Size dependence and odd/even effect in the melting of 2D lamellar crystals

Size-effect melting;Nanocalorimetry;Odd/Even Effect;Self-Assembly;Melting point depression;Magic-SizeFormation;2D Crystals

de la Rama, Lito

关键词: Size-effect melting;    Nanocalorimetry;    Odd/Even Effect;    Self-Assembly;    Melting point depression;    Magic-SizeFormation;    2D Crystals;   

Others  :  https://www.ideals.illinois.edu/bitstream/handle/2142/45497/Lito_de%20la%20Rama.pdf?sequence=1&isAllowed=y

美国|英语

来源: The Illinois Digital Environment for Access to Learning and Scholarship

【 摘 要 】

The self-assembly of polymers into thin, crystalline lamellar sheets is of interest in the fundamental understanding of the properties of these materials.This research is a fundamental thermodynamic study of the self-assembly of these lamellar crystals.Our group has successfully developed a new synthesis method to grow lamellar crystals of silver alkanethiolates (AgSCn) on inert substrates with a precise control of the number of layers.The new vapor phase synthesis method systematically changes the amount of silver and the annealing temperature to grow monodisperse lamella with specific number of layers.Together with our unique capability to do thermal analysis of nanoscale systems using the nanocalorimetry (NanoDSC) technique, the size dependent melting and effects of layer stacking in AgSCn lamellar crystals are investigated.For single layer lamella, discrete change in the lamellar thickness is achieved by changing the alkanethiol chain length.Nanocalorimetry results show size-effect melting which follows an inverse linear relationship with the lamella thickness.The discrete thickness change leads to a discrete change in the melting point.This behavior is analogous to magic size melting observed in 3D metal nanoparticles.Layer-to-layer stacking significantly changes the melting behavior of the lamellar crystals.There is a large increase (ΔT=23K for AgSC7) in the melting point between a 2-layer and a 1-layer crystal.There is also a large odd/even alternationin the melting point for the 2-layer crystals which is not observed for the 1-layer crystals.This odd /even effect persists for all stackedlamella.These results unambiguously establish that this odd/even effect is an effect of layer-to-layer stacking.These results indicate that 1-layer (unstacked) crystals of other lamellar crystals such as alkanes may not show an odd/even effect.Previous studies on alkanes have used multilayer stacked samples. A multi-variable phenomenological model for the melting of layered lamella is developed.In this model, the total excess free energy of the system is divided into the contributions from different spatially separated segments of the AgSCn lamella; the surface, interlayer interface and the central plane.The model shows a significant difference in the interfacial free energy between odd and even AgSCn which explains the large odd/even effect in melting. Understanding these lamellar systems has implications in the study of biological membranes which are self-assembled lipid bilayer lamella.A new small volume capillary-based liquid calorimeter with fast heating rates is developed.This new device is used in the study of the phase transitions of a model lipid bilayer vesicle and several proteins in solution.Anomalous multiple peaks in the phase transition of the vesicle are observed at fast heating rates.