Phase Behavior Of N-Heneicosane And N-Tricosane Under Nanocofinement

Size effect,interface interactions,and pore morphology have profundity influence on the thermodynamic properties and structural changes of n-alkanes confined in nanopores.This work mainly focuses on the variation of the phase transition temperature,the degree of supercooling,and enthalpy change as a function of the size of the n-alkanes in the nanopores,together with the reasons of the above phenomena.It also includes the phase behaviors of the n-alkanes under the nanoconfinement.The porous materials used in the experiments contain CPGs,SBA-15 and SG,KIT-6,C-SBA-15,MCM-41 with a series of pore size.The model materials are n-heneicosane and n-tricosane.Using low-temperature DSC calorimetry,the phase change temperatures,the degree of supercooling,and enthalpy change of the alkanes in variation of the size,the interface interactions,and the pore morphology are investigate.In combined with the XRD diffractometry,the changing of the structures and the phase during phase transitions is studied.The overarching aim of the study was improved understanding of pore confinement on the properties of a representative normal alkane.The main work of this experiment is as follows.（1）The n-alkanes(n-C₂₁H₄₄ or n-C₂₃H₄₈)are filled in the mesoporous material at a filling rate of 90%by a melting method.And then it is placed in a blast drying oven having a temperature higher than the melting point of the normal alkane and n-alkanes were adsorbed into the mesoporous material.Meanwhile,Samples containing n-alkanes adsorbed in the mesoporous of the materials successfully prepared in different pore diameters.（2）DSC was used to characterize the thermodynamic properties of n-alkanes in the bulk and confined after they were adsorbed in different mesoporous materials.The phase transition temperature and enthalpy change with the pore size were measured during the heating and cooling processes.The result is that as the pore size decreases,both the phase transition temperature and enthalpy change decrease,and the degree of supercooling increases.The phase transition temperature is insensitive to the polarity pore wall but varied with the pore geometry.For the first time,the influence of the pore morphology is quantified,using data for n-alkanes in the well-defined 1D channels of SBA-15 as a reference.The difference in the slope of plots of melting point versus 1/d and s-s transition temperature versus 1/d reflect entropy gains of the corresponding bulk transitions.（3）The samples were characterized using synchrophonic radiation source to determine the effect of nano-confinement on the lamellar structure of the n-alkanes.The results show that the lamellar ordering of n-alkanes confined the nanopores is disturbed,with the lamellar disorder enhanced in very small pores.It is likely caused by longitudinal spread of the molecular chains and the introduction of defects such as end-gauche,kinks,effects which are amplified in small pores.（4）Using variable temperature XRD analysis,the phase structures of n-alkanes were characterized.As a result,in CPG（12 nm）and SBA-15（14.4 nm）,n-C₂₁ shows a new rotator R_II,and the mixed O₂₁ and R_I phases.The phase sequence is L（?）R_II（?）R_I（?）O₂₁+R_I.Compared with the bulk,the XRD measurenents of n-C₂₃ in CPG（12 nm）and SBA-15（14.4 nm）did not trace O_dci and R_V phase.This does not mean that the two phases are not present only,possibly because also they are turning too fast or point temperature is too low.The possible phase sequence is L（?）R_II（?）R_I（?）O₂₃+R_I（?）O₂₃.The phase change temperatures by the XRD analysis are roughly consistent with those determined by the DSC measurements.