Hemijska postojanost polifosfatnog stakla u različitim sredinama

Abstract

Proces rastvaranja stakla je veoma kompleksan fenomen, koji zavisi od više faktora: sastava stakla, postupka njegovog dobijanja, površine stakla, temperature, pH rastvarača, brzine protoka rastvarača, vremena dejstva i dr. Ovaj proces se odvija u više faza, te je moguće kontrolisati ukupno vreme procesa, favorizovanjem ili suzbijanjem neke od ovih faza. Iako fosfatna stakla imaju odlične optičke osobine, njihova šira primena je bila ograničena zbog njihove niske hemijske postojanosti. Poslednjih godina, fosfatna stakla su privukla pažnju istraživača zbog toga što poseduju bioaktivnost i biokompatibilnost, što ih svrstava u red materijala pogodnih za primenu u medicini, stomatologiji i poljoprivredi. Upravo zbog toga je veoma značajno da se poznaje ponašanje ovih stakala pri procesu rastvaranja u različitim rastvaračima. Hemijska aktivnost fosfatnih stakala u procesima koji nastaju pri kontaktu sa raznim rastvorima pomaže pri dizajniranju i proizvodnji novih materijala. Dosadašnja istraživanja u oblasti hemijske postojanosti neorganskih stakala su pokazala da i manje postojana stakla mogu da budu od velike koristi u slučaju njihove kontrolisane rastvorljivosti. Glavna prednost stakala je što im se lako može promeniti hemijski sastav. Promenom sastava, odnosno uvođenjem novih komponenti u sastav stakla, moguće je kontrolisati kinetiku i mehanizam rastvaranja. U ovoj doktorskoj disertaciji prikazani su rezultati proučavanja fenomena rastvaranja polifosfatnog stakla sastava: 45 P2O5·3 SiO2·25 K2O·15 CaO·10 MgO·1 ZnO·1 MnO u dejonizovanoj vodi, 2 % rastvoru limunske kiseline i simuliranoj telesnoj tečnosti (SBF) u temperaturnom intervalu 15-50 C za vremena 0,5-720 h. Ispitivanja su izvršena pri stacionarnim uslovima u nezasićenim rastvorima. U ispitivanjima su korišćene dve grupe praškastih uzoraka granulacija: 0,1-0,3 i 0,3-0,65 mm i kompaktni uzorci stakla. Za analizu rezultata eksperimenata korišćene su metode: atomska apsorpciona spektroskopija (AAS), spektrofotometrijska metoda (SF), infracrvena spektroskopija (FTIR), diferencijalno-termijska analiza (DTA), skenirajuća elektronska mikroskopija (SEM), energetska disperziona spektroskopija (EDS), analiza specifične površine staklenog praha i određivanje pH vrednosti sredine. Analizirani su: hemijski sastav, struktura i značajne fizičke osobine izabranog stakla, promene mase uzoraka sa vremenom i temperaturom, promene koncentracija jona u rastvorima sa vremenom i temperaturom, promene pH rastvora sa vremenom i temperaturom i promene morfologije i sastava površinskih slojeva uzoraka sa vremenom i temperaturom pri eksperimentima rastvaranja uzorka u dejonizovanoj vodi, 2% rastvoru limunske kiseline i simuliranoj telesnoj tečnosti (SBF). Zavisnosi normalizovanog smanjenja mase stakla usled rastvaranja i normalizovane koncentracije jona u rastvorima od vremena kod svakog uzorka u sva tri rastvarača pokazuju tri uočljive faze. Pri kratkim vremenima uočavaju se linearne (najveće) promene normalizovanih koncentracija sa vremenom - "početna" faza. Naredna faza je "prelazna" faza u kojoj promene normalizovanih koncentracija opadaju sa vremenom, a takođe i brzina rastvaranja stakla. U trećoj - "krajnjoj" fazi, promene normalizovanih koncentracija su takođe linearne i male a brzine rastvaranja su nekoliko desetina puta manje od početne brzine. Proces rastvaranja počinje formiranjem rupa na površini uzorka. Pri porastu temperature i vremena formira se saćasta struktura. Dalje se proces odvija formiranjem belog sloja na površini uzorka. Debljina ovog sloja raste tokom vremena, kako proces rastvaranja napreduje. Na uzorcima rastvaranim u dejonizovanoj vodi i SBF-u uočava se veća gustina rupa i otvora u saćima u odnosu na uzorke rastvarane u 2% rastvoru limunske kiseline. Takođe, stepen pokrivenosti površine kao i debljina belog sloja su primentno veći kod uzoraka rastvaranih u dejonizovanoj vodi i SBF-u. Uticaj temperature na koncentracije jona pri rastvaranju u sva tri rastvarača, uočava se kao suženje "početne" i "prelazne" faze dok se "krajnja" faza širi. Početne brzine rastvaranja u dejonizovanoj vodi kreću se od 0,063-2,55 gm-2h-1, u 2% rastvoru limunske kiseline 2,03-28,78 gm-2h-1 i u SBF od 0,32-0,84 gm-2h-1. Vrednosti energija aktivacije difuzije katjona u dejonizovanoj vodi kreću se u intervalu 60-130 kJmol-1, a u 2 % rastvoru limunske kiseline u intervalu 31-76 kJmol-1. U "prelaznoj" fazi uočava se naglo smanjenje brzine gubitka mase i brzine otpuštanja jona u odnosu na početne brzine. Smanjenje brzine zavisi od temperature i veće je pri višim temperaturama. Pri rastvaranju u dejonizovanoj vodi brzina rastvaranja se smanjuje od 2 do 10 puta, u 2 % rastvoru limunske kiseline od 4 do 20 puta i u SBF od 4 do 10 puta "Krajnju" fazu rastvaranja karakteriše sporo smanjenje brzine rastvaranja. Dominantan mehanizam koji deluje u ovoj fazi je hidroliza mreže stakla. Brzine otpuštanja jona u "krajnjoj" fazi su u dejonizovanoj vodi u intervalu 5,08*10-4-6,57*10-2 gm-2h-1, u 2 % rastvoru limunske kiseline u intervalu 1,06*10-3-6,92*10-2 gm-2h-1 i u SBF 8,74*10-4-7,44*10-3 gm-2h-1. Vrednosti energija aktivacija otpuštanja jona u "krajnjoj" fazi procesa rastvaranja u dejonizovanoj vodi su u intervalu 41-66 kJmol-1, a u 2 % rastvoru limunske kiseline u intervalu od 42-85 kJmol-1.

The dissolution of the glass is a complex phenomenon that depends upon numerous parameters, such as glass composition, glass preparation procedure, surface condition, temperature, pH of the solution, leaching solution volume, flow rate, exposing time of the glass to the solution, etc. The process occurs in several steps. It allows the regulation of total dissolution time through the favoring or suppressing some of these steps. Although phosphate glasses have excellent optical properties, their wider application is restricted because of poor chemical durability. In recent years, the remarkable bioactivity and compatibility of phosphate glasses have attracted attention due to their potential application in medicine, dentistry and agriculture. Therefore, detailed knowledge of the dissolution behavior of phosphate glasses in different solutions is very important. Different chemical activity of phosphate glasses, during the contact with different solutions, provides for the opportunity to design and manufacture new materials. Previous studies in the field of chemical durability of inorganic glass show that even less durable glasses can be of great importance if their dissolution process is controlled. The main advantage of glasses is their flexible behavior in response to changes of the chemical composition. This allows the controlling of the kinetics as well as mechanism of dissolution of glass through the introduction of new components in different amount. The subject of this thesis is the dissolution behavior of polyphosphate glass of composition 45 P2O5·3 SiO2·25 K2O·15 CaO·10 MgO·1 ZnO·1 MnO in deionized water, 2 % solution of citric acid and simulated body fluid (SBF) at the temperature interval of 15-50 C for time 0.5-720 h. The dissolution experiments were conducted under static conditions in the unsaturated solution. Two glass powder samples, granulation 0.1-0.3 and 0.3-0.65 mm, and the bulk glass samples, were investigated. The methods employed for investigation were: atomic absorption spectroscopy (AAS), spectrophotometry (SF), infrared spectroscopy (FTIR), differential thermal analysis (DTA), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), analysis of specific surface areas and pH measurements. The experiments on sample dissolution in the deionized water, 2 % solution of citric acid and simulated body fluid (SBF) provided analysis of the chemical composition, structure and physical properties of the parent glass. The changes in ions concentration in the solution during time and with temperature, changes in pH measurements during time and with temperature, and changes in morphology and composition of surface layers in the samples during time and with temperature were determined in those experiments. Dependence of the normalized mass release and the normalized ions concentration in the solution on dissolution time in each solvents showed three phases. For shorter dissolution times, the normalized ions concentration values showed the linear (largest) changes, this phase is a so-called "initial" stage. The "initial" stage is followed by a "transitional" stage. The trend of changes in the value of normalized concentration indicates a significant decrease in the dissolution rate in the "transitional" stage. In the third stage, named "final", linear dependence of normalized concentration can be observed, but changes are small. The dissolution process starts with the formation of pits in the surface area of a sample. For prolonged treatment, the honeycomb-like structure covers the whole surface of the glass. The white layer can be observed on the surface. Thickness of this layer grows over time, as the dissolution process continues. The samples tested in the deionized water and SBF show bigger density of pits and holes in the honeycomb-like structure, compared to the samples treated in 2 % solution of citric acid. Also, the extent of covered surface, as well as the thickness of white layer, are notably bigger with the samples treated in the deionized water and SBF. The temperature impact on concentration of the dissolved ions during dissolution in all three solvents is observed as narrowing of the "initial" and "transitional" stage, while the "final" stage is expanding. The initial dissolution rates in deionized water start from 0.063-2.55 gm-2h-1; in 2% solution of citric acid from 2.03-28.78 gm-2h-1 and in SBF from 0.32-0.84 gm-2h-1. The anion diffusion activation energy values in the deionized water vary within the interval of 60-130, while in the 2 % solution of citric acid vary within the interval of 31-76 kJ/mol. The sudden decrease in dissolution rate and ions release rate is noted in the "transitional" stage. The dissolution rate decreases 2 to 10 times during dissolution in the deionized water, 4-20 times in the 2 % solution of citric acid and 4-10 times in the SBF. The slow decrease in dissolution rate characterizes "final" dissolution stage. The glass matrix hydrolysis is a dominant mechanism in this stage. The ions release rates in the deionized water are in the 5.08*10-4-6.57*10-2 gm-2h-1 interval; in the 2 % solution of citric acid in the 1,06*10-3-6,92*10-2 gm-2h-1 interval, while in the SBF 8,74*10-4-7,44*10-3 gm-2h-1. The values of the ion diffusion activation energy in the final stage of dissolution process in the deionized water are in the 41-66 kJmol-1 interval, while in the 2 % solution of citric acid are in the 42-85 kJmol-1 interval.