Highly proton conductive phosphoric acid-nonionic surfactant lyotropic liquid crystalline mesophases and application in graphene optical modulators.

Proton conducting gel electrolytes are very important components of clean energy devices. Phosphoric acid (PA, H(3)PO(4) · H2O) is one of the best proton conductors, but needs to be incorporated into some matrix for real device applications, such as into lyotropic liquid crystalline mesophases (LLCMs). Herein, we show that PA and nonionic surfactant (NS, C(12)H(25)(OCH(2)CH(2))(10)OH, C(12)E(10)) molecules self-assemble into PANS-LLCMs and display high proton conductivity. The content of the PANS-LLCM can be as high 75% H(3)PO(4) · H2O and 25% 10-lauryl ether (C(12)H(25)(OCH(2)CH(2))(10)OH, C(12)E(10)), and the mesophase follows the usual LLC trend, bicontinuous cubic (V1)-normal hexagonal (H1)-micelle cubic (I1), by increasing the PA concentration in the media. The PANS-LLCMs are stable under ambient conditions, as well as at high (up to 130 °C) and low (-100 °C) temperatures with a high proton conductivity, in the range of 10(-2) to 10(-6) S/cm. The mesophase becomes a mesostructured solid with decent proton conductivity below -100 °C. The mesophase can be used in many applications as a proton-conducting media as well as a phosphate source for the synthesis of various metal phosphates. As an application, we demonstrate a graphene-based optical modulator using supercapacitor structure formed by graphene electrodes and a PANS electrolyte. A PANS-LLC electrolyte-based supercapacitor enables efficient optical modulation of graphene electrodes over a range of wavelengths, from 500 nm to 2 µm, under ambient conditions.

Effect of hygroscopicity of the metal salt on the formation and air stability of lyotropic liquid crystalline mesophases in hydrated salt-surfactant systems.

It is known that alkali, transition metal and lanthanide salts can form lyotropic liquid crystalline (LLC) mesophases with non-ionic surfactants (such as CiH2i+1(OCH2CH2)jOH, denoted as CiEj). Here we combine several salt systems and show that the percent deliquescence relative humidity (%DRH) value of a salt is the determining parameter in the formation and stability of the mesophases and that the other parameters are secondary and less significant. Accordingly, salts can be divided into 3 categories: Type I salts (such as LiCl, LiBr, LiI, LiNO3, LiClO4, CaCl2, Ca(NO3)2, MgCl2, and some transition metal nitrates) have low %DRH and form stable salt-surfactant LLC mesophases in the presence of a small amount of water, type II salts (such as some sodium and potassium salts) that are moderately hygroscopic form disordered stable mesophases, and type III salts that have high %DRH values, do not form stable LLC mesophases and leach out salt crystals. To illustrate this effect, a large group of salts from alkali and alkaline earth metals were investigated using XRD, POM, FTIR, and Raman techniques. Among the different salts investigated in this study, the LiX (where X is Cl(-), Br(-), I(-), NO3(-), and ClO4(-)) and CaX2 (X is Cl(-), and NO3(-)) salts were more prone to establish LLC mesophases because of their lower %DRH values. The phase behavior with respect to concentration, stability, and thermal behavior of Li(I) systems were investigated further. It is seen that the phase transitions among different anions in the Li(I) systems follow the Hofmeister series.

Highly conducting lyotropic liquid crystalline mesophases of pluronics (P65, P85, P103, and P123) and hydrated lithium salts (LiCl and LiNO3).

Demand for ionically conducting materials, as membranes and electrodes, is one of the driving forces of current research in chemistry, physics, and engineering. The lithium ion is a key element of these materials, and its assembly into nanostructures and mesophases is important for the membrane and electrode technologies. In this investigation, we show that hydrated lithium salts (such as LiCl·xH2O and LiNO3·xH2O, x is as low as 1.5 and 3.0, respectively) and pluronics (triblock copolymer such as PX where X is 65, 85, 103, and 123) form lyotropic liquid crystalline mesophases (LLCM), denoted as LiY·xH2O-PX-n (Y is Cl(-) or NO3(-), and n is the salt/PX mole ratio). The structure of the mesophase is hexagonal over a broad salt concentration and transforms to a cubic mesophase and then to disordered gel phase with an increasing salt content of the mixtures. The mesophases are unstable at low salt contents and undergo a phase separation into pure pluronics and salt-rich LLCMs. The salt content of the ordered mesophase can be as high as 30 mole ratio for each pluronic, which is a record high for any known salted phases. The mesophases also display high ac ionic conductivities, reaching up to 21 mS/cm at room temperature (RT), and are sensitive to the water content. These mesophases can be useful as ion-conducting membranes and can be used as media for the synthesis of lithium-containing nanoporous materials.

Lyotropic liquid crystal to soft mesocrystal transformation in hydrated salt-surfactant mixtures.

Hydrated CaCl2, LiI, and MgCl2 salts induce self-assembly in nonionic surfactants (such as C12H25(OCH2CH2)10OH) to form lyotropic liquid-crystalline (LLC) mesophases that undergo a phase transition to a new type of soft mesocrystal (SMC) under ambient conditions. The SMC samples can be obtained by aging the LLC samples, which were prepared as thin films by spin-coating, dip-coating, or drop-casting of a clear homogenized solution of water, salt, and surfactant over a substrate surface. The LLC mesophase exists up to a salt/surfactant mole ratio of 8, 10, and 4 (corresponding to 59, 68, and 40 wt% salt/surfactant) in the CaCl2, LiI, and MgCl2 mesophases, respectively. The SMC phase can transform back to a LLC mesophase at a higher relative humidity. The phase transformations have been monitored using powder X-ray diffraction (PXRD), polarized optical microscopy (POM), and FTIR techniques. The LLC mesophases only diffract at small angles, but the SMCs diffract at both small and wide angles. The broad surfactant features in the FTIR spectra of the LLC mesophases become sharp and well resolved upon SMC formation. The unit cell of the mesophases expands upon SMC transformation, in which the expansion is largest in the MgCl2 and smallest in the CaCl2 systems. The POM images of the SMCs display birefringent textures with well-defined edges, similar to crystals. However, the surface of the crystals is highly patterned, like buckling patterns, which indicates that these crystals are quite soft. This unusual phase behavior could be beneficial in designing new soft materials in the fields of phase-changing materials and mesostructured materials, and it demonstrates the richness of the phase behavior in the salt-surfactant mesophases.

Origin of lyotropic liquid crystalline mesophase formation and liquid crystalline to mesostructured solid transformation in the metal nitrate salt-surfactant systems.

The zinc nitrate salt acts as a solvent in the ZnX-C(12)EO(10) (ZnX is [Zn(H(2)O)(6)](NO(3))(2) and C(12)EO(10) is C(12)H(25)(OCH(2)CH(2))(10)OH) lyotropic liquid crystalline (LLC) mesophase with a drastic dropping on the melting point of ZnX. The salt-surfactant LLC mesophase is stable down to -52 °C and undergoes a phase change into a solid mesostructured salt upon cooling below -52 °C; no phase separation is observed down to -190 °C. The ZnX-C(12)EO(10) mesophase displays a usual phase behavior with an increasing concentration of the solvent (ZnX) in the media with an order of bicontinuous cubic(V(1))-2D hexagonal(H(1))--a mixture of 2D hexagonal and micelle cubic(H(1) + I)-micelle cubic(I)-micelle(L(1)) phases. The phase behaviors, specifically at low temperatures, and the first phase diagram of the ZnX-C(12)EO(10) system was investigated using polarized optical microscopy (POM), X-ray diffraction (XRD), differential scanning calorimetry (DSC), Fourier transform infrared (FTIR), and Raman techniques and conductivity measurements.

The role of charged surfactants in the thermal and structural properties of lyotropic liquid crystalline mesophases of [Zn(H2O)6](NO3)2-C(n)EO(m)-H2O.

The mixtures of [Zn(H(2)O)(6)](NO(3))(2) salt, 10-lauryl ether (C(12)H(25)(OCH(2)CH(2))(10)OH, represented as C(12)EO(10)), a charged surfactant (cetyltrimethylammonium bromide, C(16)H(33)N(CH(3))(3)Br, represented as CTAB or sodium dodecylsulfate, C(12)H(25)OSO(3)Na, SDS) and water form lyotropic liquid crystalline mesophases (LLCM). This assembly accommodates up to 8.0 Zn(II) ions (corresponds to about 80%w/w salt/(salt+C(12)EO(10))) for each C(12)EO(10) in the presence of a 1.0 CTAB (or 0.5 SDS) and 3.5 H(2)O in its LC phase. The salt concentration can be increased by increasing charged surfactant concentration of the media. Addition of charged surfactant to the [Zn(H(2)O)(6)](NO(3))(2)-C(12)EO(10) mesophase not only increases the salt content, it can also increase the water content of the media. The charged surfactant-C(12)EO(10) (hydrophobic tail groups) and the surfactant (head groups)-salt ion (ion-pair, hydrogen-bonding) interactions stabilize the mesophases at such salt high and water concentrations. The presence of both Br(-) and NO(3)(-) ions influences the thermal and structural properties of the [Zn(H(2)O)(6)](NO(3))(2)-C(12)EO(10)-CTAB(or SDS)-H(2)O LLCM, which have been investigated using XRD, POM (with a hot stage), FT-IR and Raman techniques.

Lyotropic liquid-crystalline mesophases of [Zn(H2O)6](NO3)2-C12EO10-CTAB-H2O and [Zn(H2O)6](NO3)2-C12EO10-SDS-H2O systems.

The mixture of two surfactants (C12EO10-CTAB and C12EO10-SDS) forms lyotropic liquid-crystalline (LLC) mesophases with [Zn(H2O)6](NO3)2 in the presence of a minimum concentration of 1.75 H2O per C12EO10. The metal ion/C12EO10 mole ratio can be increased up to 8.0, which is a record high metal ion density in an LLC mesophase. The metal ion concentration can be increased in the medium by increasing the CTAB/C12EO10 or SDS/C12EO10 mole ratio at the expense of the stability of the LLC mesophase. The structure and some thermal properties of the new mesophase have been investigated using XRD, POM, FTIR, and Raman techniques.

Phase separation in liquid crystalline mesophases of [Co(H2O)6]X2:P65 Systems (X = NO3-, Cl-, or ClO4-).

Transition-metal aqua complex salts [M(H2O)6]X2 (where M is Mn(II), Co(II), Ni(II), Zn(II), or Cd(II) and X is NO3-, Cl-, or ClO4-) can be dissolved in triblock poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) copolymers (Pluronics, such as P65) to form homogeneous liquid crystalline (LC) mesophases. However, the [Co(H2O)6]X2:P65 LC mesophases slowly undergo phase separation into a disordered ion-free phase and an ordered ion-rich LC mesophase. The phase separation also takes place in the two-salt systems [Co(H2O)6](NO3):[Co(H2O)6](ClO4)2:P65 in which the ion-free disordered domains separate out from the initially ordered homogeneous mesophase. The phase separation results in a physical mixture of a hexagonal nitrate-rich and cubic perchlorate-rich LC and disordered ion-free domains in the mixed salt systems. The driving force in the phase separation in the [Co(H2O)6]X2:P65 system is Co(II)-catalyzed aerobic oxidation of P65 into ester and/or other oxidation products. The separation of ions in the [Co(H2O)6](NO3)2:[Co(H2O)6](ClO4)2:P65 system is related to the mesostructures of the two-salt systems that are different, hexagonal in the [Co(H2O)6](NO3)2:P65 system and cubic in the [Co(H2O)6](ClO4)2:P65 system. There is no visible phase separation in the other transition-metal salt:P65 systems. The phase separation in the [Co(H2O)6]X2:P65 systems can also be eliminated by keeping the mesophase under a N2 atmosphere.