Isotherm charts for material selection and method development with molecularly imprinted polymers and other sorbents

Abstract

A simple and efficient method is presented for assessing molecularly imprinted polymers (MIP) and other sorbents from the point of view of practical applications. The adsorption isotherms of the compounds, which need to be separated or detected in an application, are constructed from a small number of measured points on a log-log chart and then are compared graphically. Despite its simplicity and robustness this method reveals the information needed for optimal selection between MIPs and alternative sorbents. The design of separation or detection methods with MIPs is also supported by the proposed graphical isotherm comparison. Many experimental isotherms are presented supporting the proposed method.

The Selectivity of Polymers Imprinted with Amines

“…One may plot and compare in the same diagram the isotherms of several compounds on the same MIP, or the isotherms of a single compound on an MIP and its NIP, respectively, or the isotherms of the same compound on two different MIPs. Like in many other publications, we have also found [ 11 ] that such log c -log q plots are often linear in a quite wide range of the respective concentrations. (Note that this linearity means that the system observes the Freundlich equation, but, in the present context, there is no attempt to interpret the isotherms with any model.)…”

“…By looking at Figures 4 and 5 of our earlier paper [ 11 ], one may additionally observe that the log c -log q adsorption isotherms of different amines on the propranolol MIP are not only straight lines, but they are also approximately parallel with each other. The isotherms of the same amines on the NIP are also approximately parallel, albeit with a different slope, than on the MIP.…”

“…We have shown recently [ 11 ] that the adsorption isotherms of MIPs can conveniently be compared in log c -log q plots, i.e., in figures where the logarithm of the equilibrium adsorbed concentration of an investigated compound on an MIP (or on a NIP) is plotted as a function of the compound’s logarithmic equilibrium solution concentration. One may plot and compare in the same diagram the isotherms of several compounds on the same MIP, or the isotherms of a single compound on an MIP and its NIP, respectively, or the isotherms of the same compound on two different MIPs.…”

“…(Note that this linearity means that the system observes the Freundlich equation, but, in the present context, there is no attempt to interpret the isotherms with any model.) We have also shown [ 11 ] that the vertical distance between the isotherms may be interpreted as the logarithm of the selectivity (when the adsorption of two compounds is studied on the same polymer) or as the logarithm of the imprinting factor (when the adsorption of the template is plotted on the MIP and the NIP, respectively). This interpretation of the imprinting factor is very close to that of Lanza and Sellergren [ 25 ].…”

“…As we have recently shown, Ref. [ 11 ], claims about MIP selectivity need to be carefully stated. It was found that an MIP imprinted noncovalently with the beta blocker compound propranolol could bind only about five times as much propranolol as dibenzylamine (DBA) under identical conditions.…”