840
R. E. BENSON AND J. E. CASTLE
561 - 68 = 493. Thus, we believe that 68 cm.-l is a likely value for the torsional frequency but it cannot be regarded as certain. This frequency corresponds to a cosine potential barrier of approximately 6000 cal./mole. CF3CH2C1.-The infrared spectrum in the low frequency range is shown in Fig. 3. The strong band a t 350 cm.-l was reported by Nielsen, Liang and Smith2 a t -355 cm.-l. We did not find the weak band they reported a t 330 cm. -l although we cannot claim it definitely absent because of an interference with a line due to a trace of HzO. The lower frequency portion of the spectrum is new. The strong band at 180 cm.-' is presumably the same fundamental as is observed a t 191 em.-' in the Raman spectrum of the liquid. The 11 cm.-l shift is larger than is usually observed. The band at 212 cm.-l is probably the overtone of the torsion observed a t 109 in the Raman effect. The 260 cm.-l band could be accounted for by difference combinations but it seems more likely to be the fundamental previously assigned to 330 em. -l. While the vibrational assignment of Nielsen, Liang and Smith is undoubtedly correct in most
Vol. 62
features, it seems to us unlikely a t a few points. The approximate product rule was applied to C1 for H substitution from CF3CH3 and the results tended to confirm the need of some revision. The revisions we thought most likely in view of trends from other molecules, however, are in conflict with the polarization data for the Raman spectrum. For example, the CH2 symmetrical bending frequency is usually near 1450 and the CH2 twisting near 1300 cm.-l; whereas, in this case the polarization values require the reverse. I n view of these difficulties we shall not attempt a complete assignment. CF3CHCI2.-The spectrum for this substance is given in Fig. 4. All of the bands correspond t o those observed in the Raman spectrum by Nielsen, Liang and Smith,2who also observed the 376 cm. -l band in the infrared. We have not examined their assignment as a whole but agree with respect to all of the low frequency bands in our spectrum. We doubt, however, that the torsional fundamental is as high as the 141 ern.-' value they select. This value would appear more reasonable for the overtone and we tentatively select a value of about 72 cm.-l for the fundamental.
1
REACTIONS OF FRESHLY FORMED SURFACES OF SILICA BYR. E. BENSON AND J. E. CASTLE Contribution No. 464 from The Central Research Department, Experimental Station, E. I. du Pont de Nemours and Company, Wilmington,Delaware Received March 14, 1068
The rupture of silicon-to-oxygen bonds in silica has been shown to produce active sites that are capable of reacting with olefins and with alcohols. These reactions can be effected by grinding silica in the presence of organic liquids or by simple mixing of the organic materials with silica that has been ground and stored in an inert atmosphere. A probable mechanism for these reactions is presented.
Several phenomena have been observed that sug-' Fand organophilic product containing organic magest that certain freshly formed surfaces have unu- terial equivalent to 1% styrene and having a weight sua1 properties. For example, atomic nitrogen has median diameter of 0.4 p was obtained, using conbeen detected in the triboluminescent glow pro- ventional ball-milling techniques. Analytical data, duced during fracture of sugar crysta1s.l The including surface area determinations, .indicated grinding of silica in air has been reported t o give that approximately one-half of the surface was atomic oxygen,2 and sugar has been stated t o be covered by organic product and that the degree of adsorbed from aqueous solution by freshly ground polymerization of styrene was about 10. The orquartz.a The adsorbed sugar was not removed by ganic material was not removed by solvent extracwashing with water and only a small portion was tion or vacuum drying and was largely retained afremoved by washing with alcohol. Effects ob- ter refluxing in water in the presence of a wetting served on grinding quartz in the presence of alcohol agent. Similar, though less organophilic, products and other organic compounds recently have been were obtained by grinding fused silica with such reported4 with reference to our work5 which is now vinyl monomers as chloroprene, butadiene, 2described in detail. Our study was undertaken t o methyl-5-vinylpyridine and P-dimethylaminoethyl examine the reactivity of freshly formed surfaces methacrylate. The products derived from the two vinyl monomers that contained basic groups could toward organic reagents. It was found that grinding fused silica in the pres- be dyed with conventional acid dyes to give colored ence of certain organic compounds produced silica silica powders that were washfast. This effect ofpowders having chemically combined organic fers striking visual evidence of surface modificagroups. I n the instance of styrene, a hydrophobic tion. was ground in When fused (1) F. G. Wick, J . Opt. Soc. Am., 27, 275 (1947); H. Longchambon, hydrophobic and organophilic powder was also obCompt. rend., 174, 1633 (1927); 170, 691 (1923). (2) W. A. Weyl, Reasarch, 3, 230 (1950). tained. I n contrast to styrene-modified silica, (3) Von W. Engel and L. Holzapfel, Kolloid 2.-11%160 (1950). became hydrophilic on refluxing with this (4) H. Deuel and R. Gentile, Helu. Chim. Acta, 39, 1586 (1956). water. Products having less organophilic proper(5) L. M. Amett, M. F. Bechtold and R. E. Benson, U. 8. 2,728,732 ties were obtained from hydrocarbons including (December 27, 1955).
.
July, 1958
841
REACTIONS OF FRESHLY FORMED SURFACES OF SILICA TABLE I BALL-MILLINQ EXPERIMENTS
Time, Reactants hr. 400 g. fused silica 70 460 ml. styrene 400 g. fused silica 48 300 g. styrene 60 g. fused silica 48 100 ml. a-methylstyrene 100 g. fused silica 48 112 ml. p-dimethylaminoethyl methacrylate 50 g. fused silica 48 75 ml. 2:methyl-5-vinylpyridine 100 g. fused silica 46 100 ml. methyl methacrylate 100 g. fused silica 66 125 ml. acrylonitrile 50 g. fused silica 40 38.3 g. chloroprene 110 g. fused silica 7 80 ml. butadiene (-70 to -30’) 50 g. fused silica, 15 50 ml. 1-butanol 100 g. fused silica 20 178 ml. 2-octanol 50 g. fused silica 15 50 ml. toluene 100 g. fused silica 60 180 ml. benzene 100 g. fused silica 60 180 ml. cyclohexane 100 g. fused silica 20 150 ml. cyclohexylamine 100 g. fused silica 60 200 ml. acetic acid 100 g. fused silica 20 200 ml. tetrahydrofuran 400 g. glass sand 24 800 ml. styrene
Organophilic testa ++(Be)“
+
+++(Be)
+ + +(Be)
Particle size,) P
1.2 (2.24 m.a/g.) 0.74 (3.7m.B/g.) 0.30 (9.1 m.Vg.)
Analyses, % OGginal product HydrolyEed product H C H ti 0.86 0.18 1.53 0.20 (1.05) ( .14,d .98 .17 .18 1.46 (1.08) ( .15)d -17 1.09
+(Be)
0.35
.13
.I6
.17
+(Be)
.63
.15
.39
.18
+ +(Bu)
.21
.ll
+(Bu)
.12
.09
.26
a19
.25
,120
.13
.ll
.08
.ll
.05
.08
.08
.10
.40
.14
.29
.08
.17
.10
.45
.13
.ll
.ll
.22
.07
.10
.ll
.28
.09
.28
.ll
.47
.08
++(Be) + +(Be)
6.3
14.5
+++(Bu)
+++(Bu) +(Be)
+ + (Bu) + (Bu) + +(Bu) - (Bu)
++(Bu) +++(Be)
Organophilic test after hydrolysis
a Organophilic test is based on partitioning product between water and immiscible organic liquid (Be = benzene, Bu = I-butanol). I n this test a small amount of the product is placed in a test-tube and 5 ml. of water is added. The mixture is shaken and an equal volume of the indicated organic liquid is added. The mixture is shaken again, allowed to stand, and the location of the silica product is observed. ff , entire product goes to liquid-liquid interface; major about one-third of product goes to liquid-liquid interface; -, little, portion of product goes to liquid-liquid interface; f, From surface area measurement? (BET method). Poor fracture. if any, of product goes to liquid-liquid interface. Analysis was made on material that suspended readily in benzene. d Data for product extracted with benzene in Soxhlet apparatus for 24 hours,
+
benzene, toluene and cyclohexane. The organic content of these products was considerably less than those derived from styrene. In extension of this reaction, it also was found that polishing a large quartz crystal in styrene resulted in the formation of a hydrophobic surface as indicated by wetting-angle measurements. These reactions promoted by fracture are believed to occur on highly reactive sites formed by rupture of silicon-to-oxygen bonds. Life studies of these active centers indicated that they were capable of reacting with styrene and with 1-butanol for more than 260 hours after formation, if kept in an inert atmosphere. However, they were essentially unreactive toward aromatic and saturated aliphatic hydrocarbons unless these materials were present during fracture. Experimental Materials and Method.-Fused silica (4-8 mesh) was obtained from the Amerisil Company. In most experiments “Burundum” cylinders (available from U. S. Stoneware Company) were used as the grinding aid. Unless othdrwise noted, vinyl monomers containing the usual stabilizing agents were used in this investigation. I n the general procedure, the fused silica and grinding aid were placed in a conventional porcelain ball mill and covered with the organic liquid. The vessel was flushed
++,
with nitrogen, sealed and rotated for the indicated time. The vessel was opened, solvent was added, and the product was decanted from the grinding aid. The solid was suspended in a hot solvent that was suitable for removal of any low molecular weight polymer present and wa8 isolated by centrifugation. This extraction was repeated twice. The product was dried in air and finally in a vacuum oven at 130’ for 24 hours. The analytical data for these products are given in Table I. Hydrolysis studies were conducted in the presence of a wetting agent (usually “Triton” X-100). After being refluxed in water for 24 hours, the product was isolated by filtration or centrifu ation. The solid was dried in air and washed with fre& solvent that had been used for the extraction process. The analytical data are recorded in Table I. In many cases it was difficult to avoid some loss of fine particles since they tended to centrifuge or filter poorly. Undoubtedly these small particles had a proportionately higher organic dontent than the bulk of the material. Thus, loss of this finely divided material may account in part for the lower carbon values that were obtained after some extractions and, particularly, after hydrolysis experiments. Particle Size Measurements.-Particle size was determined by nitrogen absorption (BET method).o I n one instance, the weight median particle diameter, as determined by microscopic examination, was 3.8 p but nitrogen absorption measurements indicated a weight median di(6) S. Brunauer, P. H. Emmett and E. Teller, J . Am. Chem. Soe., 60,309 (1038).
842
R. E. BENSON AND J. E. CASTLE
ameter of 0.76 fi (3.6 m.”g.). Examination of the product with an electron microscope showed it to be composed of aggregates of small particles that were not visible with a light microscope, thus explaining the discrepancy in particle size measurements. The extent of surface coverage was determined by the method of Shapiro and Kolthoff .7 The styrene-modified silica powder having a surface area of 3.6 m.2/g. and containing O.So/, styrene was found to have a hydroxyl area corresponding to 1.5 m.2/g. This indicates that approximately one-half of the surface is covered by organic material and that the degree of polymerization of styrene is about 10, based on the area covered. Life Study of Active Sites.-Fused silica was ground in a helium atmosphere. A portion of the solid was removed at the indicated time intervals and added under helium to test-tubes containing various liquids. The resulting mixtures were agitated for 24 hours in the absence of grinding. The products were removed by filtration, washed with suitable solvents, and allowed to dry. The styrene product was extracted twice with warm benzene. The results obtained with silica powder that had been aged for periods of 1-260 hours are summarized in Table 11. The surface sites remained active to styrene and to butanol for more than 260 hours after formation as indicated by positive organophilic tests. For control purposes, a portion of the ground silica was aged in moist air for 24 hours and then added to styrene and to butanol. The resulting products were not organophilic.
TABLE I1 LIFE OF ACTIVE SITES~ Time
After 1 hr.
Organic compd.
Organophilic test
Analyses, % C H
(Be)
++
Styrene 0 . 2 6 0.17 Ethylbenzene .05 .06 To1uen e .os .09 Acrylonitrile Slightly .04 .OS 1-Butanol After 120 hr. Styrene 1-Butanol After 260 hr. Styrene Slightly 1-Butanol Control < .05 .05 a Silica was ground under helium and added to the reagents a t the indicated time intervals. The resulting mixture was agitated in the absence of grinding for 24 hours.
+
+++ + ++ + +
Similar results were obtained when quartz was ground under helium and added to various organic liquids. Polishing of Quartz Crystals in the Presence of Styrene.A quartz crystal (2 cm. square) was polished by rubbing against a lass surface with a silicon carbide-styrene paste. The polisaed crystal was placed in a beaker containing warm benzene, washed by decantation several times, and then carefully wiped dry. For control purposes another quartz crystal was polished in a similar manner using a paste of silicon carbide in water. The crystal which had been polished in the presence of styrene had a wetting angle with water of 55’ compared to a wetting angle of 13’ for the control. An unpolished quartz crystal that had been allowed to stand under styrene and then washed with benzene had a wetting angle of 27’. These data indicate that the surface of the quartz cryst,al that had been abraded in the presence of styrene had been modified. Electron diffraction studies of these crystals were not.conclusive, but a less pronounced quartz pattern was found for the styrenepolished crystal than for either the water-polished crystal or the un olished crystal that had merely stood under styrene. &nee the depth of covering is important in electron diffractionstudies, it is possible that more of the surface was covered with styrene than was indicated by wetting angle measurements. Dyeing of Silica Powders Prepared by Grinding Silica in the Presence of Basic Vinyl Monomers.-The silica powder obtained from the grinding of silica in the presence of 2methyl-5-vinylpyridine was refluxed for 4 hours with 100 (7) I. Shapiro and I. M. Kolthoff, J . A m . Chem. Soc., 72, 776 (1950).
Vol. 62
ml. of a 0.5% solution of a conventional blue dye of the acid type at a p H of 1. The product was removed by filtration, washed with water, acetone and finally with water. The solid was washed with warm water containing “Triton” N100, with water, and finally with acetone. The product was a light blue powder. A light red silica powder resulted when the methylvinylpyridine-silica product was treated with a conventional red dye. Similar results were obtained when the product derived from silica and dimethylaminoethyl methacrylate was treated with these dyes. In control experiments, unmodified silica was subjected to the above dyeing conditions and subsequently washed with water and solvent to give a colorless powder. In addition, attempts to dye styrene-modified silica powders under these conditions were unsuccessful.
Discussion During the fracture of silica, silicon-to-oxygen bonds are broken, and Wey12 has postulated the formation of fragments of the type -0
\
-0-Si@
-0
/
+ eOSi-0-,O\
0-
on the surface of the particles. Weyl has pointed out that it is not possible for these fragments to stabilize themselves through loss or gain of electrons in an inert atmosphere. I n our work it has been shown that the sites formed by rupture of siliconto-oxygen bonds by grinding in an inert atmosphere have an appreciable life. Thus, they are capable of reacting with styrene andwith 1-butanol for more than 260 hours after formation. These organic compounds are chemically combined, as indicated by failure to remove the organic material by solvent extraction or by vacuum drying. I n addition, the silica product derived from styrene retained its hydrophobic properties after treatment with water, while the product derived from butanol became hydrophilic. These results indicate that styrene is banded through carbon-tosilicon bonds which are not sensitive to water, whereas 1-butanol is bonded through hydrolyzable silicon-to-oxygen bonds. These data are consistent with a mechanism involving attack of the electrophilic reagent %Ossi@ on styrene t o yield =osSi-CH2CH-CeH5 which can react further with styrenee according t o well-established, acid-catalysed polymerization mechanisms* to give ~ 0 3 Si(CH2CH),CH2CHC6HS. Termination can occur
l
e
CsH6 by reaction of the growing chain with =O3SiO- on the surface of the particle, or disproportionation, etc. I n the case of 1-butanol, attack of the electrophilic reagent would yield the ester derivative =O3SiOC4H9 which would hydrolyze tfo give 1butanol with subsequent loss of hydrophobic character for the silica powder. A mechanism involving free-radical processes is less attractive since vinyl monomers containing reagents known t o inhibit free-radical polymerization have been used in this study. I n addition, it would be expected that radical att’ack on 1butanol would occur on carbon rather than on the oxygen atom. Furthermore, the monomers that are most effective for surface modification are those (8) L. B. Hammett, “Physical Organic Chemistry,” Hill Book Co., Inc., New York, N. Y., 1940, p. 307.
McGraw-
-
July, 1958
VOLUMEEFFECTS ON MIXINGIN LIQUIDBi-BiCl, SYSTEMS
that are known to undergo polymerization catalyzed by electrophilic reagents. On the basis of data available, it is not possible t o indicate a mechanism by which surface modification takes place when silica is ground in the presence of hydrocarbons such as benzene and cyclohexane. In view of the energy required to break the silicon-to-oxygen bond it seems probable that under grinding conditions highly reactive sites are formed. Thus, temperatures of short duration
'
843
approaching 1000" have been noted during friction of silica surfacesQand the ball-milling of quartz jn a calorime,ter has indicated quartz t o have a specific surface energy of 107,200 ergs/sq. cm.l0 I n view of these high energy requirements, it appears probable that a dual thermal-ionic process is operating during the grinding of silica in the presence of various organic compounds. (9) F. P. Bowden, Nature, 166, 330 (1950). (10) A. I
