The long-range orientational order of some fluoro-substituted liquid crystals with different molecular structure was determined on the basis of the optical birefringence measurements. The high values of the second-rank order parameter in the whole mesophase range for the liquid crystals studied were found. The liquid crystals with the negative dielectric anisotropy were mixed with an arachidic acid at various molar fractions. The behavior of the mixtures in monolayers formed by using Langmuir-Blodgett technique was investigated.