DRYING OF LINSEED OIL ELECTRON DIFFRACTION STUDY D. H. CLEWELL Massachusetts Institute of Technology, Cambridge, Mass. PLATE
E
MAGAZINE
LECTRON diffraction makes possible a simple method for the study of surface structure of various materials independently of the body structure because of the low penetrating power of a n electron beam. The interpretation of electron diffraction patterns follows the general methods used with x-rays even though the exceedingly low transparency of most substances to electrons results in many anomalous diffraction effects that often make correct interpretation difficult. Electrons interact with crystals like very short waves; the actual wave length is determined by the velocity of the electrons striking the crystal which, in turn, is a function of the accelerating voltage. The relation
x
=
FIGURE 1. DESIGNOF DIFFRACTION CAMERA U
optically centered; t o obviate the difficulty, a small bar magnet is clamped in the vicinity of the apparatus to deflect the beam into its proper direction. To bring the specimen up into the electron beam and adjust the former to the proper angle of incidence, a support controlled by two concentric shafts leading into the diffraction chamber through a pressure grease bushing was provided. Rotation of the inner shaft raised and lowered the specimen surface through a rack and pinion arrangement while a rotation of the outer shaft adjusted the angle of the specimen surface. The camera accommodat,ed three lantern slide plates that could be elevated one at a time into position for photographing the diffraction pat,tern. With all plates lowered into the magazine, the electron beam struck the fluorescent willemite screen, 8, for visual observation. Lantern slide plates were used because of their thin emulsions and the relatively small amount of absorbed gases that would, if present in large quantities, give vacuum troubles. There is apparently no advantage in using thicker emulsiom since the electrons penetrat'e the emulsion for such a short distance t h a t only the surface grains are exposed. Then, too, the slowness of the lantern slide plates to fog makes them more convenient to handle, and again nothing is lost by using slow emulsions since all emulsions seem to be about equally sensitive t o electrons. The exposure times ranged from a fraction of a second to 30 seconds. A vacuum was maintained throughout the system by a mercury diffusion pump backed by a Cenco Hyvac. A constant source of direct-current potential was obtained from a standard transformer-kenetron arrangement used in the operation of x-ray t,ubes. The linseed oil films were prepared by coating the polished surface of a small brass block with oil coats of various thicknesses, a smooth film surface being secured by the use of a Fpatula. The surfaces were photographed at intervals during the drying by slipping the brass blocks into a small clip on the specimen holder. Pure lithographic varnishes were used as specimens (pure polymerized linseed oil).
l/iRv
expresses the wave length in .&strom units as a function of the voltage, V . The procedure is to direct a narrow beam of electrons upon the specimen surface a t grazing angles of incidence, whereupon the beam is diffracted according to the arrangement of the surface atoms. The diffracted rays register the diffraction pattern on a photvgraphic plate as a series of black spots, lines, or curves. The penetration of the beam into the surface depends on the accelerating voltage; for example, 35,000-volt electrons grazing the surface of a rock salt crystal penetrate to a depth of approximately 1.2 X 10-7 cm. which is of the order of 5 to 10 atom layers ('7). Voltages of the order of hundreds of volts may penetrate only the first atom layer. It thus seemed reasonable that some new information concerning the little-understood physical changes that take place in a drying linseed oil film might be revealed by an electron diffraction study of the oil surface.
Apparatus The experimental apparatus was designed primarily for the investigation of surfaces by the reflection method, but aside from this consideration the general design of diffraction cameras used by previous investigators was followed (4, 8):
Diffraction Patterns
Figure 1 shows that a heated tungsten firament, F , serves as a source of electrons which are accelerated by a potential of 30,000 volts t o the pinhole, PI,in the anode. The resulting beam is further defined into a narrow pencil of rays by the second
pinhole, Pz,and is then immediately incident upon the specimen surface, M . A focusing cup surrounding the filament serves to concentrate the electron beam upon the first pinhole and allows a relatively small filament current t o be used for a given beam intensity. The pinholes are spaced 2 inches apart; the first has a diameter of 4/1000 and the second a diameter of 8 / 1 ~ inch. The holes were drilled in l/az-inch sheet copper and mounted on removable tapered plugs t o facilitate occasional cleaning. Stray magnetic field will deviate the electron beam from its path through the two pinholes even though they are
~
650
Diffraction photographs were first made of the uncoated brass block to be certain that its s'urface was sufficiently polished to give nothing but the diffuse scattering of amorphous material. The diffraction pattern of a freshly applied oil film consisted of two diffuse rings typical of a n organic long-chain liquid although the rings do not have the usual ~sin O/A values encountered in x-ray work; in the latter case the interference pattern is of intermolecular rather than intramolecular origin. The diameter of the rings agreed quite well with the intensity maxima in the simple Ehrenfest-
JUNE, 1937
INDUSTRIAL AND ENGINEERING CHEMISTRY
Keesom law which describes the diffraction of radiation hy a liquid on the basis of certain constantly repeated spacings occurring in the liquid: X = 2a (0.814) sin ff
vhere 8 X a
=
half-angle of diffraction as used in the ordinary Bragg formula
= wave length = distance between
diffracting centers
The two rings were found to correspond to the 1.54 A. and the 2.54 A. spacings of the carbon-chain molecule. As the oil dried, the first distinctive pattern to follow the liquid pattern consisted of an inverted V with an apex angle of about 120" occurring in several orders. As the drying progressed, the apex angle of the inverted V approached 180" as a limit, and the arms of the V were curved slightly downward towards the specimen surface. The final pattern (Figure 2 ) of the completely dried oil marked the completion of the transition in that the diffraction pattern was now a system of parallel straight lines, in turn parallel to the specimen surface. I n some cases the lines appeared in RS many as six orders simultaneously. The intensity along the evenorder lines was a maxiirium at tlie inidpoint while the intensity of tlie odd-order lines was a niinimuni in this region. (The first-order line does not appear on the plrotographs.)
F I ~ ~ I J2. R EFINAL PLETELY
P.TTERN
OF
DRIEDOIL
COM-
Considering t,he pattern of the dried oil, it was immediately evident that the lines were the result of diffracting centers occurring in the oil a t regular intervals below the surface. Since au array of carbowchain molecules parallel to one another and perpendicular to tlie film surface will present the required condition for the observed diffraction, calculations we= made using the Lane eiwatiorr for the diffraction of radiation along the c axis: where
c (cos .~ yo - errs y) = n X angles of incidence and reflection, respectively n = an integer c = a distance of repotit,ion normal t o tlie suritlce
70, y =
After allowance was made for a refractive index of 8.5 volts, it was found tlmt-the position of the diffraction lines gai;e c a value of 2.54 A,, corresporiding exactly with the 2.54 A. spacing of alternate carbon atoms in the chain molecule (Figure 3). A more rigorous calciilaiion of the diffraction produced by :L system of parallel carbon chains arranged a t rilndom distances from one another, where account is taken of the zigzag nature of the carbon chain, explains the variation in intensity along each of the pattern lines (6). Thus it seems evident t.1ia.t with the drying of the oil, chaiii.moleciiles which have a perfectly random arrangement in the wet oil orient themselves parallel t o one another and per-
651
pendicular to the film surface. I n no instance, regardless of the viscosity of the lithographic varnish or the rete of drying, did the final pattern appear until the oil had completely dried to a glassy, hard material. The two intermediate patterns were accounted for by assuming that at any time during the drying interval all of the orienting molecules may be described as making an angle of less than 6 with the surface normal, and that as the final dry state is reached 6 approaches zero. Thus when 6 is approximately 30", the pattern of inverted V's is obtained, and in the liquid state S is 90".
FIGURE3. Zioziti STRUCTURE f m THE CARBONCEAIN
All lithographic varnishes ranging from No. OM) to t.he very highly polymerized materials exhibited the above orientation on drying. I n general, the more viscous varnishes dried in less time than the lighter oils. However, the diffuse background associated with all diffract,ion patterns was less for the lighter hodicd oils, suggesting that in thc latter case a higher percentage of the surface mat,erial had probably undergone orientation. In tlie f i l m of the oils of very low viscosity the orientarion occurred to such a marked degree that the intensity of the first and strongcst layer line of the linoxyn pattern tended to concentrate at the midpoint of the line. This was interpreted as a tendency toward a crude crystallizat,ion of the surface molecules in addit,ion to their simple parallel alignment. A possiible explanation of the observed atomic arrangement could be the accumulation within the drying oil film of polar molecules which are subsequently marshaled into position by an electric field existing at the a i r 4 surface. The electric field is definitely present at the surface, since from a measurement of the electron index of refraction an inner potential of 8.5 volts WRS calculated for linseed oil. And since the oxidation process is necessary for orientation to occur, i t mnst he concluded that the polar products were not present in the original oil. Since the force tending to turn a polar molecule into the normal position is proportional to its dipole moment., it may he supposed that t.he greater speed of orientation that is characteristic of the molecules in the heavy-hodied oils indicates that these oils may yield oxidation products of large dipole moments, a supposition not necessarily in disagreement m5th chemical data since the bodying of the oils is believed to involve polymerization. The tendency towards crystallization that was found in the lighter bodied oils might be explained on the assumption that very short molecules obey the forces of lateral attraction between themselves and longer molecules more readily than the orienting force of the electric field at the surface. This crude crystal formed By the lateral attraction could then swing into position as a unit. The increase in diffuse background associated with lhe very viscous oils is probably the result. of a tangling of the very conrplex polymerization or oxidation products to such an extent that coniplete orientation cniriiot. be cffeeted.
Chemistry of Linseed Oil Considcring briefly the cliemistry of linseed nil, we note that tho oil is a mixture of the glycerides of linolenic, linolic, and oleic acids, together with the glycerides of the saturated organic acids, palmitic, stearic, and possibly myristic and
652
INDUSTRIAL AND ENGINEERING CHEMISTRY
arachidic (i). The exact chemical reactions that take place as the oil dries are not clearly understood although it has been definitely established that from 15 to 20 per cent of oxygen is absorbed from the air. I n the first stages of the oxidation, peroxides are formed by the attachment of two oxygen atoms at the double linkages existing in the unsaturated acid chains making up the glycerides (6). The peroxides are only an intermediate step; the work of D'Ans
FIOUHX:4. CnrmAmIzA,mos
OF
4
V.,rtsrsn FILM PKiMESTED IVITN ALUMINUMHYDROXZDB (Z) am1 Eibiier (9) seems to show thnt, upon further oxidation, acids of low moleciilr~rweight, are ariaing the compounds formed by a scissioii of the carbon chain of tile glyceride a t one of the double linkages. On the other hand, Bradley ( 1 ) reports evidence 1.0show t,hat a definite il1ree-dirnensiona1 polymerization should be associated with t,he drying of t,he oil. Since the drying of the oil to a hard film was correlated vith a gradual alignment of the carbon chains normal to the surface and not with just an increase in the total number of chains existing in t.he perpendicular position, it seems reasonable to conclude that the orientation is characterized by a factor more complicated than the simple formation of fatty acid molecules which, because of their polar nature, would he subject to orientation. If the drying procees were to result in setting free acid molecules, they would probably swing immediately into the normal position; and as the drying continued, the number of perpendicular molecules would simply increase. Turning to the idea of three-dimensional polymerization, it seeins entirely within reason that, as the chain molecules gradually join together, the resulting solid network will assume a position relati-i-e to the surface such that a large nuiriber of chains will be normal to the surface. This viewpoint seems more in accord with the diffraction informatioii since, as the solid lattice grorrs, the chain compounds of the liusecd oil will probably orient gradually. It must be reinernhered that the reactions baking pla.ce in the oxidat.ion of the oil are very comples XIKI any simple expla.nation such as has been offered here can be only approximate. A series of infrared absorption measurements made (luring the drying process would probably be valuable in furt,her understanding the formation of the solid linosyn since tlie appearance or disappearance of certain types of bonding can often he readily ascertaiiied by such measurements. For example, the saturation of the double linkages in the oxidat.ion reaction should be evidenced by a reduction in the intensity of an absorption band ai. 6.2 microns; 011 the other hand, if polar molecules are being formed, an absorption band associated with the particular atomic grouping characteristic, of the polar group should appear.
VOL. 29, 30. 6
Tendency towards Crystallization It has already been stated that a teiidency towards cryst,allization had been observed in the light-bodied oils. In a few instances a pigmentation of the oil greatly enhanced this t,endency, particularly in the case of a Venetian red pigment containing a large amount of alumina hydrate and incorporated into a light varnish vehicle. The parallel layer lines of the linoxyn pattern were now broken up into a series of spots, indicating that the carbon chains had coagulated into small crystals although tlie chains were still perpendicular to the film surface. Figure 3 shows that tlie carbon chain has a zigzag structure giving rise to two parallel rows of atoms spaced 0.87 A. apart. To explain the variation of intensity along any horizontal line of the linoxyn pattern, i t was necessary to make use of the fact that each, row of atoms had a neighhoring row at a distance of 0.87 A. I n a crystallization process each row of atoms will possess several additional neighboring rows at definite distances. This will result in such an increased variation of intensity along the diffractioii line that it is actually broken up into distinct spots. The spacings uf the spots obtained wit,h the pigueiited films were found to agree very closely with the patt.ern predicted hy the orthorhombic solid hydrocarbon struoture. Two other variiish filius pigmeiitetl xith alumina hydrate were studied, although only one evidenced crystallization (Figure 4). I t was impossible to obtain complete formulas of these two inks at tlie time so that no explanation of their different boliavior will he attempted. It was considered possible that the brass block used as backing material in all of the experiments might have had some influence on tlie orientation. To test this possibility, a very t,hick film of varnish was applied to the brass block and allowed to dry until its top surface had skinned over. A photograph of the top surface indicated that the orientation had progressed about two-thirds of the way towards completion. Cpoii removal of the skin, the wet oil underiieath gave the usual liquid pattern, from which it was concluded that the orientation was not caused by the attraction of substances within t,lie oil to the brass surface.
Conclusion Linseed oil surfaces, both dry and liquid, are readily studied by electron diffract.ion. Distinct diffraction patterns indicate that an unoxidized oil probably contains few polar products, siuce the surface structure of a wet filrri is completely amorphous. As tlie oil absorbs oxygen in the drying pmcers, a gradual orientation of carbon chain molecules normal to the film surface occurs. Complete orientation does not exist until the fiim has eompletely dried. Diffraction patterns taken a t various stages in the drying show that,at any time the orienting molecules make an angle less than 6 with the surface normal, and that,, as 6 varies continuously from 90" to O", the oil changes from the liquid state to the solid linoxym. Since drying is correlated with a gradual orientation and not with just an increase in the number of perpmdicular surface molecules, it is suggested that the alignmrnt is better explained on the basis of a three-di~nensionalpol?. merization than by the simple splitt,ing off of fatt,y mids. Pigmentation may in some instances induce a crysta11'loatian of the carbon chains of t,he oil. The diffraction pat.terris obtained in the course of tliis n'ork do not show, a high degree of resolution, but this behavior can probably be at.tributct1 largely to the fact that linseed oil is a complex mixture of various substances. In t,he calibration of the diffraction camera, patterns of reasonahly good resolution were obtainable. By increasing the
JUNE, 1937
INDUSTRIAL AND ENGINEERING CHEMISTRY
resolution of the apparatus and using as specimens pure triesters of glycerol, which are known to give linseed oil its drying properties, more detailed information of the linoxyn structure will undoubtedly be uncovered.
Acknowledgment The author wishes to acknowledge gratefully the many helpful suggestions given during the course of this work by Professors B. E. Warren and Arthur C. Hardy, and to thank the International Printing Ink Company for their courtesy in supplying specimens.
653
Literature Cited (1) Bradley, T. F., ISD. ENG.CHEM., 29, 440 (1937). (2) D’Ans, Angew. Chem., 41, 1193 (1928). (3) Eibner, “ D a s Oeltrocknen,” Berlin, Allgemeinen Industrieverlag, 1928. (4) Germer, L. H . , Rev. Sci. Instruments, 6, 138 (1935). (5) Morrell and Wood, “Chemistry of Drying Oils,” pp. 47, 85, N e w York, D. Van Nostrand Co., 1925. (6) Murkon, C. A . , Phil. Mag., [7] 17, 201 (1934). (7) Thomson, G. P., Proc. Roy. SOC.(London), A133, 19 (1931). (8) Yearian and Howe, Rev. Sci. Instruments, 7, 26 (1936). RECEIVBD March 6, 1937.
GLUCONIC ACID PRODUCTION Effect of Pressure, Air Flow, and Agitation on Gluconic Acid
Production by Submerged Mold Growths P. A. WELLS, A. J. MOYER, J. J. STUBBS, H. T. HERRICK, AND 0. E. MAY Industrial Farm Products Research Division, Bureau of Chemistry and Soils, Washington, D. C.
A
PREVIOUS communication from this division ( 3 ) showed t h a t the production of gluconic acid from glucose by submerged mold growths under increased air pressure greatly reduces the time required to complete the fermentation. This work was carried out on a laboratory scale in special glass apparatus. More recently a new type of rotary drum fermenter for carrying out submerged mold fermentations under pressure was described (1). The development of the rotary drum fermenter for this purpose was the result of continued effort to find the type of equipment best suited for conducting such a process on a commercial scale. The preliminary results presented a t that time on the production of gluconic acid showed that this type of equipment possessed decided advantages over shallow-pan vessels used for this process ( 2 ) . Table I presents some results previously obtained on the production of gluconic acid by different methode. These
TABLE
I.
Type o$ Fermentation
OF GLCCONICACID B Y DIFFERENT results show the advantages of the submerged growth type of METHODS OF MOLDFERMENTATIOW
PRODUCTION
Organism
Fermentation Vessel
Yield of dcid (Theo-
retical)
% Surface Submerged (pressure)
A study of the effect of air flow, agitation¶ and air pressure on the production of gluconic acid from glucose by submerged mold growths in rotary aluminum drum fermenters has revealed that the fermentation rate is, to a large extent, dependent on the proper adjustment of these factors. Under the best conditions found, using 15 per cent glucose solutions, yields of gluconic acid in excess of 84 per cent based on the glucose available and 97 per cent based on the glucose consumed, were obtained in 18 hours from the time of inoculation with germinated spores of Aspergillus niger. The fermentations were carried out in a type of vessel which, it is anticipated, can be easily adapted to large-scale operation.
Penicillium luteum p u r purogenum Penicillium ehrysogenun
Fermentation Period Days
Shallow pan (aluminum)
57.4
11
Glass bottle
80.4
8
80.0
2.2
(sintered glass false bottom) Submerged PeniciZZium Rotary drum chrysogenum (aluminum) (pressure) a Ueing 20 per cent glucose solutions.
fermentation over the surface type. A much greater decrease in time required is obtained when the submerged fermentation is carried out in a rotary drum fermenter instead of in the glass culture vessels used on a laboratory scale. The experimental work on this problem was continued, with the result that the average time required to ferment a 15 per cent glucose solution completely to gluconic acid was reduced to about 35 hours. However, the results obtained from week to week, while satisfactory from the standpoint of yields obtained and time required, were not consistent. Moreover, it was concluded that for commercial-scale operation, the organ-
