Ikkeideelle

Ikkeideell is a Norwegian term used to describe systems, models, or materials that do not conform to the simplifying assumptions of an ideal model. The concept is widely applied in science and engineering to acknowledge and quantify deviations from ideal behavior.

In thermodynamics, ikkeideell behavior is most often discussed with gases and fluids. An ideal gas obeys PV = nRT at all conditions, but real gases exhibit deviations due to finite molecular size and intermolecular forces. These deviations are captured by real-fluid equations of state, such as the van der Waals, Redlich–Kwong, or Peng–Robinson equations. A common measure is the compressibility factor Z = PV/(nRT), which differs from 1 for ikkeideelle gases, especially at high pressures or low temperatures and near phase transitions.

In solutions and mixtures, ikkeideell behavior appears as departures from Raoult’s law. The concept of activity and activity coefficients (gamma_i) is used to describe how interactions between different species affect chemical potential and composition. In such systems, the effective concentration driving force is a_i = gamma_i x_i rather than the ideal a_i = x_i.

The term also applies to non-ideal fluids in rheology, where viscosity and flow behavior differ from Newtonian models, and to broader modeling contexts where interactions, finite size, or structural effects lead to deviations from ideal assumptions. Understanding ikkeideell behavior is essential for accurate engineering design, process simulation, and material science.