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Surface Freezing in n-Alkanes: Experimental and Molecular Dynamics Studies

Modak, Viraj Prakash
http://rave.ohiolink.edu/etdc/view?acc_num=osu1449013699

Abstract Details

2015, Doctor of Philosophy, Ohio State University, Chemical Engineering.
Crystallization from the melt is a common process encountered in both industrial and natural settings. Nucleation is the first step in the process and hence, understanding where nucleation occurs is critical to controlling the process. For systems with free surfaces, like droplets, nucleation can occur on the surface or throughout the bulk. This aspect of crystallization has been extensively debated for water droplets because of its implications in the atmospheric sciences. For intermediate chain length n-alkanes (14 < n < 50), experiments show that surfaces can freeze above the melting point. Since an organized surface can then template freezing of the bulk, these alkanes are hard to supercool. There are competing theories regarding the physics that dive the phenomenon, but both state that shorter alkanes will not surface freeze. The goal of this work is to investigate surface freezing in n-alkanes, from both experimental and theoretical perspectives. Experiments will identify if surface freezing occurs for short chain n-alkanes containing 8 to 10 carbon atoms. Molecular dynamics will help identify the driving force. In experiments, an alkane carrier gas-vapor alkane mixture flows through a supersonic nozzle and cools at a rate of ~106 K/s. Eventually the vapor alkane condenses initially forming liquid nanodroplet aerosols, that can then freeze if the temperatures are cold enough. Static pressure measurements characterize the flow; whereas x-ray scattering and Fourier transform infrared spectroscopy, characterize the aerosol. Experiments yielded evidence for surface freezing in C8H18 to C10H22 droplets as well as estimates for the surface and volume based nucleation rates. In n-decane, decreasing the inlet conditions eventually led to the formation of nanoparticles that had a fractal-like structure and were not fully crystalline. Molecular Dynamics (MD) simulations at the united atom level provided both visualization of surface freezing as well as estimates of the thermodynamic quantities and understand the driving force behind the phenomenon. Droplet simulations, for example clearly showed an organized monolayer developed within ~4 ns on a supercooled droplet of n-octane followed by freezing of the adjacent liquid in a layer-by-layer manner, confirming our experimental hypothesis. To understand the driving force quantitatively, MD simulations were done crystals and slabs, to determine surface free energies of the liquid-vapor (LV) and the solid-vapor (SV) interfaces. The usual pressure tensor approach sufficed for LV interfaces, but a new, less computationally intensive method was developed for the SV surface free energies. The new method works well for the LJ solid and n-octane, but overestimates SV surface free energy for n-nonadecane. Simulations also provide estimates for the entropic changes associated with surface freezing. Overall these results suggest that the entropic contribution to the driving force is significant for n-octane but not for n-nonadecane.
Barbara Wyslouzil (Advisor)
275 p.
Chemical Engineering

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Citations

  • Modak, V. P. (2015). Surface Freezing in n-Alkanes: Experimental and Molecular Dynamics Studies [Doctoral dissertation, Ohio State University]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=osu1449013699

    APA Style (7th edition)

  • Modak, Viraj. Surface Freezing in n-Alkanes: Experimental and Molecular Dynamics Studies. 2015. Ohio State University, Doctoral dissertation. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=osu1449013699.

    MLA Style (8th edition)

  • Modak, Viraj. "Surface Freezing in n-Alkanes: Experimental and Molecular Dynamics Studies." Doctoral dissertation, Ohio State University, 2015. http://rave.ohiolink.edu/etdc/view?acc_num=osu1449013699

    Chicago Manual of Style (17th edition)

osu1449013699
799
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This open access ETD is published by The Ohio State University and OhioLINK.