Amphipathic surface active molecules

Association colloids are aggregates or associations of amphipathic surface active molecules. These molecules are soluble in the solvent, and their molecular dimensions are below the colloidal size range. When present in solution at concentrations above a certain critical value (the critical micelle concentration), these molecules tend to form association colloids (micelles) (Fig. 1). 

The basic experimental observation is the following. A surface-active molecule usually contains a polar head group and a nonpolar tail. Such molecules are known to reduce the surface tension of water when they are added to water—hence the term surface-active or surfactants. The main reason they do so is their tendency (more precisely, the tendency of their nonpolar part) to avoid contact with water and to seek a nonaqueous environment. These molecules are sometimes referred to as amphiphiles, but more recently the term amphipaths has been found more appropriate to describe the contrasting behavior of the two parts of the same molecule toward the solvent. 

What is a surface-active agent (or a surfactant) Why is it often referred to as an amphipathic molecule ... 

Surface-active agents (surfactants) are substances which, at low concentrations, adsorb onto the surfaces or interfaces of a system and alter the surface or interfacial free energy and the surface or interfacial tension. Surface-active agents have a characteristic structure, possessing both polar (hydrophilic) and non-polar (hydrophobic) regions in the same molecule. Thus surfactants are said to be amphipathic in nature. The wide range of uses for surfactants in pharmaceutical products and systems is the subject of this article. 

The Gibbs equation (see equation 19) predicts that a substance that reduces the surface (interfacial tension) will be the adsorbed at the surface (interface). Surface-active substances (especially long-chain fatty acids, detergents, and surfactants) decrease the surface (interfacial) tension. Amphipathic molecules (which contain hydrophilic and hydrophobic groups) become oriented at the interface. At solid-water interfaces, Ae orientation depends on the relative affinities of the adsorbate for water and the solid surface. The hydrophilic groups (sulfate carboxylate, hydroxyl, etc.) may—if the hydrophobic tendency is relatively small—interact coordinatively with the functional groups of the solid surface (Ochs et al., 1994 Ulrich et al., 1988). 

Surface-active compounds are characterised by having two distinct regions in their chemical structure these are termed hydrophilic water-liking ) and hydrophobic ( water-hating ) regions. The existence of two such moieties in a molecule is referred to as amphipathy and the molecules are consequently often referred to as amphipathic molecules. 

The dual stmcture of amphipathic molecules is the unique feature which is responsible for the characteristic behaviour of this type of compound. Thus their surface activity arises from adsorption at the solution/air interface -the means by which the hydrophobic region of the molecule escapes from the hostile aqueous environment by protmding into the vapour phase above. Similarly, adsorption at the interface between aqueous and non-aqueous solutions occurs in such a way that... 

The surface activity at the air/solution interface has been reported for a wide variety of dmgs. This surface activity is a consequence of the amphipathic namre of the dmgs. The hydrophobic portions of the dmg molecules are in general more complex than those of typical surfactants, often being composed of... 

A surface-active agent (or surfactant) is a substance that lowers the surface or interfacial tension of the medium in which it is dissolved. Surfactants have a characteristic molecular structure consisting of hydrophobic and hydrophilic groups. This is known as an amphipathic structure, and causes not only concentration of the surfactant at the surface and reduction of the surface tension of the solvent, but also orientation of the molecule at the surface with its hydrophilic group in the aqueous phase and its hydrophobic group oriented away... 

Phospholipids are amphipathic molecules that play important roles in living organisms as membrane components, emulsifying agents, and surface active agents. There are two types of phospholipids phosphoglyc-erides and sphingomyelins. 

Surfactants. Some compounds, like short-chain fatty acids, are amphiphilic or amphipathic that is, they have one part that has an affinity for the nonpolar media (the nonpolar hydrocarbon chain), and one part that has an affinity for polar media, that is, water (the polar group). The most energetically favorable orientation for these molecules is at surfaces or interfaces so that each part of the molecule can reside in the fluid for which it has the greatest affinity (Figure 4). These molecules that form oriented monolayers at interfaces show surface activity and are termed surfactants. As there will be a balance between adsorption and desorption (due to thermal motions), the interfacial condition requires some time to establish. Because of this time requirement, surface activity should be considered a dynamic phenomenon. This condition can be seen by measuring surface tension versus time for a freshly formed surface. 

Proteins are biopolymers that are encountered in many applications, such as food emulsions, hair conditioners, photographic emulsions, and various medical diagnostic products. Many of these applications are frequently based on the unique surface activity of the proteins, which is reflected in functional properties such as foaming, emulsification, and gelling. The proteins are composed of polymeric chains containing many hydrophobic and hydrophilic domains, often giving the molecules an amphipathic structure somewhat similar to that of polymeric surfactants. 

VVe observe that surface-active substances that form micelles possess one common feature, namely they are amphipathic, which means that the molecule consists of two parts, one of which is highly soluble in the medium concerned and the other insoluble. We shall deal mainly with aqueous solutions, although it is important to realise that micellisation is not confined to water as solvent but can occur in many non-polar media. In the case of aqueous systems the surfactant molecule consists of a hydrophilic group (head group) to which is attached a hydrophobic hydrocarbon group (tail). 

Remember when dealing with proteins that (1) the small physical size of the molecule is below the continuum limit and thus the molecule is not susceptible to shear induced degradation (2) structures that control activity are a result of relatively weak hydrophobic interactions and hydrogen bonds that result in an unstable active structure and finally (3) proteins are surface active due to the amphipathic nature, and thus large surface-to-volume ratios should be avoided. 

FIGURE 4.14 Surface and interfacial active molecules and their amphipathic regions. 

Due to the relatively weak adsorption of homopolymers at the L/L interface, and in some cases at the S/L interface, homopolymers are seldom used as emulsifiers or dispersants. For this purpose, the molecule is modified to include some specific units that have strong adsorption to the surface. A good example is partially hydrolysed poly (vinyl acetate), which is commercially referred to as poly(vinyl alcohol) (PVA). The polymer contains 4-12% acetate groups (i.e. 96-88% hydrolysed) and these groups impart an amphipathic character to the chain. The polymer becomes surface-active at the L/L interface and hence it can be used as an emulsifier. In addition, on a hydrophobic surface such as polystyrene, the acetate groups become preferentially adsorbed on the surface of the particles, thus leaving the PVA units dangling in solution as loops and tails . The latter provide the required steric stabilization. 

Proteins are by nature amphipathic or amphiphilic molecules that is, they contain both a hydrophobic (nonpolar) and a hydrophilic (polar) moiety. However, natural proteins per se are not used as commercial surfactants. Rather, proteins are modified by chemical or enzymatic means to products with surface-active properties. The use of modified proteins based on casein, soybean, albumen, collagen, or keratin is not new [5]. The Maywood Chemical Company introduced commercial protein-based surfactants (PBS) in the United States in 1937. They were primarily condensation products of fatty acids with hydrolyzed proteins [5], Renewed interest in PBS has occurred not only as products based on renewable raw materials (i.e., proteins and fatty acids), but also as a solution to waste disposal for animal and vegetable protein byproducts [5], Among the commercial PBS, the following trade names have been active Crotein, Lexein, Magpon Polypeptide, Protolate, Sol-U-Teins, and Super Pro. 

The formation of particles in polymer colloids ordinarily is accomplished by the free radical polymerization of an organic monomer in a liquid which is a non-solvent (diluent) for the polymer. A surface active material, such as a soap or other amphipathic molecule, is usually added to stabilize the colloidal particles as they are formed. The particle size distribution varies from very narrow to extremely broad depending primarily upon the solubility of the monomer in the diluent, the stabilizer concentration and the ionic strength. 

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