Formation of calcium crystallites in dry natural rubber particles

In this paper, the effects of drying and aging of natural rubber particles of Hevea brasiliensis are studied. The evolutions of the particle morphology and of the elemental distribution are investigated using electron-energy-loss spectroscopy imaging in a low-energy transmission electron microscope (ESI-TEM). It is found that when the sample is aged, calcium salt crystallites are formed around the particles. Fusion of these crystallites with time to form larger crystals shows evidence of ion mobility in the dry rubber matrix. Electron diffraction patterns and elemental mapping analysis indicate the crystals to be calcium sulfate. These crystallites are closely associated with membrane materials of the rubber particle and are surprisingly compatible with the hydrocarbon matrix of the rubber particle. It is proposed that polar sites on the membrane materials provide nucleation sites for the crystallization of calcium and sulfate ions from the latex serum.     

Adhesive transitions in Newton black films: a computer simulation study

We report molecular dynamics simulations of Newton black films (NBFs), ultra thin films of aqueous solutions stabilized with two monolayers of ionic surfactants, sodium dodecyl sulfate. We show that at low water content conditions and areas per surfactant corresponding to experimental estimates in NBFs, homogeneous films undergo an adhesion "transition," which results in a very thin adhesive film coexisting with a thicker film. We identify the adhesive film with the equilibrium structure of the Newton black film. We provide here a direct microscopic view of the formation of these important structures, which have been observed in experimental studies of emulsions and foams. We also report a detailed investigation of the structural properties and interfacial fluctuation spectrum of the adhesive film. Our analysis relies on the definition of an "intrinsic surface," which is used to remove the averaging effect that the capillary waves have on the film properties.     

Cyclic vinyl p-tolyl sulfilimines as chiral dienophiles: theoretical study on the stereoselectivity, Lewis acid catalysis, and solvent effects in their Diels-Alder reactions

The theoretical study reported in the present work deals with chiral cyclic vinyl sulfilimines and their reactivity as dienophiles in [4 + 2] cycloaddition reactions, using B3LYP/6-31G(d)//AM1 and B3LYP/6-31G(d)//B3LYP/6-31G(d) model chemistries. Consideration of Lewis acid catalysis, illustrated by BF(3), decreases the activation energies of the cycloaddition process while the charge transfer from the diene to the sulfilimine is augmented. The [4 + 2] cycloaddition reactions of sulfilimines with both furan and cyclopentadiene occur in the gas phase with endo stereoselectivity, which is more pronounced with the latter diene. Endo-exo energy differences in the gas phase with the B3LYP/6-31+G(d)//B3LYP/6-31+G(d), B3LYP/6-31G(d)//B3LYP/6-31G(d), and B3LYP/6-31G(d)//AM1 model chemistries are almost the same. Solvent effects are responsible for the inversion of the stereoselectivity in the reactions of sulfilimines with furan because of the great difference in the dipole moments in endo and exo approaches.     

New Ecotoxicologically and Biogenetically Relevant Terpenes of the tropical green seaweed Caulerpa taxifolia which is invading the Mediterranean

The tropical green seaweed Caulerpa taxifolia (VAHL ) C. AGARDH (Caulerpales) which is invading the Mediterranean is shown to contain trace amounts of two further novel terpenes, 7,7-C-didehydro-6-hydroxy-6,7-dihydrocaulerpenyne (= (4S, 6S,1E)-3-[(Z)-acetoxymethylidene]-6-hydroxy-11-methyl-7-methylidenedodeca-1,10-dien-8-yne-1,4-diyl diacetate; 3a ) and taxifolione (= 6-methylhept-5-en-3-yn-2-one; 4 ). The former is the most active of the toxins so far isolated from this seaweed, both as an in vitro inhibitor of the growth of marine bacteria and as a cytotoxic agent toward marine ciliate protists. This suggests a central ecotoxicological role for triacetate 3a as an adjuvant factor in the invasion of the Mediterranean by this seaweed. Moreover, the almost equally toxic 10,11 -epoxycaulerpenyne ( 2 ) which is scarcely available from Nature for bioassays can now be obtained by peroxy-acid epoxidation of caulerpenyne ( 1 ), along with the 6,7-epoxycaulerpenynes 6b and 6a . The latter are very labile, 6a giving triacetate 3a , suggesting epoxides to be late biogenetic intermediates in C. taxifolia.     

Workshop on coherent synchrotron radiation in storage rings

The annual UK SR User meeting at Diamond Light Source welcomed 250 members of the international synchrotron community to the site in South Oxfordshire, UK. This is the first time that Diamond has hosted this event as an operational facility.     

The use of rheology in the study of the composition effects on the fresh behaviour of hydraulic lime grouts for injection of masonry walls

The injection of grouts inside multi-leaf stone masonries is a technique widely used for structural consolidation. To ensure an adequate flow of the grout inside the masonry, it is crucial to assure good fresh grout properties, such as good rheological behaviour. The scope of this paper is to provide preliminary indications and valuable data about the effects of specific hydraulic lime grout composition on their rheological behaviour with the purpose of a successful injection process. Through the use of rotational rheometer together with the Taguchi method, it was possible to study the influence of water/binder ratio, the type and dosage of superplasticizer and the partial replacement of hydraulic lime by silica fume, upon the grout rheological properties. The study leads to the conclusion that polycarboxylate-based superplasticizers present better performance from a rheological point of view than the naphthalene-based superplasticizer and that the water/binder ratio and superplasticizer dosage are the most determinant factors in the fresh grout rheological behaviour. On the other hand, silica fume dosage turned out to be the factor with the least contribution to improve the grout rheological behaviour compared to the other two factors reported in this study. The results summarised in this paper are part of a larger study and precede the analysis of the performance of those grouts when injected into different porous media that simulate old masonries.     

Alpo4 and Alpo4-Al2O3 as New Heterogeneous Catalysts for the Solvent-Free Tetrahydropyranylation of Alcohols and Phenols

OH-protection of alcohols and phenols by heating the hydroxylated compound with 3,4-dihydro-2H-pyrane in the presence of an AlPO₄ or AlPO₄-Al₂O₃ heterogeneous acid catalyst is described. Isolated yields, not optimized, were in the range 50–99%.     

Studies on the Stereoselectivity of the Preparation of α,β-UNSATURATED ESTERS FROM α-Lithio-α-Silyl Esters

The reaction o f the lithium enolate o f a-trimethylsilyl esters with aldehydes and ketones has been shown to be an excellent entry into a, B-unsaturated esters as a result of the facile elimination o f the B-oxidosilane intermediate produced in the initial step of the reaction. (eq. 1) More recently it was shown that the bromomagnesium enolate o f trimethylsilyl acetates a1 lowed for the highly stereoselective preparation of (E) a, B-unsaturated esters via the isolable B-hydroxy-a-silyl ester, which was treated with BF₃0Et₂ to effect a trans elimination of the elements of trimethylsilanol⁵ (eq. 2) If this elimination were carried out with base, however, the stereoselectivity is much lower.     

EFFECTS OF SURFACTANTS ON THE TERMINAL VELOCITY OF PHENOL SOLUTION DROPS FALLING IN n-BUTYL ACETATE

ABSTRACT

Measurements of formation times and terminal velocities of aqueous phenol solution drops falling through a continuous phase of n-butyl acetate at 20°C are presented. The influence of two commercial surfactants, Medialan KF and Triton X-100 on the indicated parameters is also reported. A comparison is made between experimental results and predicted values from correlations available for terminal velocities and critical diameters, in the absence of surfactants.