Die Makromolekulare Chemie 109 (1967) 249-252 (Nr. 2536)

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

Kurzmitteilung

Refractive Index Increments for Polymers in Solutions in Multicomponent Solvents

By HANS VINK and GUNILLA DAHLSTROM

(Eingegangen am 24. April 1967)

The use of multicomponent solvents in physico-chemical measurements on polymers renders necessary some caution in the interpretation of the experimental data. Especially, in molecular weight determination by light scattering, it is necessary t o use a proper value for the refractive index increment. Such a value may be obtained by applying a correction to the conventional refractive index increment, or more directly from differential refractive index measurements between a solution and sol- vent brought to dialysis equilibrium. Although the theoretical back- ground of this procedure has been known for some time1-I2) there still seems to be some confusion about its application in actual experiments. In the present work refractive index increments were measured for some polysaccharides in multicomponent aqueous solutions. The measure- ments were carried out both with dialyzed and undialyzed solutions.

Experimental

The refractive index measurements were carried out with a differential prism refracto- meter, which was built a t this institute and has been described elsewhere13). The prism cell was calibrated with aqueous solutions of Na,SO,, using the refractive index values of K R U I S ~ ~ ) . The dialysis experiments were carried out in a cell described in15), using com- mercial cellophane membranes. All refractive index measurements and dialysis experi- ments were carried out a t 25°C.

Materials The following substances were used in the measurements. Cellulose. The sample was obtained from high D P (m5500) cotton linters, which were

subjected to a mild chlorite bleach and subsequently degraded by acid hydrolysis to about DP = 1100.

Dextran. This was a commercial fractionated sample (from Pharmacia, Uppsala, Sweden) with the following characteristics: [Yj]HzO = 0.43 dl/g, DP, = 1460.

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H. VINK and G. DAHLSTROY

Hydroxyethyl celhlose (HEC). The sample was purified by precipitation from an aqueous solution with acetone and had the following characteristics: DS = 0.88, MS = 1.6716), DP, M 2200.

Sodium carbozymethyl cellulose (NaCMC). The sample was purified by precipitation from an aqueous solution with acetone and had the following characteristics: DS = 0.96, DP, w 4400.

The stock solution of the cellulose solvent cuoxen (CED) was prepared by dissolving commercial cupric hydroxide in an aqueous solution of freshly distd. ethylene diamine (en). It had the composition ccu = 0.259 M , cen = 0.490 M . The solutions used in the measure- ments were obtained from the stock solution by dilution with water.

The cellulose solvent cadoxen was prepared by saturating an aqueous solution of freshly distd. en with commercial cadmium oxide a t 0°C. To the stock solution no sodium hydro- xide was added. It had the composition CCd = 5.1 yo and cen = 30 Yo. The h a 1 solutions were diluted 1:l with water.

The results of the measurements, together with some data from the literature, are listed in Table 1.

Table 1. Refractive index increments in various solvents

Polymer c (g P.)

Dextran 4.927

10.301 8.930

11.896 5.168 4.953 9.826

11.037 2.227 7.415 4.712 2.025

Cellulose 3.288 1.703 2.014

HEC 5.044 2.010

NaCMC 6.856

Solvent composition

H.20 NaClO.500 M NaCl 1.00 M NaCl 2.00 M NaOH 0.250 M NaOH 0.750 M Ba(OH), 0.0503 M Ba(OH), 0.1080 M cadoxen 0.237 M Cd cadoxen 0.237 M Cd CED 0.0518 M Cu CED 0.0776 M Cu

cadoxen 0.237 M Cd CED 0.0518 M Cu CED 0.0776 M Cu C~oxam'~) 0.205 M Cu FeTNal*)

cadoxen 0.237 M Cd CED 0.0518 M Cu

cadoxen 0.237 M Cd

undialyzed solutions A=436mp

0.1504 0.1476 0.1447 0.1361 0.1456 0.1437 0.1465 0.1415

0.1235 0.1323

0.1317 0.1352

0.117

0.1269 0.1314

0.1398

A=546my

0.1481 0.1454 0.1424 0.1342 0.1433 0.1415 0.1441 0.1394

0.1219

0.1295

0.110

0.1247

0.1373

dialyzed solutions A=436 mp

0.1494 0.1470 0.1435 0.1374 0.1584 0.1646 0.1642 0.1738 0.1907 0.1842 0.2525 0.2625

0.1927 0.2574 0.2653 0.233

0.1504 0.1631

0.1896

A=546mp

0.1472 0.1445 0.1413 0.1351 0.1558 0.1621 0.1618 0.1708 0.1875 0.1809

0.1890

0.245

0.1478

0.1861

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Refractive Index Increments for Polymers in Multicomponent Solvents

Discussion

From the results in Table 1 we find that in many cases there is a con- siderable difference between the dn/dc-values measured with dialyzed and undialyzed solutions. The difference is due to the unequal distribu- tion of permeable solutes in the membrane equilibrium, which arises from the binding of the permeable solutes by the polymer and from the DONNAN and excluded volume15) effects. The difference is most pro- nounced when appreciable solute binding occurs and the permeable solute has a high refractive index increment.

In measurements with undialyzed solutions the refractive index incre- ment in general decreases with increasing permeable solute concentration. This is due to the increasing refractive index of the solvent and is easily understood from the GLADSTONE-DALE law.

I n the case when no binding of permeable solute is expected, practically the same results are obtained from measurements with dialyzed and undialyzed solutions. This is the case with dextran in NaC1-solutions.

With HEC a relatively small difference between the dn/dc-values for dialyzed and undialyzed solutions is observed. This shows, as expected, that ion binding is hindered by substitution of the hydroxyl groups, although the DS level of the sample is not high enough to preclude the binding completely.

In the case of NaCMC in cadoxen the difference between the dn/dc- values for dialyzed and undialyzed solutions is due to the exchange of Na+-ions during dialysis against Cd-en: ‘-ions, the latter having a much higher refractive index increment. The higher dn/dc-values in this in- vestigation as compared to those in ref.l1) are due to differences in the cadoxen solvents used. I n the present investigation no sodium hydroxide was added to the solvent, which, however, was the case in ref.11).

Considering finally the refractive index increments for cellulose and dextran in various alkaline solvents it is interesting to observe that in CED (as well as in cuoxam and FeTNa) the difference between dn/dc- values for dialyzed and undialyzed solutions is very large, whereas in the case of cadoxen a much smaller difference is observed, which is close t o the values found in pure alkalies. This gives additional support to the view19) that in cadoxen complex formation is slight and that cadmium is bound principally by an acid-base reaction. The proposition of BROWN^^), in which the supposedly high dn/dc-value of cellulose in cadoxen is consider- ed to prove the existence of covalent cellulose-cadmium complexes, can not be sustained.

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H. VINK and G. DAHLSTROM

1) R, H. EWART, C. P. ROE, P. DEBYE, and J. R. MCCARTNEY, J. chem. Physics 14 (1946)

2) H. C. BRINKMAN and J. J. HERMANS, J. chem. Physics 17 (1949) 574. a) J. G. KIRKWOOD and R. J. GOLDBERG, J. chem. Physics 18 (1950) 54. 4, W. H. STOCKMAYER, J. chem. Physics 18 (1950) 58. 5, H. SHOGENJI, Busseiron Kenkyu 62 (1953) 1. 6, T. 001, J. Polymer Sci. 28 (1958) 459. 7, T. A. OROFINO and P. J. FLORY, J. physic. Chem. 63 (1959) 283. 8) B. E. READ, Trans. Faraday SOC. 56 (1960) 382. 9, C. STRAZIELLE and H. BENOIT, J. Chim. physique 58 (1961) 675.

687.

lo) A. VRIJ, These Universitk, Utrecht (1959). 11) W. BROWN, D. HENLEY, and J. OHMAN, Ark. Kemi 22 (1964) 189. 12) H. YAMAKAWA, J. chem Physics 46 (1967) 973. 13) P. H. NORBERG and L.-0. SUNDELOF, Makromolekulare Chem. 77 (1964) 77. la) A. KRUIS, Z. physik. Chem. B 34 (1936) 13. 15) H. VINK, Acta chern. scand. 17 (1963) 2524. 16) W. BROWN, Ark. Kemi 18 (1961) 227.

1 8 ) L. VALTASAARI, Tappi 48 (1965) 627. Is) H. VINK, Makromolekulare Chem. 76 (1964) 66. ao) W. J. BROWN, Tappi 49 (1966) 367.

N. GRAL~N, Inaugural Dissertation, Uppsala, 1944.

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