Analysis of the orientational order effect on n-alkanes: Evidences on experimental response functions and description using Monte Carlo molecular simulation
Résumé
Short-range correlations of the molecular orientations in liquid n-alkanes have been extensively studied from depolarized Rayleigh scattering and thermodynamic measurements. These correlations between segments induce structural anisotropy in the fluid bulk. This phenomenon, which is characteristic of linear chain molecules when the constituting segments are nor freely jointed, but interact through a given angular potential, is then present in the linear n-Cn series, increasing its magnitude with chain length, and it is therefore less relevant or even completely absent in branched alkanes. This intermolecular effect is clearly revealed in second-order excess magnitudes such as heat capacities when the linear molecule is mixed with one whose structure approaches sphericity. The mixing process of different aspect ratio chain molecules is thought to modify the original pure fluid structure, by producing a diminution of the orientational order previously existing between pure n-alkane chains. However, second-order thermodynamics quantities of pure liquids C P, (∂ν/∂T)P, and (∂ν/∂P) P are known to be very sensitive to the specific interactions occurring at the microscopic level. In other words, the behavior of these derived properties versus temperature and pressure can be regarded as response functions of the complexity of the microscopic interactions. Thus, the purpose of the present work is to rationalize the orientational order evolution with both temperature and molecular chain length from the analysis of pure fluid properties. To this aim, we focused on two linear alkanes, n-octane (n-C 8) and n-hexadecane (n- C16), and two of their branched isomers, i.e., 2,2,4-trimethylpentane (br- C8) and 2,2,4,4,6,8,8- heptamethylnonane (br- C16). For each compound, we propose a combined study from direct experimental determination of second-order derivative properties and Monte Carlo simulations. We performed density ρ , speed of sound c, and isobaric heat capacity CP measurements in broad ranges of pressure and temperature allowing a complete thermodynamic characterization of these compounds. Monte Carlo simulations provide a link between the molecular scale model and the experimental thermodynamic properties. Additional information about the microscopic structure of the simulated fluid model was derived, through the calculation of the radius of gyration and average end-to-end distances. Orientational order is clearly revealed by the experimental residual heat capacity trend of pure linear alkanes. The close agreement observed between this experimental macroscopic property and the calculated theoretical structural parameters support the conclusion that the orientational order between segments of linear molecules should be regarded as a conformational effect due to the flexibility of the chain.
Mots clés
Angular potential
Branched alkanes
Branched isomers
Chain molecules
Conformational effect
Depolarized rayleigh scatterings
Derived properties
End-to-end distances
Experimental determination
Heat capacities
Intermolecular effects
Isobaric heat capacity
Linear alkanes
Linear chain
Linear molecules
Macroscopic properties
Microscopic interaction
Microscopic levels
Microscopic structures
Mixing process
Molecular chains
Molecular scale models
Molecular simulations
MONTE CARLO
Monte Carlo Simulation
N-alkane chains
n-Alkanes
N-hexadecane
N-octanes
Orientational orders
Pressure and temperature
Pure fluids
Pure liquids
Radius of gyration
Residual heat
Response functions
Second order derivatives
Second orders
Short-range correlations
Simulated fluids
Specific interaction
Speed of sounds
Structural anisotropy
Structural parameter
Thermodynamic characterization
Thermodynamic measurements
Trimethylpentane Engineering controlled terms: Aspect ratio
Chain length
Computer simulation
Fluids
Isomers
Molecular orientation
Monte Carlo methods
Nanoindentation
Polymer blends
Specific heat
Structural properties
Temperature
Thermodynamics Engineering main heading: Paraffins