Handbook of Solvents - George Wypych - ChemTech - Ventech!

Solvent Transport Phenomena
Many industrial processes rely on dissolution of raw materials and subsequent removal of
solvents by various drying process. The formation of a solution and the subsequent solvent
removal depends on a solvent transport phenomena which are determined by the properties
of the solute and the properties of the solvent. Knowledge of the solvent movement within
the solid matrix by a diffusion process is essential to design the technological processes.
Many of the final properties, such as tribological properties, mechanical toughness, optical
clarity, protection against corrosion, adhesion to substrates and reinforcing fillers,
protective properties of clothing, the quality of the coated surface, toxic residues,
morphology and residual stress, ingress of toxic substances, chemical resistance, depend
not only on the material chosen but also on the regimes of technological processes. For these
reasons, solvent transport phenomena are of interest to modern industry.
7.1 INTRODUCTION TO DIFFUSION, SWELLING, AND DRYING
George Wypych
ChemTec Laboratories, Inc., Toronto, Canada
Small molecule diffusion is the driving force behind movement of small molecules in and
out of the solid matrix. Although both swelling and drying rely on diffusion, these processes
are affected by the surfaces of solids, the concentration of small molecules in the surface
layers, the morphology of the surface, and the interface between phases in which diffusion
gradient exists. For these reasons, swelling and drying are treated as specific phenomena.
7.1.1 DIFFUSION
The free-volume theory of diffusion was developed by Vrentas and Duda. 1 This theory is
based on the assumption that movement of a small molecule (e.g., solvent) is accompanied
by a movement in the solid matrix to fill the free volume (hole) left by a displaced solvent
molecule. Several important conditions must be described to model the process. These
include the time scales of solvent movement and the movement of solid matrix (e.g.
polymer segments, called jumping units), the size of holes which may fit both solvent
molecules and jumping units, and the energy required for the diffusion to occur.
The timescale of the diffusion process is determined by the use of the diffusion
Deborah, number De, given by the following equation:
De = τ
τ
M
D
[7.1.1]
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