5062
Langmuir 1998, 14, 5062-5069
Electrokinetic Properties and Colloidal Stability of Cationic Amino-Containing N-Isopropylacrylamide-Styrene Copolymer Particles Bearing Different Shell Structures Lahcen Nabzar,† David Duracher,‡ Abdelhamid Elaı¨ssari,*,‡ Guy Chauveteau,§ and Christian Pichot‡ ICS, 6 Rue Boussingault, 67083 Strasbourg Cedex, France, CNRS-BioMe´ rieux, ENS de Lyon, 46 Alle´ e d’Italie, 69364 Lyon Cedex 07, France, and Institut Francais du Pe´ trole, BP-311, 92506 Rueil-Malmaison Cedex, France Received February 27, 1998. In Final Form: May 6, 1998 The electrokinetic properties and colloidal stability behavior of styrene/N-isopropylacrylamide and styrene/ N-isopropylacrylamide-co-aminoethyl methacrylate core-shell latexes were investigated. The electrophoretic mobility was first measured as a function of pH, temperature, and ionic strength. On the basis of the results obtained for the electrokinetic measurement, a charge distribution model (volume charge distribution) was proposed to help the suggested interpretation. By use of Eversole and Bordman’s equation, the shear plane positions were estimated as a function of ionic strength and good correlation was found between the calculated values and the values obtained from the quasi-elastic light scattering measurement. The colloidal stability of those core-shell latexes was examined above and below the low critical solution temperature (LCST) (∼32 °C). Below the LCST, stabilization was mainly attributed to the combination of both electrostatic and steric stability. Whereas, above the LCST, only the electrostatic stability was the driving parameter. In addition, an apparent Hamaker constant of the core-shell latex was experimentally determined above the LCST.
1. Introduction Polymeric microspheres are used as a convenient and versatile tool in academic studies as a colloidal model and in medical diagnostics1,2 as carriers for biological macromolecules due to their large surface-to-volume ratio. Poly[N-isopropylacrylamide] hydrogels with well-defined colloidal characteristics have received much attention because poly[NIPAM] has a low critical solubility temperature (LCST) at 32 °C and offers potential for several applications: in the biomedical field as a drug delivery system3 and in medical diagnostics as a good support for grafting and targeting nucleic acids;4 such latex particles are also useful for studying the protein-polymer interaction (physical adsorption or covalent binding) as was recently reported by Kawaguchi et al.5,6 Many academic studies have been recently performed to understand the many complex phenomena involved as a function of temperature according to the thermosensitive properties7,8 of poly[NIPAM] hydrogel and core-shell with * To whom correspondence should be addressed. E-mail: Hamid.
[email protected]. † ICS. ‡ ENS de Lyon. § Institut Francais du Pe ´ trole. (1) Singer, J. M. The use of polystyrene latexes in medicine. Future Directions in Polymer Colloids; El-Aasser, M. S., Fitch, R. M.; NATO ASI Series; Springer: New York, 1987; Vol. 138, p 372. (2) Arshady, R. Biomaterial 1993, 14, 5. (3) Park, T. G.; Hoffman, A. J. Appl. Polym. Sci. 1992, 46, 659. (4) Delair, T.; Meunier, F.; Elaı¨ssari, A.; Charles, M. H.; Pichot, C. Colloids Surf., in press. (5) Kawaguchi, H.; Fujimoto, K.; Mizuhara, Y. Colloid Polym. Sci. 1992, 270, 53. (6) Fujimoto, K.; Mizuhara, Y.; Tamura, N.; Kawaguchi, H. J. Intell. Mater. Syst. Struct. 1993, 4, 184. (7) Pelton, R. H.; Chibante, P. Colloids Surf. 1986, 20, 247. (8) Meunier, F.; Elaı¨ssari, A.; Pichot, C. Polym. Adv. Technol. 1995, 6, 489.
poly[NIPAM] in the shell. An extensive study was performed by Kawaguchi et al.5,6 and Pelton et al.7,9 on the effect of temperature on the electrophoretic mobility. It has been pointed out by Pelton et al.9,10that the electrophoretic mobility and particle size are very temperature-sensitive. The increase of electrophoretic mobility as a function of temperature has been discussed and correlated to the particle size variation. A theoretical analysis of the electrophoretic mobility (µe) of the so-called “soft particles” has been proposed by Ohshima et al.11,12 According to Ohshima’s theory, the electrophoretic mobility of a particle bearing a shell of polyelectrolyte materials depends on (a) the Donnan potential of the polyelectrolyte surface layer, (b) the potential at the boundary between the surface layer and surrounding solution, (c) the density of the fixed charges, and (d) the friction parameter. The proposed µe formula seems to be consistent with the observed experimental electrophoretic behavior. In this study monodisperse latex particles composed of a poly[styrene] core and a poly[NIPAM] or cross-linked poly[NIPAM-co-AEM] hydrogel layer13 were characterized in terms of hydrodynamic particle size (as a function of salinity and temperature), electrophoretic mobility (as a function of pH, ionic strength, and temperature), and colloidal stability (as a function of temperature). In addition, the objective of this work was to complete our preliminary work on the surface and colloidal character(9) McPhee, W.; Tam, K. C.; Pelton, R. J. Colloid Interface Sci. 1993, 156, 24. (10) Pelton, R. H.; Pelton, H. M.; Morphesis, A.; Rowell, R. L. Langmuir 1989, 5, 816. (11) Ohshima, H.; Makino, K.; Kato, T.; Fujimoto, K.; Kondo, T.; Kawaguchi, H. J. Colloid Interface Sci. 1993, 159, 512. (12) Ohshima, H. J. Colloid Interface Sci. 1994, 163, 474. (13) Duracher, D.; Sauzedde, F.; Elaı¨ssari, A.; Perrin, A.; Pichot, C. Colloid Polym., Sci. 1998, 276, 219.
S0743-7463(98)00244-3 CCC: $15.00 © 1998 American Chemical Society Published on Web 08/06/1998
Colloidal Stability of Particle Shell Structures
Langmuir, Vol. 14, No. 18, 1998 5063
Figure 1. Transmission electron micrographs of (a) DD1 and (b) DD4 latex particles. Table 1. Characteristics of Polymer Latexes DD1 and DD4a
ization of cationic amino-containing N-isopropylacrylamide-styrene copolymer particles.14 2. Experimental Section Latex Preparation and Characterization. The cationic amino-containing N-isopropylacrylamide-styrene copolymer latexes13 were synthesized under emulsifier-free emulsion polymerization. The polymerization was carried out in a 200 mL round-bottomed four-necked flask equipped with a glass anchorshaped stirrer, condenser, and nitrogen inlet. The polymerization temperature was stabilized at 70 °C, and the agitation rate was set at 200 rpm. DD1 latex was prepared using the batch polymerization process using styrene, N-isopropylacrylamide (NIPAM), and 2,2′-azobis(amidinopropane) dihydrochloride (V50) as an initiator. DD4 latex was prepared by a shot-growth polymerization process. The polymerization was carried out in the beginning as for DD1, then after 89% conversion, N-isopropylacrylamide, N,N-methylenebis(acrylamide) (MBA), and aminoethyl methacrylate hydrochloride (AEM) mixture were added. A detailed procedure, recipe, and kinetic study on the polymerization of such systems has been already reported.13 All latexes were cleaned by repetitive centrifugation and redispersion using deionized Milli-Q water, before any characterization study in order to remove free electrolytes and water soluble polymers. Latex particle size (2rh) was measured by quasi-elastic light scattering (QELS) using N4MD equipment from Coultronics, France. The particle size and dispersity index (PDI) were also determined using transmission electron microscopy (TEM) at the CMEABG, Claude Bernard University, Lyon, France (from (14) Duracher, D.; Sauzedde, F.; Elaı¨ssari, A.; Pichot, C.; Nabzar, L. Colloid Polym., Sci., in press.
sample DD1e DD4
2rh 2rh 20 °C 50 °C 2rTEM (nm) (nm) (nm) 352 552
310 378
306 303
PDIb
δhc σ σd (nm) (µmol g-1) (µmol m-2)
1.007 1.011
21 87
12.30 14.60
0.75 1.34
a The error on the particle size measured by QELS was (5 nm. The polydispersity of latex particles from TEM. c δh (nm) ) (rh(20 °C) - rh(50 °C)). d Calculated using particle size at 20 °C. e Latex particles bearing only amidine groups.
b
Philips). As shown in Figure 1 and Table 1, both latexes exhibit narrow size distribution with a polydispersity index close to 1. The surface charge density was determined by colorimetric titration using the SPDP methods13,14 and the results obtained are reported in Table 1. Electrophoretic mobility (µe) of latex particles15 was performed with the Zeta Sizer III (from Malvern Instruments, England) as a function of pH, ionic strength, and temperature. The zeta potential (ζ) used in this work was calculated from electrophoretic mobility using Smoluchowski’s equation,15 where η is the viscosity of the medium and 0 and r are the permittivity of a vacuum and the relative permittivity of the medium, respectively. The measurements were carried out with latex samples highly diluted conditions (latex concentration lower than 25 × 10-3 g L-1).
µe )
0r ζ η
(1)
(15) Hunter, R., J. Zeta Potential in Colloid Science Principles and Applications; Academic Press: London, 1981.
5064 Langmuir, Vol. 14, No. 18, 1998
Nabzar et al.
Figure 2. Particle size (2rh) as a function of ionic strength log(Cs) for DD1 and DD4 latexes (pH 6.0, temperature 20 °C). (The standard deviation is (5 nm.)
Figure 3. Particle size (2rh) variation as a function of temperature for DD1 and DD4 latexes (pH 6.0, ionic strength Cs ) 10-3 M NaCl). (The standard deviation is (5 nm.)
The stability of colloidal dispersions16 is usually expressed in terms of the stability ratio (W) defined as the reciprocal fraction of successful (leading to permanent contact) collisions. At low ionic strength, coagulation proceeds under surface-reactioncontrolled kinetics with an initial rate constant (ks) and an activation energy determined by the height of the energy barrier of the interaction potential (slow coagulation). However, at high ionic strength the energy barrier is suppressed and coagulation kinetics become diffusion-controlled with an initial rate constant (kf) (fast coagulation). The stability ratio is then given by the ratio of the aforementioned rate constants W ) ks/kf. Experimental W values were obtained using turbidity monitoring during coagulation upon the addition of an electrolyte. Coagulation experiments have been carried out at different ionic strengths (Cs) for all latexes described by Duracher et al.14 at two temperatures, i.e., below (20 °C) and above (43 °C) the LCST of the NIPAM polymer. After the latex suspension and electrolyte solution (NaCl) were rapidly mixed, the turbidity (τ) variation as a function of time (at 600 nm wavelength using an UVIKON 930 spectrophotometer) was continuously recorded. The stability ratio (W) values were then calculated from the ratio of the initial turbidity slopes (dτ/dt) versus time for fast and slow aggregation runs, and the critical coagulation concentrations (CCC) were deduced from the stability curves (log(W) versus log(Cs)).
(ii) Decrease of the hydrodynamic diameter Dh upon increasing electrolyte concentrations can also be attributed to a hairy particle model in which the polymer chains at the particle surface are polyelectrolytes. The interfacial polymers were immobilized in a complex conformation with tails and loops extending far into the surrounding solution. Added salt (NaCl) screens the electrostatic repulsion between the ionic groups (inter- and intrapolymer chains) of the polyelectrolyte hairs, due to the decrease in the hydrodynamic size of the particle as recently discussed by Seebergh et al.18 According to these above two phenomena, the effect of ionic strength was more effective on the shot latex than on the batch one, as illustrated in Figure 2. The shot latex prepared using AEM monomer was found to exhibit higher hydrogel layer and hairy polyelectrolyte chain than that prepared using the batch process (Table 1). In both cases, effects (i) and (ii) can be taken into account. Nevertheless, the decrease in particle size of the shot latex can be attributed to the effect of salt on the solubility of poly[NIPAM], since the amidine groups are anchored in the end chains according to the polymerization process. A complex combination of both contributions dictates the stability, the equilibrium, and swelling state of such hydrogel shells in a specific environment. 3.2. Effect of Temperature on Particle Size. The temperature dependence of the hydrodynamic diameter of the core-shell latex was examined, and the results obtained are reported in Figure 3. In the range from 27 to 40 °C, a continuous decrease in the hydrodynamic diameter with increasing temperature was observed. This behavior was attributed to poly[NIPAM] polymer in the latex shell. Below 27 °C, the shell was swollen and the particle size was not affected. Whereas, above 27 °C, the hydrogel shell was deswollen and the particle size decreased drastically up to 40 °C. Above 40 °C, the hydrodynamic diameter slightly decreases. The thermal sensitivity of the shell has been largely discussed and attributed to the dehydration of amide moieties and the transition temperature corresponds to the LCST of poly[NIPAM], which was found to be 32 °C in the case of linear polymer3 and close to 34 °C in the case of cross-linked poly[NIPAM] hydrogel.7,8 Because of the presence of a polystyrene core, the LCST cannot be determined adequately for hydrogel particles by measuring particle size
3. Results and Discussion 3.1. The Effect of Ionic Strength on the Particle Size. The particle diameter 2rh (from QELS) as a function of monovalent electrolyte concentration (NaCl) shows a marked variation as reported in Figure 2, in which the particle diameters as a function of NaCl concentration at 20 °C have been reported. The hydrodynamic diameter is found to decrease when the electrolyte concentration is increased. The effect was more marked in the case of two-step polymerization (DD4) than for the batch method (DD1). The magnitude of particle size variation (2rh versus salinity) was attributed to the hydrogel layer (δ ) 23 for DD1 and δ ) 96 for DD4) on the polystyrene core as reported in Table 1. Moreover, the effect of salinity on the hydrodynamic size variation can be explained on the basis of two phenomena. (i) As extensively reported by several authors,9,17 increasing the electrolyte concentration caused a reduction of the medium solvency of poly[NIPAM] even below the LCST. As a result, high salinity resulted in poly[NIPAM] precipitation. (16) Verwey, E. J. W.; Overbeek, J. Th. G. Theory of the Stability of Lyophobic Colloids; Elsevier: Amsterdam, 1948. (17) Tirrel, D. A.; Schild, H. G. J. Phys. Chem. 1990, 94, 4352.
(18) Seebergh, J. E.; Berg, J. C. Colloids Surf. 1995, 100, 139.
Colloidal Stability of Particle Shell Structures
Figure 4. Electrophoretic mobility of latex DD1 and DD4 as a function of pH at a constant ionic strength 10-3 M NaCl and at 20 °C. (The standard deviation is (0.05 × 10-8 m2/(V s).)
Langmuir, Vol. 14, No. 18, 1998 5065
Figure 5. Electrophoretic mobility of latex (DD1 and DD4) as a function of temperature at pH 5 and ionic strength 10-3 M NaCl. (The standard deviation is (0.05 × 10-8 m2/(V s).)
µe ≈ -
Ne 4πηκrh2
as a function of temperature. The effect of temperature was found to be more effective in the case of core-shell latex prepared in shot than in batch polymerization process. This was attributed to the quantity of poly[NIPAM] incorporated onto the polystyrene core. 3.3. Effect of pH on the Electrophoretic Mobility. Figure 4 shows the electrophoretic mobility (µe) of latex particles as a function of pH, in 10-3 M NaCl solution at 20 °C. Particles exhibit a positive electrophoretic mobility up to pH 10 for both latexes. The µe of amidine latex particles (DD1) drastically decreases as the pH of the medium increases, whereas the electrophoretic mobility of aminated latex particles (DD4) was found to be pH independent until pH 9.5 at which the µe dramatically decreased and became negative above pH 10.5. The observed negative electrophoretic mobility for both latexes was due to the hydrolysis of amidine groups originating from the initiator (V50).18 The isoelectric points (IEP) for both latexes were found to be 10.5. The IEP of amidine latex particles is close to that already reported.19 The IEP for aminated latex was found to be close to the pKa of the functional monomer aminoethyl methacrylate hydrochloride (AEM).19 In addition, there is no direct correlation between the surface charge density and the electrophoretic mobility at the plateau value (in the acidic pH range). This anomalous result can be attributed to the complexity of the shell structure and to the surface morphology as pointed out in ref 13. 3.4. Effect of Temperature on the Electrophoretic Mobility. The electrophoretic mobility of latex particles was measured as a function of temperature, in 10-3 M NaCl at pH 5.0 as shown in Figure 5. As already reported by Pelton et al.,10 both latexes exhibit an increase in the electrophoretic mobility when the temperature increases from 20 to 40 °C (above the LCST of poly[NIPAM]). This phenomenon was attributed to the shrinkage of the hydrogel layer (reduction of hydrodynamic diameter, 2rh), which induces an increase of the surface charge density (σ) and then in the electrophoretic mobility (µe). Indeed, according to the simple model of charge distribution previously described, and using both Helmoltz’s and Smoluchowski’s formula,15 the electrophoretic mobility (µe) may be related to the surface charge density (σ) and hydrodynamic particle size (2rh) through the following equation:
where κ is the reciprocal Debye length, η the viscosity of the medium, and N the number of charged groups. This simple relationship qualitatively accounts for the previous experimental observations. The effect of temperature on electrophoretic mobility is significantly less marked with DD1 latex than with DD4. The difference can be attributed to the thicker hydrogel surface layer borne by DD4 particles, whose swelling and shrinkage are expected to induce great changes in the hydrodynamic particle size and surface charge density (Figure 3 and Table 1). Thus, it seems that the thicker the poly[NIPAM] hydrogel surface layer, the more pronounced the effect of temperature on hydrodynamic size and electrophoretic mobility as already reported by Pelton et al.10 and Ohshima et al.11,12 3.5. Effect of Ionic Strength on the Electrophoretic Mobility. The effect of ionic strength on the electrophoretic mobility was investigated at room temperature and the results obtained for DD1 and DD4 samples are shown in Figure 6. For batch latex (DD1), the electrophoretic mobility versus ionic strength exhibits a marked maximum. However, no such maximum but rather an increase at low ionic strength was observed in the case of the DD4 sample whose electrophoretic mobility values are much lower despite its high fixed charge content. Similar unusual behavior was reported by Tirrel et al.17 for negatively charged poly[NIPAM] hydrogel latexes, which behave like DD1 above the LCST and DD4 below this temperature. Note that DD4 latex was obtained by “shot-growth” polymerization at a high conversion level and is characterized by a thick and charged cross-linked poly[NIPAM] hydrogel surface layer. The maximum in the electrophoretic mobility as a function of ionic strength has also been reported by many authors both for negatively and positively charged20 polystyrene latexes and different models including surface conductance, surface roughness, ion adsorption, and hairy layer have been proposed to account for this nonideal behavior. Among these models, the hairy layer model, which postulates a shift in the shear plane position as a consequence of the presence of polymer chains at the interface, is the most widely invoked for polymer colloids.
(19) Sauzedde, F.; Ganachaud, F.; Elaı¨ssari, A.; Pichot, C. J. Appl. Polym. Sci. 1997, 65, 2331.
(20) Hidalgo-Alvarez, R.; De Las Nieves, F. J.; Van Der Linde, A. J.; Bijsterbosch, B. H. Colloids Surf. 1986, 21, 259.
(2)
5066 Langmuir, Vol. 14, No. 18, 1998
Nabzar et al. Table 2. Effect of Ionic Strength on the Shear Plane Position (∆) for DD1 Latex at 20 °Ca Cs
2rh (nm)
∆ (nm)b
∆QELS (nm)c
2rc (nm)d ) 2(rh - ∆)
10-5 10-4 10-3 10-2 10-1
430.00 360.00 352.00 348.00 340.00
46.20 11.62 3.68 1.73 1.54
46 11 7 5 1
336 336 344 344 337
a The error on the particle size measurements was (5 nm. b ∆ (nm) calculated from eq 3. c ∆QELS (nm) ) rh - [rh(0.1 M) - 1]. d 2rc (nm)c ) 2(rh - ∆b).
ζ
Figure 6. Electrophoretic mobility of DD1 and DD4 latexes as a function of ionic strength at a constant pH 6.0 and at 20 °C. (The standard deviation is (0.05 × 10-8 m2/(V s).)
Figure 7. Variation of -Ln(tanh(eζ/4kT)) as a function of k (reciprocal Debye length) at 20 °C.
As a result of the polymerization process, it is likely that these latexes bear a hairy layer on their surfaces. In the framework of this model, Eversole and Boardman’s equation21
tanh(eζ/4kT) ) tanh(eψo/4kT) exp(-κ∆)
(3)
is commonly used22,23,24 to provide a rough estimation of the shear plane position ∆. In this equation, ζ is the experimental zeta potential and ψo the surface potential. With an ideal system, the plot of -ln(tanh(eζ/4kT)) versus κ should give a straight line with slope ∆ and an estimation of ψo can be reached from the ordinate at zero ionic strength. With the hairy latex, the postulated increase in ∆ values at low ionic strength results in an outward shift of the shear plane position with a concomitant decrease in ζ values. From Figure 7, where such plots for DD1 and DD4 latexes are shown, we can conclude that (i) both latexes display an ideal behavior at high ionic strength and (ii) DD1 latex exhibits a concave profile as expected from the hairy layer model, while DD4 latex (21) Eversole, W. G.; Boardman, W. W. J. Chem. Phys. 1941, 9, 798. (22) Snowden, M. L.; Marston, N. J.; Vincent, B. Colloid Polym., Sci. 1994, 272, 1273. (23) Rosen, L. A.; Saville, D. A. J. Colloid Interface Sci. 1992, 149, 542. (24) Bastos-Gonzales, D.; de Las Nieves, F. J. Colloid Polym., Sci. 1994, 272, 592.
does not, indicating that a rough application of the hairy layer model is definitely inadequate for such a latex and illustrating the paramount effect of the core shell structure on the electrophoretic behavior. Note however that in using eq 3, it is assumed that the presence of a hairy layer does not disturb the ionic structure of the diffuse layer.23 This assumption is more acceptable for DD1, which bears weakly (only amidine end groups) charged hairs, than for DD4 with amine charged groups distributed along the polymer chains. For DD1 latex, an extrapolation to zero ionic strength of the linear part obtained at high salinity gives for ψo a value of 78 mV that, according to the Gouy-Chapman model of the diffuse double layer, leads to a surface charge density of 0.85 µmol m-2 at 0.1 M NaCl in excellent agreement with the experimental value of 0.75 µmol m-2. By use of this ψo value in eq 3, the ∆ value was calculated at each salinity, and the results reported in Table 2 show that (i) at high salinity there is an irreducible hairy layer of 1.5 nm and (ii) at low salinity, where the hairy layer is fully expanded, there is an excellent agreement between the ∆ values from electrokinetic data and those from QELS. So, the particle core size at 20 °C should be near 336 nm. This value is much higher than the dried particle size value of 306 nm derived from TEM measurements. The difference may be attributed either to core shrinkage upon drying or to the presence of a 15 nm thick rigid-like coil globule poly[NIPAM] layer around a nonshrinking polystyrene hard core. As previously indicated, the fixed charge content of DD4 is located within the surface layer. Although successfully applied by Makino et al.25 to the same but negatively charged material, the “soft particle” model for a polyelectrolyte layer of significant thickness and uniform segment density distribution proposed by Ohshima et al.11,12,26 cannot describe our experimental results. Indeed, it was not possible to fit our experimental data using a single pair of parameters, volumetric charge density (zN) and softness parameter (λ). However, it should be noted that Ohshima’s formula worked well at high salinity, giving at 0.1 M NaCl a ψo value of 1.7 mV, very close to the experimental ζ potential value of 1.5 mV, and a “softness” parameter of 0.2 nm, characteristic of a rigid layer. The apparent discrepancy is probably due to ionic strength induced changes in both the structure and the charge distribution inside the surface layer. Moreover, owing to the high reactivity of MBA, the cross-linking density and thus the polymer segment and volumetric charge densities are believed to be nonuniform by being higher at the core surface and decreasing as we move away from the hard core surface. So, the DD4 particle may alternatively be viewed (as illustrated in Chart 1) as consisting of a core (radius rc), a rigid-like cross-linked (25) Makino, K.; Yamamoto, S.; Fujimoto, K.; Kawaguchi, H.; Ohshima, H. J. Colloid Interface Sci. 1994, 166, 251. (26) Ohshima, H. J. Colloid Interface Sci. 1997, 185, 269.
Colloidal Stability of Particle Shell Structures
Langmuir, Vol. 14, No. 18, 1998 5067
Chart 1. Schematic Description of Charge Distribution in DD1 and DD4 Hydrophilic Layers, Respectively
Figure 8. Stability factor (W) as a function of electrolyte concentration (Cs) for DD1 (9) and DD4 (b) latexes at 20 °C (NaCl electrolyte, pH 6.0).
layer (radius rr), and finally a “soft” layer (radius rh) with both rr and rh depending on ionic strength. If only those charges located inside the rigid layer at a distance less than κ-1 from the surface are considered, the surface charge density is obtained at each ionic strength from
σ≈-
3Nr
[ ( )]
rc 4πrr κ 1 rr 3
3
(4)
where Nr is the number of fixed charges, assumed to be uniformly distributed inside the rigid layer. The remaining Ns ) N - Nr charged groups are assumed to be located in the soft layer, where N is the total number of charges. The overall electrophoretic mobility is then
µ e ≈ µr + µs
(5)
Figure 9. Stability factor (W) as a function of electrolyte concentration (Cs) for DD1 (9) and DD4 (b) latexes at 43 °C (NaCl electrolyte, pH 6.0). Table 3. Experimental Values for CCC and Stability Curve Slopes at 20 and 43 °C
where µr and µs are the contribution from the rigid and soft part, respectively. Note that, at high salinity, the contribution from µs should be negligible and eq 5 accounts well for the observed low electrophoretic mobility of DD4 owing to the high value of κ. 3.6. Colloidal Stability Behavior. Figure 8 shows the experimental stability diagrams (log W versus log Cs plots) for DD1 and DD4 latexes at pH 6 and at a temperature below the LCST of poly[NIPAM] chains (20 °C). Similar results of experiments conducted at a temperature above the LCST (43 °C) are shown in Figure 9. As theoretically expected,27 straight lines with negative slopes are obtained at low ionic strengths and horizontal lines (W ) 1) at high ionic strengths. From the transitions between these two coagulation kinetic regimes, marked by the breakpoints in the stability curves, the critical coagulation concentrations of the two latexes were determined below and above the LCST. The obtained values are listed in Table 3 together with the slopes of the stability curves in the slow coagulation regime. The results show that the two latexes display similar stability behavior above the LCST of poly[NIPAM] chains.
Both the CCC values and the sensitivity to ionic strength, measured by the stability curve slope, are similar and of the same order of magnitude as those usually reported for polystyrene latexes (0.11 mol L-1).28 At a temperature below the LCST, the CCC values are much higher and, once again, they are almost similar, although slightly higher for DD4 latex obtained by “shot-growth” polymerization. However, the stability of DD4 latex particles exhibits a much higher sensitivity to ionic strength change below the CCC. This can be accounted as a mere consequence of the more hydrophilic nature of the surface layer, which is expected to undergo a more rapid and greater swelling when ionic strength decreases. The driving forces of the ionic hydrogel shell swelling are the solvent-poly[NIPAM] and the segment-segment interactions ruled by the Flory interaction parameter χ, which increases with increasing salinity of the medium
(27) Sonntag, H.; Strenge, K. Coagulation Kinetics and Structure Formation; Plenum Press: New York, 1987.
(28) Rubio-Hermandez, F. J. J. Non-Equilib. Thermodyn. 1996, 21, 153.
CCC (M)
slope
T ) 43 °C
sample at 20 °C at 43 °C at 20 °C at 43 °C 1020Aeff (J) ψd (mV) DD1 DD4
0.37 0.48
0.12 0.17
-2.79 -4.5
-2.03 -2.05
0.30 0.22
13.9 13.0
5068 Langmuir, Vol. 14, No. 18, 1998
Nabzar et al.
and the osmotic pressure due to the charged groups. This is expected to improve the stability in three different ways: (i) lowering the effective Hamaker constant of the system through the swelling of the shell, (ii) increasing the number of charged groups contributing to the diffuse layer potential, and (iii) creating a non-DLVO additional stabilization term (probably due to a hairy layer at the low cross-linked periphery of the shell). Interestingly, it may be seen from Figure 6 that a linear extrapolation in the high ionic strength range gives zero mobility (or ζ-potential) at an ionic strength of 0.34 M NaCl for both samples; i.e., a value practically equal to the CCC value of DD1 but lower than that for DD4. This suggests that electrostatic repulsions are a prerequisite to stabilize the DD1 while the DD4 seems to owe its high stability, at least between 0.34 M NaCl and the CCC, to the solution properties of its thicker (∼90 nm) and hydrophilic poly[NIPAM] hydrogel layer. Similar conclusions were reported by Snowden et al.22 for poly[NIPAM] hydrogel particles having carboxylate surface charged groups. Assuming a “contact” separation of 0.2 nm, they calculated a “contact” energy of about 1.6 kT and concluded that thermal energy is enough to stabilize their hydrogel particles below the LCST. If the Hamaker constant of the swollen hydrogel is assumed to be close to that of the solvent,22 the effective Hamaker constant of DD4 could be approximated by the theoretical retarded Hamaker constant of the polystyrene core at a separation distance equal to twice the hydrogel layer thickness. At the CCC, the surface layer thickness amounts to ∼40 nm, so that the effective Hamaker constant should be equal to 10-21 J.29 In the case of DD1 sample, the stability at 0.1 M NaCl can be reasonably assumed to rise mainly from the balance between electrostatic repulsions and van der Waals attractions. So, an effective Hamaker constant, Aeff, for this sample can also be estimated using the empirical correlation of stability ratio with the high Φmax of the energy barrier opposing aggregation.30
log(W) ) 0.4
(
Φmax -1 kT
)
(6)
Using the experimental ζ value at 0.1 M NaCl (13.4 mV) in the Hogg et al.31 formula for the electrostatic interactions and the unretarded expression for the van der Waals interactions, Φmax, and thus W (eq 6), can be numerically computed as a function of Aeff. A value of 2.14 × 10-21 J was deduced for Aeff, by adjusting the experimental W value at 0.1 M NaCl. Note that even for polystyrene latexes, the experimental Hamaker constants are found to be much lower than the theoretical ones, ranging from 10-21 to 10-20 J, i.e., vary by an order of magnitude.32 Therefore, although the effective Hamaker constants are given here only by way of illustration, the obtained values are quite acceptable. When the temperature is increased above the LCST, the poly[NIPAM] surface layer shrinks as a consequence of an increase in the Flory parameter χ and hence provides a more favorable segment-segment contact. This was previously shown to result in a decrease in the hydrodynamic diameter and an increase in the electrophoretic (29) Russel, W. B.; Saille, D. A.; Schowalter, W. R. Colloidal Dispersions; Cambridge University Press: Cambridge, 1989. (30) Prieve, D. C.; Ruckenstein, E. J. Colloid Interface Sci. 1980, 73, 539. (31) Hogg, R.; Healy, T. W.; Fuerstenau, D. W. Trans. Faraday Soc. 1966, 62, 1638. (32) Hidalgo-Alvarez, R.; Martin, A.; Fernandez, A.; Bastos, D.; Martinez, F.; De Las Nieves, F. J. Adv. Colloid Interface Sci. 1996, 67, 1.
mobility. At this temperature, the particles may be considered as rigid spheres22 whose stability is ruled by the classical DLVO theory. According to this theory, the slope of the stability curve is related to the potential ψd at the inner edge of the diffuse layer through the approximate relationship by Reerink and Overbeek33 (eq 7) and the CCC related to the effective Hamaker constant through eq 8
p)
∂ log W ay2 ) -Cte 2 ; ∂ log c z CCC ) Cte
y ) tanh γ4 z6Aeff2
( ) zeψd 4kT
(7)
(8)
These equations were used to make an estimation of the pair (Aeff,ψd) for the two latexes at 43 °C. Although eq 7, which states an inverse relationship between particle size and diffuse potential, is subject to controversy and some doubt has been cast on its use,30,34 the values obtained (reported in Table 3) are consistent with the expected trend; i.e., an increase in the effective Hamaker constant is observed for both samples when the temperature is increased above the LCST. Moreover, as expected, the increase is much higher for the DD4 sample, which shrinks by 177 nm (from 535 to 358 nm), while the DD1 sample shrinks by only 46 nm (from 356 to 310 nm). 4. Conclusion The colloidal characterization of core-shell latexes bearing a thermosensitive shell has been investigated by the combination of electrophoretic mobility measurement and colloidal stability as a function of pH, ionic strength, and temperature. The results obtained enabled the charge distribution in the interfacial shell, the structure of the hydrogel layer, and the driving parameter for colloidal stability behavior of such core-shell latexes to be understood. At first, the effect of ionic strength and temperature on the particle size was examined. The particle shell was found to be sensitive to the salinity of the medium and the temperature. This pointed out that salinity drastically decreases the layer thickness of core-shell latex particles. The effect of temperature was found to be more marked than the ionic strength. The phenomenon observed was attributed to the effect of salinity on the LCST of the shell. Indeed, an increase in salinity leads to a decrease in the LCST. Then, the combination of both salinity and temperature reduces the solvency of the shell bearing poly[NIPAM] polymer. Second, the electrokinetic of these core-shell latexes was investigated as a function of pH, ionic strength, and temperature. The electrophoretic mobilities of both latexes were found to be positive in a large pH domain, and the determined isoelectric points were pH 9.5 for amidine and pH 10.5 for amine-containing latex particles. The effect of temperature on the electrophoretic mobility behavior was investigated at a constant pH and ionic strength. In relation with the effect of temperature on the particle size, the electrophoretic mobility was increased when the temperature increased. This was attributed to the increases of surface charge density induced by the shrinkage of latex particles. As regards the effect of ionic (33) Reerink, H.; Overbeek, J. Th. G. Discuss. Faraday Soc. 1954, 18, 74. (34) Tsuruta, L. R.; Lessa, M. M.; Carmona-Ribeiro, A. M. J. Colloid Interface Sci. 1995, 175, 470.
Colloidal Stability of Particle Shell Structures
strength on the electrophoretic mobility behavior, an amazing phenomenon was observed. Both latexes exhibit hairy layers and different electrophoretic profiles versus ionic strength. Therefore, the presence or the absence of the maximum in the electrophoretic mobility versus ionic strength cannot only be discussed in terms of the hairy layer, but the concentration of free interfacial chains should be taken into account. By using Eversole and Boardman’s equation, an estimation of shear plane position was determined from the extrapolation at zero ionic strength. The values obtained were found to correlate with the calculated values from the proposed model charge distribution. Finally, the colloidal stability of these core-shell latexes was investigated using monovalent salt at 20 °C and at
Langmuir, Vol. 14, No. 18, 1998 5069
43 °C. Below the LCST of the hydrogel shell, the particles exhibited electrosteric stabilization; the electrostatic stabilization is derived from the amidine and amine groups, whereas the steric stabilization was from the hairy layer induced by the hydrophilic shell. Above the LCST, the stability of this core-shell latexes was principally attributed to electrostatic stabilization. Indeed, the particles are in a shrunken state at this temperature and the obtained CCCs are lower compared with the values obtained at 20 °C. In addition, the effective Hamaker constants determined at 43 °C for both latexes were lower than those for polystyrene latexes; this was attributed to the hydration of the hydrogel shell even above the LCST. LA980244L
