Solute-solvent interactions crystallization from

In contrast to solid state crystallization, crystallization from vapor, solution, and melt phases, which correspond to ambient phases having random structures, may be further classified into condensed and dilute phases. Vapor and solution phases are dilute phases, in which the condensation process of mass transfer plays an essential role in crystal growth. In the condensed melt phase, however, heat transfer plays the essential role. In addition to heat and mass transfer, an additional factor, solute-solvent interaction, should be taken into account. 

The basic criteria to consider in crystal growth from vapor, solution, and melt phases are therefore whether the phase is condensed or dilute, and whether the phase involves a solute-solvent interaction or not. 

Difference in the environmental phases. Since the interface roughness will be different for the same crystal species depending on whether the crystal was grown from the melt, solution, or vapor phases, different growth forms are expected for different environmental phases. This implies that the Tracht of the same crystal species will depend on the structure of the environmental phases, the degree of condensation, and the solute-solvent interaction. 

Even if crystals grow from the same aqueous solution, there are differences in Habitus. NaC103 crystals, for example, grow easily as polyhedral crystals, whereas NH Cl crystals always grow as dendrites, and NaCl crystals appear as hopper crystals. If Pb or Mn ions are added, cubic crystals of NaCl bounded by flat 100 faces may be obtained quite easily, but if NaCl is grown in pure solution all crystals take a hopper form, unless great care is taken to keep the supersaturation very low. These differences occur because the solute-solvent interaction energies, and, as a result, the values of Ap,/kT and A/x/kT, are different for different crystals. 

The reasons why we have Habitus variation for vapor and solution phases, and for different solvents when the crystals grow from the solution phase, can therefore be understood in terms of the factor of steps because of the different solute-solvent interaction energies. 

Williams and Amidon investigated a method that introduces estimates of solvent-solvent, and solute-solvent interactions into the basic log-linear expression. In their approach, cosolvent-water interactions are estimated from vapor pressure data of the solvent mixtures. The data are obtained from literature sources, if available, or determined experimentally. Solvent-solute interactions are estimated from experimental solubility data. An alternate approach, as described by Khossravi and Connors, divides the free energy of solubility into crystal, cavity, and solvation components. While the free energy associated with... 

CDCI3, CCI4, and CS2 only a small amount of free OH groups related to the cc conformer could be detected. However, the spectra in ( 03)280 indicated, that in more polar solvents there is a competitive solute-solvent interaction, which leads to a switch from the cb to the cc form. X-ray data of 3,7-dimethyl-3,7-diazabicyclo[3.3.1]nonan-9-ol 2H20 EtOH have shown that both nitrogen atoms take part in hydrogen bonds, either with water or ethanol, and the molecule adopts a cc conformation. With IR spectroscopy of a water-free crystal, the cb conformation was assigned to the same molecule, and this is stabilized by an intramolecular N H—O bond (136). 

In this analysis of the nmr data we have that the results for a variety of solvents and over a wide temperature range may be interpreted as arising from small changes in the crystal field environment of the iron atom due to two solvent interactions - a term which is an intrinsic property of the solvent and a second term arising from a solute-solvent interaction. Although the application of the model has been simplified the results nevertheless give an insight into the effect of the solvent on the nmr shifts of these iron dithiocarbamate complexes. 

X-Ray diffraction from single crystals is the most direct and powerful experimental tool available to determine molecular structures and intermolecular interactions at atomic resolution. Monochromatic CuKa radiation of wavelength (X) 1.5418 A is commonly used to collect the X-ray intensities diffracted by the electrons in the crystal. The structure amplitudes, whose squares are the intensities of the reflections, coupled with their appropriate phases, are the basic ingredients to locate atomic positions. Because phases cannot be experimentally recorded, the phase problem has to be resolved by one of the well-known techniques the heavy-atom method, the direct method, anomalous dispersion, and isomorphous replacement.1 Once approximate phases of some strong reflections are obtained, the electron-density maps computed by Fourier summation, which requires both amplitudes and phases, lead to a partial solution of the crystal structure. Phases based on this initial structure can be used to include previously omitted reflections so that in a couple of trials, the entire structure is traced at a high resolution. Difference Fourier maps at this stage are helpful to locate ions and solvent molecules. Subsequent refinement of the crystal structure by well-known least-squares methods ensures reliable atomic coordinates and thermal parameters. 

Real polymer processes involved in polymer crystallization are those at the crystal-melt or crystal-solution interfaces and inevitably 3D in nature. Before attacking our final target, the simulation of polymer crystallization from the melt, we studied crystallization of a single chain in a vacuum adsorption and folding at the growth front. The polymer molecule we considered was the same as described above a completely flexible chain composed of 500 or 1000 CH2 beads. We consider crystallization in a vacuum or in an extremely poor solvent condition. Here we took the detailed interaction between the chain molecule and the substrate atoms through Eqs. 8-10. 

Most solvents have unshared pairs of electrons, and they are polar. Therefore, they have the ability to attach to metal ions or interact with anions. As a result, when many solids crystallize from solutions, they have included a definite number of solvent molecules. When this occurs in water, we say that the crystal is a hydrate. An example of this is the well-known copper sulfate pentahydrate,... 

The more sterically demanding phosphide KPH(mes ) crystallizes solvent-free, even from solutions containing THF, as the ladder polymer [K PH(mes ) ]x (25) (72). Coordination about each K atom is completed by multihapto interactions (approximately if ) between the cation and the mesityl ring of an adjacent phosphide ligand. A... 

This leaves option 3b to be scrutinised closely. When the present writer did this, he realised that his puzzlement had arisen because he like others, had fallen into the trap of which he had frequently warned his students and which he has emphasised in his writings it is a serious error to attempt to understand electrochemical phenomena by thinking of ions in isolation, because this puts them putatively into a vacuum. But the ions of concern to us do not exist in a vacuum. Ions would not leave their positions of low energy in a crystal lattice to go into solution or be formed from neutral molecules by the transfer of a charged fragment from one molecule to another if those processes were not made exo-energetic by the interaction of the ions with polar or polarisable species in their environment, most commonly the solvent. For that reason, one should always think, and indeed talk, about... 

Recent experimental and theoretical studies on crystal growth, especially in the presence of tailor-made inhibitors, provide a link between macroscopic and microscopic chirality. We shall discuss these principles in some detail for chiral molecules. Furthermore, we shall examine whether it is indeed feasible today to establish the absolute configuration of a chiral crystal from an analysis of solvent-surface interactions. Since these analyses are based on understanding the interactions between a growing crystal and inhibitors present in solution, we shall first illustrate the general mechanism of this effect in various chiral and nonchiral systems. 

In examining a crystalline structure as revealed by diffraction experiments it is all too easy to view the crystal as a static entity and focus on what may be broadly termed attractive intermolecular interactions (dipole-dipole, hydrogen bonds, van der Waals etc., as detailed in Section 1.8) and neglect the actual mechanism by which a crystal is formed, i.e. the mechanism by which these interactions act to assemble the crystal from a non-equilibrium state in a super-saturated solution. However, it is very often nucleation phenomena that are ultimately responsible for the observed crystal structure and hence we were careful to draw a distinction between solution self-assembly and crystallisation at the beginning of this chapter. For example paracetamol, when crystallised from acetone solution gives the stable monoclinic crystal form I, but crystallisation from a molten sample in the absence of solvent... 

---