Adsorbate-Induced Anchoring Transitions of Liquid Crystals onSurfaces Presenting Metal Salts with Mixed Anions

摘要

We report that metal salts composed of mixtures of anions of differing coordination strength can be used to increase the sensitivity and selectivity of adsorbate-induced anchoring transitions of liquid crystals (LCs) supported on surfaces decorated with the metal salts. Specifically, the dynamics of anchoring transitions triggered by the adsorbate dimethyl methylphosphonate (DMMP) on surfaces of aluminum (III) salts were analyzed within the framework of a model for mass transport to reveal that the sensitivity of a nitrile-containing nematic LC to DMMP increased from 250 parts-per- billion (ppb) to 25 ppb when the composition of the (counter) anion was changed from 100% perchlorate to 90% nitrate and 10% perchlorate (by mole percent). To provide insight into these observations, Polarization-Modulation Infrared Reflectance-Absorbance Spectroscopy (PM-IRRAS) was used to show that the intensity of the absorption band in the IR spectrum corresponding to the coordinated state of the nitrile group (but not the position of the peak) decreased with increase in mole fraction of the strongly coordinating anion (nitrate) in the anion mixture, thus suggesting that the addition of the strongly coordinating anion decreased the number of coordination interactions(per unit area of the interface) but not the strength of the individualcoordination interactions between the metal cation and the LC. We also measuredthe incorporation of the nitrate anion into the metal salt to decrease theeffect of humidity on the dynamic response of the LC to DMMP, a result that isconsistent with weaker interactions between the nitrate anion and water ascompared to the perchlorate anion and water. Finally, the bidentate anionacetylacetonate was measured to cause a similar increase in sensitivity to DMMPwhen mixed with perchlorate in a 1:1 ratio (the resulting sensitivity of thesystem to DMMP was 100 ppb). Overall, these results suggest that tailoring theidentity of the anion represents a general and facile approach for tuning theorientational response of LCs supported on metal salts to targeted analytes.