CORROSION PROPERTIES OF
CALCIUM-BISMUTH-BORATE GLASSES WITH SILVER
V. SIMON*, M.
Spinu**, R. ŞTEFAN**
* Faculty of Physics, “Babeş-Bolyai” University, 400084
**University
of Agricultural Sciences and Veterinary Medicine, 400372
Abstract. Quaternary Ag2O-CaO-Bi2O3-B2O3
glasses are investigated with regard to release behaviour and local structure.
The dissolution behaviour in water and physiological serum shows that the cations are released rapidly or gradually and points out a
multi-step process, generally characterised by higher rates in water than in
physiological serum. The structural effect of silver addition to bismuth-borate
glasses is observed from infrared spectroscopic data. The antibacterial
activity of the investigated samples was tested on five bacteria.
Key
words: corrosion, oxide glasses, microstructure, silver.
introduction
Over the past
two decades a general experimental and theoretical framework has been developed
for understanding the corrosion behaviour of glass in diverse environments [5].
Novel systems allowing controlled release of components are intensely
investigated [1]. Various inorganic antibacterial materials containing silver
have been developed and some of them are in commercial use. More chemically
durable materials, which slowly release the silver ion for a long period, are
developed for medical applications. The silver ions are rapidly released or
they can be gradually released into the water at a controlled rate and show an
excellent antibacterial property [10]. Silver is capable of killing over 650
different forms of bacteria, viruses, Candida, and molds
[14]. Bismuth trace element is used for
eye/ear infection and sore throats. The antibacterial activity of bismuth is
also known [4]. Although boron is potentially toxic to all organisms, and, as
boric acid and borax, has been used as a pesticide and food preservative,
higher animals usually do not accumulate boron because of their ability to
rapidly excrete it [13]. It appears that boron may influence the production of
hormones (including the active form of vitamin D), improve brain and
psychological functions, and exert immune-boosting activity [15]. Calcium is an
essential element entering into the cell walls and bones. It is important for
blood clotting and it is implied in the complicated mechanisms of long-term
memory and learning [3]. The composition and local structure at the surface of
the samples is of great importance in their dissolution properties.
The aim of this
paper is to investigate the dissolution behaviour and the antibacterial effects
of quaternary Ag2O-CaO-Bi2O3-B2O3
glasses in correlation with their microstructure.
MATERIALS AND METHODS
The investigated
samples belong to xAg2O·(5-x)CaO·10Bi2O3·85B2O3
system (1 ≤ x ≤ 4 mol
%). They were obtained from homogenized mixtures of AgNO3, CaCO3,
Bi(NO3)3·5H2O and H3BO3
p.a. chemical reagents by melting at 1050 °C for 15 minutes in sintered
corundum crucibles in an electric furnace and quickly undercooling
at room temperature by pouring onto stainless steal plates. All as prepared
samples were transparent and colourless.
The corrosion
behaviour was followed in static regime by immersion of disk samples in
different solvents (desalinised water, physiological serum and chlorine acid
solution with pH = 1.5) by measuring the mass of samples maintained in the
incubation solutions at 40 °C for different times up to 68
hours. The samples mass was determined by using an analytic balance sensitive
to 0.1mg. The glass surface area to solution volume ratio was around 15 m–1
for all samples. The dissolution rate, DR, was calculated [9] from the measured
mass loss Dm (g), samples
surface area A (cm2) and the immersion time t (min) using the
equation DR = Dm/(A·t).
IR spectra were
recorded at room temperature from powdered glass samples pressed in tablets
with KBr, in the wave number range 400 – 4000 cm–1,
using a UR-20 Carl Zeiss spectrophotometer with a
resolution of 0.6 cm–1 at 1000 cm–1.
The sensitivity
of different bacteria to the released components was tested in aerobiosis, by use of nutrient agar plates with wells
filled with powdered glass samples. Gram-positive (Streptococcus sp, Bacillus
sp, Bacillus anthracis strain R 1190) and
Gram-negative (E. Coli, Salmonella sp. and Pseudomonas
pyocyanea) bacteria were inoculated on the agar
surface. Diameters of the growth inhibition areas were determined in cm, after
24 hours incubation at 37 °C.
Results And discussion
The results of
the corrosion test carried out in water and physiological serum are shown in
Figures 1 and 2. The
dissolution rate decreases in water from the average value 5 μg/cm2·min
to 0.5 μg/cm2·min as the Ag2O content in sample
Fig. 1.
Time dependence of specific mass loss in water from xAg2O·(5-x)CaO·10Bi2O3·85B2O3
glass samples. The lines are only guide for eyes.
Fig. 2. Time dependence of specific mass loss in physiological serum. The lines are only eye guide.
increases from 2 to 4 % mol, while in physiological serum the DR values are constant lower, under 1 μg/cm2·min, excepting the sample with 3 mol % Ag2O for which DRav = 2.6 μg/cm2·min. Mass losses could not be measured after immersion in HCl solution because all samples softened in the first 15 hours.
Some data
concerning the releasable cations of the investigated
samples, which are of interest with respect to role they could play in oxide
glass systems, are summarised in Table 1. The cationic field strength is
expressed by the ratio of the cation charge to the
square of the ionic radius and is an indication of relative bond strengths. The cations
belonging to the conventional glass former oxides are characterised by high
field strengths as compared to the cations entering
as modifiers. Both calcium and silver ions act as glass network modifiers. One
also remarks that Ca2+ and Ag+ have very close values for
ionic radii but they considerably differ with respect to their relative
bond strengths.
Table 1.
Coordination number, ionic radius, field strength, Pauling electronegativity and single bond strength for the cations [7,
11, 16] entering xAg2O·(5-x)CaO·10Bi2O3·85B2O3 glasses.
|
Cation |
Coordination number |
ionic
radius (Å) |
Cation field strength (Å–2) |
Electro-negativity (Pauling units) |
Single bond strength M-O (kJ×mol–1) |
|
B3+ |
4 6 |
0.25 0.41 |
48 35.69 |
2.04 |
808.8
±20.9 |
|
Bi3+ |
6 8 |
1.17 1.31 |
2.19 1.75 |
2.02 |
337.2
±12.6 |
|
Ca2+ |
6 8 |
1.14 1.26 |
1.53 1.26 |
1.00 |
402.1
±16.7 |
|
Ag+ |
4 6 |
1.14 1.29 |
0.77 0.60 |
1.93 |
220.1
±20.9 |
The structural units built around the glass former cations can be evidenced by vibrational
spectroscopy. The IR spectra (Fig. 3) show a strong band around 700 cm–1 assigned to the B-O-B bending
vibrations and some very weak ones appearing around 800 and 1250 cm–1
which could be attributed to the BO3 triangular unit vibrations. The
BO4 unit vibrations [6, 8] were recorded from all samples between
900 and 1100 cm–1. This result denotes that the change of the boron
coordination from BO4 to BO3 units is insignificant in
the investigated composition range. The weak infrared bands
in the 500–670 cm–1 spectral range was correlated with the presence
of [BiO6] octahedral [2, 12].
Fig. 3.
The IR spectra recorded from xAg2O·(5-x)CaO·10Bi2O3·85B2O3 glass samples.
The sensitivity of different bacteria to the released components was evaluated based on the growth inhibition diameters (d) after 24 hours of incubation. A more pronounced inhibitory effect is being observed for Gram negative bacteria than for Gram positive ones. The results evidence that the bacterial sensitivity depends on sample composition. A somewhat lower antibacterial activity of the sample containing 4 mol % Ag2O might be due to the higher dissolution stability of the corresponding glass as showed by corrosion tests.
In order to explain the different release of cations from the glass network in the investigated dissolution media, beside the composition of glasses and solvents is necessary to take into account the short-range order characterising the samples.
The structural stability of cations in glass matrices is correlated with their local symmetry. The local order in glass and crystalline compounds of the same composition has both similarities and differences. The differences are assigned to the loss of long range order, to the deviation from a perfect crystalline structure. The atoms arrangement in the first coordination sphere is determined by the chemical composition and peculiarities of chemical bonds and generates the local order around them, that is very important to depict the vitreous systems.
conclusions
The IR results show that the local structure
of the investigated samples preponderantly consists of tetracoordinated
BO4 units. BiO6 structural units were also evidenced. The
silver addition to the calcium-bismuth-borate host glass preserves the vitreous
structure, evidencing high ability to accept a relatively large Ag2O
content.
The dissolution
behaviour of xAg2O·(5-x)CaO·15Bi2O3·85B2O3
vitreous system in water and physiological serum indicates a multi-step process
generally characterised by higher rates in water than in physiological serum. The
addition of Ag2O to the lime-bismuth-borate matrix influences the
dissolution rate of samples.
The
antibacterial effect of the released cations is more
pronounced on Gram-negative bacteria. The magnitude of this effect is in
agreement with the cations release rate. The
results evidence that the bacterial sensitivity depends on sample composition.
A somewhat lower antibacterial activity of the sample containing 4 mol % Ag2O
might be due to the higher dissolution stability of the corresponding glass as
showed by corrosion tests.
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