Kinetic effects of ultrasound in aqueous acetonitrile solutions
Date
2013-08-02
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
Abstract
Tänapäeval leiab ultraheli laialdast rakendust nii laborites kui tööstuses tänu ainulaadsele kavitatsioonifenomenile, mis kiirendab ja soodustab mitmeid keemilisi ja füüsikalisi protsesse. Peale mehhaaniliste efektide võib ultraheli initsieerida mitmeid homogeenseid ja heterogeenseid reaktsioone, genereerides vabu radikaale, mis kutsuvad lahuses esile ahelreaktsioone. Vastavalt kehtivatele sonokeemia tõekspidamistele, homogeenne polaarne (iooniline) reaktsioon, mis pole võimeline ümberlülituma radikaalsele rajale, ei tohiks olla tundlik ultraheli suhtes. Siiski on leitud mitmeid näiteid ultraheli toimel kiirenenud homogeensetest polaarsetest reaktsioonidest (enamasti hüdrolüüsi ja solvolüüsi reaktsioonid). Ultraheliefekte on peamiselt täheldatud vesi-orgaaniliste segasolventide kasutamisel reaktsioonikeskkonnana. Intuitiivselt arvati, et heliefektid on seotud ultraheli solvendistruktuuri lõhkuva toimega, mistõttu muutub solvatatsioon soodsamaks ja reaktsioonikiirus kasvab. See arvamus leidis kinnitust ka antud töös, kuid juba molekulaarsel tasandil.
Enamjaolt on kineetilised ultraheliuuringud läbi viidud vesi-etanool segudes. Käesolevas töös oli vaatluse all ultraheli mõju estri neutraalse hüdrolüüsi reaktsioonile vesi-atsetonitriil segudes. Kuna atsetonitriil on polaarsem kui etanool, kuid samas ei moodusta hästi vesiniksidemeid, siis atsetonitriili segudes võib eeldada teistsuguseid solvendiefekte. Ultraheliuuringute teostamiseks arendati antud töös välja originaalne on-line reaktsioonikineetika mõõtmise süsteem.
Leiti, et ilma ultrahelita vähenes hüdrolüüsi kiirus ühtlaselt atsetonitriili kontsentratsiooni kasvades, samas kui ultraheliga kiiritades muutus reaktsioonikiirus üllatuslikult vähe. Seega ultraheliefekt, kson/knon (ultraheliga ja ilma ultrahelita mõõdetud kiiruskonstantide suhe), suureneb solvendiefekti kasvades.
Leiti, et reaktsiooni kiiruse väike muutumine ultraheli toimel, nn tasandusefekt on üldine nähtus, kuna see esines erinevate reaktsioonide korral erinevates vesi-orgaanilistes solvendisegudes. Võib väita, et ultraheli toob esile reaktsiooni keskkonnas muutusi, mistõttu substraadi solvatatsioon ja seega reaktsiooni kiirused ultraheli all jäävad enamvähem samaks.
Solvatatiooni nähtuste paremaks mõistmiseks vesi-orgaanilistes solventsegudes teostati molekulaardünaamika (MD) arvutisimulatsioonid etüülatsetaadile, kui mudelestrile, vesi-etanool ja vesi-atsetonitriil keskkondades. Selgus, et estri molekul on eelissolvateeritud kaassolvendi molekulide poolt kõikides uuritud segudes, kusjuures esineb erinev solvatatsioon vastavalt etanooli ja atsetonitriili korral.
Etüülatsetaadi happekatalüütilise hüdrolüüsi kiirus langeb etanooli hulga suurenemisega solventsegus. Leiti, et kiiruse vähenemine on lineaarselt seotud etanooli sisalduse kasvuga estri esimeses solvaatkihis. Atsetonitriili korral tekkis lisaks esimesele solvaatkihile estri ümber teinegi atsetonitriilirikas solvendikiht. Kaassolvendi sisalduse suurenemine solvaatkattes põhjustab vee aktiivsuse langemist selles mikrokeskkonnas ja seega reaktsiooni kiirus kahaneb.
Etüülatsetaadi lahusele vesi-etanooli segus teostati MD simulatsioonid lisaks kahel erineval temperatuuril, 280 K ja 330 K. Temperatuuri tõustes asendus etanool esimeses solvaatkattes suures ulatuses veega. Tegemist on termo-solvatokromismi nähtusega, mil temperatuuri tõustes reagendi solvaatkate vaesub orgaanilise solvendi poolest.
Kõige eelneva põhjal järeldati, et ultraheli põhjustab lahuses olevate osakeste kineetilise energia suurenemise, mis toob esile muutused substraadi solvatatsioonis ning seega mõjutab reaktsiooni kiirust. Samas ilmnes, et ultraheli ei pea ilmtingimata lõhkuma solvendi klastreid lahuses täielikult, nagu varem oletati. Vastupidi, märgatava reaktsiooni kiiruse muutmiseks ei ole vaja rakendada ekstreemseid energiaid.
Lisaks leiti märkimisväärsed kineetilised ultraheliefektid heli intensiivsustel, mis ei ole võimelised tekitama kavitatsiooni. Esmakordselt tõestati eksperimentaalselt, et kineetilised ultraheliefektid võivad esineda ilma kavitatisooni nähtuseta. See laiendab põhimõtteliselt sonokeemilise meetodi rakendusi, võimaldades füsikokeemilisi uuringuid erinevates keemilistes ja biokeemilistes protsessides.
In the present work kinetics of the pH-independent hydrolysis of 4-methoxyphenyl dichloroacetate was investigated with and without ultrasonic irradiation in water-acetonitrile binary mixtures with molar ratios of acetonitrile from 0.008 to 35 wt% and the kinetic sonication effects (kson/knon) were calculated. All experiments were carried out with an on-line spectrophotometric system designed for this study. Our results revealed that addition of acetonitrile to the reaction medium substantially decreased the rate of hydrolysis of the ester, while rate constants determined under ultrasound showed little susceptibility to changes in solvent composition. Evidently, the size of kinetic sonication effects are related to the size of solvent effects. In parallel, MD simulations of the structure of the water-ethanol and water-acetonitrile solutions were performed with ethyl acetate as the model ester to cast light on solvent effects in aqueous-organic solutions. Ethyl acetate was preferentially solvated by co-solvent in all of these cases, however, the different solvation patterns in acetonitrile and ethanol aqueous solutions were found. MD simulations of ethyl acetate solution in aqueous ethanol medium were performed at 280 K and 330 K. With increasing temperature, a significant proportion of alcohol molecules were replaced by water molecules in the first solvation shell. This result confirmed the principle of thermo-solvatochromism, which predicts depletion of the solvation shell of the organic solvent with rising temperature. The addition of ethanol to the aqueous solution of ethyl acetate decreased the rate of hydrolysis of the ester. It was found that the decrease of the reaction rate was almost linearly related to the increase in the ethanol content in the first solvation shell of the ester. The abundance of acetonitrile relative to water in the solvation shell grew fast with the increase in the co-solvent content in the bulk solution. In parallel to this, the formation of a second acetonitrile-rich solvation shell was observed. This mode of ester solvation caused dramatic drop in the reaction rate with the increase in the acetonitrile content in the reaction mixture, as enhanced co-solvent content in the solvation shell decreased water activity in the microenvironment and lowered reaction rate. The principal conclusion from the MD simulation study was that ultrasound enhanced the kinetic energy of molecules and through this effect shifted solvation equilibrium and affected reaction rate. At the same time it appeared that ultrasound did not completely break down the solvent shells or clusters in the solution, as was believed previously. In contrast to this, application of high energy was not required to affect reaction rates. The rate levelling effect, observed in aqueous-organic binary solvents under ultrasound radiation, was independent of the solvent composition, pointing to almost identical solvation state of the reagents that could explain this rate levelling phenomenon. Finally, significant kinetic sonication effects were found at the intensity of irradiation that was not capable to induce cavitation in the solution. This result provided direct experimental evidence that the observed kinetic effects of ultrasound could occur without cavitation. This finding could lead to a principal extension of the sonochemical method and could open new perspectives for the physicochemical investigations into variety chemical and biochemical processes.
In the present work kinetics of the pH-independent hydrolysis of 4-methoxyphenyl dichloroacetate was investigated with and without ultrasonic irradiation in water-acetonitrile binary mixtures with molar ratios of acetonitrile from 0.008 to 35 wt% and the kinetic sonication effects (kson/knon) were calculated. All experiments were carried out with an on-line spectrophotometric system designed for this study. Our results revealed that addition of acetonitrile to the reaction medium substantially decreased the rate of hydrolysis of the ester, while rate constants determined under ultrasound showed little susceptibility to changes in solvent composition. Evidently, the size of kinetic sonication effects are related to the size of solvent effects. In parallel, MD simulations of the structure of the water-ethanol and water-acetonitrile solutions were performed with ethyl acetate as the model ester to cast light on solvent effects in aqueous-organic solutions. Ethyl acetate was preferentially solvated by co-solvent in all of these cases, however, the different solvation patterns in acetonitrile and ethanol aqueous solutions were found. MD simulations of ethyl acetate solution in aqueous ethanol medium were performed at 280 K and 330 K. With increasing temperature, a significant proportion of alcohol molecules were replaced by water molecules in the first solvation shell. This result confirmed the principle of thermo-solvatochromism, which predicts depletion of the solvation shell of the organic solvent with rising temperature. The addition of ethanol to the aqueous solution of ethyl acetate decreased the rate of hydrolysis of the ester. It was found that the decrease of the reaction rate was almost linearly related to the increase in the ethanol content in the first solvation shell of the ester. The abundance of acetonitrile relative to water in the solvation shell grew fast with the increase in the co-solvent content in the bulk solution. In parallel to this, the formation of a second acetonitrile-rich solvation shell was observed. This mode of ester solvation caused dramatic drop in the reaction rate with the increase in the acetonitrile content in the reaction mixture, as enhanced co-solvent content in the solvation shell decreased water activity in the microenvironment and lowered reaction rate. The principal conclusion from the MD simulation study was that ultrasound enhanced the kinetic energy of molecules and through this effect shifted solvation equilibrium and affected reaction rate. At the same time it appeared that ultrasound did not completely break down the solvent shells or clusters in the solution, as was believed previously. In contrast to this, application of high energy was not required to affect reaction rates. The rate levelling effect, observed in aqueous-organic binary solvents under ultrasound radiation, was independent of the solvent composition, pointing to almost identical solvation state of the reagents that could explain this rate levelling phenomenon. Finally, significant kinetic sonication effects were found at the intensity of irradiation that was not capable to induce cavitation in the solution. This result provided direct experimental evidence that the observed kinetic effects of ultrasound could occur without cavitation. This finding could lead to a principal extension of the sonochemical method and could open new perspectives for the physicochemical investigations into variety chemical and biochemical processes.
Description
Väitekirja elektrooniline versioon ei sisalda publikatsioone.
Keywords
vesi, nitriilid, hüdrolüüs, lahustiefektid, ultraheli, keemiline kineetika, water, nitriles, hydrolysis, solvent effects, ultrasound, chemical kinetics