Santanu Karan

Sub–10 nm polyamide nanofilms with ultrafast solvent transport for molecular separation

Composite membranes for filtering solvents Much research has focused on finding membranes that can purify water or extract waste carbon dioxide. However, there is also a need for the removal of small molecules from organic liquids. Many existing processes are energy-intensive and can require large quantities of solvents. Karan et al. grew confined polymer layers on a patterned sacrificial support to give rippled thin films that were then placed on ceramic membranes (see the Perspective by Freger). The composite membrane showed high flux for organic solvents and good stability and was able to separate out small molecules with high efficiency. Science, this issue p. 1347; see also p. 1317 Thin, crumpled polymer films on ceramic supports are high-flux membranes for removing small molecules from organic fluids. [Also see Perspective by Freger] Membranes with unprecedented solvent permeance and high retention of dissolved solutes are needed to reduce the energy consumed by separations in organic liquids. We used controlled interfacial polymerization to form free-standing polyamide nanofilms less than 10 nanometers in thickness, and incorporated them as separating layers in composite membranes. Manipulation of nanofilm morphology by control of interfacial reaction conditions enabled the creation of smooth or crumpled textures; the nanofilms were sufficiently rigid that the crumpled textures could withstand pressurized filtration, resulting in increased permeable area. Composite membranes comprising crumpled nanofilms on alumina supports provided high retention of solutes, with acetonitrile permeances up to 112 liters per square meter per hour per bar. This is more than two orders of magnitude higher than permeances of commercially available membranes with equivalent solute retention.

Ultrafast Viscous Permeation of Organic Solvents Through Diamond-Like Carbon Nanosheets

Porous Membranes Thin semi-permeable membranes are commonly used as chemical barriers or for filtration purposes. While the size of the pores will influence which molecules are able to pass, other factors—including the surface chemistry of the pore walls, electrostatic interactions, and differences in solubility—can also affect the diffusion rates. There is also a trade-off between the thickness of the membrane regarding strength and permeation rates (see the Perspective by Paul). Karan et al. (p. 444) fabricated membranes from amorphous carbon, which showed excellent strength and could be used for filtrations involving organic solvents. Nair et al. (p. 442) observed unusual behavior in graphene-based membranes which were able to prevent the diffusion of many small-molecule gases, including helium, but showed almost barrier-free movement of water. Membranes made from diamond-like carbon are used to rapidly separate organic compounds. Chemical, petrochemical, energy, and environment-related industries strongly require high-performance nanofiltration membranes applicable to organic solvents. To achieve high solvent permeability, filtration membranes must be as thin as possible, while retaining mechanical strength and solvent resistance. Here, we report on the preparation of ultrathin free-standing amorphous carbon membranes with Young’s moduli of 90 to 170 gigapascals. The membranes can separate organic dyes at a rate three orders of magnitude greater than that of commercially available membranes. Permeation experiments revealed that the hard carbon layer has hydrophobic pores of ~1 nanometer, which allow the ultrafast viscous permeation of organic solvents through the membrane.

Synthesis of thiol capped CdS nanocrystallites using microwave irradiation and studies on their steady state and time resolved photoluminescence.

Synthesis of cadmium sulfide (CdS) nanocrystallites has been performed through the microwave (MW) assisted reaction of cadmium acetate with thiourea in N,N-dimethylformamide (DMF) in the presence of two capping agents, 1-butanethiol and 2-mercaptoethanol. Attempts were made to control the size and size distribution of the thiol capped CdS nanocrystallites by controlling the number of MW irradiations/exposures for a fixed time (duration). The prepared nanocrystallites have been characterized by UV-vis spectroscopy, FTIR, XRD, FESEM and TEM. The peak position of the absorption band of the 1-butanethiol caped CdS nanocrystals in DMF solution shifted towards longer wavelength with the increasing number of MW exposures indicating the growth of particle size. In contrast, the peak position of absorption band for the 2-mercaptoethanol capped CdS nanocrystals remained nearly at the same wavelength and only the intensity of the absorption band increased with the increasing number of MW exposures. The observed steady state photoluminescence (visible range) of the 1-butanethiol capped CdS nanocrystals in DMF solution shifted towards higher wavelength, showing a decrease in intensity, with the increase in the number of MW exposures. Whereas in the case of 2-mercaptoethanol capped CdS nanocrystals in DMF solution, the photoluminescence peak remained nearly at the same position showing a decrease in intensity with increase in the number of MW exposures. The interesting results on the size-dependent steady state and time resolved photoluminescence (PL) of the CdS nanocrystallites are discussed in the present article. Possible application of such studies in the area of biotechnology has been mentioned.

Ultrathin nanofibrous films prepared from cadmium hydroxide nanostrands and anionic surfactants.

We developed a simple fabrication method of ultrathin nanofibrous films from the dispersion of cadmium hydroxide nanostrands and anionic surfactants. The nanostrands were prepared in a dilute aqueous solution of cadmium chloride by using 2-aminoethanol. They were highly positively charged and gave bundlelike fibers upon mixing an aqueous solution of anionic surfactant. The nanostrand/surfactant composite fibers were filtered on an inorganic membrane filter. The resultant nanofibrous film was very uniform in the area of a few centimeters square when the thickness was not less than 60 nm. The films obtained with sodium tetradecyl sulfate (STS) had a composition close to the electroneutral complex, [Cd37(OH)68(H2O)n] x 6(STS), as confirmed by energy dispersive X-ray analysis. They were water-repellent with a contact angle of 117 degrees, and the value slightly decreased with the alkyl chain length of anionic surfactants. Ultrathin nanofibrous films were stable enough to be used for ultrafiltration at pressure difference of 90 kPa. We could effectively separate Au nanoparticles of 40 nm at an extremely high filtration rate of 14000 L/(h m2 bar).

Vibrational spectroscopy and ionic conductivity of polyethylene oxide-NaClO4-CuO nanocomposite.

Structure, morphology and thermal properties of polyethylene oxide (PEO) with sodium perchlorate (NaClO(4)) as electrolytic salt have been investigated by incorporating cupric monoxide (CuO) nanoparticles. Monoclinic CuO affects melting and glass transition temperatures of PEO-NaClO(4). Crystallinity and free ion concentration change with the variation of CuO concentration. The maximum ionic conductivity is observed for 10 wt.% CuO. Ionic conductivity follows Arrhenius type behavior as a function of temperature.

Bias dependent dielectric relaxation dynamics of electrically tuned large-scale aligned zinc oxide nanorods in nematic liquid crystal host

It was observed that mixed ZnO nanorods and twisted nematic liquid crystals (LCs) show highly ordered molecular system. The observed molecular relaxation arose due to a reorientation of long molecular axis of the order of 400 kHz for pure ZLI-1636, while such relaxation was shifted to a lower frequency for mixture. By using the proposed equations, f=fDB−α exp[−EV/(B−BC)] and σ=σDB−sα exp[−sEV/(B−BC)], the obtained equivalent activation energy was increased remarkably by the introduction of ZnO nanorods in pure LC. A strong long-range interaction is established in 0.01% ZnO nanorods mixed LC system.

Size effect of cubic ZrO2 nanoparticles on ionic conductivity of polyethylene oxide-based composite

A solvent free solid composite polymer electrolyte (SCPE) film consisting of high molecular mass polyethylene oxide (PEO) with sodium perchlorate (NaClO4) as electrolytic salt and cubic zirconium oxide (ZrO2) nanoparticles as the filler has been prepared by solution casting technique to influence the transport properties. X-ray diffraction and Fourier transform infrared spectroscopy confirm the formation of the SCPE film, whereas atomic force microscopy reveals the presence of a network of interconnected nanoparticles forming uniform surface feature of relatively low roughness. The highest ionic conductivity (σ = 6.96 × 10-5 S-cm-1) for PEO25 - NaClO4 with 5 wt. % ZrO2 nanoparticles of the smallest size 4.5 nm is an order of magnitude higher than the pure PEO25 - NaClO4 at room temperature. The conductivity enhancement is due to the creation of additional sites and favorable conduction pathways for ionic transport through Lewis acid-base type interactions between the polar surface groups of the ceramic fi...

Effect of cadmium sulfide nanorod content on Freedericksz threshold voltage, splay and bend elastic constants in liquid-crystal nanocomposites

This report describes how doping liquid crystals (LC) with rod-like hexagonal semiconductor nanoprisms alters the dielectric and elastic properties of the composites as compared with a pristine nematic liquid crystal (NLC). Cadmium sulfide nanorods were synthesized via the solvothermal process and blended with a NLC. Nanorods were highly miscible with NLC and produced a topological defect-free texture up to a certain limit. A good dark state was achieved during the homeotropic configuration of the cell within that limit. Appreciable changes in splay and bend elastic constants of the LCs were observed after blending with nanorods. Long-range order was established in the hybrid system, and consequently the anisotropy was increased. The threshold voltage decreased dramatically by ?31%. Dielectric study revealed a high-frequency mode, which might be due to anchoring of the LC with nanorods.

Water Transport through Ultrathin Polyamide Nanofilms Used for Reverse Osmosis

Thin-film composite membranes comprising a polyamide nanofilm separating layer on a support material are state of the art for desalination by reverse osmosis. Nanofilm thickness is thought to determine the rate of water transport through the membranes; although due to the fast and relatively uncontrolled interfacial polymerization reaction employed to form these nanofilms, they are typically crumpled and the separating layer is reported to be ≈50-200 nm thick. This crumpled structure has confounded exploration of the independent effects of thickness, permeation mechanism, and the support material. Herein, smooth sub-8 nm polyamide nanofilms are fabricated at a free aqueous-organic interface, exhibiting chemical homogeneity at both aqueous and organic facing surfaces. Transfer of these ultrathin nanofilms onto porous supports provides fast water transport through the resulting nanofilm composite membranes. Manipulating the intrinsic nanofilm thickness from ≈15 down to 8 nm reveals that water permeance increases proportionally with the thickness decrease, after which it increases nonlinearly to 2.7 L m-2 h-1 bar-1 as the thickness is further reduced to ≈6 nm.

Study of Quantum Yield and Photoluminescence of Thiol Capped CdS Nanocrystallites

For the preparation of thiol capped CdS nanocrystallites, the microwave (MW) assisted reaction of cadmium acetate with thiourea in N, N‐dimethylformamide (DMF) was controlled in the presence of two capping agents, 1‐butanethiol and 2‐mercaptoethanol. The peak position of the absorption band of the CdS nanocrystals in DMF solution shifted towards longer wavelength with increasing duration (by repeated exposure for a fixed time) of MW irradiation, for 1‐butanethiol caped CdS nanocrystals indicating growth of particle size. However, the peak position of absorption band remained nearly at the same wavelength and only the intensity of the absorption band increased with increasing duration of MW irradiation for the 2‐mercaptoethanol capped CdS nanocrystals. Photoluminescence (PL) of the CdS nanocrystals in solution with 1‐butanethiol as capping agent was observed to shift towards higher wavelength in the visible range of the spectrum showing a decrease in intensity with increase in the duration of exposure of M...