Surface-active agent definition

The amount of surface-active agent present may be so small that no measurable change in any physical property, including interfacial tension, can be detected. This is particularly true if the agent is a finely divided solid (El). Lindland and Terjesen (L4) showed that, after a definite but small concentration of surfactant had been used, further additions caused but little change in terminal velocity. 

Emulsions are colloidal dispersions of liquid droplets in another liquid phase, sometimes stabilized by surface active agents. Emulsions thus consist of a discontinuous phase, dispersed in a continuous phase. The most common types of emulsions are water-in-oil (W/O) in which oil is the continuous phase, and oil-in-water (OAV) in which water forms the continuous phase. However, this traditional definition of an emulsion is too narrow to include most food emulsions. For example, in foods the dispersed phase may be partially solidified, as in dairy products or the continuous phase may contain crystalline material, as in ice cream. It may also be a gel, as in several desserts. In addition to this, air bubbles may have been incorporated to produce the desired texture. 

However there is still a great discussion about the definition of their representative properties and even about the proper meaning of the word "microemulsion". Indeed such a name would indicate a dispersed system while microemulsions show the appearence of true solutions,i.e., of homogeneous systems. Since an essential requisite for the existence of a microemulsion is the presence of water (l), we think that a study of the fundamental properties of such systems should require the use of experimental approaches specifically apt to reveal "in primis" the behavior of water. Moreover,besides of course the hydrocarbon,being the other components necessary for the existence of a microemulsion amphiphilic compounds acting as surface active agents, also techniques suitable for the study of systems with a high surface-to-volume ratio, are requested. 

When the acid number for a cmde oil is known, we want to estimate how much soap can be generated, assuming (1) the required alkali is available, which is generally trae and (2) the total surface-active agents are converted into soap, which is generally not tme, as discussed in Section 12.9.2. Based on the definition that acid number (AN) is the amount of potassium hydroxide in milligrams that is needed to neutralize the acids in one gram of oil, the soap concentration, Csoap, in meq/mL is... 

An emulsion is a heterogeneous system, consisting of at least one immiscible liquid intimately dispersed in another in the form of droplets, whose diameter, in general, exceeds 0.1 micron (italics ours) . He has further opined that such systems possess a minimal stability, which may be accentuated by such additives as surface active agents, finely divided solids etc. [1]. The presence of a surface active agent (see below, and also Chapter 2) obviously makes the system tri-component. More recently, Dickinson [2] accepted the traditional definition of an emulsion as an opaque, heterogeneous system of two immiscible liquid phases ( oil and water ) with one of the phases dispersed in the other as droplets of microscopic or colloidal size . In spite of Becher s contention that the dispersed phase is a liquid, it has been commented that the difference between a liquid-in-liquid emulsion and a solid particle dispersion in a liquid is not entirely distinct [2]. Further, in an emulsion, the dispersed phase itself can be an emulsion, so that this multiple emulsion can be of the types water-in-oil-in water or oil-in-water-in-oil [3,4]. We can also have more than one dispersed phase in a continuous phase, e.g. two kinds of aqueous solution in oil for very short periods before collision and coalescence, which is a very important route for synthetic reactions. Examples of the varieties of emulsions relevant to solid particle preparation will be cited and discussed in later Chapters. 

The plate assay method for antibiotics is the most widely used and accepted method employing the diffusion technique. Its advantages lie in its simplicity as to labor and equipment. It has definite disadvantages in that the assay is affected by various salts, surface active agents, and solvents which tend to change diffusion characteristics of the antibiotics. With alterations in the diffusion characteristics the dose response curves of the sample and standard will no longer be parallel and the assay itself would be invalid. 

The term biosurfactant is typically reserved for surface-active agents (surfactants) that are produced by microorganisms, as part of their metabolism [3-8]. However, it is necessary to clarify that there are other surfactants - such as lung surfactants - that are secreted by other species and could also be called biosurfactants. In this part of the chapter we will concentrate on the conventional definition of biosurfactants and the use of microbes to degrade waste biomass to secrete potentially usefiil biosurfactants. 

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