RELATIONSHIP

BETWEEN

AND STRUCTURE

TI4E F R E E Z I N G

OF A Q U E O U S

POINTS

SOLUTIONS

I. THE CRYOSCOPIC STUDY OF AQUEOUS SOLUTIONS OF NONELECTROLYTES:

HEXAMETHYLENETETRAMINE-WATER,

HEXA METHYLENETETRA MIN E - BUTANOL - WATER, AND DIMETHYLFORMAMIDE-WATER A . A.

Ennan

and

V. A.

Lapshin

UDC 532.77

The aqueous nonelectrolyte systems CsHlzN 4 - HaG, DMFA - HzO, and C 6HlzN4 - n - C 4 HgOH- HzO have been studied by the cryoscopic method over a wide range of concentrations. The diagrams obtained have been interpreted on the basis of the assumption that there is a direct relationship between the crystallization temperatures and the structural changes in the aqueous solutions. The characteristic concentration ranges in the systems studied, and the boundaries between them, have been determined. Analysis of the experimental data, carried out on the basis of modern ideas regarding the structure of water and aqueous solutions of nonelectrolytes, has made it possible to detect and explain the specific influence of the structural groups of water on the nature of the change in the freezing point with i n crease in the concentration of the solutions.

Intro duc tio n In recent years, cryoscopy, as a method of physicochemical anaiysis, has occupied an important place in the study of processes taking place directly in solutions. Cryoscopy is particularly widely used to establish complex formation in nonaqueous systems~ Up to the present, however, the problem of the relationship between the crystallizaffon temperatures of solutions and the structure of the solvent has not been resolved in the literature. At the same ffme it is evident that the temperature at which a pure solvent starts to solidify is directly related to its structure in the liquid phase and depends not only on the nature of the c h e m i c a l interaction between the dissolved components. but also to a significant extent on the total concentration of the solution. Increase or decrease in concentration leads to a change in the structure of the solvent and the nature of its interaction with the dissolved components, and this in m m should be reflected in the freezing points of the solutions. There is a much smaller number of papers on the cryoscopy of a~ueous solutions, and the interpretation of these papers, which does not take account of the characteristic features of the structure of the solvent water, is not rigorous. It is known that water in solutions plays the role of the medium in which the interactions between the components develop, and also takes part in these interactions, the nature of the course of many processes in water and aqueous solutions being determined by the characteristic features of the solvent water. For example, the processes of hydration, dissolution, salting-in and salting-out, change in the dielectric constant, and diffusion have been explained only on the basis of modern theories of the structure of water as solvent [1-5]. An important feature is that the specific characteristics of many properties of aqueous solutions are preserved so long as the water structure exists. In this connection it was logical to assume that the existence or complete disappearance of the structural formations of water in solutions should be reflected in the nature of the change in the crystalIization temperatures of the solutions. The reliability of this assumption becomes more real if we take account o f t h e f a c t that the restoration of the structure

I. I. Mechnikov Odessa State University. Translated from Zhumal Struktumoi Khimii, Vol. 13, No. 13, pp. 596-601, July-August, 1972. Original article submitted September 1, 1970, 9 Consultants Bureau, a division o f Plenum P u b l i s h i n g Corporation, 227 West ]7th Street, New York, N. Y. lOOl]. A l l rights reserved. This article cannot be reproduced for any purpose w h a t s o e v e r without permission o f the publisher. A copy o f this article is available from the p u b l i s h e r for $15.00.

557

of water in water and aqueous solutions precedes the nucleation o f the crystallization centers [6, 7], and if we regard the structural groups of free water in solutions as centers bringing about a l o c a l decrease in the work of formation of nuclei of the i c e structure. T h e present study was undertaken in order to investigate the influence of the state of the water structure in solution on the form o f t h e c o m p o s i t i o n - f r e e z i n g point diagrams. The possibility o f examining this problem arose in connection with the publication o f extensive theoretical and e x p e r i m e n t a l m a t e r i a l , which makes it possible to predict with sufficient accuracy the structural changes taking place when molecules of nonelectrolytes with different polarity, shape, and dimensions are introduced into water. From our point o f view, the influence of the structure of water in solutions on their crystallization is of considerable importance for the correct interpretation of the results of cryoscopic studies of aqueous solutions. Moreover, cryoscopy, together with other methods, m a y prove to be sufficiently effective to give direct information on the strueturaI changes in aqueous solutions. EX P E R I M E N TA L We chose for study the h e x a m e t h y l e n e t e t r a m i n e - w a t e t , hexam e t h y l e n e t e t r a m i n e - w a t e r - b u t a n o l , and dim e t h y K o r m a m i d e - w a t e r systems. " A n a l y t i c a l l y pure" grade butanol, "pharmaceutical" grade h e x a m e t h y l e n e t e t r a m i n e , "pure" grade dimethylformamide, and t w i c e - d i s t i l l e d water were used for the preparation of the solutions. The experiments were carried out in a cryostat, the design of which m a d e it possible to prevent supercooling of the substance being studied and to record visually the formation of the first crystals. To e l i m i n a t e the influence of the temperature factor on the crystallization, the cooling was carried out from the same temperature of 20 ~ for all the systems, after preliminary cryst a l l i z a t i o n and melting. A mixture of dry ice and heptane was used as cooling agent. T h e freezing point was determined with totally immersed mercury and alcohol thermometers with scale division 0.1 ~ The measurements were corrected for the e m e r g e n t mercury column. The results were c a l c u l a t e d as the arithmetic m e a n from 5-6 experiments. The results obtained are g i v e n i n Figs. 1-3. DISCUSSION

OF R E S U L T S

The diagrams obtained indicate that the freezing point changes according to a fairly complex law. At the same t i m e it can readily be seen that the curves are distinctly analogous. In actual fact, with increase in the concentration of the solutions, the linear dependence of the freezing point on composition is r e p l a c e d by a nonlinear dependence on all the diagrams. The observed curvatureof the ice crystallization liquidus lines cannot be attributed to a sharp increase in the number of o s m o t i c a l l y a c t i v e particles in the concentrated solutions of the systems studied. This characteristic feature of the diagrams can be explained if we assume that there is a relationship between the freezing point and the structure of the aqueous solutions [8] and take account of the nature of the change in the structure of water with change in the concentration and temperature of the solutions [9]. It has been shown [10] that the addition to water of nonelectrolytes which, because of their size and shape, do not enter the cavities of the i c e - l i k e framework of water, leads to the production of disorder in the structure. The molecules of h e x a m e t h y l e n e t e t r a m i n e , DMFA, and butanol do not enter the cavities of the water framework [11, 12, t3], and consequently bring about deformation of its bonds. With increase in concentration ir~ the solutions studied there should be a qualitative change in the structure of the soivent towards an increase in disorder. Moreover, in the systems we can expect the development of concentration fluctuations as a consequence o f the marked differences in the dimensions of the molecules o f the components and the difference in the forces of intermolecular interaction b e tween them [14]. Consequently, within a definite range of concentration at ordinary temperatures, these systems contain microregions consisting of the structural formations of "free" water. It should be noted that decrease in the temperature of the solutions f a c i l i t a t e s both the development of concentration fluctuations and the restoration of the structure of water [6, 14]. In concentrated solutions, for which we need not discuss the structure of the water present, the restoration of the structure with decrease in temperature can only be attributed to the development o f concentration fluctuations in the solutions [6]. The d e v e b p m e n t o f concentration fluctuations should probably be regarded as a phenomenon preceding not only the pro duction of a new liquid phase in the solution [15], but also the formation of a solid phase. By taking into account the qualitative changes taking place in the solutions studied with change in their coneentration and temperature, and analyzing the diagrams obtained, we reached the conclusion that the sloping linear

558

f,~

4#

-~tO

fO

30

#0

Cd,~N~,wt.% Fig. 1. Dependence of the f r e e z ing points of the G6H12N4-HzO system on the h e x a m e t h y l e n e t e t r a m i n e cone en tra rio n.

#

g

g/

:

C~HgOH,w t J o Fig. 2. Dependence o f the freezing points of the C6H12Na-n-C4HgOH-H20 system containing 30 wt. % h e x a m e t h ylenetetramine on the butanol c o n c e n 9 tration.

sections ab (Figs. 1-3) correspond to the concentrations of solutions in which the concentration fluctuations have developed to a considerable extent. In other words, in the range of concentrations corresponding to the section ab, the sn'uctural groups of "free" water, the molecules of which do not enter the i m m e d i a t e environment of the molecules o f the -Ddissolved components, and in actual fact remain under the influence of all the components o f the solution and are c h a r a c t e r i z e d by a structure which is to some extent close to the structure of the pure solvenh are ZZ Z: ::/ :/: #6 :Z preserved. These structural groups can apparently be regarded as preDMFA, vo i.~ pared centers in t h e solution, which decrease the work of formation of Fig. 3. Dependence of the freezing nuclei o f the ice structure. The observed decrease in the freezing point points of the D M F A - H 2 0 system on in this range can be attributed to continuous breakdown of the structures the DMFA concentration. of "free r water with increase in the concentration o f the solutions. This phenomenon is detected by direct x - r a y structural measurements and also by infrared spectroscopy [6, 7]. From the two-structural m o d e l o f water, it is also obvious that the freezing point o f solutions in which the water structure is preserved is determined by the degree of disorder of the most ordered groups of this structure, since these are closest to the structure of ice. This apparently also determines the observed linear character of the decrease in the freezing point in the sections ab, In a c t u a l fact, the mechanism o f the l i n e a r e h a n g e in the freezing point can be explained if it is assumed that with increase in concentration in the solutions, structural rearrangements also take place, influencing chiefly the number, size, and degree of order of the least orderedgmups of the water structure. This assumption can be made, since it is known [16, 17] that when different nonelectrolyte molecules are introduced into water, they arrange themselves in such a way that they bring about the least breakdown in its s t r u c t u r e - i n the cavities of the water framework or in those places where the w a t e r - w a t e r bonds are a l ready weakened to a considerable extent, that is in the regions with the lowest degree of order. As a result, the less ordered microregions in the water structure, which play the part o f compensators, with increase in the concentration of the solutions to some extent help to keep constant the change in the degree of order of the most highly ordered water structures, and this is revealed in the diagrams in the form o f the linear sections ab. The sections bc in the diagrams correspond to the concentrations o f solutions in which the structural groups of "free" water have c o m p l e t e l y disappeared as a consequence o f the c o m p l e t e distribution of the water molecules between the molecules of the nonelectrolytes. With increase in the concentration in this range of compositions, there also takes place a redistribution of the water molecules in accordance with the hydrophillc nature of the dissolved molecules, and this facilitates the formation of structures which, as far as the lowest temperatures, prevent the production of the structures of "free" water in the solutions and hence the production of the solid i c e phase. This is also reflected in the diagrams in the continuous increase in the depression in the freezing point in the section bc.

~,%

It should be noted that in the C6H12N4-H20 (I) and C6H12N4-n-C4HgOH-H20 (II) systems, at concentrations of the solutions corresponding to the sections bc, structures close to the water structure may be preserved; this m a y be due ID the characteristic features of the structure of the h e x a m e t h y l e n e t e t r a m i n e molecule, which contains four hydrophilic groups [18]. We probably cannot, however, speak of the structure of the "free" solvent in h e x a m e t h y i e n e tetramine crystal hydrate. S i m i l a r l y we cannot speak o f the structure of the "free" solvent with reference to c o n c e n trated solutions, in which the water structure is determined c o m p l e t e l y by the molecules of the solute. It is known

.589

[16, 19] that the structure-formation in water around solute molecules always differs from the structure-formation in pure water in the bulk of the liquid. This antagonism between these structures is also manifested to the greatest extent in concentrated solutions with decrease in temperature and is r e f l e c t e d in the freezing points. The point b, corresponding to the freezing point of a solution with definite composition, for which increase in concentration causes the "free" water structure in the solutions to disappear c o m p l e t e l y , is characteristic of the diagrams obtained. In noting this point and defining the range of concentrations in which the structure of the "free ~ solvent is preserved, i t is appropriate for the study of aqueous systems, in our opinion, to introduce the concept of the boundary of c o m p l e t e distribution (BCD) of the water molecules between the components of the solution. Analysis of the fusion diagrams of aqueous solutions o f electrolytes [20-22] led us to the conclusion that the change in the freezing points as far as the eutectic point is analogous. Many diagrams c l e a r l y show the curvature of the crystallization liquidus line. This curvature also cannot be explained from the viewpoint of the action of osmotic particles in the solution, since in this case we would arrive at the absurd conclusion that the degree of dissociation of electrolytes increases with increase in concentration. A noteworthy feature is that for systems exhibiting the greatest ability to c o m b i n e with water, the l i n e a r sections of the diagrams are preserved as far as much lower molar concentrations. I t can also be seen from the diagrams (Figs. 1 and 3) that the linear section for the D M F A HzO system (III) is preserved as far as a concentration of ,-. 4.5 M, whereas for system I it is preserved as far as about 2.4 M. If we take account of the fact that h e x a m e t h y l e n e t e t r a m i n e is more hydrophilic than dimethylformamtde, fiats provides further e v i d e n c e that the l i n e a d t y of the decrease in the freezing points of the aqueous solutions corresponds to the concentrations at which the structure of "free" water exists in the solutions. In the diagrams (Figs. i and 2) it is possible to distinguish another characteristic point c, corresponding to the eutectic compositions of the solutions. In our opinion, in solutions with the e u t e e t i c compositions there are formed those c o m b i n e d n o n e l e c t r o l y t e water structures which to the greatest extent (to the lowest temperatures), compared with the structures of other solutions o f these systems, prevent the development o f microheterogeneity in the solutions and hence the production o f the structures preceding the nucleation of crystallization centers. The course of the diagrams in sections cd can be explained if account is taken of the fact that in the case of system I, h e x a m e t h y l e n e t e t r a m i n e hydrate crystallizes, whereas in the case o f system II, a second liquid p h a s e - b u tanol - appears at Iow temperatures. Starting from the eutectic corn position, further decrease in the concentration of water in the solutions of systems I and II apparently produces a deficit of water. There is then not enough water for its distribution between the dissolved components according to their hydrophilic character. The competition for water between the components of the solution leads to the formation of structures which compensate this water deficit. The new structural formations f a c i l i t a t e the new development of concentration fluctuations in the solutions at increasing temperatures as their concentrations move further from the e u t e e t i e composition. This in turn leads to the crystallization, at higher temperatures, of the solution s of system I. For the same reason, when a definite low temperature is r e a c h e d in solutions of system II containing more than 5.6% butanol, a butanol phase appears. The quantity of butanol remaining in the solution is the quantity required for the formation of the most stable structures. Another consequence is that the freezing points o f solutions containing more than 5.6 wt. % butanol are constant. It should be noted that the concentration fluetuations of the solutions corresponding to the sections ae show significant qualitative "differences from the concentration fluctuations of the solutions corresponding to the sections cd, since in the first case they are due to some extent to an excess of water, whereas in the second case they are due to a deficit o f water. This is also one of the reasons why for system I the solid phase on the curve ac is ice, and that on the curve ed is h e x a m e t h y l e n e t e t r a m i n e b y & a t e . In distinguishing the narrow range of compositions corresponding to the section be, in which increase in the concentration of the solutions leads to the formation of structures which to an increasing extent prevent the deveiopm e n t of microheterogeneity in the solutions, i t is apparently necessary in this case to regard the euteetic point c, bounding this range of optimum distribution of the molecules of the components of the solutions, as the boundary of optimum distribution (BOD). Thus to summarize the analysis of the diagrams, i t may be concluded that the structural groups of water i n t h e solutions have a significant influence on their freezing points. The complex character of the diagrams is difficult to understand if we do not m a k e use of the i d e a that it is possible, under certain conditions, for concentration f l u c t u a tions to develop in aqueous solutions of nonelectrolytes. It is from the nature of the change in the freezing points in the range of concentrations studied, on the basis of these considerations, that it has been possible to distinguish at

560

least three characteristic concentration ranges for the nonelectrolyte solutions studied, to determine the boundary between them, and to explain the changes in the freezing points. The existence of different concentration ranges, their extents, and also the laws governing the change in the freezing points of the solutions studied are apparently determined by the nature of the reaction of the nonelectrolyte with water, its size, and its shape, and also by the magnitude of the forces of the interaction between the noneiectrolyte molecules themselves. LITERATURE CITED 1. 2.

11. 12. 13. 14.

O. Ya. Samoilov, Zh. Fiz. Khim., 20, 1411 (1946). O. Ya. Samoilov, The Structure of Aqueous Solutions of Electrolytes and the Hydration of Ions [in Russian], Izd-vo AN SSSR, Moscow (1957). H.S. Frank and A. E. Quist, I. Chem. Phys., 34, 603 (1961). M.D. Danford and H. A. Levy, L Am. Chem. Soc., 34, 3965 (1962). Yu. V. Gurikov, Zh. Stmkt. Khim., 6, 817 (1965); 7, 8 (1966). V.I. Danilov, The Structure and Crystallization of a Liquid [in Russian], Izd-vo AN USSR, Kiev (1956), p. 108. B.A. Mikhailov and V. M. Zolotarev, in: The Structure and Role of Water in the Living Organism [in Russian], No. 2, Izd-vo LGU (1968), p. 43. V.I. Danilov and V. g. Neimark, Zh. ~ksper. Teor. Fiz., 7, 1168 (1937). L D. Bernal and R. H. Fowler, I. Chem. Phys., I._, 515 (1933). V. N Vdovenko, Yu. V Gurikov, and E. K. Legin, in: The Structure and Role of Water in the Living Organism [in Russian], No. 1, Izd-vo LGU (1966), p. 3. M.N. Buslaeva and O. Ya. Samoilov, Zh. Strukt. Khim., 4, 502 (1963). A. Frattello, lvlol Phys., 8, 6 (1964). Short Chemical Encyclopedia [in Russian], Vol. 1, Sovet-skaya Entsiklopediya, Moscow (1961), p. 814. O.A. Roshchina, in: Critical Phenomena and Fluctuations in Solutions [in Russian], Izd-vo AN SSSR, Moscow

15. 16. 17. 18. 19. 20. 21. 22.

(1960), p. 109. I.L. Krupatkin, Zh. Neorgan. Khim., 1__,1210 (1956). G.G. Malenkov, Zh. Stmkt. Khim., 7, 322 (1966). Yu. I. Nabemkhin and S. I. Shuiskii, Zh. Strukt. Khim., 8, 606 (1967). D.W. Davidson, Can. L Chem., 46__, 1024 (1968). B.V. Zheleznyi, Zh. Strukt. Khim., II___,595 (1970). G.H. Cady and I. H. Hildebrand, L Am. Chem. Soc., 52___,3843 (1930). A. Seidell, Solubilities,3rd Edition, London (1940). Gmelins Handbuch der Anorg. Chem., 8 Aufg., 1927-1940.

3. 4. 5. 6. 7. 8. 9. 10.

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