Comment on “Principal role of the stepwise aggregation mechanism in ionic surfactant solutions near the critical micelle concentration : molecular dynamics study"

This work is part of the research program of FOM and is made possible by financial support from the Netherlands Organization for Scientific Research (NWO).Senter-Novem and the Dutch Ministry of Economic Affairs through grant DFN0642300 for a joint FOM-Unilever project

Cooperative slowdown of water rotation near densely charged ions is intense but short-ranged

Accepted manuscript versionWe investigate the reorientation dynamics of water at 300 K in solutions of magnesium sulfate and cesium chloride from classical atomistic molecular dynamics simulations using the “simple water model with four sites and negative Drude polarizability” (SWM4-NDP) and accompanying ion models; for SO2− 4 , we derive SWM4-NDPcompatible parameters. Results indicate that pairs of ions have a cooperative effect on water rotation, but do not support the model based on experiment whereby ion cooperativity increases the number of very slow water molecules well beyond the ions’ first hydration shell. Instead, we find that cooperative slowdown beyond the first hydration shell is weak. Intense cooperative slowdown is limited to the first hydration shells, the magnitude of the slowdown being stronger for the multivalent ions. Cooperative effects for different salts differ in both the magnitude of rotational slowdown and the spatial range of the affected water subpopulations

Dynamics of water of hydration near disaccharides strongly depends on solute topology: mapping density fluctuations, rotational anisotropy and h-bond exchange mechanism around disaccharides

comunicação em posterDisaccharides such as trehalose are abundant components of cells and may alter the phase behavior or dynamics of phospholipid bilayers: for example, trehalose is a cryoprotectant of lipid bilayers. The origin of this and other effects of disaccharides on membranes is still under debate. One possibility is that some disaccharides alter the dynamics of water of hydration relative to the bulk, and that interactions between the water of hydration of disaccharides and the phospholipids lead to changes in bilayer properties. We address this issue by investigating the dynamics of water near disaccharides kojibiose and trehalose using classical atomistic molecular dynamics simulations and transition state theory. Our results indicate that the cryoprotectant trehalose and the non-cryoprotectant kojibiose differ in the rotational dynamics of their water of hydration, with the subpopulation of water molecules nearest to the central linking oxygen being significantly slower for trehalose. Interestingly, this effect results from differences in both solute chemistry and topology: identical functional groups may interact differently with water depending on the orientation of neighboring groups, in agreement with existing reports on proteins. In contrast to observations on topologically simple solutes, our results indicate that topologically complex solutes such as disaccharides induce unexpected changes in the free energy landscape associated with rotation of water molecules. These results suggest that theoretical models to predict water dynamics near solutes, relevant for example to understand how water dynamics influences protein folding or diffusion through polysaccharide brushes, must account for both solute chemistry and topology

Proteins maintain hydration at high [KCl] concentration regardless of content in acidic amino acids

Proteins of halophilic organisms, which accumulate molar concentrations of KCl in their cytoplasm, have a much higher content in acidic amino acids than proteins of mesophilic organisms. It has been proposed that this excess is necessary to maintain proteins hydrated in an environment with low water activity, either via direct interactions between water and the carboxylate groups of acidic amino acids or via cooperative interactions between acidic amino acids and hydrated cations. Our simulation study of five halophilic proteins and five mesophilic counterparts does not support either possibility. The simulations use the AMBER ff14SB force field with newly optimized Lennard-Jones parameters for the interactions between carboxylate groups and potassium ions. We find that proteins with a larger fraction of acidic amino acids indeed have higher hydration levels, as measured by the concentration of water in their hydration shell and the number of water/protein hydrogen bonds. However, the hydration level of each protein is identical at low (bKCl = 0.15 mol/kg) and high (bKCl = 2 mol/kg) KCl concentrations; excess acidic amino acids are clearly not necessary to maintain proteins hydrated at high salt concentration. It has also been proposed that cooperative interactions between acidic amino acids in halophilic proteins and hydrated cations stabilize the folded protein structure and would lead to slower dynamics of the solvation shell. We find that the translational dynamics of the solvation shell is barely distinguishable between halophilic and mesophilic proteins; if such a cooperative effect exists, it does not have that entropic signature

The influence of pulse duration on the stress levels in ablation of ceramics: A finite element study

We present a finite element model to investigate the dynamic thermal and mechanical response of ceramic materials to pulsed infrared radiation. The model was applied to the specific problem of determining the influence of the pulse duration on the stress levels reached in human dental enamel irradiated by a CO2 laser at 10.6 mm with pulse durations between 0.1 and 100 ms and sub-ablative fluence. Our results indicate that short pulses with durations much larger than the characteristic acoustic relaxation time of the material can still cause high stress transients at the irradiated site, and indicate that pulse durations of the order of 10 ms may be more adequate both for enamel surface modification and for ablation than pulse durations up to 1 ms. The model presented here can easily be modified to investigate the dynamic response of ceramic materials to mid-infrared radiation and help determine optimal pulse durations for specific procedures.Fundação para a Ciência e a Tecnologia (FCT) – Programa Operacional “Ciência , Tecnologia, Inovação” – POCTI/ESP/37944/2001 e SFRH/BD/4725/2001

Modelling the influence of pore size on the response of materials to infrared lasers – an application to human enamel

We present an analytical model for a ceramic material (hydroxyapatite) containing nanometre-scale water pores, and use it to estimate the pressure at the pore as a function of temperature at the end of a single 0.35 µs laser pulse by Er:YAG (2.94 µm) and CO2 (10.6 µm) lasers. Our results suggest that the pressure at the pore is directly related to pore temperature, and that very high pressures can be generated simply by the thermal expansion of liquid water. Since the temperature reached at the pores at the end of the laser pulse is a strong function of pore size for Er:YAG lasers, but is independent of pore size for CO2 lasers, our present results provide a possible explanation for the fact that the enamel threshold ablation fluences are more variable for Er:YAG lasers than for CO2 lasers, and suggest that experimentalists should analyse their results accounting for factors (like age or type of tooth) that may change the pore size distribution in their samples.Comunidade Europeia (CE) - Fundo Comunitário Europeu (FEDER).Fundação para a Ciência e a Tecnologia (FCT)

Mechanical and thermal response of enamel to IR radiation - a finite element mesoscopic model

We present finite element models of human dental enamel that account for water-pores known to exist in this material, and use them to assess the influence of these pores on the temperature and stress profiles during and after single Er:YAG (2.9 µm) and CO2 (10.6 µm) laser pulses of duration 0.35 µs. Our results indicate that the temperature maximum is reached at the water-pores at the end of the laser pulse; this maximum seems to be independent of pore size for the CO2 laser but appears to be strongly dependent of pore size for the Er:YAG laser. The pressure reached at the water pore seems to be directly related to the temperature at the pore and it is significantly higher that the stress levels reached throughout the modelled structure, which indicates that water pores should play a significant role in the ablation mechanisms, even before water vaporization takes place. These results suggest that researchers conducting enamel ablation by Er:YAG lasers - or other lasers with wavelengths for which the absorption coefficients of the mineral and the water differ significantly - may want to select their samples and analyse their results taking into account factors that may alter the degree of mineralization of a tooth, such as age or type of tooth.Fundação para a Ciência e a Tecnologia (FCT) – Programa Operacional “Ciência , Tecnologia, Inovação” – POCTI/ESP/37944/2001, SFRH/BD/4725/200.Comunidade Europeia (CE). Fundo Europeu de Desenvolvimento Regional (FEDER)

Modelling of dental laser ablation

Tese de Doutoramento CiênciasO objectivo final do trabalho descrito nesta tese consiste na determinação dos melhores parâmetros de funcionamento de lasers de modo a ser possível escavar túneis compridos e estreitos através do esmalte e da dentina, um dos procedimentos necessários para se proceder a um tratamento minimamente invasivo da cárie dentária. Primeiramente é apresentada uma revisão da literatura onde são identificadas as gamas de valores dos parâmetros de funcionamento do laser para as quais são esperados os melhores resultados, e são identificados os problemas que necessitam de ser resolvidos. Os lasers que deverão produzir os melhores resultados com o mais baixo custo financeiro são os de CO2 e os de Er:YAG, com pulsos de duração na ordem dos microsegundos e com arrefecimento por água. Os problemas a resolver incluem os danos mecânicos infligidos ao material e a possibilidade da água de arrefecimento absorver grande parte da radiação incidente e deste modo impedir que sejam produzidos túneis compridos. Após uma breve introdução ao Método dos Elementos Finitos (a ferramenta de modelização utilizada neste trabalho), são apresentados os modelos produzidos para o estudo da interacção entre os lasers de CO2 e Er:YAG e o esmalte dentário e os resultados obtidos com esses modelos, para o regime sub-ablativo. Finalmente, são apresentadas as conclusões principais obtidas com este trabalho, sendo dada particular ênfase às linhas de acção práticas que estas sugerem para se obterem melhores resultados experimentais, e são delineados trabalhos futuros que interessa desenvolver nesta área.The ultimate aim of the work described in this Thesis is to determine the optimal laser operating parameters to drill long, narrow tunnels through enamel and dentine, necessary to treat dental caries in a way that minimizes the amount of material removed from the tooth. In order to do this, a review of the literature is first presented in which the ranges of laser parameter values for which the best results exist are narrowed down and the issues to be solved are identified. It is expected that CO2 and Er:YAG lasers with microsecond pulse duration and water cooling will produce the best results at a minimum financial cost. Issues to be addressed include the extent of mechanical damage caused by the lasers and whether the cooling water will absorb a large fraction of the incident radiation and thus prevent the material from being ablated. After a brief introduction to the Finite Element Method (the modelling tool used throughout this work), the models designed to investigate ablation of dental enamel by the CO2 and Er:YAG lasers are described and the results obtained for a sub-ablative regime are presented. Finally, the main conclusions obtained by this work are given and the practical guidelines to obtain better results when ablating dental enamel are presented. A brief indication of the work to be done in the future concludes this Thesis.Fundação para a Ciência e a Tecnologia (FCT)under project no. POCTI/ESP/ 37944/2001 (supported by the European Community Fund FEDER) and by PhD. fellowship number SFRH/BD/4725/2001

Boundary conditions for 3-D dynamic models of ablation of ceramics by pulsed midiInfrared lasers

We present and discuss a set of boundary conditions (BCs) to use in three-dimensional, mesoscopic, finite element models of mid-infrared pulsed laser ablation of brittle materials. These models allow the study of the transient displacement and stress fields generated at micrometer scales during and after one laser pulse, where using conventional BCs may lead to some results without physical significance that can be considered an artefact of the calculations. The proposed BCs are tested and applied to a micrometer-scale continuous model of human dental enamel under CO2 radiation (10.6 mm, 0.35 ms pulse, sub-ablative fluence), giving rise to the following results: the highest stress is obtained at the irradiated surface of the model, at the end of the laser pulse, but afterwards it decreases rapidly until it becomes significantly lower than the stress in a region 2.5 mm deep in the model; a thermally induced vibration in the material is predicted. This non-intuitive dynamics in stress and displacement distribution cannot be neglected and has to be considered in dynamic laser ablation models, since it may have serious implications in the mechanisms of ablation.Fundação para a Ciência e a Tecnologia (FCT) – Programa Operacional “Ciência, Tecnologia, Inovação” - POCTI/ESP/37944/2001, SFRH/BD/4725/2001.Comunidade Europeia (CE). Fundo Europeu de Desenvolvimento Regional (FEDER)