The role of temperature in nucleation processes

Heat and mass transfers are coupled processes, also in nucleation. In principle, a nucleating cluster would have a different temperature compared to the surrounding supersaturated old phase because of the heat release involved with attaching molecules to the cluster. In turn a difference in temperature across the cluster surface is a driving force for the mass transfer to and from the cluster. This coupling of forces in nonisothermal nucleation is described using mesoscopic nonequilibrium thermodynamics, emphasizing measurable heat effects. An expression was obtained for the nonisothermal nucleation rate in a one-component system, in the case where a temperature difference exists between a cluster distribution and the condensed phase. The temperature is chosen as a function of the cluster size only, while the temperature of the condensed phase is held constant by a bath. The generally accepted expression for isothermal stationary nucleation is contained as a limiting case of the nonisothermal stationary nucleation rate. The equations for heat and mass transport were solved for stationary nucleation with the given cluster distribution and with the temperature controlled at the boundaries. A factor was defined for these conditions, determined by the heat conductivity of the surrounding phase and the phase transition enthalpy, to predict the deviation between isothermal and nonisothermal nucleation. For the stationary state described, the factor appears to give small deviations, even for primary nucleation of droplets in vapor, making the nonisothermal rate smaller than the isothermal one. The set of equations may lead to larger and different thermal effects under different boundary conditions, however.     

The role of specific nucleation in polypropylene photodegradation

The paper focuses on the effects of polymorphism on photodegradation of isotactic polypropylene. The starting polymer was modified by a specific α-nucleating agent, 1,3;2,4-bis(3,4-dimethylbenzylidene)sorbitol, by a specific β-nucleating agent, N,N′-dicyclohexylnaphthalene-2,6-dicarboxamide, or their combination. Samples prepared by compression moulding were then exposed to UV-irradiation in the interval from 0 to 240 h. The differences in morphology were reflected in different photooxidative behaviour. Infrared spectroscopy showed that neat polypropylene was the most sensitive to photooxidation and the sample modified solely by the β-nucleating agent was the least sensitive. The remaining two samples exhibited an intermediate sensitivity. Differential scanning calorimetry revealed that the UV-exposure led to gradual changes in crystallization mechanism specifically asserting in individual materials. This behaviour was ascribed to homogeneous nucleation of partly degraded macromolecules. Possible changes of the nucleating agent itself during UV-exposure were also discussed.     

Nucleation inhibition in attrition-enhanced Pope-Peachey type of diastereomeric resolutions

The resolution of racemic 1-(3-methoxyphenyl)ethylamine with (S)-mandelic acid by diastereomeric salt formation can be made more economical by application of the Pope-Peachey method. This resolution was further improved upon by the addition of small amounts of a nucleation inhibitor, which prevents the formation of crystals of the more soluble diastereomer. The mandelic acid left in the solution is then used for further crystal growth of the less soluble diastereomer, furnishing high yields and diastereomeric excesses. Slow cooling and grinding of the crystals formed lead to increased secondary nucleation and thus to more consumption of the less soluble diastereomer and even better results.     

Resolution of 1-arylalkylamines with 3-O-hydrogen phthalate glucofuranose derivatives: role of steric bulk in a family of resolving agents

The development of three new acidic resolving agents which are hydrogen phthalates of 1,2:5,6-di-O-isopropylidene-α-d-glucofuranose 1, 1,2:5,6-di-O-cyclohexylidene-α-d-glucofuranose 2 and 1,2-O-cyclohexylidene-5,6-O-diphenylmethylidene-α-d-glucofuranose 3 is shown for the resolution of 1-arylalkylamines 7a–k. The salts between 1, 2 and (RS)-1-arylalkylamines 7a–k selectively crystallize 1·(S) 7a–j and 2·(S) 7a–h salts, allowing us to recover the corresponding bases (S) 7a–j and (S) 7a–h, respectively, in good yield and enantiomeric excess (73–95% ee). Whereas, the salts between 3 and (RS)-1-arylalkylamines 7a–c,g–i,k selectively crystallize 3·(S)-7a–c,g–i salts to recover the corresponding bases (S)-7a–c,g–i in poor enantiomeric excess (4–35% ee). The difference between the resolving ability of 1 and 2 for 1-arylalkylamines 7a–h is very slight, but there is considerable difference compared to ortho-substituted 1-arylalkylamines 7i and 7j. The role of substituents on a family of resolving agents 1, 2 and 3 is also discussed to interpret their resolving ability.     

The role of heterogeneous nucleation in the self-assembly of oligothiophenes

The cooperative self-assembly of oligothiophenes can be characterized by heterogeneous nucleation caused by trace amounts of impurities leading to a manifold of supramolecular arrangements.     

Study of the mechanism of optical resolutions via diastereoisomeric salt formation

Racemic malic acid (I) was resolved by R-α-phenylethylamine(II). The S-(?)-I.R-(+)-II diastereoisomer was in excess in the precipitated salt. DSC curves and X-ray powder diffractograms proved that the diastereoisomeric salt mixture precipitated during the resolution was isomorphous with the optically pure S-(?)-I.R-(+)-II salt. The diastereoisomeric salt mixture containing the R-(?)-I.R-(+)-II salt in abundance bound crystal solvate (water or methanol) when produced by the total evaporation of the mother liquor, while the optically pure R-(?)-I.R-(+)-II salt crystallized without solvate. It is generally assumed that solid solution formation takes place when the two diastereoisomers are alike and the high similarity results in less efficient enantiomer separation. This paper demonstrates that efficient resolution can be accomplished in case of solid solution formation too. The diastereoisomeric salt mixtures can crystallize in isomorphic form even when the physico-chemical properties of the optically pure diastereoisomeric salts are quite different.     

Evidence for the role of ions in aerosol nucleation

Aerosol nucleation has been studied experimentally in purified, atmospheric air, containing trace amounts of water vapor, ozone, and sulfur dioxide. The results are compared with model calculations. It is found that an increase in ionization by a factor of 10 increases the production rate of stable clusters by a factor of approximately 3, probably due to ion-induced nucleation.     

Merit of estimations from DSC measurements for the efficiency of optical resolutions

Chiral drugs are generally resolved through formation of diastereoisomeric salts of different solubility. When the diastereoisomer salts from a eutectic in the solid phase, a good estimate can be made for the efficiency of the resolution (S), through a calculation based on the eutectic composition (x _eu). The eutectic composition can be computed from thermal data of a differential scanning calorimetric (DSC) measurement of the diastereoisomeric salts [1]. In this work we investigated the influence of uncertainties of thermal data on the estimation of the efficiency of resolution (S), spreading throughout the calculations involved. Performance of our error-estimating method is demonstrated on 6 diastereoisomer salt pairs. The obtained deviations are close to the deviations of resolution experiments. Thus, it can be concluded that the DSC measurements are useful means in predicting the efficiency of resolutions. Deceased     

The relation between chemical structure of branched amide nucleating agents and nucleation effect in isotactic polypropylene

Journal of Thermal Analysis and Calorimetry - The relation between chemical structure of branched amide compounds as nucleating agents and the nucleation effect of them in isotactic polypropylene...     

A new method for designing optical resolutions and for determination of relative configurations

A new method has been elaborated for designing optical resolutions via diastereoisomeric salt formation, based upon the principle of three point interaction. In our method the favourite resolving agent to a given racemate and suitable resolution method can be found, and it can be predicted which enantiomer is expected in excess in the crystallized diastereoisomeric salt.     

The role of electrochemistry in the development of pi-basic dearomatization agents

Dihapto-coordination of aromatic molecules promotes numerous organic transformations that are not observed for the free aromatics. The development of osmium, rhenium, tungsten and molybdenum complexes that are capable of such binding is described in this perspective. The stability of these complexes strongly correlates to the metal d5/d6 reduction potential and electrochemical data has played a central role in their design.     

Exploring beta amyloid cleavage enzyme-1 inhibition and neuroprotective role of benzimidazole analogues as anti-alzheimer agents

Beta amyloid cleavage enzyme-1 (BACE1) is the key enzyme involved in Aβ peptide formation in Alzheimer's disease pathogenesis. We intend to target this enzyme by exploring benzimidazole analogues against BACE1 as potential anti-Alzheimer agents. Docking studies were performed to determine the hydrogen bond interactions between the designed molecules and the target protein's active site. Research indicates the relationship between oxidative stress and Aβ effect in precipitating neurodegeneration; hence, the series was also studied in vitro to ascertain its neuroprotective role by performing the lipid peroxidation assay. In silico absorption, distribution, metabolism, and excretion studies were undertaken to assess the drug-like suitability of the analogues. To judge the effect of the synthesized analogues on central nervous system (CNS), toxicity and memory model studies were conducted on mice. Thus, overall results showcase analogues 11 and 14 as the most promising ones with the dual role of BACE1 inhibition and neuroprotection, along with memory retention.     

A simulation test of the optical Kerr mechanism for laser-induced nucleation

Recent experiments have demonstrated that intense, nanosecond laser pulses can induce crystal nucleation from supersaturated solutions that are transparent at the incident wavelengths, a phenomenon termed nonphotochemical laser-induced nucleation (NPLIN). Previous work has proposed that this effect is due to the alignment of solute molecules in solution due to the electric field of the applied laser light, promoting crystalline order. We have used simulations of NPLIN to examine how an orientational bias in solution affects nucleation with Monte Carlo simulations of a Potts lattice gas model. We examine this effect within both a classical, one-step nucleation framework as well as in the context of two-step nucleation. Our results indicate that an orientational bias can reduce the free energy barrier to nucleation within the one-step picture as well as promote the crystallization of amorphous precritical nuclei (the rate-determining step in the two-step picture). However, these effects are only present with field strengths that are much greater than those used in experiments.     

Efficient resolution of a dinuclear triple helicate by asymmetric extraction/precipitation with TRISPHAT anions as resolving agents

Tetradentate 1,2-bis[4-(4'-methyl-2,2'-bipyridyl)]ethane ligand (3) and Fe(NH4)2(SO4)2.6H2O combine in a 3:2 ratio to form the racemic helicate [Fe2L3]4+ (4), as reported by Elliott et al. We now show that the enantiomeric purity of 4 can be efficiently measured by 1H NMR by the use of the TRISPHAT (1) salt as a chiral shift reagent. Large differences in chemical shifts (deltadeltadelta of up to 0.3 ppm, 20% [D6]DMSO in CD3CN) are observed between the enantiomers of 4 upon addition of [nBu4N][delta-1]. The resolution of 4 by asymmetric extraction was attempted: addition of an organic solution of [cinchonidinium][delta-1] salt (2 equiv) to an aqueous solution of helicate 4-(SO4)2 led, after vigorous stirring, to the extraction of the homochiral diastereomer [P-4][delta-1]4 into the organic layer along with the precipitation of the heterochiral diastereomer [M-4][delta-1]4 at the interface (diastereomeric ratio>49:1 for both processes). An enantioenriched fraction of [P-4][SO4]2 remained in the aqueous layer. To obtain only two fractions of resolved helicate and develop this procedure into an efficient resolution protocol, four equivalents of [cinchonidinium][delta-1] salt were used as the resolving agent. Chemically and diastereomerically pure [P-4][delta-1]4 and [M-4][delta-1]4 helicate salts were then obtained in excellent yields.