Molecular dynamics simulations of the interactions of potential foulant molecules and a reverse osmosis membrane

Reverse osmosis (RO) is increasingly one of the most common technologies for desalination worldwide. However, fouling of the membranes used in the RO process remains one of the main challenges. In order to better understand the molecular basis of fouling the interactions of a fully atomistic model of a polyamide membrane with three different foulant molecules, oxygen gas, glucose and phenol, are investigated using molecular dynamics simulations. In addition to unbiased simulations, umbrella sampling methods have been used to calculate the free energy profiles of the membrane-foulant interactions. The results show that each of the three foulants interacts with the membrane in a different manner.It is found that a build up of the two organic foulants, glucose and phenol, occurs at the membrane-saline solution, due to the favourable nature of the interaction in this region, and that the presence of these foulants reduces the rate of flow of water molecules over the membrane-solution interface. However, analysis of the hydrogen bonding shows that the origin of attraction of the foulant for the membrane differs. In the case of oxygen gas the simulations show that a build up of gas within the membrane is likely, although, no deterioration in the membrane performance was observed

Pairs of Bloch electrons and magnetic translation groups

A product of irreducible representations of magnetic translation group is considered. It leads to irreducible representations which were previously rejected as nonphysical. A very simple example indicates a possible application of these representations. In particular, they are important in descriptions of pairs of electrons in a magnetic field and a periodic potential. The periodicity of some properties with respect to the charge of a particle is briefly discussed.Comment: 4 pages, RevTex. Latex2.09, amsfont

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Aerospace Engineering and Engineering Mechanic

Nonunital multiplier pairs and remarks on generalized group C*-algebras

In the first part of this paper we will consider a generalization of D. Hadwin and E. Nordgren\u27s work on multiplier pairs. Here we will not assume the existence of an identity, but rather just ask for the existence of a bounded approximate identity. Without the assumption of the identity, we find a new result concerning the relationship between the norm closure of the left multiplication operators and the approximate double commutant of the left multiplication operators. In the second part we will suppose f, g : T→T are continuous functions on the unit circle T and let B (f, g) denote the universal C*-algebra generated by U and V subject to the conditions that U and V are a unitary, and Uf( V)U-1 = g( V). We then will prove that this C*-algebra may be represented as a crossed product. Next we will show that under certain conditions on f or g, B (f, g) will be nuclear, weakly quasidiagonal and we will be able to compute its Ext group. In the last two sections we will give a partial description of the K1-group of B (f, g) and then using the results from [DH] calculate the free entropy dimension of B (f, g). In the third and last part of this paper we show that the standard family of independent unitary n x n random matrices remains an asymptotically free Haar unitary with respect to any state 4:Mn C →C . The result was originally stated by Voiculescu for the normalized trace. Our work here will follow the modified version of Voiculescu\u27s theorem given by D. Hadwin and M. Dostal in [DH]

Entropy on the von Neumann lattice and its evaluation

Based on the recently introduced averaging procedure in phase space, a new type of entropy is defined on the von Neumann lattice. This quantity can be interpreted as a measure of uncertainty associated with simultaneous measurement of the position and momentum observables in the discrete subset of the phase space. Evaluating for a class of the coherent states, it is shown that this entropy takes a stationary value for the ground state, modulo a unit cell of the lattice in such a class. This value for the ground state depends on the ratio of the position lattice spacing and the momentum lattice spacing. It is found that its minimum is realized for the perfect square lattice, i.e., absence of squeezing. Numerical evaluation of this minimum gives 1.386....Comment: 14 pages, no figures; J. Phys. A, in pres

Periodic and discrete Zak bases

Weyl's displacement operators for position and momentum commute if the product of the elementary displacements equals Planck's constant. Then, their common eigenstates constitute the Zak basis, each state specified by two phase parameters. Upon enforcing a periodic dependence on the phases, one gets a one-to-one mapping of the Hilbert space on the line onto the Hilbert space on the torus. The Fourier coefficients of the periodic Zak bases make up the discrete Zak bases. The two bases are mutually unbiased. We study these bases in detail, including a brief discussion of their relation to Aharonov's modular operators, and mention how they can be used to associate with the single degree of freedom of the line a pair of genuine qubits.Comment: 15 pages, 3 figures; displayed abstract is shortened, see the paper for the complete abstrac

Structural Disruption of an Adenosine-Binding DNA Aptamer on Graphene: Implications for Aptasensor Design

YesWe report on the predicted structural disruption of an adenosine-binding DNA aptamer adsorbed via noncovalent interactions on aqueous graphene. The use of surface-adsorbed biorecognition elements on device substrates is needed for integration in nanofluidic sensing platforms. Upon analyte binding, the conformational change in the adsorbed aptamer may perturb the surface properties, which is essential for the signal generation mechanism in the sensor. However, at present, these graphene-adsorbed aptamer structure(s) are unknown, and are challenging to experimentally elucidate. Here we use molecular dynamics simulations to investigate the structure and analyte-binding properties of this aptamer, in the presence and absence of adenosine, both free in solution and adsorbed at the aqueous graphene interface. We predict this aptamer to support a variety of stable binding modes, with direct base−graphene contact arising from regions located in the terminal bases, the centrally located binding pockets, and the distal loop region. Considerable retention of the in-solution aptamer structure in the adsorbed state indicates that strong intra-aptamer interactions compete with the graphene−aptamer interactions. However, in some adsorbed configurations the analyte adenosines detach from the binding pockets, facilitated by strong adenosine−graphene interactions

Elucidating the mechanisms of nanodiamond-promoted structural disruption of crystallised lipid

yesThe removal or structural disruption of crystallised lipid is a pivotal but energy-intensive step in a wide range of industrial and biological processes. Strategies to disrupt the structure of crystallised lipid in aqueous solution at lower temperatures are much needed, where nanoparticle-based strategies show enormous promise. Using the aqueous tristearin bilayer as a model for crystallised lipid, we demonstrate that the synergistic use of surfactant and detonation nanodiamonds can depress the onset temperature at which disruption of the crystallised lipid structure occurs. Our simulations reveal the molecular-scale mechanisms by which this disruption takes place, indicating that the nanodiamonds serve a dual purpose. First, the nanodiamonds are predicted to facilitate delivery of surfactant to the lipid/water interface, and second, nanodiamond adsorption acts to roughen the lipid/water interface, enhancing ingress of surfactant into the bilayer. We find the balance of the hydrophobic surface area of the nanodiamond and the nanodiamond surface charge density to be a key determinant of the effectiveness of using nanodiamonds to facilitate lipid disruption. For the nanodiamond size considered here, we identify a moderate surface charge density, that ensures the nanodiamonds are neither too hydrophobic nor too hydrophilic, to be optimal

Molecular dynamics simulations of mixed DOPC–β-sitosterol bilayers and their interactions with DMSO

ell membrane phospholipid bilayers can be damaged by the large amounts of dimethyl sulphoxide (DMSO) commonly used in cryopreservation. The interaction of DMSO with model bilayers consisting of 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) and ß-sitosterol has been studied using molecular dynamics simulations. Initially the effect of sterol concentration and temperature upon bilayers solvated in pure water was determined, and membranes containing ß-sitosterol were compared with membranes containing cholesterol. These simulations showed that the presence of sterols has a condensing effect on the phospholipids, causing a reduction in the area per lipid as the sterol concentration increases, up to a phospholipid–sterol ratio of 2[thin space (1/6-em)]:[thin space (1/6-em)]1. The incorporation of sterols into the bilayer also increased the thickness and order of the phospholipid acyl tails. DOPC–ß-sitosterol bilayers at different relative concentrations were simulated in solutions of 2.5 and 25.0 mol% DMSO. The interaction of DMSO with the bilayers caused the bilayers to expand laterally, while thinning normal to the plane of the bilayer expansion. The same qualitative behaviour has been shown to occur in pure phosphocholine bilayers. However, the presence of sterols made the membranes more resistant to the effects of DMSO, to the extent that the membranes where able to maintain their integrity in 25.0 mol% DMSO, a concentration that would otherwise cause the destruction of a pure DOPC bilayer. Increasing the concentration of ß-sitosterol within the bilayers reduced the rate of DMSO diffusion across the bilayer and, if the concentration was large enough, caused the diffusion mechanism to change. DMSO was observed to disorder the membranes enough to cause an increase in the number of sterol “flip–flops”. The findings of this work provide a more realistic description of how DMSO interacts with cell membranes and the role of the composition of the membrane