A Structural and chemical Approach Towards Understanding the

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A Structural and chemical Approach Towards Understanding the Aqueous Corrosion of Sodium Aluminoborate Glasses Saurabh Kapoor, Randall E Youngman, Kiryl Zakharchuk, Aleksey A Yaremchenko, Nicholas J. Smith, and Ashutosh Goel J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.8b06155 • Publication Date (Web): 07 Nov 2018 Downloaded from http://pubs.acs.org on November 10, 2018

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The Journal of Physical Chemistry

A Structural and Chemical Approach Towards Understanding the Aqueous Corrosion of Sodium Aluminoborate Glasses Saurabh Kapoor1, Randall E. Youngman,3 Kiryl Zakharchuk,2 Aleksey Yaremchenko,2 Nicholas J. Smith,3 Ashutosh Goel,1,* 1

Department of Materials Science and Engineering, Rutgers, The State University of New Jersey,

Piscataway, NJ, 08854, USA CICECO – Aveiro Institute of Materials, Department of Materials and Ceramic Engineering, University

2

of Aveiro, 3810-193 Aveiro, Portugal 3

Science and Technology Division, Corning Incorporated, Painted Post, NY, 14870, USA

*Corresponding author Email: [email protected]; Ph: +1-848-445-4512

1 ACS Paragon Plus Environment

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Abstract Despite an ongoing strenuous effort to understand the compositional and structural drivers controlling the chemical durability of oxide glasses, there is still no complete consensus on the basic mechanism of glass dissolution that applies to a wide composition space. One major reason for this problem is the structural complexity contained within the multicomponent silicate glasses chosen for glass corrosion studies. The non-silicate network polyhedra present in these glasses interact with one another, often in unpredictable ways, by forming a variety of structural associations, for example, Al[IV] – B[III], B[III] – B[IV], resulting in significant influence on both the structure of the glass network and related macroscopic properties. Likewise, the formation of a variety of next-neighbor linkages, as well as increasingly complex interactions involving Si and differently coordinated next-nearest neighbor cations are very difficult to decipher experimentally. Consideration of these factors motivates instead a different strategy: that is, the study of a sequence of SiO2 – free ternary or quaternary glass compositions, whose structures can be unambiguously determined and robustly linked to their corrosion properties. With this aim, the present study is focused on understanding the structural drivers governing the kinetics and mechanism of corrosion of ternary Na2O – Al2O3 – B2O3 glasses (in water) over a broad composition space comprising compositions with distinct structural features. It has been shown that the addition of Al2O3 to binary sodium borate glasses decreases their corrosion rate in water and converts their dissolution behavior from congruent to incongruent leading to formation of six coordinated alumina, and higher concentration of four coordinated boron (in comparison to pre-dissolution glasses) in post-dissolution glass samples. The drivers controlling the corrosion kinetics and mechanism in these glasses based on their underlying structure have been elucidated. Some open questions have been proposed which require an extensive analysis of surface chemistry of pre- and post-dissolution samples and will be investigated in our future work.


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1. Introduction Silicate glasses are known to possess chemical durability superior to most other glass families, which is evident from the survival of medieval window and natural glasses (e.g., obsidians and Libyan Desert glass) for thousands or even millions of years.1, 2 For this reason, and in addition to its many other beneficial properties, multicomponent silicate glasses form the back-bone of glass industry, and are used in a majority of the technological and functional applications. This includes, for example, architecture, and automotive industry, pharmaceutical and electronic packaging, laboratory and kitchenware, etc. 3-6 However, despite their high chemical durability, silicate glasses gradually dissolve and corrode when exposed to aqueous solutions, where their dissolution kinetics are a function of several thermo-chemical parameters such as glass chemistry, solution chemistry, reaction temperature, time and pH.7, 8 An in-depth understanding of the fundamental science governing the glass corrosion will allow us to design glass compositions with predictable degradation rates—behavior which can be utilized to help with critically needed advancements in, for example, immobilization of radioactive waste in glasses, or design of third generation bio-resorbable glasses for tissue engineering.9,

10

These potential benefits to

mankind have led to decades of research effort focused on understanding the compositional and structural drivers controlling the mechanism and kinetics of silicate glass corrosion. The evolution has been driven by an idea that the understanding of relationships between chemical composition of glasses and their structure at an atomistic level will not only help in unearthing the fundamental mechanisms of glass corrosion but will also enable the development of models to predict material functions and properties from first principles.11, 12 Despite an ongoing strenuous effort in this direction, there is still no complete consensus on the basic mechanism of glass dissolution that applies to a wide composition space. One major reason for this problem is the structural complexity contained within the multicomponent silicate glasses chosen for glass corrosion studies (for example, ISG

13, 14

). While these compositions are relevant for their specific

applications, they are commonly comprised of multiple network former species, for example, SiO 2, B2O3, Al2O3 and P2O5, and other non-framework cations. The non-silicate network polyhedra present in these



glasses interact with one another, often in unpredictable ways, by forming a variety of structural associations (including but not limited to Al[IV] – B[III], B[III] – B[IV], AlO5, AlPO4, and BPO4 units),15-20 resulting in significant influence on both the structure of the glass network and related macroscopic properties. Likewise, the formation of a variety of next-neighbor linkages (e.g. Si–O–Al, Si–O–B, Si–O– P and Si–O–Al–O–B linkages), as well as increasingly complex interactions involving Si and differently coordinated next-nearest neighbor cations (e.g. Si-O-P[n] where n can be a range of values denoting connectivity to the glass network), are very difficult to even experimentally measure and decipher using techniques such as 29Si, 27Al, 11B or 31P magic angle spinning – nuclear magnetic resonance (MAS-NMR) spectroscopy, much less connect structure to corrosion properties. Although attempts have been made to understand these interactions in silicate glasses using

17

O triple quantum (3Q) MAS-NMR

spectroscopy,21-23 models pertaining to these structural details are still under-developed. Due to this reason in case of multicomponent glasses, results are typically reported in the form of “composition– property relationships” through the development of empirical models,24-26 as their chemical and structural complexity complicates understanding of the fundamental material science governing these relationships. This hinders the evolution from empirical models based on “composition–property relationships” to more robust quantitative/predictive models based on “composition–structure–property relationships”.27 Consideration of these factors motivates instead a different strategy: that is, the study of a sequence of simpler ternary glass compositions, whose structures can be unambiguously determined and robustly linked to corrosion properties. One example is SiO2-free oxide glasses (for example, borates, phosphates), which provides us an opportunity to obtain deeper and clearer insight into the interactions between nonsilicate framework units and non-framework cations in the glass structure,15, 18, 28-30 and their impact on the macroscopic glass properties.31-36 This fundamental knowledge, when extended to more complex silicate glasses, will not only allow us to understand the drivers of glass dissolution based on underlying glass structure, but will also form the baseline for development of non-empirical models to predict the chemical durability of multicomponent oxide glasses. The present study is focused on understanding the dissolution mechanism and kinetics of


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corrosion of glasses in the Na2O-Al2O3-B2O3 ternary system. The focus on borate glasses is justified considering the fact that combination of borate glasses with SiO2 results in the formation of borosilicate glasses – one of the most important glass systems from scientific, technological and commercial viewpoints.14, 37, 38 In fact, it has been shown that most structural features in alkali borosilicates are similar to those of alkali borates.38 It is therefore important to understand the structure–property relationships in borate-based glass compositions before extending them to more complex borosilicates, or aluminoborosilicates.39-41 Borate glasses are also fascinating in that they do not behave as might be expected when compared to most well-studied silicate glass systems, as is evident from equations 1-3.42 In silicates, ≡Si–O–Si≡ + A2O = ≡Si–O-A+ + A+-O–Si≡ (creation of non-bridging oxygen, NBO) (1) In borates – Scenario 1, ½A2O + BØ3 = A+ + BOØ2- (creation of NBO, as in silicates)

(2)

In borates – Scenario 2, ½A2O + BØ3 = A+ + BØ4- (conversion of BIII to BIV)

(3)

where, A2O refers to an alkali oxide, and Ø represents bridging oxygen atoms that are shared between adjacent borate units. The further choice to focus on the sodium aluminoborate system has been made to enable glass compositions with a rich variety of structural features over a broader composition space and glass forming range – for example, glasses can be designed with different Al[IV]/Al[V] ratios. From a technological viewpoint, borate-based glasses have been historically confined to the realms of academic research due to their comparatively poor chemical durability, with limited advances in our understanding of the fundamental science governing the chemical durability of these glasses in the intervening years. This assertion is supported by the minimal number of research articles published on this topic (on binary and ternary borates) in the last five decades (excluding research articles investigating the in vitro/in vivo bioactivity of borate glasses).43-49 However, with the advent of multicomponent borate glasses for functional applications in human biomedicine,50-55 waste management,56,

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and cover glass

applications,58 the requirement for better understanding about the structural drivers governing the chemical durability of these glasses has become increasingly important. With the aforementioned perspective, the work presented in this paper is focused on understanding the dissolution behavior and early-stage release kinetics of sodium aluminoborate glasses



designed over a broad composition space covering per-alkaline (Na/Al>1), meta-aluminous (Na/Al = 1) and per-aluminous (Na/Al 1) at low Al2O3 content. With increasing Al2O3 concentration, the fraction of 5- and 6-coordinated aluminum increases monotonically, with the highest concentration of AlO5 units (~23%) being detected in the per-aluminous ([Na2O]/[Al2O3] 8), Al(OH)4- (aq) is formed. Therefore, the suppressed release of Al2O3 from the glasses in this study is likely attributed to this pH effect, where the observed pH change is influenced primarily—and in a competing sense—by the release of boron and sodium from the glasses. The release of boron is tantamount to additions of boric acid in solution, and has the effect of decreasing solution pH, whereas Na release will show an opposite effect on the pH of the leachate solution. The increase in pH with increasing Al2O3 in the glasses in AB series is therefore likely attributed to (i) a relative decrease in the boron concentration in the bulk glass compositions, reducing the amount of boron being released from the glasses into DI water to buffer the release of alkali, and (ii) NaOH being a strong base (pKb ~ –0.5) in comparison to the weak acidic character of boric acid (H3BO3; pKa = 9.2). Turning to the NB series of glasses, the final pH of the solution is observed to increase systematically as a function of Na concentration in the glasses (Inset: Figure 8b).84 Commensurate with the pH trends, the normalized dissolution rates of Na, Al and B from the NB glasses are also positively correlated to the Na2O content in the glasses. However, based on the structural description of these glasses as investigated using MAS-NMR and Raman spectroscopy, it is highly likely that different structural descriptors may be influencing the rate of glass dissolution depending upon their [Na2O]/[B2O3] ratio, as explained below. In the case of per-alkaline glasses with Na2O/B2O3 < 1, the decrease in chemical durability with increasing Na2O is intriguing, and its structural origin is likely attributable to several competing effects. First, there is a relative increase in N4 fraction in this regime, from ~9% up to a peak of ~23%—all things being equal, this trend might be expected to increase the connectivity of the network and its corresponding durability, as has been observed in bulk borate glasses.45,

47

However, in this case the

increasing N4 fraction with Na2O concentration is effectively offset by the overall reduction in B2O3 content in the glass composition (from 65mol% down to 45mol%), such that the absolute B IV fraction of the network increases only slightly in this portion of the composition walk. It can be concluded from the present study that the increasing dissolution rate across this series up to the N4 maximum (Na/B 0.7 agrees with the study of Zuchner et al.,16 where it has been shown that, at [Na2O]/[B2O3] > 0.7, the formation of asymmetric BO3 is favored in comparison to continued BO4 formation. The formation of these NBO-bearing structural units decreases the overall connectivity of the network, leading to an intuitive increase in dissolution rate proportional to NBO concentration, likewise observed in other systems91. The MAS NMR results suggests that hydrolysis of bridging oxygen atoms around Al atoms at the glass-water interface is accompanied by a change in the coordination number of Al, since the structure of the parent glass consists primarily of tetrahedral Al. The formation of 6-fold coordinated aluminum on the leached surface of an aluminosilicate glass was first reported by Tsomaia et al. 92 It is worth noting that Criscenti et al.93 had hypothesized the formation of AlV units as an intermediate step during the conversion of AlIV to AlVI units in aluminosilicate glass corrosion. However, neither the experimental results of Criscenti et al.93 nor the results obtained in this study exhibit the formation of any AlV units as an intermediate step during glass dissolution.



Given that the bulk structure of the pre-dissolution glasses was primarily dominated by 4coordinated aluminum, its conversion to octahedral coordination post-dissolution may occur through two different mechanisms. The first possibility is the structural re-arrangement in the aluminoborate glass network due to the selective removal of charge compensating cations (i.e., Na+ ⇋ H+) and B in the glass to form an interdiffusion layer followed by protonation and rupture of bridging bonds (e.g., Al-O-Al, BO-Al) and reconstruction of the alumina network to form the hydrated surface layer that has a different structure and reactivity than the parent material. Since the structural reorganization of the hydrated layer to form higher-coordinated Al would require the formation of AlV as an intermediate step before the formation of AlVI, the absence of such species in the NMR spectra tends to contradict this mechanism. The second possibility is the precipitation of octahedral aluminum due to breakage of the aluminoborate network at the glass-fluid interface, followed by the immediate precipitation of an Al–OHrich altered layer at the glass-fluid interface. While in the first scenario (structural re-arrangement in the hydrated glass to form 6-coordinated Al – OH layer), the concentration of 6-coordinated alumina-rich phases in post-dissolution samples should be dictated by the Al2O3 concentration in the parent glass compositions, the XRD results of post-dissolution samples do not support this hypothesis. The qualitative crystalline phase fraction (based on the peak intensity of phase reflections) in post-dissolution glass samples are positively correlated with their dissolution rates (Figure 9, S5 and S6), implying the possibility of a dissolution-precipitation mechanism at play. Such a mechanism is further supported by the fact that precipitation of boehmite at 65 ºC from a supersaturated solution has been shown to depend on the Al2O3/NaOH ratio in the solution. At Al2O3/NaOH > 1, the precipitation of boehmite takes place at higher temperatures (>180 ºC); whereas at Al2O3/NaOH

A Structural and chemical Approach Towards Understanding the

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