diffusibility

Diffusibility is the ease with which a substance spreads through a medium by diffusion. It is a property that describes how readily molecules move from regions of higher concentration to lower concentration when driven by a concentration gradient. In many contexts diffusibility is closely related to, but not identical with, diffusivity and permeability. Diffusivity (or the diffusion coefficient, D) is a physical property of the diffusing species in a given medium, while diffusibility emphasizes the observable rate of transport across a particular barrier or within a specific environment.

Under Fick's laws, the flux J of diffusing material is proportional to the negative gradient of concentration: J = -D dC/dx. A larger D or a steeper gradient yields greater diffusibility. D depends on temperature, viscosity, molecular size and shape, solubility, and interactions with the medium or barrier. In membranes, diffusibility also depends on how well the molecule partitions between phases (partition coefficient) and on membrane structure.

Measurement and relevance: Diffusibility is assessed by measuring flux across a barrier, steady-state permeability, or diffusion coefficients via methods such as tracer diffusion, FRAP, or diffusion NMR. It is a central consideration in pharmacology (drug diffusion across membranes), physiology (gas exchange, nutrient transport), and materials science (polymer diffusion, corrosion).

Limitations: Real systems may involve active transport, chemical reactions, or convective transport that modify apparent diffusibility. The term is sometimes used interchangeably with diffusivity, but practitioners should specify the medium and barrier to avoid ambiguity.