Adsorption of a Homologous Series of Aliphatic Acids on a Nonionic Adsorbent HENRY GABRIEL‘ AND ROBERT J. COOLEY2 University of Santa Clara, Santa Clara, Calif.
S
TUDIES of adsorption phenomena are becoming increasingly important with the widespread use of ion exchange, chromatography, and adsorption resins. While, in general, resins can be classified and the trend of specific properties predicted by theory, it is often desirable to study and evaluate the properties of new resins. This paper describes a study of Duolite 5-30 ( a resin produced by the Chemical Process Co., Redwood City, Calif.). This resin, which was prepared for use as an adsorbent in cane sugar refining and pineapple juice clarification, was described as a specially activated phenol-formaldehyde polymer, nonionic in character, with little affinity for strong electrolytes like hydrochloric acid and alkali salts. However, as many of the impurities which it might remove would be weak electrolytes, it was considered desirable to ascertain the adsorbability of weak electrolytes. If adsorption took place, the probable mechanism of the adsorption was to be determined as well as the effect of increasing ionic strength and lengthening of the chain. At the same time, the technique used in evaluating this resin might be applicable to other new resins as they appeared. THEORY
The theories of adsorption have been discussed in several recent publications. The points pertinent t o this investigation are briefly stated here. Two types of adsorption are usually distinguished. The first, due to pure physical van der Waals forces, called physical adsorption, is weak and the adsorbate is usually easily removed from the adsorbent. . The second type, ‘(chemisorption,” stipulates that a 0.25 chemical reaction takes place a t the surface between adsorbate and adsorbent. The adsorbate is not always easily removed and when removed may 0.20 exist in a different form-i.e., as a new complex or ion, or in a reduced or oxidized state. A third, interme0.1 5 diate type of adsorption, called “perI sorption,” is thought to exist, in which 4 the resin acts as a molecular sieve 0 E‘ and the size of the adsorbed molecule --. is a determining factor (4). xo.lo The differences between the first two types of adsorption were summarized by Brunauer ( I ) . 0.05 The operation of van der Waals adsorption and/or persorption would tend t o make size and structure relatively more important than polarity and ionization. Thus by studying a address, Chemistry Department, Biena College, Loudonville, N. Y . 2 Present address, Chemical Prooess Co., Bedwood City, Cdif.
comparatively small number of compounds which have definite structural variation and varying polarity, the nature of the adsorption process could be determined. I n this study, the adsorption isotherms were determined for a number of closely related aliphatic acids. The results were plotted for the Freundlich equation, in which a plot of log ( z / m ) against log C gives a straight line, if the equation is applicable, as well as for the Langmuir equation, which is considered applicable if a plot of log ( x / m ) (l/C) against log x / m produces a straight line. EXPERIMENTAL
Duolite S-30 is a phenol-formaldehyde polymer which has been specially activated in order to enhance its adsorptive properties. It is an extremely porous material and is inert t o most reagents and common solvents even at elevated temperatures. The resin conforms with the physical requirements of a good adsorbent. The resin, as received in this laboratory, was in granular form selectively screened to 30- t o 50-mesh size, U. S. screen standard. In order to avoid complication of possible formaldehyde contamination, the resin was soaked in distilled water a t room temperature for 1 hour and in water a t 90” C. for 0.5 hour. The resin was then “cycled” by slurrying in a 5% solution of sodium hydroxide, followed by a slurry in 5% hydrochloric acid. After standing in the acid solution overnight, the resin was rinsed repeatedly until the rinse was free from chloride and its p H was close to 7 . The resin was then dried to constant weight. Some difficulty was encountered here due to the porosity of the resin. The resin was first air-dried a t room temperature for 24 hours and then
3
0.s
C
1 Present
1.5
1 .O EQ/L
Figure 1. Freundlich curves 1. n-Butyric acid
1236
2.
Propionic acid
3.
Acetic acid
2.0
INDUSTRIAL AND ENGINEERING CHEMISTRY
June 1955
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were pi etted into each of the flasks. Each o f the 12 samples of resin and supernatant acid solution was agitated simultaneously by means of a multiple stirrer arrangement while in a constant temperature bath. At the end of a minimum of 48 hours, aliquot portions of the acid were taken and titrated with standard alkali.
0.95
0.20
The results were calculated with the Freundlich adsorption isotherm in mind. The following working formulas were used for the point determinations. (titer diff., ml.) ( N NaOH) (aliq. factor) (millieq. wt.) x / m = weight of resin sample, grams C = ( N NaOH) (equilibrium titer, ml.)
x
d 0.10
0.05
SUMMARY OF RESULTS
1.o C = EQ/L
0.5
From the adsorption isotherms in formation regarding a process of this nature may be obtained. Certain criteria must be met if the Freundlich and Langmuir equations are to describe an adsorption correctly. These criteria necessitate graphs
J
I
I
1.5
2.0
~
Figure 2. J.
2.
Freundlich curves
Trichloroacetic acid Dichloroacetic acid
3. 4.
Chloroacetic acid Acetic acid
daced in a Cenco-de Khotinskv air-circulating oven in lavers 5 b m . in depth. The temperatuie was maintained at 54" c. and in less than 24 hours constant weight was reached. A drying temperature higher than 55" C. should be avoided, as the porosity of the resin seems to be affected. The resin was kept fn tightly sealed, preheated jars placed in desiccators. The followi
