Surface-ordered phase of nonadeca-1,18-diene: Observed by molecular dynamics study

Abstract

Phase properties of nonadeca-1,18-diene were investigated by the molecular dynamics simulation study, it is found that it has a new phase, surface-ordered phase, at temperature higher than the bulk-melting-point. • Nonadeca-1,18-diene has a new surface-ordered phase at temperature higher than bulk-melting-point. • Inner part of normal chain plays major role in forming surface-ordered phase. • End group of chain affects temperature of surface-ordered phase. • Center-of-mass of chain of crystalline monolayer shows fluctuation. It is newly found that normal alkane has a surface-ordered phase (SOP) at the temperature higher than its bulk-melting-point; is the SOP a property common to all the materials made of normal chain molecules? There is no answer yet. In order to solve the valuable subject, we first investigated the free surface behavior of nonadeca-1,18-diene, C 19 H 36 , by the molecular dynamics simulation. Initial state of the system for the simulation is a crystalline film consists of three lamellate layers, by long time heating with very low heating rate, it is found that, in temperature range from 352 K to 364 K, the middle layer is always in melt state while the two surface layers remain crystalline. According to the verification of state stability, this coexistence of melt and crystalline-monolayer at same temperature is identified as the SOP of nonadeca-1,18-diene; its significance is to show that there exists an intermediate state between the solid state and the liquid state. We found that the aggregation form of molecules of the two crystalline surface monolayer is hexagonal chain side packing, direction of chain axis is approximately parallel to the surface normal. By analyzing the SOP of nonadeca-1,18-diene and that of the normal nonadecane, it is found that the function of inner part of the chain is more important than the function of the end-group in forming the SOP. The properties of the surface-ordered phase can be very useful in manufacturing mechanical sensors, optical sensors and other aspects because of the remarkable changes in surface tension and light transmittance of the material.