Grand canonical Monte Carlo (GCMC) simulations were performed to investigate the adsorption behavior of an equimolar CO2/CH4 mixture in carbon nanotubes (CNTs). Five CNTs [(6, 6), (7, 7), (8, 8), (9, 9), and (10, 10)] with diameters varying from 0.678 to 1.356 nm, seven temperatures (283, 293, 303, 313, 323, 333, 343 K), and seven pressures (1, 5, 10, 15, 20, 25, 30 MPa) were chosen to investigate the effect of temperature, pressure, and pore size on the adsorption behavior. The results show that the CNTs have a preferential adsorption of CO2 in the binary CO2/CH4 mixture. For pore size effect on the adsorption behavior, we found that the adsorption of CO2 is much larger than that of CH4 in the same CNT and that the CO2 adsorption in the CNTs increases dramatically with an increase of CNT's diameter, whereas the absolute amount of adsorbed CH4 changes little with the CNT's pore size. In the investigated temperature and pressure ranges, we observed that in the CNTs with diameters less than 1.1 nm, the temperature and pressure have little effect on the adsorption behavior of the binary mixture, whereas in larger CNTs the adsorption behavior changes with temperature and pressure significantly. In addition, the CNTs demonstrated a higher selectivity of CO2 than other materials (activated carbons, zeolites 13X, and metal−organic frameworks) reported in the literature. For example, in (6, 6) CNT at 343 K and 1 MPa, the selectivity reaches 11.2, larger than that reported in activated carbon. The selectivity in narrow CNTs (<1 nm in diameter) is fluctuating with temperature and pressure, but it remains almost the same in larger CNTs at the investigated temperature and pressure.

Molecular dynamics simulations have been performed to investigate the hydration of Li(+), Na(+), K(+), F(-), and Cl(-) inside the carbon nanotubes at temperatures ranging from 298 to 683 K. The structural characteristics of the coordination shells of ions are studied, including the ion-oxygen radial distribution functions, the coordination numbers, and the orientation distributions of the water molecules. Simulation results show that the first coordination shells of the five ions still exist in the nanoscale confinement. Nevertheless, the first coordination shell structures of cations change more significantly than those of anions because of the preferential orientation of the water molecules induced by the carbon nanotube. The first coordination shells of cations are considerably less ordered in the nanotube than in the bulk solution, whereas the change of the first coordination shell structures of the anions is minor. Furthermore, the confinement induces the anomalous behavior of the coordination shells of the ions with temperature. The first coordination shell of K(+) are found to be more ordered as the temperature increases only in the carbon nanotube with the effective diameter of 1.0 nm, implying the enhancement of the ionic hydration with temperature. This is contrary to that in the bulk solution. The coordination shells of the other four ions do not have such behavior in the carbon nanotube with the effective diameter ranging from 0.73 to 1.00 nm. The easier distortion of the coordination shell of K(+) and the match of the shell size and the nanotube size may play roles in this phenomenon. The exchange of water molecules in the first coordination shells of the ions with the solution and the ion diffusion along the axial direction of the nanotube are also investigated. The mobility of the ions and the stability of the coordination shells are greatly affected by the temperature in the nanotube as in the bulk solutions. These results help to understand the biological and chemical processes at the high temperature.

Provable Data Possession (PDP) protocol makes it possible for cloud users to check whether the cloud servers possess their original data without downloading all the data. However, most of the existing PDP schemes are based on either public key infrastructure (PKI) or identity-based cryptography, which will suffer from issues of expensive certificate management or key escrow. In this paper, we propose a new construction of certificateless provable group shared data possession (CL-PGSDP) protocol by making use of certificateless cryptography, which will eliminate the above issues. Meanwhile, by taking advantage of zero-knowledge protocol and randomization method, the proposed CL-PGSDP protocol leaks no information of the stored data and the group user's identity to the verifiers during the verifying process, which is of the property of comprehensive privacy preservation. In addition, our protocol also supports efficient user revocation from the group. Security analysis and experimental evaluation indicate that our CL-PGSDP protocol provides strong security with desirable efficiency.

Molecular dynamics (MD) simulations were performed to study the structural properties of water molecules confined in functionalized carbon nanotubes (CNTs). Four CNTs, two armchair-type (6, 6), (7, 7) and two zigzag-type (10, 0), (12, 0) CNTs, representing different helicities and different diameters, were chosen and functionalized at their open ends by the hydrophilic -COOH and the hydrophobic -CH 3 groups. The structural properties of water molecules inside the functionalized CNTs, including the orientation distributions of dipole moment and O-H bonds, the length of the single-file water chain, and the average number of hydrogen bonds, were analyzed during a process of simulations. MD simulation results in this work showed that the -CH 3 functional groups exert little special effects on the structural properties of water molecules. It is mainly due to the relatively small size of the -CH 3 group and its hydrophobic nature, which is consistent with hydrophobic CNTs. For CNTs functionalized by -COOH groups, the configurations of -COOH groups, incurvature or excurvature, determine whether water molecules can enter the CNTs. The incurvature or excurvature configurations of -COOH groups are the results of synergy effects of the CNTs' helicity and diameter and control the flow direction of water molecules in CNTs.

Equilibrium molecular dynamics simulations have been performed to investigate the structural characteristics of ethanol molecules confined in single-walled, pristine armchair carbon nanotubes with a length of 2.5 nm and diameters ranging from 0.68 to 1.35 nm in an open ethanol reservoir at 298.0 K and 100.0 kPa by all-atom and united-atom models. Both models present similar results. Structural properties of confined ethanol molecules are analyzed in terms of the average number of hydrogen bonds, radial density distributions of methyl and hydroxyl groups, orientation distributions of the methyl-methylene bond, oxygen-hydrogen bond and dipole moment, and molecular conformations as a function of the diameter of carbon nanotubes. The results indicate that the behavior of the nonpolar part of confined ethanol molecules changes monotonically with the diameter, whereas that of the polar part changes non-monotonically. The different dependence on diameter indicates that the wall-fluid interactions determine the behavior of nonpolar groups, whereas that of polar groups is determined by the fluid-fluid interactions. Only in the nanotube with a diameter of 1.081 nm did the confined ethanol molecules have a highly preferred dipole orientation. The conformational equilibrium also varies considerably with the diameter non-monotonically. The largest proportion of gauche ethanol corresponds to the most preferred dipole orientation.

The behavior of water molecules under nanoscale confinement has received considerable attention, especially for the influence caused by the modified groups of pores. To better design bionic nanodevices for future research, we anchored carboxyl acid (−COOH) groups onto the inner wall of a single-walled armchair carbon nanotube’s (CNT’s) central region to model the pore shape of aquaporin-1 and investigated the effect of modified groups on the structure of water molecules. The orientations and density distributions of water molecules in the CNTs and near the tube mouths have been studied by molecular dynamics simulation. The results indicate that water molecules confined inside the two unmodified regions have opposite and steady preferential dipole orientations pointing toward the −COOH groups on the central region of the CNT. Meanwhile the orientations of water molecules near the tube mouths which are certain distances away from the −COOH groups are also affected. This phenomenon becomes stronger as the number of −COOH groups increases and the CNT diameter decreases. In addition, the results show that the −COOH groups on the inner wall of the central region have a slight effect on the axial density distribution of the water molecules near the tube mouths, but a strong impact on that of water molecules inside the CNTs. Different distances between the −COOH groups and tube mouths can create diverse axial density distributions of water molecules.

Abstract:With the rapid development of Internet, the traditional computing environment is making a big migration to the cloud-computing environment. However, cloud computing introduces a set of new security problems. Aiming at the virtual machine (VM) escape attack, we study the traditional attack model and attack scenarios in the cloud-computing environment. In addition, we propose an access control model that can prevent virtual machine escape (PVME) by adapting the BLP (Bell-La Padula) model (an access control model developed by D. Bell and J. LaPadula). Finally, the PVME model has been implemented on full virtualization architecture. The experimental results show that the PVME module can effectively prevent virtual machine escape while only incurring 4% to 8% time overhead.

With the rapid development of cloud computing, the demand for infrastructure resources in cloud data centers has further increased, which has already led to enormous amounts of energy costs. Virtual machine (VM) consolidation as one of the important techniques in Infrastructure as a Service clouds (IaaS) can help resolve energy consumption by reducing the number of active physical machines (PMs). However, the necessity of considering energy-efficiency and the obligation of providing high quality of service (QoS) to customers is a trade-off, as aggressive consolidation may lead to performance degradation. Moreover, most of the existing works of threshold-based VM consolidation strategy are mainly focused on single CPU utilization, although the resource request on different VMs are very diverse. This paper proposes a novel self-adaptive VM consolidation strategy based on dynamic multi-thresholds (DMT) for PM selection, which can be dynamically adjusted by considering future utilization on multi-dimensional resources of CPU, RAM and Bandwidth. Besides, the VM selection and placement algorithm of VM consolidation are also improved by utilizing each multi-dimensional parameter in DMT. The experiments show that our proposed strategy has a better performance than other strategies, not only in high QoS but also in less energy consumption. In addition, the advantage of its reduction on the number of active hosts is much more obvious, especially when it is under extreme workloads.