Impact of mesogenic aromaticity and cyano termination on the alignment and stability of liquid crystal shells.

in Soft Matter (2023), 19(14), 2637-2645

We carry out a strategic and systematic variation of the molecular structure of liquid crystals (LCs) molded into spherical shells, surrounded by aqueous isotropic phases internally and externally ... [more ▼]

We carry out a strategic and systematic variation of the molecular structure of liquid crystals (LCs) molded into spherical shells, surrounded by aqueous isotropic phases internally and externally. Contrary to common expectation, based on previous studies that have almost exclusively been carried out with cyanobiphenyl-based LCs, we find that the director field aligns normal to the LC-water interface when we use an LC molecule that is entirely non-aromatic. We propose to explain this by the inability of such an LC to participate in hydrogen bonding, rendering the normal configuration favorable as it minimizes the molecular cross section in contact with the water. We also find that cyano-terminated LC molecules contribute greatly to stabilizing the LC-water interface. This explains why shells made of cyanobiphenyl LCs are much more stable than shells of LCs with non-cyano-terminated molecules, even if the latter exhibit aromatic cores. Unstable LC shells can be stabilized very efficiently, however, through the addition of a low concentration of molecules that are cyano-terminated, preferably below the threshold for dimerization. Our study provides a much clarified understanding of how the molecular structure dictates the stability and alignment of LC shells, and it will enable a diversification of LC shell research and applications to systems where the use of non-cyanobiphenyl LCs is required. [less ▲]

Measuring the Anisotropy in Interfacial Tension of Nematic Liquid Crystals

in Crystals (2021), 11(6), 687

<jats:p>Liquid crystal (LC) phases typically show anisotropic alignment-dependent properties, such as viscosity and dielectric permittivity, so it stands to reason that LCs also have anisotropic ... [more ▼]

<jats:p>Liquid crystal (LC) phases typically show anisotropic alignment-dependent properties, such as viscosity and dielectric permittivity, so it stands to reason that LCs also have anisotropic interfacial tensions. Measuring the interfacial tension ? of an LC with conventional methods, such as pendant drops, can be challenging, however, especially when we need to know ? for different LC aligning conditions, as is the case when we seek ??, the interfacial tension anisotropy. Here, we present measurements of ?? of the common synthetic nematic LC compound 5CB against water using a microfluidic droplet aspiration technique. To ensure tangential and normal alignment, respectively, we add poly(vinyl alcohol) (PVA) and sodium dodecylsulfate (SDS), respectively, as a stabilizer and measure ? for different concentrations of stabilizer. By fitting the Szyszkowski equation to the data, we can extrapolate to zero-stabilizer concentration, obtaining the ? of 5CB to pure water for each alignment. For normal alignment, we find ??=31.9?0.8 mN?m-1, on the order of 1 mN?m-1 greater than ?$_$=30.8?5 mN?m-1 for tangential alignment. This resonates with the empirical knowledge that 5CB aligns tangentially to an interface with pure water. The main uncertainty arises from the use of polymeric PVA as tangential-promoting stabilizer. Future improvements in accuracy may be expected if PVA can be replaced by a low molar mass stabilizer that ensures tangential alignment.</jats:p [less ▲]

Realignment of Liquid Crystal Shells Driven by Temperature- Dependent Surfactant Solubility

in Langmuir (2019), 35(2019), 1113211140

We investigate dynamic director field variations in shells of the nematic liquid crystal (LC) compound, 4-cyano-4′-pentylbiphenyl, suspended in and containing immiscible aqueous phases. The outer and ... [more ▼]

We investigate dynamic director field variations in shells of the nematic liquid crystal (LC) compound, 4-cyano-4′-pentylbiphenyl, suspended in and containing immiscible aqueous phases. The outer and inner shell interfaces are stabilized by the cationic surfactant, cetyl trimethyl ammonium bromide (CTAB), and by the water soluble polymer, poly(vinyl alcohol) (PVA), respectively. PVA and surfactant solutions normally promote tangential and orthogonal alignments, respectively, of the LC director. The rather high Krafft temperature of CTAB, TK ≈25 °C, means that its solubility in water is below the critical micelle concentration at room temperature in most labs. Here,we study the effect of cooling/heating past TK on the LC shell director configuration. Within a certain concentration range,CTAB in the outer aqueous phase (and PVA in the inner) switches the LC director field from hybrid to uniformly orthogonal upon cooling below TK. We argue that the effect is related to the migration of the surfactant through the fluid LC membraneinto the initially surfactant-free aqueous PVA solution, triggered by the drastically reduced water solubility of CTAB at T < TK.The results suggest that LC shells can detect solutes in the continuous phase, provided there is sufficient probability that thesolute migrates through the LC into the inner aqueous phase. [less ▲]