1536
FRANCES E. HELDERS AND JOHND. FERRY
the water surface and toward the tube. Surfacepotential measurements'O and electron micrographs" of fatty-acid films show the presence of island structures. Ryan and Shepard12 obtained autoradiographs of radioactive calcium stearate monolayers transferred at low pressures by the Blodgett technique13; large islands of liquid expanded phase surrounded by areas of gaseous phase were postulated. Conclusion The volatility, solubility and polarity of the spreading solvent cannot be directly related to the shape or position of the stearic acid isotherms. ilt intermediate and high pressures, no effects of solvent are observed; at low pressures, effects which may be influenced by the spreading solvent appear. (10) W. D . Harkins and E. IC. Fischer, J. Chem. Phys., 1, 852 (1933). (11) H. E. Ries, Jr., and W. A. Kimball, THISJOURNAL, 59, 94
(1955).
(12) J. P. Ryan and J. W. Shepard, ibid., 69, 1181 (1955). (13) K. B. Blodgett, J. Am. Chem. SOC.,57, 1007 (1935).
Vol. 60
The evidence of the pressure-area and radioactivity-area investigations, combined with related studies, suggests that the uncompressed monolayers exist in large clusters of extremely small islands. During compression, the clusters may coalesce and the islands may deform to yield a continuous solid film of low compressibility just before collapse. The structure, packing and deformation of islands during compression may give rise to the various shapes of the pressure-area isotherms in the low-pressure region. The spreading solvent used could affect the shape of the isotherm through its effect on the size, shape and distribution of island structures formed during the spreading process, rather than through solvent retention in the monolayer. Acknowledgment.-The authors thank A. T. Wilson for advice on the radioactivity measurements, Joseph Gabor for assistance in the experimental work, and C. J. Berrigan of E. I. du Pont de Nemours Csr: Company for providing the Teflon coatings.
NON-NEWTONIAN FLOW I N CONCENTRATED SOLUTIONS OF SODIUM DESOXYRIBONUCLEATE' BY FRANCES E. HELDERS~ AND JOHN D. FERRY Contribution from the Department of Chemistry, University of Wisconsin, Madison, W i s . Received J u n e 7, 1966
Non-Newtonian viscosities of salt-free aqueous sodium desoxyribonucleate solutions have been measured by capillary and falling ball methods over the concentratios range from 0.19 to 18.3 X 10-3 g./ml. The polymer molecular weight and radius of gyration were 5.8 X 106 and 2170 A., respectively. The viscosities extrapolated to zero shear rate (7) range! from 0.195 to 40,000 poises at 25'. The apparent activation energy for flow, calculated from measurements at 5 and 25 , increased approximately linearly with concentration, up to 8.6 kcal. a t 18.3 X g./ml. From the capillary measurements, taking into account the inhomogeneous shear rate, the dependence of apparent viscosity ( v 8 ) on shear rate (?) was calculated. I n form, this function agreed with direct measurements by Markovitz and Zapas at one concentration using a cone-and-plate viscosimeter which provides homogeneous shear. At concentrations from 0.75 to 3.06 X g./ml., qa/q wai a function only of ?7/c, similar to that predicted by the theory of Bueche for free-draining flexible coils. At concentrations of 0.19 and 0.38 X 10-8 g./ml., qa/q was a function of 91, similar in shape to that predicted by the theory of Saito for rigid elongated ellipsoids; and the corresponding rotary diffusion coefficient reduced to water a t 25' was 20 sec. - I , in agreement with flow birefringence measurements by Doty and collaborators in very dilute solution. At 1.5 X g./ml., 0.2 M sodium chloride diminished the relative viscosity by a factor of 10 witho,it changing 7 . l ~ as a function of $q/c. At 0.19 and 0.38 X 10-3 g./ml., the presence of 50.47, glycerol diminished the relative viscosity by a factor of seven and changed the character of the non-Newtonian flow to resemble that predicted by Bueche for shielded flexible coils.
Introduction Most investigations of the viscosity of sodium desoxyribonucleate solutions have been concerned with extremely low concentrations, seeking information about molecular shape or the effects of electrical charge^.^-^ More concentrated solug./ml., where tions, in the range from 10-4 to the molecular domains overlap but there is no ordering with liquid crystal formation, have received little attention except for an earlier paper from this Laboratory.6 In this range, the dependence of apparent viscosity on shear rate is very 1
(1) Presented in part at the 128th Meeting of the American Cheniica Society, Minneapolis, Minn. (2) General Electric Company Fellow in Chemistry, 1955-1956. (3) J. Pouyet, J . chim. phys., 48, 016 (1951). (4) M. E. Reichmann, 8. A. Rice, C . A. Thomas and P . Doty, J . Am. Chem. SOC.,76, 3047 (1954). (5) J. A. V. Butler, B. E. Conway and D. W. F . James, Trans. Faraday Soc., 50, 612 (1954). (6) 8. Katz and J. D . Ferry, J. A n . Chem. Soc., 76, 1589 (1953).
marked. The interpretation of such non-Newtonian behavior is of interest in connection with current concepts of flow in polymeric systems,' especially for concentrated systems where flexible molecules behave more nearly as free draining rather than as shielded coils. Earlier measurements in this moderately concentrated rang$6 were made in 0.2 M aqueous sodium chloride as solvent, The present paper reports data for salt-free aqueous solutions over a wide range of concentrations, as well as some measurements with salt and in a glycerol-water solvent, using a new preparation of sodium desoxyribonucleate. The results are compared with theoretical calculations of non-Newtonian flow of solutions of flexible coiled molecules' and rigid (7) F. Bueche, J. Chem. Phys., 22, 1570 (1954). (8) N. Saito, J . Phys. Soc. J a p a n , 6, 297 (1951). (9) H. A. Scheraga, J . Chem. Phys., 2S, 15% (1955).
Nov., 1950
NON-NEWTONIAN FLOW IN SOLUTIONS OF SODIUMDESOXYRIBONUCLEATE 1537 Materials and Methods
The sodium desoxyribonucleate (SDNA, sample 11) was prepared in collaboration with Dr. Sidney Katz, following the method used by Signer and Schwanderlo for their preparation VTII. (Dr. Katz's earlier preparation11 by the same method is designated sample I.) Sample 11, like sample I, was protein-free as tested by the Weber modification of the Sakaguchi reaction. Because of the emphasis on saltfree solutions in this work, a Van Slyke determination'* for chloride was made to ensure that the sodium chloride used in the course of preparation had been fully removed; this test indicated a sodium chloride content of less than 0.02%. The molecular weight of sample 11, determined from light scattering measurements by Dr. Katz together with the value of refractive index increment used by Doty and collaborators,' 0.188 (g./ml.)-l, was 5.8 X 106. The radius of gyration, calculated from the angular dependence of scattering, was 2170 A. Its relative viscosity at a concentration of 0.0149% in 10% sodium chloride at p H 6.0, 20°, and an average shear rate of 910 sec.-l, was 1.40. The corresponding values for sample I were 5.9 X loa (cqrrected with the above refractive index increment), 2120 A., and 1.254. The molecular dimensions of both samples compare well with those of the best preparations studied by Doty and collaborators*; the lower viscosity of sample I suggests that it may have undergone some delayed degradation due to slight enzymatic attack during preparation. This difference between the two samples is also reflected in the zeroshear viscosity in 0.2 M sodium chloride, as will be seen below. The SDNA was stored a t 0" with about 6% water content. After 3 years, a t the close of the measurements reported here, its relative viscosity in 10% sodium chloride was unchanged. Stock solutions were made up by weight in conductivity water, and their exact concentrations were usually determined by dry weight a t 105" in vacuo. Solutions for measurements were prepared by appropriate dilutions, introducing salt or glycerol when required. Mixing was usually accom lished by a very slow magnetic stirrer, the solutions being f e p t a t 0" with a trace of toluene until homogeneous. The p H , measured in every case with a Beckman Model G pH Meter, was 6.75 i 0.1 for the salt-free solutions with two exceptions (7.1 and 6.45) ahd 6.3 f 0.3 for those in 0.2 M sodium chloride. The dependence of viscosity on p H should be slight in this range." Very dilute salt-free solutions of SDNA have been found by Litt, Simmons and Doty to be susceptible to a denaturation at room temperature similar to that which occurs in the presence of salt at elevated temperatures.l' However, sufficient ionic strength to protect a ainst this process is supplied by the sodium ions if the SD%A concentration exceeds about 10-4 g./ml., which was the lower limit of our concentration range. The solutions for which data are reported here were stable with respect to viscosity for several days at least; in a few cases, dilute solutions displayed decreasing viscosity with elapsed time, and these were discarded. At SDNA concentrations of 5.9 X lo-* g./ml. and above, falling ball viscosity measurements were made with small glass spheres,eJ6 of diameter from 0.1 to 1 mm., obtained through the generosity of the Minnesota Mining and Manufacturing Co., and also with stainless steel bearing balls of diameter from 0.038 in. to '/IC, in., thus providing a considerable range of shear rates. The diameters of the glass spheres were measured with a microscope, and their density was obtained by weighing on a microbalance; it appeared to be uniformly 3.0 for those free of air bubbles. Nominal viscosities were calculated by the usual Faxen equation. g./ml. and below, visAt concentrations of 3.0 X cosity measurements were made with horizontal capillaries (10) R. Signer and H. Schwander, Helu. C h i n . Acta, 33, 1522 (1950). (11) S. Katz, J . Am. Chem. Soc., 5'4, 2238 (1952). (12) D. D. Van Slyke, J. BioE. Chem., 68, 523 (1923). (13) J. M. Creeth, J. M. Gulland and D. 0. Jordan, J. Chem. Soc., 1141 (1047). (14) M. Litt, N. 5. Simmons and P. Doty, private communication. (15) P. Elirlich, S. Shulinan and J. D. Ferry, J. Am. Chem. Soc., 5'4, 2258 (1952).
bent in zigzag shape for compactness and provided with small bulbs at each end.16 The capillary length was about 150 om.; the radius, 0.5 or 1.0 mm.; and the bulb volume, about 1 ml. Flow in either direction could be achieved by applying air pressure with reversing stopcocks. This pressure, regulated by a water column manostat, was varied from 6 to 45 cm. of water. An additional effective pressure due to the average liquid head in the viscometer during di.6charge of a bulb was determined by calibration wlth a liquid of known viscosity. The calibration, which also furnished the instrument constant embodying the capillary dimensions and bulb volumes, was performed as follows. Flow of a given volume of the standard liquid (a National Bureau of Standards calibrated oil, or olive oil previously. studied in an .Ostwald viscosimeter) was timed in both directions a t various a r pressures. A plot of reciprocal time (l/t) against pressure in cm. of water ( h )gave for each direction a straight line whose slope was proportional to the instrument constant K and whose intercept was the average head pressure h,. In subsequent experiments with SDNA solutions, the viscometer was filled (by a hypodermic syringe) with an identical volume, as indicated by suitable marks on the capillary. Nominal viscosities q m for the SDNA solutions were calculated from the formula qrn Kt(h f h v p l p s ) (1) where p and ps are the densities of solution and calibrating oil, respectively. I n practice the density ratio was usually unnecessary since the h, term was relatively small. In addition to the falling ball and capillary measurements in this Laboratory, measurements on two solutions were generously made by Drs. Hershel Markovitz and L. J. Zapas of the Mellon Institute of Industrial Research with their cone-and-plate viscosimeter" which provided a homogeneous rate of shear.
