INDUSTRIAL AND ENGINEERING CHEMISTRY
July, 1928
settling of fine pigments to give other forces sufficient time to offset or modify the settling tendency. Paint vehicles may be so affected by heat and other treatment as not only to increase viscosity of the liquid, but also to produce in the vehicle a system of true liquid with a very thorough colloidal dispersion of molecular complexes. While true viscosity of a liquid can, theoretically, affect only the rate of pigment settling, a system of colloidal dispersion introduces a definite yield value to our paint vehicles which give the necessary suspension to fine pigments. I n a practical way it is very probable that the use of soaps, such as aluminum stearate, to avoid settling works in this manner. The aluminum stearate is very finely dispersed as a colloid in the paint vehicle, thereby giving the vehicle a gel structure and a definite yield value to oppose settling force. DEGREE OF WETTING-After perfect mechanical dispersion of a pigment in a paint vehicle, the specific degree of wetting between the particular solid and liquid evidences itself. I n a well-designed enamel the pigment and vehicle are chosen to produce the most nearly perfect degree of wetting between the two. In cases of enamels conditions of viscosity and yield value of the vehicle, and extremely fine particle sizes (large surface must be depended upon t o oppose settling force), such an enamel system in a comparatively short time shows the presence of any larger pigment particles, which settle to the bottom of the container in a very dry and often hard cake. The remaining fine pigment is held in suspension very well over long periods of time and the intermediate phase of soft settling is not experienced a t all. [END OF
735
Less thorough wetting between pigment and vehicle is often accompanied by definite forces of pigment flocculation. Pigment flocculation results in grouping pigment particles in loosely held colonies which tend to settle as a unit. The force of flocculation may thus result in an actual increase in the amount of settled pigment, but this same force prevents hard, dry caking of the pigment found in thoroughly dispersed paints. Soft settling, although of larger amount, is much to be preferred to a less amount of hard, dry settling. The wetting power of a paint vehicle is usually sensitive to relatively small additions of soluble salts, driers, volatile solvents, etc. With a rather wide range of soluble products the desired effect may often be secured in several ways. Effect of Shape of Pigment Particle
In addition to the size of pigment particle the shape of a particle may sometimes be employed to resist settling. Asbestine contains needle-shaped crystals of value in this respect. The list of pigments with properly shaped particles is too limited to depend very greatly on this cure for settling. Conclusion
Pigment settling is only one of many paint phenomena whose control may be more certain by a better understanding of the forces at work between solid and liquid phase. More accurate methods of measuring particle size, viscosity, and yield value, and a more thorough understanding of solidliquid interface forces will most certainly have many practical benefits. SYMPOSIVAI]
Solubility of Lubricating Oil in Liquid Carbon Dioxide' Elton L. Quinn U N I V E R S I T Y OF UTAH.
M
ANY investigators have observed that solid carbon dioxide made by expanding the liquid from an ordinary commercial carbon dioxide cylinder is nearly always contaminated with lubricating oil. If the cylinder is inverted and after standing for several days or a week is carefully drained, the carbon dioxide obtained from it will still contain oil. This oil is readily recovered by distilling the ether solution which is formed during the process of making an ordinary ether carbon dioxide freezing mixture. It is quite evident that this oil must form a solution with the liquid carbon dioxide and, in spite of the fact that liquid carbon dioxide is noted for its low solvent power, the amount of oil obtained from a 20-pound cylinder of this liquid indicates that there is a considerable solvent action in this case. This investigation was started for the purpose of determining the solvent power of liquid carbon dioxide for lubricating oil and, if possible, to determine the conditions of manufacture Ivhich would decrease to a minimum the quantity of oil reaching the plant filling-station. Most manufacturers of carbon dioxide use a high-grade lubricating oil in their carbon dioxide compressors. Glycerol has been and is being used to some extent as a lubricant, but it has few advantages over a good grade of lubricating oil, and many disadvantages. The principal advantages are that it is only slightly soluble in liquid carbon dioxide and, 1 Received
February 10,1928.
SALT
LAKECITY,
UTAH
if small quantities do reach the cylinder, it is odorless and tasteless and produces no bad results when the carbon dioxide is used for charging water for carbonated beverages. Some of its disadvantages are that its cost is high, and that it is a poor lubricant, especially if a small amount of water is present, and in case of accidental heating in the cylinders of the compressor it decomposes, producing products hp,@g a very bad odor. On the other hand, the lubricating oll nuw used is a good lubricant, has no odor or taste, but seems to form a solution with liquid carbon dioxide. The presence of oil in the liquid carbon dioxide in no way decreases its value for the manufacture of soda water, as the gas readily distils from the solution in a pure condition. When carbon dioxide is used in the refrigeration industry, however, the question of oil solubility becomes of very great importance. In this case any oil carried to the expansion coils by the liquid carbon dioxide will solidify on the inside of the pipes and prevent an efficient transfer of heat. Experimental Procedure
The solubility of lubricating oil in liquid carbon dioxide was determined in much the sameway as was the solubility of naphthalene and iodine in liquid carbon dioxide, already described by the author.2 I n this case, however, some difficulty was experienced in introducing the liquid solute into the 2
Quinn, J . A m . Chcm. Soc., SO, 672 (1928).
INDUSTRIAL AND ENGINEERING CHEMISTRY
736
reaction tubes and sealing the tubes afterwards. This was finally accomplished by sealing the oil into small, thin glass bulbs which were of such a size that they readily fitted into the reaction tube. These bulbs were large enough to hold three or four drops of the oil, and it was very important that a larger amount was not used as this made a separation of the oil and the solution very difficult. After the reaction tubes had been sealed, the bulbs were broken either by shaking or by the cautious application of heat which caused them to rupture as the oil expanded.
Vol. 20, No. 7
of the oil solution. This was determined by weighing 9 known volume of the solution in a special type of pycnometer such as was used to determine the density of iodine and naphthalene solutions. Determinations were made a t three temperatures only, but the results (Table I) show that no appreciable error was introduced by assuming the density of the solution to be the same as that of the pure solvent. Table I-Density of Solutions of Oil in Carbon Dioxide TEMPERATURE DENSITY OF OIL SOLUTION DENSITY OF P U R E
c.
0 5 14
0.932 0.895 0.825
coz
0.927 0.895 0.825
The results of the solubility determinations are shown in Table 11. Relation between Solubilities and Densities
Figure 1-Solubility
of Oil in Carbon Dioxide and Densities of Pure Components
After the reaction tubes had been prepared with the oil in one leg and the proper quantity of liquid carbon dioxide in the other, the two liquids were brought together by turning the solvent into the leg containing the solute. The tubes were than agitated in a thermostat for from 6 to 8 hours, and finally the solution was carefully decanted from the oil into the measuring tube. In most cases this decantation was rather easy to accomplish, as the viscosity of the oil and its tendency to stick to the sides of the tube made a clean-cut separation possible. Immediately on the removal of tho tubes from the thermostat, the volume of the solution was read on the measuring tube and the solution then quickly frozen in a freezing mixture of solid carbon dioxide and ether. After the liquid in the tube had all changed to a solid, a small hole was melted in the tip of the tube and the whole system allowed to stand until all the carbon dioxide had escaped. The measuring leg was then cut a t a point just above that reached by the liquid carbon dioxide and the amount of oil determined by weighing before and after washing the oil out with pure carbon tetrachloride. Care had t o be taken during this washing operation to prevent the remora1 of any glass fragments that sometimes fell into the tube during the cutting. The volume of the solution was determined by weighing the water necessary to fill the tube to the mark reached by the solution. The carbon dioxide, obtained from the Pure Carbonic Company, of Berkeley, Calif., was distilled from a commercial cylinder into the reaction tubes. An analysis of the gas taken from the top of the cylinder gave a carbon dioxide content of 99.86 per cent, and showed no oil and only a small amount of water vapor. The oil used as a solute was taken from the stock tank of another plant manufacturing liquid carbon dioxide. It had a density of 0.8537 a t 20" C. and was colorless and odorless. Density Measurements
I n order to calculate the solubility of the oil in a definite weight of the solvent, it was necessary to know the density
If lubricating oil and liquid carbon dioxide are sealed in a glass tube and the temperature is gradually lowered from room temperature, a point will be reached where the solution of carbon dioxide in lubricating oil, which previously existed as a bottom layer, will rise to the surface and take the place of the solution of oil in carbon dioxide. The results of many observations indicated that this phenomenon took place a t a temperature very close to 10" C. It was impossible to determine the exact temperature of this change, as the movement of the liquids in each other was very slow. This change of position was, of course, caused by the difference in change of density of the two layers with a change in temperature, and because this phenomenon has some bearing on the question of oil separation by means of an oil trap, it seemed desirable to determine the exact temperature where it takes place. It has already been found that the density of the solution of oil in carbon dioxide was not appreciably different from that of the pure solvent a t this temperature, and it would seem reasonable to assume that the solution of carbon dixoide in the oil would not differ very greatly from that of the pure oil. If this is the case, the temperature where the two density curves cut each other would be the same as that a t which the two layers change places. Density determinations on the pure oil were made a t 30", 20°, lo", and 0" C., and Figure 1 shows the curve obtained when the results obtained were plotted against the temperatures. The density curve of the liquid carbon dioxide was obtained by plotting the values obtained by Behn3 and those obtained by Jenkin14 of Lubricating Oil in Liquid Carbon Dioxide a t Various Temperatures WEIGHT TEMPERA- COz WEIGHT OIL W E I G H T OIL PER TURE SOLN. COY FOUND 100 GRAMSCOz 0 c. cc. Grams Gram Gram
Table 11-Solubility
VOLUME
NO.
1 2 3 4
25 25 25 25
3.201 5,233 3.842 2.353
2.273 3.715 2.728 1.670
5 6 7
20 20 20
3.705 5.568 6.852
2.853 4.288 5,276
0.7039 0.6674 0.7514 0,7482 Av. 0 . 7 1 8 0,0240 0,8412 0.0359 0.8372 0.0387 0.7335(not
8 9
20 20
2.240 4.419
1.725 3.403
0.0151 0,0279
10 11 12 13
10 10 10 10
5.522 5.809 5.034 4.002
4.738 4.984 4.320 3.434
14 15 16 17 18
0
5,108 5,740 5.340 6.383 5.806
4.735 5.321 4.950 5.650 5.382
5.297 5.074
5.461 5.231
0 0 0
0
19 20
-20 -20 8
4
0.0160 0,0248 0.0205 0,0125
averaged)
0.8754 0.8199 Av. 0 . 8 4 3 0,0429 0.9054 0,0438 0.8787 0.0800 0.9260 0,0312 0.9084 Av. 0 . 9 0 4 0,0386 0.8151 0,0424 0.7971 0,0400 0.8050 0,0443 0.7840 0,0428 0.7951 Av. 0 . 8 0 0 0.0218 0.3992 0.0197 0.3766 Av. 0 . 3 8 8
Ann. P h y s i k (4) 3, 733 (1900). Proc. Roy. SOC.(London),988, 170 (1920).
July, 1928
INDUSTRIAL AND ENGINEERING CHEMISTRY
and it will be noted that the intersection takes place a t exactly 10" C. If we now plot the solubility values indicated in Table I1 against the temperature on this same figure, a most interesting fact is discovered. The solubility curve has a maximum value a t the temperature a t which the two components have equal densities. This, of course, agrees very well with the theory of solubility-that is, the mutual solublity of the components of a solution becomes greater as the properties of these components become nearer alike. Solubility of Glycerol
Several determinations of the solubility of glycerol in liquid carbon dioxide were made, but the results were very discordant, no doubt because the weights of glycerol obtained were very small. In no case was the solubility of the glycerol found to be greater than about 0.05 per cent, and in most cases the difference in weight of the tube before and after washing out the glycerol was so small that there was doubt as to whether or not this was due to error in weighing. Time was not available to continue these determinations to obtain more accurate results. Discussion of Results
The most common method of removing lubricating oil from carbon dioxide is to place a simple oil trap between the compressor and the condensers. That this method is not very efficient is indicated by the fact that most commercial samples of liquefied carbon dioxide contain oil. These traps are usually designed to permit the heavy oil droplets to separate from the gaseous carbon dioxide, but some traps are placed under such conditions that the trap may be filled with liquid carbon dioxide, in which case, of course, separation of the two liquids is impossible. A much worse condition may arise in some cases where the trap is placed so that the temperature falls a t times below 10" C.; then the oil already separated in the trap will rise to the surface and flow into the condensers with the liquid carbon
737
dioxide. This is made possible by the fact that such traps usually have outlets placed a t the top. Assuming conditions are such that the liquid carbon dioxide in a commercial cylinder becomes saturated with lubricating oil, then a 50-pound cylinder with a capacity of 2356 cubic inches would have a filling density of 50"/5 = 0.588, and from the tables of Stewart5the volume of the cylinder occupied by the liquid a t 20" C. is found to be 66 per cent. The weight of the liquid carbon dioxide in this cylinder would therefore be 43 pounds and the gas would weigh 7 pounds. From the value of the solubility of lubricating oil in carbon dioxide found in this investigation to be 0.843 gram per 100 grams of carbon dioxide, the 43 pounds of liquid carbon dioxide would contain 164 grams of oil. Summary
1-Solubility determinations of lubricating oil in , liquid carbon dioxide have been made a t various temperatures. It was found that the maximum solubility took place a t 10" C., a t which temperature the solution contains about 0.9 per cent oil. 2-Density determinations showed that the oil did not change the density of the liquid carbon dioxide appreciably a t ordinary room temperatures. 3-The solubility of glycerol in liquid carbon dioxide was found to be very small, probably less than 0.05 per cent. 4-The maximum solubility of oil in liquid carbon dioxide took place a t a temperature where the densities of the com- . ponents had the same value. Acknowledgment
The most of the experimental work in this paper was conducted a t Stanford University while the author was a guest of the Department of Chemistry, and he wishes to express his appreciation for the aid extended by the university and the members of the departmental staff. 5
Trans. A m . SOL.Mech. Eng., 30, 1111 (1008).
Improved Laboratory Condenser"' Edward S. West DEPARTMENT O F BIOLOGICAL CHEMISTRY,
WASHINGTOI
HE accompanying illustration shows a condenser which has been found strikingly superior to the old Liebig type for general laboratory and research use. The condenser is easily made from two pieces of Pyrex glass tubing so chosen that the smaller fits into the larger leaving a space of 1 to 1.5 mm. between the tubes. The larger tube, which is to serve as the mater jacket, is fitted with side tubes for the water inlet and outlet. The smaller tube, which is the condensing tube, is cut the same length as the larger jacket tube, slipped into it, and the edges of the outer and inner tubes are sealed to each other a t each end. An adapter is then sealed to one end of the condenser as shown. The outer tube should be made of medium or heavy Pyrex tubing and the condensing tube of the lighter grade. A convenient laboratory size condenser has an inner tube of 7 t o 9 mm. inside diameter, a total outside diameter of 12 to 15 mm., a water column between inlet and outlet tubes of 300 mm. length, and an over-all length of about 550 mm. 1 Received April 26, 1928 This type 0 1 condenser in convenient sizes is supplied by Arthur H Thomas Company, Philadelphia, P a .
U N I V E R S I T Y SCHOOL O F MEDICINE,
ST. LOUIS,
MO.
Such a condenser is small, compact, and easy to manipulate, requiring only buret clamps for support and in many cases no clamp support a t all. It is sturdy and much less subject to breakage than the older types, because the heavy outer jacket tube protects the inner condensing tube all the way to the end. There are no rubber connections to leak or replace. The greatest advantage is that it is much more efficient than the old Liebig type. The same quantity of
&A A n Improved Laboratory Condenser
mater passes through the condenser as in case of the Liebig, but owing to the narrow space between the condensing tube and outer jacket the circulation is much faster and the cooling layers of water next the condensing tube are changed many times more rapidly than in the case of the Liebig condenser.
