Lyotropic mixture made of potassium laurate/1-undecanol/K2SO4/water presenting high birefringences and large biaxial nematic phase domain: a laser conoscopy study

Abstract

The lyotropic liquid crystalline quaternary mixture made of potassium laurate (KL), potassium sulphate, 1-undecanol and water was investigated by experimental optical methods (optical microscopy and laser conoscopy). In a particular temperature and relative concentrations range, the three nematic phases (two uniaxial and one biaxial) were identified. The biaxial domain in the temperature/KL concentration surface is larger when compared to other lyotropic mixtures. Moreover, this new mixture gives nematic phases with higher birefringence than similar systems. The behavior of the symmetric tensor order parameter invariants σ (3) and σ (2) calculated from the measured optical birefringences supports that the uniaxial-to-biaxial transitions are of second order, described by a mean-field theory.

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

“…The maximum value of the birefringence in the N D phase decreased to~2.5 × 10 −3 for the kosmotrope ions Cl − , F − and SO 2À 4 with respect to those values for the chaotropic ions Br − , I − , NO À 3 and CIO À 4 ; , whose values were about 4 × 10 −3 and 5 × 10 −3 . As previously reported, [9,10] the higher (smaller) the birefringence the higher (smaller) micellar shape anisotropy. In this framework chaotropic ions (Figure 2(a)-(d)) lead to the formation of micelles with bigger shape anisotropy compared with those from kosmotrope ions (Figure 2(e)-(g)).…”

“…[3,4] Furthermore, in the case of a lyotropic mixture with only one surfactant, without a cosurfactant, only uniaxial N D or uniaxial N C phases are observed. [5] If there is a co-surfactant in the mixture, there is a possibility to obtain the biaxial nematic phase in addition to other two uniaxial ones, [5][6][7][8][9] depending on the temperature and relative concentrations of the constituents of the lyotropic mixture. The uniaxial (biaxial) phase present two (three) different values of the principal refractive indices, [5] which are used to define the birefringences Δn = n y − n x and δn = n z − n y , where x, y and z represent the three orthogonal axes of the reference frame when the phases are aligned (details in the Experimental section).…”

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.…”

“…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.…”

“…The temperature dependence of the birefringences is usually employed for the construction of these phase diagrams. 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.…”