Amphiphiles, small molecule

Vesicles are capsules in which the shells are composed of amphiphilic small molecules or polymers. Generally, the shell is an amphiphilic bilayer with an aqueous interior. These differ fundamentally from capsules generated in a water-in-oil emulsion because the oil phase in the vesicle system is only in the shells, which are surrounded by an outer aqueous phase. 

In one of the first studies, Yanagida et al.52 used a series of amphiphilic small molecules (Fig. 17.13) to obtain the gelation of an I / -based electrolyte composed by 0.6M l,2-dimethyl-3-propylimidazolium iodide, 0.1M lithium iodide, 0.1 M... 

Since the amphiphilic nature is essential for the phase behaviour, systems of small molecules (e.g., lipid water mixtures) and polymeric systems (e.g., homopolymer copolymer blends) share many connnon features. 

Szostak et al. worked on the basis of a simple cellular system which can replicate itself autonomously and which is subject to Darwinian evolution. This simple protocell consists of an RNA replicase, which replicates in a self-replicating vesicle. If this system can take up small molecules from its environment (a type of feeding ), i.e., precursors which are required for membrane construction and RNA synthesis, the protocells will grow and divide. The result should be the formation of improved replicases. Improved chances of survival are only likely if a sequence, coded by RNA, leads to better growth or replication of membrane components, e.g., by means of a ribozyme which catalyses the synthesis of amphiphilic lipids (Figs. 10.8 and 10.9). We can expect further important advances in the near future from this combination ( RNA + lipid world ). 

Vesicle size, bilayer fluidity, membrane permeability, microviscosity, ability to bind small molecules, suseeptibility to pore formation, flip-flop rates, extent of water penetration, lateral amphiphile diffusion, vesiele fusion, and kinetic medium effeets (some of which will be discussed briefly below) all depend on the paeking of... 

Thus far, methods of organizing small molecules into mono- and multilayers have been discussed. However, both LB and self-assembly techniques allow deposition of polymer films as well. Two approaches to preparing polymer films by LB method have been described. An amphiphilic polymer film can be... 

Higashimura et al. [70] tried to elucidate the interactions between amphiphilic miktoarm star molecules produced by cationic polymerization and small molecules using NMR techniques. In their comparison between two different architectures no distinct differences were observed for star-blocks and miktoarm stars, both species being sufficiently capable of accommodating hydrophilic molecules within their hydrodynamic volume. 

Formation of membrane structures is also due to the hydrophobic effect. When a molecule is composed of both polar and nonpolar moieties, it is said to be amphiphilic. Since the hydrophobic effect is concerned with the tendency for nonpolar molecules or nonpolar components of molecules to associate spontaneously in aqueous solutions, there is a wide range of small molecules such as fatty acids and phospho- and glycolipids which, when dissolved in water, segregate to form membranes or micelles [146]. The same type of segregation occurs in the crystalline state, as in the structures of the long chain alkyl glycosides [147], cerebrosides,... 

Emulsion stability is required in many dairy applications, but not all. In products like whipped cream and ice cream, the emulsion must be stable in the liquid form but must partially coalesce readily upon foaming and the application of shear. The structure and physical properties of whipped cream and ice cream depend on the establishment of a fat-globule network. In cream whipped to maximum stability, partially coalesced fat covers the air interface. In ice cream, partially coalesced fat exists both in the serum phase and at the air interface also, there is more globular fat at the air interface with increasing fat destabilization. Partial coalescence occurs due to the collisions in a shear field of partially crystalline fat-emulsion droplets with sufficiently-weak steric stabilization (low level of surface adsoiption of amphiphilic material to the interface per unit area). To achieve optimal fat crystallinity, the process is very dependent on the composition of the triglycerides and the temperature. It is also possible to manipulate the adsorbed layer to reduce steric stabilization to an optimal level for emulsion stability and rapid partial coalescence upon the application of shear. This can be done either by addition of a small-molecule surfactant to a protein-stabilized emulsion or by a reduction of protein adsorption to a minimal level through selective homogenization. 

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