THE JOURNAL OF
PHYSICAL(a CHEMISTRY (Registered in U. S. Patent Office)
VOLUME66
Copyright, 1962, by the American Chemical Society)
OCTOBER 18, 1962
NUMBER10
THE INTERXAL STRUCTURE OF COLLOIDAL CRYSTALS OF p-FeOOH A N D REMARKS ON THEIR ASSEMBLIES IN SCHILLER LAYERS BY JOHX H. L. WATSON AND R. R. CARDELL, JR., Edsel B. Ford Institute for S4edical Research, Detroit 9, Michigan AND
WILFRIED HELLER
Depai tment of Chemzstry, Wayne State University, Detroit, Michigan Received March 6 , 1969
It is shown how ultrathin sectioning of colloidal crystals can be ap lied to electron microscopical studies of their structure The techni ue is described and applied to tactoid-forming deposits ofp-BeOOH crystals. Cross sections of the single crystals show that &ey are square and remarkably uniform with a side of about 550 A. Their mutual orientation has been maintained during specimen preparation and the crystals are shown t o exist in an orthogonal array. il structure is demonstrated wzthzn the crystals, which is shown to be most probably that of an oriented bundle of loosely packed rods, also in an orthogonal array, wherein the repeating distance is about 60 A . The long rods are themselves grystals and are referred to as “subcrystals,” regularly arranged within the conventional crystal. They are about 30 i 5 A. thick, separated by about the same distance. Electron micrographs illustrate evidence for the substructure from both longitudinally and cross sectioned crystals, as well as unseciioned crystals. An example of fringes 11 A. apart (&lo’%) is shown, in good agreement with the a dimension (10.48 8.)in the tetragonal unit cell. The observations suggest that the crystals grow by reason of tetragonal unit cts11s forming subcrystals in such a way as to promote a uniform rate of growth in the a and b directions and in this sample about 14X more rapid growth in the c direction. There is some evidence to support the view that the subcrystals might be hollow rods. Pertinent Observations also are reported concerning the occurrence of twins and other irregularly shaped crystals (somatoids) among the P-FeOOH crystals.
Introduction The purpose of this paper is twofold: (a). t o show how a very old technique jn the preparation and study of biological and petrographic specimens, thin i~ectioning,adapted more recently to electron microscopy and renamed “ultrathin sectioning,” can be used to give useful information concerning the rnicrostructure of certain non-biological materials, notably inorganic colloidal crystals, and (b) to demonstrate the use of the technique in studying tactoid-forming deposits in P-FeOOH suspensions. The sectioning experiments were undertaken in the presen1 work1 to discover what information might be derived from them concerning (a) the packing of crystals of p-FeOOI12 in the sediments formed by slowly hydrolyzing ferric chloride solutions at room t e m p e r a t ~ r e (b) , ~ the shape and the thickness of the cross sections of the single pFeOOH crystals and their individual volumes, and (1) Presented before the 36th National Colloid Symposium, Palo Alto, California, June 25-27, 1962 The woyk IS part of a larger, con. tinuing project being conducted by Dr. Heller on the properties of schiller layers oE p-Fe0011 and other crystals. ( 2 ) The common name for p-FeOOH is fi-feriac oizde monohydrate. (3) 13. Zocher and W. Keller, Z. anorg. allgem. Chem., 186,73 (1930).
(e) the fine structure, if any, of these crystals From such data one can hope to derive, in conjunction with optical data, the exact equilibrium distances between the p-FeOOH crystals in schiller layers. Until now only approximate values could be given for these4 and they were not sufficiently accurate to be used as a basis for calculations of the interaction energy between these crystalline particles. Conventional transmission electron micrographs (Fig. 1) demonstrate the highly monodisperse nature of the crystals, a fact which will be discussed in detail in a separate publication. They also show that these crystals deviate from usual crystals by reason of their rounded or irregularly shaped ends. They therefore may be related to the crystal habitus classified by Kohlschiitter6 as somatoids, which are defined as crystals of regular internal structure but of more or less irregular external shape. Similarly, the numerous twins observed in these specimens could be referred to as (4) W. Heller, Compt. rend., 201, 831 (1935).
(5) V. Kohlsohlitter, C. Egg,and &I. Bobtelsky, H e l v . C h i n . Acta., 8 , 467 (1926).
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JOHN H. L. WATSON,R. R. CARDELL, JR., AND WILFRIED HELLER
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‘‘schiller layers.”’ These exhihit hrilliant interference colors, which has led to the quoted dcsignation. The individual crystals composing thcsc schiller layers were recognized at an early stage to represent 8-Fe00H.8 More detailed examination@ led to the following dimensions of thesunit cell of 8-FeOOH: a = b = 10.5 A,, e = 3.03 A,, in correction of the dimensions previously givcn by,Milligan and Weiser, a = 5.28, b = 10.24, c = 3.34 A. Milligan and Weiser were the first to identify the prcviously unknown crystalline compound ~ - F C O O I I . ~ ~ More recently, A. L. Mackay” and othersl2J3 confirmed and improved the values given hy Kratky and Nowotny. Mackay’s values are a tetragonal unit cell of a = b = 10.48 and c = 3.023 1. Observations and conclusions made in the present work are consistent with the earlier results but the sectioning experiments add considerably to the information available both on the ultrnstructurc of the crystals and their probable mode of packing within the schiller layers. We have recently published complete d-value data for 8-FeOOII.“ Specimen Preparation The experiment4 were carried oat with colloidal dispersions of 8-FeOOH which contained schiller layers. The
Fig. l.--Unsect.iortcd, nholo crys?:~Is of & F d i o H , X~~l,OOO.80 k v . dcctrons. (In each figure, unless i d . catcd otherwise, tlie line represente 0.1 #.) The insert shows the u p p ~ ~ m n cofe n true twin (clear arrow) conipwed with tlint of overlapping crystals (solid arrow), Xii,OOO.
garticular specimen described in this pnpor was nne which ad m FeCL concentration of 2.34 mmoles/l. and was embedded 326 days after ita origins1 prepamtion.l6 The specimen had a meawred average length of 234 rt 23 mp, an average width of 60 10 mp. nnd an average ratio of length to width of 4.0 rt 0.3. With minims1 disturbance of the layers, the supernatant 801 (nctosol) was removed by pipet. With extreme care, absolute alcohol waq ndded and after 8ome time the supernatant liquid W M removed again. Several such changes were effected O Y C B~ period of 12 l r . in order ta ensure masimum dehydration of the msterisl. The alcohol of the find change was replaced by a 3: 1 mixture of catalyzed n-butyl to methyl methacrylate and several changes of the methacrylate mixture made to ensure complete replacement of alcohol by methacrylate. Polymerization of the methaeryllrte about the crystals WLLS accomplished at
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somatoids. These micrographs give length and width of the single crystalseasily, indicate when there is mere ovrrlnpping aS with twinning, hut are able to tell little or nothing accurately ahout the dimension or shape of the crystal in the direction Of the electron beam‘ By the adA dsfinition of the term “tactoid” is frequmtiy reqursted and a i m ? a mittedly inaccurate method Of “shadow eating,”
