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The surface melting of octamethylcyclotetrasiloxane confined in controlled pore glasses as studied by 1H-NMR
KTH, School of Chemical Science and Engineering (CHE), Centres, Industrial NMR Centre.
KTH, School of Chemical Science and Engineering (CHE), Chemistry, Physical Chemistry.
KTH, School of Chemical Science and Engineering (CHE), Chemistry, Physical Chemistry.ORCID iD: 0000-0002-0231-3970
2007 (English)In: Journal of Physical Chemistry B, ISSN 1520-6106, E-ISSN 1520-5207, Vol. 111, no 7, 1574-1581 p.Article in journal (Refereed) Published
Abstract [en]

We have measured the thickness of the pre-molten surface layer that appears at the interface of octamethylcyclotetrasiloxane (OMCTS) to the matrix in controlled pore glasses with pore diameters ranging 7.5-73 nm. Except for the glass with the largest pores, the layer thickness data for different pore diameters fall on a single master curve when plotted versus T-m - T, where T-m is the size-dependent volume melting point of the pore-confined OMCTS. Hence, at a single temperature, the surface layer thickness depends strongly on the curvature of the pore wall and therefore that of the solid-liquid interface. For temperatures where it exceeds two monolayers, the layer thickness depends logarithmically on T-m - T; for the glass with the largest pores, this turns into a power law with the exponent -1/2. The results are interpreted in terms of a continuous model of the solid-liquid interface with an arbitrary curvature. Because OMCTS is a weakly polar molecule with close to spherical shape, our data also lend themselves to Lennard-Jones type simulations.

Place, publisher, year, edition, pages
2007. Vol. 111, no 7, 1574-1581 p.
Keyword [en]
1st-order bulk transitions, ice surfaces, solid-surfaces, porous silica, water, liquid, nmr, size, adsorption, spectroscopy
National Category
Physical Chemistry
Identifiers
URN: urn:nbn:se:kth:diva-6454DOI: 10.1021/jp0654765ISI: 000244225100012Scopus ID: 2-s2.0-33847787458OAI: oai:DiVA.org:kth-6454DiVA: diva2:11172
Note
Uppdaterad från submitted till published: 20100927. QC 20100927Available from: 2006-11-29 Created: 2006-11-29 Last updated: 2017-12-14Bibliographically approved
In thesis
1. Phase transitions in porous media studied by NMR
Open this publication in new window or tab >>Phase transitions in porous media studied by NMR
2006 (English)Doctoral thesis, comprehensive summary (Other scientific)
Abstract [en]

This Thesis presents studies of phase transitions ocurring in porous media. The investigated phase transitions include melting/crystallization, surface pre-melting and liquid-liquid phase separation of binary mixtures. A combination of NMR techniques, already existing and newly developed and ranging from cryoporometry to elaborate self-diffusion and spin-relaxation experiments, was applied in order to detect and quantify the effect of finite size constraints on those phase transitions. By relating the results to physico-chemical models, the difference in behaviour with respect to that of bulk was exploited and related to pore morphology and surface properties in diverse porous systems.

NMR cryoporometry is based on the detection of the melting/freezing temperature shifts with respect to those in the bulk state to obtain mean pore size and pore size distribution. We extended the size range in which this can be done in porous matrices of both hydrophilic and hydrophobic nature to a 1 μm-600 nm upper limit. This was achieved by introducing two new probe liquids namely octamethylcyclotetrasiloxane (OMCTS) and zinc nitrate hexahydrate Zn(NO)3•H2O.

The thickness of the pre-molten surface layer that appears at the interface of frozen octamethylcyclotetrasiloxane (OMCTS) to the matrix in controlled porous glasses was quantified and modeled including its temperature and wall-curvature dependence. The results reveal that the layer thickness depends logarithmically on the deviation from the pore melting point, while for the largest pore investigated this turns into a power law with the exponent of –1/2. Diverse NMR techniques were used not only to detect solely the surface layer and the evolution of the surface melting, but also to distinguish the latter from the volume melting transition within the pores.

The morphologies of two nanostructured materials, sintered films of TiO2 nanoparticles and a mesoporous foam obtained by surfactant-templated synthesis, were investigated. These two porous matrices have very different structures but fall into the size range accessible by NMR cryopormetry and their characterization plays an important role in their future applications. They were studied by exploiting the difference between melting and freezing temperature shifts ΔTm and ΔTf, respectively, with respect to that of bulk. NMR cryoporometry was shown to be a suitable alternative and an excellent complement to other porosimetry techniques, namely mercury intrusion and gas sorption porosimetries to analyze the pore structure and pore size distribution because of the unique and model-independent access to information about pore shape. By combining NMR cryoporometry with NMR diffusion experiments holds great potential for accessing information about pore interconnectivity.

By diverse NMR techniques we provided experimental evidence that corroborate that liquid-liquid phase separation of a binary mixture imbibed in porous media actually occurs within the individual pores. The size distribution of the phase-separated domains was measured.

Place, publisher, year, edition, pages
Stockholm: KTH, 2006. x, 50 p.
Series
Trita-FYK, 0603
National Category
Physical Chemistry
Identifiers
urn:nbn:se:kth:diva-4197 (URN)987-91-7178-526-8 (ISBN)
Public defence
2006-12-08, Sal D3, KTh, Lindstedtsvägen 5, Stockholm, 13:00
Opponent
Supervisors
Note
QC 20100927Available from: 2006-11-29 Created: 2006-11-29 Last updated: 2010-09-27Bibliographically approved

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