C. L. SUTULA AND L. S. BARTELL
1010
Vol. 66
Institute for Atomic Research and Department of Cheiiristril, Iowa State Uniceraity, Aines, Iowa Receized September if3*1961
An tLlcctron diffraction study has been madc of the structure arid mo1oc:ular oricntation in iiiult,imol(’cIilRrfilms of :I v:iric%y of long-chain molecules. Films were prepared sevcral ways, but principally by evaporating t i i l u t ~soliit.ions ~ on iiwt,:tl slidw Most of tho films studied were 30 to 40 molecules thick, although films considerably thinner and thicker w r e ctx:mined. Thc: films were found to consist of microcryst’als with 001 planes parallel t o the surface of the substrate, in agreement with previous studies. Careful measurenients of the lattice parameters and crystal form were made. ‘l’lirse shoired, contrary t,o srvoral previous studies involving some of the same compounds, that, the molccular packing in the colloidal films wafi chttract,crist io of t,hat and indistinguishable from t.hat occurring in the bulk crystals. Eviderice is presented which indic*ates that. t,ho rsccpt,ion:il packing found in tho earlier studies of thin films was the result of impurities rather than the cxtrenie thinness. T w o ncw methods are reported for preparing thin films with molecules aligned in a designated direction.
It is widely appreciated that the orientation and packing of anisotropic molecules at interfaces are important, factors in intcrfacial properties such as friction, adhesion, arid wettability. Sumcrous qua1it:itive studies of orientation bascd on surface potcnt ials, ;itlsorption isothcrms, coefficients of frirtioii, and contwt angles have bccn rcported hut n o w of theso methods permits a very precise c~hamcteri7,atiori. 1‘:lect ron diff raction studies, on thc ot hrr harid, thotigh rcstrictrd iii applicability, arr capable of yielding conaidcrablc dctail about the striict lire of srufaw films. 14;xtremrly informat i w diffraciioii st udicls hiive bceii nmdc hy several aut horh, on both monomolccwlar arid multimolecular films. Of particvlar irit crest have. bccn scvcral reports of multimolecular films in which molceular packing dif€ercd from that charactcristic of the bulk matcrial. l‘hc possibility that this diffcrencc arose from the colloidal character of thc interfac4al films ticscrvrd invrst igation. I n addition, in studies of multilnyrrs, suffickit attention has riot a1w:iys bren paid to the composition of the films or their thic*kncss,and the variety of (*ompounds studied has bcrn small. The presrnt investigation WLS undertukcn l o study molccular oricrit at ion and packing in films of sevrrsl homologous w i t s of n-hydrocarbon dcrivati\w 0 1 1 metal slides, by rlct%ron diffraction. I’articaiilur attention was given to the plirity of the cwmpounda. I’ilrns rmging ir! t hickrims from one molecrilar layrr to ovw 1000 A. w r e investigated. .I vrry helpfril de\ ice, riot prcviously used in st riwtrir:d studies of such films, was a polarization spect romctcr ( q a b l r of measuring the average thickness of the thiriricr films to within a small fr:wtion of a molecwl:ir length. ‘I‘hc prcparatiori of reasonably homogcncous films of IOOO A a i d o \ w W:LS not difficult, as disrussrd i n t hc folloning. The deposition of known nuin1)ws of molcc\ilar layers in the very thin arid i l l termrt1i:itr raiigrs by the l,angmuir--Blodgrl,t t rchiiiqiie v a s porriblr with thc more polar dcrivatives arid a feu examplrs of such films w r o studied. Struvt urn1 w d t s for Langmuir-Blodgett films
-
( 1 ) (a) (‘vntiibution N o 1061 \\'ark W R S ucrioimed i n the Aines I lrburntwy of t l l v I‘ Atmnic l,IlrlK) Coil~niission. (b) plrsenttd a i flit’ I {Gtll N:rtiunul Merting of t l w A n i c . i wan (’lii~nnculSoviet).,
-
1 h i t 0 1 1 RIlrssut Iiusetts Apirl h, 1 ( a ) Buued on n diTseitntiun hy C L Sutulu t o thc Grndiintc Srhuol, I o ~ aState V n i \ ersitj. i n plrrtiiil fulfillment of t h c rcquireincnts for t h e ilPnirr of Ilortor of I’liilosoDIiy, 1950
confirmed results of earlier published st,udics :tiid will not, bc discusscd in detail in the following. I n the case of non-polar molcculcs, for which the Langmuir-Blodgctt technique is not applicablr, the attainment of hornogcneous films of controlled thickness in the t,hinner range provcd mow difficult,. One met’hod which was successful in producing quit>c uniform films eve? of pure hydrocarbons from 1000 down to 30 A. or less involved t>hebuffing down of 1000-A. films with soft, tissue paper. It was found, however, that the mcchanical shear associatcd witJhthe buffing had a profound influcnce on molecular orientation, as will be discussed in a forthcoming paper. The present paper will be concerned, unless otherir-isc st>atcd, &h films which have been subjected to no externally applied mcchanical shear in their preparation.
Experimental Procedure Preparation of Surfaces.-Platinum slides and chromiurii plated steel slides were used as substrates for t,he films. They w r e polishcd by hand with increasingly smaller sizes of abrasives in the mariner customary in metallurgical polishing procedures. Positive removal of abrasive particles from the inetal surfacc was accomplished by vigorous rubbing of the entire surface of the slide with a srnall piece of polishing cloth in a jet of water. This wm followed by rubbing with a cotton applicator in a stream of distilled water. The last, tracw of greasc-like material remaining on the surface after the above operation were removed in the following way. Platinum slides were dried with soft tissue after rinsing and were flamed for about 1 min. a t a dull red color in the oxidizing portion of a flame of a meker burner. Chromium platcd slides were first nipcd wit,h tissue dampened with benzene after rinsing and were flanicd in the oxidizing port,ion o f a flame of a meker burncr for a total of 5 t o 7 sec. in I-sccr. steps. I’;lectron diffraction photographs taken of mct:d surfacvs c1o;ined by t h e above procedure showed only thc typic:il pattcrn of polished surfaces. They did not indic:tte the presence of abrasive or of oriented, grease-like substanccts. Small drops of water placed on the metal surfaces spread and ave contact angles that wcrc approximately zero. The sur% c ~ salso gave black breath figures arid films of n-octadccyl ainine ere roadily formed by t,hr drop retraction technique from 0 . 1 solutiolis of n-octadecylamine in cetane. era1 methods wore usctl in prvPreparation of Films.-S paring films. One iiivolvtxl rrading a fuecd compound ovor a warm slidc arid allowing the material to solidify. Gencrally, this method gave films too thick for convenient use i n eloctror~ tfilfraction experiments. Another method was t o spread a smd1 quantity of a solution of R part.icular trompound over the metal slidc. A film of the conipound fornicd tis t,he solvcnt evaporated. illthough such films wcrc someah:it hetcrogmaous in t.hic.ltness, largr, fairly uniform :irc:is oftcn were obscrved atid w r e scI~:ct(:d for dt~tailctls i udy .
June, 1962
MULTIMOLECULAR FILMSOF LONG-CHAIN H HYDROCARBON DER~VATIVES 1011
In the case of the acids, a few films of Langmuir-Blodgett multilayers also were investigated. The overwhelmin majority of the films studied were prepared by the seconi method, and averagcd about loo0 A. in thickness. Diffraction Apparatus.-Diffraction patterns were obtained by the reflection method with a General Electric electron diffraction unit using an accelerating voltage of 35 0.064 A.). Zinc oxide patterns were taken to dekv. ( A termine the wave length of the electrons. Measurements of Film Thickness.-A polarization spectrometer, similar to Rothen’s ellip~orneter,~ was used to dcterniinc t,he optical thickness of tho thinner films. Although its greatest utility was in the second paper of this series,‘ it is expedient to describe it here. For tilms less than 150 b. thick the scnsitivity of measurement wag of the order of f l b. The optical arrangement and the technique of taking readings ww identical to that described by Bartell, Ituch, and Betts.6 Langmuir-Ulodgctt films of neutral stearic acid and of barium stearatc deposited on highly polished surfaces of platinum and of chromium plated steel were used to calibrate the instrument. The change in optical reading between a fresh surface and :I surface covered by a film was used a~ a measure of the thickness oC the film. The substrate surface was carefully prepared for every experiment according to the method previously described, and its base optical reading was determined shortly after preparation. Immediately afterward, tt film was formed on the metal surface and its thickness was cstimated. Then the film was exanlined by electron diff rmtion. In the case of film thicker than a fcw hundred angstroms, thickncsses were estimated visually from the color of the film. Chemicals.--The hydrocarbons docosane through triacontane were estimated t.0 bc over 99% urc by mass spectrographic analysis performed by the She{ Development Company,. The remainder of the hydrocarbons and acids melted to within a degree of the values reported in the literature. Benzene, toluene, n-tetradccane, acetone, and n-octane were used as solvents for the hydrocarbon derivatives when the films were formed by evaporation from a solution. All of the above solvents were carefully redistilled. Benzene, tolucnc, and acetone, evaporated drops of which after the above treatment left no film on a freshly prepared metal surfacc, were used without further purification. The compounds n-octane and n-tctradccane u-ere percolated through a column of silica gel and aluminum oxide to remove polar impurities.
Results All compounds gave well defined electron diffraction patterns except for lauric acid, which was irregularly oriented. Patterns similar to those obtained in this investigation have been reported in a number of previous studies.6 Several analyses of such patterns have been published.7-Q The diffraction data indicated that the multimolecular films were preferentially oriented and po1ycryst:tlline. The films appeared to be composed of many contiguous submicroscopic crystallites orient,cd with their basal (001) planes parallel to trhe plane of the substrate surface. This orientat,ion of the crystallites (disrcgarding the underlying Rdsorbed monolayer in the case of acids6J) was found to bc unrelated to the thickness of the films, occurring in films ranging in average thickness from several molecular layers to films well over (3) A. Rothen, Rev. Sci. Instr., 16, 26 (1945). (4) L. S.Bartell and C . I,. Sutrila, t o be publislied. ( 5 ) L. S. Bartell and R. J. Ruch, J . Pkys. Ckem., 60, 1231 (1956); I.. S. Bartell and J. F. Bette. ihid., 64, 1075 (1960). ( R ) See for example, 1,. H. Gernier and K. 11. Storks, Proc. Natl. Acud. Sei., 23, 390 (1937); Th. Schoon, Z. physik. Chem., B39, 385 (1938).
(7) L. IJ. Clermer and K. 11. Storks, J. Ckem. Phys.. 6, 280 (1938). ( 8 ) Z. G . l’insker, “Electron Diffraction,” Riitter~vorths Scientific I’i~liiic:~ti~~ns, T~on4on.I %:%. 1‘3) (;. -2. Murison, I ’ h L Xuu., [ i ]17, 201 (193.1).
1000 A. Nevertheless, it is fair to point out that only in the case of Langmuir-Blodgett films was there any guarantee that the crystallites diffracting the electrons were as thin as the average film thickness. Moreover, the electron beam penetrates only the top few molecular layers of any flat regions in a film, leaving unexamined the lower layers. When the films were prepared by the first two methods described above, the basal axes of the crystallites were randomly arranged in the plane of the substrate surface. Two methods wera found, however, which gave films with basal planes parallel to the substrate but with molecular tilts all in virtually the same direction. These are described in the Appendix. Electron micrographs of 1000-A. films of stearic acid and octadecyl st,earate prepared on evaporated carbon substrates confirmed the polycrystalline character of the films. Crystallites of octadecyl stearate ranged from thin, flat, well developed crystals the order of a micron in width on down through much smaller plateleLs and prisms closely wedged together to flat, featureless clumps. Crystallites a few tenths of a micron in diameter often had orientations strongly correlated with those as far away as several microns even though the over-all orientation in the film was random. Crystallites of stearic acid failed to show well developed faces in the micrographs, possibly because of the deleterious effects of the water used in the preparation of the microscope specimens prior to shadowing the films. The angle which the long axis of the molecules made with the perpendicular to the plane of thc substrate surface, customarily designated by +, was used as a measure of molecular orientation. Orientations observed in the present study are listed in Table I in terms of $ No,a quantity which should be directly related to the crystallographic angle p for bulk crystals if 001 planes in the films are parallel to the surface. By means of a systematic analysis of the position and intensity of the elongated reflections occurring in the patterns, it often was possible to determine the lattice parameters a and b of the crystallites in the films. In the films investigated, reflections revealing the c-spacing were never observed on unrubbed films.lo The film results are compared in Table I with published results of investigations of bulk crystals of n-hydrocarbon derivatives by X-ray and electron diffraction. In the case of some substances for which no values are reported in the literature, values found by interpolation or by extrapolation using literature values for homologous substances are given. I n other cases structural data for similar substances are listed. The diffraction data from several films were too incomplete for a reliable determination of the parameters of the crystallites. In these cases, the cited data from the literature gave cdculated
+
(IO) In the case of a gently buffed film of octadecanol which retained the 001 orientation, the c spacings were observed once. PreHuniably crystallites were elevated iibove the average surface, exposing their sides as n d l as their top surfaces t,o the electron beam. The absence of 001 reflections in the remainder of the patterm ia additional evidenre that cryxtullites were flat uncl srnooth. with essentially no t,nll spirea.
TABLE 1 THEl'ARAME1'Y;RS
L)ETERMINED FOR CRYSTAI.I,ITES FOUNQ I N MULTIMOLECULAR FILMS OF
LONG-CIIAIS, 11-131 I>ROCARLION
DERIVATIVES Compound
(0)
(6)
(C)
... ... ...
B
...
! l , f 900
Cryrtal form
n-(k t a d f m n e (impure)" .. .. 90" Orthorhombic Homologoub compoundh 7.43 4.96 ... 90" Orthorhombic ... n-Eicosane (impure)" 7.4 5 ... Orthorhombic 90' Homologous compound* 7.43 4.96 90 " Orthorhombic ... n-lhcosane .. .. ... Triclinic ... Data from films" fit extrapolated results of hIazeec n-'l'ricosane" 5.0 7.5 ... ... 90 " Orthorhombic Smithd 4.97 62 7.48 90" Orthorhombic ... n-Hexacosane" 5.5 7.4 Monoclinic 118' Homologous compoundd 5.57 7.42 119' ... hfonoclinic ... n-Heptacosane" 5.0 7.5 ... Orthorhombic ... 9oo Smithd 4.97 72.59 7.48 90 ' Orthorhombic ... n-Octacosane" 5.5 7.4 118' Monoclinic Homologous compound' 5.57 7.42 ... 119' Monoclinic ... n-Nonaco8ane" 5.0 7.5 ... ... 0rthorhomt)ic 90" Smithd 4.97 7.48 77.70 90" Orthorhombic ... n-Triacontane" 5.5 7.4 ... ... Monoclinic 118" Homologoris compounde 5.57 ll9O 7.42 ... Monoclinic ... n-Hontriacontane" 5.0 ... Orthorhombic 7.5 *.. 90 " Smithd 4.97 7.48 ... 90" 0rthorhomt)ic ... n-lhtriacontane (impure)" 6.0 ... 7.35 Orthorhombic ... 90" Homologous compoundb 90" 4.96 ... 7.43 Orthorhombic ... Stearic acid" 9.40 5.0 ... ... 128" ~ ~ o r i o c ~ i nform i c , (3 Abrahamsson and von Sydow' 4.96 128"14' 50.76 9.35 ilionoclinic, form C ... Arachidic acid" 128-129" * . ... Monoclinic, form C Interpolated' 128" 9.3 5.0 Monoclinic, form C ... n-Heptadecanoic acid" 20 " .. ... Possibly triclinic Palmitic acida 9.48 5.0 ... ... 127- 129 Monoclinic, form C Verma" 9.68 46.86 128'57' 5.05 ... Monoclinic:, form C Thibaud and Duprc LaTourh 129" 9.41 5.00 45.9 ... Monoclinic, form C Isostearic acid" .. ... 130-135" Possibly triclinic Isopalmitic acid" 125O Possibly triclinic: .. .. *.. ... Lauric acid" patterns diffuse, no reliable data obtained .. ... ... 118" 7.53 n-Octadecyl stearate" 5.56 Monoclinic 7.40 n-Hexadecyl stearate" ... ... 118" 5.65 Monorlinic 7.5 ... ... 120' 5.68 n-llodecyl laurate" hl onocli n i (7.42 92.8 118.7' ... 5.61 n-Cetyl palmitate by Kohlhaas' Monoclinic 7.49 93 119.2' ... 5.60 n-Cetyl palmitate by Schoonf Monoclinic. 7.45 49.4 90' 90" 4.97 Orthorhombic n-Octadecyl alcohol" .. ... ... 110' nStcarsmide (impureY .. a This investigat,ion. * A. Miillrr, Proc. R o y Soc. (London), A120,437 (1928). W. M. Mazce, ref. 14. A. E. Smith, 13. M. M. Shearer and V. Vand, ref. 19. f 6. Abrahamsson and E. von Sydow, A d a Crzlst., 7,591 ( 1954). A. It ref. 13. Verma, Proc. Roy. Soc. (London), A228, 34 (1955). h J. Thibaud and I". Duprc LaTour, J. chim. p h y s . , 29, 133 (1932) i R. Kohlhaas, 2. Krist., 98, 418 (1938). 1 Th. Schoon, Z . physzk. Chem., B39, 385 (1938). k Some films of btraric acid were found to contain crystall~tesof form B as well &s C.
...
...
...
...
...
..
...
..
...
...
O
..
positions for reflections in the diffraction patterns which agreed well with the observed positions. The agreement exhibited in Table I shows that the packing of the molecules in the crystallites in the films is the same, to within cxprrimrntal error, as that observed in studies of larger crystals. No influcnce of the colloidal nature of the thinnrr films was observed on the packing of the molecules in the crystallites.
...
acid which gave patterns indistinguishable from thosr observed in the present study. Trillat and IIirsch'l have found monoclinic forms A, B, and C of stearic acid in crystalline lenses of stearic acid which were. formed on a water surface from benzene solutions. On the othcr hand, Brummagel2 has reported that crystallites in niultimolecular films of n-tetrarosane, n-triacontane, and n-tetratriacontane were of orthorhombic form, whereas in the present investigation, films of the higher even-numbered n-hydrocarbons were Discussion found to exhibit monoclinic structures, provided Several investigators have obtained data for the compoullds 'c\Tere sufficiently pure. It seems films similar to those reported here, including likely that the differencecan be attributed to effects films deposited different ways. For example, of ~ ~ o m o ~ o g oinlpurities us nhicil are extremely Germer and Storks6J have found crystallites of monoclinic forms A and C of stearic acid in multi(I 11 T r Trillnt an,^ T h . v. Irlrsch. conlpt re,ld, 216 ( 1 o w . molcr.dnr ~ , a r ~ ~ u i r - J 3 l o t i g c films tt of stcwir ( 1 2 ) IC c I~ntrrin>,rge,i+OC rto,,. sor ( r , o r l ioIl), 8188, 111 ( 1 0 )1;
difficult to remove, and which can alter the molecular packing, as discussed below. A. E. SmithI3in an X-ray study of single crystals of a series of n-hydrocarbons determined that the even-numbered n-hydrocarbons beginning with nhexacosane are monoclinic a t room temperature. The shorter, even-numbered n-hydrocarbons below and including n-hcxacosane also may crystallize in a triclinic form, Mazee14 has found n-tetracontane to be triclinic at room temperature while Muller and Lonsdale'6 have found a triclinic form for n-octadecane. Indeed, it appears for pure compounds that the orthorhombic form has been found only for the odd-numbered n-hydrocarbons and also for certain n-aliphatie alcohols. 16*17 I n this connection, Smith1* has shown that the presence of even a single homologous impurity in a concentration as low as 1% gives orthorhombic crystals for the even-numbcrcd n-hydrocarbons. Also, Shearer and Vand,lg whilc detcrmining the crystal structure of n-hexatriacontane, observed the crystals to change from an orthorhombic form to a monoclinic form upon repeated crystallization. Analogous observations were made in the present investigatiou when Langmuir-Blodgett multilayers of stearic acid were deposited from a film balance with paraffined sides. Although pure stearic acid was used, it was found that if the benzene solution in which the acid was spread on the water was allowed to spread to the paraffined sides, the Langmuir-Hlodgett multilayers subsequently formed sometimes were orthorhombic instead of monoclinic of form A or C. This undoubtedly was due to the contamination of the acid film on water by paraffin wax. Recently Takogi has made similar observations which he has explained, also, by contamination.20 Analogous effects of impurities also were found in films formed by evaporating solutions. The effects of impurities also may account for the results of Katta and RigamontiZ1who found, in disagreement with the present study of pure compounds, orthorhombic crystallites in films of many n-aliphatic acids, esters, ethers, etc., arid suggested that the orthorhombic form was characteristic of multimolecular films of all these compounds. Although several crystalline forms have been observed in multimolecular films of n-hydrocarbon derivatives, the observation of crystallites with their basal axes parallel to the plane of the substrate surface (considered to be an 001 orientation) has been remarkably consistcnt. Molecular orientations observed in multimolecular, unrubbed films agree to within 1 or 2' with the anglcs that the molecular chains form with the 001 planes in A. E. Smith. J. Chem. l'hya., 21, 2229 (1983). W. M . Masee, Rec. trav. chim., 67, 197 (1948). A. Muller and K. Lonsdale, Acta Cryst., 1, 129 (1948). D. A. Wilson and E. J. Ott, J . Chem. P h y s . , 2 , 231 (1934). (17) D. G. IColp a n d E. S. Lritton, J. A m . Chem. Soc., 73, 8593 (1951). (18) A. E. Smith, International Congress of t h e Union of Crystallography, Montreal, 1067; Alstracts of the Communications, 4, 74 (13) (14) (15) (1U)
(1957). (19) 1-1.M. M. Shearer and V. Vand. A c t o Crust., 9, 379 (1956). (20) S. Takogi. private rommiinication, 1958. ( 2 1 ) C:. Nilttu arid 11. Rigailioriti. /i. C . Accud. Lincei, [n] 2 a , 342 (l!l35~.
the larger crystals of the compounds, as determined by previous structural invest igations.22 Tho 001 surface exposcd by the films is t'hen essentially a met'hyl surface, the surface which, for a crystal, would have the lowest free energy of the crystallographically possible faces. It undoubtedly is significant that the substanccs invest'igated above normally crystallize in thin flat plates exposing 001 faceseZ3 Whether the orientation of the crystallites deposited upon the slides was determined principally by considerations of interfacial free energy, or, somehow, by the kinetics of growth of crystallites which preferentially orient parallel to the surface is not known. It will be shown in the next paper of this series, however, that surfaces of presumably higher free energy can be induced in many of the films by mechanical shear which reorients the molecules. Acknowledgments.-We are deeply indebted to Dr. A. E. Smith and the Shcll Development Company for samples of a scries of extremely pure n-hydrocarbons from docosane through triacontane and to Dr. M. Senkus and the 12. J. Reynolds Tobacco Company for a sample of pure n-hentriacontane. We wish to thank the Ohio Oil Company for its generous aid in the electron micrograph studies. It is a pleasure to acknowledge support from the American Petroleum Institute in the early phases of this investigation. Appendix Controlled Orientation from a Melt.-Films formed by cooling molten compounds spread over a surface were usually too thick for convenient diffraction studies. I t was found, however, that the following method gave films of stearic acid with smooth regions only about 10,000 A. thick and with molecular tilts all ttligncd in the same direction. Molten stearic acid was spread over the surfact of a polished pltttinum slide a t a temperature of 130-140 If one erid of the slide was held against a cold block of metal (-50"), a thin film of solid stearic acid began to form a t the cold end and sprcad down the slide, pushing i n front of it a thick wave of the molten material. The resultant film was of monoclinic form C, and the molecular chains were tilted from the normal to the surface by 38", pointing in the direction of propagation of the wave. Controlled Orientation from Evaporation of Solutions.It was discovered that similar results for stearic acid could be obtained, with thirlrler films, by evaporating a dilute solution of the acid in acetone. If a drop of solution was spread over a clean metal slide and one end of the slide wag placed against a warm surface (slightly under the boiling
.
(22) It cannot be supposed t h a t the metal slides are a t all flat on a n atomic scale. It then seems, a t first, remarkable t h a t molecular directions in a surface film should be related a t all closely t o t h e macroaverage plane of t h e surface. Nevertheless, not only are experimental mean tilts t o t h e average surface identical t o t h e crystallographic tilts, b u t the scatter from the mean cannot be very many degrees, according t o the diffraction patterns. I n all of the films investigated t h e molecular packing was essentially crystalline. It must be concluded t h a t lateral regularity is enornetically more important t h a n precise regie t r y of the low-energy methyl a i d s with the surface of t h e substrate (or in t h e case of some polar derivatives, with the upper surface of t h e adsorbed monolayer). I2lectron Inicrographs show t o within a resolving power of some tens of angstroms (roughly t h e molecular dimensions of t h e long-chain compounds) t h a t t h e polished surfacea are smooth, except for scattered scratches, a n d undulate gently. T h e horizontal tops of t h e undulations then would be t h e regions most accessible t o t h e electron beam in diffraction experimrnts. Viewed from this standpoint t h e close relation between molecular and macro directions seems not unreasonable. 123) See. for examrde, S. Amelinrkx, Acta Cryst., 8 , 590 (1958): 9 , I t i (IDSti); 9, 217 (l!):(i).
C. MYRON ARCAXTI ASD WIT,T,I.AM R. CARROLL
1014
point of acetone) a thin film of solid stearic acid began to form ut thc warm end and spread down the slide, pushing in front of it a wave of the solution. The rrsultant film was
T'ol. GO
again of monoclinic form C with the molccular chains tilted 38" from the normal, pointing in the direction of propagation of the 1r2ivc.
EXTRACTION OF SODIUM TIIIOCYASATE FROM AQUEOUS SOLUTIONS BY TRIBUTYL PHOSPHA4TE BY G.
r\rY1tos
ARCAXD W .~
WILLIAM
1%.CARROLL
Departrnmt of Chmistry, Montana Stde College, Bozeman, .Vontana Recezved September 22, 1961
The extraction of sodium thiocyanate from ayueous solutions by tri-rb-butyl phosphate has been studied and the dltta weie analyzed in several different ways. Values of AFo and K have been calculatcd at 20, 30, and 40" for the complete equilibrium and for the distribution alone. Evidence indicates that a different hydrate is extracted a t 20" than a t the higher temperatures.
Introduction Solvent extraction methods have been used to determine the nature of species existing in certain aqueous solutions. Yost and White2 postulated the existenco of HzOs05 in aqueous solution, Swift, et u L . , " ~ studied the extraction of iron(II1) by ethers to arrivc a t HFeC14 as the predominant species in 6-8 P hydrochloric acid, and Arcand5 used an extraction method to propose arsenic(II1) equilibria in hydrochloric acid solutions. Rfrlnick, et u L . , ~ investigated the extraction of ferric thiocyanate with tri-n-butyl phosphate (TBP) and as a result proposed (l?e(SCN)3), as the cxtracted specics. Baldwin, Higgins, and Soldano' investigated the extraction of various alkali metal salts by TBP and proposed an equation for the extraction Electrolyte(aq)
+ nH,O
= Electrolyte(or)
(1)
A distribution constant can be calculated if this equation is valid. The electrolyt'e in the aqueous phase is considered to be completely dissociated and that in the organic phase completely associated. The authors interpret n t o be the primary hydration number of the cation extracted. The present work shows that the BaldwinHiggins-Soldano (hereafter referred to as B-HS) analysis is valid to a point for the study of the extraction of sodium and potassium thiocyanates. However, other procedures have been developed which, in conjunction with the B - H S approach, lead to knowledge of the composition of species in the organic phase and permit evaluation of some of the thermodynamic constants associated with the transfer of the thiocyanates from t,he aqueous to the organic phase. (1) American Systems, Inc., 1625 E. 126th Street, 1Iawthorneq California. (2) D. M. Yost and R. J. White, J . A m . Chem. Soc., 60, 81 (19281. (3) R. ,I. Myera, D. E. hfeteler, and E, €1. Swift, ibid., 72, 3767 (1950). (4) See also 9. Kato and R. Isii, Sci. Papers Inal. Phue. Chem. Research, 86, 82 (1939); Chem. Abstr., 83, 7232' (1939). ( 5 ) G. h4. Arcand, J . A m . Chem. Soc., 79, 1865 (1967). (6) L.Melnick, It. Freiser, and H. F. Ileeghly, A n a l . Chem., 26, 856 (1983). (7) W. H. Baldwin, C. E. Higpins, and R . A. Soldano, J . Phus. Chem., 68, 118 (1959).
Experimental Tri-n-butyl Phosphate.-For most purposes, Eastman T B P was washed with 1 F HCI, then with 1 P NaOH, and finally with distilled water until the organic phase was clear and the washings neutral. Where high purity was desired, wmhed TB1' was distilled at 3 mm. pressure, the fraction boiling between 121 and 124" being collected. The refractive index was +D 1.4222. Solutions.-Stock solutions were prepared from reagent grade materials. Thiocyanate solutions were standardized against AgEr'03 or KIOa. Fisher Chemical Company stabilized Karl Fischer reagent was used to determine water. Distribution Measurements.-In most cases, equal volumos of aqueous solution and wet T B P were added t o 60-ml. separatory funnels and placed in a con$ant temperature water-bath controlled to within k0.1 Samples were shaken intermittently ovcr a period of 5 hr. and then were allowed to stand in the bath overnight. Weighed portions of both phases were removed for analysis. The organic phases were washed 6 timcs with 5-ml. portions of water, the washings being collected in a volumetric flask. For water determinations, equal volumes of aqueous solution and dry TBP were added to 60-ml. separatory funneh and placed in a constant temperature bath. Samples were shaken continuously for 2 hr. and then were allowed to stand in the bath for about 48 hr. Weighed portions of both phases were rcmoved for analysis. Analyses.-Aliquots of the aqueous phase acidified with HC1 were titrated with standard KIO3 in the presence of CC1, t o determine thiocyanate.8 The end- oint was taken as the disappearance of iodine color in the C&. Thiocyanate concentrations in the organic phase were determined by washing the NaSCN from aliquots of the TBP and titrating the combined washings as described above. Water in the T B P wm determined by dissolving aliquots in dry methanol and titrating with Karl Fischer reagent to an amperometric end-point.$
.
Results and Discussion The distribution data for various aqueous concentrations of sodium thiocyanate at different temperatures are shown in Tablc I. An att,empt was made to analyze these data in several diffcrcnt ways, all of thcm based on thc stoichiomctric cquation Na+(aq)
+ SCN-(aq) + nHnO = NaSCN.nHzO(or)
(2)
Method of B-H-S.-The procedure involves plotting the quantity 6 us. [KaSCK],, where 6 = 2 log (SCX-),, -
____
+ n log ( K O ) - log [NaSCN],, = log Zi' + b'[l\;aSCiV]I,, (3)
(8) R. Gaugin, A n a l . Cham. Acta, 8 , 272 (1949). (9) R. W. Freedman, A n a l . Chem., 81, 1287 (1959).
