Die Makromolekulare Chemie 110 (1967) 144-149 (Nr. 2558)

From the Institute of Physical Chemistry, University of Uppsala, Uppsala, Sweden

Phase Equilibria in Dextran-Barium Hydroxide Solutions

By HANS VINK

(Eingegangen am 29. Mai 1967)

SUMMARY: In the system dextran-barium hydroxide-water phase separations occur, in which two

liquid phases with different dextran and barium hydroxide content are in equilibrium with each other. For the system a phase diagram was determined, and the dependence of the phase equilibria on temperature and the molecular weight of dextran was also studied. From the phase equilibrium studies it was deduced that phase separation occurs when the charge density in dextran molecules increases over a definite level.

ZUSAMMENFASSUNG: Im System Dextran-Bariumhydroxid-Wasser konnen Phasenseparationen auftreten,

wobei sich zwei fliissige Phasen mit verschiedenen Gehalten von Dextran und Barium- hydroxid miteinander im Gleichgewicht befinden. Fiir dieses System wurde ein Phasen- diagramm bestimmt, und auI3erdem wurde die Abhangigkeit der Phasengleichgewichte von Temperatur und Molekulargewicht des Dextrans untersucht. Es ergab sich, daI3 fiir das Auftreten der Phasenseparationen die Ladungsdichte der Dextranmolekeln eine bestimmte Grenze iiberschreiten muI3.

Introduction

Polysaccharides, being polyalcohols, possess a weak acidic character and hence in strongly alkaline media they are to some extent ionized and behave as polyelectrolytes. Under these conditions they also share the latter’s tendency for strong binding of multivalent counterions, which may result in the precipitation of the polyelectrolyte-counterion com- plex1,2). An example of this behaviour is found in dextran-barium hy- droxide solutions. In this separation occur, in which barium hydroxide content

Materials Dextran

case rather interesting phenomena of phase two liquid phases with different dextran and are in equilibrium with each other.

Experimental

Three commercial fractionated samples (from Pharmacia, Uppsala, Sweden) were used. They were designated A, B, and C and had the following characteristics.

Sample A: [?]HzO = 0.21, DP, = 260 SampleB: [q]H20 = 0.43, DP, = 1460 SampleC: [?]HzO = 0.70, DP, w 12000

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Phase Equilibria in Dextran-Barium Hq droxide Solutions

Barium hydroxide

A BAKER’S analyzed reagent in the form of Ba(OH),.8H20 was used. From this stock solutions with different barium hydroxide concentrations were prepared, using double distd. water. The stock solutions were preserved in tightly stoppered polyethylene flasks.

Experimental procedure The solubility of dextran in barium hydroxide solutions was studied by determining the

precipitation point for a thermostated solution by gradually changing its composition. This was done by titrating with a barium hydroxide solution until a permanent turbidity appe- ared. The precipitation point was found to be very sharp, which made an accurate de- termination of the solubility curves possible.

I n the quantitative study of phase equilibria the dextran-barium hydroxide solutions were prepared by weighing the components (dextran and the stock solution of barium hydroxide) into a cylindrical flask. The flask was thermostated in a water-thermostate. It was occasionally vigorously shaken to let the phases come to equilibrium, after which they were allowed to settle. The relative volumes of the phases were determined by meas- uring the position of the interphase boundary with a cathetometer. The supernatant liquid was then analyzed for its composition. I ts barium hydroxide concn. was determined by titration with hydrochloric acid. The dextran concn. was obtained from the previously determined solubility curves and the water content was obtained by difference. Finally, the density of the supernatant phase was determined and the compositions on the weight basis of the two phases were calculated.

Results and Discussion

In the determination of the solubility curves it was found that a t a fixed temperature dextran is soluble in barium hydroxide solutions up to a certain limiting concentration of barium hydroxide. Beyond that limit a single phase is obtained only if the dextran concentration exceeds a certain minimum concentration, below which phase separation occurs. The solubility curve, which separates the single and two-phase regions has the form shown in Figs. 1 and 2. It was found in the experiments that precipitation occurred in solutions with dextran concentrations as low as c = 0.01 g/l. This indicates that for all practical purposes the curve may be considered to lack a lower branch (i.e. a t low dextran concen- trations the single phase region does not extend beyond the limiting barium hydroxide concentration; however, this is true only for homo- geneous dextran samples).

The solubility of dextran in barium hydroxide solutions is markedly influenced by temperature and the molecular weight of dextran. The temperature dependence is shown in Fig. 1 and we find that with increas- ing temperature the solubility curve is displaced to higher barium hy-

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H. VINK

droxide concentrations, its form remaining almost unchanged. The in- fluence of the molecular weight of the dextran sample is shown in Fig. 2. With increasing molecular weight the solubility curve is displaced to lower barium hydroxide concentrations. At high dextran concentrations

Fig. 1. Solubility curves for the dextran sample A at the temperatures 15, 25 and 35 "C. c = concentration of dextran, m =

concentration of Ba(OH),

Fig. 2. Solubility curves for the dextran samples A, B and C at 25 "C. c = concen- tration of dextran, m = concentration of

Ba(OH),

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Phase Equilibria in Dextran-Barium Hydroxide Solutions

the solubility curves for different molecular weight samples seem to con- verge, which indicates a decreasing selectivity in the molecular weight dependence a t high dextran concentrations. This is a t least in part due to the inhomogeneity of the dextran samples, as the solubility of a molec- ular species is affected by the presence of other molecules, the inter- ference being more pronounced a t high concentrations. It is obvious that phase separation in dextran-barium hydroxide solutions may be used for the fractionation of dextran with respect t o molecular weight, and in dilute solutions and for low molecular weights the selectivity appears to be good.

The solubility curves in Figs. 1 and 2 cover the behaviour in dextran- barium hydroxide solutions at relatively low concentrations of these com- ponents. For a more complete description of the system it is favourable to use triangular diagrams. Such a diagram for the dextran sample A is shown in Fig. 3, the basic data being listed in Table 1. In the diagram

Fig. 3. Phase diagram for dextran sample A at 25°C. The critical

point is marked by a circle

1

H20 10 20 30 40 50 -D '1. Dextran

the positions of some tie lines and the critical point have been indicated. The triangular region situated above the two-phase area is a three-phase region where a saturated barium hydroxide solution is in equilibrium with Ba(OH),.8 H,O and a concentrated dextran-barium hydroxide so- lution. The diagram is incomplete in the regions in the vicinity of pure barium hydroxide and pure dextran, where the system is ill-defined and difficult to study.

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H. VINK

Table 1. Phase equilibrium data for sample A at 25 "C

(%) 94.2 97.3 96.8 96.6 95.8 95.6 82.1

I Supernatant phase

(%) 24.7 33.9 38.7 38.7 38.9 39.1

4.2 4.4

13.6 4.3

Precipitate

Ba(OW2

(Yo) 6.6 9.9

12.6 13.4 15.3 16.2

critical point

HZO

(%) 68.7 56.2 48.7 47.9 45.8 44.7

The form of the solubility curves and the phase diagram may be under- stood in terms of the mass action law for the binding of barium hydroxide by dextran:

where m is the molar concentration of barium hydroxide, c is the concen- tration of dextran in grams per liter, p is the degree of binding and M is the equivalent weight of dextran (assuming that each glucose unit in dextran can bind at most one barium ion, we have M = 162). With the help of Eq. (l), using the value k = 1.5 for the equilibrium constant3), p-values for a number of points on the solubility curves in Fig. 2 were calculated. The results are listed in Table 2. We find that along a solu-

Table 2. Degree of binding of Ba2+-ions on the solubility curves

Dextran concn. c (SP.)

0 15 40 80

Sample A

0.159 0.137 0.155 0.134 0.155 0.144

Sample C

0.133 0.129 0.130

bility curve the degree of binding @ is nearly constant, the value decreas- ing with increasing molecular weight of the dextran sample. Thus, it may be concluded that precipitation of dextran in barium hydroxide solutions occurs when the degree of binding (or, in other terms, the charge density in a dextran molecule) exceeds a certain limit. Then virtually all dextran is precipitated from the solution, the precipitate having the form of a concentrated dextran-barium hydroxide solution. It may be assumed

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Phase Equilibria in Dextran-Barium Hydroxide Solutions

that in the precipitate dextran molecules are associated through electro- static attraction between the bivalent Ba2+-ions and ionized hydroxyl groups belonging to different dextran molecules.

Finally, in order to compare dextran with other polysaccharides and oligosaccharides some tentative experiments were carried out with barium hydroxide solutions of glucose, sucrose, raffinose, inulin and amylopektin, all at 25 "C. With the oligosaccharides no phase separations occurred even with saturated barium hydroxide solutions, whereas with inulin and amylopektin solid precipitates were obtained, the precipitation occurring at much lower barium hydroxide concentrations than with dextran. NO tendency for the separation of two liquid phases was observed.

l) €I. MORAWETZ, Fortschr. Hochpolymeren-Forsch. 1 (1958) 1. 2, J. A. RENDLEMAN, Advances Carbohydrate Chem. 21 (1966) 209. 3, H. VINK, Makromolekulare Chem. 76 (1964) 66.

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