Single Amphiphilic Chains

The conformational transitions observed in the simulations [97] resemble in some aspects the so-called zipping transitions [211], the process in which two strongly attracting strands composing the polymer come in contact in such a way as to form a bound double structure, which remains swollen and does not assume compact configurations. The cylindrical-shaped conformations in which the hydrophobic backbone is in a locally collapsed state (Figs. 41e and f) look a lot like three-dimensional zipped structures. 

Single hydrophobic-amphiphilic (HA) copolymers with the same HA composition but with different distribution of H and A units along the main hydrophobic chain were also simulated [212]. In particular, regular copolymers comprising H and A units in alternating sequence, regular multiblock copolymers composed of H and A blocks of equal lengths, and the quasirandom protein-like copolymers with a quenched primary structure were studied. These copolymers are schematically depicted in Fig. 24b,c, and d. 

There are experimental evidences of some facts predicted in the simulations [97,212]. 

Selb and Gallot [214] have demonstrated that poly(styrene)-gra/f-poly(4-vinyl-N-ethylpyridium bromide) forms unimolecular micelles in water/ methanol mixtures. These experimental data can be treated as an indirect confirmation of the simulation result [97] that sufficiently long regular copolymers with amphiphilic monomer units do form intramolecular aniso-metric micellar structures in a poor solvent. 

In a series of papers [216,217], Nakata and Nakagawa have studied the coil-globule transition by static light scattering measurements on poly(methyl methacrylate) in a selective solvent. They have found that the chain expansion factor, a2 = R2/R20, plotted against the reduced temperature, r = 1 - 0/T, first decreases with decreasing r, as it should be, but then begins to increase (see, e.g., Fig. 2 presented in [217]) In the authors opinion, the increase of 

---

In this model, the single amphiphile chain is treated as accurately as possible. Well established values for the C - C bond length, and for the relative energy and bond angles of trans and gauche conformers [1] are all included in the model. This approach was pioneer-... 

There has been considerable interest in the simulation of lipid bilayers due to their biological importance. Early calculations on amphiphilic assemblies were limited by the computing power available, and so relatively simple models were employed. One of the most important of these is the mean field approach of Marcelja [Marcelja 1973, 1974], in which the interaction of a single hydrocarbon chain with its neighbours is represented by two additional contributions to the energy function. The energy of a chain in the mean field is given by ... 

Okayama, Japan. The synthetic procedures for S-D dyads and their reference compounds shown in Fig. 6 are illustrated in Fig. 7, where a naphthalene and a ferrocene moiety are used as an S and a D moiety, respectively. For comparison of photoinduced electron transfer rates between a single alkyl chain and a triple alkyl chain as the spacers of the S-D dyads with the same length of four-carbons, S-D dyads with a rigid spacer with a bicyclo[2.2.2]octane were synthesized [39]. The synthetic procedure for the S-D dyads with the rigid spacer is also shown in Fig. 8 [39], Other amphiphilic compounds and chemicals were commercially available. 

Stable monolayers of S-D amphiphilic dyads with a single alkyl chain spacer containing a naphthalene and a ferrocene moiety... 

Liquid Condensed Films (Lco) As the area per molecule (or the distance between molecules) is further decreased, a transition to a so-called liquid condensed (Lco) state is observed. These states have also been called solid expanded films (Adam, 1941 Gaines, 1966 Birdi, 1989, 1999 Adamson and Gast, 1997). The n versus A isotherms of n-pentadecylic acid (amphiphile with a single alkyl chain) have been studied, as a function of temperature (Birdi, 1989). 

For single alkyl chain surfactants, which are of principal interest in this treatment, the factor primarily determining the CMC is the size of the hydrophobic part. The dependence of the CMC on the number of carbon atoms (nc) in the alkyl chain can for many classes of amphiphiles be written2)... 

Ikeda, I. Takayuki, K. Tanaka, K. Takeda, T. Kurosawa, H. Okahara, M., (1990) Synthesis and vesicleforming ability of an amphiphilic compounds composed of a single lipophilic chain and a crown ether ring as a hydrophilic moiety. Nippon Kagaku Kaishi 1059-1064 [Chem. Abstr. 114 26193],... 

When the packing considerations are taken into account, this model practically predicts that amphiphiles with a single alkyl chain are will form micelles or bilayers, those with two alkyl chains bilayers, and those with three alkyl chains inverted hexagonal phases.[0]... 

As the polymer concentration increases, interchain association inevitably occurs, but some amphiphilic chains can undergo a limited interchain association to form a stable mesoglobular phase that exists between microscopic single-chain globules and macroscopic precipitation. As expected, when the solvent quality changes from good to poor, intrachain contraction and interchain association occur simultaneously and there exists a competition between these two processes. Such a competition depends on the comonomer composition and distribution on the chain backbone and also depends on the rate of micro-phase separation. When intrachain contraction happens quickly and prior to interchain association, smaller mesoglobules are formed. A proper adjustment of the rates of intrachain contraction and interchain association can lead to polymeric colloidal particles with different sizes and structures. 

In the case of ordered mesoporous oxides, the templating relies on supramolecular arrays micellar systems formed by surfactants or block copolymers. Surfactants consist of a hydrophihc part, for example, ionic, nonionic, zwitterionic or polymeric groups, often called the head, and a hydrophobic part, the tail, for example, alkyl or polymeric chains. This amphiphiUc character enables surfactant molecules to associate in supramolecular micellar arrays. Single amphiphile molecules tend to associate into aggregates in aqueous solution due to hydrophobic effects. Above a given critical concentration of amphiphiles, called the critical micelle concentration (CMC), formation of an assembly, such as a spherical micelle, is favored. These micellar nanometric aggregates may be structured with different shapes (spherical or cylindrical micelles, layered structures, etc. Fig. 9.8 Reference 70). The formation of micelles. 

On the other hand, polyfpropylene oxide) (PPO) is hydrophilic at low temperatures and hydrophobic at high temperatures. Thus, water is a good solvent for PPO below approximately 15 °C, whereas at higher temperatures aggregates are formed in aqueous solutions. With blocks of PEO and blocks of PPO combined into single polymer chains, one can therefore expect amphiphilic characteristics and aggregation phenomena. It has to be emphasized that, for this triblock copolymer, the outer PEO blocks are more water soluble than the inner PPO blocks, and thus the mechanism of gel formation will be different from the above mentioned triblock copolymers and is still a matter of discussion. 

K. Yamauchi, Y. Yoshida, T. Moriya, K. Togawa and M. Kinoshita, Archebacterial Lipid Models Formation of Stable Vesicles from Single Isoprenoid Chain-Amphiphiles, Biochim. Biophys. Acta, 1193 41 (1994). 

The small fraction of single free chains together with slow exchange kinetics essentially freezes the exchange process. A sufficiently fast exchange of amphiphiles is, however, necessary to sustain thermodynamic equilibrium. Therefore vesicles, once formed, are in a metastable, trapped, or quenched thermodynamic state. The number of amphiphiles and therefore their bilayer area is essentially constant on timescales of most experiments. An extreme case is glassy polymers with very slow lateral mobility which can even impede shape changes of polymer vesicles ( frozen vesicles). 

Small assemblies that topologically mimic the micelle architecture but that present covalently bound amphiphilic chains have been synthesized as an alternative approach to provide intrinsic stability These nanocarriers are referred to as unimolecular polymeric micelles (UPM) and consist of single macromolecules such that their formation and dissociation are intrinsically independent on polymer concentration (no apparent CMC). UPM can be obtained from both dendrimers and star-block copol)miers, with the latter form involving fewer s)mthetic steps and being simpler to prepare. 

---