Counterion and composition effects on the size and interface dynamics of discotic nematic lyotropic liquid crystals made of tetradecyltrimethylammonium halide (TTAX)-decanol (DeOH)-water-NaX, with X = Cl- and Br-, were investigated using NMR and fluorescence spectroscopies. The dynamics of the interface was examined by measuring deuterium quadrupole splittings from HDO (0.1% D2O in H2O) and 1,1-dideuterodecanol (20% 1,1-dideuterodecanol in DeOH) in 27 samples of each liquid crystal. Aggregation numbers, N-D, from 15 samples of each mesophase were obtained using the fluorescence of pyrene quenched by hexadecylpyridinium chloride. N-D of TTAB and TTAC are about 230 30 and 300 20, respectively. ND of TTAC increases with increasing concentration of all mesophase components, whereas TTAB shows no correlation between size and composition. The dimension of these aggregates prevents the occurrence of undulations, previously observed in lamellar phases. The quadrupole splitting of decanol-d(2) in TTAC is about 5 kHz smaller than in TTAB, and the splitting of HDO is observed only in TTAB. All results are consistent with a more dynamic TTAC interface. The TTAC aggregate should be more dissociated from counterions and the excess ammonium-ammonium electrostatic repulsions contribute to increase the mobility of the interface components. (C) 2007 Elsevier Inc. All rights reserved.