Liyotropik nematik fazların kararlı hale gelmesinde misel yüzeylerindeki kozmotrop-kaotrop etkileşimlerin rolü

Three series of lyotropic quaternary mixtures 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. Since the first serie consisted of KL/decanol (DeOH)/water/alkali sulfate (lithium sulfate, Li2SO4, sodium sulfate, Na2SO4, potassium sulfate, K2SO4, rubidium sulfate, Rb2SO4, and cesium sulfate, Cs2SO4), the second serie was obtained from SDS/DeOH/water/alkali sulfate lyotropic mixtures. Other mixtures were prepared by dissolution of some sodium salts of kosmotropic anions (sodium fluoride, NaF, sodium acetate, NaOAc, sodium chloride, NaCl) and of chaotropic anions (sodium bromide, NaBr, sodium nitrate, NaNO3, sodium chlorate, NaClO3, sodium iodide, NaI, sodium thiocyanate, NaSCN, and sodium perchlorate, NaClO4) in the primary mixture of TDTMABr/DeOH/water, separately. The characteristic nematic textures of uniaxial discotic nematic (ND), biaxial nematic (NB) and uniaxial calamitic (NC) were confirmed under polarizing light microscope with and without magnetic field. Laser conoscopy was employed to determine the uniaxial-to-biaixal phase transitions from the temperature dependence of the birefringences of the nematic phases. All experimental results indicated that the interactions at the micelle surfaces play a key role on the stabilization of different nematic phases. Since the kosmotrope-kosmotrope or chaotrope-chaotrope interactions between the head groups of the surfactants and the ions of the electrolytes gave rise to the stabilization of the ND phase, the opposite characters of these species, i.e. kosmotrope-chaotrope, stabilize the NB and/or NC phases. In addition, because two micellar systems, isotropic micellar solutions and lyotropic liquiud crystals, are close contact with each other, we evaluated some parameters of isotropic micellar solutions such as critical micelle concentration (cmc), degree of counterion binding (β), micellization Gibbs energy (ΔGmic) and minimum area per surfactant head group (Amin) at constant salt or electrolyte concentration and temperature. These parameters gave useful information on understanding of the interactions at micelle surfaces.