Self-assembly thermodynamics

Kubo and Nishiyabu describe the use of reversible boronate esterification to build well-ordered microparticles through supramolecular polymerisation of benzene-l,4-diboronic acid with tetraols. When pentaerythritol is used as the tetraol component for self-assembly, thermodynamically stable flower-like microparticles are produced. Surface functionalisation enables formation of nanometal-deposited heterogeneous catalysts and white-light-emissive chemosensors. 

This chapter deals with the use of differential scanning calorimetry (DSC) in the study of some of the properties of self-assembling thermodynamically stable, multicomponent systems that appear to the naked eye as homogeneous and mono-phasic. Typical examples are micellar and microemulsion systems [1]. Both of these liquids are charaeterized by the presence of a dispersed phase, whether in the form of aggregates or droplets (diameter 10 nm), a dispersing medium, and a large interphasal area that becomes increasingly important as the particle size decreases. 

Tethering may be a reversible or an irreversible process. Irreversible grafting is typically accomplished by chemical bonding. The number of grafted chains is controlled by the number of grafting sites and their functionality, and then ultimately by the extent of the chemical reaction. The reaction kinetics may reflect the potential barrier confronting reactive chains which try to penetrate the tethered layer. Reversible grafting is accomplished via the self-assembly of polymeric surfactants and end-functionalized polymers [59]. In this case, the surface density and all other characteristic dimensions of the structure are controlled by thermodynamic equilibrium, albeit with possible kinetic effects. In this instance, the equilibrium condition involves the penalties due to the deformation of tethered chains. 

Self-assembled monolayers (SAMs) [8] The layers are formed by heterologous interaction between reactive groups, such as thiols, and noble metals, such as gold or silver. Since the molecules are selectively adsorbed on these metals, film growth stops after the first monolayer is completed. The molecular aggregation is enthalpy driven, and the final structure is in thermodynamic equilibrium. 

Blockcopolymer microphase separation [9] Depending on the length of chemically different blocks of monomers in a block copolymer, ordered nanostructures can be obtained in bulk samples and thin films. The film morphology can differ significantly from the bulk morphology, but because the structure is determined by the pair-pair interaction of monomers and/or an interface, and it is a thermodynamically stable structure, it is classified as self-assembly. 

Micelles the mostly spherical nanoscale aggregates formed by amphiphilic compounds above their critical micelle concentration in aqueous solution have a narrow size distribution and are dynamic, because there is a fast exchange of amphiphiles in solution and those incorporated in micelles. However, micelles are defined as self-assembled structures, since the structure is in thermodynamical equilibrium. 

DD can be monitored by a variety of experimental techniques. They involve thermodynamic, dilatometric, and spectroscopic procedures. Molecular dynamics (MD) simulations also become applicable to self-assembled systems to some extent see the review in Ref. 2. Spectroscopic methods provide us with molecular parameters, as compared with thermodynamic ones on the macroscopic level. The fluorescence probing method is very sensitive (pM to nM M = moldm ) and informs us of the molecular environment around the probes. However, fluorescent molecules are a kind of drug and the membrane... 

The stability of a trivial assembly is simply determined by the thermodynamic properties of the discrete intermolecular binding interactions involved. Cooperative assembly processes involve an intramolecular cyclization, and this leads to an enhanced thermodynamic stability compared with the trivial analogs. The increase in stability is quantified by the parameter EM, the effective molarity of the intramolecular process, as first introduced in the study of intramolecular covalent cyclization reactions (6,7). EM is defined as the ratio of the binding constant of the intramolecular interaction to the binding constant of the corresponding intermolecular interaction (Scheme 2). The former can be determined by measuring the stability of the self-assembled structure, and the latter value is determined using simple monofunctional reference compounds. 

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... 

Many opportunities conversely are supported by reversible reactions of QM despite the noted complications. One example includes the synthesis and chiral resolution of binaphthol derivatives by two cycles of QM formation and alkylation.77 The reversibility of QM reaction may also be integrated in future design of self-assembling systems to provide covalent strength to the ultimate thermodynamic product. To date, QMs have already demonstrated great success in supporting the opposite process, spontaneous disassembly of dendrimers (Chapter 5). 

Goshe, A.J., Steele, I.M., Ceccarelli, C., Rheingold, A.L. and Bosnich, B. (2002) Supramolecular chemistry and self-assembly special feature supramolecular recognition on the kinetic lability of thermodynamically stable host-guest association complexes. Proceedings of the... 

There are two levels of self-assembly in the formation of tetra-, penta-and hexa-nuclear products from the poly-bipyridyls (L) 20 and 21 and iron(II) salts FeCl2, FeBr2 or FeS04 - the products are anion-dependent. The coordination of three bpy units, from different ligand molecules, to the Fe2+ centers produces a helical structure interaction of these helical strands with anions results in further molecular organization to form the final toroidal product. The discussion draws parallels between the helical and toroidal structures here and secondary and tertiary structure in biological systems (482). Thermodynamic and kinetic intermediates have been characterized in the self-assembly of a di-iron triple stranded helicate with bis(2,2/-bipyridyl) ligands (483). 

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