Chiral conformational fluctuations soften the twist elasticity of nematics

Achiral nematogens may exhibit transient chiral states that interconvert by thermal fluctuations due to low energetic barriers between left- and right-handed twisted conformations. Allowing for a thermodynamic equilibrium between the mirror-image enantiomers and accounting for favorable chiral interactions between like enantiomeric species, we show by mean-field theory that a mechanical twist deformation in a slab geometry deracemizes such compounds. This, in turn, leads to a softening of the twist elastic response of the nematic. We find that the twist elastic constant vanishes at a second order transition to a state where cholesteric phases with opposite handedness and enantiomeric excess coexist. Even though this transition is usually hidden by a transition to, e.g., a crystalline state, we predict that the twist constant could develop an unusual temperature dependence due to pretransitional effects.