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Medium-Range Structural Organization of Phosphorus-Bearing Borosilicate Glasses Revealed by Advanced Solid-State NMR Experiments and MD Simulations: Consequences of B/Si Substitutions Yang Yu, Baltzar Stevensson, and Mattias Edén J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.7b06654 • Publication Date (Web): 06 Sep 2017 Downloaded from http://pubs.acs.org on September 12, 2017
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The Journal of Physical Chemistry
Medium-Range Structural Organization of Phosphorus-Bearing Borosilicate Glasses Revealed by Advanced Solid-State NMR Experiments and MD Simulations: Consequences of B/Si Substitutions Yang Yu, Baltzar Stevensson, and Mattias Ed´en∗
Physical Chemistry Division, Department of Materials and Environmental Chemistry, Stockholm University, SE-106 91 Stockholm, Sweden ∗ Corresponding
author. E-mail:
[email protected] ACS Paragon Plus Environment
1
The Journal of Physical Chemistry
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Abstract
The short and intermediate range structures of a large series of bioactive borophosphosilicate (BPS) glasses were probed by solid-state nuclear magnetic resonance (NMR) spectroscopy and atomistic molecular dynamics (MD) simulations. Two BPS glass series were designed by gradually substituting SiO2 by B2 O3 in the respective phosphosilicate base compositions 24.1Na2 O–23.3CaO– 48.6SiO2 –4.0P2 O5 (”S49”) and 24.6Na2 O–26.7CaO–46.1SiO2 –2.6P2 O5 (”S46”), the latter constituting the ”45S5 Bioglass” utilized for bone grafting applications. The BPS glass networks are built by interconnected SiO4 , BO4 , and BO3 moieties, whereas P exists mainly as orthophosphate anions, except for a minor network-associated portion involving P–O–Si and P–O–B[4] motifs, whose populations were estimated by heteronuclear
31 P{11 B}
NMR experimentation. The high Na+ /Ca2+
contents give fragmented glass networks with large amounts of non-bridging oxygen (NBO) anions. The MD-generated glass models reveal an increasing propensity for NBO accommodation among the network units according to BO4 85% of the total phosphate speciation in the present Na–Ca–B–Si–P–O glasses,12 there is a non-negligible fraction of Q1P phosphate groups, whose population grows slightly for increasing B content of the glass.12 Besides retarding the phosphate release in aqueous media by the presence of a covalent O bridge at the tetrahedron (PO3 –O–X), identifying the preferential bonding partner X becomes important. For phosphosilicate glass analogs, the Q1P moieties involve P–O–Si bonds, while P–O–P fragments are normally negligible in phosphosilicate glasses with low P contents.22–25, 65 However, given the well-known strong propensity for the P5+ cation to form P–O–Al/B bonds with trivalent Al3+ /B3+ cations in alumino/boro-phosphate phases,9, 11, 53, 58, 62, 66 the anticipated bonding partner of P in BPS glasses is B rather than Si. Yet, preliminary solid-state NMR results (commented on in ref.12 ) suggested a prevalence of P–O–Si linkages, at least for Si-rich BPS glasses. Here we provide a detailed analysis by MD simulations along with heteronuclear
31 P{11 B}
solid-state NMR experimentation.
We also report a pilot study of the borate/silicate/phosphate intermixing in modifier-rich BPS glasses. Such aspects have been studied extensively for B and Si in alkali/alkaline-earth bearing M(2) O–B2 O3 –SiO2 glasses devoid of P.30–40 Since the detailed structural model of Na2 O–B2 O3 –SiO2 glasses proposed by Yun-Dell-Bray-Xiao (YDBX),28, 29 numerous experimental structural studies are presented for borosilicate glasses, encompassing reports on the {BO3 , BO4 } speciation,28–32, 35–37 the NBO partitioning among SiO4 and BO3 groups,31, 33–37 as well as the intermixing of borate and silicate network building blocks.30–40 Overall, these results suggested a preference for Si– NBO over B–NBO contacts and a growing propensity for SiO4 to interlink with other networkformers according to B[3] (ring)
