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Phase transitions in porous media studied by NMR
KTH, School of Chemical Science and Engineering (CHE), Chemistry, Physical Chemistry.
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: urn:nbn:se:kth:diva-4197ISBN: 987-91-7178-526-8 OAI: oai:DiVA.org:kth-4197DiVA: diva2:11176
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
List of papers
1. Inorganic salt hydrates as cryoporometric probe materials to obtain pore size distribution
Open this publication in new window or tab >>Inorganic salt hydrates as cryoporometric probe materials to obtain pore size distribution
2006 (English)In: Journal of Physical Chemistry B, ISSN 1520-6106, E-ISSN 1520-5207, Vol. 110, no 9, 3867-3870 p.Article in journal (Refereed) Published
Abstract [en]

 The depression of the melting temperature of Zn(NO3)(2)center dot 6H(2)O was used to obtain the pore size distributions in controlled pore glasses. Measured by H-1 NMR, the average value of the temperature depression AT and the known average pore size yield K = Delta T center dot d approximate to 11.6 K center dot nm as the material-dependent factor for Zn(NO3)(2)center dot 6H(2)O in the Gibbs-Thompson equation. The melting temperature is close to room temperature. Hence, this salt hydrate and some related other ones are better materials than water (K approximate to 50 K center dot nm) for cryoporometric studies of systems with hydrophilic pores. The data also provide 46 mN/m for the solid-liquid surface tension of this salt hydrate.

Keyword
Glass, Hydrates, Hydrophilicity, Melting, Surface tension, Temperature measurement, Zinc compounds
National Category
Chemical Sciences
Identifiers
urn:nbn:se:kth:diva-6452 (URN)10.1021/jp055915k (DOI)000235944500008 ()2-s2.0-33645697448 (Scopus ID)
Note
QC 20100920Available from: 2006-11-29 Created: 2006-11-29 Last updated: 2017-12-14Bibliographically approved
2. NMR cryoporometry with octamethylcyclotetrasiloxane as a probe liquid. Accessing large pores
Open this publication in new window or tab >>NMR cryoporometry with octamethylcyclotetrasiloxane as a probe liquid. Accessing large pores
2007 (English)In: Journal of Colloid and Interface Science, ISSN 0021-9797, E-ISSN 1095-7103, Vol. 305, no 2, 280-285 p.Article in journal (Refereed) Published
Abstract [en]

Octamethylcyclotetrasiloxane is presented and investigated as probe liquid for NMR cryoporometry or DSC-based thermoporometry. This compound which may imbibe into both hydrophilic and hydrophobic pores is shown to exhibit a melting point depression that is larger than that for other cryoporometric probe materials such as cyclohexane. The transverse relaxation time differs by more than three orders of magnitude between the solid and liquid states, separated by a sharp phase transition. Hence, as demonstrated in controlled pore glasses, octamethylcyclotetrasiloxane can provide pore size distributions for materials with pore sizes up to the micrometer range.

Keyword
Cryoporometry, DSC, Melting point, NMR, Phase transition, Pore size, Pore size distribution, Thermoporometry
National Category
Physical Chemistry
Identifiers
urn:nbn:se:kth:diva-6453 (URN)10.1016/j.jcis.2006.09.054 (DOI)000243122000011 ()
Note
QC 20100924. Uppdaterad från In press till Published (20100924).Available from: 2006-11-29 Created: 2006-11-29 Last updated: 2017-12-14Bibliographically approved
3. The surface melting of octamethylcyclotetrasiloxane confined in controlled pore glasses as studied by 1H-NMR
Open this publication in new window or tab >>The surface melting of octamethylcyclotetrasiloxane confined in controlled pore glasses as studied by 1H-NMR
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.

Keyword
1st-order bulk transitions, ice surfaces, solid-surfaces, porous silica, water, liquid, nmr, size, adsorption, spectroscopy
National Category
Physical Chemistry
Identifiers
urn:nbn:se:kth:diva-6454 (URN)10.1021/jp0654765 (DOI)000244225100012 ()2-s2.0-33847787458 (Scopus ID)
Note
Uppdaterad från submitted till published: 20100927. QC 20100927Available from: 2006-11-29 Created: 2006-11-29 Last updated: 2017-12-14Bibliographically approved
4. Pores in nanostructured TiO2 films. Size distribution and pore permeability
Open this publication in new window or tab >>Pores in nanostructured TiO2 films. Size distribution and pore permeability
2007 (English)In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 111, no 21, 7605-7611 p.Article in journal (Refereed) Published
Abstract [en]

Nanoporous films of crystalline anatase with intended application in dye-sensitized photovoltaic cells were investigated by NMR cryoporometry, NMR diffusiometry, electron microscopy, and X-ray diffraction. The nanoparticles from which the films were subsequently sintered were prepared in two ways, one with an acidic and one with a basic aqueous process environment and along different temperature regimes. The average morphology was similar in all films as indicated by the roughly identical < 2 kappa V/S > values where kappa is the mean curvature of the pore surface and S/V denotes the surface-to-volume ratio. Self-diffusion of water in the pores is strongly reduced with respect to that of bulk and is influenced both by micrometer-scale obstructions to molecular displacement and by pore-size effect in pore interconnectivity. The investigated samples exhibit different transport regimes as concerning those phenomena. In this initial study performed on a limited set of samples, we found no linear correlation between particle and pore sizes. Instead, total porosity is controlled by particle-particle jamming which, together with particle size polydispersity, may also dominate the effects that lead to the observed pore size distributions for the different samples. The rich variation of structural effects and transport properties among the few prepared films call for further studies in order to find an optimal film structure.

Keyword
nuclear-magnetic-resonance, differential scanning calorimetry, nmr diffusion measurements, field gradient, porous-media, nanocrystalline tio2, silica-gels, cryoporometry, probe, liquid
National Category
Physical Chemistry
Identifiers
urn:nbn:se:kth:diva-6455 (URN)10.1021/jp070321y (DOI)000246695400009 ()2-s2.0-34250303492 (Scopus ID)
Note
Uppdaterad från manuskript till artikel: 20100927. Tidigare titel: On the structure of porous TiO2 films. QC 20100927 Available from: 2006-11-29 Created: 2006-11-29 Last updated: 2017-12-14Bibliographically approved
5. Pore morphology and interconnectivity in a mesoporous/macroporous polyhedral silica foam material
Open this publication in new window or tab >>Pore morphology and interconnectivity in a mesoporous/macroporous polyhedral silica foam material
2008 (English)In: Langmuir, ISSN 0743-7463, E-ISSN 1520-5827, Vol. 24, no 9, 4827-4832 p.Article in journal (Other academic) Published
Abstract [en]

The pore system of a highly swollen, block-copolymer-templated, polyhedral silica foam material is investigated by a combination of transmission electron microscopy, nitrogen sorption, and NMR cryoporometry. The adsorption-desorption hysteresis and melting-freezing hysteresis data recorded by the respective methods provide pore Volume and access channel sizes that virtually coincide for the two used methods. This provides a consistent picture where polyhedral foam cells of 60-70 nm diameter are interconnected by cylindrical access channels with several characteristic sizes for the latter.

Keyword
nitrogen sorption, porous solids, mercury porosimetry, mesoporous materials, nmr cryoporometry, adsorption branch, size distribution, intrusion, probe, thermoporometry
National Category
Physical Chemistry
Identifiers
urn:nbn:se:kth:diva-6456 (URN)10.1021/la702318y (DOI)000255432000062 ()2-s2.0-44249112883 (Scopus ID)
Note
Uppdaterad från manuskript till artikel: 20100927. Tidigare titel: Pore morphology and interconnectivity in a surfactant-templated mesoporous silica material. QC 20100927Available from: 2006-11-29 Created: 2006-11-29 Last updated: 2017-12-14Bibliographically approved
6. Liquid-liquid phase separation in micropores
Open this publication in new window or tab >>Liquid-liquid phase separation in micropores
2004 (English)In: Current applied physics, ISSN 1567-1739, E-ISSN 1878-1675, Vol. 4, no 2-4, 370-372 p.Article in journal (Refereed) Published
Abstract [en]

Phase separation of binary liquids with an upper critical temperature in porous materials is studied by H-1 NMR cryoporometry and cross-relaxation spectroscopy and by N-15 NMR spectroscopy. The first method provides domain size distributions of the separating minority component while the other ones verify the segregation of the two liquids on molecular level. We find that metastable structures are formed manifested by bimodal domain size distributions. The kinetic arrest of domain growth may be imposed upon by bottlenecks in the porous structure that block diffusion of entire droplets. Practical applications, if one of the liquid components is polymerized, include mobile polymer particles trapped in porous matrix that can serve, e.g., as filters with microscopically amphiphilic pathways.

Keyword
porous materials, phase separation
National Category
Physical Chemistry
Identifiers
urn:nbn:se:kth:diva-6457 (URN)10.1016/j.cap.2003.11.051 (DOI)000220831300072 ()2-s2.0-1542610502 (Scopus ID)
Note
QC 20100921Available from: 2006-11-29 Created: 2006-11-29 Last updated: 2017-12-14Bibliographically approved

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