RELATIONS BETWEEN MUTUAL AND TRACER DIFFUSION COEFFICIENTS
1693
Relations between the Mutual and Tracer Diffusion Coefficients Associated with Isothermal Systems of Two Nonelectrolytes in an Un-ionized Solvent for the Limiting Case When the Physical and Chemical Properties of the Solutes Become Indistinguishable'
by Peter J. Dunlop2 Chemistry Department and Institute for Enzyme Research, University of Wisconsin, Madison, Wisconsin 63706, arbd Department of Physical and Inorganic Chemistry, Adelaide University, Adelaide, South Australia (Received December 8, 1964)
Isothermal ternary diffusion may be described in terms of flow equations which contain four measurable diffusion coefficients. However, the assumption of microscopic reversibility requires that these coefficients be linearly dependent. Using one form of the equation which relates the four measurable diffusion coefficients, an equation is derived which relates the mutual and tracer diffusion coefficients associated with a ternary system to certain limiting cross-term diffusion coefficients and thermodynamic properties of the system. The general case in which the components are miscible in all proportions is considered. Graphs which illustrate various forms of the derived equation are included.
this paper to derive from one form of the relation For the past two decades, values for the variation between the four diffusion coefficients another equawith concentration of the mutual3 and tracer4 diffusion tion which relates the mutual and tracer diffusion cocoefficients have been reported for systems of two and three components, respectively. Usually these systems have consisted of electrolytes, nonelectrolytes, and (1) This investigation was supported in part by National Science Foundation Research Grant GP-179, by National Institute of Arthritheir mixtures in nonionized solvents. Many equatis and Metabolic Diseases (U.S.P.H.S.) Research Grant AM-05177, tionsj-ll have been proposed to relate the aboveand by the Study Leave Fund of the University of Adelaide, South Australia. mentioned diffusion coefficients and several of these (2) Department of Physical and Inorganic Chemistry, Adelaide Unirelations have been tested e ~ p e r i m e n t a l l y . ~ ~How-~~ versity, South Australia. ever, only onel9 equation suggested to date has uti(3) Here we wish to distinguish (see ref. 33) between the mutual diffusion coefficient of a two-component system and the various binary lized any of the general formulations for describing diffusion coefficients associated with the same system. diffusion which have been suggested by Onsager and (4) The tracer diffusion coefficient of one component in a ternary sysFuoss20 and by Onsager.21 Here, in order to describe tem is de6ned in footnote 34. isothermal ternary diffusion, we use experimental flow (5) G. S. Hartley and J. Crank, Trans. Faraday SOC.,45, 801 (1949). equations22 which have been tested e~perimentally~~(6) A. W.Adamson, J . Phys. Chem., 58, 514 (1954). (7) L. S.Darken, TTans. A I M E , 175, 184 (1948). and which are very similar to those suggested by On(8) 0.Lamm, Acta Chem. Scand., 6, 1331 (1952). sager.21 Four diffusion coefficients appear in these (9) S. Prager, J . Chem. Phys., 21, 1344 (1953). flow equations, but it has been shown20Vz1from the as(10) A. W.Adamson and R. R. Irani, J . chim. phys., 55, 102 (1958). sumption of microscopic reversibilityZ4 that such co(11) R.J. Bearman, J . Phys. Chem., 65, 1961 (1961). efficients are linearly dependent. Various forms of (12) P. A. Johnson and A. L. Babb, ibid., 60, 14 (1956). the equation which relates these four diffusion coef(13) P. C. Carman and L. H. Stein, Trans. Faraday SOC.,52, 619 ficients have been d i s c ~ s s e d . ~ ~ -It ~ 7is the purpose of (1956). Volume 69,Number 6
M a y 1966
PETERJ. DUNLOP
1694
efficients associated with systems of two nonelectrolytes (the solutes) diffusing in an un-ionized solvent. S~ndelofl-~ has already obtained the equation for the case that the two solutes have different properties. The equation obtained here describes the limiting case when the chemical and physical properties of the two solutes become indistinguishable. It should be emphasized that similar equations which apply to other limiting physical situations for a given ternary system can be obtained by simply renumbering the components.
Flow Equations, Definitions of Diffusion Coefficients, and Nomenclature We first define the diffusion coefficients which are used to describe relative motion between the components in isothermal ternary diffusion. Binary diffusion is a special case of ternary diffusion when the concentration of one component becomes zero. It has been suggested2zand experimentally verifiedz3 that the flows (Jl)V and (JZ)v of solutes 1 and 2, respectively, in an un-ionized solvent 0, may be described for the volume frame of reference (see eq. 3) by the two flow equations28 (J1)V
=
- (~)11)V(bCl/bz)T,P,C2,t(D12)v (
(J2)v
=
- (Dz1)v(bC,/bz>,,,,c,,t
m / b z )T ,P,c1,t (1)
(D2z)v (bCz/bz)T,P,c1,t
where (D& and (DZJv are the main diffusion coefficients and (D12)v and (DZI)Vare the cross-term diffusion coefficients. The (bCi/bz)T,p,c,zcare solute concentration gradients for unidimensional diffusion a t constant temperature, T, and constant pressure, P. Each concentration, C,, is expressed in moles of component i per unit volume, so that 2
CC,Pl = 1
where the P, are the partial molar volumes of the components. The volume frame of reference is defined by
(3)
We choose the volume frame of reference for the diffusion coefficients because it becomes identical with the cell or experimental reference frame if the partial molar volumes of all components are independent of the solute concentrations for the conditions of the experiment. It has also been indicated experimentallyzg-31 and shown theoreti~allyl~ that The Journal of Physical Chemistry
-+
(4%)
0 as Cz +0
(4b) Now in order to discuss the various limiting values of the four diffusion coefficients defined by eq. 1 and 3, we introduce the notation32 (D21)~
kl
(DtJv = (D0)v (i, j = 1, 2) (ii)C1+0 cp Ck+O (k,1 = 0, 1, 2)
(5)
i.e., the left-hand side of eq. 5 denotes the limiting value of the diffusion coefficient (D& as Ck jirst approaches zero and then C l approaches zero. The following limiting values of the four general diffusion coefficients in eq. 1are of special interest in this paper. 2(Dll)V =
(DMl0)V
‘(Dll)V = ‘(D22)V
=
(DM0l)V
(6)
(7)
(DT1)V
= (DM20)V = (DMX0Z)V
(8)
= (DT2)V
(9)
‘(D2Z)V
1(D& = 2(D21)v= 0 (see eq. 4) (14) L. Miller andP. C. Carman, Trmns. Furuduy Soc., 55,1831 (1959). (15) P. C. Carman and L. Miller, ibid., 55, 1838 (1959). (16) L. Miller and P. C. Carman, ibid., 58, 1529 (1962). (17) R. R. Irani and A. W. Adamson, J. Phys. Chem., 64,199 (1960). (18) R. Mills, ibid., 67, 600 (1963). (19) L.-0.Sundelof, Arkiv Kemi, 20, 369 (1963). (20) L. Onsager and R. M. Fuoss, J . Phys. Chem., 36, 2689 (1932). (21) L. Onsager, Ann. N . Y . Acud. Sci., 46, 241 (1945). (22) R. L. Baldwin, P. J. Dunlop, and L. J. Gosting, J . Am. Chem. SOC.,77, 5235 (1955). (23) P. J. Dunlop and L.J. Gosting, ibid., 77, 5238 (1955). (24) (a) L. Onsager, Phys. Rev., 37, 405 (1931); (b) ibid., 38, 2265 (1931). (25) G. J. Hooyman, Physicu, 22, 751 (1956). (26) J. G. Kirkwood, R. L. Baldwin, P. J. Dunlop, L. J. Gosting, and G . Kegeles, J. Chem. Phys., 33, 1505 (1960). (27) D. G. Miller, J. Phys. Chem., 63, 570 (1959). (28) When the concentration of component 2 (say) becomes zero then ( D z I ) = ~ 0 (see eq. 4) and (t)C2/t)z))~,p = 0. Thus eq. 1 be-
comes Fick’s first law for binary diffusion
i-0
i=O
(D& +0 as C1+0
(Jl)V
= -(oll)V(
dCl/bs)T,P
= -(DMlO)V( dCl/bZ)T.P = -(DMOl)V(
dCl/br)T,P
where (DMIo)~ is the mutual diffusion soefficient of component 1 in component 0 and ( D M o Iis) ~the numerically identical mutual diffusion coefficient of compoenent 0 in component 1 (see ref. 33). This mutual diffusion coefficient is a function of CI. When C1 or CO approaches zero, then the mutual diffusion coefficient has the limiting values ~ ( D M and I o ~)(~D M I orespectively. )~, (29) P. J. Dunlop, J . Phys. Chem., 61, 994 (1957). (30) I. J. O’Donnell and L. J. Gosting, “The Structure of Electrolytic Solutions,” W. J. Hamer, Ed., John Wiley and Sons, Inc., New York, N. Y., Chapman and Hall, London, 1959. (31) L. A. Woolf, D. G. Miller, and L.J. Gosting, J. Am. Chem. SOC., 84,317 (1962). (32) When only one concentration variable approaches zero then only one superscript is used on the ( D i j ) ~ .
RELATIONS BETWEEN MUTUAL AND TRACER DIFFUSIONCOEFFICIENTS
1695
composition C1. As we shall see later, for this limiting case when the properties of 2 and 1 become indis2 ( D ~and ~ ) l ~( D ~ J ~ tinguishable, certain equations which follow are I n eq. 6 to 9, (DM1O)V and (DM~o)v denote the mutual33 greatly simplified. It should be emphasized that we diffusion coefficients of components 1 and 2, respeccan never actually perform the limiting experiments tively, in the solvent 0, while (D& and (DT& denote in which components 1 and 2 have identical properties, the tracer34diffusion coefficients of components 1 and 2, since in that case these two components can no longer be distinguished from each other. However, by making respectively, in solutions of the other component^.^^ the properties of component 2 (the tracer component) Thus (D& denotes the tracer diffusion coefficient approach those of component 1 as close as is experiobtained when a vanishingly small amount of commentally possible, useful information which closely ponent 2 diffuses in a mixture of components 0 and 1. Figure 1 illustrates these diffusion coefficients in terms approximates@ the limiting results expressed by the of three-dimensional diagrams. Other limiting difequations can be obtained. I n the laboratory, comfusion coefficients are given by eq. 5: e.g., 0 2 ( D ~ J ~ponent 2 is usually chosen to be a radioactive isotope is the value of (Dll)v first extrapolated to Co = 0 of component 1. and then to C:, = 0. This value corresponds to the We shall now derive a general relation between the diffusion coefficients and certain thermodynamic quanmutual diffusion coefficient of component 1 in a tities for a system in which a trace amount of comvanishingly small amount of component 0. It is ponent 2 diffuses in a solution of composition C1. possible to relate O(D11)v and O(DZ2)v(see Figure 1) The result will first be obtained for the case that 1 to the mutual diffusion coefficient of components 1 and 2. This may be achieved36 by differentiating eq. 2 with respect to x and utilizing eq. 1 together with (33) Binary diffusion of components 1 and 0 may be described in
o ( D ~ land ) ~ O(DZZ)V
2
CC,dPi = 0
i=O
(10)
the relation for the limiting case that Co = 0. The results obtained are3' (DMl2)V
= O(D1l)V
- (pl/pZ)['((o12)V]
(11)
where2a (DM1Z)V = (DM2l)V
(13)
The two tracer3* diffusion coefficients (D& and and the three mutual diffusion coefficients (DMOl)V, (DMIZ)V, and (DMz~)v, which have been mentioned above, are all functions of concentration. Some of the limiting values of these quantities are shown in Figure 1 (superscripts indicate that a given concentration is zero). For example, ODT~(the subscripts V have been omitted in Figure 1) is the limiting value of the tracer diffusion coefficient of component 1 when Coapproaches zero, and it may be seen from eq. is equal to 0 0 M 1 2 , the value of 4a, 7, and 11 that the mutual diffusion coefficient of components 1 and 2 when C1is zero. In the discussion up to this point the two solute components 1 and 2 have been assumed to have different physical and chemical properties. However, we now wish to consider the caseasin which the properties of component 2 (say) approach those of component 1 when a trace amount of 2 is diffusing in a solution of (DT2)V
terms of only one mutual diffusion coefficient; (.e., that for the volume frame of reference. Otherwise two unequal binary diffusion coefficients are required to describe relative motion between the components. (34) The tracer diffusion coefficients of components 1 and 2 in a solvent 0 are denoted by ( D T I )and ~ ( D T z ) respectively. ~, (DTi)v, for example, is equal to the value of ( D I I )in ~ eq. 1 when C1 becomes zero. Experimentally (Di1)v is measured when Ci is very small and the result so obtained is assumed to be equal to ( D T I ) ~ .It should be noted that ( D T ~ )isv a function of Cz and thus this quantity may be l to Co = 0 to give ~ D T(for I extrapolated to CZ = 0 to give z D ~ or convenience the subscript V is often dropped from the diffusion coefficients). Similar quantities are defined for DTZ. The tracer diffusion coefficients D T ~and DTZ do not depend on the frame of reference. The subscripts V are only used on these coefficients to be consistent with eq. 5. (35) A different choice of the solvent, 0, simply requires a renumbering of components. (36) F. 0. Shuck and H. L. Toor, J . Phys. Chem., 67, 540 (1963). (37) These equations can also be obtained in the form O(D1l)V
-
(i71/i7z)~Y~1z)vI= O(D2z)v
- ( i7z/VdP(Dz1)vl
by taking the limit of eq. 14 as Co tends to zero and then Utilizing eq. 50 of ref. 25. (38) There is, of course, a measurable tracer diffusion coefficient for component 0 which does not appear in Figure 1. If either of components 1 or 2 were chosen as the solvent, then the diagrams corresponding to that choice would contain the tracer diffusion coefficient of component 0 in the present choice of components. (39) This type of limit differs operationally from the limit of (D12)v as CZ (say) approaches zero. The concentration variable CZis continous and may have all values that are physically possible. However the properties of component 2 cannot, except in thought, be varied continuously. These properties can nevertheless be changed in finite steps by utilizing radioisotopes of that component. (40) That only a p p r o z i m t e values of tracer diffusion coefficients are sometimes obtained by using radioisotopes is illustrated by the data of Wang (J. H. Wang, J . Am. Chem. Soc., 73, 510 (1951); J. H. Wang, C. V. Robinson, and I. S. Edelman, ibid., 75, 466 (1953)). Using H22016 and HzlO18 as tracer components he obtained 2.13 x l o d and 2.83 X lod, respectively, for the self-diffusion coefficient of water. In this case, of course, the effect is quite pronounced due to the very low molecular weight of water.
Volume 69, Number 6 M a y 1966
PETERJ. DUNLOP
1696
I
I I
JC2 I I I
I I
I
@A I
II
I I
co=0 f FIGURE I
FIGURE 2
Figure 1. Ideal graphs of the limiting values of (D&, (D&, (D&, and (D& for a hypothetical system in which the components are miscible in all proportions; the three cases indicated are C1-+ 0, CB-+ 0, and Co-c 0 (the subscript V has been omitted from all the ( D i j ) v ) . The graphs are for the general cme t h a t the properties of components 1 and 2 are different and are ideal in the sense that, for the purpose of illustration only, all diffusion coefficienta have been assumed to be linear functions of the concentrations. All the coefficients indicated are defined by eq. 1 to 5. For a given composition of the system the dashed line between graphs indicates the diffusion coefficients which are related by eq. 17. Figure 2. Corresponding graphs of Drl, DB, 0 2 1 , and Dla for the special limiting caae that the properties of component 2 become indistinguishable from those of component 1. Figure 2 illustrates this limiting case of Figure 1. Figures 2A and 2B are the mirror images of Figures 2D and 2C, respectively. The dashed line between graphs indicates those quantities which are related by eq. 23.
The Journal of Physical Chemistry
RELATIONS BETWEEN MUTUAL AND TRACER DIFFUSIONCOEFFICIENTS
and 2 have different properties and then for the limiting case in which these properties become indistinguishable. I n the derivation component 2 is chosen as the tracer component, but the result is symmetrical in 1 and 2. The discussion is limited to diffusion of two nonelectrolytes in an un-ionized solvent.
A General Equation Relating the (D& for Vanishingly Small Values of C2 (1) Components 1 and 2 Have Di$erent Properties. It has previously been indicated that, as a result of the assumption of microscopic r e ~ e r s i b i l i t y ,a~ rela~ tion exists between the four diffusion coefficients defined by eq. l. For the volume frame of reference this relation has the form2'~31~41~42
bii(Diz)v
+ biz(D22)v = b22(D21)v + b21(D11)v
(14)
2
bjE atk = &k
+
=
Cawkr
(i,.i,IC,
( C * ~ k / C O ~ O ~
Pi$ =
(144
k=l
= 1,2)
(14b) (144
(bPi/bC*)T,P,CkZO,j
where the pc are the chemical potentials of the solute components. The validity of eq. 14 has been tested for a number of three-component systems27J1~s6~43-46 and in each case it was verified within the estimated error of measurement. Here we choose to accept this relation as a starting point and then take the limit as Cz tends to zero as a first step in seeking a relation between the mutual and tracer diffusion coefficients associated with a three-component system. The result is easily obtained if we remember that p12 and p21 remain finite in the limit for nonelectrolytes and then utilize eq. 6 and 9 and the relation lim (C2p2,) = RT
(15 )
(2-0
1697
values. Equation 17 relates the mutual diffusion coefficient, D M ~of ~component , 1 in a solvent 0 to the trace diffusion coefficient, D T ~of , component 2 in a solution of composition C1. Sundeloflg has derived an equivalent equation and discussed in considerable detail its significance for three-component systems of electrolytes and nonelectrolytes in un-ionized solvents. Figure 1 illustrates these diffusion coefficients which are related by eq. 17 (see dashed line between graphs) * (2) The Properties of Component d Approach Those of Component 1. We now wish to take the limit of eq. 17 as the properties of component 2 become identical with those of component 1. It has been indicated previously that this limiting situation is usually approximated experimentally by using for component 2 a radioisotope of component 1. As a fmt step in obtaining the result we examine the chemical potential derivatives peg and the solute partial molar volumes V ifor the special limiting case in which components 1 and 2 become indistinguishable. Because the chemical potentials, pi, cannot be measured directly they are usually related to the solute concentrations by equations of the form pj
pio
+ RT In Cjyt
m
m
Vi
=
j=O
m
m
CBi&?Czk k-0
(19b)
where the A,oo = 0 and the B,oo are constants for a given three-component system. These expansions
Equation 14 finally becomes
+
WV RTCoVo(L)] i piz)
(012~11
(18)
in which the ptoare the chemical potentials of the standard states for the volume concentration scale, and the yc are the corresponding activity coefficients. I n general each ye is a function of both concentrations C1and Cz. Now it is found experimentally for mixtures of nonelectrolytes that the logarithm of each solute activity coefficient, In ye, and each solute partial molar volume can be expressed as a Taylor series expanded about C1 = C2 = 0. Thus
where R is the gas constant. Another necessary limiting equation is obtained with the use of eq. 4b and 1'Hospital's rule
[
(i = 1, 2)
cro
(17)
For convenience in these equations and those that follow we choose to drop the subscript V from all the (D&
(41) P. J. Dunlop, J . Phy8. Chehem., 68,26 (1964). (42) A relation of this general form exista for other frames of references, see ref. 25-27. (43) P. J. Dunlop and L. J. Gosting, J. Phys. Chem., 63, 86 (1959). (44)P. J. Dunlop, %%id., 63, 612 (1959). (45) R.P.Wendt, ibid., 66, 1279 (1962). (46) F.J. Kelly, Ph.D. Thesis, University of New England, Armidale, New South Wales,Australia, 1961.
Volunze 69,Number 6 May 1066
PETERJ. DUNLOP
1698
are valid for any mixture of 1 and 2 in the solvent 0 when C1and CZare finite or zero. But as the properties of components 1and 2 approach one another, the values of Vl and V2must become identical, as must also the values of In yl and In yz. Thus in this limit we argue that the coefficients in eq. 19 become independent of the value of i for both the activity coefficients and the partial molar volumes, respectively. Therefore, the coefficients of all terms in each series of order ( j k) must be subject to the restrictions
+
=
Aljk
AZjk
[M(j+,)/j!k!I
=
(20a)
(j,k = 0 , l , . . . , m )
[No+~)/j!k!I (20b) where the MO.+k)and the No+,) are constants. EquaBljk
= Bzje =
tions 20 ensure that eq. 19 become power series C2) when the properties of in one variable C = (Cl 1 and 2 become indistinguishable. Thus in this limit eq. 19 become
+
i
= (j
+ k)
c = (Cl+ C2) m
Vsolute =
C(Nt/i!)Ci
(21b)
i=O
where M o = 0 and N o is a constant. It follows from these remarks that, in the limit when the properties of 1and 2 become identical, then
Vl y1z =
=
Vz
y21 = y11
(224 (22b)
= yzz
where
(b In y~/bCj)T,P,CkZO,j = yu (i,j= 1, 2)
(2%)
Equations 22 may now be substituted in eq. 17 to give
- coVo(ezl) + ["(cl-l + yl1) 1 +
D M ~=ODTZ
12
(Vl
Yll)
cz=o
(23) Equation 23 is a general relation between the mutual diffusion coefficient of component 1, D M I ~= zD1l, for composition C1 and the tracer diffusion coefficient of component 2, DTZ = 2D22,at the same concentration. The term in brackets gives the variation with concentration of the difference between these two measurable quantities. Figure 2 illustrates the limits of the quan1 a8 the properties of 2 and 1 become tities in identical (see eq. 23). It should also be emphasized The Journal of Physical Chemistry
that when components 1 and 2 have identical properties, certain symmetry relations appear in the diffusion coefficients which have been discussed above, i.e.
Dii(C1 = Diz(C1
U;
= U;
b)
=
D22(C1 = b;
=
U)
(24)
Cz = b)
=
Dzl(C1 = b; Cz =
U)
(25)
Cz
=
C2
ODTZ = ODT~= Dsl = Ds2
(26)
where a and b are values of C1and Cz which define any composition of the system and Dsl and Dsz are the identical self-diffusion coefficients of components 1 and 2, respectively. It is interesting to take the limits of eq. 23 as C1and Coapproach zero, respectively. (1) To obtain the limit as C1 -+ 0, we note that CoVo--+ 1, yll remains finite and that because of eq. 4a = lim (Dlz/CJcl=0 = (bDl~/dC1)c2=cl=o
21012
c1+0
(27)
These relations then enable eq. 23 to be written
Thus the mutual and tracer diffusion coefficients are identical at infinite dilution of both components 1 and 2 since both the partial derivatives in brackets, 21e21 and 21t91z, are equal. This follows because eq. 23 is symmetrical in 1 and 2 because the properties of components 1and 2 are identical. (2) When Co --t 0, eq. 23 become^,^' with the help of eq. 6 and 26
+
'DMOI = Dsi 'OD12 (29) since (CJ-' and V1 are identical when Co = Cz = 0. Equation 29 indicates that the mutual diffusion coefficient of component 1 in a vanishingly small amount of 0 is related in a rather simple manner to the seZfdiffusion coefficient of component 1, Dsl, and a crossterm diffusion coefficient 20012 which is the limitingvalue of D12as both Czand Coapproach zero. The validity of eq. 23 which contains both mutual and tracer diffusion coefficients together with crossterm diffusion coefficients and thermodynamic factors should be subject to direct experimental verification. However, at present it would be rather difficult to measure 2D12and 2821 with sufficient accuracy. The case when Cz = Co = 0 is of importance because eq. 29 should enable 20D12to be obtained quite accurately48 (47) Equation 29 is a specid case of eq. 11-13. (48) Probably the best data available for utilizing eq. 29 are those of Mills (see ref. 18) together with those of C. L. Sandquist and P. A. Lyons, J . Am. Chem. SOC.,76,4641 (19%). Mills has measured the
VAPORPHASERADIOLYSIS OF ETHANOL
1699
since it is to both ODMol and Dsl with "& is a Measan accuracy of Several tenths Of 1%. .. of the &flerence between the forces experienced by limiting amounts of Components 0 and 1, respectively, in excess quantities of component 1.
tracer diffusion coefficient of benzene in solutions of biphenyl in benzene and Sandauist and Lyons have measured the corresponding mutual diffusion- coefficient; If we designate biphenyl as the solvent, 0, tagged benzene the tracer component, 2, and pure benzene the other solute, 1, then from the data given in the above papers
Acknowledgment. The author wishes to thank professorL. J , ti^^ for many stimulating discussions and suggestions.
is a measure of the diference between the forces experienced by limiting amounts of biphenyl and benzene, respectively, in excess quantities of benzene. The above figure for zDD~zshould be accurate i o better than 0.5%.
*4012 = (1.558
- 2.247) X
10" = -0.689 X 10-6
The Vapor Phase Radiolysis of Ethanol1
by L. W. Sieck and R. H. Johnsen Department of Chemistry, Florida State University, Tallahassee, Florida
(Received December 10, 1964)
Initial and nonscavengable yields for the radiolysis of ethanol vapor at -45 mm. (25") have been measured. The contribution made by ionic species to the reaction products is discussed.
Introduction Although extensive radiolytic investigations of ethanol have been carried out in condensed phases, little information is available concerning the vapor phase decomposition. Ramaradhya and Freemanz8 have determined product distributions at high pressures and temperatures (108"), using a-particles. An interpretation of the chemistry was proposed in terms of a free-radical mechanism. A second investigation by these same workerszbconsidered the effect of added cyclohexene and benzene on the CO, CH,, and Hz yields. More recently, Myron and Freeman have studied the ethanol system using a high intensity ysource employing pentadiene as a scavenger.3 In the present work, initial yields have been determined, and nitric oxide has been used as a scavenger as well as oxygen and propylene. These studies were carried out at 25"' where reactions with significant activation energies would be relatively unimportant. The use of NO was prompted by Hoare's observation4 that complete scavenging occurred in the photolysis
of acetone-nitric oxide admixtures, suggesting no peculiar effects in oxygenated systems.
Experimental Apparatus. The irradiation vessel was fashioned from a brass cylinder equipped with 0-ring-sealed, 0.0127-cm. aluminum foil windows, through which the electron beam passed along an axis perpendicular to the longitudinal axis of the cell. Filling and evacuation was achieved through Whitey Teflonseated needle valves fitted with metal tapered joints for connection to a vacuum and sample handling system. The cell contained a volume of 2040 cc. and could be evacuated to mm. with no detectable leakage after a 24-hr. period. The entire apparatus (1) This work was supported i n part by the U. S. Atomic Energy Commission under Contract AT-(40-1)-2001. (2) (a) J. M. Ramaradhya and G . R. Freeman, Can. J. Chem., 39, 1836 (1961); (b) ibid., 39, 1843 (1961). (3) J. J. J. Myron and G . R. Freeman, ibid., 43, 381 (1965). (4) D. E. Hoare, ibid., 40, 2012 (1962).
Volume 69,Number 6 Mag 1966
