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I N D U S T R I A L AND ENGINEERING CHEMISTRY
Vol. 15, No. 8
Surface &Tensionof Alkaline Soap Solutions' By E. B. Millard MASSACHUSSTTS INSTSTUTE
OF
TECHNOLOGY, CAMBRIDGE,
MASS.
This conclusion is not supHE use of some alIt has long been known that alkaline material increases the deterported by the writer's prekaline materia1 in gent power of soap. Salts of weak acids, which produce mild atkaliminary measurements of conjunction with line solutions upon hydrolgsis, are most commonly used for this drop volumes at 25' C., in soap in the ordinary washwhich 0.05 per cent of soap purpose. I t has been found that those substances which produce decreased the interfacial tening of white cotton fabrics the greatest decrease in surface tension are the most eficient detersion of water against kerosene has become somewhat gengents, and therefore the cleansing power of these alkaline "addi45 per cent, and further addieral practice, and long expetion agents" can be measured in this way, as has been found by tion of soap caused only a rience seems to have shown slight further decrease in the previous inoestigators, although their results do not in all respects surface tension, indicating an that such cleansing is done agree with those of the writer. approach to surface saturaas well and more cheaply The writer has determined the interfacial tension of oarious soap tion. Similar results were obwhen a mild alkali is used solutions against benzene by measuring the drop oolume, comparing tained by White and Martin4 with a diminislied quantity in connection with solutions the ideal drop oolume with such a solution, and calculating by means of soap. of soap than when soap of the equation 2s r y = mg. The results show that sodium The measurements of E1alone is employed. SiliCarbonate is the most eflective saltfor reducing surface tension. ledae and Isherwoode uDon cate of soda has long been drip volumes of solution; of used as a builder in cake soap and salts of weak acids soap to make possible a larger cake of soap at a given price and a t 100' C. refer to concentrations of soap and added alkaline salts as an alkaline material. Most of the substances commonly which are higher than those used in this work, but they show an increase in drop number (corresponding to a decrease of surface used in power laundries for this purpose are salts of weak acids tension) upon the addition of soap alone to water. In their work -such as washing soda, borax or sodium phosphate-which no mention is made of the use of a correction curve such as those by hydrolysis produce solutions of mild alkalinity and con- developed by Lowenstein and by Harkins, so that their drop siderable reserve of alkalinity. That is, considerable un- volumes are not strictly proportional to surface tensions. These state that when equivalent quantities of the various adhydrolyzed salt remains in solution and this material hydro- writers dition agents are used, the same decrease in drop volume is pro: lyees progressively as the first quantity of alkali becomes ex- duced regardless of the nature of hausted. These materials have an advantage over sodium the salt used. The work reported hydroxide in that they produce far less decrease in strength of below does not agree with this conclusion; for example, the interthe cotton upon repeated washing. facial tension of 0.03 per cent soap Probably the chief function of soap in washing is that of an plus 0.12 per cent Na3P04.12Hz0 emulsifying agent far the greasy material which surrounds the against benzene is 11.3dynes, and dirt, holding it in suspension until it is removed by mechan- the interfacial tension when 0.05 b cent sodium carbonate (an ical agitation of the solution. While the decrease in surface per equivalent amount) is used with tension at an interface when soapy water is one of the phases 0.03 per cent soap solution in place is probably not an accurate or complete measure of the cleans- of sodium phosphate, is 14 dynes. Shorter and Ellingworth5 have ing power of the aqueous phase, those substances which prosome experiments of a simiduce large decreases of surface tension when dissolved in made lar character, in many of which water are probably more efficient detergents than others which the fatty acid was in excessover have less effect upon the surface tension. The decrease in total alkali. Their paper contains surface tension produced by an "addition agent" when a a full discussion of the emulsifysoap solution of mixed concentration is used, has been consid- ing action of soaps. ered a measure of the cleansing power until some more defiEXPERIMENTAL nite way of measuring this property i s established.2 InterThe apparatus used in these facial tensions against benzene at 40" C. have been calculated from drop volumes measured for solutions containing measurements (Fig. 1) consists 0.03 per cent of soap, together with varying concentrations of a measuring pipet sealed t o a of several alkaline materials which are in more or less common capillary tube the lower end of use in connection with soap. This soap concentration is which is of circular cross secabout equal to t h a t used in ordinary laundry practice. Some tion and ground at anangle of unpublished work done in the institute laboratories has shown 90 degrees to the axis of the that these benzene-soap interfacial tensions are of the same' tube.6 All the measurements order of magnitude as when kerosene or emulsifiable oils were made with a tip having a radius of 0.343 cm. This tip is such as olive oil are substituted for the benzene. immersed in a flask of benzene PREVIOUS WORK supported upon a stopper having BottazziS found from his work with sodium and potassium an exit for displaced air; and FIG.1 oleates, using a stalagmometer, that pure soaps do not decrease the the whole lower portion of the surface tension of water even when concentrated enough to form apparatus is immersed in a water thermostat held a t 40" C. a skin on the surface, and that sodium or potassium hydroxide gradually decreases the surface tension. He states that only by means of an electric heater. To the upper end of the unhydrolyzed molecules of soap lower the surface tension, and measuring pipet is attached a small air chamber into which
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that alkalies act only to diminish the hydrolysis of the soap.
J . Phys. Chem., 24, 617 (1920). Proc. Roy. SOC.London, WdA, 231 (1916). 0 The apparatus and method are adapted from Harkins and Brown, J . A m Chem. SOC, SS, 246 (l916), as a more rapid but less accurate procedure suitable for first exploration of the field covered. 4
Received March 9, 1923. 2 This same assumption was adopted by Elledge and Isherwood, Tms JOURNAL, 8, 793 (1916). a Alti accad. Lincei, 21,II, 365 (1913); C.A , , 7, 3041 (1913). I
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August, 1923
INDUSTRIAL A N D ENGINEERING CHEMISTRY
811
mercury can be dropped, which by displacing air causes the slow formation of drops on the end of the capillary tip. Before filling the pipet witha solution the constant temperature bath and benzene flask are set aside and the benzene is wiped from the dropping tip with a clean cloth. The tip is then immersed in the solution, which is drawn into the pipet by drawing off mercury a t a until the meniscus stands near the top mark of the pipet. Then the tip is again wjped carefully on the sides, but not on the bottom, and mercury is allowed to flow into the air chamber c through a capillary nozzle controlled by stopcock b, until a drop of solution is suspended from the tip. This drop should wet the whole bottom of the tip and none of the side-that is, it shouldlook like tip m and not like n or p of the drops sketched in the figure. In case the drop does not completely cover the circumference of the capillary, the tip must be cleaned and a perfect drop produced. Then the flask of benzene is carefully raised into position and held by the stopper while the constant temperature bath is brought up around it. TENSION AGAINST BENZENE AT 40‘ c., TIP RADIUS 0.343 Radius Fraction Ideal Surface (Drop of Ideal Drop Tension % ume Volume)1/3 Drop Volume Dynes .... 0.282 0.524 0.643 0.439 26.8 0.592 0.197 0.628 0.313 19.1 0.143 0.615 0,232 0.656 14.1 0.076 0.117 0.836 0.600 7.1 0.244 0.383 0.638 0.550 23.4 0,206 0.328 20.0 0.628 0.580 0.193 0.626 0.594 0,308 18.8 0.148 0.240 14.6 0.651 0.616 0.078 0.130 0,600 7.9 0.803 0.271 0.423 0.642 0.530 25.8 0.266 0.415 0.641 0.533 25.3 0,255 0.398 0.639 0.541 24.3 0.636 0,239 0,376 22.9 0.654 0,633 0.227 0,359 0.562 21.9 0,263 0.534 0,641 0.410 25.0 0.636 0.240 0.377 0.552 23.0 0.631 0.216 0,342 0,572 20.9 0.626 0.096 0.191 0.305 18.6 0.596 0.629 0.327 0.206 0 . 5 8 2 0.030) 20.0 0.622 0.275 0 . 0 6 0 NaaPO4.- 0 . 1 7 1 16.8 0.618 0.616 0 . 0 9 0 12Hz0 0.138 0.226 13.7 0.664 0.608 0 114 0.187 11.4 0.708 0.634 0.363 22.1 0.660 Modified 0 . 2 3 0 0.632 0.220 0.348 21.2 0 . 1 2 soda 0.568
TABLE I-INTERFACIAL
---- +
---. Drop Vol-
SOLUTION Soap Builder
9%
0:03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.30 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03
s
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i
After sufficient time has elapsed so that the benzene and the tip are at 40” C., stopcock b is adjusted so that mercury flows into the air chamber very slowly, causing the drop on the tip to increase in size. When the drop falls off from its own weight, the stopcock is closed quickly and the position of the nieniscus is read. Then drops are caused to fall very slowly and they are counted until 10 or 20 have fallen, when the stopcock is closed quickly and the volume read again. As only 3 or 4 drops fall per minute a slight variation in the nearly uniform reaction time of the observer in closing the stopcock is not important, and the drop volumes obtained are substantially correct. Drop volumes so obtained are shown in Table I. The concentrations of soap and salts are in per cent by weight of the material as it is actually sold on the market, since it is these which are of primary interest to users of washing chemicals. In case a comparison of chemical equivalents is desired, these may be computed from the following data: Sodium carbonate is the C . P. calcined product; trisodiumphosphate means the hydrate NaSP04.12H20; “BW” silicate of soda contains 20 per cent NanO, 30 per cent SiOz, and 50 per cent water; “Star” silicate of soda contains 10.5 per cent NazO, and 26.7 per cent SiOz. The “modified soda” is a mixture of equal parts of sodium carbonate and sodium bicarbonate, which approximates the composition of mixtures sold to the laundry trade but is not identical in composition with any of them, so far as the writer is aware. A good grade of commercial soap, containing 7 per cent water and free from excess alkali or fillers or builders of any kind, was used, and the solutions were freshly prepared. Mixtures of the desired compositions were obtained by diluting the proper quantities of 0.1 per cent soap solution and
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0.02 B
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004 O.0b 008 P E R C CNT B Y WEIGHT
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0.10
&I2
OFADDELI
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AL7-
FIG.2
From the fact that an alkaline silicate of soda produces a greater decrease of surface tension than a less alkaline silicate, a determination of the actual alkalinity or hydroxide-ion concentration of these solutions as a function of the surface tension would be of great interest. Such experiments are in progress in another laboratory in the institute. Experiments upon the’ effect of mixtures of soap and sodium carbonate on repeated washing of cotton cloth are also in progress. 1
J . Am. Chem. Soc., 41, 499 (1919).
