Role of kosmotrope-chaotrope interactions at micelle surfaces on the stabilization of lyotropic nematic phases

Abstract

Three lyotropic quaternary systems of ionic surfactants were prepared to investigate the role of kosmotrope-chaotrope interactions at the micelle surfaces on stabilizing the different nematic phases. The ionic surfactants were potassium laurate (KL), sodium dodecylsulfate (SDS) and tetradecyltrimethylammonium bromide (TDTMABr), where KL is a kosmotrope surfactant, and others are chaotrope. The first system consisted of KL/decanol (DeOH)/water/alkali sulfate and the second of SDS/DeOH/water/alkali sulfate. The third system was prepared by adding sodium salts of chaotropic or kosmotropic anions to the primary mixture of TDTMABr/DeOH/water, separately. The characteristic textures of discotic nematic (N ), biaxial nematic (N) and calamitic nematic (N ) phases were identified under polarizing light microscope. Laser conoscopy was employed to determine the uniaxial-to-biaxial phase transitions. The kosmotrope-kosmotrope or chaotrope-chaotrope interactions between the head groups of the surfactants and the ions of the electrolytes led to the stabilization of the N phase. On the other hand, kosmotrope-chaotrope interactions stabilize the N and/or N phases.

Maier-Saupe model for a mixture of uniaxial and biaxial molecules

“…In some recent publications, Akpinar, Reis, and Figueiredo-Neto [23][24] reported X-ray and optical characterizations of biaxial nematic structures in a large class of quaternary liquid-crystalline mixtures. From these measurements, it has been possible to establish many novel phase diagrams in terms of temperature and molar fraction of the components, which represents a real advance with respect to the early work of Yu and Saupe for a ternary The present calculations, for the elementary MS6 lattice model, at the mean-field level, are a contribution to the understanding of the effects of shape fluctuations on the thermodynamic behavior of complex liquid-crystalline systems.…”

Experimental Conditions for the Stabilization of the Lyotropic Biaxial Nematic Mesophase

“…The alkali salts (sodium salts) were Li2SO4, Na2SO4, K2SO4, Rb2SO4 and Cs2SO4 (Na2SO4, NaF, NaCl, NaBr, NaNO3, NaClO4 and NaSCN). The results [14] indicated that when the surfactant head groups and the ions bound to them at the micelle's surface exhibit the same property in terms of the chaotropic or kosmotropic The results discussed in [88] may be considered as preliminary. For this reason, complementary investigations were needed to understand the effect of the specific interactions between the ionic species at the micelle surfaces on the stabilization of the lyotropic nematic phases, especially the biaxial.…”

“…In the phase diagrams of lyotropic mixtures presenting the three nematic phases, the N B phase is mainly located between the other two uniaxial phases [1,2,[7][8][9][10][11][12][13][14][15]. The transition from the uniaxial (N D or N C ) to the biaxial phase is of second order, as predicted by the Landau-type mean-field theory [16,17].…”

Anomalous Behavior in the Crossover between the Negative and Positive Biaxial Nematic Mesophases in a Lyotropic Liquid Crystal

“…The subscripts 1, 2, and 3 refer to the three orthogonal axes of the laboratory frame. [11] In the N D and N C phase domains, Dn = 0, dn ¼ 6 0, and Dn ¼ 6 0, dn = 0, respectively. In the N B phase, Dn ¼ 6 0, dn ¼ 6 0, and N À B ðN þ B Þ is characterized simply by dn > Dn (Dn > dn).…”

“…The crossover from N À B to N þ B corresponds to the point at which the relative biaxiality in the refractive index passes through an extreme. [11,14] The partial phase diagram of the mixture investigated is shown in Figure 4. The temperature domain of the N B phase is almost in-dependent of the percentage molar fraction of the main amphiphile; nevertheless, at low concentrations, the N B domain is larger than that at high concentrations.…”