Surface-active agent structure

Surface active agents are important components of foam formulations. They decrease the surface tension of the system and facilitate the dispersion of water in the hydrophobic resin. In addition they can aid nucleation, stabilise the foam and control cell structure. A wide range of such agents, both ionic and non-ionic, has been used at various times but the success of the one-shot process has been due in no small measure to the development of the water-soluble polyether siloxanes. These are either block or graft copolymers of a polydimethylsiloxane with a polyalkylene oxide (the latter usually an ethylene oxide-propylene oxide copolymer). Since these materials are susceptible to hydrolysis they should be used within a few days of mixing with water. 

Modem commercial detergents are mixtures. Their most important component is a surfactant, or surface-active agent, which takes the place of the soap. Surfactant molecules are organic compounds with a structure and action similar to those of soap. A difference is that they typically contain sulfur atoms in their polar groups (4). 

As with surface-active agents, the detailed chemistry of these products is a good deal more complicated than is indicated by the nominal structures frequently quoted. Most commercial products are mixtures of which the nominal structure represents a basic type only. Indeed, the detailed chemistry of the more complex products is still only partially understood. These provisos should be borne in mind when considering the structures given below. 

A foam is a colloidal dispersion of gas bubbles trapped in a liquid. To produce a stable foam, several characteristics of the liquid are necessary. For example, a viscous liquid facilitates the trapping of gas bubbles. The presence of a surface active agent or stabilizer that, for structural reasons, preferentially locates on the surface of the gas bubble also provides a more permanent foam. A low vapor pressure for the liquid reduces the likelihood that the liquid molecules (particularly those surrounding the bubble) will easily evaporate, thus leading to the collapse of the foam. 

At their critical micelle concentrations, surface active agents (such as sodium dodecyl sulfate, Triton X-100, lysolecithin, and bile salts) self-associate into spherical or rod-shaped structures. Because dilution to below the c.m.c. results in rapid disassembly or dissolution of these detergent micelles, micelles are in dynamic equilibrium with other dissolved detergent molecules in the bulk solution. 

Any conclusions about the organization of different components within the dispersions should take the ultrastructure of the systems into consideration. The surface-active agents that act as stabilizers for the nanoparticles are often able to form additional colloidal structures, such as vesicles or micelles, by self assembly. In addition to a potential importance in the formation and stability of the dispersions, such structures contain lipophilic domains that may represent alternative compartments for the localization of incorporated drugs. As a consequence, their presence may affect drug incorporation and release. 

It is therefore apparent why the physical chemistry of surfaces and the structure and activity of surface-active agents are also of interest to the medicinal chemist. Antimicrobial detergents and many disinfectants exert their activity by interacting with biological surfaces and are important examples of surface-active drug effects. 

Spherical particles of various metal phosphate particles can be prepared by precipitation using urea as a homogeneous precipitation agent. Surface-active agents, such as SDS and CTAC, are effective in preparation of uniform-size spherical particles. The formed spherical particles are amorphous and contain OH- and H20, except cobalt phosphate particles with layered structure. These panicles are agglomerates of primary particles, and have pores of different sizes ranging from ultramicropore to mesopore. 

SOAPS. Chemically, a soap is defined as any salt of a fatly acid containing 8 or more carbon atoms. Structurally a soap consists of a hydrophilic (water compatible) carboxylic add which is attached to a hydrophobic (water repellent) hydrocarbon. Soap molecules thus combine two types of behavior in one structure part of the molecule is attracted to water and the other part is attracted to oil. This feature underlies the function of these materials as surface active agents, or surfactants. Soaps are one class of surfactants. The other classes generally are called detergents. See also Colloid Systems and Detergents. 

Synthetic detergents (syndets) belong to the group of surface active agents [which are substances which affect (usually reduce) surface tension when dissolved in water or in water solns] and have structurally unsymmetrical molecules contg both hydrophilic, or water-soluble, groups and hydrophobic, or oil-soluble hydrocarbon chains... 

Interfacial rheology deals with the flow behavior in the interfacial region between two immiscible fluid phases (gas-liquid as in foams, and liquid-liquid as in emulsions). The flow is considerably modified by surface active agents present in the system. Surface active agents (surfactants) are molecules with an affinity for the interface and accumulate there forming a packed structure. This results in a variation in physical and chemical properties in a thin interfacial region with a thickness of the order of a few molecular diameters. These... 

T. Walker, The influence of surface active agents on the structure of water, J.Colloid Interface Sci. 45 (1973) 372-377. 

Klevens, H.B. (1953) Structure and aggregation in dilute solutions of surface active agents. /. Am. 

Tn recent years, it has frequently been observed that the toxicity of A herbicides can be affected markedly by the presence of various types of surface-active agents (10, II, 12, 13). Jansen (II, 12) has paid particular attention to the effect of the structure of the surface-active agent on activity and thus implicitly, to that measure of the structure known as hydrophile-lipophile balance (HLB) (I). However, we do not believe that the explicit correlation has been shown to exist. 

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