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Heterogeneous ice nucleation is an important process in atmospheric science, food preservation, and other areas of research. Muscovite mica is a commonly occurring mineral, and although its ice nuc...

Heterogeneous ice nucleation is an important process in environmental, biological, and atmospheric science. A variety of mineral, organic, and biological materials function as ice nucleating partic...

Molecular dynamics simulations are employed to investigate the influence of inorganic salts on ice nucleation by the Al surface of kaolinite, terminated with hydroxyl groups. Seven salt solutions (LiI(Cl), NaI(Cl), KI(Cl), and NH

Experiments have shown α-Al2O3 (corundum) to be a very efficient ice-nucleating agent. Molecular dynamics simulations are used to explore the microscopic origins of its ice-nucleating ability. This...

All lithium halides exist in the rock salt crystal structure under ambient conditions. In contrast, common lithium halide classical force fields more often predict wurtzite as the stable structure. This failure of classical models severely limits their range of application in molecular simulations of crystal nucleation and growth. Employing high accuracy density functional theory (DFT) together with classical models, we examine the relative stability of seven candidate crystal structures for lithium halides. We give a detailed examination of the influence of DFT inputs, including the exchange–correlation functional, basis set, and dispersion correction. We show that a high-accuracy basis set, along with an accurate description of dispersion, is necessary to ensure prediction of the correct rock salt structure, with lattice energies in good agreement with the experiment. We also find excellent agreement between the DFT-calculated rock salt lattice parameters and experiment when using the TMTPSS-rVV10 exchange–correlation functional and a large basis set. Detailed analysis shows that dispersion interactions play a key role in the stability of rock salt over closely competing structures. Hartree–Fock calculations, where dispersion interactions are absent, predict the rock salt structure only for LiF, while LiCl, LiBr, and LiI are more stable as wurtzite crystals, consistent with radius ratio rules. Anion–anion second shell dispersion interactions overcome the radius ratio rules to tip the structural balance to rock salt. We show that classical models can be made qualitatively correct in their structural predictions by simply scaling up the pairwise additive dispersion terms, indicating a pathway toward better lithium halide force fields.

Background Few risk factors are associated with the risk of relapses of Crohn’s disease in children. Aims The aims of this retrospective cohort study were to describe the rate of relapses in children with Crohn’s disease, its evolution over the past decade and to determine risk factors associated with relapse. Methods Patients under 18 years old and diagnosed between 2009 and 2019 were included. Patients clinical, endoscopic, histological, and laboratory characteristics, as well as their treatments, where collected from their medical records and the prospective CHU Sainte-Justine inflammatory bowel disease registry. Survival analyses and Cox regression models were used to assess the impact of those risk factors on relapse. Results 639 patients were included. There was a decrease in the clinical relapse rate over the past decade: 70.9% of patients diagnosed between 2009 and 2014 experienced a relapse compared to 49.1% of patients diagnosed between 2015 and 2019 (p Conclusions Relapse risk was significantly associated with baseline clinical, endoscopic, histological and laboratory data and treatment strategies. These results could help better select treatment options for pediatric Crohn’s disease at induction and maintenance. Kaplan-Meier curve representing patients time to relapse according to the mean infliximab level in post-induction. Funding Agencies NoneFonds Recherche Santé Québec / Fondation du CHU Sainte-Justine

Background The natural evolution of Crohn’s disease is incompletely understood in the pediatric population. Data on factors influencing time-to-remission are very limited in the literature. Aims The aim of this retrospective cohort study was to describe the time to clinical remission in children with Crohn’s disease as well as changes over the past decade and to identify factors associated with time to clinical remission. Methods Patients under 18 years old diagnosed between 2009 and 2019 were included. All data were collected from the patients’ medical records and the CHU Sainte-Justine inflammatory bowel disease registry. Survival analyses and linear regression models were used to assess the impact of clinical, laboratory, endoscopic, histological and therapeutic factors on time to clinical remission. Results 654 patients were included in the study. There was no change in the time to clinical remission over the past decade. Female sex in adolescents (ajusted bêta regression coefficient (aβ)= 31.8 days, p= 0.02), upper digestive tract involvement (aβ= 46.4 days, p= 0.04), perianal disease (aβ= 32.2 days, p= 0.04), presence of active inflammation on biopsies (aβ= 46.7 days, p= 0.01) and oral 5-ASA exposure (aβ=56.6 days, p= 0.002) were all associated with longer time to clinical remission. However, antibiotic exposure (aβ= -29.3 days, p=0.04), increased eosinophils on biopsies (aβ= -29.6 days, p=0.008) and combination of exclusive enteral nutrition and TNF- alpha inhibitors as induction therapy (aβ= -36.8, p=0.04) were associated with shorter time to clinical remission. Conclusions Time to clinical remission did not improve during the decade and was associated with baseline clinical and histological data and treatment strategies. Combination of enteral nutrition and TNF-alpha inhibitors was associated with faster clinical remission. Kaplan-Meier curve representing the time to clinical remission of patients according to the first induction treatment administered. Funding Agencies NoneFonds Recherche Santé Québec / Fondation du CHU Sainte-Justine

Background Crohn’s disease (CD) triggers are incompletely understood and the incidence of the disease has been increasing. Aims The aims of this study were to describe the trends in the clinical, endoscopic, histological, and laboratory characteristics of pediatric CD during the last decade and to describe the seasonal variation of disease presentation at diagnosis. Methods Patients under 18 years old and diagnosed between 2009 and 2019 were included. Patients clinical, endoscopic, histological, and laboratory data were collected from the medical records. Data were analyzed for the cohort as a whole and according to diagnostic periods (2009–2014 and 2015–2019) and seasons. Results 654 patients were included in the study. The total number of incident CD cases significantly increased yearly. Patients diagnosed between 2015 and 2019 were younger at diagnosis (OR: 2.30, p There were fewer CD diagnosis during winter. The highest vitamin D levels in patients occurred in summer and fall, but the majority of patients had, regardless of the season of diagnosis, severe vitamin D deficiency (the median vitamin D level was 60.0 nmol/L in summer and fall compared to 47.0 nmol/L in winter-spring, p=0.003). Vitamin D levels at diagnosis are inversely correlated with PCDAI (Pearson correlation coefficient = -0.19, p=0.03) and SES-CD (-0.20, p=0.04). Patients diagnosed in fall had lower PCDAI and SES-CD scores, less failure to thrive, less digestive symptoms and less extensive digestive involvement. Colonic disease was significantly more frequent during summer and fall (27.3% of patients diagnosed in summer and fall versus 18.2% of cases in winter and spring, p=0.01). Conclusions The disease phenotype has changed over the years and there are important seasonal trends in the frequency and severety of the disease suggesting possible disease triggers. Our findings provide interesting avenues for future research, such as identifying the clinical significance of granulomas, vitamin D deficiency and microbiota on pediatric CD activity. PCDAI at diagnosis according to the season. Funding Agencies NoneFonds Recherche Santé Québec / Fondation du CHU Sainte-Justine

Background Crohn’s disease (CD) triggers are incompletely understood and the incidence of the disease has been increasing. The aims of this study were to describe the trends in the clinical, endoscopic, histological, and laboratory characteristics of pediatric CD during the last decade and to describe the seasonal variation of disease presentation at diagnosis. Methods Patients under 18 years old and diagnosed between 2009 and 2019 were included. Patients clinical, endoscopic, histological, and laboratory data were collected from the medical records. Data were analyzed for the cohort as a whole and according to diagnostic periods (2009–2014 and 2015–2019) and seasons. Results 654 patients were included in the study. The total number of incident CD cases significantly increased yearly. Patients diagnosed between 2015 and 2019 were younger at diagnosis (OR: 2.30, p There were fewer CD diagnosis during winter. The highest vitamin D levels in patients occurred in summer and fall, but the majority of patients had, regardless of the season of diagnosis, severe vitamin D deficiency (the median vitamin D level was 60.0 nmol/L in summer and fall compared to 47.0 nmol/L in winter-spring, p=0.003). Vitamin D levels at diagnosis are inversely correlated with PCDAI (Pearson correlation coefficient = -0.19, p=0.03) and SES-CD (-0.20, p=0.04). Patients diagnosed in fall had lower PCDAI and SES-CD scores, less failure to thrive, less digestive symptoms and less extensive digestive involvement. Colonic disease was significantly more frequent during summer and fall (27.3% of patients diagnosed in summer and fall versus 18.2% of cases in winter and spring, p=0.01). Figure 1:PCDAI at diagnosis according to the season. Figure 2:Proportion of patients with L1-L2-L3 disease locations by the Paris classification according to the season of diagnosis. Conclusion The disease phenotype has changed over the years and there are important seasonal trends in the frequency and severety of the disease suggesting possible disease triggers. Our findings provide interesting avenues for future research, such as identifying the clinical significance of granulomas, vitamin D deficiency and microbiota on pediatric CD activity.

Molecular dynamics simulations of the dissolution of aspirin in water are reported. Crystals initially cubic and cylindrical in shape are considered, and the influence of temperature is examined. All simulations are carried out in a manner designed to prevent the build up of any significant concentration of aspirin in solution, physically corresponding to sink conditions. It is found that aspirin dissolution follows a three stage mechanism similar in most respects to earlier observations for nanocrystals of NaCl and urea. In the initial stage, molecules are first lost from corners and edges of the crystal, the crystal is solution annealed into a particular limiting shape, that then persists throughout a large fraction of the dissolution process. For aspirin, initially cubic crystals anneal into a cylindrical shape, in contrast with the near spherical shapes attained by cubic NaCl and urea crystals. In an intermediate stage, during which most of the crystal dissolves, the dissolution rate is well described by a simple classical model which assumes that the rate is proportional to the active surface area of the crystal. For aspirin, the active surface area is identified as the curved surface of a cylinder for both crystal shapes. In the final stage, the small remaining crystal looses its structure and rapidly dissolves. Given that a similar three stage mechanism applies to crystals as varied as NaCl, urea, and aspirin, we conjecture that this mechanism is possibly quite general, and might apply to many ionic and molecular crystals. As for NaCl and urea, an analysis of the activation energy associated with aspirin dissolution strongly suggests that detachment of molecules from the crystal is the rate determining step, at least under the sink conditions.

Ice clouds play an important role in the Earth’s radiative budget and hence climate. Heterogeneous ice nucleation, a major pathway for ice formation in cirrus and mixed-phase clouds, is induced by active sites present on atmospheric aerosol particles termed as ice-nucleating particles. Feldspars have been shown to be highly ice nucleation active. Despite the importance of mineral dusts for ice nucleation, the role of atmospheric aging (e.g. surface alteration due to interactions with chemical species) on their ice nucleation efficiency is largely unknown. This is primarily due to the lack of microscopic level insight into nucleation from laboratory/field-based experiments, due to the inability to experimentally access the small spatial and temporal scales at which nucleation process occurs – a problem that can be potentially tackled with computer simulations. We utilize direct Molecular Dynamics simulations (GROMACS 5.1.4) to investigate the interactions of solutes with different surfaces of potassium feldspar mineral (microcline) and the corresponding interfacial water structure at a microscopic scale. We investigated the interactions of monovalent cations (H3O+, (NH4)+, Li+, K+, Cs+) with various surfaces of microcline, and subsequent effects on the near-surface water structure at 300 K. The investigated surfaces include the perfect cleavage planes, (001) and (010), as well as the high energy plane (100) of microcline. Feldspar is modeled as semi-rigid (lattice atoms fixed expect K+ and H of surface OH) and as fully flexible (all lattice atoms free to move) with the CLAYFF force field, and the TIP4P/Ice model is employed for water. Results show that on simulation timescales, lattice vibration is necessary for ion exchange between added cation and lattice K+, albeit at different exchange rates for the 3 planes. None of the 3 flexible surfaces show any preference for over K+ in terms of ion exchange within the simulation timescale. Both the semi-rigid and flexible surfaces show higher adsorption of molecular cations ((NH4)+ and H3O+) compared with the simple spherical cations. In addition, we do not observe ice nucleation on modified microcline surfaces (both semi-rigid and flexible) at a supercooled temperature of 230 K within the simulation timescale. To conclude, the presented work provides an improved understanding of the processes modifying the feldspar surfaces in water and aqueous solutions and its possible relevance for ice formation.

Background The natural evolution of Crohn’s disease is incompletely understood in the pediatric population. Data on factors influencing time-to-remission are very limited in the literature. The aim of this retrospective cohort study was to describe the time to clinical remission in children with Crohn’s disease as well as changes over the past decade and to identify factors associated with time to clinical remission. Methods Patients under 18 years old diagnosed between 2009 and 2019 were included. All data were collected from the patients’ medical records and the CHU Sainte-Justine inflammatory bowel disease registry. Survival analyses and linear regression models were used to assess the impact of clinical, laboratory, endoscopic, histological and therapeutic factors on time to clinical remission. Results 654 patients were included in the study. There was no change in the time to clinical remission over the past decade. Female sex in adolescents (ajusted bêta regression coefficient (aβ)= 31.8 days, p= 0.02), upper digestive tract involvement (aβ= 46.4 days, p= 0.04), perianal disease (aβ= 32.2 days, p= 0.04), presence of active inflammation on biopsies (aβ= 46.7 days, p= 0.01) and oral 5-ASA exposure (aβ=56.6 days, p= 0.002) were all associated with longer time to clinical remission. However, antibiotic exposure (aβ= -29.3 days, p=0.04), increased eosinophils on biopsies (aβ= -29.6 days, p=0.008) and combination of exclusive enteral nutrition and TNF- alpha inhibitors as induction therapy (aβ= -36.8, p=0.04) were associated with shorter time to clinical remission. Figure 1: Kaplan-Meier curve representing time to clinical remission in patients 13 years old and older by sex. Figure 2: Kaplan-Meier curve representing the time to clinical remission of patients according to the first induction treatment administered. Conclusion Time to clinical remission did not improve during the decade and was associated with baseline clinical and histological data and treatment strategies. Combination of enteral nutrition and TNF-alpha inhibitors was associated with faster clinical remission.

Background Few risk factors are associated with the risk of relapses of Crohn’s disease in children. The aims of this retrospective cohort study were to describe the rate of relapses in children with Crohn’s disease, its evolution over the past decade and to determine risk factors associated with relapse. Methods Patients under 18 years old and diagnosed between 2009 and 2019 were included. Patients clinical, endoscopic, histological, and laboratory characteristics, as well as their treatments, where collected from their medical records and the prospective CHU Sainte-Justine inflammatory bowel disease registry. Survival analyses and Cox regression models were used to assess the impact of those risk factors on relapse. Results 639 patients were included. There was a decrease in the clinical relapse rate over the past decade: 70.9% of patients diagnosed between 2009 and 2014 experienced a relapse compared to 49.1% of patients diagnosed between 2015 and 2019 (p Figure 1: Kaplan-Meier curve representing patients time to relapse according to the first maintenance treatment administered. Conclusion Relapse risk was significantly associated with baseline clinical, endoscopic, histological and laboratory data and treatment strategies. These results could help better select treatment options for pediatric Crohn’s disease at induction and maintenance. Figure 2: Kaplan-Meier curve representing patients time to relapse according to the mean infliximab level in post-induction.

International audience; Large-scale molecular dynamics (MD) simulations are reported for aqueous t-butanol (TBA) solutions. The CHARMM generalized force field (CGenFF) for TBA is combined with the TIP4P/2005 model for water. Unlike many other common TBA models, the CGenFF model is miscible with water in all proportions at 300 K. The main purpose of this work is to investigate the existence and nature of microheterogeneous structure in aqueous TBA solutions. Our simulations of large systems (128000 and 256000 particles) at TBA mole fractions of 0.06 and 0.1 clearly reveal the existence of long-range correlations (> 10 nm) that show significant variations on long time scales (∼ 50 ns). We associate these long-range, slowly varying correlations with the existence of supramolecular, domain-like structures that consist of TBA-rich and water-rich regions. This structure is always present but continually changing in time, giving rise to long-range, slowly varying pair correlation functions. We find that this behavior appears to have little influence on the single particle dynamics; the diffusion coefficients of both TBA and water molecules lie in the usual liquid state regime, and mean square displacements provide no indication of anomalous diffusion. Using our large system simulations, we are able to reliably calculate small angle x-ray scattering (SAXS) and small angle neutron scattering (SANS) spectra, except at very low wave vector, and the results agree well with recent experiments. However, the present paper shows that simulation of the relatively simple TBA/water system remains challenging. This is particularly true if one wishes to obtain properties such as Kirkwood-Buff factors, or scattering functions at low wave vector, which strongly depend on the long-range behavior of the pair correlations.

Molecular dynamics simulations are used to investigate the factors that influence the nucleation of NaCl crystals in a supersaturated aqueous solution. We describe a methodology for detecting solidlike NaCl clusters (potential nuclei) and following their evolution in time until they achieve nucleation (which is very rare) or dissolve back into solution. Through an analysis of cluster lifetimes and multiple nucleation events, we demonstrate that cluster size is not the only property that influences cluster stability and the probability of achieving nucleation. We introduce a parameter called cluster crystallinity, which is a measure of the solidlike order in a particular cluster. We show that cluster order (as measured by this parameter) has a strong influence on the lifetime and nucleation probability of clusters of equal sizes, with the lifetime and probability of nucleation increasing with increasing crystallinity. These observations remain true for clusters as small as six ions, showing that the structural factors are important even at the earliest stages of crystal birth.

Nucleation of ice by airborne particles is a process vital to weather and climate, yet our understanding of the mechanisms underlying this process is limited. Kaolinite is a clay that is a significant component of airborne particles and is an effective ice nucleus. Despite receiving considerable attention, the microscopic mechanism(s) by which kaolinite nucleates ice is not known. We report molecular dynamics simulations of heterogeneous ice nucleation by kaolinite (001) surfaces. Both the Al-surface and the Si-surface nucleate ice. For the Al-surface, reorientation of the surface hydroxyl groups is essential for ice nucleation. This flexibility allows the Al-surface to adopt a structure which is compatible with hexagonal ice, Ih, at the atomic level. On the rigid Si-surface, ice nucleates via an unusual structure that consists of an ordered arrangement of hexagonal and cubic ice layers, joined at their basal planes where the interfacial energy cost is low. This ice structure provides a good match to the atomistic structure of the Si-surface. This example is important and may have far-reaching implications because it demonstrates that potential ice nuclei need not be good atomic-level matches to particular planes of ice Ih or cubic ice, Ic. It suggests that surfaces can act as effective ice nuclei by matching one of the much larger set of planes that can be constructed by regular arrangements of hexagonal and cubic ice.

Molecular dynamics (MD) simulations are employed to investigate crystal nucleation and growth in oversaturated aqueous LiF solutions. Results obtained for a range of temperatures provide evidence that the rate of crystal growth is determined by a substantial energy barrier (∼49 kJ mol-1) related to the loss of water from the ion hydration shells. Employing direct MD simulations, we do not observe spontaneous nucleation of LiF crystals at 300 K, but nucleation is easily observable in NVT simulations at 500 K. This contrasts with the NaCl case, where crystal nucleation is directly observed in similar simulations at 300 K. Based on these observations, together with a detailed analysis of ion clustering in metastable LiF solutions, we argue that the ion dehydration barrier also plays a key role in crystal nucleation. The hydration of the relatively small Li+ and F- ions strongly influences the probability of forming large, crystal-like ion clusters, which are a necessary precursor to nucleation. This important factor is not accounted for in classical nucleation theory.

Molecular dynamics simulations are used to determine the mechanism of urea crystal dissolution in water under sink conditions. Crystals of cubic and tablet shapes are considered, and results are reported for four commonly used water models. The dissolution rates for different water models can differ considerably, but the overall dissolution mechanism remains the same. Urea dissolution occurs in three stages: a relatively fast initial stage, a slower intermediate stage, and a final stage. We show that the long intermediate stage is well described by classical rate laws, which assume that the dissolution rate is proportional to the active surface area. By carrying out simulations at different temperatures, we show that urea dissolution is an activated process, with an activation energy of ∼32 kJ mol-1. Our simulations give no indication of a significant diffusion layer, and we conclude that the detachment of molecules from the crystal is the rate-determining step for dissolution. The results we report for urea are consistent with earlier observations for the dissolution of NaCl crystals. This suggests that the three-stage mechanism and classical rate laws might apply to the dissolution of other ionic and molecular crystals.

Molecular dynamics simulations are used to investigate the dissolution of NaCl nanocrystals (containing ∼2400 ions) in water. We focus on systems under sink conditions at 300 K, but the influences of concentration and temperature are also investigated. Cubical, spherical, tablet-shaped, and rod-shaped nanocrystals are considered, and it is shown that the initial shape can influence the dissolution process. Dissolution is observed to occur in three stages: an initial period where the most exposed ions are removed from the crystal surface, and the crystal takes on a solution-annealed shape which persists throughout the second stage of dissolution; a second long intermediate stage where dissolution roughly follows a fixed rate law; and a final stage where the small residual crystal (≲200 ions) dissolves at an ever increasing rate until it disappears. The second stage of dissolution which applies for most of the dissolution process is well described by classical rate equations which simply assume that the dissolution rate is proportional to an active surface area from which ions are most easily detached from the crystal. The active area depends on the initial crystal shape. We show that for our model NaCl nanocrystals the rate-determining step for dissolution under sink conditions is ion detachment from the crystal, and that diffusion layers do not exist for these systems.

In this work, a general theory to account for any kind of polarization arising from polar as well as ions induced interactions in fluid mixtures is proposed. The general treatment is based on the self-consistent mean field theory (SCMF) that was originally proposed and applied for pure components using integral equation theories and molecular simulation studies. The extended SCMF is consistent with theory-based equations of state applied to hard chain mixtures. The theory is extended to mixtures and compared to molecular simulation data. The comparison to molecular simulation data shows good to excellent results for phase co-existence properties, energy and effective dipole moment. The validity of the theory is demonstrated by studying VLE of highly non-ideal mixtures of aldehyde and ketones using statistical association fluid theory.

Heterogeneous ice nucleation at solid surfaces is important in many physical systems including the Earth's atmosphere. AgI is one of the best ice nucleating agents known; however, why AgI is such an effective ice nucleus is unclear. Using molecular dynamics simulations, we show that a good lattice match between ice and a AgI surface is insufficient to predict the ice nucleation ability of the surface. Seven faces modeled to represent surfaces of both β-AgI and γ-AgI, each having a good lattice match with hexagonal and/or cubic ice, are considered, but ice nucleation is observed for only three. Our model simulations clearly show that the detailed atomistic structure of the surface is of crucial importance for ice nucleation. For example, when AgI is cleaved along certain crystal planes two faces result, one with silver ions and the other with iodide ions exposed as the outermost layer. Both faces have identical lattice matches with ice, but in our simulations ice nucleation occurred only at silver exposed surfaces. Moreover, although hexagonal ice is often the only polymorph of ice considered in discussions of heterogeneous ice nucleation, cubic ice was frequently observed in our simulations. We demonstrate that one possible mechanism of ice nucleation by AgI consists of particular AgI surfaces imposing a structure in the adjacent water layer that closely resembles a layer that exists in bulk ice (hexagonal or cubic). Ice nucleates at these surfaces and grows almost layer-by-layer into the bulk.

Molecular dynamics simulations are used to investigate the behavior of aqueous tert-butanol (TBA) solutions for a range of temperatures, using the CHARMM generalized force field (CGenFF) to model TBA and the TIP4P/2005 or TIP4P-Ew water model. Simulation results for the density, isothermal compressibility, constant pressure heat capacity, and self-diffusion coefficients are in good accord with experimental measurements. Agreement with the experiment is particularly good at low TBA concentration, where experiments have revealed anomalies in a number of thermodynamic properties. Importantly, the CGenFF model does not exhibit liquid-liquid demixing at temperatures between 290 and 320 K (for systems of 32 000 molecules), in contrast with the situation for several other common TBA models [R. Gupta and G. N. Patey, J. Chem. Phys. 137, 034509 (2012)]. However, whereas real water and TBA are miscible at all temperatures where the liquid is stable, we observe some evidence of demixing at 340 K and above. To evaluate the structural properties at low concentrations, we compare with both neutron scattering and recent spectroscopic measurements. This reveals that while the CGenFF model is a definite improvement over other models that have been considered, the TBA molecules still exhibit a tendency to associate at low concentrations that is somewhat stronger than that indicated by experiments. Finally, we discuss the range and decay times of the long-range correlations, providing an indication of the system size and simulation times that are necessary in order to obtain reliable results for certain properties.

Molecular dynamics simulations are employed to investigate the structure of supercooled water (230 K) in contact with the (001), (010), and (100) surfaces of potassium feldspar (K-feldspar) in the microcline phase. Experimentally, K-feldspar and other feldspar minerals are known to be good ice-nucleating agents, which play a significant role in atmospheric science. Therefore, a principal purpose of this work is to evaluate the possibility that the K-feldspar surfaces considered could serve as likely sites for ice nucleation. The (001) and (010) surfaces were selected for study because they are perfect cleavage planes of feldspar, with (001) also being an easy cleavage plane. The (100) surface is considered because some experiments have suggested that it is involved in ice nucleation. Feldspar is modeled with the widely used CLAYFF force field, and the TIP4P/Ice model is employed for water. We do not observe ice nucleation on any of the K-feldspar surfaces considered; moreover, the density profiles and the structure of water near these surfaces do not exhibit any particularly icelike features. Our simulations indicate that these surfaces of K-feldspar are likely not responsible for its excellent ice nucleating ability. This suggests that one must look elsewhere, possibly at water-induced surface rearrangements or some other "defect" structure, for an explanation of ice nucleation by K-feldspar.

Molecular dynamics simulations are used to investigate the mechanism of crystal nucleation in NaCl solution. Our observations support a two-step mechanism, similar to that recently proposed for other systems [P.G. Vekilov, Nanoscale 2 (2010) 2346]. The two sequential steps consist of a concentration fluctuation resulting in a dense ion “patch” followed by spatial ordering. The second, spatial-ordering step is rate determining, consistent with some protein solutions. The “size” of the critical NaCl nucleus is ∼ 1 nm, including ∼ 75 ions at ambient temperatures. Given that evidence for the two-step mechanism is found for solutes as different as NaCl and proteins, it appears to be an important mechanism of homogeneous crystal nucleation.

Molecular-level Monte Carlo simulations are performed to validate equation of state approaches for the description of the N2/CH4/C2H6 vapour–liquid equilibria under conditions relevant to Titan’s lower atmosphere. The Monte Carlo simulations confirm the validity of the equation of state approaches, so that both provide a reliable description of the unknown composition of cloud droplets in this region of Titan’s atmosphere. Furthermore, the models are compared with experimental data from laboratory studies of aerosol droplets that contain N2, CH4 and C2H6. Good agreement is also found here.

Aggregation and microheterogeneity in dilute aqueous solutions of 2-butoxyethanol (BE) are investigated employing molecular dynamic simulations. Solutions ranging in concentration from infinite dilution to the BE mole fraction, X BE = 0.06, are considered, over the temperature range 300–338.13 K. This range includes temperatures both below and above the lower critical solution temperature (LCST) of BE-water solutions. At all temperatures considered, BE association begins at a very low concentration and proceeds very rapidly up to X BE ≈ 0.02, then continues more slowly with the BE aggregates increasing in size with increasing concentration. Physically, the aggregates are loosely formed structures with significant amounts of water remaining in the BE-rich regions. The aggregation appears mainly driven by association of the hydrophobic tails of BE. As is characteristic of entropy driven hydrophobic association, BE aggregation occurs more rapidly and the aggregates increase in size with increasing temperature. Both the aggregate size and their growth with increasing temperature are in qualitative with size estimates from small angle neutron scattering (SANS) experiments. At temperatures above the experimental LCST, we observe demixing at a value of X BE that lies within the two phase region. At higher temperatures and concentrations, the aggregate size approaches the length of the simulation cell, even when 64,000 molecules (water plus BE) are included in the sample. Finite-size effects come into play when the length scales associated with aggregation and microheterogeneity become comparable with the length of the simulation cell, and this important simulation issue is discussed.

We investigate pressure driven flow rates of water through a (6,6) carbon nanotube (CNT) for the TIP3P, SPC/E, and TIP4P/2005 water models. The flow rates are shown to be strongly model dependent, differing by factors that range from ∼6 to ∼2 as the temperature varies from 260 to 320 K, with TIP3P showing the fastest flow and TIP4P/2005 the slowest. For the (6,6) CNT, the size constraint allows only single-file conduction for all three water models. Hence, unlike the situation for the larger [(8,8) and (9,9)] CNTs considered in our earlier work [L. Liu and G. N. Patey, J. Chem. Phys. 141, 18C518 (2014)], the different flow rates cannot be attributed to different model-dependent water structures within the nanotubes. By carefully examining activation energies, we trace the origin of the model discrepancies for the (6,6) CNT to differing rates of entry into the nanotube, and these in turn are related to differing bulk mobilities of the water models. Over the temperature range considered, the self-diffusion coefficients of the TIP3P model are much larger than those of TIP4P/2005 and those of real water. Additionally, we show that the entry rates are approximately inversely proportional to the shear viscosity of the bulk liquid, in agreement with the prediction of continuum hydrodynamics. For purposes of comparison, we also consider the larger (9,9) CNT. In the (9,9) case, the flow rates for the TIP3P model still appear to be mainly controlled by the entry rates. However, for the SPC/E and TIP4P/2005 models, entry is no longer the rate determining step for flow. For these models, the activation energies controlling flow are considerably larger than the energetic barriers to entry, due in all likelihood to the ring-like water clusters that form within the larger nanotube.

Silver iodide is one of the most effective ice nuclei known. We use molecular dynamics simulations to investigate ice nucleation by AgI disks and plates with radii ranging from 1.15 to 2.99 nm. It is shown that disks and plates in this size range are effective ice nuclei, nucleating bulk ice at temperatures as warm as 14 K below the equilibrium freezing temperature, on simulation time scales (up to a few hundred nanoseconds). Ice nucleated on the Ag exposed surface of AgI disks and plates. Shortly after supercooling an ice cluster forms on the AgI surface. The AgI-stabilized ice cluster fluctuates in size as time progresses, but, once formed, it is constantly present. Eventually, depending on the disk or plate size and the degree of supercooling, a cluster fluctuation achieves critical size, and ice nucleates and rapidly grows to fill the simulation cell. Larger AgI disks and plates support larger ice clusters and hence can nucleate ice at warmer temperatures. This work may be useful for understanding the mechanism of ice nucleation on nanoparticles and active sites of larger atmospheric particles.

Ternary solutions consisting of urea, tert-butyl alcohol (TBA), and water are investigated employing molecular dynamics simulations. The main purpose of the present paper is to investigate the effect of urea on TBA aggregation and by extension its influence on hydrophobic interactions. The aggregation of TBA can be detected from the concentration dependence of structural properties such as first-shell TBA-water coordination numbers and TBA-TBA hydrogen-bond numbers, as well as through changes in the translational diffusion coefficients of TBA. It is found that urea acts to delay the association of TBA to concentrations greater than those required to cause TBA aggregation in binary TBA-water systems. It is shown that urea acts through a direct mechanism, whereby it preferentially binds to TBA replacing water from the first coordination shell. TBA-urea hydrogen bonds can be as strong as, or stronger than, those of TBA-water, and urea binds to both the hydrophilic and hydrophobic moieties of TBA. Our observations are qualitatively consistent with experimental results for urea-TBA-water solutions and with recent simulation studies of urea's action as a protein denaturant.

Heterogeneous ice nucleation is an important phenomenon in the physical environment influencing atmospheric and biological processes. Despite this relevance, the mechanism of heterogeneous ice nucleation is not understood at the microscopic level, and what exactly constitutes a good ice nucleus is an open question. Employing molecular dynamics simulations, we demonstrate that an electric field, which acts very near a surface, can create an effective ice nucleus in models of supercooled liquid water. To serve as an ice nucleus, the field must polarize only a very thin water layer (∼10 A), and the field strength required is realistic on the relevant length scale. Our results support the idea that local electric fields could play a major role in heterogeneous ice nucleation, particularly for the very rough particles with many surface structure variations, that serve as ice nuclei in environmentally realistic situations.

Molecular level Monte Carlo simulations have been performed with various model potentials for the CH4–N2 vapor–liquid equilibrium at conditions prevalent in the atmosphere of Saturn’s moon Titan. With a single potential parameter adjustment to reproduce the vapor–liquid equilibrium at a higher temperature, Monte Carlo simulations are in excellent agreement with available laboratory measurements. The results demonstrate the ability of simple pair potential models to describe phase equilibria with the requisite accuracy for atmospheric modeling, while keeping the number of adjustable parameters at a minimum. This allows for stable extrapolation beyond the range of available laboratory measurements into the supercooled region of the phase diagram, so that Monte Carlo simulations can serve as a reference to validate phenomenological models commonly used in atmospheric modeling. This is most important when the relevant region of the phase diagram lies outside the range of laboratory measurements as in the case of Titan. The present Monte Carlo simulations confirm the validity of phenomenological thermodynamic equations of state specifically designed for application to Titan. The validity extends well into the supercooled region of the phase diagram. The possible range of saturation levels of Titan’s troposphere above altitudes of 7 km is found to be completely determined by the remaining uncertainty of the most recent revision of the Cassini-Huygens data, yielding a saturation of 100 ± 6% with respect to CH4–N2 condensation up to an altitude of about 20 km.

Grand canonical Monte Carlo simulations are employed to investigate the structure of water adsorbed on kaolinite surfaces with trenchlike defects. The results obtained for two water models (SPC/E and TIP5P-E) at 235 K are essentially the same. Calculation of water density profiles in all three dimensions shows that a dense ordered state is present in our trench systems. The narrowest trenches have the highest water density and display clearly layered structures along the width and depth of a trench. The water within a trench shows distinct proton order and is strongly ferroelectric. These ordered structures might be important in the initial stages of nucleation and growth of ice on kaolinite surfaces.

Recent laboratory studies of water adsorption on kaolinite at 296 K, and at relative humidity (RH) values relevant for the atmosphere, have reported coverages ranging up to tens of monolayers. In contrast, recent simulations have suggested that atomistically smooth kaolinite surfaces uptake only monolayers (some slightly overgrown) at similar RH values. In an effort to possibly explain the laboratory data, we have performed water adsorption calculations on kaolinite surfaces containing trenchlike structures using the grand canonical Monte Carlo simulation method at 298 K. The results obtained show that the granularity of the surfaces can play a major role in the adsorption of multiple layers of water. For all trenches considered, multilayers of water were observed over a large range of RH. The narrowest trench investigated remained filled with water even in the very low RH regime (or=0.0003%). Increasing the trench width resulted in partial or complete trench filling depending on the RH value, with large water mounds growing on the step edges. This strong affinity for water is explained by very attractive water-lattice interactions inside the trenches, especially near the walls. Our calculations suggest that water adsorption in trenches, and possibly in other similar defects, can offer an explanation of the experimental results.

We employ molecular dynamics simulations and the reference interaction site model (RISM) integral equation theory to study the solvation structure and solvation thermodynamics of the transfer process from water to a water-urea mixture. Simple positive and negative ions together with uncharged species of the same size are used as crude models for the hydrophilic and hydrophobic groups of a protein. We find that urea preferentially solvates positively charged species. The solvation free energies obtained indicate that larger solutes favor the transfer from water to a water-urea mixture. The decomposition of the transfer free energy into the energetic and entropic terms shows that the energetic part is much larger than the entropic one and tends to dominate the transfer process, supporting the direct mechanism of urea-denaturation. In addition, the effect of urea on the water liquid structure is discussed from the viewpoint of solvation entropy.

A simple functional form for a general equation of state based on an effective near-neighbor pair interaction of an extended Lennard-Jones (12,6,3) type is given and tested against experimental data for a wide variety of fluids and solids. Computer simulation results for ionic liquids are used for further evaluation. For fluids, there appears to be no upper density limitation on the equation of state. The lower density limit for isotherms near the critical temperature is the critical density. The equation of state gives a good description of all types of fluids, nonpolar (including long-chain hydrocarbons), polar, hydrogen-bonded, and metallic, at temperatures ranging from the triple point to the highest temperature for which there is experimental data. For solids, the equation of state is very accurate for all types considered, including covalent, molecular, metallic, and ionic systems. The experimental pvT data available for solids does not reveal any pressure or temperature limitations. An analysis of the importance and possible underlying physical significance of the terms in the equation of state is given.

Grand canonical Monte Carlo calculations are employed to investigate water adsorption on kaolinite at 298 and 235 K. Both basal planes (the Al and Si surfaces) as well as two edge-like surfaces are considered. The general force field CLAYFF is used together with the SPC/E and TIP5P-E models for water. Problems that occur in single slab simulations due to arbitrary truncation of the point charge lattice are identified, and a working remedy is discussed. The edges and the Al surface adsorb water at subsaturation in the atmospherically relevant pressure range. The Si surface remains dry up to saturation. Both edges have a very strong affinity for water and adsorb continuously up to monolayer coverage. The Al surface has a weaker affinity for water but forms a subsaturation monolayer. On the Al surface, the monolayer is formed in an essentially sharp transition, and strong hysteresis is observed upon desorption. This indicates collective behavior among the water molecules which is not present for the edges. Binding energies of singly adsorbed water molecules at 10 K were determined to understand the differences in water uptake by the four kaolinite surfaces. Binding energies (SPC/E) of -21.6, -46.4, -73.5, and -94.1 kJ/mol, were determined for the Si surface, Al surface, unprotonated edge, and protonated edge, respectively. The water monolayer on the Al surface, particularly at 235 K, exhibits hexagonal patterns. However, the associated lattice parameters are not compatible with ice Ih. Water density and hydrogen bonding in the monolayers at both 298 and 235 K were also determined to better understand the structure of the adsorbed water.

Antibody-mediated rejection (AMR) consensus criteria are defined in kidney and heart transplantation by histological changes, circulating donor-specific antibody (DSA), and C4d deposition in affected tissue. AMR consensus criteria are not yet identified in small bowel transplantation (SBTx). We investigated those three criteria in 12 children undergoing SBTx, including one retransplantation and four combined liver-SBTx (SBTx), with a follow-up of 12 days to 2 years. All biopsies (91) were evaluated with a standardized grading scheme for acute rejection (AR), vascular lesions and C4d expression. Sera were obtained at day 0 and during the follow-up. C4d was expressed in 37% of biopsies with or without AR, but in 50% of biopsies with severe vascular lesions. In addition, vascular lesions were always associated with AR and a poor outcome. All children with AR (grade 2 or 3) observed before the third month died or lost the graft. DSA were never found in any studied sera. We found no evidence that C4d deposition was of any clinical relevance to the outcome of SBTx. However, the grading of vascular lesions may constitute a useful marker to identify AR that is potentially resistant to standard treatment, and for which an alternative therapy should be considered.

Objective: Small bowel (SB) transplantation (Tx), long considered a rescue therapy for patients with intestinal failure, is now a well recognised alternative treatment strategy to parental nutrition (PN). In this retrospective study, we analysed graft functions in 31 children after SBTx with a follow-up of 2–18 years (median 7 years). Patients: Twelve children had isolated SBTx, 19 had combined liver–SBTx and 17 received an additional colon graft. Growth, nutritional markers, stool balance studies, endoscopy and graft histology were recorded every 2–3 years post-Tx. Results: All children were weaned from PN after Tx and 26 children remained PN-free. Enteral nutrition was required for 14/31 (45%) patients at 2 years post-Tx. All children had high dietary energy intakes. The degree of steatorrhoea was fairly constant, with fat and energy absorption rates of 84–89%. Growth parameters revealed at transplantation a mean height Z-score of –1.17. After Tx, two-thirds of children had normal growth, whereas in one-third, Z-scores remained lower than –2, concomitant to a delayed puberty. Adult height was normal in 5/6. Endoscopy and histology analyses were normal in asymptomatic patients. Chronic rejection occurred only in non-compliant patients. Five intestinal grafts were removed 2.5–8 years post-Tx for acute or chronic rejection. Conclusions: This series indicates that long-term intestinal autonomy for up to 18 years is possible in the majority of patients after SBTx. Subnormal energy absorption and moderate steatorrhoea were often compensated for by hyperphagia, allowing normal growth and attainment of adult height. Long-term compliance is an important pre-requisite for long-term graft function.

Alport syndrome is a collagen type IV disease caused by mutations in the COL4A5 gene with the X-linked form being most prevalent. The resultant alpha5(IV) collagen chain is a component of the glomerular and skin basement membranes (SBMs). Immunofluorescent determination of the alpha5(IV) chain in skin biopsies is the procedure of choice to identify patients. In 30% of patients, however, the mutant protein is still found in the SBM resulting in a normal staining pattern. In order to minimize or eliminate false results, we compared the distribution of the alpha2(IV) chain (another SBM component) and the alpha5(IV) chain by standard double label immunofluorescence (IF) and by confocal laser scanning microscopy. The study was performed on 55 skin biopsies of patients suspected of Alports and five normal control specimens. In normal skin, IF showed the classical linear pattern for both collagens along the basement membrane. Additionally, decreased alpha5(IV) was found in the bottom of the dermal papillary basement membrane. Confocal analysis confirmed the results and show alpha5(IV) focal interruptions. In suspected patients, both techniques showed the same rate of abnormal alpha5(IV) expression: segmental in women and absent in men. Our results show a physiological variation of alpha5(IV) location with focal interruptions and decreased expression in the bottom of the dermal basement membrane. Comparison of alpha5(IV) with alpha2(IV) expression is simple and eliminates technical artifacts.

The structural and energetic properties of solutions containing water, urea, and trimethylamine-N-oxide (TMAO) are examined using molecular dynamics simulations. Such systems are of interest mainly because TMAO acts to counter the protein denaturing effect of urea. Even at relatively high concentration, TMAO is found to fit well into the urea-water structure. The underlying solution structure is influenced by TMAO, but these perturbations tend to be modest. The TMAO-water and TMAO-urea interaction energies make an important contribution to the total energy in solutions where counter-denaturing effects are expected. TMAO-water and TMAO-urea hydrogen bonds have the largest hydrogen-bond energies in the system. Additionally, TMAO cannot hydrogen bond with itself, and hence it interacts strongly with water and urea. These observations suggest that the mechanism of TMAO counter denaturation is simply that water and urea prefer to solvate TMAO rather than the protein, hence inhibiting its unfolding.

Large-scale molecular dynamics simulations (64 000 particles) are used to examine the microscopic mechanism of crystal nucleation and growth in a slightly supersaturated solution of NaCl in water at 300 K and 1 atm. Early-stage nucleation is observed, and the growth of a single crystal is followed for ∼140 ns. It is shown that the nucleation and growth process is better described by Ostwald’s rule of stages than by classical nucleation theory. Crystal nucleation originates in a region where the local salt concentration exceeds that of the bulk solution. The early-stage nucleus is a loosely ordered arrangement of ions that retains a significant amount of water. The residual water is slowly removed as the crystal grows and evolves toward its stable anhydrous state.

In dilute aqueous solution, tert-butyl alcohol (TBA) tends to aggregate but trimethylamine-N-oxide (TMAO) does not. Given that both molecules have very similar geometry with hydrophobic and hydrophilic groups, it is interesting to ask why they behave so differently in aqueous solution. To explore this question, we use molecular dynamics simulations to study two models representing TBA and TMAO that differ essentially only in their electrostatic properties. It is shown that this difference is sufficient to give the different solution behavior. Furthermore, the principal difference identified is that the hydrophilic group of TMAO (the oxygen) interacts on average much more strongly with water than the corresponding group (the hydroxyl) of TBA. A hydrogen-bond analysis shows that water-TBA and water-TMAO hydrogen bonds are similar in number, but that the hydrogen-bond energy is much more negative for water-TMAO. In all likelihood, this accounts for the different behavior in dilute aqueous solution.