By enthalpic

The value of EM for a cooperative self-assembled structure provides a measure of the monomer concentration at which trivial polymeric structures start to compete, and therefore EM represents the upper limit of the concentration range within which the cooperative structure is stable (Scheme 2). The lower limit of this range is called the critical self-assembly concentration (csac) and is determined by the stoichiometry of the assembly and the strength of the non-covalent binding interactions weaker interactions and larger numbers of components raise the csac and narrow the stability window of the assembly (8). Theoretical treatments of the thermodynamics of the self-assembly process have been reported by Hunter (8), Sanders (9), and Mandolini (10). The value of EM is lowered by enthalpic contributions associated with... 

The exchange of BTM-n cations is mainly governed by enthalpic effects in contrast to alkylammonium exchange, the behaviour of which is primarily determined by entropy changes (All being essentially zero). This suggests that electrostatic interactions, which are enhanced due to better delocalization, are also important. dgp spacings of 1.45 nm confirm a flat orientation of these ions. 

Electroneutral combinations are often favored by enthalpic against entropic contributions, however with noticeable exceptions. Thus, crown ether 18-C-6 complexes with e.g. free amines in methanol show stability constants around lgK= 2.5 0.1, quite independent of the amine structure, but very sizeable differences in AH, ranging from 1 or 2 kJ/mol for secondary or tertiary amines to 30 kJ/mol for primary amines with surprisingly favorable entropic TAS values of up to 13 kJ/mol. Protonated amines show increased lg K values of up to 4.4, mainly due to enthalpic advantages, and not. as one might expect for the more polar combinations, due to entropic effects, although these are also sizeable.[38]... 

The interest in hydrophobic interactions was stimulated by their unusual thermodynamic properties it was argued and believed that they are governed, not by enthalpic, but by entropic features, characterized by the undesirable entropy decrease of water in the vicinity of nonpolar groups (Frank and Evans, 1945 Kauzmann, 1959 Franks, 1975 Tanford, 1980). This conclusion was reached largely from consideration of solvation effects at room temperature. 

Further, data of Nishi and Kwei (68) show that the LCST for the polystyrene-poly(vinyl methyl ether) system is constant to within 10°C when the polystyrene Mw lies between 50,000 and 1,000,000. This result again suggests that entropic contributions to the phase transition are of secondary importance when the component molecular weights are high and that the phase instability at LCST is governed by enthalpic consider-... 

Many of these complexes are more stable than those formed with the corresponding open-chain ligands. This macrocyclic effect is determined by enthalpic as well as by entropic contributions. Still higher stability constants are obtained when side chains with additional donor groups are attached to the macrocycle thus, tetraacetic acid 22 forms complexes with most metal ions, including alkaline earth ions and transition metal ions these complexes show stabilities among the highest known. 

Gradient high-performance liquid chromatography (HPLC) has been useful for the characterization of copolymers (14-19). In such experiments, careful choice of separation conditions is a conditio sine qua non. Otherwise, low resolution for the polymeric sample will obstruct the separation. However, the separation in HPLC, dominated by enthalpic interactions, perfectly complements the entropic nature of the SEC retention mechanism in the characterization of complex polymer formulations. 

All theoretical deductions discussed so far show that a surface layer caused by entropic as well as by enthalpic forces must be very thin and can only be of the order of a few nanometers. From the discussion in Sect 2 it became evident that the consequences of this small alteration of the radial density profile for the scattering curves show up only in the immediate vicinity of the match point. At high contrast the curves are shifted parallel to the ordinate as is the case for homogeneous particles and SAXS-experiments conducted under these conditions would be misleading. Near the match point, however, the maxima are shifted in a characteristic fashion parallel to the abscissa which allows to distinguish between a homogeneous particle and a particle with an inhomogeneous density distribution. This point has been discussed already in Sect. 2 (cf. also the discussion in Ref. [52]). 

As for adsorption chromatography (LAC), where separation is directed by adsorptive interactions between the macromolecules and the stationary phase, an ideal case may be defined as well. In ideal LAC conformational changes are assumed to be zero (AS = 0) and the distribution coefficient is exclusively determined by enthalpic effects ... 

In a more general sense, the size exclusion mode of liquid chromatography relates to a separation regime, where entropic interactions are predominant and TAS > AH. In the reverse case, AH > TAS, separation is mainly directed by enthalpic interactions. As both separation modes in the general case are affected by the macromolecule size and the pore size, a certain energy of interaction may be introduced, characterizing the specific interactions of the monomer unit of the macromolecule and the stationary phase, e is a function of the chemical composition of the monomer unit, the composition of the mobile phase of the chromatographic system, and the temperature. 

The physical nature of hydrophobic effects was previously considered to be entropic. Based on this hypothesis, it has often been claimed that the thermal stabilization of proteins in thermophiles may be correlated with an increase in the number of hydrophobic residues. A critical analysis proved the differences to be statistically insignificant the recent dramatic increase in sequence data from complete genomes of mesophilic and (hyper-) thermophilic bacteria and archaea clearly confirmed this finding (see below). Considering the real meaning of the word hydrophobic, it is clear that the aversion of nonpolar solutes to water becomes more ordinary and less entropy-driven at extreme temperatures, whereas in the mesophilic temperature regime the hydrophobic effect is indeed entropic. Maximum aversion arises at the temperature at which the ftee energy of transfer of nonpolar solutes into water shows its maximum. Under this condition, the entropy (i.e., the temperature derivative of AG) equals zero, so that the hydrophobic effect must be driven by enthalpic contributions, attributable to van der Waals forces in the core of the protein. ... 

Limitations for the applicability of SEC for certain molecules are the high shear forces due to the dense column packing, which may influence molecular assemblies and lead to degradation. Furthermore, the column packing material can induce unwanted interactions between the stationary phase and the molecules, thus shifting the separation mechanism from pure size exclusion towards separation by enthalpic interactions. Especially in cases of multifunctional polymers like dendritic molecules with a high number of end groups, delayed elution or complete adsorption can be observed. 

While the influence of CO2 sorption on the systems studied here results in phase segregation by a depression of the lower critical transition, the results do not exclude the probability that for some systems CO2 sorption at high pressure (or low temperature) will promote miscibility near the UODT or UCST where the transition is dominated by enthalpic considerations. At liquid-like densities, its influence could mimic that of common liquid solvents by mitigating unfavorable contacts. Indeed this is the case for block copolymers of polyisoprene and polystyrene. ... 

It is important to note the elution at 48% THF Retention becomes completely independent of molecular mass, and all fractions elute with the column s liquid voliune Vf. At this transition point, which is termed critical point of adsorption (CPA), entropy driven size-exclusion effects are completely compensated by enthalpic adsorption interactions. Although both types of interaction strongly depend on molecular weight, this mutual compensation phenomenon leads to molecular weight independent retention at CPA. This phenomenon allows for practical implementation of isocratic polymer separation other than sec. Thus, because of chromatographic invisibility of a main polymer chain, effective separation of telechehc polymers based on types of end-groups can be performed (Ref 7, p. 125). The same principle is applied for isocratic separation of some block copolymers (Ref 7, p. 151). 

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