NOTEB
March, 1957
entropy change which results from the uncurling of the hydrocarbon portion of the detergent molecule when it enters a micelle. Choice of a smaller value for the entropy change would reduce the predicted sharpness of micelle formation. Although no critical micelle concentrations were observed in this work, the data in Fig. 2 suggest that a mean micelle size which is concentration independent is achieved at a lower concentration with sample L than with the unfractionated detergent or fraction H. This is expected on the basis of solubility considerations and is predicted by Reich. The very large difference between the micelle weights for fractions L and H is surprising, and shows that the ratio # / A , .which should be identical for the two fractions and the unfractionated sample, is not the primary factor determining the most probable size of a stable micelle. Thc higher micelle weight for the lower molecular weight fraction shows that the mean degree of association increases as the hydrophilic group is shortened. The micelles formed by L contain a t least four times as many detergent molecules as those formed by sub-fraction H. Such er large difference, if the hydrocarbon portions of the molecule are identical in the two fractions, probably reflects a different typo of packing and micelle shape. THE CHEMICAL REACTIONS OF CALCIUM HYDROXIDE, SILICA AND WATER MIXTURES AT €352' BY SIDNEY A. GREEN BERG^ John#-Manvilla Reaearch Center, Manvills. Now Jsrssv Recsiwsd September 17, f O M
373
and 12.3 and remained at this level for the greater part of the reaction period. The conductivity of the solution containing Sp.B. silica decreased from 3.0 to 1.3 X lo-* ohms-' in four hours. In contrast the solution with S.L. silica showed a small increase from 3.8 to 4.3 X lo-* ohms-' in agreement wit.h the pH measurements. The electrical conductance of the solution with D.S. fluctuated between 4.0 and 5.8 X lo-* ohms-' during the course of the reaction. Essentially, therefore, most of the reaction proceeds in a saturated solution of calcium hydroxide.2 Solid Phases.-The DTA curves showed endothermic peaks at 490' for the decomposition of calcium hydroxide and at 825" exothermic peaks were found which correspond to the conversion to 8-wollastonite of tobermorite.2.4 The peak heights at 490" decrease with reaction time for samples containing S.L. and D.S. silicas; however, only a very small peak was found for the sample with Sp.B. silica after 20 minutes of reaction. The peak heights a t 825" increased rapidly with reaction time for the S.L. and Sp.B. samples, and somewhat less rapidly for the mixture with D.S. silica, but after 3 hours of reaction all the peaks reached the same height. There is a rapid rise in surface area for the mixture with D.S. silica from 15 to 78 sq.m./g. in 210 minutes. In Table I the surface area of products after 3 to 4 hours of reaction are listed. TABLEI I Silioa type
S.L. Sp. B.
D.S. Aerogel
Budace area#,
Reaotion
Burfacs areaa,
nq. m./g. SiO:
time, hr.
750 383 22 247
4
61
3 3 3
64 61 46
8q.
m./& clampla
The chemical reaction of calcium hydroxide with colloidal silicas in the presence of excess water was investigated by examining periodically the liquid and solid phases during the course of the reaction. Electrical conductivity and pH measurements were used to follow the changes in composition of the liquid phases. The formation of the solid products was studied by differential thermal analysis (DTA), surface area, weight loss and X-ray techniques.
The 1000° weight losses of the solid phases initially dried at 115' decrease with reaction time and level off a t 12.5 to 13.8% (g. loss/lOO g. ignited sample) after two hours of reaction. After 45 minutes of reaction the X-ray patterns of the mixtures with S.L. and Sp.B. silicas showed evidence for the pregence of tobermorite in lines at 3.05 and 1.82 AB4 N0 such lines were found in the pattern of the sample containing D.S. silica. Experimental Conclusions.-The reactions rates are different Materials.-The Mallinckrodt Sp.B., S.L. and aerogel but are not necessarily proportional to the surface silicas have been described .* The diatomaceous silica D.S. (Celite 403, Johns-Manville Corp.) was mined at Lompoc, areas of the silicas. (It should be noted that the Calif. Both Baker A.R. calcium hydroxide and some pre- absolute rate of reaction is slower for these large pared from Mallinckrodt S.L. calcium carbonate were used. crystallite size, chemically-pure, calcium hydroxides Equipment.-MoRt of the apparatusjhas been described.* than for fine-particle size, commercial grade The differential thermal analysis equi ment is essentially like one in the laboratory of Professor $. Kerr at Columbia limes.6) Although the products show essentially the same surface areas (Table I), those with D.S. or University.8 Procedure.-In each experiment 25 6. of calcium hy- Sp. B. silicas are thixotropic gels,b but high surface droxide was allowed to react with an equimolar quantity of area, S.L. silica does not form such a gel. It is clear Si02 in 800 ml. of water at 82". Samples were removed that tobermorite forms immediately on reaction. periodically and the phases were separated by filtration. Both the thixotropic behavior and electron microsResults and Conclusions copys indicate that the products are plate-like crysLiquid Phases.-After 20 minutes of reaction tals. Although it is difficult to propose a complete the pH values of the solutions were between 12.1 mechanism, nevertheless because of the insolubility of calcium silicate in the presence of excesss calcium (1) Chemistry Department, Seton Hall University, South Orange, New Jersey. (2) 8. A. Greenberg, THISJOURNAL,68, 362 (1954); 60,325 (1956).
(a) P. Kerr, et ai.. Preliminary Roports, Reference Clay Minerals, American Petroleum Inatitute. Research Project 49, Columbia Univeraity, New York, N. Y.,1951.
(4) H. F. M. Taylor, J . Chem. Soc., 3682 (1960); I63 (1858). (5) 8. A. Greenberg, unpublished results. ( 0 ) A. Orudemo, in article b y J. D. Bernal, "The Structures of Cement Hydration Compound," Proceedings of the Third International Symposium on the Chemistry of Cement, London, 1952.
374
hydroxideP2it seems reasonable to assume that reaction of colloidal silica with calcium hydroxide solutions must occur in situ and not in solution. (The same kind of reaction would be expected for the hydrolysis of tricalcium silicate.) According to this hypothesis calcium hydroxide would first be chemisorbed by silica, then in the presence of the Ca++ and OH- ions and water, which would diffuse into the silica structure, hydrolysis of SiOSi bonds results (pH 12.3 f 0.2) and simultaneously the SiO, tetrahedra reorient into the tobermorite structure. Acknowledgments.-The assistance of Mr. J. Pellicane in the performance of most of the experimental work is gratefully acknowledged. Thanks are also due to Mr. J, McGourty of this Laboratory for the DebyeScherrer patterns and to Mr. G. Reimschussel for the surface area measurements. THE CRITICAL PRESSURE AND TEMPERATURE OF DIMETHYL OXALATE BY S. ALEXANDER STERNAND WEBSTERB. KAY Department of Chemica2 Enpinrering, The Ohio State Univcrrily, Columbua, Ohto Received October 10, 1066
The critical pressure and temperature of dimethyl oxalate, (COOCH3)2, are reported in the International Critical Tables‘ as 9.48 atmospheres and 260°, respectively, based on older experimental data.21a A critical pressure of 9.48 atmospheres appears abnormally low for this compound when compared with that of other esters of organic acids. Consequently] it was decided to measure the critical constants of dimethyl oxalate in order to determine whether the low value reported for the critical pressure is in error or whether it represents a possible anomalous behavior. Experimental Eastman Kodak Co. research grade dimethyl oxalate was fractionally crystallized from ethyl alcohol five times. The purified fraction was then degassed under high vacuum (less than 1 X 10-6 mm.) and a portion was distilled into the experimental tube, and was frozen, and the remainder of the tube filled with mercury preparatory to the determination of the critical constants. The experimental method for the determination of the critical constants consisted essentially in the determination of the temperature at which the liquid-va or interface disa peared when the sample was heated. T\e apparatus has gee, described elsewhere.‘ Dimethyl oxalate was found to be thermally unstable above 200”, decomposing t o form a dark colored liquid and a relatively insoluble gas. It was necessary therefore to raise the ’temperature of the sample rapidly to the critical temperature in order to minimize theeffect’of the decom osition products on the value of the critical constants. W h e this procedure results in a fair approximation of the critical temperature, the pressure observed is considerably higher than the true critical‘pressure because of the presence of the gaseous decomposition roduct. To obtain a value o?the pressure consistent with the experimental critical temperature the critical pressure was calculated from a Clausius-Cla eyron vapor pressure equation of the form log P = A - &T established at lower tem(1) “International Critical Tables,” Vol. 111, MoGraw-Hill Book Co., N e w York, N. Y.. 1928, p. 248. (2) H.V. Regnault, Msm. Acad. Roy. Bci., 26,336 (1862). (3) I?. Weger, Ann., Psi, 61 (1883). f), Kay and 0 , M. Rarnboaek, In& En#, Chrm,, 46, 221 (4)
(lSbO),
Vol. 61
NOTES
peratures. Such a procedure is justified, as the above equation is known to fit the va or pressure data of a lar e number of substances reasona&y well up to the critic8 point, even though the basic assumptions usually made m its derivation are not valid over so extensive a ran e.6 While of a lower order of accurac the above mettod is capable of bracketing the vages of the critical constanta within reasonable limits.
Results In order to evaluate the constants, “A” and “B,” of the equation log P = A B / T , the following vapor pressure data were obtained for dimethyl oxalate.
-
Temp., “C. Pressure, atm.
163.3 1.03
180 1.66
200 220 3.99 8.87
The values of “A” and “B” were calculated by the least-squares method and found to be 5.2240 and 2280, respectively, when the pressure is given in atmospheres and the temperature in OK. The average deviation of the vapor pressures computed by means of the equation from the experimental data is less than &3%. The critical temperature of dimethyl oxalate was observed to be 355 f 701 while the critical pressure, calculated by means of the vapor pressure equation as the pressure at the critical temperature, was estimated to be 39.3 f 4 atm. The pressure observed at the critical temperature was 65.4 atm. indicating the presence of gaseous decomposition products. Using the empirical methods for estimating critical constants developed by Lyderson,’ the critical temperature and critical pressure of dimethyl oxalate were calculated to be 366’ and 39.4 atmospheres, respectively, in fair agreement with the values reported here. It is concluded, therefore] that the critical temperature and pressure previously reported for dimethyl oxalate are grossly in error. (6) B. F. Dodge, “Chemical Engineering Thermodynamics,” l o b Ed., McGrsw-Hill Book Co., Ino., New York, N. Y., 1844, p. 248. (6) 0. A. Rougen, K. M. Watson and R. A. Ragata, ”Chemical Process Principles,” Vol. I, John Wiley and Sona, Inc., Ncw York, N . Y., 1964, p. 87.
SHEAR DEPENDENCE OF VISCOSITY OF NATURAL RUBBER SOLUTIONS BY MORTONA. GOLUB Contribution from fhe R. F. Goodrich Company Reaearch Center, Brackauills, Ohio Received Oclober 18, 1968
I n an earlier paper’ viscosity measurements on dilute Alfin polyisoprene solutions a t quite low shear rates were reported. It was shown that the limiting slope of the intrinsic viscosity us. rate of shear plot at low shear rates, (A[q]/AD)~+o,.ls proportional to the square of the zero shear intrinsic viscosity [7lO2.This relationship suggested the D+O possibility of using the slope of the (A [q]/m) us. [qIo2plot as a molecular weight-independent parameter expressing the magnitude of the shear dependence of intrinsic viscosity, and thus as a possible means for studying polymer microstructure. It (1)
M,A, CIolub, T ~ r JOURNAL, r 00, 481 (1966),
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