Classification of surfactants

American Society for Testing and Materials, Standard for Qualitative Classification of Surfactants by Infrared Absorption, D2357-74 (Reapproved 1989). 1916 Race St., Philadelphia, PA 19103. 

Particularly useful is the physical classification of surfactants based on the hydrophile-lipophile balance (HLB) system [67,68] established by Griffin [69,70]. More than 50 years ago he introduced an empirical scale of HLB values for a variety of nonionic surfactants. Griffin s original concept defined HLB as the percentage (by weight) of the hydrophile divided by 5 to yield more manageable values ... 

The classification of surfactants in common usage depends on their electrolytic dissociation, which allows the determination of the nature of the hydrophilic polar group, for example, anionic, cationic, nonionic, and amphoteric. As reported by Greek [18], the total 1988 U.S. production of surfactants consisted of 62% anionic, 10% cationic, 27% nonionic, and 1% amphoteric. 

In this overview, I start with the general classification of surfactants and their unusual properties. This is followed by some examples to illustrate the application of surfactants in some chemical industries. 

A simple classification of surfactants based on the nature of the hydrophilic group is commonly used. Four main classes may be distinguished, namely, anionic, cationic, zwitterionic, and nonionic. A useful technical reference is McCutchen. Another useful text, by van Oss et al., gives a list of the physicochemical properties of selected anionic, cationic, and nonionic surfactants. The handbook by Porter is also a useful book for classification of surfactants. Another important class of surfactants, which has attracted considerable attention in recent years, is the polymeric type. A brief description of the various classes is given below. 

Classification of Surfactants. Surfactants are classified according to the nature of the polar (hydrophilic) part of the molecule, as illustrated in Table III. In-depth discussions of surfactant structure and chemistry can be found in references 33-35. 

Surfactant additives have been studied intensively in recent years because of the self-reparability or self-assembly of their micro structures after degradation by mechanical or extensional stresses. This ability has led to many studies of their applications in DHC recirculation systems. Classifications of surfactant DR As and their self-assembly nature are described. Also discussed in this section are the main research results on microstructures, rheological properties, HTR of surfactant DR solutions, and approaches to enhance heat transfer coefficients. Significant field tests around the world are reviewed. 

Cetyltrimethylammonium bromide CH3(CH2)i5N(CH 3)381 is a typical cationic surfactant. Dissociation of this compound results in amphiphilic cations and small hydrophilic anions. This classification of surfactants is similar to the classification of strongly interacting compounds (other than surfactants) in Sections II and III into cations (Section II A), anions (Section II B) and electroneutral and zwitterionic organic compounds (Section III). The adsorption of anionic surfactants is indeed enhanced, when the adsorbent carries high positive surface charge (at low pH for materials hsted in Tables 3.1, and 3.3-3.5), and adsorption of cationic surfactants is more pronounced at high pH, and the adsoiption of nonionic surfactants is often rather insensitive to the pH. However, the mechanisms of surfactant adsorption, and experimentally observed adsorption isotherms of surfactants are very different from the compounds discussed in Sections II and III. 

The classification of surfactants into denaturing and nondenaturing poses the question as to the origin of the distinction between the two types of behavior. Since all surfactants have an amphipathic structure, why do synthetic ionic surfactants denature proteins while natural ionics and synthetic nonionics do not The answer must clearly... 

Yang, S. and Khaledi, M.G, Chemical selectivity in micellar electrokinetic chromatography characterization of solute-miceUe interactions for classification of surfactants. Anal. Chem., 61, 499, 1995. 

The most common and widely accepted classification of surfactants is based on the nature of the hydrophilic (head) group of the surfactants. They can belong to either of the following classes ... 

The most popular classification of surfactants is based on its ionic characteristics they are either anionic, cationic, or nonionic. Since ions are normally found in aqueous fluids, ionic characteristics of surfactants are found in the hydrophilic head thus, an anionic surfactant would have negatively charged species in the hydrophilic heads. Because of the requirement of charge neutrality in an overall fluid system, an anionic surfactant would have its positive counterion within its vicinity. This counterion is subject to various ion-exchange mechanisms either with other types of counterions in the fluid or on solid surfaces. It is possible for nonionic species to be hydrophilic, because of their polar and hydrogen-bonding interactions with water molecules. 

The role of surfactants in stabUizing emulsions, as well as the relationship between demulsifier structure and performance, has been identified for over 50 years [J9]. The classification of surfactants as well as demulsifiers is quite arbitrary, but a commonly used one is based on chemical structure [20, 21], Chemical types include nonionic, anionic, and cationic. A brief summary of the evolution in demulsifier chemistry over the years and the effective concentration range is presented in Table 1. The development of chemicals which are more surface active has allowed for reductions in the average dosages. 

There are many other tests for the classification of surfactants, and Rosen and Goldsmith [4] give a comprehensive selection. However, it is usually more convenient to proceed to a separation by ion exchange and examine the isolates for the four main classes. 

Table 1 Classification of surfactant additives according to the interface at which they are active...

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