Supercritical Water Confined In Graphene Nanochannels

Transcription

1 Superritial Water Confined In Graphene Nanohannels J. Sala*, E. Guàrdia, and J. Martí Departament de Físia i Enginyeria Nulear, Universitat Politènia de Catalunya, B4-B5 Campus Nord, Barelona, Catalonia, Spain jonas.sala@up.edu & FAX: We report results of a series of moleular dynamis simulations of water inside a narrow graphite hannel at superritial onditions. A wide range of densities ( gm -3 ) at the superritial temperature of 673 K have been onsidered. A thorough analysis of the struture, hydrogen bonding, dieletri and dynami properties of the systems was performed. Our present results are ompared with previous studies of liquid water onfined in graphene nanohannels at ambient and high temperature onditions. INTRODUCTION Reent advanes in the design of nulear reators are related to the so alled Superritial Water Reator, where onfined superritial water is employed as the oolant medium [1]. Our ontribution will present brand new omputer simulations of superritial water onfined in graphene slabs as an extension of previous works [2,3,4]. Struture and hydrogen bonding, as well as dieletri and dynamial properties of superritial water onfined in those graphene slabs have been investigated by moleular dynamis (MD) simulations. In this paper we will be foused on the struture, hydrogen bonding and dieletri properties. Dynami properties will be published elsewhere [5]. I - COMPUTATIONAL DETAILS We onsidered N water moleules embedded into two parallel graphite plates with no defets to model two sheets of highly oriented pyrolyti graphite (HOPG). We set up a simulation box in the x, y, and z diretions of 34.4, 34.1 and 31 Å, respetively. These values orrespond to the geometry of HOPG in the real system. We assumed the z oordinate to be perpendiular to the graphite layers, and the usual periodi boundary onditions were onsidered only in the x and y diretions. The temperature of the system was kept at T=673K. Different number of moleules were assumed ranging from N=100 to N=800 to over a range of densities from ρ= 0.08 to ρ=0.66 gm -3. Snapshots of typial MD onfigurations are depited in Figure 1. Water-water inter- and intramoleular interations were modeled with a flexible simple-point-harge (SPC) potential whih was speifially reparametrized to reprodue the main trends of the infrared spetrum of water at ambient onditions [6]. This flexible SPC potential has a ritial point (T = 643 K, ρ = 0.32 gm -3 ) whih is very lose to the experimental one (T = 647 K, ρ = gm -3 ) [7]. Water-arbon fores were assumed to be of the Lennard-Jones type with the same parametrization employed in previous studies of water near graphite (see for instane Ref. [8]). 1

2 Figure 1: Snapshots of the simulation box with the water moleules and the graphene walls at different densities (from left to right and from top to bottom ρ=0.66, 0.33, 0.16 and 0.08 g/m 3 ). To arry out the simulations we employed the integration algorithm of Berendsen et al. [9] with a time step of 0.5 fs and a thermal bath oupling parameter of 10 fs. Short-ranged fores were trunated at half the box length and the Ewald summation tehnique as proposed by Spohr [10] was applied to aount for the long ranged Coulomb interation. Eah run onsisted of an initial equilibration period of 50 ps and a prodution period of 300 ps to ollet statistially meaningful properties. II RESULTS AND DISCUSSION During the simulations, we separately analyzed the properties of two different groups of water moleules diretly related to their loations with respet to the graphite walls: adsorbed for water in the losest layer and for water far from the graphite. The exat definition of these regions in eah ase omes from the orresponding oxygen density profiles. (see Figure 2). As an be seen from Figure 2, the peaks related to the interfaial struture beome less well defined as the density of the system diminishes. 2

3 Figure 2: Oxygen density profiles of water moleules at T=673K and different densities Oupation and Residene Times In Table 1 the perentages of water moleules loated in the different regions are reported and they are ompared with previous results at ambient [2] and high temperature [4] onditions. Aording to our findings, the oupation of the adsorbed layer is almost insensitive to the temperature. On the other hand, at T=673K we observe that this oupation systematially inreases as the density diminishes. (Note that at T=298K and T=473K an intermediate layer loated between the adsorbed and the entral like regions was observed [2,4]). The residene time of a water moleule in a given region (τ res ) may be desribed as the average time spent by the moleule in suh region before moving away. The τ res values listed in Table 1 were determined following the proedure desribed by Impey et al. [11]. At all temperatures, τ res in the adsorbed regions are signifiantly lower than in the ones. As a general trend, τ res diminishes as temperature inreases. At the superritial temperature, we find that the residene time at the adsorbed region is muh less dependent on the density than that orresponding to the region. We think that this is due to the fat that τ res in the adsorbed layer is dominated by the strength of graphite-water interations. On the ontrary, τ res in the region is mainly due to water-water interations leading to a signifiant diminution of the residene time as ρ diminishes. 3

4 Table 1: Perentage of oupation of the different regions, residene times of the water moleules, mean number of hydrogen bonds per moleule and stati dieletri onstant. T/K ρ/gm -3 region % oupation τ res /ps n HB ε adsorbed adsorbed adsorbed adsorbed adsorbed adsorbed intermediate adsorbed intermediate adsorbed a 17 a 60 a a 14.0 a a a 3.50 a 3.55 a 3.48 a b 30 b 35 b 85 b a from Ref. [2], b from Ref. [3], from Ref. [4] Hydrogen Bonding As in previous studies of aqueous systems at superritial onditions [12, 13], we adopted a geometri definition of the hydrogen bonds, i. e. we assumed that two water moleules were H-bonded if the next three geometrial onditions were fulfilled: 1. The distane R OO between the oxygen atoms is smaller than R OO, 2. The distane R OH between the "aeptor" oxygen and the hydrogen "donor" atoms is smaller than R OH, 4

5 3. The H--O...O angle ϕ is smaller ϕ. The ut-off distanes R OO = 3.5 Å and R OH = 2.4 Å were obtained from the first minima of the orresponding radial distribution funtions goo ( r ) and goh ( r). The angular o ut-off was hosen to be ϕ = 30. As an be seen from Table 1, the mean number of hydrogen bonds per moleule (n HB ) is slightly lower in the adsorbed layer than in the entral like region. The same tendeny was found at ambient onditions [3]. On the other hand, at T=673K, the differene between n HB for the two regions beomes less signifiant as density diminishes. It is worthy to remark that at the lowest density, i.e. for ρ= 0.08 gm -3, the obtained value, namely n HB =0.25, should orrespond to a pratially monomeri struture. Stati Dieletri Constant The stati dieletri onstant ε an be omputed as [14,15] 2 4π < M > ε = 1+ (1) 3Vk B T 2 where < M > is the averaged square dipole moment, V is the aessible volume, and k B is the Boltzmann fator. The values of ε due to water moleules in adsorbed layers and -like water are reported, together with the overall value, in Table 1. At all temperatures, the value for ε is larger in the absorbed layer than in the region. This suggests the existene of a preferential orientation of the moleules lose to the graphite walls. However, as temperature inreases, the overall values of ε beome loser to those from the -like region, basially due to the inrease of moleular disorder indued by the high thermal energies, whih produe a loss of the orientational order of the moleular dipoles. Finally, at the superritial temperature, we observe a drasti diminution of the stati dieletri onstant as density diminishes. Similar findings were obtained in a previous study of dieletri properties in (unonfined) superritial water [15]. ACKNOWLEDGEMENTS The authors gratefully aknowledge finanial support from the Generalitat de Catalunya (Grant 2005SGR-00779), from the Ministerio de Eduaión y Cienia of Spain (Grant FIS C02-01), and from European Union FEDER funds (UNPC-E015). J.S. is a reipient of a FPI Spanish fellowship. REFERENCES [1] WAS, G. S., AMPORNRAT, P., GUPTA, G., TEYSSEYRE, S. WEST, E. A., ALLEN, T. R., SRIDHARAN, K., TAN, L., CHEN, Y. REN, X., PISTER, C., J. Nul. Mater., vol. 371, 2007, p

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