Adsorbate-Induced Ordering Transitions of Nematic Liquid Crystals on Surfaces Decorated with Aluminum Perchlorate Salts

We report an investigation of interfacial physicochemical phenomena underlying adsorbate-induced ordering transitions in nitrile-containing liquid crystals (LCs) supported on surfaces coated with metal perchlorate salts. When the mass density of Al(ClO4)(3) deposited onto a surface was low (0.39 +/- 0.03 ng/mm(2)), we measured 20 mu m thick films of nematic 4-cyano-4'-n-pentyl-biphenyl (5CB) to initially exhibit perpendicular (homeotropic) ordering, consistent with the influence of coordination interactions between the nitrile groups of 5CB and Al3+ ions on the surface on the ordering of the LC. Furthermore, exposure of freshly prepared LC films to vapors of an adsorbate that coordinates strongly to Al3+ ions (dimethylmethylphosphonate, DMMP) triggered an ordering transition in the LC films, supporting our conclusion that the initial perpendicular orientation of the LC was induced by nitrile-Al3+ coordination interactions. Subsequent equilibration of the LC on the surface (hours), however, resulted in a slow, time-dependent ordering transition in the absence of DMMP that corresponded to the tilting of the LC away from the surface normal. Measurements of the solubility of Al(ClO4)(3) in nematic 5CB (saturation value of 1.7 mu mol/mL) supported our hypothesis that the slow ordering transition observed in the absence of DMMP was due to the loss of metal ions from the surface into the LC film (dissolution). In contrast, the solubilization capacity of a 20 mu m thick film of 5CB was determined to be insufficient to dissolve 2.14 +/- 0.24 ng/mm(2) of the salt from a surface, and we measured the homeotropic ordering of nematic films of 5CB on these surfaces to persist for days. Equilibration of these samples, however, was accompanied by a loss of response to DMMP (perpendicular orientation of the LC before and after exposure to DMMP). Control experiments performed with a noncoordinating metal perchlorate salt (sodium perchlorate) confirmed our proposition that the loss of response to DMMP was due to formation of an electrical double layer that promoted perpendicular ordering of the LC and thus masked the effects of changes in coordination interactions induced by DMMP on the ordering of the LC (the electric field of the double layer promotes the perpendicular orientation of the 5CB). Finally, by coating surfaces with Al(ClO4)(3) at loadings that were intermediate to those reported above (1.13 +/- 0.09 ng/mm(2)), we observed (i) perpendicular ordering of the LC in the absence of DMMP, and (ii) reversible ordering transitions induced by DMMP during storage of samples over 4-5 days. These results, when combined, indicate that the ordering of the LC on the metal-salt-decorated surfaces is strongly dependent on the loading of metal salt, with key interfacial physicochemical processes being metal ion-nitrile coordination interactions, dissolution of salt into the LC, and formation of electrical double layers. The results of this study provide guidance for the design of LC films that respond to specific chemical analytes and suggest principles for passive chemical sensors.