Mechanistic Study of the Anchoring Behavior of Liquid Crystals Supported on Metal Salts and Their Orientational Responses to Dimethyl Methylphosphonate

Abstract

This paper reports an investigation of the orientational behavior of films of nematic liquid crystal (LC) 4‘-pentyl-4-biphenyl-carbonitrile (5CB) supported on a variety of metal perchlorate salts and their orientational response to dimethyl methylphosphonate (DMMP). It is found that 5CB supported on perchlorate salts comprised of metal ions with high electron affinities (>15.64 eV), such as Cu2+, Zn2+, Cd2+, Ni2+, Co2+, La3+, Al3+, Eu3+, and Fe3+, assumes a homeotropic orientation whereas 5CB supported on salts of metal ions with low electron affinities, including Mn2+, Mg2+, Ag+, Cs+, and Na+, assumes a planar or tilted orientation. The role of the nitrile group of 5CB in determining the orientational behavior of the LC via its coordination with the metal ions is supported by polarization-modulation infrared reflection−absorption spectroscopy (PM-IRRAS). A new nitrile stretching peak is measured with Cu2+, Ni2+, and Cd2+ (which cause homeotropic anchoring) but not with Ag+, Na+, and Mg2+ (which cause tilted or planar orientations of 5CB). Upon exposure to 100 ppb of DMMP, the 5CB supported on the perchlorate salts of Al3+, Eu3+, and Cu2+ undergoes a homeotropic-to-planar anchoring transition while 5CB supported on the other metal ions shows no measurable orientational response to DMMP. These results, when combined, support the view that the orientational response of 5CB to DMMP on metal perchlorate salts results from a competitive binding of 5CB and DMMP to the cations of the metal salts.