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IUPERESCES (1) .ISI)REAS, J . AI.>HALSER, E. .I., ASD TL-c R , W.B.: J. Phys. Cheni. 42, 1001 (193s). (21 BRIDOXIS!P . : The P h v s i c s of High P r e s s u r e , 11. 58. The Macniillan Company, S e w T o r k (1031). (3) B L - ( . K I S G H . S J I , IT.D., A S D DEIBERT, C. It.: J. P h o t . SOC. 12, 610 (1946). ( 4 ) I C I L E R T S , c. Ei.,et d . : .Im.Gas .Issoc. AIonthlj- 28, 435 (1946). ( 5 ) KEYES,F. G . : Proc. Am. Acad. h r t s Sci. 68, 529 (1932-331. F. li;.: J. Phys. Cheni. 45, S46 (1941). (6) llarr;, G. L., Davrs, J. K . , ASI) BARTELL, ( i )POULTER: T. C . : Phys. Rev. 35, 297 (19301. (8) SMITH,G . R.: J. Phys. Cheni. 48, 168 (1944). (0) SMITH,C:. IT., A N D SORG,L. V , : J. Phys. Chetn. 45, 671 (1941).
SILICIC CHElIISTRT’,2
IiRSST A . HAUSER licpcct~triieiit of C‘heiriicnl E n y i i i e c r i n g . Slasscichitsetts Zrzstit i i t c of l’echiioloyi/,
Cnnrbridgc, Jlassnchusetts Keccii’ed Febricaty 20, 1948
“In science the credit goes t o the man u-ho convinces the world, not to the man t o Jvhoni the idea first occurs.” These ivords ivere spoken by the British physician Sir Killiam Osler in the last lecture he gave before the Royal Society of Jledicine in 1918. Onc could hardly find a more striking example of their significance than by offcring a tirief historical reyieiv of the colloid chemistry of silicon. In 1861, Thomas Graham read before the Royal Society in London a paper on “Liquid diffusion applied t o analysis.” €IC differentiated therein betn-een matter lvhich would and substances ivhich ~\-ouldriot diffuse through a membrane. References t o this lecture (3), lvhich must be regarded as the foundation of colloid science, are mostly limited t o the follon-ing statement: “-4s gelatine also appears to he of the non-diffusing type, it is proposed to designate substances of the calass as COLLOIDS, and t o speak of their peculiar form of aggregation as the C‘OLIAOIDAALCOSDITIOS OF lIATTER.” What has droppcd into oblivion, however, is the following sentence: “The comparatively Jired class as regards diffusion, is represented hy a different order of clieniical substances. marked out by the ahscnce of t h e power t o crystallize. .Imong t h e Presented before the Symposium on Colloid Clhemistry of the Silicates, which was held under t h e auspices of the Division of Colloid Chemistr>-a t t h e 112th 1Ieeting of t h e ;\nierican Chemical docietj-, S c w l-ork C i t y . Septemhcr 15-19. 1917. This paper is based on t h e hook Silicic C h e v i s l q by Itheir analogy t o organic substances. ”
Three years later Thonias (;r:iham i v r o t e another paper (4),from which the following excerpts have heen I kcn: 6
“We have no degrees of solutiility t o s p c w k of‘ wit11 respect t o silicic acid, like t h e t l e g r c ~ ~ of solubility of a salt. . . . The ultimate pt~ctizatioti( i f silicic a c i d is prcceded b y :I gradu:11 thickening i n the liquid itself. Thc~p r o d u c t i o n of ihr compounds of silicic acid drscritwd, indicates t h e possession of a i v i d c r i~aiigeof affiriitJ- 11ya colloitl t h a n could well he anticipated. T h e organic colloitls iii’c, I I O douht iiivcsti,ll w i t h simil:ir .\-itle po\\-cr? of conihirint i on.”
Thomas Graham iinquest ionnhly ( l ~ s e i ~ vcwclit c ~ foi, having heen the first to ,convince the irorld tliat :I cdloicial ctonditioii of maitw exists. That siliceous compounds exhibit propertics c*hai~acteristi(* of this st atc hxcl : ~ l i y d yheen mentioned eighty-tivo years prior t o (;rahnm’s first tlisclostiiv. in the dissertation of the Swedish scientist Torhrrn T k g m a n , .‘Ilc Terra Siliwci,” ivhich a fen- yeaw later \\-as reprinted in one of his textliooks ( 1 ) . Tlic follo\i-ing is m i excerpt of the most pertinent part therefrom: ..Fiiially I must still tliiiili of thorc. iiicompictc phcriomt~ii:i,\vliicii tlcpeiid o n a srcmiiig soluhility. This silicious liquor [ a l l d i silicate s ~ i l u t i o n - - I ~ : . ~j ~ is. Iprecipitated ~. hy all acids. hccttuse t h e alkali prefers t o httiig o n t o t h n n r a t h r r t h a n t o til(, g r a v r ~ l . This p r e e i p tateti gravel tias a very expanded aiid loose t r r t u r c . is filled I v i t h ~ a t r ~sor .t h a t it is twelvr times as heavy when moist, t h a n \\-hen d r y . IIowi,vcr it’ one first atltls more M-ater hef(1i.c adding acid, t h e solution remains clear, even if mor(’ arid is theii :iddctl as ivoultl be needed t o neutralyze t h e alkali. THIS IS .\ Pl~X~L71dI.41< I’1II~:SO~II~:SOS : ~ n dthe reason for i t probahly is t h e following: Through t h e dilution w i t h n-:iter thci siliciuus IiarticIPs are very much separated from each other, or iiiade f i i i p r sritl t)etti,r tlistrihuted throughout tlir liquid. =\lthough the particles. hriiig hr,avier t h a n thcx l i q u i t l . should s c t t l r o u t , t h e y can in this case not overcome t h e resistancr due t o f r i c t i o n , tii~c.:tusc~ a greater force n-ill tw needed t o accelerate setiimeritatioii tli:in til(, o n c resulting fi,oiii the difference in specific gravity. The silica particles reni.iiri srispcritlctl i n t h e liqnitl arid at t h c sanic time invisible due t o thc,ir fineness and trrriisp:ircric-y.”
We colloid chemists must hlnmc oiiiwh-es that it took ,so long, nearly a century, for it t o dawn upon us that tlic colloidal properties of siliceous matter are just as important for the explanation of some plienomcna it eshihits as they are with many high-molecular organic vompounds. This is the more sui,prising if onc takes into consideration the fact that silicon in comhination with oxygen account s for 76 per cent of the earth’s crust. Structural chemistry, largely based on the results of chemical analysis combined with x-ray diffraction studies, has shown that silicon is tetracovalent in its chemical behavior. This is in accord with its position in the fourth group of the Periodic System of elements. Silicon, however, differs from carbon, because its maximum covalency can he 6, as exemplified by SIFT-. The tivo elements also differ in their reartivity ton-ardhydration and hydrolysis of their halides and the case of rupture of silicon-to-silicon bonds by water or hydroxyl ions. The most plausible explanation for these differences is based on the size of the atoms (silicon = l . l i 2 A\.; cnrhon = 0 . i i -1.) and the
SILICI(' CHEMISTRT
1167
correspondingly greater screening of the nucleui. charge of the‘ silicon. It d s o should be noted that of all the single bonds bet\veen tetracovalent silicon nncl other elements, the Si-0 h n d with an energy value of 8'3.3 kg.-tal. pci. molt>is hy far the strongest ( I 1). .Inother tool for structural research ivhirh is becoming more :ind mol.(' important is infrared spectroscwpy. \-ibrations involving the Si-0 honcl ha\-e shoivri that the intensity ofthe honds is about five times stronger than that of t h e ('-.--O 1)ondS. .Is far as the ionic. character of silicon 1)onds is concwnetl. ine:isiirciiic~iits of i-rlutive intensities of absorption in the infr:ircd permittc'cl cstinxiting t ixt io of the ionic character of the Hi-0 h n d t o that of the ( ' - 4 h n)t l t o 1~ 2 . 3 ( 1 4 ) . This is in line n-ith the relationship hetiveen electronegativity clifY'c~i.mcy~ancl prr writ ionic c h a i w t e r . For the Si-0 1)ontl it amounts to 51 per cwit, iuicl f o i ' the, ('-0 iwnd t o 22 per ( w i t . The ratio .il:'22 = 2.3. I n the (*:is('of Si-kI: :i11(1 ('-F the ratio, although smaller, is ,still 1 .(i( I 1 ) . 1 hese facts should suffice t o point out lie\\. futile it \voiiltl 1)c t o :ittcmpt t o apply thc. laws of classic>alorganic c~1iemiiti.yt o the c1iemisti.y of silirori ( 1 2 ) . Thew are, ho\veveia, also many siniilarities l)et\v~cnt h c elements cn:ii-l)on a i i c l silicon and their c~ompoiinds1vhic.h are i v o r t h i ~ ~ ) i d i n g'I.'o psphin thpm in spite of the difference het \wen the homeo- n r i t l heteiwpoliii, ljindingb of t h r w rlements, the colloidal state miist be taken more into cwnsideixtiori thwn h ~ i s.SO e. silicon atom is knon-n to share four electi.ons \vith ncighRut eacali oxygen atom needs tivo electrons for satiii,ation. '1'11~1 silicwn-oxygen tetrahedron (figiire 1 ) is not satiirated. If the oxygcn :[toms : I I Y ~ i~eplacedivith hydiusyl groups, the tetixhedron then is s u t i i i x t r d h y a i i s e c ~ i c . l i Ii>diwxyl needs only one elect ron. T h u s , hy sharing the right niim1)or of 0 1 p ( s trons, t \ v o or moi'e atonis :ire firmly hel(l togctheiy in this spwifir r:isp. t y 4 ' c i . c ~ t i c ~ o is grm.ixlly made to a chemicul hontl 01' 21 (#ovalent1)oncl. If sex-era1 silicon-oxygen tetri-lhedix iii'c c*omhined so that t i i ~ :il\vays ) s1i:ityX one oxygen, LL chain-like striirtiii,e i,esiilts. In such :I cahain. tivo of thc osygpn atoms helonging t o one silicon atom remain unsatiu:itcd. This tlt~fic*irnyyma\' bc) compensated hy attaching 21 sodiiim atom to ?very iinsatui,atcd oxyg:r'n. .\ tihrillar aggregate is thus formed, \vhicsh, if clispersetl in ivate1.. i ~ c p e s o ~ i tt ls i ~ stiurture of sodium silicate 01' \\.atel' glass (figiii-e 2). The f a c a t t h u t thc socliiim ion hydixttls appreciahly :ind the presence of sucah long-c*hain:iggr~g:itoa ofl'clr. :I simple explanation for the soluhility of sodium silic and the high yiscosity of its solutions. 'The entanglement of these filirillar :i pg;itcs on clcsic*c;1tion;~lsci esplains the formation of coherent sodium silicate filnis. Their. strurt i i w is comparable t o films obt ainecl from long-chain h i g h - m o l ~ c ~ i i l u i ~ - \ ~oi,g:itiica ~~igl~t cwmpounds s1ic.h as nihber or c.ellulosr. If' sodium silicntr. i.q l~eac~tr~cl it1 soliit ion lvith a solution of t i n arid, the sodium will exchange for thch :[rid hycli,ogen, t tills forming ii silicsic: acid chain (figure 3). If t \ v o 01' moi-cl of thcsc togr,ther or tangle up, condensat ion het\veen neigh1)oririg h y d i ~ ) IVater is split off' and the chains are no\v held together by shai A loose, unoriented, hut nevertheless rigid t i~idimcns;ion:ilst rii(4tiiiv ly~slilt,s,i l l i t 1 :I hyclroiis silic:~grl has hccw foimc~tl. r ,
I
\ I ,
.. ,
, . I , / ,
.
I
,
I
...
/
\
>
' , . . , I . .
,
SILICIC CHEMISTRY
1169
glass is obtained. The oxygen in the vertex position of every tetrahedron only remains unsaturated. Similar sheets may be formed by joining magnesium or aluminum octahedra together (figure 1). If a sheet of aluminum octahedra is superimposed on a hydrated silica sheet, they are held together by secondary valence forces, but they can also condense. This is how the most abundant clay, kaolinite, n-as formed. \Then a sheet of aluminum hydroside octahedra is sandn-iched betn-een two silica sheets and condensation follows, the clay mineral pyrophyllite is formed. If the aluminum hydroxide is replaced by magnesium or iron hydroside, the minerals talc and nontronite, respectively, result by condensation. If some of the silicon is replaced by an ion of lower valency, ITe have built up the basic structure for mica. If we replace the aluminum with an ion of lower valency, the nucleus for the highly reactive clay mineral bentonite has been formed (5).
*-Si @-OH FIG.4. ('oiitlensation of silicic acid chains t o form silica sheet
TTTeon-e these deductions t o the combination of results from chemical analysis and x-ray diffraction studies (2). The first xrisual proof for the assumed platey shape of these clay particles has only fairly recently been offered by electronmicroscopic studies ( 5 ) . -A11 this information, however, is still inadequate to offer a satisfactory esplanation for many of the properties exhibited by siliceous matter, just as classical organic chemistry alone is unable to account for some properties eshibited by many organic colloids, as for example gelation, the high viscosity of their solutions, and elasticity. It is here that the importance of colloid science becomes self-evident. It is the bridge which spans the gap in our knodedge of the properties eshibited by matter present in the range of dimensions we may no longer neglect. To avoid coining new terms for this branch of colloid science, let us see if one cannot adopt some of the terminology already familiar to scientists specializing in the chemistry of high-molecular organic compounds. \Ye shall classify the silicon hydroxide tetrahedra and the octahedra of magnesium, aluminum, and iron hydroxides as monomers (figure 5 ) comparable t o isoprene or styrene. By
1170
ERNST A. HAUSER
condensation of these monomers individually we produce a polymer comparable t o natural rubber or to polystyrene (figure 5 ) . If the silicon hydroxide polymer reacts with the aluminum or magnesium hydroxide polymer, or mixtures of these by further condensation, the result is an inorganic copolymer, comparable to the copolymers of butadiene and styrene, or of isobutylene and isopreiie (figures 6 and 7). The resulting particle now represents the nucleus of a colloidal micelle. Its properties will depend on the distribution of electrical charges in its surface. The finer the particle, the more surface is exposed and the more pronounced will be the tendency t o have its electric charge, caused by the unbalanced electron
I
6 OH 4 Af 6 OH
OH 4 AI
4 OH
4
si
6 0 (a!
(d
I
FIG. G . C o p o l p i e r s : (a) prior t o condensation (halloysite) ; (b I after condensation (kaolinite).
distribution, neutralized. K h a t this implies can best be explained by offering a few examples. The question why solutions of alkali silicate polymers have such high viscosities and set t o a gel like many high-molecular organic colloids has baffled chemists for a long time. Quite recent Ivork proves that the comparatively high viscosity of mater glass solutions and their gelation upon the addition of acids or electrolytes carrying cations of higher valency, or less hydratability, than sodium ion are based solely on the morphology of the alkali silicate particle. Sodium silicate, for example, owes the viscosity of its solutions t o the water hulls, or lyospheres, carried by the sodium ions, which must be considered as ionizable counter ions and not as fixed components of the silicate structure. These lyospheres also prevent the colloidal particles from condensing and forming larger particles. Upon the addition of an acid or a multivalent cation, a base-exchange reaction
6 0 4 SI'
4O+ZOH
6 M8 4 0 i ZOH 4 si
6 0
/ K 60
3 Si f / A ( 4 OfPOH 4 A( 40fZOH 3 Si f / A /
6 0 l K (C
SILICIC CHEJIISTRY
1173
Other fields which n-ould benefit by paying more attention t o the colloid chemistry of siliceous matter are geolog;v, petrography, the ceramic and the paper industries, agriculture, highway construction, soil solidification (13), and flood control, t o mention only a few. They all depend on the surface composition of clays and the colloidal phenomena based thereupon. I n ceramics it is plasticity and shrinkage ( 5 ) of the calay slips, in the paper industry dispersion and coating ( C ) , in agriculture the fertility of the soil, in flood control the assurance that sufficient permeability is maintained (13). In geology and petrography it is the phenomenon of rheopexy \vhich d e ‘ s e r ~ ~pecial attention. This is particularly eo, sincc it has been shown that its owuiwnce is most pronounced if colloidal clay is contaminated u-ith colloidal iron oxide ( 5 ) . ThiP phenomenon is probably responsible for the layer-type deposits of lays and in petrography for the petrifaction of such delicate structures as jellyfish. The Solnhofen slates are famous for these and it has been proven that they LW’ largely composed of c*olloidalparticles of siliceous matter ( 5 ) . In closing this survey, I should like to wfci, t o a statement made nearly thirty years ago hy I: famous A i m t r i m chemist I h . Rudolf Kegscheider. He said: # $predict I that thp day \vi11 come \\-hen the> chemistry of Silicon n3ll lie a serious competitor t o organic chemistry.” It is hoped that colloid chemists Tvho have heen active in iwearch \\-ith c~ompountlxof siliceous compositions will realize th:it this day h a s come, and that it therefore is not presumptuous t o coin a nen- name for this branch of colloid science?-namely, “silicic chemistry.” Silicic chemistry has lieen I m n and is groiving fast, and as ivith the development of every child, \ye X I Y in for many surprises! ~
SlXJLiRI-
T h e history of the colloid chemistry of silicon is reviev.ed t o prove that this hranch of the science of colloids is even older than the term *‘cxolloid’’itself. The opinion that certain properties of matter, such as the formation of solutions of high viscosity and the ability to form gels, are limited to organic compounds is disproved. -ittention is dran-n to the similarity of the reactions responsible for the formation of organic’ and siliceous colloids and a uniform terminology for these reactions is suggested. Examples of diceous compounds exhibiting the same properties as organic ones are offered. T’arious colloidal phenomena characterist ic of silicon compounds and their importance to science and industry are discussed. Attention is dran-n t o the versatility of this ne\\- field, and the suggestion is made t o give it the name “silicic :hemistry.”
.I ticioi (17 I ( Only a fen- days before the author presented this paper, he was visited hy Professor S i 1 9 Hnst of the Royal Technical 1-niversity and the Sohel Institute ’or Physics, in Stockholm, Sweden. Professor Hast gave him a detailed clescrip-ion of his new technique for studying the structure of clays and diatomaceous hells u-ith the electron microscope (Sature 159, 354, 370 (1947)). This new
1174
ERSST A , HAUSER
method of sample preparation has enabled Professor Hast t o increase the microscope’s resolving power far beyond what has been possible so far and t o make the changes which the original structure undergoes during hydration and subsequent dehydration act’ually visible. Besides this, it has enabled him even t o make the individual layers of clay particles discernible and also the process by which they are stacked up. R E F I ~I ~< Sl (~ (1) BERGMAS, T. : Kleine phipische umi cheniische W’erkc, Vol. 3, p. 391.
F r a n k f u r t a/lIain (1785). ( 2 ) BRAGG, W. If., ASD BRAGG, W. I,.: X-liciys arid Cr!gstnl Stritctrci.c. h. Bell and Sons, L t d . , London (1925). (3) G R A H A ~ IT, H . :Phil. T r a n s . 1861, 183; l’roc. l i o y . Soc. (London) 11, 243 (1861). (4) GRAHAXI, TH.: Proc. Roy. Soc. (London) 13, 335 (1864). (5) HALXER, E. h.:Chem. Revs. 37, 287 ( l W 5 ) . (6) HAI-SER, E . -1.: Paper T r a d e J . 8-12-193i; S-24-1939. ( 7 ) H A ~ S EE R,,A. : t-.S.patents 2,266,636; 2,266,637; 2,266.638; 2:317,685. (8) HACSER, E, A , , A S D DASNESBERC, 1;. 11.:1’. S. patent 2,101,348. (9) H ~ I - S E E. R ,A , , A S I ) LE DEAI-, D. S.: .J. Pliys. C:heiii. 42,961,1038 (1938) ; 83,1037 (1939) ; 45, 54 (19-11);J . AUessnder’sColloid Chcviietr!j, Vol. V I , p. 191, Reinhold I’utilishing Corporation, S e x T o r k (19-16). (10) LARSEX,D . €1,: I n J. .ilesander’s Colloid Cherriist~’y,Vd,V I , p . 509. Iteinholtl l’uhlishing Corporation, ?;ew Toric (19iG). (11) PACLING, L . : T h e S a t r c r e o j the Cherjiical Noritl. Cornell University Press, Ithnca, ?;ew York (1940). (12) ROCHOW, E. 1.: A n I n t ~ ~ o d u c t i oion t h e ChcrriisLry oj l i t e Silicorzcs. John Wiley a n d Sons, Inc.; Sen- T o r k (1946). (13) WISTERKORN,H. F . : I n J . Alexander’s C d l ~ i t Cheniis/ry, l Vol. V I , p. 459. Ikinholtl Publishing Corporation, S e \ y York (1946). (14) \ v R I G H T , x., A N D H n - T E R , 11. I-.: J . .\I?I, Chem. S O C . 69, 803 (1947).
