Hydrophobic units

As early as 1969, Wlieeler and Widom [73] fomuilated a simple lattice model to describe ternary mixtures. The bonds between lattice sites are conceived as particles. A bond between two positive spins corresponds to water, a bond between two negative spins corresponds to oil and a bond coimecting opposite spins is identified with an amphiphile. The contact between hydrophilic and hydrophobic units is made infinitely repulsive hence each lattice site is occupied by eitlier hydrophilic or hydrophobic units. These two states of a site are described by a spin variable s., which can take the values +1 and -1. Obviously, oil/water interfaces are always completely covered by amphiphilic molecules. The Hamiltonian of this Widom model takes the form... 

Chain-Growth Associative Thickeners. Preparation of hydrophobically modified, water-soluble polymer in aqueous media by a chain-growth mechanism presents a unique challenge in that the hydrophobically modified monomers are surface active and form micelles (50). Although the initiation and propagation occurs primarily in the aqueous phase, when the propagating radical enters the micelle the hydrophobically modified monomers then polymerize in blocks. In addition, the hydrophobically modified monomer possesses a different reactivity ratio (42) than the unmodified monomer, and the composition of the polymer chain therefore varies considerably with conversion (57). The most extensively studied monomer of this class has been acrylamide, but there have been others such as the modification of PVAlc. Pyridine (58) was one of the first chain-growth polymers to be hydrophobically modified. This modification is a post-polymerization alkylation reaction and produces a random distribution of hydrophobic units. 

HydrophobicaHy Modified, Ethoxylated Urethane. HEUR associative thickeners are in effect poly(oxyethylene) polymers that contain terminal hydrophobe units (66). They can be synthesized via esterification with monoacids, tosylation reactions, or direct reaction with monoisocyanates. There are problems associated with aH of the methods of synthesis. The general commercial procedure for their synthesis is by a step-growth addition of... 

These are molecules which contain both hydrophilic and hydrophobic units (usually one or several hydrocarbon chains), such that they love and hate water at the same time. Familiar examples are lipids and alcohols. The effect of amphiphiles on interfaces between water and nonpolar phases can be quite dramatic. For example, tiny additions of good amphiphiles reduce the interfacial tension by several orders of magnitude. Amphiphiles are thus very efficient in promoting the dispersion of organic fluids in water and vice versa. Added in larger amounts, they associate into a variety of structures, filhng the material with internal interfaces which shield the oil molecules—or in the absence of oil the hydrophobic parts of the amphiphiles—from the water [3]. Some of the possible structures are depicted in Fig. 1. A very rich phase... 

The formation of a microphase structure leads to a surface-active effect [31]. The surface tension of water is considerably lowered when amphiphilic copolymers are dissolved. The surface-active effect appears more significantly in the copolymers with more hydrophobic units. 

The microphase structure was clearly observed in transmission electron micrographs of the film of amphiphilic copolymers cast from aqueous solutions [29, 31]. An important finding was that no microphase structure was observed for the film cast from organic solutions. This difference indicates that a microphase structure is formed in aqueous solution, but not in organic solution. Different hydrophobic groups showed considerably different morphological features i.e. whether microphase separation leads to a secondary or higher structure depends on the type of hydrophobic units in the copolymers [31],... 

NMR line broadening occurred more pronouncedly for copolymers with aromatic hydrophobic units than for those with aliphatic ones [31]. This implies that polycyclic aromatic rings tend to be more tightly packed when self-aggregating than do aliphatic groups. 

Two other series of biodegradable polymers that depend upon chain degradation are the polyanhydrides and the polyorthoesters. Both of these polymers contain hydrophobic units linked together along the polymer chain by functional... 

A class of systems extensively investigated by means of PFG-NMR are colloids. They are usually hydrophobically modified water-soluble polymers, that is, polymers with a water-soluble skeleton bearing one or more hydrophobic units, which allow the self-assembling of the polymer in water solution and the interaction with surfactants.77... 

Abstract Protein-like copolymers were first predicted by computer-aided biomimetic design. These copolymers consist of comonomer units of differing hydrophilicity/hydro-phobicity. Heterogeneous blockiness, inherent in such copolymers, promotes chain folding with the formation of specific spatial packing a dense core consisting of hydrophobic units and a polar shell formed by hydrophilic units. This review discusses the approaches, those that have already been described and potential approaches to the chemical synthesis of protein-like copolymers. These approaches are based on the use of macromolecular precursors as well as the appropriate monomers. In addition, some specific physicochemical properties of protein like copolymers, especially their solution behaviour in aqueous media, are considered. 

HEUR associative thickeners are in effect poly(oxyethylene) polymers that contain terminal hydrophobe units. They can be synthesized via esterification with monoacids. tosylation reactions, or direct reaction with monoisocyanates. 

Other copolymers that may show micellization similarto that of BAB triblock copolymers include random copolymers if the hydrophobic units are located near both ends of the polymeric structure (Schild and Tirrell, 1991 Ringsdorf et al., 1992 Chung et al., 1999 Jones and Leroux, 1999), and graft copolymers if the hydrophobic blocks are grafted near both ends of a hydrophilic backbone. 

As described previously for dansylleucine-modified CDs, the hydrophobic side chain of leucine affected the guest binding of the fluorescent CDs. To examine the presence of hydrophobic units near the dansyl moiety, monensin-incorpo-rated dansyl-L-lysine modified (3-CD (41) was prepared as an environment-re-... 

This energy is supplied by the condensation of hydrophobic units from the aqueous medium and by the energy of polymerization,... 

The main conclusion drawn from the simulations [170] is that in the presence of monovalent counterions, the charged protein-like copolymers can be soluble, even in a very poor solvent for hydrophobic units. There are three temperature regimes, which are characterized by different spatial organization of polyions and their conformational behavior. 

All known life forms on Earth are surrounded by bilayer membranes that exploit the unique behavior of molecular structures that have, in one part, hydrophobic units built primarily from nonpolar carbon-carbon and carbon-hydrogen bonds and, in another part, polar bonds involving heteroatoms. The first part of the molecule is insoluble in water and therefore aggregates. The second part is soluble in water and presents itself to bulk water solvent, sequestering the hydrophobic parts of the molecule to the interior. 

Bilayer vesicles, especially the non-isotonic ( stressed ) kind or those which contain large amounts of dissolved cholesterol, can also be made to leak with the aid of molecular harpoons . These are membrane-disruptive surfactants comprised of a rigid, wedge-shaped hydrophobic unit attached to a hydrophilic chain, e.g. 22. It was discovered that just one harpoon molecule per ten... 

Figure 6.12. Selected anticonvulsants for the development of a pharmacophore model. The essential structural elements are indicated by dotted rectangles. 1 = phenytoin 2 = carbamazepine 11 = lamotrigine 13 = zonisamide 60 = rufinamide inset, 58 = remacemide. R, hydrophobic unit D, electron-donor group HAD, hydrogen donor/acceptor unit. (AfterRef 281.)...

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