Using Silica to Demonstrate Hydrogen Bonding / / / / '\ y '\:
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The efficiency of a. multitude of hydrogen bonds, even though of relatively low bond strength, can be dramatically demonstrated by camparing the fluid character of mineral oil to the more solid character of the same \o/ \o 0 mineral oil t o which has been added a small amount of finely divided silica hearing a large proportion of surface hydroxyls. The hydrogen bonding involved a he pictured as shown in the f i ~ r ein , which these relatively low strengLh bonds occur in such number that they effectively form a three/H dimensional fence through which the molecules of mineral oil have difficulty moving. The effectiveness of the system is demonstrated by tipping a half-filled vial of the mineral oil-silica system on its side showing that / this sample, which appears to he the same as the pure mineral oil sample, does not flow. By tapping the vial with one'spalm, it can be further shown that applicationof energyis c%pableofbreaking the hydrogen bonds and causing the thixotropic mixture t o flow. The re-formation of new hydrogen bonds within the fluid in its altered position readily takes place and a new "shapepe"of the fluid is observed.
41\0/
7'
O \Si
' Si\
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cm%>
Experimental
'ITsing a. typeA33-45-58Barnoh and Lomh refrrtctometer a t 19"C, the refractive indexes of the sample so prepared and of the mineral oil alone were both found to he 1.466.
Discussion Cnho~ilhlSiT, i.i a fumed silica wlth no inrernal surfwe srcu;' hence, thc hgdn,xyls on itsenrire tiurfwr nren of e wr reqdily n r d s h l e f t w hydroget. honding. It w l s fonnd rlrnt hen tin^ tlrp d i m at 1G;"C for :4 hr to rcmorr l noted 111.~1 adcnOl,rtiOl.s of rilira awl mirccral r d used I t s h ~ u h Iw if & bizher viseositv m i n e d oil were used s. lesser amount of silica would be rewired \ I , irnpcrrant niprvt f the derr~mitmtionfrom rlw .tandpoinr ~f thv desi&d cilPvr i- th:xt there be no d~flercnce i n refrqctivc index c l i rlrr m~nemloil and the mincrul d - 4aysrem, ~ ~inJirntiq thar rlw lluil ltsclf war ul.gltercd hy addition of the silica. It can he called t o the attention of students that the hydrogen honding is responsible for the nowNewtonian characteristics of the system and that the useof silica fillers is common in preparation of greases of various kinds. 2fii m? ~~
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Csbot Corporation Industrial Bullet,in, CGen-5 (1968). (2nd Ed.), Reinhold Publishing Corporation, New York, 1967, p. 52.
'HEFTMANN, E., "Chromittography"
Volume 49, Number 6, June 1972
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