A phenomenological model was developed for the average volume fraction profile through the mixing zone between turbulent gas and liquid in horizontal, layered flow. A probability distribution (PDF) of the height of the interface was defined in terms of a ratio between the interfacial energy (system energy) and the turbulent kinetic energy (driving energy), representing an analogy to the Boltzmann distribution in statistical physics. A potential advantage with this modelling approach is that it offers an alternative to (often inaccurate) entrainment correlations that are used in boundary conditions for dispersion models. A unified PDF and dispersion model was tested against X-ray tomography data, with satisfactory results. (C) 2013 Elsevier Ltd. All rights reserved.