Langmuir 1988, 4 , 1128-1130
1128
niques. The assembling numbers of AmPcl and AmPc2, which depend on the preparation condition of LB films, are evaluated by using a modified molecular exciton theory as ca. 5-14 for AmPcl and ca. 5 for AmPc2. LB film dichroisms prepared by either the vertical dipping or horizontal lifting can be consistently seen in UV-vis and/or IR regions. Dichroic ratios and ,A, values of &-bands that correlate to the asssembling number are sensitive to LB film preparation conditions, such as film pressures, transfer rate of substrates, additives, and film-transfer techniques. Molecular arrangement and orientation models of AmPcl LB film prepared by vertical dipping are proposed, on the basis of polarized UV-vis and IR spectra, force-area isotherms, and mechanical stylus methods. The transfer steps a t the meniscus, which are involved in the vertical dipping and horizontal lifting techniques, strongly suggest the cause of the in-plane dichroism of AmPcl and AmpC2 LB films. They suggest that dichroic ratios and their phases are independent of the relative position between the moving barrier and meniscus.
Experimental Section Preparation of nickel 4,4’,4”,4”’-tetrakis((n-octadecy1amino)carbony1)phthalocyanine (AmPcl) and nickel 4,4‘,4‘‘,4’‘’-tetrakis(n-nonadecanoy1amino)phthalocyanine(AmPc2)was described in the literature.’, Kyowa Kaimen Kagaku (Model HBP-AP) was used as a film balance and consists of a Teflon-coated trough, a Wilhelmy-type glass float, and vertical dipping and horizontal lifting units. Subphase temperature was regulated at 18 “C by using a Lauda RCS2O thermostat. Doubly distilled pure water was used as the
subphase. Optically polished fused quartz and CaFzwere used as solid substrates. Hydrophobic quartz was obtained by treatment with octadecyltrichlorosilane. A CHCl, solution containing either AmPcl or AmPc2 and/or other additives was passed through a Millipore 0.45-wm filter and spread on water. Two techniques were employed for building up films at a desired constant film pressure. One is the conventional LB technique in which only Y-type multilayers are deposited by the vertical dipping technique. The other is a modified LB technique in which only X-type multilayers can be formed by the horizontal lifting technique with a small tilt angle.I6 The transfer ratio of AmPcl by the vertical dipping LB method was equal to unity. Electronic absorption intensities of the built-up f i i s were proportional to the dipping or lifting number. An LB film thickness per layer was evaluated by using a mechanical stylus method (Sloan, model dektak 3030; stylus force, 1 mg; stylus radius, 12.5 pm) to obtain the film thickness of a multilayer. Electronic absorption spectra and their second-derivative spectra were recorded by using a Hitachi 330 UV-vis/near-infrared spectrophotometer. IR absorption spectra were recorded with a Perkin-elmer 1800 FT IR spectrometer. Polarized electronic and IR absorption spectra, in which the incident angle of the light beam is normal, were recorded by using a Glan prism, a gold wire grid polarizer, and a goniometer. In order to obtain an absorption spectrum of the film, the difference spectral technique was employed with a single-ratio technique. In the method, two spectra, of a substrate and of the substrate with film stored in the respective memories, and an absorption spectrum of the film was calculated from them. Registry No. AmPcl, 106725-59-1;AmPc2, 99144-77-1. (16) Nakahara, H.; Fukuda, K. J . Colloid Interface Sci. 1979, 69, 124-133.
Domain Size in Ordered Packing of Monodispersed Particles S. Y. Chang and T. A. Ring* Department of Chemical Engineering, University of Utah, Salt Lake City, Utah 84112 Received May 18, 1987. I n Final Form: April 14, 1988 Diffraction line broadening has been adapted to light diffraction to measure ordered domain size. Domain sizes of -25 pm were observed for TiOz powders and of -40 pm for SiOz powders. Scanning electron microscopy was used to confirm the ordered domain size of both SiOz and TiOz.
Introduction Using light diffraction, Hiltner and Krieger’ showed that the interparticle spacing in an ordered packing of latex particles could be obtained by using the Bragg equation (commonly used for X-ray analysis of crystalline materials): nX = 2d sin 8 (1) where n is the order of diffraction, X is the wavelength of the incident light, d is the distance between the diffracting planes, and 0 is the angle between the incident light and the diffraction planes. In the usual experiment X is known and 0 is measured, allowing the calculation of d when n is assumed to be 1. In this paper we extend a phenomena called line broadening in X-ray diffraction to light diffraction. Line broadening is used in powder X-ray dif(1)Hiltner, P. A.; Krieger, I. M. J.Phys. Chem. 1969, 73,2386-2389.
fraction to determine the crystalite size; the smaller the diffracting crystals the broader the diffraction peak.
Theory The first treatment of line broadening was due to Scherrer,2 who derived the equation kX p(28) = L cos 0 where /3 is the peak breadth a t half-height in radians, L is the crystal size (the volume average crystal dimension perpendicular to the diffracting planes), and k is a constant about which there has been considerable disagreement. Values given include 0.94 (Scherrer, 1920), 0.89 (Bragg, 1933), 0.92 (Seljalkow, 1925), and 1.42 (Laue, 1926; Jones, (2) Scherrer, Kolloidchemie by Zsigmondy, 3rd ed.; Leipzig: Berlin, 1920; p 394.
0743-7463188/ 2404-1128$01.50/0 0 1988 American Chemical Society
Langmuir, Vol. 4, No. 5, 1988 1129
Domain Size in Monodispersed Particles 1938).3 The value of k depends on the shape of the crystals and the particular reflection for nonspherical particles. For spherical crystals made of spherical particles, Stokes and Wilson3 gave k = 1.0747. Since all of these values of k are very near 1.0, most users of this theory simply neglect k in eq 3 and refer to L as the crystallite size. From the Scherrer equation, one can see that for a given crystal size the broadening becomes most pronounced a t high values of 8. In addition, X-ray line breadth contains information on latice strain (due to impurities), lattice defects, and thermal vibrations of the crystal structure. The chief problem to determine crystallite size from line breadth is the determination of P(28) from the diffraction profile, since broadening can also be caused by the instrument. To correct the instrumental broadening in the pattern of the sample, it is convenient to run a standard peak from a sample in which the crystal size is large enough to eliminate all crystallite size broadening. The standard peak is run under instrumental conditions identical with those of the sample, so that the broadening of the standard can be equated with the instrumental broadening. By use of a convolution integral method3 with these two patterns, the crystalite size broadening can be isolated. However, with these ordered colloid experiments a large “crystal size” standard is not available. Attempts to determine the broadening due to the instrument optics are detailed. In order to investigate the domain size effect in the ordered packing of monodisperse colloidal particles, a HeNe laser analogue to an X-ray diffractometer is used. As with X-ray diffraction, light diffraction line broadening will contain information on the lattice strain caused by defects and thermal vibrations. In contrast to X-ray diffraction, light diffraction line broadening will contain additional information on lattice strain due to a nonuniform particle size distribution. However, the effects of all types of latice strain will be modified by the presence of the fluid between the particles. To verify this theory for colloidal suspensions we will use data that are contained in E. Barringer’s Ph.D. T h e ~ i s .These ~ data are not published in the open literature. For this reason a short discussion of his experimental procedure is included.
Experimental Procedure The apparatus was based on the diffraction system described by Hiltner and Krieger.’ The dispersion was illuminated with a 5-mW HeNe laser (X = 632.8 nm) having a 0.8-mm beam diameter (Coherent 80-2H, Palo Alto, CA), and the reflected intensity was measured by a photodetector (PIN 5D, United Detecter Technology, Culver City, CA). These elements were mounted on supports that were connected to an alignment system that maintained an identical angle for the source and the detector relative to the cell. The cell was constructed of Delrin and is schematically shown in Figure 1. A solid glass half-cylinder (Pyrex) was used as the cell window to prevent beam refraction prior to striking the glass/suspension interface. The laser light entered the cell along the radii of the semicylindrical window, refracted at the glass/suspension interface, diffracted from the particle array, and exited the suspension with one final refraction at the interface, shown in Figure 1. An attempt to determine the instrument broadening was made by measuring the broadening when the suspension was replaced by water. This gave a broadening of 0.001 rad at the critical angle of reflection for the cell. This small value for optics broadening had a negligible effect on the results for ordered domain size. (3) Lispon, H.; Steeple, H. Interpretation of X - R a y Powder Diffraction Patterns; Macmillan: London, 1970; p 261. (4) Barringer, E. A. Ph.D. Thesis, Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA, 1983.
8ii‘...
.....
...‘ 8
+-&p~L.++~
e‘
Ordered dispersion
Figure 1. Schematic of diffraction cell with the beam path through the window into the dispersion. From B a r ~ i n g e r . ~ The Si02powder was synthesized by using the Stober et al.5 method. The TiOz powder was s y n t h e ~ i z e dby~ ~ using ~ the initial reagent concentrations of 0.15 M Ti(OC2H5), and 0.5 M HzO. After being formed, the powders were washed with water for purification and stored in a COz-free environment. Solutions of KC1, KOH, and HCl were prepared by using analytical grade reagents and deionized water. Except for the HCl solution, these solutions were outgassed with N2 to remove absorbed COz and then were stored in a C02-free environment. Sample preparation for the diffraction cell was conducted in a glovebag under N2 atmosphere. The cell volume was calibrated to be 7.0 mL. The cell was filled, by a pipet, with the powder dispersion extracted from the ordered portion of the purified powder sediment. The cell was sealed, and the particles were allowed to settle below half of cell volume (Le., 3.5 mL). The clear supernatant at the top was removed until the meniscus was at the 3.5-mL mark, and then 3.5 mL of a 0.0001 M KCl solution was added to the cell. An additional two drops of 0.01 M KOH solution (pH 12) was added to the cell to raise the pH above 7.0, which enhanced the repulsive interparticle interaction. The cell was resealed, and particles were allowed to settle until a stable equilibrium condition was reached (i.e., no change in sediment volume over a 24-h period). Then fmt-order (0,) and second-order (0,) diffraction peaks, as well as the critical angle (03 were recorded for several positions in the sediment.
Results Figure 2 shows a scan of detector output voltage versus 8 for ordered TiOz in pH 9.0 and 5 X M KCl (-54 mV (5) Stober, W.; Fink, A.; Bohn, E. J. Colloid Interface Sci. 1968, 26, 62-69. (6) Barringer, E. A.; Bowen, H. K. Langmuir 1985, I , 414-420. (7) Jean, J . H.; Ring, T. A. Langmuir 1986, 2, 251-255. (8) Barringer, E. A.; Bowen, H. K. J. A m . Ceram. Soc. 1982, 65, ~199-~201. (9) Jackson, J. D. Classical Electrodynamics, 2nd ed.; Wiley: New York, 1975; p 282.
Chang and Ring
1130 Langmuir, Vol. 4, No.5, 1988 io: 632 8 nm
nD = 1.474
IO
20
30
40
50
M)
m
Angle.8 (degrees) Figure 2. Scattered intensity (detector output voltage) versus scatering angle for the ordered Ti4and the ordered SiOzshowing the fust and second-order diffraction peaks and the critical angle for total internal reflection. From Barringer.'
Figure 3. SEM analysis of tup surface of a SiOzordered structure (bar = 5 pm).
Table 1. Results lor S O l
ordered detector output, V 8, deg 33.8 61.2
8'. deg 28.5 60.8
base line peak height @(28).rad 0.05 0.05
3.8 0.267
0.01745 0.03316
domain size, wm 43.6 39.6
Table 11. Result8 for TiO, ~~
detector output, V
e. deg 8', deg base line peak height 8(28), rad 27.6 56.4
23.2 55.1
0.17 0.17
0.61
0.28
0.0262 0.0436
ordered domain
size, wm 27.3 26.2
(potential, 0.119 volume fraction, D spacing 0.717 pm) and ordered Si02 in pH 7.0 and lo4 M KC1 (-70 mV ( potential, D spacing 0.583 pm). Both scans were performed a t the top of the ordered sediment, -0.2 mm below the top of the ordered region. Since the light is refracted a t the window-suspension interface, Snell's law must be applied to relate the goniometer angle 8 to the true diffraction angle 8': n. sin ( r / 2 - 8') = n, sin ( r / 2 - 8 ) (3)
Figure 4. SEM analysis of top surface of a TiO, ordered structure (har = 1 pm). From Barringer!
smaller than the -40 pm measured with line-broadening experiments. However, this is only a top surface view of the particle packing; an interior view may reflect a different ordered domain size. The TiOz packing, shown in Figure 3,has only one ordered domain visible in the 9 X 13 pm view of the packing. Line-broadening experiments gave a -20-pm ordered domain size for TiOz, which is beyond the view of Figure 3.
where n, is the refractive index of the cell window (1.474 Conclusions for Pyex at 632.8 nm) and n, is the refractive index of the dispersion. n, can be evaluated from the critical condition Line broadening has been adapted from X-ray analysis of total internal reflection, 8,. by the following f o r m ~ l a : ~ for use in light diffraction to measure ordered domain size. Ordered domain size was measured for both TiOz and Si02 n. = nr sin ( r / 2 - 8,) (4) monodisperse powders produced by the hydrolysis of alAn analysis of this data using eq 2 is shown in Table I for koxides and sedimented in agueous salt solution. The SiOz and in Table I1 for T i 0 2 The ordered domain size ordered domain sizes measured for TiOz and SiOz by line calculated for Si02 is -40 pm and that for TiOp is -25 broadening were similar to those observed in SEM pictures pm. These results are very similar for both the first and of the top surfaces of ordered packings of these particles. second diffraction peaks. Acknowledgment. This work was funded by the NaScanning electron m i c r m p e (SEM) analysis of the SiOz tional Science Foundation Contract number 8617500 and and T i 0 2 ordered structures is shown in Figures 3 and 4, uses experimental data from E. A. Barringer's Ph.D. therespectively. For SiOpvarious ordered domains oriented sis.' in different directions are shown in Figure 2. These orRegistry No. TiOz, 13463-67-7; SOp,7631-86-9. dered domains vary in size from 5 to 20 pm. which is
