Amphiphiles lipids/colloids

It should be noted that surfactants in solution are usually classed as association colloids . In this book they are considered in a separate chapter (Chapter 4) because a detailed consideration of their phase behaviour, properties and applications is merited by their industrial importance. Biological amphiphiles (lipids) are also considered in Chapter 4. 

Liposomes and micelles are lipid vesicles composed of self-assembled amphiphilic molecules. Amphiphiles with nonpolar tails (i.e., hydrophobic chains) self-assemble into lipid bilayers, and when appropriate conditions are present, a spherical bilayer is formed. The nonpolar interior of the bilayer is shielded by the surface polar heads and an aqueous environment is contained in the interior of the sphere (Figure 10.3A). Micelles are small vesicles composed of a shell of lipid the interior of the micelle is the hydrophobic tails of the lipid molecules (Figure 10.3B). Liposomes have been the primary form of lipid-based delivery system because they contain an aqueous interior phase that can be loaded with biomacromolecules. The ability to prepare liposomes and micelles from compounds analogous to pulmonary surfactant is frequently quoted as a major advantage of liposomes over other colloidal carrier systems. 

Low-molecular-weight surfactants ( emulsifiers ) are important ingredients in food products. The types of surfactants most commonly studied in food colloids research are phospholipids (lecithin), mono/diglycerides (particularly glycerol monostearate), polysorbates (Tweens), sorbitan monostearate or monooleate (Spans), and sucrose esters. These small lipid-based amphiphiles can typically lower the interfacial tension to a greater extent than the macromolecular amphiphiles such as proteins and certain gums (Bos and van Vliet, 2001). 

FIG. 2 Observed SAXS curve for lipid IVa vesicles and its approximation function. (Reprinted with permission from the paper entitled Scattering studies on colloids of biological interest (amphiphilic systems), by O. Glatter. Progr. Colloid Polym. Sci. H4 52. Copyright 1991 Steinkopff Publishers, Darmstadt, FRG.)... 

One important class of molecules abundant in foods is that of amphiphilic molecules (including fats, lipids and proteins), also known as surfactants. In the following sections some novel insights are presented into the formation of lamellar phases (colloidal structures with varying topologies depending on thermodynamic and flow conditions), and the formation of anisotropic colloidal protein structures is discussed. 

The formed mixed micelles can diffuse to the unstirred water layer that lines the epithelium, where the micelles disintegrate and lipid amphiphiles are ahsorhed. Bile salts are recycled hack into the lumen and continue to interact with lipid digestion. Thus at any given time during lipid digestion, a complex mixture of different colloid phases is present in the intestinal lumen (Rigler et al., 1986). 

As is well known, the bilayer structure of cell manbranes exhibits hydrophobic properties in the hydrocarbon part. This means that those molecules that must interact with the membrane interior must be hydrophobic. Anesthesia is brought about by the interaction between some suitable molecule and the lipid molecules in the biological membrane at the cell interface. The effect of pressure has been reported to be due to the volume change of membranes, which reverses the anesthesia effect. Local anesthetics are basically amphiphile molecules of tertiary amines, and some have colloidal properties in aqueous solution. The anesthetic power is determined by the hydrophobic part of the molecule. Surface tension measurements showed a correlation with the anesthetic power for a variety of molecules dibucane < tetracainebupivacainemepivacaine < lidocaine < procaine (aU as HCl salts). ... 

The structural units of liposomes are amphiphile molecules, mainly phospholipids, Alec Bangham and co-workers observed self-closed lipid structures after they had been dissolved in water [357- This first observation took place after egg yolk lecithin had been dispersed in water. According to D.D.Lasic and D. Papahadjopoulos, liposomes are self-assembling colloidal particles in which a lipid bilayer encapsulates a fraction of the surrounding medium [36],... 

For decades, colloid and surface scientists have known that amphiphilic molecules such as phospholipids can self-assemble or self-organize themselves into supramolecular structures of bilayer lipid membranes (planar BLMs and spherical liposomes), emulsions, and micelles [2-4]. As a matter of fact, our current understanding of the structure and function of biomembranes can be traced to the studies of these experimental systems such as soap films and Langmuir monolayers, which have evolved as a direct consequence of applications of classical principles of colloid and interfacial chemistry. As already mentioned in Section I, the seminal work on the self-assembly of planar lipid bilayers and bilayer or black lipid membranes was carried out in 1959-1963. The idea started while one of the authors was reading a paperback edition of Soap Bubbles by C. 

The methods of preparing lipid vesicles are well known [2,5,9]. Vesicles are made predominantly from amphiphiles, a special class of surface-active molecules, which are characterized by having a hydrophilic (water soluble) and a hydrophobic (water insoluble) group on the same molecule. A typical vesicle-forming molecule, such as lecithin (see Fig. 7), has two hydrocarbon chains, also called hydrophobic or nonpolar tails, attached to a hydrophilic group, often named the polar head. In general, most of these molecules are not soluble in water however, instead of solutions they form colloidal dispersions [15]. 

Cations bind to nanoscopic colloidal assemblies, supra-molecular solid lipid nanoparticles of amphiphilic calix[4]arene derivatives, leading, in certain cases, to the clustering of such assemblies in the presence of divalent cations Mg and Ca . Such clustering was imaged by atomic force microscopy. Such effects have particular relevance to the use of such transport systems in physiological media. 

The use of surfactants or amphiphilic molecules in electrochemistry dates back over four decades [1,2]. Extensive research on electrochemistry in surfactant systems has been reported primarily in the last 20 years. Surfactant systems are ubiquitous. The aggregation of surfactant molecules may produce a variety of systems including micelles, monolayers and bilayers, vesicles, lipid films, emulsions, foams, and microemulsions. Developments in the area of electrochemistry in such association colloids and dispersions have been documented by Mackay and Texter [3]. Mackay [4] reviewed the developments in association colloids, particularly micelles and microemulsions. Rusling [5,6] also reviewed electrochemistry in micelles, microemulsions, and related organized media. This chapter focuses on microemulsions and does not deal with micelles, monolayers, emulsions, and other surfactant systems per se. 

Interfaces between aqueous phase and the volumes confined by amphiphilic molecules [288]. In vitro, these refer to lipid vesicles and micelles, lipid lamellae, cubic and hexagonal phases, Langmuir-Blodgett (LB) films, which are important in colloid science and in extraction technology. In vivo, these are the surfaces of biological membranes. 

A related field of research where NMR investigations have yielded a large variety of information are soft colloidal volume systems, so-called association colloids, composed of amphiphilics (surfactants, lipids) or copolymers, e.g. block-copolymers. Structures such as micellar solutions, microemulsions, or liquid crystalline phases possess internal interfaces where the arrangement of molecules can be governed by similar principles as at a solid/liquid interface. NMR methods have very successfully been employed in this area, as has been reviewed [8, 9], and some ideas and approaches can be transferred to liquid interfacial layers in colloidal particle dispersions. 

Proteins and polar lipids coexist in biological systems, mainly unassociated with each other but also as composite structures with specific actions [137]. They have a very important physical property in common an inherently amphiphilic nature, which provides the driving force for the formation of associative structures of lipids as well as for stabilizing some food colloids. 

The term amphiphilic is used to describe surfactant-like molecules having a dual polarity a polar group in one end and a long hydrocarbon chain in the other end. Chemists have stretched it to describe polar and nonpolar interactions. If the term amphiphilic/amphiphobic is used, the author is advised to speedy the liquids used. As for lipophilic, we find it limits in scope because lipo usually means fats and lipids. Although some have extended it to hydrocarbon oils, we feel that hydrocarbon oils are already represented by oleo. We also feel that the term lyophilic is not appropriate. It was used to describe affinity between colloids and their dispersed media and doesn t appear to be too relevant. 

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