1334
FRANK E. YOUNG AND FRANCIS T. JONES
SUCROSE HYDR,ATES THESUCROSE-WATER PHASE DIAGRAM' FRANK E. YOUNG AND FRANCIS T. JONES Western Regional Research Laboratory,2Albany, California
Received January 19, 1949
The work reported in this paper was undertaken as the first step in investigation of phase equilibria of basic importance in the freezing preservation of foods. A survey of the literature (1, 2, 4, 5, 6, 7, 9, 10, 12, 15, 18) indicated that the phase diagram of the sucrose-water system was apparently well established as one involving a simple eutectic of ice and sucrose without hydrate formation. This system was chosen as a starting point, because it offered a n opportunity t o check our technique on a system of importance in freezing preservation. Although the phase equilibrium data in the literature were not always in good agreement, particularly in the vicinity of the eutectic, there was no indication of the complexity subsequently encountered. Apparently no crystalline hydrate of sucrose bas previously been isolated and identified in the solid state, although several types of measurements have been interpreted as indicating the presence of hydrates in aqueous solution (3, 8, 11, 14, 16, 17). Seven previously unreported solid phases have been shown to exist by the well-defined eutectics with ice obtained during this study. Although the data are incomplete because of difficulties discussed later in this paper, solubility curves have been determined for four phases. Additional evidence has been obtained for the existence of three of these phases by microscopic observations and for two of these phases by x-ray difiraction photographs. The data also indicate a complex region below -15"C., the interpretation of which is very uncertain. For convenience in presentation, the material in this paper is re.ported chronologically. EXPERIMENTAL
Sucrose solutions were prepared from distilled water and commercial granulated table sugar containing less than 0.1 per cent impurities. The principal impurities were: moisture, 0.01 per cent; ash, 0.01 per cent; reducing sugars, 0.04 per cent. Three experimental methods were used in this study: solubility measurements, warming curves, and observations with a polarizing microscope. Solubilities were measured refraetometrically on solutions which had been allowed to come to equilibrium from both undersaturation and oversaturation in a 1 Parts of this work were presented a t the 28th Annual Meeting of the Pacific Division of the American Association for the Advancement of Science a t San Diego, California, June 18, 1947, and at the 113th National Meeting of the American Chemical Society a t Chicago, Illinois, April 23, 1948. * Bureau of Agricultural and Industrial Chemistry, Agricultural Research Administration, U. S. Department of Agriculture.
SUCROSE HYDRATES
1335
constant-temperature bath controlled to =tO.O3"C. These measurements always agreed within = t O , l per cent. Warming curves were obtained with the apparatus shown in figure 1, in conjunction with a potentiometer and a copper-constantan thermocouple
h f
j
k
a FIG.la. Amaratus for warming curves : a , thermocouple; b , pulley for stirrer; c, bear.. ing for stirrer shaft; d , ground joint for removal of inner tube in case of breakage; e, doublewalled glass tube which can be evacuated t o reduce warming rate; f , stirrer shaft (stainlesssteel tube, in. O.D. x 0.01 in. wall); g, rotary stirrer. FIG.l b . Stirrer assembly: h, bearing to center thermocouple in stirrer; i , thermocouple protection tube (stainless steel, A in. O.D. x 0.01 in. wall welded closed at the bottom); j , depth of sample; k, end of thermocouple tube and position of junctions. I
+
calibrated by standard thermometric procedures. A precision potentiometer and single-junction thermocouple were used until shortly after the discovery of phase I, a t which time a recording potentiometer and three-junction thermopile were substituted. The usual procedure in obtaining a warming curve was to
1336
FRANK E. YOUNG AND FRANCIS T. JONES
cool a 20-ml. sample in the apparatus shown in figure 1, stir in seed crystals, and allow crystallization to proceed for a period of hours (or days if necessary) in a refrigerated room. The sample tube was then brought up to the laboratory in a cold alcohol bath, connected to the potentiometer, stirring motor, and vacuum pump, and allowed to warm in an ice bath. Where the primary break was above -3"C., a water bath a t a convenient:temperature was used. At the usual stirring rate of 60 R.P.M. and with the space between the walls evacuated to about 0.05 mm. pressure, the warming rate varied from about 0.5" per minute
TIME
FIG.2. Typical warming curves. Curves, a, b, c, d were obtained with the manual potentiometer; curve e with the recording potentiometer.
a t -25°C. to about 0.1" per minute a t -3°C. for sucrose solutions containing no solid phase. It was found that the breaks occurred a t the same temperatures whether the sample was warmed rapidly or slowly. Several typical warming curves obtained on sucrose solutions are shown in figure 2 ; all of the examples shown except figure 2e mere obtained on the manual potentiometer and are drawn through the experimental points. These points were rarely more than 0.05"C. away from the smooth curve except for an occasional point a t the break, as shown in figure 2b. Although breaks in warming curves (caused by the disappearance of solid phase) were occasionally as weak as that shown in figure 2a, it mas found that such breaks were always in good agreement with breaks on more typical curves such as figure 2b. Curves 2c and 2d show multiple breaks typical of those obtained in the regionbelow - 15"C.,
1337
SUCROSE HYDRATES
which will be discussed later. Figure 2e was obtained with the recording potentiometer and is typical of the curves obtained with phases I to VII. Breaks in the warming curves were usually reproducible within fO.l°C., except those below -15°C. which are not on the ice curve; the latter had a spread of about f0.15"C. Eutectic plateaus, such as the one shown in figure 2e, lasted from 3 to 101 min., during which time the temperatures rose only 0.15"C. or less. In warming runs in which the eutectic was of fairly long duration, the eutectic +50
+ 40
1.
I
I
I
I
1
I
M
I
I
Anh#ous Sucrose
'*L
-301
I
I
I
I
I
I
I
I
I
I
I
COMPOSf TON PercentSucrose by Weight)
FIG.3. Sucrose-water phase diagram. 0 , warming-curve measurement; A, solubility measurement; 0 , published values for sucrose solubilities; V , melting-point determination; - - -, uncertain portions of curves.
mixture was initially a hard crystalline solid (and occasionally expanded enough to break a glass container), but it usually softened rapidly enough so that it could be stirred easily by the mid-point of the plateau. These eutectic plateaus were usually reproducible to &0.15"C. RESULTS AND DISCUSSION
The data obtained in this investigation are shown in the phase diagram (figure 3) which, for convenience, also includes solubilities of anhydrous sucrose above 0°C. obtained from published data (4, 6, 9, 12, 18). These solubilities
1338
FRANK E. YOUNG AND FRANCIS T. JONES
are indicated by squares on the diagram. The other points were obtained from warming curves (circles), solubility measurements (triangles), and melting points (inverted triangles). Two regions of this diagram are shown on a larger scale in figures 4 and 7. The concentrations of sucrose in solutions a t equilibrium with the various solid phases can be read from the phase diagram (figure 3) and are shown in table 1. Because all of the curves in the phase diagram have been followed for considerable distances into regions where the corresponding TABLE 1 Sucrose concentration in solutions at equilibrium with the various solid phases
1
(a) ICE
1 zaeight per ccirt
0.0 10.0 20.0 30.0 40.0 50.0 60.0 70.0 80.0
'I I
'
(b) ANHYDROUS SUCROSE Concentration
0.0 -0.6 -1.5 -2.66 -4.4 -7.05 -11.6
1
-19.05
I (-30)
63.6 63.75 64.4 65.4 66.8 G8.6 70.4 72.3
(C) PEASE I Concentration
"C.
weight per cent
-13.95 E* -10.0 0.0 +10.0 +20.0t +30. O t +40. Ot f50.07
58.06
weigh1 per cent
(e) PEASE I11
(d) PEASE I1 --
Temperature
I 58.3 63.25 68.5 73.8s 79.46 88.37
(f)
PEASE
Temperature
"C. -10.5 E -10.0 0.0 +lO.O +20.0 +30.0 +40.0 f45.7 M Iv
.____
Concentration
-
-
zc'eig/it p e r L T
56.2 63.15 70.65 78.25 84.41
I __Temperature
1
1 ~
OC.
-9.55 E 0.0 f10.0 +20.0 (f27.8) M
Temperature
Concentration
weight per cent
"C
weight per cent
55.1 58.55 62.95 67.5 72.2
-9.06 E -5.0 0.0 +5.0
54.0 56.95 62.76 69.3
Concentration
I
Temperature "C.
-8.56 E
-5.0 0.0 +5.0
+10.0
E indicates eutectic with ice.
* This eutectic has not been obtained experimentally. M indicates melting point. t These values were obtained from published data (4, 6, 9, 12, 18). solid phases are metastable, solid lines have been used for all of the well-defined curves in both stable and metastable regions. Broken lines have been used only for uncertain portions of the curves rather than for their metastable portions. It is a striking feature of this system that, in the absence of crystals of other solid phases, each phase may be studied far into regions where it is metastable without transition to another phase. Thus ice is a metastable phase from B to L in figure 3.
Ice and anhydrous sucrose At least two results in good agreement were obtained for each point shown on the ('ice" curve, A L in figure 3. I n the absence of sucrose hydrates these points
I
SUCROSE HYDRATES
1339
are easily obtained without seeding for solutions containing less than 72 per cent sucrose, but at higher concentrations increasing viscosity makes seeding difficult and crystal growth very slow. At no time did sucrose crystallization occur during cooling or warming runs while the ice curve was being studied. Ice points below -21°C. have not been obtained from warming curves. However, a few ice crystals found growing in an 80 per cent solution a t -34°C. did not melt visibly during 2 weeks' storage a t -32°C. but melted almost completely during 1 week a t -29°C. After a few days at -31"C., the remaining crystals appeared to be regrowing slowly. On this basis, the temperature a t which ice is a t equilibrium with an 80 per cent sucrose solution should be approximately -30°C. The measurements reported here for the ice curve below -5°C. are appreciably higher than those previously published, the discrepancy increasing with decreasing temperature. This difference is probably due to supercooling; all of the previous authors appear to have used cooling methods, in which supercooling is a serious difficulty, aggravated in this system by high viscosity. I n the present work, the results from both cooling and warming curves on the ice line were in good agreement down to -11°C. if supercooling was minimized. As the amount of supercooling increased, breaks on the cooling curves occurred at progressively lower temperatures than the corresponding breaks on warming curves. Below -11°C. the degree of supercooling was invariably so large that cooling curves were useless. The slow growth of crystals a t low temperatures in highly viscous solutions and difficulties in seeding are additional disadvantages of the cooling-curve method which are greatly reduced by the use of warming curves. The ice-anhydrous sucrose eutectic shown in table 1 is the point of intersection of the solubility curves of ice (AL) and anhydrous sucrose ( J M ) . We were unsuccessful in numerous attempts to obtain thermal evidence of the existence of an ice-anhydrous sucrose eutectic mixture. No eutectic was obtained after several weeks' storage at -18°C. from a sucrose solution which was heavily seeded with powdered sucrose after ice had crystallized. No eutectic plateaus were obtained on warming curves of intimately ground mixtures of powdered ice and 300-mesh sucrose. Other methods have similarly failed to give eutectics. It must be concluded that the anhydrous sucrose-ice eutectic is very unlikely t o form in sucrose solutions a t low temperatures.
Phases with ice-eutectic temperatures below - 15°C. I n studies of the ice curve a t low temperatures, forty-three solutions of concentrations ranging from 58.3 to 77.8 per cent sucrose showed fairly sharp breaks at 24.0"C. with a spread of about 0.3"C. (line Y Z of figures 3 and 4). Most of these warming curves also showed breaks corresponding to the ice curve, suggesting that -24°C. is the eutectic temperature for ice and another solid phase. This phase has been designated phase VIII. After a number of breaks had been obtained a t this temperature, the warming curves began to show other breaks as well (figure 4). In many of these warming curves, three or even four consecutive breaks were found, as shown in figures 2c and 2d. I n others, only a single break occurred (figures 2a and 2b). Several
1340
FRANK E. YOUNG AND FRANCIS T. J O N E S
curves showed breaks flattened almost to a characteristic eutectic plateau (figure 2d), but in the others there was merely a change of slope. Breaks a t the same temperatures were occasionally obtained on consecutive warming curves with solutions of the same concentration; many of these breaks were duplicated in later curves. They could not, however, be reproduced a t will. Because of the high viscosity, seeding was very difficult and was used on only a few samples in this region but without any obvious improvement in reproducibility. No solid phase except ice was ever observed in microscopic examinations of portions of these samples a t -34"C., but another phase, if present, may have'gone into solution when some of the excess ice was melted by warming one I
I
I
I
I
1
Q
Y
-25
-
\ I
I
I
60
65
70
I 75
\
.
\ \
I
80
t=€Rc€ivr.WCRO.SE By weigh!) FIG.4. Sucrose-water phase diagram below -15'C. - - -,possible grouping of points. ~~. Number of observations-0, 1; A,-2; 0 , 3; a, 4; A ,5 ; W, 6 . The shaded areas represent ~
0.3"C.
edge of the sample to obtain a field suitable for microscopic observation. When the slide was allowed to cool again during observation, only ice crystals like those shown in figure 5b re-grew. The breaks in this region were probably not due to mechanical peculiarities of the apparatus, since warming curves on mercury, carbon tetrachloride, alcohol, and solutions of several salts whose phase diagrams are well known showed no unexpected breaks in this temperature range; some warming curves of sucrose solutions also failed to show breaks in this region. If any factors which would be influenced by high viscosity (such as stirring rate) were responsible for the peculiar breaks in this region, the apparently well-established isotherm Y Z would not have been obtained, but instead, the breaks would have occurred at progressively higher temperatures as the concentration (and viscosity) increased. The temperature at which the viscosity became low enough to permit rotation
_
SUCROSE HYDRATES
1341
of the stirrer was also found to be unrelated to the temperatures of the breaks obtained. The same results were obtained with both the manual and the recording potentiometers, indicating that the measuring system was not the source of these breaks. Several warming curves obtained with solutions prepared with highpurity sucrose obtained from the Bureau of Standards showed multiple breaks a t some of the same temperatures as the other curves in this region, indicating that these breaks are not due to impurities in the sucrose. It thus appears that these unexplained breaks are due to some changes taking place in the sucrose solutions. Most of the points in this region appear to be grouped about several temperatures (indicated by dashed lines in figure 4). If this grouping is significant, the sucrose-water phase diagram is extremely complex.
FIG.5a. Phase I (sucrose heniipentahydrate) ; photomicrograph (75X) ; note twinning FIG. 5b. Ice; photomicrograph (75X) FIG.5c. Sucrose grown a t -125°C.; photomicrograph (75x1
Phases with ice-euiectic temperalures above - 15°C. After several months of microscopic study a t -3i"C., crystals of a new type (figure 5a) distinctly different from ice (figure 5b) and sucrose (figure 5c) were observed. We shall call this new type of crystal phase I. At first phase I was believed to be the second phase in the eutectic a t -24"C., but other sucrose solutions seeded with these crystals gave breaks on warming curves determining curve H N in figures 3 and 7 (see also table IC).The same warming curves usually showed arrests a t -10.52"C. (line H X ) lasting from 3 to 101 min. (table 2). It should be mentioned that the solubility of phase I at SG.06"C. was determined only from the undersaturated side; equilibrium from the supersaturated side was not reached before anhydrous sucrose appeared as a solid phase. Phase I was shown to be the hemipentahydrate (CleHzzO11.2.5HeO) by the following closely agreeing analyses made on clean, dry, single crystals: carbon (microcombustion), 37.3 per cent (theory, 37.2 per cent) ; moisture (Fischer),
1342
FRANK E. YOUNG AND FRANCIS T. JONES
*0
m
?
e2 h h
X
X b u5 v
1343
SUCROSE HYDRATES
11.W per cent (theory, 11.63 per cent); sucrose (index of refraction of solutions containing weighed amounts of water and hydrate), 88.8 per cent (theory, 88.4 per cent). A single crystal was observed to melt slowly a t +45.7"C. without changing t o anhydrous sucrose. This measurement was made on a special microscope hot stage in which a measuring thermocouple was adjacent to the crystal. Figure 6a shows phase I changing into anhydrous sucrose a t 23°C. Figures Gb and 6c taken 3 days apart show the reverse change taking place a t -12°C. in another sample. Three days after figure Gc was obtained, the anhydrous sucrose TABLE 2 Eutectic temperalures PHASE'
___
. .
1 j
I.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11... . . . . . . . . . 111... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ......................... .............................
1
AVERAGE TEMPERATURE
NUMBER O F OBSERVATIONS
min.
"C.
-10.5 -8.5s
-6.5
VERAGE LENGTK O F ARREST (*O.OB")t
14 48 21 10 4 6 4 42
18 23 19 15 26
9 10
3
* Ice is the second component of these binary eutectic mixtures. I n most cases, for convenience, t h e warming runs were st.arted before the eutectic had crystallized completely; the average length of arrest thus indicates the average amount of eutectics present. 3 All changes of slope; no eutectic arrests.
phase had disappeared. This behavior is in accord with the phase relations shown by the diagram (figure 7). The breaks usually obtained from warming curves of samples containing less than 70 per cent sucrose, seeded with phase I, did not lie on curve H N but determined a separate smooth curve GO (figure 7 and table Id). These same runs showed eutectic breaks (line BB-W) a t -9.55"C., 1" above the ice-phase I eutectic temperature. This behavior thus indicated the existence of another solid phase, which is designated phase 11. A G5 per cent sucrose solution seeded with phase I a t -4°C. first formed typical unsymmetrical crystals of sucrose hemipentahydrate; when these had almost stopped growing, small spherulites resembling cauliflower started t o grow. These spherulites, shown in figure 8a, grew without change in appearance when separated from phase I and trans. ferred to fresh solution. Crystals of this spherulitic phase are small and slender with symmetrical, pointed terminations (figure 8b). Attempts t o grow large crystals of this phase have been unsuccessful. Air-drying a mass of these crystals to constant weight on a porous plate a t -4°C. gave a solid containing 3.5 moles of water per mole of sucrose, as shown by closely agreeing analyses for moisture
1344
FRANK E. YOUNG AND FRANCIS T. J O N E S
(Fischer), 15.3 per cent (theory, 15.5 per cent), and for sucrose (index of refraction, as for phase I), 84.6 per cent (theory, 81.1 per cent). This corresponds to M
+I5
i AMIYOROUS
0
N"
0
0
-5 R PHASE S PHASE H
A
-- ICE ICE
-10
-I 5
45
50
55 60 65 70 COMPOSITION (Percent Sucrase by Weight)
75
80
FIG.7. Sucrose-water phase diagram between +16"C. and -15°C. 0 , warming-curve measurement; a,solubility measurement; 0 , published values for sucrose solubilities; V, melting-point determination; - - -, uncertain portions of curves.
a FIG,Sa. Typical growth of phase I1 (sucrose hemiheptahydrate) ; one-half natural size FIG. Sb. Single crystals of phase 11; photomicrograph (1OOX)
the hemiheptahydrate (C12H&1-3.5 HZO). The melting point of phase 11, approximately +27.8"C., is less certain than that of phase I. Because of the high viscosity of the resulting liquid, melting is best observed by changes in
SUCROSE HYDRATES
1345
birefringence, an observation made difficult by the extremely small size and the aggregation of the crystals. Warming-curve breaks observed on 58 per cent and 60 per cent solutions were far enough above curve GO to cause suspicion. After reproduction of these breaks, some of the crystals were removed to seed other solutions which then gave warming-curve breaks determining curve FP and eutectic line AA-V (figure 7 and tables l e and 2). This phase is designated as phase 111. Crystals which appear to be typical of this phase are shown in figure 9. This phase has not yet been isolated in a pure state; hence its composition and melting point have not been determined. Occasionally during warming runs, other eutectic and primary breaks were observed corresponding to phases IV, V, VI, and VI1 (figure 7). When one of these eutectic breaks was observed, a portion of the sample was removed while
FIG. 9. Phase 111; photoniicrogrnph (75X)
a large part of the eutectic was still present and stored for future investigation in a room maintained a t - 12°C. Several warming runs were made with portions of these solutions for seed, but further investigation was postponed until the solubility curve for phase 111 was more definitely established. Later, when warming curves were run on solutions seeded with these stored samples, only phase I1 eutectics were obtained, suggesting that the other phases had transformed to phase I1 during storage. Only with phase IV have enough primary points been obtained to draw a solubility curve. The points above the solubility curves between 50 per cent and 64 per cent and the point a t +4.250C. and 69.Gbper cent sucrose probably lie on the solubility curves for the remaining phases. Further attempts to obtain crystallization of these phases have been unsuccessful, usually resulting in phase 11. Thus the disproportionately large number of phase I1 eutectics were obtained. Phase I1 has occasionally been found to grow spontaneously and rapidly in solutions seeded with crystals of another phase. If the crystals of the other phase are relatively large, they grow slowly while the invariably minute crystals of phase I1 grow more rapidly. When warmed, the small phase I1 crystals (in
1346
FRANK E. YOUNG AND FRANCIS T. JONES
large number) melt rapidly, causing a sharp break, and the few large crystals of the other phase melt slowly, giving a slight break or none a t all. For this reason, it is possible to seed a solution with crystals of some other phase and obtain phase I1 breaks. Samples removed from such solutions a t the time of the eutectic break, when examined microscopically show the fine crystals of phase I1 disappearing rapidly with the few large crystals of the other phase dissolving slowly. In several runs, however, two or three well-defined eutectic plateaus corresponding to as many phases have been observed on the same warming curve. The simultaneous existence of several such eutectics may be partly explained by the slowness of the transitions caused by the close proximity of the eutectic breaks in both temperature and composition, together with the high viscosity of the solutions. Further work is required to elucidate the puzzling features of this region of the phase diagram. It should be emphasized that the original crystallization of phase I was accidental and that it may require months for i t to reappear spontaneously. This is also true of the other phases.
Optical and crystallographic properties Crystals of phases I, 11, and I11 (figures 5a, 8, and 9) all show weak birefringence and parallel extinction with the slow ray crosswise for their ordinary views. Crystals believed to belong to phases IV, V, VI, and VI1 appear to resemble phase I1 in size, appearance, and optical properties. Differentiation has not yet been possible. Only phase I has yielded single crystals suitable for crystallographic determination. The other crystalline phases appear to be orthorhombic, but sufficiently good interference figures or end views have not been obtained from their crystals to exclude the possibility that they may be monoclinic. Anhydrous sucrose is monoclinic and ice is hexagonal. Crystals of sucrose hemipentahydrate (phase I ) are shown in figure 5a. Figure 10 shows the appearance of cross-sections obtained by cutting the elongated crystals with a scalpel. The cut or fractured surfaces are conchoidal, but almost perfectly centered acute bisectrix views can be obtained by propping the crystal section up in a slot in a thin slide, Correlation of the optical and crystallographic properties requires that this compound belong to the monoclinic crystal system. The fact that it has only one twofold axis of symmetry places it in Class 4 (hemimorphic), which is consistent with the fact that sucrose is optically active. Twinning is characteristic of this substance. One portion of the twin is often smaller than the other. Figure 5a shows ordinary views of both twinned and untwinned crystals. The edge views of twins are uncommon. The twinning habit and optic orientation are shown in figure 10. The extinction angle of 14O is measured between the @-direction and the edge which is usually the longest. The trace of the twinning plane and the crystallographic a-axis have this direction also. The axial planes in the two portions of the twin are symmetrically related to the twinning plane and make an angle of 28" with each other. The twinning plane is also a cleavage plane; cracks parallel to this plane may sometimes be seen in pieces cut for cross-sections. The ordinary views of the crystals have an
SUCROSE HYDRATES
1347
obtuse end angle of 109" which is bisected by the a-index direction which is also the b-crystallographic axis. The crystallographic angle 0 is 58". The refractive indices given in table 3 were determined by immersion methods in sodium light. The crystals were placed in the oil a t +l"C., and then brought to room temperature for examination. It was necessary to work rapidly because
C,,H2,0,;
2L,H,O Untwinned
C12Hof?0I1-2fHf?0 Twinned FIG.10. Cross sections of twinned and untwinned crystals of sucrose hemipentahydrate.
anhydrous sucrose crystals began to grow in the hydrate crystal in most cases. The hydrate would be completely converted in an hour or two. X-ray powder photographs (figure 11) show that phases I and I1 are distinctly different from each other and from anhydrous sucrose. The powder diffraction data are given in table 4.
Occurrence in frozen foods The discovery of sucrose hydrates explains the white "mold- or fondant-like" growth (figure s a ) sometimes appearing in frozen fruit packed in sucrose or
1348
F R A N K E. YOUNG A N D FRANCIS T. JONES
TABLE 3 Optical and cr~stallographic properties of sucrose hemipentahydrate Crystal system: monoclinic ; Class 4 (hemimorphic) Beta angle = 58". Flattened prismatic tablets elongated parallel to b-axis. Obtuse end angle = 109". Refractive indices (5593 CY = 1.5170 i 0.0005 /3 = 1.5255 i 0.0005 y = 1.5275 i 0.0005
.&.; 25°C.):
Optic axial angle: Observed
2E = 82"
Calculated from sin V = Calculated from sin2 V =
sin E ~
a
2V = 51'
d(y2
- P2)
P2(y2
-
2V = 51"
a2)
Optical character: ( - j Dispersion: (r
> v)
faint on both sides. No detectable crossed dispersion.
Optical orientation: CY coincides with b . Ordinary views show CY and y'; the interference figure shows Bro off center; axial plane lengthwise. End views show Bz,centered. Axial plane makes 104" angle with a-axis or twinning plane. Angle of 28" between axial planes of twinned portions.
FIG. 11. X-ray powder photographs of sucrose, phase I (sucrose hemipentahydrate) and phase I1 (sucrose heniiheptahydratej.
sucrose sirup and stored a t or below - 18°C. for several months. For over twenty years these growths have puzzled those working with frozen fruit (13). The appearance of this' material has been considered objectionable and has occasion-
1349
SUCROSE HYDRATES
ally caused the rejection of frozen fruit. Although warming curves on sucrose solutions seeded with sucrose hydrates found growing in several kinds of frozen fruit gave the phase I1 eutectic break, it is quite possible that different hydrates may occur in other samples. TABLE 4 Powder diflraction data SUCROSE EEMIPENTAHYDPATE
SUCROSE HEIIBEPTAHYDRATE
________
d
12.50 7.31 6.82 6.19 5.56 5.10 4.56 4.20 3.96 3.81 3.62 3.43 3.27 3.11 2.91 2.75 2.65 2.53 2.37
Est. I
d
RlW*
19.89 11.88 7.72 7.17 6.57 5.12 4.28 4.03 3.84 3.57 3.40 3.27 3.12 2.92 2.81 2.66 2.57 2.50
S M MS W S M MS MS N Nl W
M MS MW
M WIW W MW
Est. I
W W
S MW M
S M S M MW
vw
M MW W W W W
W
MW
Copper K , radiation I X = 1.542). * S = strong; MS = medium strong; M = medium; W = weak; MW = medium weak; VW = very weak. SUMMARY
The sucrose-water system has been investigated by warming curves, solilbility measurements, and microscopy. Two crystalline hydrates of sucrose have been isolated and identified as sucrose hemipentahydrate (C12Hz2011-2.5 HzO) and sucrose hemiheptahydrate ( ClzHzzOll. 3.5 HzO) Solubility-temperature relationships have been determined for ice, anhydrous sucrose, and for these two hydrates, as well as for two additional solid phases not yet identified. Photomicrographs have been made of three of the phases. Crystallographic and optical properties have been measured for sucrose hemipentahydrate. X-ray powder photographs are shown for sucrose hemipentahydrate and hemiheptahydrate. Indications have been obtained for the existence of several more solid phases.
.
The authors wish to acknowledge the very considerable assistance of Dr. Fred Stitt, who contributed many valuable suggestions and criticisms, Miss Jane
1350
WILLIAM D. HARKINS, ROSE MITTELMANN, AND M. L. CORRIN
Waterman, Mrs. Bernice Platt, Mrs. Martha Atkins, Mrs. Elizabeth Green, and Mr. Harold Lewis, who made most of the thermal measurements, Dr. Kenneth Palmer and Miss Merle Ballantyne, who took the x-ray powder photographs, Mr. Lawrence White and Miss Elizabeth McComb, who made analyses establishing the compositions of the hydrate crystals, and Mr. Jay T. Allison and Mr. Eugene J. Gatze, who did much of the photographic work. REFERENCES BABINSKI,J.: Gae. Cukrownicea 38, Nos. 34-8; Centr. Zuckerind. 32, 782 (1924). COLE,W. C.: J. Agr. Research 66, 137 (1938). FREUNDLICH, H., AND SCHNELL, A , : 2. physik. Chem. 133, 151 (1928). GRUT,E. W.: 2. Zuckerind. Eechoslovak. Rep. 61,356 (1937). GUTHRIE,F.: Phil. Mag. [5] 2,216 (1876). H R U BR., ~ , AND KASJANOV, V.: 2.Zuckerind. Eechoslovak. Rep. 63,187 (1939); Intern. Sugar J. 42, 21 (1940). (7) JONES, H. E., AND GETMAN, F. H.: Z. physik. Chem. 49,385 (1904). (8) KOLTHOFF,I. M.: Verslag. Akad. Wetenschappen Amsterdam 36, 281 (1926). R., AND EITEL,H.: Rec. trav. chim. 42, 539 (1923). (9) KREMANN, (10) LEIGHTON,A.: J. Dairy Sci. 10, 219 (1927). (11) MCBAIN,J. W., AND KISTLER,S. S.: J. Phys. Chem. 33, 1806 (1929); Trans. Faraday SOC.26, 157 (1930). (12) MONDAIN-MONVAL, P.: Compt. rend. 181, 37 (1925). W., AND DIEHL,H. C.: Western Canner and Packer 36 f4), 55 (April, 1944). (13) RABAK, G.: J. Am. Chem. SOC.43, 2406 (1921). (14) SCATCHARD, (15) SHORT,B. E.: The Specific Heat of Foodstuffs.I . Publication No. 4432, Bureau of Engineering Research, University of Texas, Austin, Texas (August 22, 1944). (16) SUGDEN, J. N.: J. Chem. SOC.129,174 (1926). (17) VAND,V.: J. Phys. Colloid Chem. 62, 314 (1948). (18) VERHAAR, G.: Areh. Suikerind. Nederland en Ned.-Indie 1, 324 (1940); Intern. Sugar (1) (2) (3) (4) (5) (6)
J. 43, 50 (1941).
TYPES OF SOLUBILIZATION I N SOLUTIONS OF LONG-CHAIN COLLOIDAL ELECTROLYTES WILLIAM D. HARKINS, ROSE MITTELMANN,
AND
M. L. CORRIN
George Herbert Jones Chemical Laboratory, University of Chicago, Chicago, Illinois Received January 13, 194.9 I . INTRODUCTION : PRINCIPAL TYPES OF SOLUBILIZATION
Solubilization is a term used to indicate the process by means of which substances which are not very soluble in a given solvent dissolve in solutions of long-chain electrolytes to an extent appreciably greater than in the solvent itself, The solubility is the amount dissolved at equilibrium with the solute. 1 This investigation was carried out under the sponsorship of the Reconstruction Finance Corporation, Office of Rubber Reserve, in connection with the synthetic rubber program of the United States Government.
