Effect of Hofmeister anions on the existence of the biaxial nematic phase in lyotropic mixtures of dodecyltrimethylammonium bromide/sodium salt/1-dodecanol/water

Abstract

Lyotropic mixtures including different sodium salts of Hofmeister anions were studied in order to investigate the effect of these anions on the existence of biaxial nematic phase and on the uniaxial to biaxial phase transitions. For this purpose, these sodium salts were added singly into different mixtures of dodecyltrimethylammonium bromide (DDTMABr)/1-dodecanol/water, keeping the relative molar concentration of all the constituents constant. The uniaxial to biaxial phase transitions were determined from the temperature dependence of the birefringences by laser conoscopy. Micellar shape anisometry and average micellar volume were evaluated from small-angle X-ray scattering measurements. The results indicated that the Hofmeister anions were bounded to the head groups of DDTMABr molecules at the micelles' surfaces, which significantly affect the different orientational fluctuations responsible for the formation of different nematic phases, biaxial phase domains and uniaxial to biaxial phase transition temperatures.

Experimental Conditions for the Stabilization of the Lyotropic Biaxial Nematic Mesophase

“…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. Sodium salts of some Hofmeister ions were added [12] to the host lyotropic mixture of dodecyltrimethylammonium bromide (DTMABr)/dodecanol (DDeOH)/water. The results showed that the interactions between the positively charged head group of DTMABr, trimethylammonium, and the Hofmeister series kosmotropic (SO4 2− , F − , Cl − ), and chaotropic (Br − , NO3 − , I − , ClO4 − ) anions play an important role on stabilizing the lyotropic nematic phases.…”

“…Although the results of [12] provided useful results about the stabilization the different nematic phases, it was necessary to generalize the role of specific interactions on the micelle's surfaces in terms of the kosmotropic and chaotropic properties of both the surfactant head groups and the counterions/ions. For this purpose, three surfactant molecules were selected, two of them with chaotropic head groups (one positively charged, the other negatively charged) and one with a negatively charged kosmotropic head group.…”

“…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].…”

“…In the case of the specific interactions between head groups of the surfactants and the counterions/ions of strong electrolytes, in terms of their kosmotropic and chaotropic characters, choosing the surfactant and the electrolyte with slightly opposite (strongly same) character may help to stabilize a lyotropic mixture of the N B (N D ) phase. Strongly opposite characters of both ionic species, head groups and the ions present in the mixture, stabilizes the N C phase [12,14].…”

Effect of Dopant Structure on Birefringences and Symmetric Tensor Invariants of Lyotropic Nematic Phases

“…Three different LNPs are defined in the literature. They are discotic nematic (ND), calamitic nematic (NC) and biaxial nematic (NB) phases [6][7][8][9]. From the optics point of view, the first two ones are uniaxial and the latter one is biaxial.…”