TERNARY LABELED ALANINE-ALANINE-WATER SYSTEMS
with polystyrene in alcoholic solutions.24 The chargetransfer energy for each of two benzene-silver bonds formed in the silver perchlorate-benzene complex has been estimated as ca. 16 kcal mole-' of silver ion.2s While this Ordinarily does not form in the presence Of water, when the benzene rings and Ag+ ions are con-
2299
strained to favorable configurations as might occur in highly cross-linked exchangers, the formation of such charge - transfer complexes is conceivable. (24) D. F.Evans, J . Chem. Phys., 24,1244 (1956). (25) H.G. Smith and R. E. Rundle, J . Am, Chem. SOC.,80, 5075 (1958).
Measurements of the Intradiffusion Coefficients' at 25" of the Ternary Systems (Labeled L-ct-Alanine)-(DL-ct-Alanine)-water and (Labeled @-Alanine)-( pAlanine)-Water
by J. G. Albright2 DiffusionResearch Unit, Research School of Physical Scklaees, Auatralian NatWnaE University, Canberra, Australia, and the Institute for EnResearch. University of Wisconsin, Madkon, Wisconsin 63706 (Received January $1, 1966)
Intradiff usion coefficients have been measured at 25" for the systems (labeled ca-alanine)(Dca-alanine)-water and (labeled Balanine)-(P-alanine)-water over concentration ranges of 0.0-1.5 and 0.0-5.0 moles/l., respectively. The four diffusion coefficients for these ternary systems are calculated at a series of concentrations from the intradiffusion coefficients obtained in this study and some previously reported data for the mutual-diffusion coefficients. The results show that at higher concentrations in the system with &alanine the cross-term diffusion coefficients may be an appreciable fraction of the main-term diffusion Coefficients. Thermodynamic transport coefficients related to solute-solute interaction and to solvent-solute interaction have been calculated and discussed.
Introduction In recent years interest has developed in the study of isothermal diffusion in three-component systems. It is hoped that such studies will help yield a better understanding of the complex transport phenomena which occur in biological systems as well as improve the understanding of diffusion in industrial processes. A system composed of a solvent containing two solutes which are chemically equivalent but isotopically different constitutes a three-component system of particular interest. I n this case the four diffusion coefficients, which are necessary to describe a three-com-
ponent system, may be calculated in a simple manner from the values of the intradiffusion coefficients and mutual-diff usion coefficients for the system.a Data (1) The term intradiffusion was introduced and explicitly defined in ref 3. IntraMusion coefficients are equal to tracer-diffusion coefficients for the systems described here where the unlabeled solute is chemically equivalent to the labeled solute. (2) The experimental work reported in this article was performed by the author while visiting the laboratory of Dr. R. Mills at the ANU during the tenure of the NIH Postdoctoral Fellowship No. 4-F2GM-19,747-02. Subsequent tceatment of the data was performed at the author's present addreas, Institute for Enzyme Research, and supported in part by Public Health Service Research Grant AM06177 from the Institute of Arthritis and Metabolic Diseases.
Volume 70, Number 7 July 1966
J. G. ALBRIGHT
2300
obtained for this type of system should be an interesting supplement to the data obtained for the more complicated case of three chemically different components. There are, however, few data on intradiffusion coefficients available in the literature, especially for aqueous solutions of organic solutes. Thus the systems (labeled L-a-alanine)-(m-a-alanine)-water and (labeled p-alanine)-(p-alanine)-water, where the labeled components contained CI4,were investigated in this s t ~ d y . ~
Experimental Section Materials. Fluka "puriss" reagent grade D L - ~ alanine, L-a-alanine, and p-alanine were used in preparing the nonradioactive solutes. These materials were purified by recrystallization in which ethanol was added to warm (60") concentrated aqueous solutions of each of the materials to form the crystals. The mother liquor was removed from these solutions, when they had been cooled to room temperature, by centrifugal drainage. Conductance measurements made over extended periods of time on concentrated solutions of each of these mat,erials showed that ionic impurities were negligible. There was only a small increase of conductance with time which indicated that growth of biological organisms would not be an important factor in the diffusion experiments. The materials were dried under vacuum at room temperature for 2 days and then stored open to an atmosphere dried by PzOs in a desiccator. Small quantities of L-a-alanine and p-alanine labeled with carbon-14 were obtained from the New England Nuclear Corp. Each sample had an activity of 0.5 mcurie. By adding these samples to 1.0-g nonradioactive portions of the same chemical materials and purifying by the procedures described above, but on a smaller scale, 0.5-g samples of the two materials were obtained. For the first six experiments on the system (labeled L-a-alanine)-(DL-a-alanine)-water, the radioactive sample had been recrystallized once. It was then discovered that when the sample was recrystallized a second time, the measured values of intradiffusion coefficients appeared to be slightly higher. Thus, for the remaining four experiments, doubly recrystallized radioactive samples were used. For all of the experiments on the system (labeled 0-alanine)(0-alanine)-water, the radioactive material was recrystallized twice. The KC1 used for the calibration of the diaphragm cell was recrystallized once from water followed by centrifugal drainage. Naphthalene for the scintillation solution was recrystallized once from methanol. The rest of the materials used in preparing the solutions for counting were of reagent grade quality. The water The Journal
of
Physical Chemistry
which was used to prepare the solutions had been distilled and then deionized. Apparatus. Two diaphragm cells with magnetic stirring of the type developed by Stokes5 were used. The cells had Pyrex glass sinters of porosity No. 4 (5-10 p ) . The total volume of either cell was about 100 cc and the ratio of the volumes of the two chambers in each cell was within 3% of unity. A special bottom plug as described in ref 3 was used in all experiments. The stirring rods were rotated at 60 rpm. The cells were mounted in a thermostat which was maintained a t 25' to within =kO.Ol" with the diaphragms always within a 1" angle of being horizontal. As a precaution against the growth of microorganisms, the diaphragms were cleaned with nitric acid after each diffusion experiment. Concentrations of the solutions of KCl from calibration experiments were determined by precise conductance measurements with a Jones bridge. The oil thermostat for the conductivity cells was maintained a t 25 0.01". A new unit for liquid scintillation counting was designed and built for these experiments. This unit, which is immersed in a thermostat and held at a set temperature to within *0.02", contains two photomultiplier tubes (EM1 Type 95365) and can count radioactive events in two counting bottles a t one time. Four specially designed counting bottles are kinematically mounted in a bottle-holding assembly which in turn has four kinematic mounting positions relative to the photomultiplier tubes. Each counting bottle may be set before either photomultiplier tube. In the counting operation a pair of counting bottles, one bottle for each of the two solutions from a diffusion experiment, is used in conjunction with each photomultiplier tube. Radioactive events in each member of the pair are counted for alternate equal intervals (usually 100 sec) of time by shifting the bottle-holder assembly between two mounting positions. This procedure reduces error in the measurement of the relative counting rates of the two solutions arising from drift in the electronic apparatus. Error arising from the difference in optical efficiency of the members of each bottle pair is canceled by reversing the solutions in the pair of bottles after a first set of counting intervals and obtaining a second set of counts over a period of time equal to that for the first set of counts. After the background counting
*
(3) J. G. Albright and R. Mills, J . Phys. Chem., 69, 3120 (1965). (4)Here nbor-alanine designates an equal mixture of the D and L forms of or-alanine. (5) R. H. Stokes, J . Am. Chem. floc., 7 2 , 7 6 3 (1950).
TERNARY LABELED ALANINE-ALANINE-WATER SYSTEMS
rate, which is separately measured, is subtracted, a ratio of counting rates for the two solutions is calculated from the ratio of the number of counts accuniulated from each solution. Identical sets of electronic apparatus were used in conjunction with the two photomultiplier tubes. These consisted of a Fluke high-voltage power supply, llodel 412 A, a Franklin linear amplifier and preamplifier, Model 358, an RIDL scaler, Model 49-44, and an RIDL timer, lSlodel54-8. Counting Methods. Scintillation solutions were prepared by dissolving 20-50 g of naphthalene, 4 g of PPO (2,5-diphenyloxazole), and 0.4 g of POPOP [2,2’-p-phenylenebis(5-phenyloxazole) ] in 1 1. of dioxane. These solutions were added to the aqueous solutions from the experiments in an accurately measured ratio of about 2 : l . Difficulty with solubility was experienced with the more concentrated solutions for both of the experimental systems. It was necessary to dilute the aqueous solutions with water and to lower the concentration of naphthalene in the scintillator solutions in order to achieve solubility. After preparation of the solutions it was important to scrutinize them carefully to be sure that there was no separation of phases. Greater solubility was obtained for the more concentrated solutions of p-alanine by the addition of dilute sulfuric acid.6 I n this case, a chemical reaction seemed to occur in the solutions as evidenced by a high and decreasing background counting rate and by some color change, but after a sufficient time the color and background rate returned to normal and stable counting rates were obtainable. In these cases the solutions were allowed to stand for several days before counting. T~ reduce arising from coincidence effects, the radioactive of the two solutions from a diffusion experiment was first diluted with nonradioactive but chemically identical stock solution so that the resulting two solutions had approximately the same level of radioactivity. The dilution ratio, based on an expected value of the diffusion coefficient, was accurately determined by gravimetric measurements. Gravimetric procedures were used in all stages of the preparation of these two solutions, as well as a portion of stock solution for counting background events, so that the resulting complex solutions still had identical chemical composition. Before use, the exteriors of the counting bottles were always carefully cleaned with a cloth of a type that is commercially sold for cleaning eye glasses. The interiors were cleaned by rinsing in turn with dioxane, water, and acetone and then dried in vacuo. For most experiments the pulse height discriminator
2301
on the Franklin amplifier was set so that approximately 1000-1500 counts/sec were obtained. Lower counting rates were used in several instances to make the background counting rate a smaller fraction of the total counting rates for the solutions. Two sets of bottle pairs were used in the measurement of the ratio of the concentrations of the tracer component in the two solutions from a diffusion experiment. I n these measurements over a million counts were obtained for each solution in each bottle. The ratio measured with each of the two pairs of bottles was used to calculate a diffusion coefficient. An average of these two results gave the measured diffusion coefficient for the experiment. The difference of the two results indicated the experimental error of the counting measurements. For most experiments this error was d=O.l% or less. Calibration. By following the method described by Stokes,’ the diaphragm-cell constants were determined by calibration experiments where KC1 in a 0.5 M aqueous solution was allowed to diffuse into pure water. These constants were about 0.17 and 0.18 cmV2for the two cells that were used in this experimental work. Calibration experiments were performed at regular intervals in order to obtain the cell constants as a function of the accumulated stirring time of the cells. There was only a minor variation of the cell constants with time, and the measured constants were fitted to a straight line with a rms (root-mean-square) deviation of less than 10.1%.
Supplementary Data Mutual-Difusion Coeficients. Gutter and Kegeles8 have measured the mutual-diff usion coefficients for the system (m-a-alanine)-water by the Gouy interferometric method. They expressed their experimental results polynomia1 in by the
+
D , X 10’ = 0.9146 - 0.14277~ 0.01943~~(1) Here the subscript v designates the volume-fixed frame of reference. Donoian and Kegeless have measured the mutualdiffusion coefficient for the system (p-alanine)-water by the Gouy interferometric method. For use in the analysis of the data to be present’ed here, their data ~~~
(6) This acid was originally used because it was a safe strong acid
that did not excessively quench the scintillation process or form insoluble complexes with components of the solution. However, because of the slow chemical reaction a more suitable choice could undoubtedly be found for future experiments on similar systems. (7) R. H.Stokes, J. Am. Chem. SOC.,73, 3527 (1951). (8) F. J. Gutter and G. Kegeles, ibid., 75,3893 (1953). (9) H.C. Donohn and G. Kegeles, ibid., 83, 255 (1961).
Volume 70,Number 7 July 1966
2302
J. G. ALBRIGHT
were expressed by the following polynomial in molarity.
+
D, X lo5 = 0.939 - 0.1549~ 0.05535~20 . 0 1 1 8 ~ ~ 0.000827~~ 0 c 5.4 (2)
+
<
