By using electron spin resonance spectroscopy to measure the concen tration of the aromatic radical formed during a reaction, the electron-attract ing properties of the zeolite can be compared for different cage cations. In a series of experiments, the cation is changed systematically through the alkaline earth and transition series. Dr. Richardson finds an exponential relationship between the number of radicals formed and the electron affin ity of the cation. He cautions that the electron affinity of a cation is modified by the dielectric and crystal field properties of the surface, but it may still be considered representa tive of the charge attraction. The degree of ionization also is exponentially related to the ioniza tion energy of the aromatic. Thus, the ionization mechanism is similar to charge transfer complex forma tion, Dr. Richardson points out. This is because the driving force to ionize an aromatic molecule involves the difference between the ionization po tential of the molecule and the elec tron affinity of the cation in the zeo lite. His experimental results display an interesting synergistic effect when small amounts of copper ion are added to ±he zeolites, he says. The electron affinity of the copper increases as other cage cations change from sodium through the alkali and alkaline earth series to magnesium. Dr. Richardson believes this effect is due to perturbations in the copper energy levels caused by neighboring oxygen ions which are themselves perturbed by the ionic potentials of the major cations. An understanding of this relationship could play an im portant part in unraveling promotion effects in catalysis.
Dr. James T. Richardson Change cation systematically
MW distribution by osmodialysis Scientists at Esso Research and Engi neering have developed a quick and versatile way to determine the molecu lar weight distribution of low-molecu lar-weight polymers. The method— osmodialysis—consists of following the progress of dilute toluene solu tions of polymers as they permeate a gel cellophane membrane. This is done by using a conventional self-bal ancing osmometer to plot apparent osmotic pressure against time. Dr. Boyd E. Hudson of Esso's ana lytical research division told the Di vision of Colloid Chemistry during the 152nd ACS National Meeting that osmodialysis works with molecular weights from 500 to 25,000. This range makes the method useful in studies of many oils, gums, adhesives, plasticizers, and mastics. Osmodialysis has several advantages over other molecular weight methods. It works in a range below that pos sible with classical membrane os mometry. Also, it is faster than frac tional precipitation, as individual frac tions do not have to be isolated. And, unlike gel permeation methods, os modialysis is independent of the chemical composition of polymers; and it works with polymer mixtures. Dr. Hudson explains that osmosis and dialysis, the two phenomena com bined in the method, have been con sidered by scientists to be mutually preclusive. This is because full the oretical osmotic pressure—a measure of molecular weight—cannot be de termined while permeation is taking place. However, the plots of the de cay of apparent osmotic pressure ob tained by the new method with time can be simply interpreted to give molecular weight distributions. The interpretation depends on the follow ing facts about the behavior of dilute solutions of low-molecular-weight polymers: • The apparent osmotic pressure (P) does not depend on molecular weight ( M ) , but it depends on the weight concentration of diffusible polymer in the solution. • The plot of log Ρ vs. time of a narrow molecular weight fraction is a straight line of characteristic slope or half-life (ii/ 2 ). • 11/2 is proportional to M 2 for a given cell membrane; gel cellophane 600 was used in this Esso work. • In about four half-lives, the poly mer having the lowest molecular weight is so depleted that the next higher molecular weight group can be characterized on the basis of its per meation rate. To calculate molecular weight dis tribution, the Esso workers plot the log of apparent osmotic pressure
Esso's Hudson Molecular weights from 500 to 25,000
against time—four hours, for example. They then draw the tangent to the lower part of this slope—say from two to four hours—and extrapolate it back to zero time. The slope of this line gives the molecular weight of the highest molecular weight frac tion. The point where this extrap olated tangent cuts the zero time axis gives the amount of this fraction in the intial polymer solution. The Esso workers then subtract the contribution of this tangent to ap parent osmotic pressure from the meas ured total pressure and plot a new graph. They draw the tangent to the lowest part of the new curve and so identify the second highest molec ular weight fraction. They repeat this process until three or four frac tions are identified for molecular weight and abundance. A simple arithmetic calculation then gives them the mean molecular weight of the entire system. Dr. Hudson points out that osmo dialysis has one weakness. It may give mean molecular weights that are low. This is because high-molecularweight components that will not permeate the membrane may be pres ent. Esso workers are trying to de velop a simple method to assay such "heavy ends." SEPT. 19, 1966 C&EN 23
