106
WALTERBAUER,NEILLWEBERAND STEPHEN E. WIBERLEY
Jenkins and Rideal did not pretreat their gases in this way and they report half times in the order of a few minutes, Both of these groups report an activation energy of about 10 kcal. (10.7 and 10.2 kcal.), but the activation found in this study was only 8 kcal. There are several rationales possible for this difference but nothing conclusive can be said. The 8 kcal. figure was obtained on precarbided films which were baked out with hydrogen between runs. These films should have been in a condition similar to those of Jenkins and Rideal but possibly quite different from those of Beeck, Smith and Wheeler, who did not precarbide their films. The most likely reason for the 2 kcal. discrepancy is that the films used in this study may have been slightly poisoned by some unknown contaminant, probably oxygen. The usual theoretical discussion for an
Vol. 62
idealized surface suggests an increase in activation energy resulting from poisoning. But on a real surface, with a spectrum of adsorption energies, it might be that the activation energy could decrease since the poison molecules would preferentially cover those sites with the highest heats of adsorption.7 An attempt was made to see whether it would be possible to distinguish now between the Rideal and Langmuir-Hinshelwood mechanisms. *Itwas concluded that it is not possible t o do so. Furthermore, because of their marked similarity it does not seem possible to devise a way of distinguishing between them.4 The authors wish to acknowledge the generous financial assistance of the California Research Corporation. (7) J. G. Foss, ibid., 60, 1012 (1956).
FLOW PROPERTIES AND STRUCTURE OF PEPTIZED ALUMINUM SOAP-HYDROCARBON GELS BY WALTERH. BAUER,NEILLWEBERAND STEPHEN E. WIBERLEY Rensselaer Polvtechnic Institute, Troy, New Yovk. Received September 80, 1967
Characteristic properties of aluminum soap-hydrocarbon gels, which appear above a critical concentration region, may be explained by a network structure with weak binding forces developed a t points of contact between non-coiling olymeric aluminum soap polymer units of constant diameter and of average length increasing with soap concentration. k g h viscosity, Newtonian flow and elastic properties are exhibited a t very low shearing stresses. These are replaced by flow properties typical of concentrated solutions of high polymers as increasing shearing stress during flow and increasing temperature supply the requisite energy to overcome intermolecular linkage forces, and as network structure formation is reduced by interaction between peptizer molecules and soap polymer units. Flow measurements in capillaries were made on aluminum dilaurate soap toluene gels at various temperatures and with varying amounts of m-cresol, and on aluminum di-3,5-dimethylhexanoate gels in toluene. At very low shearing stresses initial elastic extension, viscous flow at constant stress, and elastic return on release of stress were measured for the aluminum dilaurate toluene gels and the gels containing ni-cresol as peptizer. Results are interpreted in terms of the slructure postulated. Rate of shear iersus shearing stress curves show that a t high rates of shear flow properties are those expected from concentrated solutions of long rigid polymer particles whose degree of alignment increases and whose network linkage decreases as shear rate increases. Peptizers reduce low rate of shear viscosities, and elastic moduli, a t low shearing stresses and reduce the critical shearing stress between Newtonian and shear rate sensitive flow regions. Energy of activation for viscous flow varies with temperature interval, shear rate and peptizer added, corresponding to changes in amount of network structure.
The flow behavior of solutions of aluminum soaps in hydrocarbons1 is similar in many respects to that shown by polymeric solutions such as the cellulose nitrate in butyl acetate studied by Philippoff and Hess2 and by suspensions showing structure, as the bentonite water systems reported by Rehbindere3 The time dependent elastic properties and the viscous flow exhibited a t low rates of shear, and the flow with shear rate thinning found above a critical stress indicate that the aluminum soaps are dispersed polymerically with a network structure a t low rates of shear resulting from cohesion of the dispersed units a t favorable points. Primary differences in nature of the aluminum soap polymer and typical coiling hydrocarbon polymers are that the soap polymer is weakly bonded and varies in length in dynamic equilibrium as the result of the opposing actions of bonding on favorable collisions and of disruptional thermal forces, while in the case of the long chain hydrocarbon (1) N. Weber and W. H. Bauer, THISJOURNAL, 60, 270 (1956). (2) W. Philippoff and K. Hess, 2. phyaik. Chem., B81, 837 (lQ36). (3) P, R&bj~der,Disc. Faraday $ocI, is, 151 (19541,
polymers the molecular weight does not vary reversibly with concentration and thermal action. Linear aluminum soap polymer molecules, as proposed by McGee4 and Ludke,6 are formed by comparatively weak bonding forces involving aluminum atoms, oxygen atoms from hydroxyl groups, and shared carboxyl oxygen atoms from the fatty acid molecules. Debye, Barber and Autrey6 concluded from diffraction and viscosity measurements on dilute solutions of aluminum dilaurate in cyclohexane that the soap was present priqarily as elongated compact micelles of about 40 A. in diameter,, increasing in average length from 200 to 1500 A. over a concentration range of 0.16 to 1 g. of soap per 100 cc. The structural viscosity markedly exhibited in solutions containing 1% or more aluminum soap in hydrocarbons may be ascribed to the network structure with weak (4) C.G. McGee, J . A m . Chem. Soc., 71,278 (1949). (5) W. 0. Ludke, 8. E. Wiberley, J. Goldenson and W. H. Bauer, THISJOURNAL,69, 222 (1955). (6) P.Debye, W. A. Barber and A. P. Autrey, Summary Report to Army Chemical Center, Maryland, Contrapt QA-18-108-CML-2144.
Jan., 1958
FLOW PROPERTIES OF PEPTIZED ALUMINUM SOAP-HYDROCARBON GELS
binding forces developed a t points of contact or ends of such aluminum soap micelles. These micelles may have the structure of the polymer unit suggested by Ludke5 or several such units may be aligned in a group along the aluminum octahedrally coordinated oxygen atom axis.6 Assuming the mechanism of peptization proposed by Ludke,5 addition of molecules with available protons, such as alcohols or cresols, to aluminum soap-hydrocarbon gels should result in interaction of some of the added peptizer molecules and the soap polymer. It would be expected that polymer growth a t the ends of soap polymer rods would be blocked on bonding of the peptizer molecules, and that coordinate linkages in the soap polymer units would be weakened or broken with attendant reduction in micelle length. The purpose of this investigation was to study the effect of peptizer action on the elastic behavior and flow properties of aluminum hydroxydilaurate in toluene. The peptizer chosen was m-cresol, for which the effect on infrared absorption of aluminum soap-hydrocarbon gels has been de~cribed.~ For comparison of the action of peptization with that of a change in soap nature, toluene gels of aluminum di-3,5-dimethylhexanoatewere also prepared and flow properties measured. Experimental
106 I
I
-'1
'
I
I
I
I/
I
IO'
Y
n
fl#
4
Materials.-Aluminum hydroxydiIaurate was prepared as previously described.? Aluminum analysis showed that the ratio of moles of fatty acid to moles of aluminum was 1.98 to 2.01 in the samples of soap used. Toluene and m-cresol were prepared as described by L ~ d k e . The ~ 3,5-dimethylhexanoic acid was prepared by Dr. Milton Orchin, Bureau of Mines, Pittsburgh, Pennsylvania. Gel Preparation.-Preparation of the aluminum soap toluene gels was carried out as previously described.' When peptized gels were prepared, the rn-cresol was dissolved in the toluene before addition of soap. As shown by flow properties, the major portion of the peptizer action occurred in the 24 hour aging period at 55". However, further slow changes took place in the peptized gels during a period of a year, indicating that complete equilibrium had not been reached even in that time. Flow Measurement.-A capillary viscometer with floating driving piston and interchangeable capillaries was used for flow measurements at shear rates above 1 to 10 cm.-l.l At very low shear rates, measurements of axial extension under stress, Newtonian flow and elastic return on stress release were made by means of a Gaertner short range cathetometer on a volume of gel in a partially filled capillary, connected with a reservoir of gel to which fixed driving pressures were suddenly applied or released. In some cases, efflux from the ca illary tube was collected and weighed to determine small lopow rates.
Experimental Results Flow curves shown in Fig.. 1 were obtained for gels containing 5% by weighcof aluminum dilaurate in toluene with 1, 2 and 3 moles of m-cresol per mole of soap (calculated using the formula Al(0H)Rz). Results for an unpeptised 5% gel are shown for comparison. The maximum shearing stress a t the wall, 7 , and the mean rate of shear, D, were calculated from P,'the mean driving pressure corrected for kinetic energy losses and from Q, the flow rate, by means of the relationship
I
I
107
m - 2 E - 3 P - 0
n-
j
I
m - 2
d 0"
4
lo-' ,o
SHEARING S T R E S S , y ( dynes cm' ). Fig. 2.-Flow curves of aluminum di-3,bdimethylhexanoate in toluene a t 25': dashed line, 5y0 aluminum dilaurate in toluene; dot-dashed line, 5% aluminum dilaurate in toluene with 2 moles of m-cresol per mole of soap.
and
RP
7 s -
2L
(7) W. W. Harple. S. E. Wiberley and W. H. Bauer, Anat, Chew., 84, 635 (1952).
L and R are the length and the radius of the capillary tube. As was found with unpeptized gels,' flow curves for rates of shear above approximately
108
WALTERBAUER,NEILLWEBERAND STEPHENE. WIBERLEY
I
i
axial return, the value of x measured along the axis of the tube at r = 0, then
i
1
Capillary Radius = 0.0643m? = s e a r Stress a t wall (cm")
004
-.
E'
s
z 0.03 0 fn z w IX W
-J
5002
X Q
001
0
I
I
0
200
Vol. 62
400
i 6 00
TIME fsec).
Fig. 3.-Creep curve, 5y0 by weight aluminum dilaurate in toluene at 25". Shear stress suddenly released at times corresponding to points marked x.
100 set.-' were markedly dependent on the capillary length to radius ratio. Results obtained a t high shear rates for gels containing 1.84% of aluminum dilaurate in toluene with 0, 1 and 2 moles of m-cresol per mole of soap are also shown'in Fig. 1. Flow curves obtained for gels of aluminum 3,5-dimethylhexanoate in toluene are plotted in Fig. 2, together with comparison curves for peptized and unpeptized 5% aluminum dilaurate in toluene gels. Typical creep curves based on measurement of elastic axial extension and Newtonian flow under constant stresses and of elastic return after stress release for gels in partially filled capillaries are shown in Figs. 3 and 4. I n all cases, approximately linear behavior was shown over the interval of stress applied, in respect to the initial deformation and elastic recovery. Taking the time dependent recovery after release of stress as 2, the distance of movement measured along a coordinate parallel to the axis of the tube, r the distance measured from the axis toward the periphery of the tube, P the fixed driving pressure, and L the capillary length filled with gel, the relation between the time dependent strain recovery, (- dx/dr)t and the shearing stress Pr/2L is given by
Jt is a time dependent compliance. If X is the
From measurements of the axial recovery with time after stress release for various applied fixed stresses, values of the compliance, Jt, were calculated for the gels studied. The results are shown in Figs. 5 and 6. Discussion Elastic Properties and Viscosity at Low Rate of Shear.-As is the case with unpeptized gels, the aluminum soap-toluene gels containing m-cresol flow with constant viscosity over an interval of increasing stress a t low rates of shear, but the viscosity is drastically reduced by the action of the peptizer (Fig. 1). The compliance of the peptized gels is greatly increased compared to that of the gels without m-cresol, and the compliance changes more rapidly with time, as shown in Fig. 5. These effects are similar to those resulting from lowering the concentration of the soap in the gel, illustrated in Figs. 4 and 6. To the reduction in contact interaction of the soap micelles on dilution corresponds a reduction of the network structure arising when m-cresol molecules, by hydrogen bonding, lower the average soap micelle length and the forces of attraction at polymer ends. The occurrence of a maximum in these effects is ascribed to increasing mutual hydrogen bonding of the mcresol molecules as their concentration in the toluene increases, with a corresponding increase in the viscosity of the solution. From the initial compliance measured at 10 seconds, the initial shear modulus of rigidity, Glo seC, may be calculated, since GOsea
=
I
1
1 JIOsea
Values of the shear modulus of rigidity and the Newtonian viscositv. T / D . a t low shear rates were calculated for the Yariou; gels, and are listed in Table I. TABLE I ELASTIC AND FLOWPROPERTIES OF ALUMINUMSOAPTOLUENE GELS Wt. % soap in toluene
Moles m-cresol per mole soap
Viscosity, r / D at D
10-1
sea.-'
Aluminum dilaurate 2 0 5 x 108 3 0 36 x 103 5 0 280 x 103 5 1 13.X loa 5 2 1 . 9 x 10s 5 3 2 . 7 x 103 Aluminum di-3,5-dimethylhexanoate 5 . 3 x 103 1.5 0 35 x 108 2.0 0
Apparent vis., r / D at D = 5 x 108 sec.-L
Shear modulus
of rigidity, G~osec., dyne cm.-*
0.40 0.93
80 360 1100 630 270 580
0.45 0.85
60 200
0.19 0.45 1.41
...
Apparent Viscosity at High Rates of Shear.Above a critical shearing stress, all the gels show a rapid increase in rate of shear of several decades with a small increase in shearing stress, followed by
1
Jan., 1958
FLOWPROPERTIES OF PEPTIZED ALUMINUM SOAP-HYDROCARBON GELS
109
om
015
--5 z 8
3
z
010
-I
4
X
005
zoo
400
600
TIME (secl.
Fig. 4.-Creep curve, 5% by weight aluminum dilaurate in toluene with 2 moles of m-cresol added per mole of soap. Shear stress suddenly released a t times corresponding to points marked X.
1 I
i
No.
Moles M - C r e s o l per mol0 a m p
I
0
lI-0
I
Curve
2 3
i
*-.
__
I
0 X -3-
0
I
50
I
T I M E (set,).
Fig. 5.-Time dependent compliances of 5% by weight aluminum dilaurate in toluene with various amounts of added m-cresol; effect of peptization on elastic properties.
50
100 TIME
(sac. ).
Fig. 6.-Time dependent compliances of aluminum soaps in toluene; effect of dilution on elastic properties.
a regime of flow in which shear rate thinning occurs and the apparent viscosity approaches that of the solvent a t high shear rates. In the critical stress region, shearing stresses are great enough that the soap micelles are broken, facilitating alignment of the micelles and reducing their length. Both of the latter effects make possible the greatly reduced apparent viscosity a t high shear rates, which is of the order of 1.41 for the 5% aluminum dilaurate gel a t D = 5 X lo3 sec.-', compared to the viscosity of 2.8 x lo5 poise at D = 0.1 see.-', as listed in Table I. The effect of peptization in the high rate of shear flow region is much reduced, since the apparent viscosity a t D = 5 X lo3 see.-' changes from 1.41 for the 5% unpeptiaed aluminum dilaurate-toluene gel to 0.40 when 2 moles of m-cresol are added per mole of soap added, while the corresponding change a t D = 0.1 see.-' is from 2.8 X lo6 to 1.9 X lo3. The relatively reduced effect of peptization a t high rates of shear indicates that network structure is greatly destroyed under these conditions and that the peptizer is acting chiefly to reduce the micelle length. I n the aluminum soap-toluene gels to which mcresol has been added, the critical shearing stress a t which shear rate thinning sets in is much lower than is the case with unpeptized gels, consistent with a weakening of the gel structure by action of the peptizer. Energy of Activation for Viscous Flow.-If the separate factors in the postulated mechanism of flow of the aluminum soap-hydrocarbon gels, such as strain and destruction of reversible network
110
JULES V. HALLUM AND HARRY V. DRUSHEL
structure, reversible breaking of polymeric linkages, and micelle alignment in flow are affected in varying degrees by peptization, then values of E v i q the activation energy for flow, calculated from n = A eEvisIRT, should vary with the amount of peptizer present. This should be especially true a t lower rates of shear at which structural elements have not been as completely destroyed as under higher flow rates. Some values of Evis are shown in Table 11, calculated from flow data on 1.84% aluminum dilaurate-toluene gels with various amounts of mcresol. Over the temperature interval of from -17.8 to 25", the value of E ~ is8 lowered with increasing peptization, especially a t the lower shear rate. The low values of Evi,are consistent with the remarkably small changes in apparent viscosity of the gels below 25". Between 25 and 50°, the gels studied lost much of the structural viscosity, becoming comparatively free flowing at low shear rates. This is reflected in the changed values of Eviawhen calculated over the higher temperature interval. It is indicated that at some point between 25 and 50" the temperature is high enough so that thermal forces largely disrupt the intermicellar structure, corresponding to a sort of gelsol transformation. While the equation r] = d e E v i s / R T clearly is not applicable over the range of temperatures, calculations of are useful in pointing out the changing mechanism of flow with peptization, and further study of temperature effects on flow rates should be valuable. Conclusions It is concluded that the primary action of m-
Vol. 62
cresol in peptization of aluminum soap-toluene gels is in modification of the length of soap polymeric micelles and blocking off of attractive forces a t ends of the soap polymers. Such action reduces the amount of network structure, with the major effect of reducing the low rate of shear visTABLE I1 ENERGY OF ACTIVATION FOR VISCOUSFLOW 1 .84T0BY WT. IN TOLUENE ALUMINUM DILAURATE Moles cresol mole soap
D X 10-8, dyne cm.-*
cal. - 17.8-25'
EVh,cal.
0
1 1 1 1 4 4 4 4
1900 1000 800
4200
'/2
0 2 0 '/a
1 2
Evil,
720 1700
25-50'
4400
1800 1800 1200
cosity and shear modulus and the critical shear stress a t which shear rate sensitive flow sets in. The action of m-cresol is similar in important features to dilution or to critical increase in temperature. Acknowledgment.-Acknowledgment is made to Dr. Huntington Jackson8 for use of data on which temperature calculations of Eviswere based, and to Wladimir Philippoff for helpful advice and suggestions. This work was conducted under contract between the Chemical Corps, U. s. Army, and Rensselaer Polytechnic Institute. (8) Huntington Jackson, Ph.D. Dissertation, Rensselaer Polytechnic Institute, Troy, N. Y.,1951.
THE ORGANIC NATURE OF CARBON BLACK SURFACES' BY JULES V. HALLUM AND HARRY V. DRUSHEL Contribution from the Colzimbian Carbon Fellowship, Mellon Insliliitc of Industrial Research, Pitlsbzirgh, Pa. Receiued October 4, 1967
Evidence is presented for the existence of quinone groups and aromatic hydroxyl groups on the surface of carbon black particles. This evidence is based largely upon polarographic analyses of slurries of carbon blacks. A mechanism for the chemical interaction of carbon blacks with elastomers is proposed on the basis of these functional groups.
Introduction A recent paper by Garten and Weiss2has shown that some of the reactions of the surfaces of carbon black particles can be explained as reactions of hydroquinone and quinone structures. Most recently, Studebaker, Huffman, Wolfe and Nabors3 have shown by analysis with diazomethane and other reagents that as much as 18% of the oxygen found on carbon blacks may be present in a 1,4quinone form. Independently, by the use of infra' red and polarographic analyses, the same qualitative conclusions have been reached in these laboratories. These techniques were used to identify (1) Presented i n part a t t h e 130th National Meeting of ACS, Atlantic City, N. J., Sept. 20, 1956. (2) V. A. Garten and D. E. Weiss, Australia?~J . Chem., 8, 6 8 (1955). ( 3 ) M. L. Studebaker, E. W. D. Huffman, A. C. Wolfe and L. C . Nabors, Ind. Eng. Chsm., 48, 102 (1956).
functional groups on the carbon black particle surface. On the basis of the presence of these functional groups, a mechanism for the chemical in6eraction of carbon blacks with elastomers is proposed. Discussion of Results Evidence from Infrared Spectra.-In general, infrared analyses were unsuccessful, perhaps due to absorption and scattering of the radiation by the particles. However, in the case of a channel black of 8-10 mp particle diameter and which had a high percentage of chemisorbed oxygen the method proved to be practical. Curve A in Fig. 1 shows the infrared spectrum obtained from a Nujol mull of this black. The baiid at 6.3 p is attributable to either condensed aromatic ring systems or to hydrogen-
.
