59 research outputs found
Relating structure with morphology: A comparative study of perfect Langmuir–Blodgett multilayers
No full textAtomic force microscopy and X-ray reflectivity of metal-stearate (MSt) Langmuir-Blodgett films on hydrophilic Silicon (1 0 0), show dramatic reduction in 'pinhole' defects when metal M is changed from Cd to Co, along with excellent periodicity in multilayer, with hydrocarbon tails tilted 9.6° from vertical for CoSt (untilted for CdSt). Near edge X-ray absorption fine structure (NEXAFS) and Fourier transform infra-red (FTIR) spectroscopies indicate bidentate bridging metal-carboxylate coordination in CoSt (unidentate in CdSt), underscoring role of headgroup structure in determining morphology. FTIR studies also show increased packing density in CoSt, consistent with increased coverage
Chemistry at Air/Water Interface versus Reaction in a Flask: Tuning Molecular Conformation in Thin Films
No full textAtomic force microscopy and X-ray reflectivity studies of cobalt stearate Langmuir−Blodgett (LB) films (CoStp) deposited from a preformed bulk sample on quartz substrates showed formation of a Volmer−Weber type monolayer but no multilayers as compared to the excellent multilayers of cobalt stearate films (CoStn) deposited at the air/water interface by the usual LB technique, in spite of both showing bidentate bridging type coordination of cobalt ions with the carboxylate group. The difference is attributed to existence of different headgroup conformers, observed from Fourier transform infrared (FTIR) studies. The CoStp films had a higher energy ‘boat’ conformation with linear O−Co−O linkage, whereas CoStn formed a low energy conformer with a bent O−Co−O configuration (bond angle of 105°). Present results support the necessity of bidentate bridging coordination in multilayer deposition, but rejects its sufficiency by bringing out the crucial role played by air/water interface. Differences in surface pressure−molecular area isotherms and hydrocarbon tail−tail interactions (evident from FTIR spectra) of the films support the above statement. Methyl−methyl interactions observed in CoStn samples suggest hierarchy of supramolecular chemistry at the air/water interface in tuning the C−O−Co bond angle essential to satisfy the wetting condition with the substrate and subsequently form LB multilayers
Solidlike and liquidlike behavior in monolayers and multilayers of metal-bearing amphiphiles
No full textAtomic force microscopy (AFM) of cadmium stearate (CdSt) and cobalt stearate (CoSt) Langmuir-Blodgett films show differences in their in-plane morphologies. CdSt films, with a huge number of in-plane “pinhole” defects, follow self-affine behavior, whereas CoSt films, which are almost void of such in-plane defects, show deviation from self-affinity especially at small length scales, suggesting liquidlike behavior, imparting flexibility to the system, in plane. Phase images of CoSt obtained from tapping mode AFM show gentle undulations or hemispherelike features in contrast to its smooth topography, unlike the CdSt system where both height and phase images show self-affine domains. Near edge x-ray absorption fine structure spectroscopy indicates no preferred in-plane orientation of the head group in CoSt films. The undulating features in CoSt is explained by invoking a radially symmetric orientational distribution in the tilt of adjacent hydrocarbon tails, causing a small in-plane density variation which shows up in the phase image. These orientational disorders in adjacent tails probably allow “filling up” of in-plane defects thereby giving rise to its excellent in-plane coverage and hence a “liquidlike” behavior in CoSt. Brewster angle microscopy shows that parent Langmuir monolayers of stearic acid in the presence of Cd and Co ions in the aqueous subphase behave as two-dimensional “solids” and “liquids,” respectively, suggesting the phenomena to be inherent in the amphiphiles and probably independent of their organization as monolayers and multilayers
Intramolecular and Intermolecular Rearrangements in Nanoconfined Polystyrene
No full textVacuum ultraviolet spectroscopy reveals an intramolecular rearrangement involving a change in "physical dimers" of adjacent pendant benzene rings of atactic polystyrene (aPS) from "oblique" to "head-to-tail" (ht) configuration in aPS films spin coated on fused quartz, as film thickness R goes below 4R g (R g = unperturbed polystyrene gyration radius ≈ 20.4 nm). Simultaneously, transverse layering of molecular "gyration spheres", for film thickness R ≤ 4R g, causes an increase in free energy (reduction in cohesion) that follows a (R g/R) h dependence with b ≈ 3, a clear deviation from planar confinement. The variation of in-plane and outof-plane cohesive energy over a film of a given thickness is explained by invoking a fixed-range, repulsive, modified Pöschl-Teller intermolecular potential, with the strength of this potential decreasing with increase in R. Possible reduction of "dimer" dipole moment due to ht configuration is consistent with reduction of cohesion between aPS molecular gyration spheres
Long-timescale dynamics of thiol capped Au nanoparticle clusters at the air-water interface
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In-situ synthesis of Au nano particles of co-existing morphologies in liquid crystalline matrix
No full textTransition from a complex to a simple two dimensional liquid: Isotherm and Brewster angle microscopy studies
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