Particle size-induced phase transitions

The effect of ionic strength on the adsorption of protein onto poly[NIPAM] is more complex than was expected. In fact, salinity affects not only electrostatic interactions but also the colloidal properties of such thermally sensitive particles (1) the increase in ionic strength leads to a reduction in particle size induced by lowering the volume phase transition temperature (i.e., the LCST of linear thermally sensitive polymer decreases as the salinity of the medium increases) and (2) salinity affects the degree of attractive and repulsive electrostatic interactions. As a result, the adsorption of proteins onto thermally sensitive microgel particles is generally and dramatically reduced as salinity increases, irrespective of temperature (as illustrated for P24 [Figure 9.26] adsorption onto poly(NIPAM) particles). 

Experimental problems with TGA are usually connected with sample preparation for instance, homogeneous or very disperse particle sizes may yield different results, while the presence of humidity adsorbed on the surface of the particles may mask or alter the response. Deliquescent or highly hygroscopic samples yield poorly reproducible results because it can be difficult to discriminate between removal of wetting solvent and removal of structural solvent. It is useful to accompany DSC experiments with TGA experiments. Heat absorption in a DSC plot may correspond to solvent loss and not to a phase transition (see above). Importantly, as shown below, a desolvation process may sometimes induce the formation of another polymorph or pseudo-polymorph not otherwise attainable. 

Furthermore, it has recently been found that the discrete nature of a molecule population leads to qualitatively different behavior than in the continuum case in a simple autocatalytic reaction network [29]. In a simple autocatalytic reaction system with a small number of molecules, a novel steady state is found when the number of molecules is small, which is not described by a continuum rate equation of chemical concentrations. This novel state is first found by stochastic particle simulations. The mechanism is now understood in terms of fluctuation and discreteness in molecular numbers. Indeed, some state with extinction of specific molecule species shows a qualitatively different behavior from that with very low concentration of the molecule. This difference leads to a transition to a novel state, referred to as discreteness-induced transition. This phase transition appears by decreasing the system size or flow to the system, and it is analyzed from the stochastic process, where a single-molecule switch changes the distributions of molecules drastically. 

Of importance in using this molecular dynamics technique and other molecular dynamics techniques that utilize periodic boundary conditions are issues with artificially-induced correlations due to the finite size of the computational cell. Artificial phase transitions or other dynamics can be observed when the computational cell dimensions are sufficiently small to allow a particle to interact with its (correlated) periodic image. Artificial effects can be circumvented by making the computational cell sufficiently large that periodic atomic images are separated by a distance greater than the atomic correlation length in the material. 

The dependence of transverse correlation radius Rex on the particle radius calculated on the basis of Eq. (4.73) is reported in Fig. 4.36. At temperatures T correlation radius Rex diverges at critical radius Rer (T) as anticipated from Eq. (4.73), see curves 1-3. The divergence corresponds to the size-induced ferroelectric phase transition. At temperatures T > 7), transverse correlation radius monotonically increases with the particle radius due to R) increase, see curves 4 and 5. 

Piazza et al. [27] have also shown that the micelle depletion-induced fluid-solid phase transition can be profitably exploited to perform an efficient size fractionation of latex particles. 

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