Kinetics Study of Formation of the Permselective Polyamide Layer in Thin Film Composite Membranes

Kinetics study of formation of the permselective polyamide layer in Thin Film Composite membranes

Subhalaxmi Behera, A.K Suresh

Department of Chemical Engineering, Indian Institute of Technology Bombay, Mumbai - 400076

email: subha.b@iitb.ac.in, aksuresh@iitb.ac.in

Abstract

Thin film composite membranes, used extensively in reverse osmosis and other separation applications, depend for their function on a thin (100-200 nm) dense layer, which is formed on the surface of a macroporous support membrane by an interfacial polycondensation reaction. The properties of this dense layer depend on the conditions under which the interfacial reaction is carried out, through the kinetics. However, the kinetics of this process are extremely difficult to study, firstly because it occurs in a very thin region on the support membrane (and possibly inside the pores), and secondly because it is a very fast reaction with timescales of the order of seconds. A method to study the kinetics of the reaction, and hence understand the effect of preparation parameters on the rate and evolution of structure, would thus be of great value, not only for the membrane application, but also in other areas such as microencapsulation where such reactions are used.

Our approach to the study of interfacial polycondensation kinetics is developed in three parts. Firstly, we study the reaction in isolation in a liquid-liquid system, separated from the support membrane, to facilitate measurements. Secondly, we control the overall rate of monomer consumption by controlling the interfacial area available. These two steps are similar to our earlier studies of the polyurea microencapsulation reaction. Thirdly, we exploit the fact that the reaction involves (a) consumption of an alkaline monomer (a diamine) and (b) release of an acid byproduct (HCl), both of which should contribute to a decrease in pH as the reaction progresses, and hence use a fast-response on-line pH probe to follow the reaction. The method thus involves conducting the reaction in a dilute emulson, in which the aqueous phase forms the continuous phase, with the pH of the continuous phase being followed with time. A calibration curve, to allow measured pH to be translated to monomer conversion, is developed by measurements of pH of samples, which simulate different conversions. While the reaction configuration used is precisely that of microencapsulation, the determination of true kinetics requires a clean separation of reaction kinetic and transport influences on the rate. This is achieved here by conducting the reaction at high shear rates (5000 RPM in a homogenizer), so that the polymer formed is instantaneously stripped off the interface and transport limitations are eliminated.

The above methodology has been demonstrated in this work on the reaction between an aqueous solution of m-phynelenediamine (MPDA) and an organic solution of trimesoyl chloride (TMC). The variation of pH with conversion has been theoretically modeled and experimentally established. It is shown that the method does yield consistent and reproducible kinetic data. The method has been used to study the effect of surfactant concentration and monomer concentration ratio on the kinetics. Surfactant concentration is shown to influence the rate of the reaction, and hence the rates of the same reaction could be different when used for microencapsulation, from when used for TFC membranes. A limitation of the method is that it cannot be used when the aqueous monomer is in large excess, but this condition can be approached through kinetic modeling. The kinetic modeling is done assuming it's a reaction controlled process.

 

Keywords: Interfacial polycondensation kinetics, thin film composite membrane, Micro-encapsulation