Linseed Oil at the Surface of Titanium Dioxide Pigment Study by High Magnification Electron Micrography WILLIAM R. LASKO and LAWRENCE S. WHITE Research Laboratory, Titanium Division, National Lead Co., Sayreville,
N. J.
to almost complete dryness with a glass muller 3 inohes in diameter on a 12-inch square of plate glass. A spatula was used to eolloct the mixture in the center of the glass plate, and amyl acetate was added to compensate for that lost by evaporation. The mulling process was then repeated; in mast eases the reuisite degree of dispersion was attained bv mulline the mixture %re, times to near dryness After the final mulling, amyl acetate was added in sufficient
Eleotmn micrographs of particulate materials often give a very fuzzy appearanoe which cannot he completely assooiated with the intrinsic surface of the material. By means of a high magnification technique it has been found that suoh surface characteristics may he due to the prerenoe of adsorbed dispersant. A study has been made of the surface roughness of titanium dioxide pigment particles, as well as the fuzziness developed upon the addition of various amounts of unpolymerized and polymerized linseed oils, used as dispersants. In some instances the nonuniform adsorption of these oils indicates the heterogeneous nature of the particle surface. The need for the use of controlled amounts of dispersant when studying the intrinsic surface oharaeteristies of particulate materials is indicated. Specimen contamination by the action of the electron beam is shown to he absent under the oonditions employed in this study.
I
N T H E electron microscopy of many particulate materials
the edges of the particles have often seemed to have a rough or fumy appearance. For example, Schuster and Fullam (4) have noted that the use of dispersing oils to achieve adequate dispersion in the microscopy of pigments produces fuzziness around the edges of the pigment partioles. I n studies with titanium dioxide pigment, various electron micrographs have revealed roughness and fuzziness an the peripheries of the pigment partioles. Titanium dioxide is a white pigmentary material of high hiding power, widely used in the paint and allied industries. This pigment consists of crystalline particles having a narrow range of skes centered a t about 0.2 mioron. The well defined surfaces and crystalline nature of, this material make it well suited for use in the study of surface structure 8s differentiated from surface fwziness imparted by dispersants. In order t o make visible the detail required for this study, it was necessary t o employ a technique for achieving a final magnification of 2OO,OOOX, which is higher than any of those normally used in most electron microscopy. This paper deals with an exploratory study of some examples of the phenomena described above, undertaken with the view that the techniques employed could be of considerable value in the microscopy of particulate materhls, and indicates the import.ance of using oontrolled amounts of dispersant when studying the surface characteristics of such materials.
rnater solution. The function of this coating was to enable the water employed in stripping to penetrate between the nitrocelhlase film and the glass slide. Before the solvent from the pigment dispersion had evaporated; the film on the glass slide was made uniformly thin by drawing down with the straight edge of a spatula. The film was dried in a vertical position, and one end of the microscope slide was wiped clean of film to facilitate subsequent stripping. Who" drying had been corn leted, the cleaned end of the slide was immersed in a Petri dis! filled with distilled wrLter, care being taken that only the edge of the film was in contact with the water. . The slide was held in a tilted position a t an angle of approximately 75' to the vertical. After a few seconds in the water. the edre of the film came free, followed by a major portion of thk film. -The free part of the film floated o n the-sufface of the water, the remainder being attached to the glass slide, where it R&S not immersed in the water. Standard specimen grids i200-mesh. woven stainless steel wire disks. l/a inch in diameter) &ere placed on the slide beneath selected areas of the film. The
SPECIMEN PREPARATION
The objective of specimen preparation in the electron microscopy of pigments is to obtain a well dispersed sample in a thin supporting film, The film should he thin, so that maxbnwn contrast and optimum resalutinn can be attained in the electron micrograph; the thickness is determined hy the requirement that the film shalk not rupture in the electron beam. For all the preparations except those for which no oil was used, the required quantity of oil was added to a weighed amount of pigment (-100 mg.) by means of a calibrated dropper from a 0.3% solution of the oil in amyl acetate. After the pigment had been mixed with the oil solution, i t was mulled with a 2% golutian of nitrocellulose (Parlodion) in amyl acetate with addit~onal amyl met& as required to produce a workable mixture. Where no oil was used, the procedure began with the mulling of the pigment with the nitrocellulose solution. The mixture was mulled
Figure 1.
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Electron Miomgraph of broup of rigment Particles
Firstands-ndataqeaol ~ n I n = p ~ m . n t a t M , O W X ( i ~ =and ~ ) 2W.WOX. Arrows represent 0.1 micron. F u ~ ~ i n e aarisea s from p~emenceof oil used for dispersion
1631
ANALYTICAL CHEMISTRY
1632
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slide was then withdrawn from the water so that the film wa8 spread smoothly upon the slide and covered the specimen grids; during this operation care was taken that the specimen grids did not slide beneath the film from the select.ed locations. Aft,er
eter was used in order to increase the contrast in the f i n d image, The microscope wm carefully aligned, so that both voltage and magnetic centers were very close together; the alignment was checked a t frequent intervals. All critical parts of the electron optical system (apertures, pole pieces, specimen holder, etc.) were cleaned a t frequent intervals in order to maintain optimum resolution. The focusing magnifier ws.8 used as an aid in setting the controls for focus and then, to ensure that optimum focus was attained in one of the five frames of the photographic plate, a focal series was made including settings above and below that selected with the aid of the focusing magnifier. Kodak lantern slide medium d a t e s were used and develomd in Kodak Versstol
V O L U M E 26, N O . 10, O C T O B E R 1 9 5 4 developer (1 part of Versatol to 3 parts of water). The electron micrographs were taken a t an initial electronic magnification of 4000X; were made optical enlargement tono 8 magnification prints 40,000x, Theseby were photographed change in magnification, and the resulting negatives were printed with enlargement to a final magnification of 200,OOOX.
As an illustration, Figure 1 shows electron micrographs of a particular group of pigment particles a t magnificationsof40,00OX and 200,OOOX. Comparison of these electron micrographs clearly shows the increased detail available st the higher m a g 6 fi(cation
1633 (- 30 seconds) and a 15-minute exposure to the electron beam. (The intermediate of the series are indistinguishable from these two.) There is no evidence of specimen contamination in these micrographs. In addition, examination of many electron micrographs of pigments supported in nitrocellulose films has revealed no signs of contamination, This lack of contamination might, in part, be attributable to the method of specimen preparation, since Watson ( 5 ) has reported that much less contamination is abserved with carbon black when the specimen is mounted in t,he
SPECIMEN CONTAMINATION
Under certain conditions electron microscope specimens are ubject to contamination from the effects of the electron beam 1-5, 5). For example, in a micrograph of zinc oxide smoke reiorted by Ennos (2, Figure 1, eta, a t a magnification of Z0,OOOX) here is evidence of gross contamination after a 15-minute exPlosure to the electron beam. It was therefore important that t,he contaminating conditions existing in the microscope be studit:d, as the presence of such contamination might easily result in t,he misinterpretation of surface irregularities.
Figure 5. Electron Micrograph of Pigment A with 1.5% Unpolyrnerized Linseed Oil, 200,OOOX
I n view of results with both types of specimen mounting, filmless (zinc oxide smoke) and in the film (pigments), there seems to be no doubt as to the validity of the surface features described below. PIGMENT PARTICLE SURFACE FREE O F OIL
IPigure 4. Electron Micrograph of Pigment A with 1.5% Unpolymerized Linseed Oil, 200,nnn x
As a test of contaminating conditions zinc oxide smoke was fumed onto a standard stainless specimen grid and a series of electron micrographs, also a t a magnification of 2O,OOOX, was obtained of the same field, the first one after the minimum exposure to the electron beam necessary to locate the field and focus (- 30 seconds), and then the remainder a t &minute intervals during a 15-minute exposure to the beam. The intensity of the beam on the specimen was higher than that employed for making micrographs of the pigments described below. Figure 2,A and B, shows the first and last of the series of micrographs of this zinc oxide; it represents, respectively, minimum exposure
In order to examine the extent of interaction between pigment and oil it is necessary to have some knowledge of the appearance of the particle surface free of oil, 60 that differentiation may be made between the roughness inherent in the particle surface and the fuzziness produced by ail. The natural or inherent roughness of the surfaces of titanium dioxide pigment particles may vary, but this roughness is distinct from that produced by oils. Such roughness, however, cannot be unambiguously studied by electron microscopy unless the pigments are prepared by mounting them free of oil within the nitrocellulose film, using the technique described. As an example, two pigments mounted free of oil in a nitrocellulose film are shown in Figure 3,A and B . In the case of pigment A the surface is seen to be smooth, while pronounced roughness is present on the surface of pigment B. For pigment A the grey areas of the edges of some of the particles (indicated
1634
ANALYTICAL CHEMISTRY
by arrows) are apparently thin enough to allow electrons to be transmitted. These gray areas are seen to he different in structure from the fuaeiness associated with the presence of oil a t the surface of pigment particles. This smooth surfaced pigment (pigment A) was employed in the experiments described below, in which oil was added to the pigment during preparation for electron microscopy. PIGMENT PARTICLE SURFACE IN THE PRESENCE O F OIL
0.5% Unpolymerized Linseed Oil. In order to study the nature of the fuzziness produced a t the surface of pigment particles in the presence of oil, the smooth surfaced pigment shown in Figure 3,A (pigment A) was employed. For this study i t was estimated that ahout 1% unpolymerized linseed oil (by weight on a pigment basis) was required to form a monolayer on this pigment, considering it to have an average particle size of about 0.2 micron. At surface coverages of less than a monolayer, adsorption of the oil molecules would he more likely t o occur a t the more active or bigher energy portions of the surface. To observe this 0.5% ail was used for initial electron micrographs. The eleotron micrograph of pigment A using 0.5% unpalymeriaed linseed oil is shown in Figure 4. (This is a varnish oil refined by mechanical bleaching; the Gardner-Holdt viscosity is A and the acid number is 2 to 3.) The surface of the pigment is partly covered with adsorbed oil which extends from about 20 A. t o about 40 A. from the surface. There m e portions of the surface where the oil appears more concentrated, with the result that the thickness of the adsorbed layer appears to be greater. The particle inserted in the lower left-hand portion of Figure 4 is taken from Figure 3 4 , of the smooth surfaced pigment (mounted free of oil in the nitrocellulose film) in order to provide a comparison between the pigment surface with oil and the surface free of oil. The electron micrograph of pigment A n.it,h 1.5% polymeiized 1.5% Unpolymerized Linseed Oil. In order t o study t,he eflinseed oil (Figure 6) shows that this oil is adsorbed to a greater fects of larger amounts oi oil, the concentration of oil was indegree than the unpolymerized oil itt the same concentration, creased from 0.5% t o 1.5%. The specimen was prepared in the In Figure 6, as in Figure 5, there is evidence of oil dissolved or same manner as that used for the lower oil concentration, The dispersed in the nitrocellulose film around t h r pigment particles. electron micrograph of pigment A with 1.5% unpolymerized The nature of the adsorbed layer of unpolymerized linseed oil linseed ail is shown in Figure 5. In this case the same general is different from that of tho polymerieed linseed oil. In genersl, fuzziness exi& as with the lesser amount of linseed oil, except the unpolymerized oil appears to exist RS a thin (-20 A.) layer that the coating of oil is more dense. The light background in with a few filamentary projections ertending ns much as 40 A. the nitrocellulose film surrounding the particle is indicative of dissolved or dispersed oil. 1.5% Polymerized Linseed Oil. It is known that a polymerized linseed o i l ( p r o c e s s e d by controlled p o l y m e r i z a t i o n with a Gardner-Holdt viscosity of 2-2 and an acid number of 13 to 18)differs from unpolymerized linseed oil in certain aspects of i t s behavior with t i t a n i u m d i o x i d e pigments. These differences have been attributed to the presence of polymers in the oil. It was, therefore, of interest to study the interaction between pigment and this type of oil. The technique employed was the same as that used for the unpolymerized oil with a 1.5% concentration of polymer-. ' ized ail. Figure 7. Electron Mierogi ,aph of P i g m e n t with 20% Polymerizbd Linseed Oil, 200,OOOX
V O L U M E 26, N O . 10, O C T O B E R 1 9 5 4 from the surface, while the latter oil seems to form somewhat thicker (-50 A.) layers with significantly more and longer filaments projecting from the surface. These observations are espected, since unpolymerized linseed oil consists essentially of compound. of low molecular weight, while the polymerized linseed oil consists of larger molecules. Effect of Increased Amounts of Oil. For information concerning the interaction of pigment with great excess of oil, a specimen was prepared a i t h 207, polymerized linseed oil. It can be seen in Figure 7 , that the la1 er of oil extends, for the most part, to about 150 A. from the surface of the pigment, while certain areas on the particles elhibit relatively little adsorbed oil. Thii difference in amount of oil adsorbed a t the particle surface again demonstrates the heterogeneous adsorption characteristics of the pigment under consideration. Particle Morphology. The effect of over-all particle morphology on the variation of total oil thickness presented to the electron beam was also considered, but this effect does not necessarily hold true for the titanium dioxide pigment particles employed in this investigation. I n order to explain the differences in the amount of oil adsorbed at the surface solely on the basib of over-all particle morphology, it would be necessary to assume that regions of high oil adiorption correspond to particle faces perpendicular to the plane of the micrograph which present large total 011 thickness to the electron beam. A region of low oil adsorption then might be a similarly perpendicular face of smaller estent in the direction of the electron beam or the intersection of two faces which are not ptipendicular to the plane of the micrograph. Such a p:trticle qtiucture, m-hile possible, does
1635 not seem to be characteristic of titanium dioside pigment particles, and has not been revealed in examination of many metalshadowed pigment dispersions. CONCLUSIO3S
I t has been shown by a high magnification electron microscope technique that the rough or fuzzy appearance of the edges of particles arises from the natural roughness of the surface as well as from the presence of adsorbed oil. Therefore, it is important in those cases where a dispersant is required to obtain optimum dispersion of a particulate material, as for the study of particle characteristics, that a minimum amount of dispersant be employed. A difference has been observed between unpolymerized and polymerized linseed oil in the nature of the fuzziness a t the pigment surface, and also in the estent of fuzziness depending upon the amount of oil used in specimen preparation. Specimen contamination by action of the electron beam has been shorvn to be absent under the conditions employed. LITERATURE CITED
(1) Cosslett. V. E., J . 4 p p Z . Phys., 18, 844-6 (1947). (2) Ennos, A. E., Brit.J . -4ppZ. Phys., 4, 101-6 (1953). (3) Hillier, J.. J . A p p l . Phys., 19, 226-30 (1948). (4) Schuster, M. C., and F u l l a m , E. F., IND.ENG.CHEM.,A N . i L . ED., 18,653-7 (1946). ( 5 ) W a t s o n , J. H. L., J . AppZ. Phys., 18, 153-61 (1947). RECEIVED for review February 9 , 1954. Accepted July 10, 1954. Presented a t the National Meeting of the Electron Nicroscope Society of America. Pocono Manor, P a . , November 5 , 1953.
Fluorometric and Colorimetric Microdetermination of Uranium in Rocks and Minerals J. A. S. ADAMS‘ and WILLIAM J. MAECK2 Department o f Chemistry, University o f Wisconsin, Madison, W i s .
A system of operations and manipulations has been developed for analyzing geologic specimens containing less than 10 p.p.m. of uranium. Means of opening the sample, as well as cellulose chromatographic and ethyl acetate extraction procedures for the separation of uranium from interfering elements, have been investigated and compared. The fluorometric procedure has been critically studied and compared with Yoe’s colorimetric procedure. In general, samples containing less than 10 p.p.m. of uranium are best analyzed fluorometrically; more than 10 p.p.m. colorimetrically.
I
K THE pazt few ).ears many a1,tirlee have appeared
011 the determination of uranium. RIoat of these articles have drnlt with the determination of macro or semimicro amounts of urnnium. For microdeterminations of uranium, the fluorometric3 method n-as intensively investigated and developed. The Trace Elements Laboratory of the U. S. Geological Survey has‘publiahed a number of investigations (6) in this field of fluorometry, as has the S e n . Brunswick, S.J., lahorntory of the Atomic Energy Comniission (8-10). For the past 3 years this laboratory has been engaged in studies of the geochemistry of uranium. These studies necessitatrd the determination of uranium in a large variety and
Present address. The Rice Institute, Houston, Tex. Present address. r S ,5,> -I.;? 886. 9710 TSL-, Det. 2 , A r m s Chwnicsl Center. \Id. 1
2
number of rocks and minerals, most of which contained only 0.1 to 10 p.p.m While making several thousand such determinations, various methods and procedures were investigated. The optimum conditions for several important steps were quantitatively determined. A system of procedures has been developed which permits the routine analysis of many different types of geologic specimens. The presentation of these procedures and their comparison are the purpose of this paper. OPEXING OF SAMPLE
The specimens dealt with in this work were mostly natural silicates and the opening of the sample played an important part in drterniining the uranium content. Radioautographic techniques ( 2 ) show that generally uranium may occur in rocks either as a “paint” around the individual crystalline grains of a rock or it may be bound within the crystalline lattices. Usually, the paint is easily taken into solution with dilute acid, while crystalline silicates-e.g., zircons-are difficult to bring into solution. HYDROFLUORIC-hITRIC ACID TREATMEYT
The conventional procedure of hydrofluoric-nitric acid treatment n a s used for the opening of siliceous materials such as granites and obsidians because it leaves much less salt than does an alkali fusion. For a 1-gram sample, three hydrofluoricnitric acid treatments followed by three nitric acid treatment. \I eie wffivient to open most sprcimc=ns,ewluding zircons.
