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Na/Ca Intermixing around Silicate and Phosphate Groups in Bioactive Phosphosilicate Glasses Revealed by Heteronuclear Solid-State NMR and Molecular Dynamics Simulations
Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK), Physical Chemistry.ORCID iD: 0000-0003-4041-2803
Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK), Physical Chemistry.
Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK), Physical Chemistry.
2015 (English)In: Journal of Physical Chemistry B, ISSN 1520-6106, E-ISSN 1520-5207, Vol. 119, no 17, 5701-5715 p.Article in journal (Refereed) Published
Abstract [en]

We characterize the intermixing of network-modifying Na+/Ca2+ ions around the silicate (QSin) and phosphate (QPn) tetrahedra in a series of 16 Na2O–CaO–SiO2–P2O5 glasses, whose P content and silicate network connectivity were varied independently. The set includes both bioactive and bioinactive compositions and also encompasses two soda-lime-silicate members devoid of P, as well as two CaO–SiO2 glasses and one Na2O–SiO2–P2O5 glass. The various Si/P↔Na/Ca contacts were probed by molecular dynamics (MD) simulations together with heteronuclear magic-angle-spinning (MAS) nuclear magnetic resonance (NMR) experimentation utilizing 23Na{31P} and 23Na{29Si} REDOR, as well as 31P{ 23Na} and 29Si{23Na} REAPDOR. We introduce an approach for quantifying the extent of Na+/Ca2+ ordering around a given QPn or QSin group, encoded by the preference factor 0⩽ PM ⩽ 1 conveying the relative weights of a random cation intermixing (PM = 0) and complete preference/ordering (PM = 1) for one of the species M, which represents either Na+ or Ca2+. The MD-derived preference factors reveal phosphate and silicate species surrounded by Na+/Ca2+ ions intermixed nearly randomly (PM ≲ 0.15), except for the QSi4 and QSi1 groups, which manifest more significant cation ordering with preference for Na+ and Ca2+, respectively. The overall weak preferences are essentially independent of the Si and P contents of the glass, whereas PM primarily correlates with the total amount of network modifiers: as the latter is increased, the Na/Ca distribution around the {QP0, QSi1, QSi2} groups with preference for Ca2+ tend to randomize (i.e., PCa decreases), while the PNa-values grow slightly for the {QP1, QSi3, QSi4} species already preferring coordination of Na. The set of experimental preference factors {PCa} for the orthophosphate (QP0) groups extracted from 31P{23Na} REAPDOR NMR-derived M2(P–Na) dipolar second moments agrees well with the MD-generated counterparts. Our results on the Na/Ca intermixing in soda-lime-silicate glasses are discussed in relation to previous reports, highlighting the dependence of the conclusion on the approach to data evaluation.

Place, publisher, year, edition, pages
2015. Vol. 119, no 17, 5701-5715 p.
National Category
Physical Chemistry
Research subject
Physical Chemistry
Identifiers
URN: urn:nbn:se:su:diva-116269DOI: 10.1021/acs.jpcb.5b01130ISI: 000353931000039OAI: oai:DiVA.org:su-116269DiVA: diva2:805905
Available from: 2015-04-17 Created: 2015-04-17 Last updated: 2017-12-04Bibliographically approved
In thesis
1. Composition-Structure Correlations of Bioactive Glasses Explored by Multinuclear Solid-state NMR Spectroscopy
Open this publication in new window or tab >>Composition-Structure Correlations of Bioactive Glasses Explored by Multinuclear Solid-state NMR Spectroscopy
2015 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This PhD thesis presents a study of structure-composition correlations of bioactive glasses (BGs) by employing solid-state Nuclear Magnetic Resonance (NMR) spectroscopy.

Silicate-based Na2O−CaO−SiO2−P2O5 BGs are utilized clinically and are extensively investigated for bone regeneration purposes. Once implanted in the human body, they facilitate bone regeneration by partially dissolving in the body fluids, followed by the formation of a biomimetic surface-layer of calcium hydroxy-carbonate apatite (HCA). Eventually, the implanted BG totally integrates with the bone. The bioactivity of melt-prepared BGs depends on their composition and structure, primarily on the phosphorus content and the average silicate-network connectivity (NC). We explored these composition-structure relationships for a set of BGs for which the NC and phosphorus contents were varied independently.

The short-range structural features of the glasses were explored using 29Si and 31P magic-angle-spinning (MAS) NMR spectroscopy. 31P MAS NMR revealed that the orthophosphate content is directly proportional to the total P content of the glass, with a linear correlation observed between the orthophosphate content and the silicate network connectivity. The bearings of the results for future BG design are discussed.

By using multiple-quantum coherence-based 31P NMR experiments, the spatial distribution of orthophosphate groups was probed in the melt prepared BGs, as well as in two mesoporous bioactive glasses prepared by an evaporation-induced self-assembly technique. The results evidence randomly distributed orthophosphate groups in the melt-prepared BGs, whereas the pore-walls of the mesoporous bioactive glasses constitute nanometer-sized clusters of calcium phosphate. The distribution of Na+ ions among the phosphate/silicate groups were studied by heteronuclear dipolar-based 23Na−31P NMR experiments, verifying that sodium is dispersed nearly randomly in the glasses.

The phosphorus and proton environments in biomimetically grown HCA were investigated by using 1H and 31P MAS NMR experiments. Our studies revealed that the biomimetic HCA shared many local structural features with synthetic and well-ordered hydroxy-apatite.

Place, publisher, year, edition, pages
Stockholm: Department of Materials and Environmental Chemistry (MMK), Stockholm University, 2015. 68 p.
Keyword
bioactive glasses, glass structure, orthophosphate distribution, solid-state NMR, dipolar interactions, dipolar recoupling, multiple-quantum spin counting, READOR, REAPDOR
National Category
Natural Sciences
Research subject
Physical Chemistry
Identifiers
urn:nbn:se:su:diva-116232 (URN)978-91-7649-174-4 (ISBN)
Public defence
2015-06-03, Magnéli Hall, Arrhenius Laboratories, Svante Arrhenius väg 16 B, Stockholm, 13:00 (English)
Opponent
Supervisors
Note

At the time of the doctoral defense, the following paper was unpublished and had a status as follows: Paper 4: Accepted.

Available from: 2015-05-11 Created: 2015-04-15 Last updated: 2015-06-23Bibliographically approved

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