The increment of the temperature of maximum density of water by addition of small amounts of tert-butanol: Experimental data and microscopic description revisited.

The temperature of maximum density, TMD, of aqueous solutions of tert-butanol has been experimentally determined in the pressure range of 0-300 bars and up to 0.025 tert-butanol mole fraction. At atmospheric pressure, this quantity increases for low alcohol mole fractions, reaches a maximum at intermediate concentrations, and then quickly falls. The new experimental results are basically in agreement with previous data in the literature by Wada and Umeda [G. Wada and S. Umeda, Bull. Chem. Soc. Jpn. 35, 646 (1962)], except at very low mole fractions, where these authors reported a stronger density anomaly. Our measurements also confirm the known effect of pressure, p, on the variation in the temperature of maximum density with respect to that of pure water, ΔTMD: this quantity increases with p over the whole composition range. In addition, molecular dynamics simulations were performed between 0 and 2000 bars and from 238 to 328 K using a recently proposed model for the tert-butanol/water system. It has been found that our model reproduces qualitatively the experimental behavior of the ΔTMD, but for pressures above 1000 bars. A detailed structural analysis showed that the addition of tert-butanol promotes the low density water structure, and this promotion is somewhat hampered as the temperature increases at high pressure (ΔTMD > 0) and mostly independent of temperature at low pressures (ΔTMD < 0). Our analysis shows that the ultimate factor determining changes in the TMD is the temperature dependence of the low density water structure enhancement. We have also carried out a local structure analysis in which in addition to solid-like structures, low density liquid water ones have also been considered.

Clustered yellow papules in the posterior axilla of a middle-aged woman.

Naevus lipomatosus cutaneous superficialis (NLCS) of Hoffman-Zurhelle is a rare hamartomatous benign condition first described in 1921. Two clinical variants have been described: a classical form of multiple yellow papules that coalesce to form larger plaques with segmental distribution, and a solitary form also known as pedunculated lipofibroma. We present a case of early-stage NLCS with characteristic histopathological and dermoscopic features.

'Rainbow pattern': a dermoscopic sign of invasive melanoma.

BACKGROUND: The 'rainbow pattern' was initially described as a highly specific dermoscopic feature of Kaposi sarcoma. Since then, it has been reported in many benign and malignant cutaneous tumours, including a few malignant melanomas (MMs). AIM: To determine the frequency and presentation of this dermoscopic pattern in primary cutaneous MMs in comparison to other cutaneous tumours. METHODS: The presence of a rainbow pattern was evaluated in a sample of 1100 dermoscopic images of different melanocytic and nonmelanocytic cutaneous neoplasms. RESULTS: The rainbow pattern was observed in 23 of 245 (9.4%) MM and 44 of 855 (5.1%) non-MM neoplasms. MMs presenting this feature were generally thicker: 82.6% > 1 mm and 43.0% > 2 mm. Compared with non-MMs, rainbow pattern in MMs was more commonly focal (82.7% vs. 36.4% nonfocal, P = 0.001) and associated with > 2 dermoscopic structures associated with MM (100% vs. 9% with fewer, P = 0.001). CONCLUSION: The rainbow pattern is a dermoscopic sign that can occasionally be observed in invasive MMs. In MMs, this feature is usually associated with other dermoscopic criteria of MM and located in a focal and eccentric area, as opposed to a diffuse and isolated presentation in non-MM neoplasms.

Lentiginous melanoma (lentigo maligna and lentigo maligna melanoma) in Australia: clinicopathological characteristics, management and recurrence rates after 10-year follow-up at a tertiary centre.

BACKGROUND: Lentiginous melanoma or lentigo maligna is a slow-growing type of melanoma frequently arising in sun-damaged skin and often first diagnosed in the elderly. Few studies report long-term follow-up. OBJECTIVES: To define characteristics of lentiginous melanoma in situ (LM) and invasive lentiginous melanoma (LMM) in Australian patients managed at a tertiary centre and describe local recurrence or treatment failure rates after long-term follow-up. METHODS: Retrospective single-centre study of LM/LMM patients evaluated between January 2005 and March 2007. Medical and photographic records were reviewed. RESULTS: One hundred two patients were included, with a total of 117 lesions (70 LM and 47 LMM). Seventy-nine were new primary LM/LMM, and 38 were recurrences. Primary cases were mostly pigmented (71%), while 77% of recurrent cases were partially pigmented/light brown or amelanotic. The margins were clinically ill-defined in the majority of cases (64% of primary cases and 94% of recurrent cases). Dermoscopy of the primary LM/LMM showed either classic 'common' melanoma features (33%) or classic LM/LMM features (41%), while 95% of recurrent cases had no features for melanoma or LM/LMM. Primary cases that were initially excised (113, 97%) had mean histopathological clear margins of 4.9 mm (range 0.1-22 mm). The median follow-up time was 7.5 years (95% CI 5.2-10.0) with more than 10-year follow-up in 32% and 5-10 years in 24% of patients. There were 44 (38%) recurrences over the entire follow-up period. Half of the patients who recurred did so within the first 3.8 years after the first treatment. CONCLUSION: LM/LMM often recur late and are clinically subtle; therefore, careful monitoring and long-term follow-up are required.

Rosettes in actinic keratosis and squamous cell carcinoma: distribution, association to other dermoscopic signs and description of the rosette pattern.

BACKGROUND: Rosettes, a dermoscopic structure characterized by four white points arranged as a 4-leaf clover, supports the dermoscopic diagnosis of actinic keratosis (AK) or squamous cell carcinoma (SCC). OBJECTIVE: The association of rosettes with other dermoscopic structures in AK or SCC and their distribution has not been analysed yet. METHODS: We conducted a prospective study of patients with histologically proven AK or SCC who presented dermoscopic rosettes at initial evaluation. RESULTS: A total of 56 tumours were collected (94.6% AK and 5.4% SCC). Thirty-seven (66.1%) lesions were non-pigmented and 19 (33.9%) pigmented. The most common dermoscopic findings were erythema (53; 94.6%) and scale (42; 75%). White circles were present in 21 lesions (37.5%); pigmented pseudonetwork in 18 (32.1%) and multiple grey to brown dots and globules in 14 (25%). Rosettes were distributed focally in 9 (16.1%) and generalized in 47 (83.9%). The rosette pattern (rosettes as the main structure) was observed only in AK (19; 35.8%). LIMITATIONS: The analysis was not blinded. The distinction between focal distribution (up to 3 rosettes) or generalized could be considered arbitrary. CONCLUSION: The rosette pattern identified in AK may be a specific pattern for AK.

Temperature of maximum density and excess properties of short-chain alcohol aqueous solutions: A simplified model simulation study.

We perform an extensive computational study of binary mixtures of water and short-chain alcohols resorting to two-scale potential models to account for the singularities of hydrogen bonded liquids. Water molecules are represented by a well studied core softened potential which is known to qualitatively account for a large number of water's characteristic anomalies. Along the same lines, alcohol molecules are idealized by dimers in which the hydroxyl groups interact with each other and with water with a core softened potential as well. Interactions involving non-polar groups are all deemed purely repulsive. We find that the qualitative behavior of excess properties (excess volume, enthalpy, and constant pressure heat capacity) agrees with that found experimentally for alcohols such as t-butanol in water. Moreover, we observe that our simple solute under certain conditions acts as a "structure-maker," in the sense that the temperature of maximum density of the bulk water model increases as the solute is added, i.e., the anomalous behavior of the solvent is enhanced by the solute.

Temperature of maximum density and excess thermodynamics of aqueous mixtures of methanol.

In this work, we present a study of representative excess thermodynamic properties of aqueous mixtures of methanol over the complete concentration range, based on extensive computer simulation calculations. In addition to test various existing united atom model potentials, we have developed a new force-field which accurately reproduces the excess thermodynamics of this system. Moreover, we have paid particular attention to the behavior of the temperature of maximum density (TMD) in dilute methanol mixtures. The presence of a temperature of maximum density is one of the essential anomalies exhibited by water. This anomalous behavior is modified in a non-monotonous fashion by the presence of fully miscible solutes that partly disrupt the hydrogen bond network of water, such as methanol (and other short chain alcohols). In order to obtain a better insight into the phenomenology of the changes in the TMD of water induced by small amounts of methanol, we have performed a new series of experimental measurements and computer simulations using various force fields. We observe that none of the force-fields tested capture the non-monotonous concentration dependence of the TMD for highly diluted methanol solutions.

Demixing and confinement of non-additive hard-sphere mixtures in slit pores.

Using Monte Carlo simulation, we study the influence of geometric confinement on demixing for a series of symmetric non-additive hard spheres mixtures confined in slit pores. We consider both a wide range of positive non-additivities and a series of pore widths, ranging from the pure two dimensional limit to a large pore width where results are close to the bulk three dimensional case. Critical parameters are extracted by means of finite size analysis. As a general trend, we find that for this particular case in which demixing is induced by volume effects, the critical demixing densities (and pressures) increase due to confinement between neutral walls, following the expected behavior for phase equilibria of systems confined by pure repulsive walls: i.e., confinement generally enhances miscibility. However, a non-monotonous dependence of the critical pressure and density with pore size is found for small non-additivities. In this latter case, it turns out that an otherwise stable bulk mixture can be unexpectedly forced to demix by simple geometric confinement when the pore width decreases down to approximately one and a half molecular diameters.

Reverse Monte Carlo modeling in confined systems.

An extension of the well established Reverse Monte Carlo (RMC) method for modeling systems under close confinement has been developed. The method overcomes limitations induced by close confinement in systems such as fluids adsorbed in microporous materials. As a test of the method, we investigate a model system of (36)Ar adsorbed into two zeolites with significantly different pore sizes: Silicalite-I (a pure silica form of ZSM-5 zeolite, characterized by relatively narrow channels forming a 3D network) at partial and full loadings and siliceous Faujasite (which exhibits relatively wide channels and large cavities). The model systems are simulated using grand canonical Monte Carlo and, in each case, its structure factor is used as input for the proposed method, which shows a rapid convergence and yields an adsorbate microscopic structure in good agreement with that of the model system, even to the level of three body correlations, when these are induced by the confining media. The application to experimental systems is straightforward incorporating factors such as the experimental resolution and appropriate q-sampling, along the lines of previous experiences of RMC modeling of powder diffraction data including Bragg and diffuse scattering.

Adsorption of probe molecules in pillared interlayered clays: experiment and computer simulation.

In this paper we investigate the adsorption of various probe molecules in order to characterize the porous structure of a series of pillared interlayered clays (PILC). To that aim, volumetric and microcalorimetric adsorption experiments were performed on various Zr PILC samples using nitrogen, toluene, and mesitylene as probe molecules. For one of the samples, neutron scattering experiments were also performed using toluene as adsorbate. Various structural models are proposed and tested by means of a comprehensive computer simulation study, using both geometric and percolation analysis in combination with Grand Canonical Monte Carlo simulations in order to model the volumetric and microcalorimetric isotherms. On the basis of this analysis, we propose a series of structural models that aim at accounting for the adsorption experimental behavior, and make possible a microscopic interpretation of the role played by the different interactions and steric effects in the adsorption processes in these rather complex disordered microporous systems.

A computational study of electrolyte adsorption in a simple model for intercalated clays.

A pillared interlayered clay is represented by a two-dimensional quenched charged disordered medium, in which the pillar configuration is produced by the quench of a two-dimensional electrolyte and the subsequent removal of the anions (that act as a template). The cation charge is counterbalanced by a neutralizing background that is an ideal representation of the layer's negative charge in the experimental system. In this paper we investigate the adsorption of electrolyte particles in this charged disordered medium resorting both to the use of the replica Ornstein-Zernike equation in the hypernetted chain approximation and grand canonical Monte Carlo simulations. The theoretical approach qualitatively reproduces the simulated behavior of the adsorbed fluids. Theoretical estimates of the material porosities obtained for various types of pillar distributions are in good agreement with the simulation. We investigate the influence of the matrix on correlation functions and adsorption isotherms.

Topological considerations on microporous adsorption processes in simple models for pillared interlayered clays.

The microporous structure of pillared interlayered clays is determined by their interlayer separation and the distribution of the pillars that separate their layers. The pillars provide stability to these quasi-two-dimensional high surface area materials. In this work we present a topological analysis of available and accessible volumes within various simple models of pillared interlayered clays. Each model is characterized by a distribution of pillars. Both fully ordered structures and disordered pillar distributions with either attractive or repulsive interpillar correlations are considered. Particular attention is paid to the problem of accessibility. In systems with similar degrees of porosity, even when cavities within each model might be able to host the same adsorbate molecules, their accessibility will strongly depend on the pillar distribution. The theoretical analysis presented in this work may facilitate the interpretation of experimental results, pointing out those quantities that are key to describe the texture of the porous material.

Phase behavior of a simple lattice model with a two-scale repulsive interaction.

The properties of a simple one-dimensional lattice model with two repulsive ranges are studied in terms of its analytic solution. Its phase behavior is characterized by the presence of a disorder-order-disorder transition (or a fluid-solid-fluid transition in lattice gas language). A similar situation was discussed by Hemmer and Stell [Phys. Rev. Lett. 24, 1284 (1970)] when considering the purely repulsive version of their ramp potential. The melting of the solid phase, when pressure is increased along an isotherm, is a feature common to both models and one of the characteristic features of water.