Modeling the behaviour of confined dipolar and ionic systems

Abstract

In this doctoral thesis confined dipolar and ionic systems have been modelled and investigated, namely dipolar tubes and helices composed of dipolar hard spheres and ionic liquids. Mutual for those systems is the fact that their structure and behaviour are dominated by long-range interactions, i.e., dipole-dipole interaction in case of dipolar systems and Coulombic interaction in case of ionic systems. The feature of pronounced ordering of formed configurations and possibility of manipulating them via externally applied fields (magnetic/electric field in case of magnetic/electric dipoles and electric field in case of ions) attract attention of condensed matter physics. The first part of this doctoral thesis, which is dedicated to dipolar systems, is dealing with the investigation of the structure and cohesive energy in tubes and helices composed of dipolar hard spheres. A complex dependence of cohesive energy on surface packing fraction and dipole moment orientation has been observed. In case that single-thread helices are considered, the lowest cohesive energy is achieved at the highest surface packing fraction. Besides that, an interesting non-monotonic behaviour of the cohesive energy as a function of the surface packing fraction has been obtained. In case of multi-thread helices, a new phase, showing remarkably lower cohesive energy, has been determined. This phase is referred to as ZZ tube and it consists of threads following the confining cylinder’s axis, labeled as the z axis, in terms of both spatial and dipole moment orientation. Actually, in case of ZZ tubes dipolar hard spheres are arranged into a local triangular lattice, with densely packed threads following the z axis. In the scope of this doctoral thesis dipolar configurations under the condition of cylindrical confinement are considered, meaning that dipolar spheres are placed on a confining cylinder. First question which arises is what are the possible configurations, since there is an interplay between the two components, i.e., positioning and dipole moment orientation of the particles. For a fixed geometry, only certain dipole moment orientations lead to stable configurations. The above mentioned two components can be treated as independent variables, while dependent variable is the cohesive energy emerging from the dipole-dipole interactions. Accordingly, geometry is kept fixed, while dipole moment orientation is varied and the dependence of cohesive energy on dipole moment orientation is determined. In an analogous way, dipole moment orientation is kept fixed, while geometry is varied, i.e., the radius of confining cylinder, leading to the dependence of cohesive energy on geometry. Besides the cohesive energy, some other dependent variables are considered, like the total polarization of a dipolar configuration. Except from the theoretical relevance, dipolar tubes and helices represent model systems which might be useful for other scientific areas. Modeling of dipolar tubes and helices might provide better understanding of certain biological structures (for example, microtubules) or macromolecules (protein folding is dominated by electric dipole-dipole interactions). Due to the mechanical flexibility of dipolar structures and possibilities of manipulating them by external electric or magnetic fields, they might be useful in the synthesis of electronic devices...

U ovoj doktorskoj tezi modelovani su i ispitani prostorno ograniqeni dipolni i jonski sistemi, taqnije dipolne tube i heliksi sastav eni od dipol- nih qvrstih sfera i jonske teqnosti. Zajedničko za ove sisteme je činjenica da su im struktura i ponašanje pretežno određeni dugodometnim interakci- jama, dipol-dipolnom interakcijom u sluqaju dipolnih sistema, odnosno Ku- lonovom interakcijom u sluqaju jonskih sistema. Odlika naglaxenog struk- turnog uređivača formiranih konfiguracija i mogunost manipulacije ima preko eksterno primećenih poa (magnetno/elektriqno polje u sluqaju magnet- nih/elektriqnih dipola, odnosno elektriqno po e u sluqaju jona) privlaqe pažnju sa aspekta fizike kondenzovane materije. Prvi deo ove doktorske teze, koji je posvećen dipolnim sistemima, bavi se istraživanjem strukture i kohezione energije u tubama i heliksima koji su sastav eni od dipolnih qvrstih sfera. Dobijena je kompleksna zavisnost kohezione energije od povrxinske gustine pakovanja i orijentacije dipolnih momenata. U sluqaju jednostruko namotanih heliksa, najniža koheziona en- ergija postignuta je pri najvixoj povrxinskoj gustini pakovanja. Pored toga, dobijena je zanim iva nemonotona zavisnost kohezione energije od povrxinske gustine pakovanja. U sluqaju vixestruko namotanih heliksa otkrivena je nova faza, koja pokazuje primetno niu kohezionu energiju. Ova faza nazvana je ZZ tuba, a sastoji se od niti koje prate osu konfinirajućeg cilindra, oznaqenu kao z osa, u smislu prostorne i orijentacije dipolnih momenata. Zapravo, u sluqaju ZZ tuba dipolne qvrste sfere su ureene u lokalno trougaonu rexetku, sa gusto pakovanim nitima koje su paralelne sa z osom. U okviru ove doktorske teze razmatrane su dipolne konfiguracije pri uslovu cilindriqnog prostornog konfiniranja, xto znaqi da su dipolne sfere postav ene na konfinirajući cilindar. Prvo pitanje koje se postava jeste koje su mogue konfiguracije, sa obzirom da postoji preplitanje dve kom- ponente, prostornog pozicioniranja i orijentacije dipolnih momenata qes- tica. Za fiksiranu geometriju, samo određene orijentacije dipolnih momenata dovode do stabilnih konfiguracija. Pomenute dve komponente mogu se treti- rati kao nezavisne promen ive, a zavisna promen iva je koheziona energija usled dipol-dipolnih interakcija. Prema tome, geometrija je fiksirana, a dipolna orijentacija se varira i odreuje se zavisnost kohezione energije od orijentacije dipolnih momenata. Na analogan naqin, dipolna orijentacija je fiksirana, a geometrija se varira, recimo radijus konfinirajueg cilindra, xto dovodi do zavisnosti kohezione energije od geometrije. Osim kohezione energije, razmatrane su i druge zavisne promen ive, kao xto je ukupna po- larizacija dipolne konfiguracije. Pored teorijskog znaqaja, dipolne tube i heliksi predstav aju modelne sisteme koji mogu biti korisni u drugim nauqnim oblastima. Modelovanje dipolnih tuba i heliksa moe pruiti bo e razumevanje određenih bioloxkih struktura (na primer, mikrotubula) ili makromolekula (savijanje proteina dominantno je odreeno elektriqnim dipol-dipolnim interakcijama). Usled mehaniqke fleksibilnosti dipolnih struktura i mogunosti za manipulaciju ima preko eksternih magnetnih ili elektriqnih po a, date strukture mogu biti korisne u sintezi elektronskih uređaja...