and Hν geometries, using eqs 5 and 7, respectively, appear in Table 3. Given the asymmetry of the molecules (for an electron micrograph, see Figure 1 of ref 32) and the fact that they are not truly monodisperse, the agreement to within about (10% among the three values is satisfactory. PTFE/NaPSS/Salt Dispersions. In the latex/polymer/ solvent systems, the scattering power of PTFE dominates. In Vν experiments, PTFE scattering exceeds that from even the most concentrated NaPSS solution by a factor of about 70. In the Hν mode, essentially all the scattering arises from the PTFE, as the NaPSS depolarizes very weakly. Figure 4a shows Γ h Vν/q2 vs q2 for the polarized studies, while Γ h Hν vs q2 results appear in Figure 4b for the depolarized measurements. Values for DtVν were determined from the intercepts of the plots in Figure 4a. The DtHν and DrHν values were taken from the slopes and intercepts, respectively, of the plots in Figure 4b. Equations 5 and 7 were used to convert all the diffusion coefficients into hydrodynamic radii. The results, shown in Table 3, should be regarded as apparent Rh values because the viscosity of water was used. Choosing to
Interactions between PTFE and NaPSS
Figure 4. Apparent diffusion coefficients from Vν measurements (a) and decay rates from Hν measurements (b) plotted against squared scattering vector magnitude, for various NaPSS concentrations.
Figure 5. Viscosity as a function of NaPSS concentration for various added salt molarities, indicated.
display apparent Rh simplifies the comparison of translational and rotational diffusion data, but one must remember the true meaning of these numbers: they are inversely related to either translational or rotational diffusion coefficients. (Equations to accomplish the back conversion to diffusion data are provided in the legend.) Scanning across the table, trends with added salt are not immediately apparent. Scanning down the table reveals a significant increase of apparent Rh with concentration. This surely reflects increased solution viscosity, but there is more to it than that. Discussion of the Dynamics Results. The viscosity of the present solutions increased with concentration and decreased with added salt, as shown in Figure 5. In a continuum fluid, the product of viscosity and probe diffusion coefficient would be constantsi.e., not vary with salt or with concentration. Such reasoning is valid only
Langmuir, Vol. 14, No. 2, 1998 269
when the particles are not aggregating, as has been confirmed here by static light scattering for solutions containing NaPSS. Indeed, the increased apparent hydrodynamic radii from the translational diffusion measurements approximately track the viscosity, insofar as its concentration dependence is concerned. It is difficult to identify a clear trend for the salt dependence of the translational diffusion coefficients. Before discussing the unusual behavior revealed by the rotational diffusion, we consider approaches that do not explicitly involve the viscosity.16,59-65 For if viscosity were the sole determinant of mobility, then small probes would not perfuse gels, which they do. A frequent theme is the obstruction caused by the polymer. A representative expression to describe the decrease in translational diffusion of a particle with added polymer is D ≈ Dφ)0(1 - φ), where φ is the polymer matrix volume fraction. Accordingly, the rate of diffusion of a particle is just proportional to the volume available for diffusion. The amount of added salt can affect the total obstruction presented by the polymer chains, the overall solution viscosity, or both. Chain extension due to chargecharge repulsions along the backbone could raise the solution viscosity at low salt. To understand the obstruction effect, consider that a probe particle can closely approach a like-charged polymer chain at high salt, where the Coulombic forces are screened, but must avoid the chain under conditions of low salt. Translational diffusion of bovine serum albumen through DNA solutions was found to increase with added salt.66 Chain contraction should have been minimized by the inherent stiffness of DNA, so the simplest explanation is that obstruction effects were reduced as the negatively charged zone surrounding the DNA was collapsed by salt addition. The comparative flexibility of NaPSS prevents a similar separation of viscosity and obstruction effects, but it is still possible to demonstrate the importance of the ionic atmosphere surrounding the polymer on the latex dynamics. This is done by forming products of the macroscopic solution viscosity and a diffusion coefficient as a function of salt. It is the rotational diffusion that proves interesting, and in this case, “harassment” might be a better word than “obstruction” to describe the effect of the unbound polymer matrix. Figure 6 shows DrHνη vs cNaPSS at the highest and lowest salt conditions. Within error, the viscosity “corrects” for the loss of rotational motion as NaPSS is added; the plots are flat with concentration. However, two data sets at different added salt do not coincide. The dynamic probe-polymer interaction clearly changes with added salt. When salt is added to a solution that originally contains none, η decreases, but the increase of Dr does not keep pace. We postulate that the rotational diffusion of the probe is retarded, despite the lower overall solution viscosity, by closer approach of the polymer matrix. A similar conclusion favoring the significance of ion atmosphere effects for rotational diffusion was reached (59) Phillies, G. D. J. J. Phys. Chem. 1989, 93, 5029-5039. (60) Johansson, L.; Elvingson, C.; Lofroth, J. E. Macromolecules 1991, 24, 6024-6029. (61) Ogston, A. G.; Preston, B. N.; Wells, J. D.; Snowden, J. M. Proc. R. Soc. London, Ser. A 1973, 333, 297-316. (62) Cukier, R. I. Macromolecules 1984, 17, 252-255. (63) Altenberger, A. R.; Dahler, J. S.; Tirrell, M. Macromolecules 1988, 21, 464-469. (64) Pickup, S.; Blum, F. D. Macromolecules 1989, 22, 3961-3968. (65) Mackie, J. S.; Meares, P. Proc. R. Soc. London, Ser. A 1955, 232, 498-509. (66) Wattenbarger, M. R.; Bloomfield, V. A.; Bu, Z.; Russo, P. S. Macromolecules 1992, 25, 5263-5265.
270 Langmuir, Vol. 14, No. 2, 1998
Figure 6. Product of rotational diffusion coefficient and viscosity as a function of NaPSS concentration for two added salt molarities, indicated.
by Sohn et al., in a study of magnetic latex particles dispersed in aqueous solutions of the same NaPSS polymer.42 Conclusion The presence of dilute NaPSS in colloidal PTFE dispersions prevents or reverses salt-induced aggregation, yet there is no evidence for polymer binding to the colloidal particles. While depletion repulsion is now an accepted part of colloid-polymer behavior and while restabilization has been observed at high polymer concentrations, the curious aspect to the present observations is the restabilization of coagulated latex at low concentrations of added polymer. After the initial restabilization, translational diffusion of the colloidal PTFE probes decreased with added polymer, approximately following the Stokes-
Jamil and Russo
Einstein law in the limited concentration range explored. The dependence of translational diffusion on added salt was not clearly identified. Rotational diffusion measurements were sensitive to the ionic atmosphere in the vicinity of the PTFE particles. It appears that closer approach of the polymer matrix to the polymers slows the rotational diffusion when salt is added, after allowance is made for the change in solution viscosity. The Hν experiments described herein are simple and reasonably precise because of the high depolarization of the crystalline colloidal PTFE. Perhaps that fact will help to dispel the commonly held notion that depolarized DLS is a troublesome experiment. This impression is based on early experiments with very weakly depolarizing biomolecules, in which depolarized DLS appeared somewhat less precise than traditional methods, such as electric or flow birefringence. Experiments on weak depolarizers can produce good results,67 but DLS experiments are always tedious when signal levels are comparable to the solvent scattering. Given strongly depolarizing probes, depolarized DLS becomes a simple and robust tool for studying polymer-colloid interactions. Its most important advantage, that no external perturbation need be applied, is especially relevant for polyelectrolyte systems containing delicate ionic atmospheres. Acknowledgment. This work was supported by NSF Grant DMR-9221585. We are very grateful to Professor Mark McLaughlin and Mr. Alfonso Da’vila of this department for preparing the labeled NaPSS polymer. LA960841C (67) Zero, K.; Pecora, R. In Dynamic Light Scattering; Pecora, R., Ed.; Plenum: New York, 1985.
