(396h) 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