VISCOSITY OF AQUEOUSSOLUTIONS
to publish the data obtained for the glass powder. We are also pleased to acknowledge the help provided by Dr. M. Martin (GBopBtrole, Paris) for the realization of this work. We wish to thank Dr. J. Chaus-
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sidon from C.N.R.A. (Versailles, France) and Prof. H. Laudelout from this university for many helpful discussions. G. P. is indebted to I.R.S.I.A. for a Ph.D. fellowship.
Viscosity of Aqueous Solutions. IV. Chloroammineplatinum(IV) Salts. Influence of Ionic Charge on the Viscosity B-Coefficient
by E. R. Nightingale, Jr.,I and J. F. Kuecker2 Research & Engineering Co., L h h , New Jersey, and Univereity of Nebraska, Lincoln, Nebraska (Received December 1 , 1964)
E880
The viscosities of aqueous solutions of Pt(NH&Cl4, [Pt(NH3)6C1]Cla,[Pt(NHa)4NH2Cl]Cln, and Na2PtClehave been measured at 20, 25, and 30" in the concentration range 0.005 to 0.1 m. The viscosity data have been interpreted in terms of the Jones-Dole equation for strong electrolytes. The viscosity B-coefficients for the platinum ions at 25" are calculated to be: Pt(NHa)s4+, +0.406; Pt(NH3)&13+, 0.397; Pt(NH3)4NH2C12+,0.301; and PtC1C2-, 0.218 l./mole. The ionic activation energies for viscous flow are +370, 290, 480, and -290 cal./mole, respectively. The dependence of the ionic B-coefficient on ionic charge has been analyzed as a function of ionic size, structure, and charge type. It is demonstrated that for large ions with hydrophobic surfaces the B-coefficient is determined primarily by the size of the ion and is essentially independent of ionic charge. Polyatomic ions with hydrophilic groups such as -C1, -NH2, or -OH which can more effectively delocalize the ionic charge exhibit diminished B-coefficients. For small monatomic ions which are peripherally hydrated, the B-coefficient is a function of the electric field about the ion and increases with increasing ionic charge and decreasing crystal radius. The limiting value for the B-coefficient is determined by the electric field required to polarize the water molecule to form hydrolytic species M(OH),'-n.
-~ Previous discussions from our l a b ~ r a t o r i e s ~have emphasized that transport processes can provide significant information concerning the nature of ionsolvent interactions and the effective size of hydrated entities. However, transport parameters and infrared spectra indicate that ionic interactions with water are highly specific and greatly dependent upon the charge, size, and structure of the ions. The current contribution is one of a series designed to elucidate the effects of ion parameters on the viscosity of aqueous
solutions and also to provide insight into the nature of the ion-solvent interactions which prevail. The influence of a strong electrolyte upon the vis(1) T o whom inquiries should be addressed: Esso Research & Engineering Co., P. 0. Box 121, Linden, N. J. (2) Presented in partial fulfillment of the requirements for the M.S. degree, University of Nebraska, 1962. (3) E. R. Nightingale, Jr., J. Phys. Chem., 6 3 , 742, 1381 (1959). (4) E. R. Nightingale, Jr., and R. F. Benck, ibid., 6 3 , 1777 (1959). (5) E. R. Nightingale, Jr., ibid., 6 6 , 894 (1962).
Volunu 69, Number 7 July 1866
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E. R. NIGHTINGALE, JR.,AND J. F. KUECKER
cosity of a polar solvent is given by the Jones-Dole equations 7/70 =
1
+ A1/C + BC
(1)
where 9/70 is the viscosity of a salt solution relative to that of the solvent water, C is the molar concentration, and A and B are constants characteristic of the electrolyte. The A-coefficient represents the contribution from interionic electrostatic forces,’ and the B-coefficient measures the order or disorder introduced by the ions into the solvent structure. The latter coefficient is a specific and approximately additive property of the ions of a strong electrolyte a t a given temperatures although no satisfactory theoretical treatment has yet been given. The ionic B-coefficient is proportional to the partial molar entropy of h y d r a t i ~ n , ~and , g it recently has been demonstrated that the B-coefficient is also a measure of the radius and, hence, the effective volume of an ion in aqueous s ~ l u t i o n . ~While the ionic charge obviously affects the entropy of hydration and effective ionic volume, no previous studies have attempted to explain the role of ionic charge on the viscosity B-coefficient. With the exception of the Cs+, Rb+, I 0) is said to be affected by peripheral solvation. It is interesting to note that an estimate can be made of the maximum value of the B-coefficient for monatomic ions by extrapolating a plot of rz us. B , (at constant charge). As shown in The Journal of Physical Chemistry
E. R. NIGHTINGALE, JR., AND J. F. KUECKER
Figure 1, the upper limit for monatomic ions is B ‘v 0.8, bec:tuse ions with very intense electrical fields polarize the water molecules sufficiently to form hydrolytic species M(OH),“-”. The effect of the smaller polyatomic ions on the solvent striicture is also determined primarily by the surface charge density which is never exactly spherically symmetrical. Curve D in Figure 1 illustrates the variation of the B-coefficient for X O p ions with r, = 2.9 A. and with approximately 50% *-character in the X-0 bond.24 The latter parameter is a necessary criterion for the comparison of such polyatomic ions. The relation between AE* and dB/dT has been given4by
from which it may be seen that the sign of AE* is the negative of that for dB/dT. The positive activation energies for viscous flow shown in Table I1 for the Pt(IV)‘+, Pt(IV)3+,and Pt(IV)2+ions closely approximate that calculated previously for the (CZH5)4N + ion.6 As the temperature is increased, the ordered solution structure about these ions is diminished more than is that of the pure solvent, and the relative viscosity and, hence, the B-coefficient decreases. A negative activation energy such as is observed for the PtCl2- ion has heretofore only been associated with peripherally hydrated ions about which the electric field is sufficiently large to permit a further ordering of the water molecules as the’solvent structure is destroyed by an increase in temperature.* It is clear that a negative activation energy for an ion with aperipheral hydration implies a condition in which the ‘(apparent” size of the ion (i.e.j the B-coefficient) increases with temperature. Since such an ion does not induce extensive “ice-likeness” in the solvent about the ion, it appears that the condition of positive Bcoefficients accompanied by negative activation energies can only occur for species with hydrophilic or partially hydrophilic surfaces (e.g., those with surface groups such as -OH, -NH2, or -C1). Ions of this type most nearly represent an Einstein solute in that their effect upon the solution viscosity is closely proportional to the volume fraction of the solute. (22) Literally, “hydrated” a w a y from the surface. The behavior of RIN+ ions in enhancing the solvent structure away from the ion surface has been described by Frank (ref. 21). We shall define “structure ordering” as resulting from peripheral (Li +, F-, etc.) or aperipheral (RIN +, Pt(NHs)s‘+, etc.) solvation although ions with aperipheral “hydration” are actually unhydrated. (23) F. J. Kelley, R. Mills, and J. .M.Stokes, J. Phys. Chem., 64, 1448 (1960). (24) E. R. Nightingale, Jr., ibid., 64, 162 (1960). (26) A. Einstein, Ann. Physik, 19,289 (1906); 34,591 (1911).
