Microscopic order as a function of surface coverage in alkyl-modified silicas:  spin probe studies

Wright, Paul; Lamb, Edward.; Dorsey, John G.; Kooser, Robert G.

doi:10.1021/ac00031a015

Cited by 22 publications

(9 citation statements)

References 31 publications

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“…EPR studies using two different spin probes found that the rotational mobilities of the probes decreased with increasing surface coverage . This study, along with others, has postulated the existence of at least two types of regions in the interphase due to aggregation or ordering of the alkyl chains. − This effect is especially pronounced against a nonwetting solvent such as water, where the chains have been shown to collapse nearly into the plane of the substrate …”

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confidence: 64%

Lateral Diffusion of Molecules Partitioned into Silica-Bound Alkyl Chains: Influence of Chain Length and Bonding Density

Hansen

Harris

1996

Anal. Chem.

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Lateral diffusion of a hydrophobic fluorescent molecule partitioned into monomeric alkyl chains bound to a planar silica substrate was measured as a function of chain density and chain length. Measurement of fluorescence recovery after patterned photobleaching was used to observe the diffusional relaxation of a concentration profile of probe molecules over distances of micrometers. The diffusion rate of the probe molecule partitioned into C-18 chains decreased with decreasing chain coverage. As the chain length was reduced from C-18 to C-8 and C-4, the rate of diffusion also decreased. These results, when combined with results from a previous study of the effect of overlaying solvent on diffusion rate (Hansen, R. L.; Harris, J. M. Anal. Chem. 1995, 67, 492-498), are consistent with a domain model for long-range transport of partitioned molecules through the bound ligands. Fluorescence recovery experiments in which diffusion is monitored over a distance of micrometers offer a unique means to probe long-range structure of surface-immobilized alkyl chains.

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mentioning

confidence: 64%

Lateral Diffusion of Molecules Partitioned into Silica-Bound Alkyl Chains: Influence of Chain Length and Bonding Density

Hansen

Harris

1996

Anal. Chem.

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“…At high bonding density, the molecules become crowded, less mobile, and more disordered and project a rigid and rough surface to probe fluids. Decreases in mobility of attached molecules ("freezing") with increased bonding density have been observed by solid-state NMR, 58,59 ESR, 60 fluorescence, 61 and molecular dynamics. 62 The literature offers additional support to our conjecture concerning molecular roughness and rigidity.…”

Section: Resultsmentioning

confidence: 96%

“…At high bonding density, the molecules become crowded, less mobile, and more disordered and project a rigid and rough surface to probe fluids. Decreases in mobility of attached molecules (“freezing”) with increased bonding density have been observed by solid-state NMR, , ESR, fluorescence, and molecular dynamics 3 Contact angle hysteresis for water (·), CH 2 I (▪), and C 16 H 34 (▴) on silicon wafers treated with Me 3 SiNMe 2 for various durations. …”

Section: Resultsmentioning

confidence: 99%

Trialkylsilane Monolayers Covalently Attached to Silicon Surfaces: Wettability Studies Indicating that Molecular Topography Contributes to Contact Angle Hysteresis

1999

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Chemically grafted monolayers of trialkylsilanes were prepared by reaction of (primarily) alkyldimethylchlorosilanes with silicon wafers under three conditions: in the vapor phase at elevated temperature (60−70 °C), in toluene in the presence of ethyldiisopropylamine (EDIPA) at room temperature, in toluene/EDIPA at 60−70 °C. It was determined that reactions at the solution−solid interface are very slow in the later stages of the reaction and that long reaction times are necessary to achieve maximum bonding density. The bonding density is determined and can be controlled by the reaction conditions. The highest carbon content on the surface (assessed by X-ray photoelectron spectroscopy) as well as the highest contact angles were obtained using vapor phase reactions. A series of nine H(CH2) n Me2Si− surfaces was prepared with n = 1, 2, 3, 4, 8, 10, 12, 18, and 22. Water contact angles (θA/θR = ∼105°/∼94°) are independent of chain length, indicating that these surfaces project disordered methyl groups toward the probe fluid and that water does not penetrate the monolayers. Hydrophobization is achieved topologically: the monolayers prevent water from penetrating and interacting with residual silanols. n-Hexadecane and methylene iodide contact angles decrease with increasing chain length for this series, indicating that these probe fluids penetrate the monolayers and interact with methylene groups. These chemically grafted monolayers differ in structure from those prepared by self-assembly in that the distance between molecules is significantly greater and that all molecules are covalently attached to the substrate. The contact angle hysteresis for these surfaces is a function of alkyl group structure and bonding density: mobile surfaces with flexible chains or rotational mobility and rigid surfaces that pack well exhibit low hysteresis, whereas rigid surfaces that cannot pack well exhibit high hysteresis. We argue that molecular level topography (roughness and rigidity) is responsible for the observed hysteresis.

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“… where 〈cos 2 (θ)〉 is the mean angular fluctuation of the θ angle between the direction of the z main magnetic axis and the D // direction. Under the approximation of an axial symmetry for the nitrogen hf tensor, S can be estimated from the measurable EPR parameters through the Eqn (6)27, 28 where the A' // and A' ⟂ are the apparent N hf tensors submitted to motional averaging, and A zz , A xx , A yy are the real principal values determined by computer simulation of the rigid limit spectrum.…”

Section: Resultsmentioning

confidence: 99%

Synthesis of spin labelled silica and EPR molecular dynamics study at silica–polymer interfaces

Dondi

Pepori

Buttafava

et al. 2011

J of Physical Organic Chem

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Information concerning the interface structure in filler/polymer composites is of key importance for the rationalization of reaction mechanisms in mechano-chemical (extrusion, blending, etc.), thermal or radiation induced free radical processes and for elucidating the factors underlying the reinforcing mechanism. The analysis of the chain dynamics is a suitable tool for undertaking such investigations because any reactivity parameter (rate constants, collisional frequencies, activation energies) and bonding interactions are strictly related to the mobility of the interacting centres. EPR spectroscopy coupled with specific spin labelling at the filler/polymer interface is a tool for making such novel perspective available. In this work, a spin labelling study of the molecular motion at the filler-rubber interface in a silica-SBR blend is reported. Spin labels of different length, spanning a 9-11 Å depth and linked to the surface of silica particles, were prepared and used for determining the rotational diffusion tensors, the T 50 and order parameter in silica/SBR interfaces. The measurements carried out as a function of the temperature in comparison with unbound spin probes dispersed in the rubber matrix have afforded information consistent with the structure of the interfaces predicted by molecular-level theoretical models.

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Microscopic order as a function of surface coverage in alkyl-modified silicas: spin probe studies

Cited by 22 publications

References 31 publications

Lateral Diffusion of Molecules Partitioned into Silica-Bound Alkyl Chains: Influence of Chain Length and Bonding Density

Lateral Diffusion of Molecules Partitioned into Silica-Bound Alkyl Chains: Influence of Chain Length and Bonding Density

Trialkylsilane Monolayers Covalently Attached to Silicon Surfaces: Wettability Studies Indicating that Molecular Topography Contributes to Contact Angle Hysteresis

Synthesis of spin labelled silica and EPR molecular dynamics study at silica–polymer interfaces

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