THE HYDRATION OF THE CRYSTALLIKE FIBERS O F SOAP CURD J. K. hIcBAIX, H. I. BULL,
~ N D L.
S. STADDON’
Dppartment o j C h e m i s t r y , Stanford 1-niiersizy, C a l i f o r n i a Receii ed F e h m a r g 23, 1934
The commonest occurrence of soap is in the form of curd fibers embedded in a liquid crystalline solution, as in many hous4old soaps. Some toilet soaps consist almost exclusively of curd fibers. These solid and concentrated forms of soap are the most difficult to cliaracterize scientifically, and their nature is most open to dispute (5). Curd fibers, by x-ray evidence (12, 13, 14, l5), are crystalline, exhibiting three spacings as distinguished from the one of liquid crystals. They have been shown to consist of hydrated neutral soap (4). They exhibit a definite solubility which is, however, very greatly dependent upon temperature and to a certain extent upon previous history. In our phase rule studies (6) we have found it possible to treat them as a phase or, rather, as a series of phases, although this is considered erroneous or iiicomprehensible by This is partly due to the fibers not being massive; for although they may be centimeters long, they are finely microscopic or, more usually, ultramicroscopic in breadth and thickness. They thus present a large surface available for sorption, and all electrolytes are noticeably sorbed by them. Even saturated sodium chloride solution, n-hicli completely displaces other more dilute electrolytes from the curd fibers, does not prevent all non-electrolytes so far studied from being appreciably sorbed, glycerol being the sole exception. One highly reproducible measurement has been made by four previous independent methods (3, 7, 9, 11). Sodium palmitate fibers at 90°C. in the presence of a saturated solution of sodium chloride are found to contain 2.1 mols of water to 1mol of sodium palmitate. They also contain between 1 and 3 per cent of sodium chloride. Thus the strong negative sorption of sodium chloride by sodium palmitate is in reality a slight positive sorption by the hydrated fibers. Using glycerol, Bennett (1) found that a t room temperature the hydrated Experimental n ork carried out a t Bristol University, England, in 1923-1927. example, Ostwald and Erhring: Kolloidchem. Eeihefte 31, 345-46 (1930); however, they are doubtless depending upon their own assumption, “Da der Wassergehalt dieser Kernfasern zweifellos kontinuierlich variiert.” 1075
* For
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J. W. MCBAIS, H. I. B C L L AND L . S. STADDON
sodium palmitate fibers separating from ordinary isotropic (0.25 -JT11 solution have tlie composition S a p . 10HzO,whereas those separating from more concentrated (1.0 AYIL) anisotropic liquid solutions (liquide h conique) eshibit an empirical formula Nap. 4.3H20. The present cornniiiiiication employs the dew point method of nieasuriiig vapor pressure in studying the hydration of curd fibers of sodium palmitate and sodium laurate xvith and without the presence of a saturated solution of sodium oxalate. The results are unexpected in that tliey appear to indicate definite hydrates or amounts of water of crystalliza t'ion. E X P E R I b l E S T A L METHOD
The dew point apparatus is that first designed by Cuniming ( 2 ) n-ith tlie essential modifications by McBain and Salmon (8) which convert it into ft differential method. It is s h o ~ nin i diagram in figure 1. A liiglily polished silver test tube is fitted with a rubber stopper in which is inserted a thermometer, reading to 0.01"C., and two glass tubes through which a rapid current of water is circulated by a power pump from and t o a thermostat of adjustable temperature. The silver tube is held in a large rubber stopper fitted into an outer glass vessel which contains the closefitting, thin-walled shell holding the solution or curd to be studied. Tlie top of the outer vessel rises about one inch above the stopper, so that the closed space is completely immersed in distilled water in the transparent thermostat. A glass capillary tube, sealed to a stopcock, passes through the stopper holding the silver tube and may be connected x-ith a water pump and the outer vessel thus evacuated if required. The glass outer vessel should be as sinal1 as possible so that equilibrium can be quickly attained. The silver tube should be kept highly polished b y a silversmitli. Hand polishing requires practice. The motion should be along the length of the tube so as to avoid any disturbing transverse scratches. For cleaning, tlie tube was immersed overnight in freslily distilled thiophene-free benzene and lightly polished with silk which had been extracted with benzene. The effects of traces of hydrogen sulfide have to be removed b y polishing with jen-eler's rouge. The device of preventing formation of dew on a diagonal portion of the lower part of the silver test tube by coating it with an invisible film from boiling conductivity water was used in the manner originated by McBain. Illumination has to be carefully adjusted for optimum visibility of the boundary betn-een the bright surface and that dinimed by incipient formation of dew. Too much dew may necessitate recleaning. The silver tube was adjusted to l.O°C. above the temperature of the thennostat before assembling as in figure 1. To hasten attainment of equilibrium the outer tube was evacuated. Under good conditions the differeiice between appearance and disappearance of den- was only 0.03'C.,
HYDRATION O F CRYSTALLINE FIBERS O F SOAP CURD
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the mean being taken as the dew point. The zero correction, for differences between thermometers, and personal error, was found by one of us (H. I. B.), but not by previous workers, to depend slightly upon the condition of the silver tube, which was therefore subjected to an exact routine of cleaning and treatment. It is determined by measuring the dew point lowering of
TO REGULATED RE SERVOlR
.......-.-.........
\& %\
WATER IN TRANSPARENT THERMOSTAT
-SILVER TUBE
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\
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-SOAP '\
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FIG 1. DEWPOIUTA P P A R ~ T UFOR S STUDYIX~; VAPORP R E ~ S UOF R ESOLUTIONS OR SOLIDSCOXT.IISISGKATER
conductivity water itself ; these measurements were repeated a t frequent intervals as a check. T o insure avoidance of false equilibria with viscous solutions it is necessary to shake or stir the specimen being measured. K t h curds the most satisfactory device is to allow the curd t o form in the vessel where practi-
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J . W . MCBAIN, H. I. BULL A S D L. S. STADDOX
cable and then to cut a n a y tlie upper surface and the middle portion of the curd itself as shonn in figure 1. Water diffuses better in this way than when the curd is a heap of separate lumps. The curd is allowed t o remain in contact with the closed (evacuated) vapor space for several hours before taking a measurement. I n the later experinients not more than 30 mg. of water vias removed from a curd at a time; or, if large amounts were removed, the curd was left for a week in a closed vessel to recover internal equilibrium. Dehydration of the curd n ithin tlie dew point apparatus 11 as found inadvisable and was rejected in favor of dehydrating the curd by placing its container in a vacuum desiccator and weighing from time to time. The chief disadvantage is that the exact composition at which a break occurs in the vapor pressure is missed and has to be surmised from iiispection of the graphs of dew point lowering against gross composition of the curd. The curds were subsequently analyzed by the methods and using tlie precautions described in previous communications. They n-ere firct dissolved in boiled-out neutral water, decomposed u-ith excess of acid, the fatty acid titrated in 80 per cent alcohol, and the aqueous filtrate with standard alkali. Then if sodium oxalate was present, it TTas determined with permanganate. A quicker method used in the later experiments on curds containing no oxalate n-as complete dehydration to constant weight. Thus with curd 25A (H. I. B.) complete analysis of 0.4232 g. of such completely dehydrated curd yielded 0.4230 g. of sodium palmitate. Thus from the weight a t any time, the amount of n-ater present vias knoxyn by difference. Each dew point lowering recorded is the mean of from six to twenty individual readings of which many thousand were required for the present work.
Results in presence of saturated sodaum oxalate3 Sodium oxalate crystals n-ere present in excess for the purpose of serving as an indicator to show when the last amount of free solution just evaporated. Up to this point the dex point has to remain constant, but thereafter any water n-ithdran-n comes from the dry fibers of the hydrated soap curd and the vapor pressure falls suddenly. The sodium palmitate a i d sodium laurate were special preparations made for us by I
