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The Hofmeister effect describes how different ions make solutes more or less hydrophobic. The effect is thought to occur due to structural changes in the solvent induced by the ion's presence, particularly in water. In this study, the structural changes in water due to the presence of ions are investigated by molecular dynamics simulations of various monatomic ions in the SPC/E water model. Structural analyses reveal specific orientations of solvating waters around each of the ions studied. Using a new method, these orientations are quantified by a set of pair correlation functions that describe dipole–ion correlations in structure. These correlations are shown to contribute to the potential of mean force between waters and the ion of interest, and therefore to the free energy of the system. The magnitude of this free energy is found to result in a Hofmeister series for the various ions studied, therefore demonstrating a Hofmeister effect with respect to water's structure that is quantified by pair correlation functions. Most crucially, the pair correlations that lead to this Hofmeister effect also contribute to the hydrophobic effect (the entropy of hydrophobic solvation) [Liu et al., J. Chem. Phys., 2015, 142, 114117], and those which dominate the hydrophobic effect are modulated by an ion's presence, therefore demonstrating a mechanistic link between the two effects. [ABSTRACT FROM AUTHOR]

Based on the continuity equations and Poisson's equation, we developed a numerical model for perovskite solar cells. Due to different working mechanisms, the model for perovskite solar cells differs from that of silicon solar cells and Dye Sensitized Solar Cells. The output voltage and current are calculated differently, and in a manner suited in particular to perovskite organohalides. We report a test of our equations against experiment with good agreement. Using this numerical model, it was found that performances of solar cells increase with charge carrier's lifetimes, mobilities and diffusion lengths. The open circuit voltage (Voc) of a solar cell is dependent on light intensities, and charge carrier lifetimes. Diffusion length and light intensity determine the saturated current (Jsc). Additionally, three possible guidelines for the design and fabrication of perovskite solar cells are suggested by our calculations. Lastly, we argue that concentrator perovskite solar cells are promising. [ABSTRACT FROM AUTHOR]

Photovoltaic cells with absorbing layers of certain perovskites have power conversion efficiencies up to 20%. Among these materials, CH3NH3PbI3 is widely used. Here we use density-functional theory to calculate the energies and rotational energy barriers of a methylammonium ion in the α or β phase of CH3NH3PbI3 with differently oriented neighbouring methylammonium ions. Our results suggest the methylammonium ions in CH3NH3PbI3 prefer to rotate collectively, and to be parallel to their neighbours. Changes in polarization on rotation of methylammonium ions are two to three times larger than those on relaxation of the lead ion from the centre of its coordination shell. The preferences for parallel configuration and concerted rotation, with the polarisation changes, are consistent with ferroelectricity in the material, and indicate that this polarisation is governed by methylammonium orientational correlations. We show that the field due to this polarisation is strong enough to screen the field hindering charge transport, and find this screening field in agreement with experiment. We examine two possible mechanisms for the effect of methylammonium ion rotation on photovoltaic performance. One is that rearrangement of methylammoniums promotes the creation and transport of charge carriers. Some effective masses change greatly, but changes in band structure with methylammonium rotation are not large enough to explain current–voltage hysteresis behaviour. The second possible mechanism is that polarization screens the hindering electric field, which arises from charge accumulation in the transport layers. Polarization changes on methylammonium rotation favour this second mechanism, suggesting that collective reorientation of methylammonium ions in the bulk crystal are in significant part responsible for the hysteresis and power conversion characteristics of CH3NH3PbI3 photovoltaic cells. [ABSTRACT FROM AUTHOR]

A series of new metallodendrimers built around a ruthenium phthalocyanine core has been prepared. Employing a convergent synthetic strategy, pyridine-containing ligands were prepared and then assembled onto the ruthenium phthalocyanine through axial ligand coordination. The growing shell of oligoethylene glycol chains surrounding the lipophilic core allows solubilisation in water. Photophysical studies show that all the metallodendrimers are strongly phosphorescent and the deactivation pathway of their triplet state depends on the medium in which the compounds are dissolved. On one hand, quenching of the triplet state by the dendritic shell is observed and found to be substantially enhanced in aqueous media. On the other, the dendrimer shields the phthalocyanine from oxygen. This notwithstanding, the phthalocyanines are able to generate singlet oxygen in less polar environments such as in CHCl3or THF solution, while in water the generation of singlet oxygen is almost completely switched off. [ABSTRACT FROM AUTHOR]

Measurements with different techniques point to a strong affinity of hydroxide ions for interfaces between water and hydrophobes, but some spectroscopic experiments do not detect excess hydroxide at the interface, while others do. Hydroxide ions are unusual in that they reduce the relative permittivity of an electrolyte solution more than other monovalent, monatomic ions. This implies that they suppress the collective dipole-moment fluctuations of nearby waters. We show that the absence of these fluctuations leads to a Hamaker-like force on the hydroxide ion that attracts it to regions where dipole-moment fluctuations are smaller than in bulk water, in other words, to regions of low relative permittivity. We show also that there is no contradiction between the picture of the basic, negatively charged interface and spectroscopic measurements. This is, in part, because the hydroxides are mostly below the outermost molecular layers. By combining a simple model for this fluctuation force with a modified Poisson–Boltzmann equation, we reproduce the dependence of the ζ-potential on pH, including the low isoelectric point, the approximate magnitude of the experimental surface charge density, and the Jones–Ray data for the dependence of surface tension on electrolyte concentration. We discuss also the apparent contradiction between molecular-dynamics simulations that deny and experiments that support a basic, negatively charged interface. [ABSTRACT FROM AUTHOR]

The claim that the behaviour of the fluoride ion at aqueous hydrophobic interfaces contradicts the fluctuation force model described in our paper is dismissed on four grounds. (1) The dielectric decrement of NaF solutions remains uncertain. (2) Even if it were as large as that of NaOH, this would not disprove our theory (as explained in our paper), partly because there is no reason to assume that it interacts with surrounding water molecules in the same way as does the hydroxide ion, despite their having similar ionic radii. In fact, experimental evidence shows the solvation structures around the two ions are quite different, a difference not captured by simulations of the type used by Vácha et al.(3) There is no experimental evidence that fluoride is not adsorbed at the interface at millimolar concentrations. (4) The models and methods of calculation used by Vácha et al.in their comment appear to be flawed, and this casts some doubt on the rather firm conclusions drawn from similar simulations in previous works. [ABSTRACT FROM AUTHOR]

The claim that the behaviour of the fluoride ion at aqueous hydrophobic interfaces contradicts the fluctuation force model described in our paper is dismissed on four grounds. (1) The dielectric decrement of NaF solutions remains uncertain. (2) Even if it were as large as that of NaOH, this would not disprove our theory (as explained in our paper), partly because there is no reason to assume that it interacts with surrounding water molecules in the same way as does the hydroxide ion, despite their having similar ionic radii. In fact, experimental evidence shows the solvation structures around the two ions are quite different, a difference not captured by simulations of the type used by Vácha et al.(3) There is no experimental evidence that fluoride is not adsorbed at the interface at millimolar concentrations. (4) The models and methods of calculation used by Vácha et al.in their comment appear to be flawed, and this casts some doubt on the rather firm conclusions drawn from similar simulations in previous works. [ABSTRACT FROM AUTHOR]

The claim that the behaviour of the fluoride ion at aqueous hydrophobic interfaces contradicts the fluctuation force model described in our paper is dismissed on four grounds. (1) The dielectric decrement of NaF solutions remains uncertain. (2) Even if it were as large as that of NaOH, this would not disprove our theory (as explained in our paper), partly because there is no reason to assume that it interacts with surrounding water molecules in the same way as does the hydroxide ion, despite their having similar ionic radii. In fact, experimental evidence shows the solvation structures around the two ions are quite different, a difference not captured by simulations of the type used by Vácha et al.(3) There is no experimental evidence that fluoride is not adsorbed at the interface at millimolar concentrations. (4) The models and methods of calculation used by Vácha et al.in their comment appear to be flawed, and this casts some doubt on the rather firm conclusions drawn from similar simulations in previous works. [ABSTRACT FROM AUTHOR]

The claim that the behaviour of the fluoride ion at aqueous hydrophobic interfaces contradicts the fluctuation force model described in our paper is dismissed on four grounds. (1) The dielectric decrement of NaF solutions remains uncertain. (2) Even if it were as large as that of NaOH, this would not disprove our theory (as explained in our paper), partly because there is no reason to assume that it interacts with surrounding water molecules in the same way as does the hydroxide ion, despite their having similar ionic radii. In fact, experimental evidence shows the solvation structures around the two ions are quite different, a difference not captured by simulations of the type used by Vácha et al.(3) There is no experimental evidence that fluoride is not adsorbed at the interface at millimolar concentrations. (4) The models and methods of calculation used by Vácha et al.in their comment appear to be flawed, and this casts some doubt on the rather firm conclusions drawn from similar simulations in previous works. [ABSTRACT FROM AUTHOR]