Microsecond Time-Resolved Infrared Study of the Electric-Field-Induced Reorientation of Nematic Liquid Crystals. Part II: Dependence of the Motion of Liquid Crystals on the Applied Electric Field

The electric-field dependence of the reorientation motion of a nematic liquid crystal, 5CB (4- n-pentyl-4'-cyanobiphenyl), has been studied by microsecond time-resolved infrared spectroscopy. A rectangular pulsed electric field with a short pulse duration (2 ms) and a low repetition rate (5 Hz) was used to examine the liquid crystal (LC) response in a silicon cell. The motion of the rigid core part (the cyanobiphenyl group) of 5CB was monitored by the CN stretch band and that of the flexible part (the pentyl group) by the pentyl CH stretch band. The response of the LC to the pulsed electric field consists of two components, the slow component and the fast component. The slow component is common to the rigid core and the flexible parts of SCB. The voltage dependence of the slow component exhibits a clear threshold, indicating that this component corresponds to a cooperative motion of the 5CB molecules. The fast component is specific to the flexible part and shows exponential rise and decay behavior patterns. This observation suggests that the fast component corresponds to some noncooperative motions which are characteristic of the pentyl group. It is most likely that the internal rotation around the C(biphenyl)-C(pentyl) bond is responsible for the fast component. It is suggested that the LC molecules near the cell interface play a key role in the primary stage of the reorientation motion under an applied electric field.

35Cl NQR and Crystal Structure Studies of Salts of Chlorodifluoro- and Dichloroacetic Acid

The 35Cl NQR spectra of several chlorodifluoroacetates were studied as a function of temperature, including the acid ClF2CCOOH. The cations were: Ammonium, guanidinium, paramethylanilinium. Also some acid salts M⊕ClF2CCOO⊖ • n - ClF2CCOOH ( n > l ) were studied by 35Cl NQR. The bleaching temperatures of the NQR signals were determined. In the para-methylanilinium salt and in the guanidinium salt a phase transition has been observed. The crystal structure of guanidinium chlorodifluoroacetate has been determined at room temperature (a = 1089 pm, 6 = 845 pm, c = 832 pm, space group Pnma, Z = 4). For comparison, guanidinium dichloroacetate was studied by 35Cl NQR and by X-ray diffraction, too: P21/c, Z = 4 , a = 804pm, b = 1202 pm, c = 1080 pm, ß = 131.58°. For guanidinium chlorodifluoroacetate and chlorodifluoroacetic acid, the 35Cl spin lattice relaxation time T1 and the line width have been followed up as a function of temperature. Therefrom, the activation energies of the reorientation motion of the group -CF2C1 have been determined to be 14 kJ • mol-1 (from T1) and 12.5 kJ • mol- 1 (from Δv) for the pure acid and 9.2 kJ • mol-1 and 8.8 kJ • mol-1 , respectively, for the guanidinium salt.

Compressibilities of aqueous electrolyte solutions

The Tammann-Tait-Gibson (TTG) model was used to derive and analyse an equation for the isothermal compressibility of aqueous electrolyte solutions as a function of temperature and pressure. The linear equation of Φk in c½ (Φk is the apparent molal compressibility, c is in mol 1-1) is shown to be inadequate. The best function in the square-root of the concentration [either in (mol kg-1) or c] is degree three. This gives the correct limiting slope predicted by the TTG model, viz., Sv/(BT + 1), where S, is the Masson slope for apparent molal volumes and BT is the Tait parameter for pure water. This slope was verified previously by comparison with the limiting slope obtained experi- mentally, and can be predicted from the standard ionic entropies. The difference in the TTG slope and the Debye-Hiickel point-charge slope is proportional to changes in the reorientation motion of water molecules close to the ionic surface. The electrostriction component, Vcelect, of the limiting partial molal volume is equal to - K°(BT+ 1), where K° is the limiting partial molal compressibility. Values of V°elect calculated from this relationship are compared with values from other models. The TTG model was used to derive internal pressure functions which could be used to analyse deviations of V°elect from the NIH (non-interacting homomorph) model. The TTG equations were used to calculate the isothermal compressibilities of 15 electrolytes. The agreement with experimental values is good (deviations are less than 0.1 × 10-6cm3 g-1 bar-1 for βv for the most reliable data). Values of Φk at 200 bar were calculated also and are in good agreement with the corresponding experimental values.