T H E E QV I LI B R I =1 USD ER L TI S G THE SOX P-B 0I L I S G P R 0C E SSE S. THE SYSTE31 POTASSIUT\l L-1URSTE-POTASSIU~I CHLORIDE-TF-AATER BY JAMES WILLIAM J I c B A I S A S D JI.1LCOLJI CLIFFORD F I E L D
The study of soap solutions has shown that they exist in a number of forms, each with very striking characteristics.' I n pursuing the systematic study of the constitution of these liquids, jellies. liquid crystals and curds it is essential t h a t the conditions under which each form exists should be exactly delimited. Further the processes of soap boiling depend upon the existence of this series of well-defined equilibria. A11 soaps, pure, mixed commercial, whether potassium or sodium, are essentially of the same type. The differences are in degree and not in kind and the equilibria are so similar in the various cases that by means of numerical factors the effect of any electrolyte or mixture of electrolytes can be fairly closely predicted.2 Finally, to a large extent, the behaviour of a mixture of soaps can he inferred from that of its pure constituents taken separately. I t is found also that the results from such small scale laboratory experiments as are described in the present paper are in numerical agreement with similar results obtained on a full commercial scale. Two preliminary contributions from this laboratory3 dealt with sodium laurate and sodium palmitate in which the chief results were to find the various kinds of soap systems which exist and t o indicate some of the limits for the existence of ordinary isotropic soap solutions a t various temperatures with and without the presence of salt. McBain and Elford4 describe a more thorough study of systems derived from potassium oleate which like potassium laurate possesses the advantage of being so soluble that all the possible forms can be observed a t room temperature, Reference should be macle t o the papers cited for justification of the applicability of the phase rule t o the external equilibria of reversible colloids and for a description of the special methods of investigation which have been developed in this study. The present installment dealing with potassium laurate, includes the first successful delimitation of the field of existence of the important anisotropic liquid (liquid crystal, liquide B conique) middle soap5 and also the first information obtained as t o the upper limits of existence of that other anisoThe state of knowledge of this subject was summarised 11:- one of LIS in 1 9 2 2 (Fourth Colloid Report of Brit .hsocn for the =Idvancement of Sclence, 244), now under revision and a more recent account revised u p to date has been contributed as chapter 1-in 1-01. I . (pp 137-164) of Jerome Alexander's "Colloid Chemistrl-" (1926'. McBain and Fitter: J. Chem. Soc 129. 893-898 (1926). JIcBain and Burnett: J. Chem. Soc.. 121, 1320 (1910); 1IcBain and Langdon: 127, 852 (1925). ,J. Chem. Soc., 129, 421 (1926). Discovered by 1IcBain and Langdon. ~
1546
J.13IES WILLIAM 3ICBAIN A S D MAL C O L N CLIFFORD F I E L D
tropic liquid the soap boiler's neat soap. Potassium laurate is so soluble that solutions less than 2S, cannot be salted out a t room temperature by potassium chloride as the latter cannot reach a high enough concentration. Concentrations of potassium laurate higher than zS, a t room temperature and all solutions a t temperatureq above 60' can be salted out. All these phases exist equally ne11 in the absence of salt and the simplest procediire is to begin n i t h the two-component system laurate and water. in which most of the typical equilibria appear. Accordingly, this will be discussed first, together n-ith observations made upon individual phases. followed by the data obtained with systems in which potassium chloride is added as a third component. Materials Throughout thiq ~i-orkone sample of potassium laurate was used. specially prepared bp Kahlbaum. I t had been carefully freed from alcohol, its purity and neutrality checlied antl found to be satisfactory, data from two further qamples being found t o agree \Tithin the experimental error. The potassium chloride used was also from Iiahlbauni antl was tlriecl by careful warming in a silica dish. The soap, as a dry powder. was weighed into a thick-nalled duro glaqs tube 6 t o 9 incheq long antl M t o inches in diameter. sealed a t one end in the form of an ordinary testtube. P o t a 4 u m chloride (if any) \vas nest ac!cletl as required and the tube reneighed. Finally I to 3 grams of 1Tater (conductivity) nere introduced by means of a I cc pipette according to the concentration of soap required. and the tube again neighed and sealed. Xfter preparation of the expeliment, the tube was heated and shaken until the contents were transformed into a single homogeneous liquid. For this purpose a 'ZI nter bath waq sometimes sufficient, but \Then higher temperatures Tvere required either a glycerine bath (up t o zoo') or an electric oven was employed. The latter was made by winding a resistance coil round a holloTv cylinder of sheet asbestos (length 8 t o 9 inches, diameter 3 t o -1. inches) containing two mica window through which the system could be examined. The temperature was read by a thermometer. the bulb of which mas kept in contact with the soap tube. Stirring was effected by slowly shaking the furnace as a whole. s%
Methods of Observation I n every system, as in the case of all other coap' studied, it was possible t o observe a temperature T, on heating to n hich a clear, transparent, homogeneous, isotropic liquid n-as formed. On alloning this liquid to cool s l o ~ ~ lthe y, critical temperature for the separation of a qecontl phase (in the more concentrated soap systems) or for the formation of two liquid layers (in the more dilute soap systems with salt) could he determined. Thus, for any concentration of soap above approximately 2 . 3 S , , , with or without potassium chloride, on cooling a point was reached where the clear system became turbid, slight heating destroyed the turbidity, but re-cooling caused it t o reappear a t the
POTAbSIU31 LATRATE-POTASSIUM CHLORIDE-V-.%TER
I547
same teni1:eratiu-e. For concentrations of soap heloir- z.jS,,. a similar point was reached on cooling. provided the concentration of potassium chloride present was a t least OS,^. I n this latter case the cloudiness was more apparent, and on further cooling, the tube contained globules of two liquid phases, which gradually separated into two layers. the relative volumes of n-hich depended on the temperature. Determination of this turbidity temperature delimits the boundary between homogeneous isotropic solutions antl other phases Il-hich exist. I n all cases the turbidity temperature can be approached either on cooling or heating, provided the latter is carried out sufficiently slowly? showing (as has been observed by previous workers) that the system is in true equilibrium, and that no supersaturation occurs on cooling. The two methods of observation n.ere employed nhich are described in 1IcBain and Elfortl's pay,er (loc. cit.) : namely. the direct visual ob-2ervation of Ti and the microscopic method of examining a, sample of soap solution in a small flattened glass bulb between crossed Sicols, the system being heated by a small electric furnace on the state of the microscope. The determination of T h 3 the temperature to n-hich a wholly anisotropic solution has to be heated in order that an isotropic phase begins t o separate from it, may be approximately determined by noting the fall in rigidity that accompanies formation of the isotropic liquid. The microscopic methcd is more accurate but even here difficultj- is ezFeriencet1 in ohtaining a thin enough sample. -1combination of the two methods however gives a very good indication of the temperature T h .
Eyicl'7l'brirr in the two-componcrit system potassl'um Iutrrnte-zrnfer. It will he seen from n comparison of Fig. I n.ith that of Fig. I of the paper 1 3 IIcBain ~ ant1 Elfortl (loc. cit.) or pp. 1 4 2 , 143 A1lexander (loc. cit.) that exactly the same phases occur in both soap systems: namely, hydrated crystals, neat soap, mitltlle soap antl isotropic homogeneous solutions. Only the limiting concentrations nhich mark off these fields differ for the various soaps thus making a difference in the size of the corresponding fields. Thus for potassium laurate solutions below about 2.3Sl,- are composed of a single homogeneous iFotropic phase for all teni1:eratures down to room temperature; solutions hetn-een 2.jS,,. antl AS,,- are composed of tn-o phases at room temperature. one optically anisotropic, the other isotropic. On heating these solutions a homogeneous isotropic solution is reatlily obtained. Similarly if the system is wholly anisotropic imidtlle soap) a t room temperature, it hecoines hetercgeneous on heating t o a definite temperature T h . while a t a slightly higher tem1:erature Ti it finally becomes homogeneous and isotropic. These transformations are rerersihle antl may be represented thus : ;Inisotropic phase
Th
*
Heterogeneous (anisotropic ant1 iqotropic phase)
TI
Isotropic phase
, cooling to rooin remperature leads For Foal) concentrationq above j .OS,, either t o the partial. or total zeparation of lamellar crystals of hydrated
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JAMES TVILLIAN MCBAIiX AND MAL COLRI CLIFFORD FIELD
potassium laurate. Thus, 9 . 0 s ~potassium laurate a t room temperature, exists in the tube as an upper portion of anisotropic liquid (about j o 2 ) and a lower portion of solid white hydrated crystals which exhibit a distinct sheen when interlocked together. McBain and Elford' described the hydrated crystals of potassium oleate as being wax-like; the hydrated crystals of potassium laurate are on the contrary much more solid. T'orlander2 tabulated the melting points of many fatty acids and soaps but omitted that of potassium laurate. Pure potassium laurate in its white powdery condition was introduced into a fine melting point tube and attached t o the bulb of a thermometer, which was placed in the electric furnace. At 264' the soap lost its white powdery appearance and formed a transparent gelatinous solid; this would be termed by T'orlander the first melting point of the soap, but it is really the transition point t o the conic anisotropic liquid, neat soap. At 3;6' the true melting point to an isotropic liquid was observed, this being the lowest temperature at which a meniscus existed. Care was taken to observe the soap a t the bottom of the melting point tube because contact with air at this temperature decomposes it. The so-called first melting point represents the highest temperature at which hydrated potassium laurate crystal can exist and is therefore a point on the boundary of the neat soap field, The upper neat soap boundary has been completely determined in this two component system, by observing the temperature when the last trace of hydrated crystal dissolves and forms the homogeneous neat soap phase (Tc). The systems were heated in the electric oven until they formed a homogeneous isotropic liquid, they were then alloxed t o cool slowly until the first trace of hydrated crystal separated (considerable supersaturation always occurred, the crystal separating many degrees below the temperature at which it dissolved). The system containing a trace of hydrated crystal in neat soap was reheated and the temperature observed when the last trace of solid matter disappeared, If complete separation of hydrated crystal took place, i. e. the system was allowed to cool down to room temperature, a mass of bubbles was formed interspersed with crystals, and on reheating this completely obscured the point a t which the crystals finally disappeared. Calculations have shown that the degree of hydration of the crystal cannot exceed one molecule of water per molecule of potassium laurate; thus the homogeneous hydrated crystal boundary is exceedingly narrow. The highest soap concentration as yet examined (approx. 43Kn or 92% soap) still contained a trace of anisotropic liquid together with hydrated crystal. Examination under the microscope has shown that two types of crystal exist ( I ) hexagonal lamellae which are apparently hydrated potassium laurate crystal; when the system is crystalline it consists almost entirely of this type, but traces of ( 2 ) small needles or rods have been noticed, and are probably acid soap produced by hydrolysis. S o method of exact delimitation of the lower neat soap Loc. cit. or Alexander: L'Colloid Chemistry," pp. Ber , 43, 3120 (1910).
132,
133 (1926).
POT.1SSIUM LA~RATE-POTASSIC31CHLORIDE-WATER
I549
boundary has yet been practicable in the case of potassium laurate and thus the shape of this one particular line in the diagram can only be taken as approximate. The position of one end was determined in the two-component system. For concentrations of soap below ;. jKnhydrated crystal separates if the system is cooled in melting ice. Table I includes all the data obtained for the aqueous potassium laurate systems. T h n a s determined by the viscosity method, and checked by the niicroscope method. T , was determined by direct observation of the turbidity
HOMOC ENEO U S
FIG.I Equilibrium diagrnm foi the tn-o-component
