Tímea Nagy

Monte Carlo simulation of the electric double layer : dielectric boundaries and the effects of induced charge

To model the double layer near an electrode, theories and simulations must include the different dielectric coefficients of the electrode, the commonly-postulated ‘inner’ layer, and the electrolyte. Recently, Boda et al. [D. Boda, D. Henderson, K.-Y. Chan, D.T. Wasan. Phys. Rev. E, 69, 046702, (2004)] developed a technique to include inhomogeneous dielectric coefficients in arbitrary geometries in a simulation. Here, Monte Carlo simulation results based on this method are reported for the density profiles of 1:1, 2:2 and 2:1 aqueous electrolytes. The simulations include two dielectric boundaries, one from an inner layer of low dielectric coefficient and one from an uncharged metal electrode. In addition, an extension of a Poisson–Boltzmann (PB) type theory due to Onsager and Samara [L. Onsager, N.N.T. Samara. J. chem. Phys., 2, 528, (1934)] is developed and compared with our simulation results. This approach works best for 1:1 salts at low concentrations.

Simulation of an Electrical Double Layer Model with a Low Dielectric Layer between the Electrode and the Electrolyte

We report Monte Carlo simulation results for double layers of 1:1 and 2:1 electrolytes near an electrode with an inner layer that has a dielectric constant, ε(2), smaller than that of the electrolyte, ε(3). The electrolyte is modeled in the implicit solvent framework (primitive model), while the electrode is a metal electrode in this study (ε(1) → ∞). The charged hard sphere ions are not allowed to enter into the inner layer. We show that the capacitance of the inner layer is C(δ) = ε(0)(ε(2) + ε(3))/2δ, where δ is the thickness of the inner layer. This result is different from that obtained from solutions of the Poisson-Boltzmann equation (ε(0)ε(2)/δ), indicating that interpretation of experimental data with a fitted ε(2) dielectric constant of the inner layer must be done using a different equation. We also show that the properties of the diffuse layer are not independent of the value of ε(2), which is a usual assumption of the Poisson-Boltzmann theory. This is mainly because the repulsive image charges repel both the counterions and the co-ions, while the electrode charge attracts the counterions and repels the co-ions.

Roadside verges as habitats for endangered lizard-orchids (Himantoglossum spp.): Ecological traps or refuges?

Alterations in traditional land use practices have led to severe declines in the area of semi-natural grasslands, thereby seriously threatening plant and animal species dependent on these habitats. Small anthropogenic managed habitats, like roadsides can act as refuges and might play an important role in conserving these species. Colonization of roadside verges by endangered lizard orchids (Himantoglossum spp.) has long been known, but few studies have systematically explored the suitability of roadside habitats for these orchids and the impact of roads on them. In this paper we present results of targeted surveys of three lizard orchid taxa on roadsides from eight European countries. During these surveys we searched for lizard orchids inhabiting roadside verges and recorded their distance from road, aspects of the roadside environment, as well as vegetative and reproductive characteristics of individual plants. We found large numbers of lizard orchids on roadside verges. Distance from roads was not uniformly distributed: orchids occurred more closely to roads than expected by chance. This suggests that regular management of roadsides (e.g. mowing) might enhance colonization and survival of lizard orchids. On the other hand, we also found that close proximity to roads negatively affects reproductive success, suggesting that the immediate vicinity of roads might act as an ecological trap (i.e. favorable in terms of colonization and survival but unfavorable in terms of reproduction). Nonetheless, the fact that significant and viable populations are maintained at roadsides suggests that traditionally managed roadside verges may allow long-term persistence of lizard orchid populations and may serve as refuges in a landscape context.

Turkish graveyards as refuges for orchids against tuber harvest

Abstract Harvest of orchid tubers for salep production is widespread in southwestern Asia and the Balkans and constitutes a major conservation risk for wild orchid populations. Synanthropic habitats, such as graveyards, are important refuges for orchids and other organisms and could offer protection from salep harvesting because of their special cultural role. However, little is known about the occurrence and factors influencing harvesting of salep in graveyards. During field surveys of 474 graveyards throughout Turkey, we observed 333 graveyards with orchids, 311 graveyards with tuberous orchids, and salep harvest in 14 graveyards. Altogether, 530 individuals of 17 orchid species were collected, representing 9% of the individuals recorded. Harvesting intensity was relatively low, and populations were usually not wholly destroyed. However, some species were clearly more affected than others. Salep harvesting risk of orchid species was significantly associated with flowering time, with early‐flowering species being more affected. A marginally significant positive relationship between harvesting risk and species‐specific tuber size was also detected. Our data suggest that graveyards might offer some protection against salep harvesting in Turkey, but they also show that some orchid taxa are much more affected than others. Overall, our observations add more weight to the conservation value of these special habitats.

The occurrence of Spiraea crenata and other rare steppe plants in Pannonian graveyards

Abstract Spiraea crenata was categorised as a species extinct from Hungary at the end of the 20th century. This stepperelic species was rediscovered in 2000 in a rural graveyard (Pusztamonostor). As a result of our recent survey of 294 Pannonian graveyards, we found further 12 localities of S. crenata. We also found 27 populations of further protected plant species, mainly with pontic, pontic-pannonian, Eurasian or continental distribution. We found that the total scrub cover of graveyards with S. crenata was significantly higher than graveyards without this species; this is obviously related to the fact that the individuals of S. crenata were found mostly in edges of the graveyards, where they could survive mowing among high and dense scrubs or small trees. Other factors (geographic position, altitude above sea level, area of graveyards, proportion of grasslands, and proportion of territory covered by graves) were not related to the presence of the species. The total number of other protected species was significantly higher in graveyards with S. crenata. Most of the sites with S. crenata functioned as graveyards during the 2nd Military Survey of the Austrian Empire (1806–1869; 10 out of 13 sites), and the 3rd Military Survey (1869–1887; 12 of 13 sites). The long usage history of these graveyards suggests that the S. crenata individuals – along with other remarkable species – might be remnants of the original steppe vegetation rather than the result of plantations for ornamental purposes. Our results highlight the role of graveyards in the preservation of steppe flora, one of the most endangered component of the European flora.

Resurrection and typification of Elatine campylosperma (Elatinaceae), a long-forgotten waterwort species

The name Elatine campylosperma Seub. is generally treated as one of the synonyms of E. macropoda Guss. However, recent morphological, phylogenetic and karyological studies indicate that this judgement should be revised. In the present paper we typify the name E. campylosperma, review its taxonomic history and provide a thorough description, with compilation of previously published data and our new measurements from in vitro cultures. Based on our herbarium survey, we outline its Atlantic-Mediterranean distribution area (Spain, France, Italy, Greece, Turkey and Algeria). Habitat preferences are summarized from our field observations, water quality measurements and the label information of the herbarium specimens examined. Intact E. campylosperma seeds were found in faecal samples of the Eurasian Coot (Fulica atra L.) in southern Spain and two of them were germinated, suggesting that E. campylosperma has a capacity for long distance dispersal via endozoochory.

Precipitation and timing of flowering in ghost orchids (Epipogium aphyllum Sw.)

The rare, fully mycoheterotrophic Ghost Orchid, Epipogium aphyllum is only visible during its short flowering and fruiting season, which lasts for a few weeks between May and October. Due to the apparent unpredictability of its flowering, decades may pass between subsequent observations at the same locality. The factors affecting timing of flowering in this enigmatic species remain largely unexplored. In Hungary, it is an extremely rare species: between 1924 and 2014 only 25 dated observations from 15 locations are known. Hungary is located on the edge of the species’ distribution area where higher precipitation may occur only in higher regions of mountains. Hence, the spatial and temporal pattern of precipitation might limit the emergence of generative shoots. In this paper we compared rainfall patterns in relation with the Ghost Orchids’ observations to multiannual precipitation averages. The year of flowering and the month preceding flowering (but not the year before flowering and the month of flowering) were characterised by significantly more rainfall than the multi-annual average precipitation. These results suggest that the appearance of the species in Hungary is precipitation-dependent.

Correction to “Simulation of an Electrical Double Layer Model with a Low Dielectric Layer between the Electrode and the Electrolyte”

I a recent paper, we reported a Monte Carlo (MC) simulation study on the properties of a model double layer with an inner layer (of width δ = 3 Å) introduced between the electrode and the electrolyte (the diffuse layer). The ions were not allowed to enter the inner layer. We used different dielectric constants for the electrode (ε1 → ∞), the inner layer (ε2), and the diffuse layer (ε3 = 80). The double layer geometry is usually simulated in a planar geometry, where periodic boundary conditions are applied in the dimensions parallel to the electrode surface. This requires taking the effect of charges outside the central simulation cell into account in order to mimic an infinite homogeneous system in those dimensions. We used the charged sheet method to handle these long-range corrections. In the case of the three dielectric slabs geometry, however, extra care must be taken to collect all the polarization charges induced on the ε1|ε2 and ε2|ε3 dielectric boundaries. The polarization charge is computed by our Induced Charge Computation (ICC) method, which is a numerical procedure that considers only a finite segment of the dielectric boundary cut out by the central simulation cell (note that there are faster ways to handle this electrostatic problem; see the work of dos Santos and Levin and references therein). Corrections to the polarization charges that fall outside the simulation cell must be included, however, in order to achieve charge neutrality in the bulk region in the middle of the simulation domain. The corrections were computed by taking fundamentally wellestablished values of the total polarization charges (obtained from Gauss’s law) and deducting the value given by ICC. The total induced charges (sum rules), however, have been reported erroneously in our paper (eqs 7, 8, and 29). The correct sum rules are