Mass transport across a phase boundary (solid–liquid, liquid–gas, liquid–liquid, or solid–gas) driven by differences in concentration, partial pressure, or chemical potential, where the interfacial region and its local equilibrium or kinetics control the net flux between phases.

Principle

Net interfacial flux equals the product of a driving potential (difference in concentration, fugacity or chemical potential between adjacent phases or between a bulk and the interface) and an interfacial conductance (mass‑transfer coefficient); if interfacial chemical equilibrium holds then interface concentrations are related by equilibrium relations, otherwise kinetic resistance at the interface modifies flux independently of bulk transport.

Demonstration

Illustrative scenario — Gas absorption into a stirred liquid with a soluble gas: The gas concentration in the bulk gas sets a surface partial pressure; Henry’s law relates surface concentration in the liquid if local equilibrium is assumed (Recognition). A concentration boundary layer in the liquid impedes transport from the bulk liquid to the interface; if stirring increases, the boundary layer thins and absorption rate rises (Action). Observed uptake depends on both interfacial equilibrium/kinetics and boundary‑layer diffusion (Consequence).

Misapplication

Using bulk phase concentrations or partial pressures as though they were equal to interface concentrations (for example substituting bulk liquid concentration into an equilibrium relation) when boundary‑layer resistance or finite interfacial kinetics exist; this neglects the distinct resistances in series and overestimates transfer rates.

Consequence

Misjudging interfacial control leads to incorrect design of mass‑transfer equipment (absorption columns, stripping, extraction), inefficient solvent usage, inadequate contactor sizing, or failure to meet separation specifications under operating conditions.

Reversal

When turbulence or active mixing eliminates boundary layers so rapidly that bulk and interface concentrations equilibrate on the timescale of interest, interface resistance becomes negligible and bulk transport controls; alternatively, when rapid interfacial reactions consume species immediately at the surface, equilibrium assumptions fail and reaction‑limited models are required.

Boundary

Clearly within: gas–liquid absorption with significant boundary layers, liquid–liquid extraction where interfacial resistance limits rate, adsorption at solid surfaces with finite kinetics. Boundary case: slow agitation where both interface kinetics and bulk diffusion contribute comparably. Clearly outside: homogeneous bulk diffusion with no phase boundary or well‑mixed batch reactor where phases are miscible and no lasting interface exists.

Semantic Tension

Equilibrium assumption versus kinetic limitation: choosing an equilibrium model simplifies analysis but may ignore finite interfacial kinetics or boundary‑layer resistance; the correct choice depends on timescales, mixing, and reaction at the interface.

Synthesis

Practical assessment requires separating resistances in series (gas film, interface, liquid film, bulk) and determining which is rate‑controlling; design and scale‑up decisions follow from whether interface kinetics, film diffusion, or bulk mixing dominates under operating conditions.