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Abstract:
The interfacial adsorption properties of several different dopants in cyanobiphenyl liquid crystals
have been measured using specular neutron reflection. It was found that a partly fluorinated
analogue of 11OCB, called F17, adsorbed strongly at the interface between 5CB and air but it
was not adsorbed at the interface between 5CB and a solid substrate treated with cetyl trimethyl
ammonium bromide (CTAB). The concentration dependence of the adsorption at the air interface
was well described by the Brunauer, Emmett and Teller (BET) model, adapted for solutions
rather than the gas phase. The isotherms are determined by two equilibrium constants: KS for
adsorption of the dopant directly at the interface and KL for adsorption onto previously adsorbed
dopant. The temperature dependence of KS indicated that the adsorption enthalpy is not
influenced by the phase of the 5CB and its value of _29 kJmol_1 is consistent with physical
adsorption. The value of KL is zero in the isotropic phase but increases rapidly on cooling in the
nematic phase suggesting that the F17 is less compatible with nematic than isotropic 5CB.
The smallest layer thicknesses (B18 A ° ) suggest that the F17 molecules are approximatel
perpendicular to the surface. The other dopants studied were components of the E7 mixture:
8OCB and 5CT. No adsorption was found for 8OCB but 5CT showed adsorption at a CTAB
treated solid interface when present in 5CB at the 10% level. In this case, the value of KS was
much smaller than for F17 but the value of KL was such that an exponential concentration
profile (predicted by the BET model) was observed with characteristic thickness of B200 A ° .
The results demonstrate the potential for very precise control of surface properties in liquid
crystal devices by using appropriate dopants
