Effect of alkyl chain length of alcohols on nematic uniaxial-to-biaxial phase transitions in a potassium laurate/alcohol/K₂SO₄/water lyotropic mixture

Abstract

Lyotropic liquid crystalline quaternary mixtures of potassium laurate (KL), potassium sulphate (K 2 SO 4 )/alcohol (n-OH)/water, with the alcohols having different number of carbon atoms in the alkyl chain ( n), from 1-octanol to 1-hexadecanol, were investigated by optical techniques (optical microscopy and laser conoscopy). The biaxial nematic phase domain is present in a window of values of n = n KL ± 2, where n KL = 11 is the number of carbon atoms in the alkyl chain of KL. The biaxial phase domain got smaller and the uniaxial-to-biaxial phase transition temperatures shifted to the relatively higher temperatures on going from 1-nonanol to 1-tridecanol. Moreover, these new mixtures present high values of the birefringences comparing to other lyotropic mixtures. This result is expected to be related to the micellar shape anisotropy. Our results are interpreted assuming that alcohol molecules tend to segregate in the micelles in a way that depends on the relative value of n with respect to n KL .The larger the value of n, the more alcohol molecules tend to be located in the curved parts of the micelle, favoring the uniaxial nematic calamitic phase with respect to the biaxial and uniaxial discotic nematic phases.

Investigation of the interaction of alkali ions with surfactant head groups for the formation of lyotropic biaxial nematic phase via optical birefringence measurements

“…The large value of the birefringence in the N D phase of the Na 2 SO 4 mixture (~8×10 -3 at 25°C) indicates the existence of more anisometric micelles (or reveals a larger molecular segregation in the micelle). 15,18,19,36 The reason why we could not obtain nematic phases with Li 2 SO 4 may be related to the crystallization process, since Li+ highly binds to the head group of KL. This situation was also reported for LiBr in the mixture KL/dodecyltrimethylammonium bromide/alkali bromide/water.…”

“…As reported before, the relation between n s and n a is important to have different types of nematic phases. 19 For instance, if a lyotropic mixture presents a nematic phase, in the case of n s > n a a N D phase is most likely to be observed and in the reverse situation, n s < n a , a N C phase is more probable.…”

“…20 Lyotropic liquid crystalline mixtures of KL/M 2 SO 4 /UndeOH/water were prepared as discussed in Refs. [18,19]. SDS/M 2 SO 4 /DDeOH/water mixtures were similarly produced.…”

“…18 It is well known that in lyotropic liquid crystals the phase transitions between uniaxial nematics and biaxial nematic occur by changing the temperature and/or the relative concentrations of the mixture components. In a previous study, 19 we showed that the alcohol-chain length (present as a co-surfactant in the lyotropic mixture) with respect to the main surfactant alkyl chain has an important effect on the topology of the phase diagram. In that study, it was observed that, at a given temperature, an increase in the alkyl chain length of the alcohol causes the transition from the N D phase to first N B and then N C phase.…”

Shear viscosity and rheology of ternary and quaternary lyotropic liquid crystals in discotic and calamitic nematic phases

“…The N D phase was observed until (decreasing) temperatures of about 15 • C. The phases and phase transitions were determined by laser conoscopy (Galerne and Marcerou 1983) and optical microscopy observations. Aside the corresponding N C ternary sample, named Nc3, the remaining samples are part of a family of new quaternary mixtures (Akpinar et al 2012b), namely KL/K 2 SO 4 /1-decanol/H 2 O. Notice here two features, first the presence of salt (potassium sulfate) and second that we have kept the same cosurfactant (1-decanol) of the classical mixture in the particular quaternary system chosen.…”