DIFFRACTION O F
LIGHTBY
1881
NONAQUEOUS ORDERED SUSPENSIONS
that calculations might resolve this problem is not very practical because of the infinite number of possibilities.
5. Summary We have presented a spin-free formulation of perturbation theory for interaction energies. We discussed two techniques for treating the symmetry problem raised in section 3. One method utilized explicitly the nonuniqueness resulting from (2.11) while the
1~‘~’)
second method placed certain restrictions on the for n 2 1. The fact that there exist an infinite number of possible perturbation schemes and that there exist no criteria outside of computation for evaluating them, poses a serious problem in perturbation theory for interaction energies.
Acknowledgments. The authors acknowledge helpful discussions and hospitality at the Theoretical Chemistry Institute at the University of Wisconsin.
Diffraction of Light by Nonaqueous Ordered Suspensions
by P. A. Hiltner,* Y. S. Papir, and I. M. Krieger Department of chemistry and the Division of Macromolecular Science, Case Western Reserve University, Cleveland, Ohio 44108 (Received November 1% 19’70) Publication costs assisted by the Public Health Service
A technique is described for resuspending latex particles in nonaqueous liquids. Suspensions of a monodisperse latex in some polar liquids are iridescent and give Bragg diffraction peaks. The particle separation D in the ordered array, the particle diameter DO,and the volume fraction @ obey the relationship ~ ( D / D o ) ~ = 0.74, the value 0.74 being the volume fraction occupied by spheres in a close-packed arrangement. The order is attributed to electrostatic repulsion between particles as a result of partial dissociation of ionic surface groups. Intrinsic viscosity measurements indicate that swelling is negligible in most polar liquids. Suspensions in nonpolar liquids are either unstable or highly swollen and do not show Bragg diffraction.
Bragg diffraction by ordered colloidal suspensions was reported in an earlier paper.l llleasurements on electrolyte-free monodisperse latexes showed that the particles are in a close-packed arrangement which persists throughout the suspension, even when the particles are several diameters apart. The latex particles are charged, owing to bound initiator fragments, and the long-range order is attributed to interaction of the electrical double layers. I n the absence of shielding electrolyte, the electrostatic repulsion was found to be effective over distances of several particle diameters. The magnitude and range of the interparticle potential were varied experimentally by addition of electrolyteq2 The interparticle potential, as well as other properties of the suspension, should depend on the nature of the suspending medium. The present work describes the preparation of stable latex suspensions in nonaqueous liquids and their characterization by optical diffraction.
rene with 0.5-10.0% divinylbenzene was emulsionpolymerized in the presence of both ionic and nonionic surfactants; polymerization was initiated by the thermal decomposition of potassium persulfate. The resulting aqueous suspensions were about 50% polymer by volume and highly iridescent. Particle sizes obtained by electron microscopy ranged from 0.15 to 0.25 p ; the uniformity index (ratio of weight average diameter to number average diameter) was always less than 1.01. To redisperse the latex particles in nonaqueous media, the aqueous suspension was initially deionized by addition of a monobed ion-exchange resin (Amberlite AIB-3, Rohm and Haas Co.) in the ratio of 1 g of resin per 25 g of latex. After 24 hr the resin was removed by filtration. This procedure removes both
Experimental Section Details of the preparation of monodisperse crosslinked latexes have been described previ~usly.~Sty-
(2) P. A. Hiltner and I. M. Krieger, “Order-Disorder Behavior in Monodisperse Colloids,” in “Polymer Colloids,” R. Fitch, Ed., Plenum Press, New York, N. Y . , in press. (3) Y . S. Papir, M . E. Woods, and I. M. Krieger, J . Paint Technol.,
(1) P. A. Hiltner and I. M. Krieger, J . Phys. Chem., 73, 2386 (1969).
42, 571 (1970).
The Journal of Physical Chemistry, Vol. 76, No. 18, 1972
1882 free electrolyte and ionic surfactant from the suspension. The only ionic species remaining are the bound surface charges and an equal number of free hydrogen ions. To promote dissociation of the surface charges in low dielectric solvents, the hydrogen ions were replaced by quaternary ammonium ions according to the following procedure. A column of cation-exchange resin (Amberlite IR-120, Rohm and Haas Co.) was converted into the hydrogen ion form and eluted with a 5% aqueous solution of tetraethylammonium hydroxide until the eluent was basic. The resin in the ammonium form was removed from the column, mixed with the deionized latex for at least 6 hr, and removed by filtration. The latexes were redispersed in organic media, both in the acid form and in the quaternary ammonium form. The aqueous suspension mas first dialyzed against methanol using regenerated cellulose tubing 27/32 in. in diameter. The dialyzate was stirred constantly and replaced daily for at least 1 week. After dialysis, the suspension was diluted if necessary with methanol to yield a solids content of l5-20%, put into fresh dialysis tubing, and immersed in a large excess of the desired solvent. A comparable volume of the medium was often added to the methanol suspension. When the organic medium was miscible with water, identical results were obtained if the methanol dialysis was omitted and the aqueous suspension was dialyzed directly against the organic liquid. Polymer contents of the suspensions were determined by drying accurately weighed samples t o constant weight at 110". Less volatile liquids were removed in a vacuum oven at 150". A value of 1.045 g/cc for the density of the polymer was used to convert weight fraction into volume fraction; densities of the solvents werc: those specified by the manufacturer. Existence of a stable suspension was verified by intrinsic viscosity measurements and by the appearance of the characteristic iridescent diffraction colors which had been observed in the aqueous suspensions. Intrinsic viscosity measurements were made with internaldilution viscometers at 30.08". Flow times were measured by a stop watch, while dilutions were made using weighing burets. The data were plotted as (qr - 1)/ 4 us. 4, where qr is the relative viscosity and 4 the volume fraction; the intrinsic viscosity was obtained from the intercept. For those redispersed suspensions which exhibited iridescent colors, Bragg diffraction maxima were observed, indicating that the particles maintained an ordered three-dimensional arrangement. I n the diffraction experiment, which has been described previously,' the intensity of a beam of monochromatic light reflected from a plane surface of the suspension is measured as a function of the incident angle. The angular position of the intensity maximum is determined at several wavelengths; the angle is related to the centerThe Journal of Physical Chemistry, Vol. 76, No. 18, 1071
P. A. HILTNER, Y. S.PAPIR, AND I. RI. KRIEGER to-center distance between the particles in the array by means of the Bragg equation, modified to include refractive index effects. Electrophoretic measurements were made on a qualitative basis to detect dissociation of the ionic surface groups. A drop of the resuspended latex and a drop of the pure liquid were deposited on a microscope slide. Two thin wire electrodes were inserted, one in each drop, and a thin bridge of solvent was drawn between the drops. A potential was applied, and the movement of the boundary observed with a microscope at 960X ; the polarity was then reversed and the boundary was again observed.
Results Stable suspensions of latex particles were obtained in a wide variety of nonaqueous media (Table I). There was no indication of settling or coagulation even after standing several months. Many suspensions in polar liquids exhibited the iridescent colors characteristic of aqueous suspensions, indicating an ordered arrangement of the particles. The particle separation D in the array, calculated from diffraction measurements, was always significantly larger than the particle diameter DO as determined by electron microscopy. Upon dilution, the diffraction maxima shifted in the direction of larger spacings until a maximum separation was obtained at a volume fraction &. When diluted further, the suspension lost the iridescent colors and no diffraction maxima could be detected. Table I1 gives D values for the same latex in several different media at various volume fractions. Superposition of the data for different particle sizes was obtained when a reciprocal reduced volume vr-l = (D0/D)3was graphed against the volume fraction 4 (Figure 1). For a close-packed array of uniform spheres, the theoretical volume fraction is 0.74. The straight line in Figure 1 represents the equation +vr = 0.74. The correspondence between the experimental points and the theoretical line implies that a closepacked arrangement of the particles persists throughout the entire volume of the suspension. Upon dilution, the particles move apart while maintaining the closepacked configuration. This behavior is analogous to that observed in deionized aqueous suspensions.l There the close-packed order persists in suspensions as dilute as 1% polymer by volume, where the particle separation is four times the particle diameter. I n the aqueous case, the order has been attributed to long-range electrostatic repulsion between the negatively charged particles in the absence of any shielding electrolyte. It must be concluded that the particles in the nonaqueous suspensions are also charged and, therefore, that the surface ionic groups are at least partially dissociated. Additional evidence for the existence of negativery charged particles was given by the moving boundary electrophoresis experi-
DIFFRACTION OF LIGHTBY
1883
NONAQUEOUS ORDERED SUSPENSIONS
Table I : Latex Suspensions in Various Media Medium
(25')
H + form
1.89 2.02 2.21 2.24 2.28 2.38 4.76 4.81 6.02 6.59 11.8 13.1 13.3 17.0 17.4 17.8 18.3 20.1 26.5 32.6 36.7 37.7 46.6 78.5 109 182
Unstable
t
n-Hexane Cyclohexane 1,4-Dioxane Carbon tetrachloride Benzene Toluene Anisole Chloroform Ethyl acetate Methyl benzoate m-Creso1 Benzyl alcohol n-Hexanol 3-Pentanone Acetophenone Benzaldehyde Cyclohexanone 1-Propanol Benzonitrile Methanol DMF Ethylene glycol DMSO Water Formamide N-Methylformamide
+m,
%
(Et)rN + form
17
Unstable Unstable Stable
...
Stable Stable Stable Stable
>50 >50 >50
...
...
...
Stable Stable Stable
20 30 31
...
*..
... ... ... ... ... Unstable Stable
