Langmuir 1989,5, 816-818
816
tion. The ability to strengthen the hydrophobic effect depends on the protein concentration and alcohol chain length. The effect increases for a given alcohol with decreasing protein concentration. For a given protein concentration, two characteristic regions appear in the ability to strengthen HI as a function of alcohol chain length. In the first region, the effect increases linearly with increasing chain length up to octanol, with the ability tending to level off at dodecanol. In the second region, the effect increases sharply and linearly from tridecanol to hexadecanol. No such discontinuity or curvature was observed in the corresponding effect in a surfactant solution as a function of alcohol chain length.
Taking the effect in the first region to be representative of the effect anesthetic agents have on HI in the ionic or hydrophilic channels in membrane proteins, the "cutoff" effect in anesthetic potency in homologous alkanols is rationalized.
Acknowledgment. This research was supported by the University of Kuwait Grant no. SC027. Registry NO.Ethanol, 644-17-5;l-prOpanOl, 71-23-8; 1-butanol, 71-36-3; 1-hexanol, 111-27-3; 1-tetradecanol, 112-72-1;l-pentadecanol, 629-76-5; 1-octanol, 111-87-5; 1-decanol, 112-30-1; 1dodecanol, 112-53-8; 1-tridecanol, 112-70-9; 1-hexadecanol, 36653-82-4;urea, 57-13-6.
Particle Sizes and Electrophoretic Mobilities of Poly(N-isopropylacrylamide) Latex R. H. Pelton,*it H. M. Pelton,? A. Morphesis,* and R. L. Rowells McMaster University, Department of Chemical Engineering, Hamilton, Ontario, Canada, L8S 4L7,and Department of Chemistry, University of Massachusetts, Amherst, Massachusetts 01003 Received September 12,1988. In Final Form: January 25, 1989 Average particle diameters and electrophoretic mobilites of poly(N-isopropylacrylamide) latex were measured as a function of temperature. Diameters decreased from 788 nm at 10 "C to 380 nm at 50 "C in 0.001 M KC1; the corresponding electrophoreticmobilities increased from -0.193 X lo4 (18 "C) to -3.06 x lo4 m2V-' s-' (47 OC). The most dramatic changes with temperature occurred around 31 "C, the lower critical solution temperature of poly(N4sopropylacrylamide) in water. The increased electrophoreticmobility with temperature reflected increasing charge density when the particle diameter decreased. Charge density increased with decreasing particle diameter because the number of charged groups per particle was constant.
Introduction We have been interested in the development of sterically stabilized aqueous latexes which can be flocculated by heating. As a first step, latexes based on cross-linked poly(N-isopropylacrylamide)(polyNIPAM) were prepared by surfactant-free emulsion polymerization.' The term latex is perhaps misleading for a t room temperature polyNIPAM is water soluble and the polyNIPAM-based particles appear to be water-swollen colloidal gel particles. In the absence of added electrolyte, the latex was colloidally stable over an extended temperature range, whereas in the presence of 0.1 M CaCl,, the latex coagulated when the temperature was raised above 31 "C, thus demonstrating the desired temperature-sensitive colloidal stability. The temperature sensitivity of polyNIPAM latex properties is a reflection of the lower critical solution temperature (LCST) of 31 "C for polyNIPAM in water. From the coagulation behavior, it was concluded that below the LCST the latex particles were colloidally stabilized both by steric stabilization and electrostatic stabilization whereas above the LCST only electrostatic stabilization was operative. On the basis of imprecise electron micrographs and turbidity measurements, it was postulated that the polyNIPAM latexes underwent significant reduction in particle size as the temperature rose above the LCST. In this work, particle size and electrophoretic mobilities of polyNIPAM latex are reported as a function of temperature.
'McMaster University.
* University of Massachusetts. 0743-7463/89/2405-0816$01.60/0
Table I. PolvNIPAM Latex Reciw' NIPAM 14 g L-' methylenebis(acry1amide) 1.4 g L-' potassium persulfate 0.83 g L-' distilled water 720 mL at 70 O
C
Experimental Section All measurements were made by using a latex prepared by the surfactant-free emulsion polymerization of N-isopropylacrylamide-see Table I. More details of monomer purification and latex polymerization along with electron micrographs of this latex are given in ref 1. Latex dispersions were cleaned by successive ultracentrifugation, decantation, and dispersion in doubly distilled
water. Particle size measurements were made with a h4alvern M2OOO photon correlation spectrometer (PCS) and a Spectra Physics 124B, 15-mW helium-neon laser. All data were collected at a scattering angle of 90° and at one latex concentration. Electrophoretic mobilities were measured by the Pen Kem System 3000 electrokinetic analyzer. Care was taken to ensure that temperature equilibrium was reached before recording measurements.
Results and Discussion Particles sizes are shown in Figure 1 as a function of temperature. The diameters of particles dispersed in 0.001 and 0.01 M KC1 gradually decreased from 788 nm at 10 "C to 604 nm at 32 OC, after which the particle size dropped to 492 nm over a range of 3 "C. Above 40 OC, the diameter decreased very slowly with increasing temperature. There was little distinction between the two KC1 concentrations. (1)Pelton, R. H. Colloids Surf. 1986,20, 247.
0 1989 American Chemical Society
Langmuir, Vol. 5, No.3, 1989 817
Characteristics of Poly(N-isopropylacrylamide)Latex 0,
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Figure 2. Electrophoretic mobility of polyNIPAM latex as a function of temperature.
At low temperature, the particle diameters were lower in 0.1 M CaC12,with a value of 569 nm at 10 "C decreasing to 457 nm at 31 "C. Above 34 "C, the diameters increased dramatically and were irreproducible. This behavior is an indication of latex coagulation above 34 "C. Electrophoretic mobilities of the polyNIPAM latexes were measured as function of temperature, and the results are shown in Figure 2. At 20 "C, the mobility in 0.001 M KC1 was -0.17 X lo4 m2 V-' s-l and increased slowly with increasing temperature up to 32 "C. Further increasing temperatures gave a dramatic increase to -2.8 X m2 V-' s-l at 42 "C. In 0.1 M CaC12,the absolute value of the mobility also increased with increasing temperature; however, the magnitude of the increase was much less. The current results are consistent with the picture of polyNIPAM latex behavior presented previously. That is, as the temperature increases past the cloud point of polyNIPAM in water, the latex particles expel water and contract. Temperature-dependent swelling was postulated on the basis of indirect evidence, whereas the results in Figure 1 show directly that particle size is a function of temperature. It is of interest to consider why the particle size decreased in the temperature range 10-30 "C, which is below the LCST of polyNIPAM. Heskins and Guillet reported ultracentrifuge studies of polyNIPAM dissolved in water. Their results showed that the reciprocal of the sedimentation coefficients decreased linearly by a factor of 2 when the temperature was increased from 20 to 33 0C.2 Therefore, at temperatures 10 "C below the LCST, the polymer coils appear to undergo significant contraction with increasing temperature, which is consistent with the particle size results in Figure 1. The same authors reported that the viscosity molecular weight of their polyNIPAM in water increased nearly by 2 when the temperature was increased from 25 to 33 "C. They attributed this behavior to the association of polymer molecules below the LCST. In the case of polyNIPAM particles, such association behavior would appear as flocculation below the LCST-there was no evidence of this even in the presence of 0.1 M CaC12. The influence CaCl, on particle size can be understood in terms of electrostatic effects. The polyNIPAM particles
were prepared by a method analogous to the surfactantfree polymerization of styrene, which results in polymer particles stabilized by sulfate surface charge groups originating from the potassium persulfate initiator. Thus, it is expected that many of the end groups on the polyNIPAM chains will be sulfate groups. The negative electrophoretic mobility exhibited by the latex is evidence for negatively charged groups, which we presume are sulfates. I t is reasonable to expect that the sulfate end groups will contribute to the swelling and expansion of the polymer chains and that these electrostatic effects will be retarded by the presence of high electrolyte concentrations. The electrophoretic mobility of the NIPAM latexes increased dramatically with decreasing particle diameter, and it is of interest to relate this behavior to electrokinetic models. PolyNIPAM latex is unusual because the total number of charges per particle is constant in spite of large changes in particle diameter with temperature. By contrast, latexes with a polyacrylate surface layer will show large changes in mobility with pH; however, the total number of dissociated carboxyl groups per particle also varies with P H . ~ To apply electrokinetic models to the present results, it is necessary to have information about the spatial distribution of charges in the latex particles as a function of temperature. I t has been demonstrated that in the case of persulfate-initiated emulsifier-free polystyrene essentially all the covalently bonded sulfate groups are located at the polystyrene/water interface. Presumably, this situation arises because the sulfate groups are very incompatible with polystyrene and there is sufficient polymer chain mobility during the polymerization to allow charge migration to the interface. By contrast, the core of polyNIPAM latexes should be quite hydrophilic, even at 70 "C, the temperature during latex synthesis, and it is possible that many sulfate groups are located in the particle interiors. The charge distribution is likely to fall between the followingtwo extremes: (1) charge is uniformlydistributed throughout the particle volume, and when the diameter decreases at elevated temperatures the volumetric charge
__ (2) Heskins,M.; Guillet, J. E. J.Mucromol. Chem. 1968, A2(8),1441.
(3) Buscall, R.; Corner, T.; McGowan, I. J. In Effect of Polymers on Dispersion Properties;Tadros, Th., Ed.; Academic Press: New York, 1982; p 379.
818 Langmuir, Vol. 5, No. 3, 1989
Pelton et al.
density increases; ( 2 ) all charge is located near the hydrodynamic surface of the particle, and when the latex diameter decreases the surface charge density increases. The uniform charge model can be applied to theories developed for the electrophoretic behavior of polyelectrolytes" and polyelectrolyte-coated particles."' For example, the Hermans and Fujita expression for a large polyelectrolyte as given by Buscall et aL3is shown in eq 1. Note eq 1does not include the factor 0.35 given by Buscall et al. to account for counterion condensation because counterion condensation is unlikely to be operative in the low charge density polyNIPAM latex system
3 ~ +~2 b~ i-bb3~ b = f 3~~ + 3 ~ ~ b where ~1 is the electrophoretic mobility, e is the electronic charge, f is the friction factor per repeat unit, K is the Debye screening parameter, b = ( f c / ~ ) " ~c ,is the number of charge gioups per unit volume, and 9 is the viscosity. In the application of eq 1 to the current results, there are two unknowns: N , the total number of charges per particle, and R, the hydrodynamic radius of the repeat units. f is calculated from R by Stokes' law, and c is obtained by dividing N by the particle volume. Note that f and c are temperature dependent, whereas R and N are assumed not to be. The case in which all the charges are located on the hydrodynamic plane (i.e., case 2 above) can be described by the following simple model, assuming all the charges are located on the surface of the hydrodynamic equivalent sphere:
-e 3 K 3 -
where N is the number of sulfate groups per particle, r is the particle radius, and u is the charge density. Assume charge density is related to potential by the Helmholtz formula (i.e., potential is low):
= t$K@o (3) where to is the permittivity of a vacuum (SI units), D is the dielectric constant, and is the surface potential. Combining eq 2 and 3 gives -Ne 00 = (4) qpK(47rr2) U
Finally, assuming that the c potential equals the surface potential and that electrophoretic mobility is related to the c potential by the Smoluchowski equation p=-
4% 9
(5)
Combining eq 4 and 5 gives
( 4 ) Hermans,J. J.: Fuiita, H. Kon. Ned. Akad. Wetensch h o c . Ser. B 1955,58, 182. ( 5 ) Jones, I. J. J. Colloid Interface Sci. 1979,58, 451. (61 Ohahima. H.:Kondo. T. J. Colloid Interface Sci. 1987. 116.305. (7) Sharp, K: A.; Brooks, D. E.Biophys. J . i985,47,563.
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