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The cages, dynamics, and structuring of incipient methane clathrate hydrates
Walsh, Matthew R.,Rainey, J. Daniel,Lafond, Patrick G.,Park, Da-Hye,Beckham, Gregg T.,Jones, Michael D.,Lee, Kun-Hong,Koh, Carolyn A.,Sloan, E. Dendy,Wu, David T.,Sum, Amadeu K. Royal Society of Chemistry 2011 Physical chemistry chemical physics Vol.13 No.44
<P>Interest in describing clathrate hydrate formation mechanisms spans multiple fields of science and technical applications. Here, we report findings from multiple molecular dynamics simulations of spontaneous methane clathrate hydrate nucleation and growth from fully demixed and disordered two-phase fluid systems of methane and water. Across a range of thermodynamic conditions and simulation geometries and sizes, a set of seven cage types comprises approximately 95% of all cages formed in the nucleated solids. This set includes the ubiquitous 5<SUP>12</SUP> cage, the 5<SUP>12</SUP>6<SUP><I>n</I></SUP> subset (where <I>n</I> ranges from 2–4), and the 4<SUP>1</SUP>5<SUP>10</SUP>6<SUP><I>n</I></SUP> subset (where <I>n</I> also ranges from 2–4). Transformations among these cages occur <I>via</I> water pair insertions/removals and rotations, and may elucidate the mechanisms of solid–solid structural rearrangements observed experimentally. Some consistency is observed in the relative abundance of cages among all nucleation trajectories. 5<SUP>12</SUP> cages are always among the two most abundant cage types in the nucleated solids and are usually the most abundant cage type. In all simulations, the 5<SUP>12</SUP>6<SUP><I>n</I></SUP> cages outnumber their 4<SUP>1</SUP>5<SUP>10</SUP>6<SUP><I>n</I></SUP> counterparts with the same number of water molecules. Within these consistent features, some stochasticity is observed in certain cage ratios and in the long-range ordering of the nucleated solids. Even when comparing simulations performed at the same conditions, some trajectories yield swaths of multiple adjacent sI unit cells and long-range order over 5 nm, while others yield only isolated sI unit cells and little long-range order. The nucleated solids containing long-range order have higher 5<SUP>12</SUP>6<SUP>2</SUP>/5<SUP>12</SUP> and 5<SUP>12</SUP>6<SUP>3</SUP>/4<SUP>1</SUP>5<SUP>10</SUP>6<SUP>2</SUP> cage ratios when compared to systems that nucleate with little long-range order. The formation of multiple adjacent unit cells of sI hydrate at high driving forces suggests an alternative or addition to the prevailing hydrate nucleation hypotheses which involve formation through amorphous intermediates.</P> <P>Graphic Abstract</P><P>The dominant cages of clathrates are classified and the formation of multiple sI unit cells is reported from large-scale simulations. <IMG SRC='http://pubs.rsc.org/services/images/RSCpubs.ePlatform.Service.FreeContent.ImageService.svc/ImageService/image/GA?id=c1cp21899a'> </P>
Marcus A. Florez(Marcus A. Florez ),Brian De(Brian De ),Bhavana V. Chapman(Bhavana V. Chapman ),Anussara Prayongrat(Anussara Prayongrat ),Jonathan G. Thomas(Jonathan G. Thomas ),Thomas H. Beckham(Thom 대한방사선종양학회 2023 Radiation Oncology Journal Vol.41 No.1
Purpose: There has been limited work assessing the use of re-irradiation (re-RT) for local failure following stereotactic spinal radiosurgery (SSRS). We reviewed our institutional experience of conventionally-fractionated external beam radiation (cEBRT) for salvage therapy following SSRS local failure. Materials and Methods: We performed a retrospective review of 54 patients that underwent salvage conventional re-RT at previously SSRS-treated sites. Local control following re-RT was defined as the absence of progression at the treated site as determined by magnetic resonance imaging. Results: Competing risk analysis for local failure was performed using a Fine-Gray model. The median follow-up time was 25 months and median overall survival (OS) was 16 months (95% confidence interval [CI], 10.8–24.9 months) following cEBRT re-RT. Multivariable Cox proportional-hazards analysis revealed Karnofsky performance score prior to re-RT (hazard ratio [HR] = 0.95; 95% CI, 0.93– 0.98; p = 0.003) and time to local failure (HR = 0.97; 95% CI, 0.94–1.00; p = 0.04) were associated with longer OS, while male sex (HR = 3.92; 95% CI, 1.64–9.33; p = 0.002) was associated with shorter OS. Local control at 12 months was 81% (95% CI, 69.3–94.0). Competing risk multivariable regression revealed radioresistant tumors (subhazard ratio [subHR] = 0.36; 95% CI, 0.15–0.90; p = 0.028) and epidural disease (subHR = 0.31; 95% CI, 0.12–0.78; p =0.013) were associated with increased risk of local failure. At 12 months, 91% of patients maintained ambulatory function. Conclusion: Our data suggest that cEBRT following SSRS local failure can be used safely and effectively. Further investigation is needed into optimal patient selection for cEBRT in the retreatment setting.