Apparatus for Differential Thermal Analysis of Lubricating Greases DAVID
B. COX
and JAMES F. McGLYNN
Technical Service Laboratory, Socony Mobil Oil Co., Inc., Brooklyn, N. Y. Differential thermal analysis is an excellent technique for the study of phase changes in metallic soap-oil systems such as lubricating greases. An automatic apparatus specifically designed for differential thermal analysis of lubricating greases is described. Experimental results reported include a re-examination of the phase change behavior of pure lithium stearate and a new phase diagram far the model grease system lithium stearate-n-hexadecane.
M..
present-day commercial luhricatmg greases consist of a lubricating oil thickened to a grease consistency by a colloidal dispersion of a crystalline metallic soap. The application of differential thermal analysis (DTA) to such soap-hydrocarbon systems has been reported by several investigators (8, 6, 9). They have shown that several lithium and sodium soaps in various hydrocarbons exhibit distinct phase changes that occur reprodncihly when the systems are heated. Such changes are caused by stepwise breakup and solution of the soap crystal structure (g). Determination of the phase change temperatures in a given soap-hydrocarbon system permits constrnction of a phase diagram with its solubility information. M a s urements of the latent heat of phase transformation may be made, and the distinctiveness of differential thermograms for specific soaps or soap-solvent mixtures permits their use for identification purposes. This paper reports the heginning of an investigation into lithium soap-oil systems using differential thermal analysis as the principal tool. The DTA apparatus has two advantages over those described previously ( 3 , 6 , 7 ) . All experimental data are recorded automatically, and five different samples may be analyzed in a single experimental run. OST
MATERIALS
LITHIUMSTEARATE. Hystrene 5-97 stearic acid (Atlas Powder Co.) was recrystallized twice from methyl ethyl ketone, the second time using. Norit A decolorizing charcoal. A thud recrystallieation from Sovasol No. 2 (a 960
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ANALYTICAL CHEMISTRY
Figure 1.
D1IA apparatus
Light nnphthn, hoiling point 60" to 90° C.) gave strnrir wid with Iodine No. (Hanus) Acid No. Melting point (micro-
scope hot stage)
The cells were then heated in an oil bath until all the soap dissolved in the hydrocarbon.
Nil 197.6 69.5-70.0' C .
A solution of stearic acid in Formula 30 ethyl alcohol was neutralized to a phenolphthalein end point by addition of a carbonate-free, 50% aqneons alcohol solution of lithium hydroxide (lithium hydroxide monohydrate, Foote Mineral Co.). After the crystalline soap precipitate which formed had been stirred and digested for 20 minutes, it was filtered and dried. The soap was recrystallized either from methanol or Formula 30 ethyl alcohol and dried in a vacuum desiccator a t room temperature. It was stored in the desiccator over sodium hydroxide and soda lime. n-HEXADECANE. Du Pant cetane was dried over Drierite. SOAP-OILSAMPLES.To ensure intimate mixture of lithium stearate-nhexadecane samples before analysis, the samples were predissolved. The soap and hydrocarbon were weighed into borosilicate glass cells having a ground joint cap with stopcock. The cells were then carefully evacuated to about 2- or 3-mm. pressure t o remove entrained air and refilled with dry nitrogen. The evacuation and nitrogen flush were repeated several times.
Figure 2.
Pair of cells
The apex of the peak in the differential thermogram marked the completion of melting, and the actual cell temperature corresponding to the peak apex was within 1' C. of accepted values for the melting points o,f these standards (stearic acid, 70 C.; benzil, 95" C.). This accuracy was independent of either the heating rate or the peak amplitude. The heating rate was varied from 0.55" to 2.5" C. per minute, and the peak size was varied by mixing the standards with alumina. The identity of the peak apex with the accurate phase change temperature follows logically from the placement of the thermopile and thermocouple in the physical and thermal center of the sample.
117".
,
197"-
i247°C.1 TIME
Figure
3. Heating thermogram for pure lithium stearate Heating rate, 1.5' C. per min.
APPARATUS
Differential Thermal Analysis. The DTA apparatus shown in Figure 1 consists of three basic parts: the sample and reference cells ant1 their thermal environment, the he,tt-control mechanism, and the temperature recording mechanism. The heating bath is a cylinder of Duralumin which is heated from the center by a gOO-n-att, two-element cartridge heater and is insulated on the side and bottom by a thick layer of dense magnesia. The cylinder contains wells for five pairs of sample and reference cells. All wells are parallel to and equidistant from the axis of the cylinder. The sample and reference cells (Figure 2) are made of stainless steel tubing, machined to a wall thickness of 0.03 inch, and have a sample capacity of 4 cc. The cells are closed, but not sealed, by stainless steel, twist-lock caps. The temperature-sensing elements enter directly into the center of the cells through stainless steel shield tubes, integral with the cap. Each sensing element consists of a five-junction, iron-constantan thermopile for differential temperatures plus a single iron-constantan thermocouple to measure the actual cell temperature. The thermopiles and thermocouples are insulated by woven glass sleeves tied into a narrow bundle and coated with Teflon. The Teflon was applied as an aqueous dispersion and sintered to produce a thin, smooth, inert surface on the sensing element. The heating rate of the Duralumin cylinder is controlled by a RIinneapolisHoneywell Regulator Co. Type 152 ilir0-Line, time-temperature program controller. This unit operates a variable transformer through which current is supplied to the heater. Part of the total heating current is supplied through a second variable transformer which is set manually for each different heating rate. This arrangement permits close automatic control of heating rates throughout the experimental runs. The maximum heating rate of about 2.5' C. per
minute is set by the maximum output of the cartridge heater. Lower heating rates may be selected by suitable choice of controller settings. Cooling determinations so far have used the natural cooling rate of the heating block, which varies from about 1' C. per minute a t 220' C. to about 0.3" C. per minute a t 100' C. However, there is a coil of copper tubing wound around the block inside the insulation through which a fluid coolant may be circulated. A 16-point potentiometer (Minneapolis-Honeywell Regulator Co.) automatically records all differential temperatures, cell temperatures, and the heating cylinder temperature on the same strip chart. The recorder reads and prints all 16 points in about 30 seconds, thus providing sufficient frequency of reading to define the temperature curves. The range of the recorder 1s -73" to 260" C. Visual Observations. Visual features of phase changes in experimental samples were observed in a microscope hot stage using crossed polarleers. The hot stage was based on the design of Vold and Doscher (8). All samples were sealed in flat borosilicate glass capillaries to minimize oxidation. Temperatures in the hot stage were measured by an iron-constantan thermocouple connected to a self-balancing precision potentiometer. Calibration of the hot stage with A. H. Thomas Co. melting point standards showed that a t temperatures of 200' C. and above, a thermal gradient of about 3' to 4 O C. existed within the stage. Melting and other phase changes, therefore, appeared to take place over a wider range than actually exists. Calibration of DTA Apparatus. The various operational characteristics of the DTA apparatus were determined using pure stearic acid and benzil as melting point standards. At first technical white oil or n-hexadecane was used as the reference material. Later. levigated alumina was substituted for the fluids.
Calculations of latent heat of fusion for stearic acid and benzil, using either the equation of Evans, Hutton, and RIatthews (3) or the T'old method (Y), showed that the present apparatus is not suitable for accurate calorimetry. Best results XTere only accurate to within ~ k t l O 7of~ accepted values. RESULTS AND DISCUSSION
Lithium Stearate. The phase change behavior of dry lithium stearate was determined by a series of differential thermal analysis runs made under varied conditions. Mixtures of lithium stearate with alumina having a soap content from 10 to 50% were run in addition to 100% soap samples. Also, duplicate samples were run a t heat rates 1%-hichvaried from 0.55" to 1.5" C per minute. I n all cases the resulting thermograms showed three distinct peaks. Figure 3 is a typical thermogram for a pure soap sample, replotted from the strip chart record. I n all the runs the first peak occurred a t 117" C. and the second a t 200" C. However, the third peak's apex temperature varied from 229" to 225' C., depending on the heat rate and the amount of admixed alumina in the sample. The thermograms of soap without alumina all showed 229" C. for the third peak. The variation in the third peak is explained by the fact that there were two different phase changes concealed in the one peak. The first of these occurred probably a t 225 " C. or slightly higher and the second at 229' C.; the apparatus was not able to resolve the heat effects into two peaks. The existence of the 225" C. transition was verified in three separate batches of lithium stearate by cooling thermograms and by visual observations in the microscope hot stage. Figure 4 shows the cooling thermogram for the same sample heated in Figure 3. The 229" C. peak of Figure 3 was resolved by different amounts of supercooling into two peaks, one a t 223" and the other a t 207 " C. The phase change which occurred a t 200" C. on heating was lowered markedly to 137" C. on VOL. 29, N O . 6, JUNE 1957
961
cooling, while the 117" C. heating transition was only slightly supercooled to 108" C. The actual physical nature of these phase changes as seen in the hot stage between crossed polarizers is as follows. On being heated, lithium stearate went through a crystalline rearrangement at 117" C. At 200" C. the soap crystal presumably became somewhat disordered, but there was no obvious visual change except for a possible small increase in light transmittance. At 225" C. the soap softened to a jellylike, brightly birefringent, liquid-crystalline phase, and at 229" C. the soap finally melted to a mobile isotropic liquid. The melting followed the previous change to the liquid crystal state so closely that it was possible to see three phases within the sample at one time. The thermal gradient in the hot stage persisted even with slow heating rates, so the assignment of the two phase change temperatures was based on the first appearance of the liquid crystal phase and on its complete disappearance. On being cooled, the sample went through the reverse sequence of phase changes a t temperatures near those found by differential thermal analysis even when the cooling rate was roughly four times as great. The liquid-crystal phase was clearly homogeneous within its temperature range. Vold and Vold reported only three phase changes in lithium stearate (9). These changes, on heating, were said to be a n intercrystalline transition a t 114" C., the beginning of a mesomorphic or waxy phase a t 185" C., and final melting at 224" C. The lower temperatures assigned by Vold and Vold to the phase changes are explained by the fact that they took the sample temperatures at the beginning of phase change peaks in differential thermal analysis thermograms as the phase change temperatures. AI. J. Vold had already pointed out ( 7 ) that this practice gave low results with that apparatus. I n the cooling thermogram reported by Vold and Vold for lithium stearate, there were still only three peaks (9). The Vold apparatus may not have been able to resolve the two highest phase changes into two peaks. The existence of two phase changes too close together for resolution by differential thermal analysis on heating m s reported by Evans, Hutton, and LIatthems ( S ) , who observed the phenomenon in a technical lithium stearate in oil. I n this case also, the two phase changes were detected when supercooling effects allowed their resolution into two peaks in the cooling thermogram. Lithium stearate might be expected to have a liquid-crystal type mesomorphic phase on the basis of analogy
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962
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ANALYTICAL CHEMISTRY
.
0-3oUMIN.
IOCW MIN.
223'CC. I
93 o c . Figure 4. stearate
Cooling thermogram
Cooling rate decreased from 1
240
E
120
l o80 o
I 260°1
177OC. TEMPERATURE
"
for
pure lithium
to 0.3' C. per min.
I
B'hE
I
i
A &E
t
90
100 Figure 5. A.
B.
I
I
80
70
I
I
I
60 50 40 MOLE PERCENT SOAP
I
I
I
30
20
IO
Phase diagram for lithium stearate-n-hexadecane Crystalline lithium stearate II C. Crystalline lithium stearate I D. E. Isotropic soh.
to other alkali metal soaps. Vold and Vold studied a series of alkali metal palmitates by differential thermal analysis (IO) and noted that lithium palmitate showed only three phase change peaks (at 103", 190", and 223" C.). Lithium palmitate apparently lacked the liquid-crystal phases, called neat or subneat, found in the other members of the series. They interpreted this to mean that lithium palmitate underwent two structural changes at the melting point. Because the palmitates resemble the stearates so closely, perhaps lithium palmitate may have a n equilibrium liquid-crystal phase which exists at a temperature slightly below the melting temperature. System Lithium Stearate-nHexadecane. Figure 5 shows a n equilibrium phase diagram for the system lithium stearate-n-hexadecane. The major boundaries in this diagram were determined by differential ther-
W a x y phase Liquid-crystal phase
mal analysis of soap-oil samples pretreated as described above. Visual observations of these samples between crossed polarizers in the hot stage supported the differential thermal analysis data. Because the observed phase change temperatures did not vary with the heating rate and were readily reproducible, they were assumed to be equilibrium values. The broken boundaries outline a possible eutectoid system but are not proved experimentally, The nomenclature of the phases was taken from Vold and Vold, except where noted differences occur. The negligible interaction between crystalline lithium stearate I1 and n-hexadecane was shown by the failure of the solvent to lower the temperature of the intercrystalline shift at any concentration observed. The transition from crystalline lithium stearate I to the waxy phase was lowered from 200" to 1'79" C. in the dry soap by as little as
12.5 mole % of n-hexadecane. The constancy of the temperature of this phase change throughout the concentration range studied suggested a eutectoid between the crystal I phase, the waxy phase, and the liquid-sol phase. The phase boundary marking the transition from waxy phase to the liquid-crystal mesomorphic phase was lowered gradually by increasing percentages of nhexadecane up to about 60 mole 70. At lower soap concentrations the boundary was constant at 190" C. The temperature of final solution of lithium stearate in n-hexadecane was lowered in a gradual manner by increasing concentration of the hydrocarbon throughout the range studied. The phase diagram for lithium stearate-n-hesadecane reported by Vold and Vold (9) showed a phase island called nonaqueous middle soap, by analogy to a similar phase reported for the system sodium stearate-n-hexadecane (I) and to aqueous, sodium-soap systems (4). The middle soap for lithium stearate was described as liquidcrystalline and was said to be completely bounded by the waxy and isotropic solution phases. The esperimental observations of the present study clearly indicated that the liquidcrystal phase had the same character as the Vold and Vold middle soap, but that the phase was continuous with the liquid-crystal phase of the dry soap and persisted to as low a soap concentration as measured. There was no evidence for any phase island. The differential thermal analysis thermogram of the 3.9 mole yo soap sample still clearly showed four peaks marking all the major phase boundaries. Visual observations of samples of low soap content in the hot stage was soinen hat ambiguous because birefringence in the liquid-crystal phase
becomes vanishingly dim a t these low soap contents. Holvever, there was a sharp transition between the jelly structure of the liquid-crystal phase and the fluid mobility of the solution. The visually observed jelly-sol transition in the low soap samples occurred at the same temperature as the top peak in thermograms. The present disagreement concerning the phase behavior of lithium stearate in n-hexadecane may be compared to a similar case involving the system sodium stearate-n-hexadecane. Strosb and Abrams (6) reportpd that this system contains no phase islands, all phases being continuous with ones occurring in the pure soap. Their report, which was based mainly on differential thermal analysis data, stands in contrast to those of Doscher and Vold ( 1 ) and of Smith and McBain (b), who reported phase islands in sodium soap-hydrocarbon systems. These latter investigators based their phase diagrams on visual observation alone. Stross and Abrams stated that a possible reason for the discrepancy between their findings and those of Doscher and Vold might lie in the failure of the latter to exclude all traces of water from the system. The major differences between results of the present lithium stearate system and those of Vold and Vold stem from the differing number of phases found in the pure soap and the different shape found for the solution boundary. These differences, in turn, seem to be explained by the greater accuracy and sensitivity inherent in the differential thermal analysis apparatus described above. SUMMARY
An apparatus for differential thermal
analysib of lubricating greases operates automatically in controlling heating rates and recording all pertinent temperatures and differential temperatures. Five different determinations may be run simultaneously, allowing rapid collection of data. The cell and thermopile design permits high sensitivity and accuracy in locating phase changes. Differential thermal analysis of lithium stearate revealed a previously unreported mesomorphic phase occurring b e h e e n 225" and 229" C. A differential thermal analysis study of the system lithium stearate-n-hesadecane yielded a phase diagram n-hich appcars to be more accurate than one previowly reported. LITERATURE CITED
PI.,Vold, It. D., J . Colloid Sci. 1, 299 (1946). (2) Doscher, T. &I..Vold. R. D., J . Phiis. & Colioid Chem. 52, 97 (1948). 13) Evans. D.. Hutton. J. F.. Matthrns. J. B'., J : A p p l . Chem. ?London) 2 ,
(1) Doscher, T.
\
,
252 (1952).
(4) McBain, J. W., Vold, R. D., Vold, hl. J., J . Am. Chem. SOC.60, 1866
(1938). (5) Smith, G. H., RlcBain, J. K., J .
Phys. & Colloid Chem. 51, 1180
( 1947). (6) Stross, F. H., Abrams, S. T., J . .I m. Chem. SOC.73,2825 (1951). (7) Vold, ?vl. J., ANAL. CHEM.21, 683 (1949). (8) Vold, 11. J., Doscher, T. M., IUD. ESG. CHEX, ANAL.ED. 18, I54 (1946). (9) Vold, 11. J., Vold, R. D., J . Colloid Sci. 5 , 1 (1950). (10) Vold, R. D., Vold, &I.J., J . Phys. Chem. 49, 32 (1945). RECEIVEDfor revim July 20, 1956 Accepted January 12, 1057. Division of Analytical Chemistry, lleeting-in-Miniature, ACS, New York, N. Y., March 1956. Divisions of ilnalytical Chemistry and Physical and Inorganic Chemistry, 129th LIerting, ACS, Dallas, Tex., April 1956.
Thoron-meso-Tartaric Acid System for Determination of Thorium MARY H. FLETCHER, F. S. GRIMALDI, and LILLIE B. JENKINS
U. S. Geological Survey,
Washington 25,
b In the spectrophotometric determination of thorium with thoron, mesotartaric acid is used as a masking reagent for zirconium. The effects of different experimental variables such as the concentrations of the reagents, time, and temperature, and the behavior of 35 ions which might b e present in thorium ores are dis-
D. C.
cussed. A dilution procedure is given for the direct determination of thorium in zircon (ZrSiOd) that is also generally applicable to other materials.
T
HE determination of thorium in mineral separates such as zircon is important in Geological Survey pro-
grams on geochronology. However, the determination is usually lengthy, because zirconium is a serious interference in most methods for the determination of thorium. An earlier paper (I) stated that meso-tartaric acid should be superior to d-tartaric acid as a masking reagent for zirconium in the determination of thorium with thoron. VOL. 29, NO. 6 , JUNE 1957
963
