magistrsko delo
Vid Ravnik (Author), Miha Lukšič (Mentor), Franc Požgan (Thesis defence commission member), Barbara Hribar (Thesis defence commission member)

Abstract

S računalniškimi simulacijami Monte Carlo v kanonskem in velekanonskem ansamblu smo proučevali strukturne in termodinamske lastnosti modela vode s potencialom centralne sile (CF1). Študirali smo čisto vodo kot tudi vodo v utesnjenem okolju. Le-to smo modelirali z v prostoru zamrznjeno konfiguracijo Lennard-Jonesovih delcev dane gostote, pri čemer je bila interakcija delcev adsorbenta s kisiki vode odbojna. Rezultate za čisto vodo smo primerjali tudi z rezultati Ornstein-Zernikove integralske enačbe v HNC približku ter s približki, ki so vključevali mostovne funkcije referenčnega sistema togih krogel. Računalniške simulacije Monte Carlo dobro opišejo geometrijo čiste vode (koordinacijska števila OH in HH sta 2 in 1), dolžina vezi OH je primerljiva z eksperimentalno, kot HOH pa je nekoliko manjši od eksperimentalno določene vrednosti za vodo v parni fazi. Integralska enačba v HNC približku ne da zadovoljivih rezultatov za koordinacijo vode ter ne opiše strukture sistema dobro. Z vključitvijo mostovnih funkcij se lahko dokaj dobro približamo rezultatom simulacij pri temperaturi 300 K, vendar funkcije niso prenosljive (pri višjih temperaturah dobimo slabše rezultate kot s HNC približkom). Večjo pozornost smo namenili vodikovi vezi ter tetraedričnemu in translacijskemu ureditvenemu parametru, s katerima smo študirali strukturne anomalije modela. Definicija vodikove vezi na osnovi porazdelitvene funkcije parske energije vod da boljše rezultate kot enoparametrična definicija na osnovi razdalje. Pri 300 K dobimo povprečno vrednost števila vodikovih vezi na vodo okoli 3,8. Opazili smo, da modelna voda pri 300 K izkazuje v območju gostot med 0,95 in 1,15 g/mL strukturno anomalijo. S pomočjo velekanonske simulacije Monte Carlo smo s preprosto linearno odvisnostjo kemijskega potenciala od temperature uspeli reproducirati tudi gostotno anomalijo, odvisnost ρ(T) pa se dobro ujema z eksperimentalnimi podatki. Študirali smo tudi fazni diagram modela v μ-ρ projekciji. Pri dovolj nizkih temperaturah (pod 400 K) smo opazili koeksistenco dveh faz (plin-tekoče). V drugem delu smo proučevali vedenje modelne vode v neurejeni porozni snovi (prisotnosti ovir Lennard-Jonesovega tipa - adsorbenta). Ob prisotnosti adsorbenta se intermolekularna geometrija vode ni spremenila. Opazili smo, da ima pri dani zasedenosti prostora z delci adsorbenta velikost delcev adsorbenta znaten vpliv na ostale količine (vodikova vez, ureditveni parametri). Za adsorbent, ki ima primerljivo velikost delcev kot voda (σ = 2 ˚A), nismo opazili velike spremembe v številu z vodikovo vezjo koordiniranih vod, z naraščajočo gostoto delcev adsorbenta se tetraedrični in translacijski ureditveni parameter nekoliko zmanjšata. Večji delci adsorbenta (σ = 5 ˚A) pa so imeli večji vpliv na CF1 vodo: v primerjavi z čisto vodo je število vodikovih vezi z gostoto delcev adsorbenta naraščalo, tetraedrični ureditveni parameter je bolj strmo padal, translacijski ureditveni parameter pa je izkazoval ne-monotono vedenje. Pri visoki gostoti adsorbenta je območje strukturnih anomalij izginilo. študirali smo tudi vpliv utesnjenosti na μ-ρ projekcijo faznega diagrama in ugotovili, da prisotnost adsorbenta vodo iz sistema izrine, pri dani temperaturi pa za goste adsorbente z velikimi delci ne opazimo več faznega prehoda kot pri čisti vodi.

Keywords

voda;modeli vode;potencial centralne sile;utesnjeno okolje;adsorbent;računalniške simulacije;simulacije Monte Carlo;Ornstein-Zernikova integralska enačna;magistrska dela;

Data

Language: Slovenian
Year of publishing:
Typology: 2.09 - Master's Thesis
Organization: UL FKKT - Faculty of Chemistry and Chemical Technology
Publisher: [V. Ravnik]
UDC: 544.272(043.2)
COBISS: 82014723 Link will open in a new window
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Other data

Secondary language: English
Secondary title: Modeling water with a central force potential in a crowded environment
Secondary abstract: Monte Carlo computer simulations in the canonical and grand canonical statistical ensemble were employed to study the structural and thermodynamic properties of the central force model of water (CF1). We studied both bulk water and water in a crowded environment. The latter was modelled with a stationary configuration of Lennard-Jones particles with a given density, where the interaction of the adsorbent particles with the oxygen of the water was repulsive. The results for bulk water were also compared with the results of the Ornstein-Zernike integral equation with the HNC approximation and bridge functions based on a reference system of hard spheres. Monte Carlo simulations describe the geometry of bulk water molecule well (the coordination numbers for OH and HH are 2 and 1, respectively), the OH bond distance is comparable to experiment, and the HOH angle is slightly smaller than the experimentally determined value for water in the vapour phase. The integral equation in the HNC approximation does not give satisfactory results for water coordination and does not describe the structure of the system well. With the inclusion of bridge functions, we can obtain good agreement with the results of simulations at a temperature of 300 K, but the functions are not transferable (at higher temperatures, the results are worse compared to the HNC approximation). We focused on hydrogen bonding, tetrahedral and translational order parameters, which we used to investigate the structural anomalies of the model. Defining a hydrogen bond based on the pair energy distribution function of water gives better results than a one-parameter definition based on distance. At 300 K, the average value of the number of hydrogen bonds per water was calculated to be about 3.8. It was observed that the model water at 300 K has a structural anomaly in the density range between 0.95 and 1.15 g/ml. With the help of the grand canonical Monte Carlo simulations, we were able to reproduce the density anomaly with a simple linear dependence of the chemical potential on temperature, and the dependence ρ(T) is in good agreement with experimental data. We have also studied the phase diagram of the model in the μ−ρ projection. At sufficiently low temperatures (below 400 K), the coexistence of two phases (gas-liquid) was observed. In the second part, we studied the behaviour of model water in a disordered porous material (presence of Lennard-Jones-type obstacles). In the presence of the adsorbent, the intermolecular geometry of the water did not change. We found that the size of the adsorbent particles has a significant effect on other quantities (hydrogen bonding, order parameters) for a given fraction of the space occupied by adsorbent particles. When the adsorbent has a comparable particle size to water (σ = 2 ˚A), we did not observe a large change in the number of waters coordinated by hydrogen bonds. As the density of the adsorbent particles increased, the tetrahedral and translational order parameters decreased slightly. Larger adsorbent particles (σ = 5 ˚A) had a greater effect on CF1 water: compared to bulk water, the number of hydrogen bonds increased with the density of adsorbent particles, the tetrahedral order parameter decreased more sharply, however the translational order parameter exhibited non-monotonous behaviour. At high adsorbent density, the structurally anomalous region disappeared. We also investigated the effect of confinement on the μ − ρ projection of the equation of state. We found that the presence of the adsorbent displaces the water from the system and at a certain temperature for dense adsorbents with large particles the phase transition is no longer observed in contrast to pure water.
Secondary keywords: water;central force potential;crowded environment;adsorbent;Monte Carlo computer simulations;Ornstein-Zernike integral equation;
Type (COBISS): Master's thesis/paper
Study programme: 1000375
Embargo end date (OpenAIRE): 1970-01-01
Thesis comment: Univ. v Ljubljani, Fak. za kemijo in kemijsko tehnologijo, smer Kemija
Pages: 57 str.
ID: 13331912