THE JOURNAL OF
PHYSICAL CHEMISTRY (Registered in U. 9. P a t e n t Office)
VOLUME66
(0Copyright, 1962, b y the American Chemical Society)
NUMBER7
JULY 25, 19G2
r\;OK-SOhP GREASES. V. THE ISFLUEXCE OF n-IIEPTYL DERIVATIVES O S SILICA-SON-POLAR LIQUID GEL STABILITY BY
JAMES P. \ T I G H T M A N ’
AND
J. J. CHESSICK
Il‘illiam II. Chandler Chemistry Laboratory, Lehigh University, Bethlehem, Pennsylvania Received April 6, 1961
At concentrations as low as 10 -12 wt yohigh area, polar silicas in non-polar liquids were found to form gels. Gel formation ~ma direct consequence of at least trace amounts of water which enhanced particle-to-particle interactions by “cementing” silica particles in a three-dirncnsional network structure. The influence of n-heptyl additives on the resulting gel structure was investigated; the additives included the acid, chloride, aldehyde, amine, and alcohol. Low concentrations (ca. 2 weight % of the amine and alcohol) were sufficient to fluidize the system. On the other hand, the aldehyde and acid influenced gel consistency only slightly. The chloride had no effect. Conccntrations of these ineffective additives aa high aa 12 weight, yo were studied. Fundamental studies utilizing solution adsorption and heat of adsorption measurementa were conducted in stringently dried systems. The results of these studies led to the development of a new mechanism to explain gel breakdown by additives. The mechanism is based on the ability of the additives to remove adsorbed water from positions between the flocculated particles. Excess additive then adsorbs onto the surfaces of the silica particles, thus preventing reflocculation when water is re-introduced into the system. water. One ml. of the I IIi-Si1 > Acrcsil struins in the finished bulbs in order to minimize varintions in the exothermic heat of brcaking.7 according to adsorption and heat of wetting meas4 . Sedimentation Volumes.-The sedimentation volurcments and would appear to bc an important i i m ( % yof Hi-Si1 X303 were measured in decane and in a serics of solutions of the heptyl compounds in decane. The solu- parameter determining gel strength. tions were made up gravimetrically. In one set of eyperi2. Adsorption from Solution.- Initially it was mcrits, the solid was oven-dried and capped upon removal from the own with self-scaling rubber caps. The liquids bclicved that the effect of thclsc additives on gel were introduced through the caps with a syringe. The structure was a direct consequence of strong adsorption of the compounds influential in structural brealinittcr content of thesc systcins is known to be intermcdiate hctween that present in large gel batches and in the liquids down. ,4dsorption studies were bcguii as a conused for heats of immersion and solution adsorption. In sequence. The net adsorption, 42,l0 of heptyl comanother set of euperimcnts, the solid RU exposed to watcr vapor a t several relative pressures and then cap ed. Thc pounds from parafin oil solutions onto Hi-Si1 X303 liquids again were introduced by a syringe. A8er initial as a frinction of the equilibrium mole fraction, Xs, settling, the tubes were reshaken and allowed to stand rior (8) The use of u pioriblon penetroincter might be c r to the measurement of final sediment heights. The vofume percchntage of Hi-Si1 X303 used wa,s 3.1 in all sedimentation giounds that i t hrrs not been proxed t o be a qiiantitati\e instrument. Tho measured penetrntion value In aibitiarv units can be con\ e r t 4 t o studies. true yield values I ortuntrtuly, preclse yield \ uliit.s u orc not necessary 5 . Penetrometer Measurements of Gels.-Reproduciblc gels were prepared using Hi-Si1 X303 (10 weight %) oven here .4dditireactio. as much of a ”go ’-‘ no go” process; an effective dricd a t 100’ for 12 hr., and paraffin oil dried over PzOS. additi\e causrd co ilpiett brcskdo\rn of gel structure yielding a system Prc-mixing was accomplished in a drybox containing PnOa having rheological properties of the unloatlcd liquid itself The and flushed with dry nitrogen. Reproducible dispersions others caused smpll changes in yleld \nllle (0) A. C. Zettlemoyer, G. 3. Toung, J. J. Ctiessick, and F. H A e a l ~ y J. . PILUS.Chem., 51, 619 (1933). (7) C. 11. IIollabaugh, Ph.D. Dissertation, Leliiyli University, IDSO.
(9) J J . Chessitk A C Zettlrino>cr and ( 7 J Young, J Colloid Sei, 13, 372 (1958) (IO) dohscripts 1 and 3 iefw to ieliicle and additive molecules. re. syectivrly
is presented in Fig. 2. The term 42, in moles per gram of adsorbent, is thc product of the number of moles of both liquids initially present and the change in mole fraction due to adsorption of the heptyl compound. This change is defined as the difference between the initial and the final equilibrium bulk mole fractions. E o adsorption of the chloride occurred from paraftin oil solutions onto IIi-Si1 X303; unimolccular films of the acid, alcohol, and aminc formed readily. Monolayer capacities for the adsorption of these compounds were calculated by thrce indepcndcnt methods. Method I involved a simplification of the general equation of solution adsorption." If exclusive adsorption of one component occurs, then a t monolayer coverage n2a = &/X2 where nz8 is the monolayer capacity of the heptyl compound. Method I1 is the so-called graphical method of iVagySchaylla where monolayer capacities wero ohtaincd by extrapolation of the linear portions of the isotherms in Fig. 2 to X2 = 0. blonolayer capacities by mcthod I11 were estimated geometrically from a knowledgc of the arca of the ndsorhatc molccule and the number of adsorbent (silanol) sites capable of interacting with the adsorbate. The geometrical values were used to estimate the theoretical concentration change of the he tyl compound due to adsorption. Exclusive n sorption of the heptyl compound perpendicular to the surface (polar group down) was assumed. The calculations were made over a concentration range in excess of t-hat required to give monolayer co\ariige. The fact that close agrcemcnt, n-ithin lo%, was obtained betnwn the calculated and experimental vdues is offered as evidence for exclusive monolayer adsorption of the heptyl additives. It is significant also that once monolayers formed, the values for the amounts adsorbed could be used to predict accurately concentration changes on the introduction of a known amount of silica in solutions of much higher concentr nt'ion. The constancy of the heat of immersion values ovcr a range of concentration in excess of that required to give monolayer coverage was taken as indcpcndrnt supporting evidence for the belief that only monolayer adsorption of the alcohol, amine, and acid occurred. If this were not the case, heats of immersion would be expected to vary continuously with concentration. 3. Calorimetric Heats.-Heats of adsorption, A H 8 , listed in Table I mere calculated from the equation A H , = AH1 - AHd - AH, (1) whwe AH1 is the beat of immersion, AHd the heat of dissolution of adsorbate molecules, and AH; the enthalpy change for the formation of the adsorbate-solution interface. The application of eq. 1 assumes that mixed adsorption does not occur a t monolayer coverage.12 The heat values AH1 and AHd wcre measured experimentally. Assuming monolayer coverage and orientation of
cp
(11) TYo. Ostwald sild R do Iraguirre, Kollmd-Z., 80, 279 (1922). (lla) L. G. Nagy and G . Ycliay, X a g u n j Kemzaz Foluotrat, 66, 31
(19GO). (1%) J. J Choesiok and A. C. Zettlomoyor, Adoonces In Catalysts, X I , 263 (1959).
10
I
-
01
0.0
I
I
I
0 2
0 4
0 4
S?.
I
1
0 8
Fig. 2.--?rTet isotherms for the :tdsorption of n-hcpt't 1 compounds from paraffin oil solutions onto Hi-Si1 X303 :!t 26" : a, n-heptyl alcohol; b, ?a-heptylainiiie; e, n-heptanoic acid.
the polar end of the hcptyl compound toward the surface, the enthalpy of formation of the adsorbatesolution interface, AH,, was estimated to be - i 8 ergs/cm.2, which is the heat liberated when :i solid covered with a monolayer of heptane is immerscd in heptane a t 25". rr.\BI,E 1 H ~ A TOF S ADSORPTIOX OF HEPTYL C o ~ r w v ~A i ~~ s o n r m i ~ FnoM PARAFFIN OIL SOLUTIOSS ONTO Hi-Si1 X303 AT 2(i" (MONOLAYER COVERAGE) -AH1,
ergs
alcohols > acids > aldehydes >> chlorides. The same order of interaction of
1221
functional groups with water has been reported by KakovskyI6 from a study of the influence of the functional group (or energy of its hydration) on the absolute value of solubility in water. ButlerIB also reported that the amine and alcohol groups have the highest energies of interaction with water. The dependence of the sedimentation volumes on the amount of preadsorbed water shown in Fig. 4 supports the association mechanism; the greater the amount of water present in the gel the greater the amount of alcohol needed to effect deflocculation. Some further results published earlierg corroborate or expand on the above mechanism. Gels containing Hi-Si1 once fluidized by the alcohol or amine did not regain structure on subsequent addition of water up to 4 weight %. Evidently, sufficient additive was added initially to allow formation of a stable protective monolayer in addition to that needed to remove water from between adjacent, flocculated particles. Polar and non-polar attractive forces between adsorbed molecules and the silica surface as well as significant lateral interactions between molecules oriented polar group down contribute in part to films stable to replacement by added water. Gels containing the less polar silica, Aerosil, were fluidized also by the amine and alcohol. Addition of more water, however, displaced the additive from the surface and gel structure reformed. Rutile-paraffin oil gels were fluidized not only by the amine and alcohol but also by the acid; all adsorbed films were stable to added water. Apparently, surface polarity of the solid which is least for Aerosil and greatest for rutile plays an important role in additive action. No comprehensive studies were carried out with Aerosil or rutile gels, however. Acknowledgment.--This article is based on work performed under Air Force Contract, No. AF 33 (616)-7120, under the monitorship of Materials Central, Wright Air Development Division, WrightPatterson Air Force Base, Ohio. Grateful appreciation for aid and encouragement are extended to Messrs. H. Schmenker and J. Christian of WADD. Prof. A. C. Zettlemoyer followed the course of this research carefully and was most generous with helpful comments and criticisms. (15) I. A. Kakovaky, “Proc. 2nd Intern. Congr. of Surf. Act.,” Vol. IV, Butterworth Scientifio Publications, London, 1957, p. 310. (16) J. A. V. Butler, “Chemical Thermodynamics,” The hlacmillan Co., London, 1946.
