Nonionic surfactants, description

The description of a mixed adsorption layer of ionic and nonionic surfactants requires the appropriate adsorption isotherms. For example, the Frumkin isotherm gives... 

Florence (1983) provide a comprehensive reference for the use of surfactants in drug formulation development. The treatment by Florence (1981) of drug solubilization in surfactant systems is more focused on the question at hand and provides a clear description of surfactant behavior and solubilization in conventional hydrocarbon-based surfactants, especially nonionic surfactants. This chapter will discuss the conventional surfactant micelles in general as well as update the reader on recent practical/commercial solubilization applications utilizing surfactants. Other uses of surfactants as wetting agents, emulsiLers, and surface modiLers, and for other pharmaceutical applications are nc emphasized. Readers can refer to other chapters in this book for details on these uses of surfactant Polymeric surfactant micelles will be discussed in Chapter 13, Micellization and Drug Solubility Enhancement Part II Polymeric Micelles. 

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. 

General Description A viscous, liquid, nonionic surfactant... 

The expression o = Oq- kTJ, with J given in Table 5.2, can be used for description of both static and dynamic surface tension of ionic and nonionic surfactant solutions. The surfactant adsorption isotherms in this table can be used for both ionic and nonionic surfactants, with the only difference that in the case of ionic surfactant the adsorption constant K depends on the subsurface concentration of the inorganic counterions see Equation 5.48 below. 

In the second case, the mathematical descriptions of the adsorption layer of ionic and nonionic surfactants qualitatively do not differ. In this section, the distribution of adsorption of ionic surfactants over the weakely retarded bubble surface at Re l, Re l is analysed. In Section 9.3., the structure of the stagnant cup of rising bubbles at Re l is discussed. Conditions required for the formation of different regimes of dynamic adsorption layers at rising bubbles will be considered in Section 9.4. 

One of the major steps to enhancing the understanding of surfactant association structures is to investigate the phase equilibria of water-oil-surfactant mixtures. Since the pioneering contributions of P. Ekwall in Scandinavia and K. Shinoda in Japan, who analyzed ternary systems containing ionic and nonionic surfactants, respectively, extensive experimental work has been devoted to different kinds of microemulsion systems [7-20]. During recent decades, detailed descriptions of the phase behavior of many ternary (water-oil-surfactant), quaternary (water-oil-surfactant-alcohol), and even quinary (water-oil-surfactant-alcohol-salt) systems have been presented (for example, see Refs. 21-36). These studies have provided evidence for several new phases where the surfactant creates surfaces. Thus, in addition to bicontinuous microemulsions, one finds dilute lamellar phases (L ) [37-43] and liquid isotropic phases of randomly connected bilayers called... 

To form a microemulsion three ingredients are necessary polar solvent (water), apolar solvent (oil), and surfactant. Since typical microemulsions only occur under rather selective circumstances it is in practice necessary to have an additional tuning variable that can be adjusted to obtain optimal conditions for microemulsion formation. In the early studies of Schulman et al. (3) the amount of cosurfactant was used to tune the systems in addition to the salt concentration. This introduces a fourth (cosurfactant) and sometimes a fifth (salt) component, making the ther-modynamic description nearly intractable. Below we illustrate the basic principles by staying with three-component systems, using the temperature as the tuning variable. This situation is most easily realized in practice with nonionic surfactants of the type, where E denotes an ethylene oxide unit. 

The situation is still more complex in the presence of surfactants. Recently, a self-consistent electrostatic theory has been presented to predict disjoining pressure isotherms of aqueous thin-liquid films, surface tension, and potentials of air bubbles immersed in electrolyte solutions with nonionic surfactants [53], The proposed model combines specific adsorption of hydroxide ions at the interface with image charge and dispersion forces on ions in the diffuse double layer. These two additional ion interaction free energies are incorporated into the Boltzmann equation, and a simple model for the specific adsorption of the hydroxide ions is used for achieving the description of the ion distribution. Then, by combining this distribution with the Poisson equation for the electrostatic potential, an MPB nonlinear differential equation appears. 

Prilling, where enzyme powders are suspended in molten nonionic surfactants and later spray cooled into particles, is no longer in use. It was a preferred process in the early days of industrial enzymes for detergents but now more strict demands to dust levels of enzyme granulates have made them obsolete. The term prills is still used although, as a general description of enzyme granulates, but it is technically not correct. 

Surfactant adsorption theories are based on different physical and geometrical models of the adsorbed layer, resulting in a variety of surface equations of state or equivalently in several different adsorption isotherms. The usual approach in the theoretical description of the adsorption of ionic surfactants is the generalization of an adsorption isotherm (or equation of state) of nonionic surfactants by incorporating the electrostatic contribution in the adsorption free energy [4, 5, 6, 7, 8]. The validity of the ionic models derived is usually tested by applying the models for the description of the surface... 

With this type of nonionic surfactant, the presence of a cosurfactant is not needed to obtain a large microemulsion single-phase domain at a fixed temperature. Moreover, by varying the relative volumes of the polar and apolar parts of the surfactant, it is easy to modify the hydrophilic/hydrophobic balance (expressed as the spontaneous curvature //q, surfactant packing parameter Pq, or HLB balance R). These quantities can be modified by temperature variations only because the hydration of the polar head is temperature-dependant. Therefore, the spontaneous curvature turns from oil to water as the temperature is increased. Shinoda, Kunieda and co-workers, as well as Kahlweit s group, have given a general description of the phase behaviour obtained with C/Ey species (49). 

The stability/instability of any agrochemical dispersion is determined by the balance of three main forces (i) Van der Waals attraction that is universal for all disperse systems and which results mainly from the London dispersion forces between the peu--ticles or droplets, (ii) Double layer repulsion that arises when using ionic surfactants or polyelectrolytes, (iii) Steric repulsion that arises when using adsorbed nonionic surfactants or polymers. A description of these three interaction forces is first given and this is followed by a combination of these forces and discussion of the theories of colloid stability. The latter can account for the stability/instability of the various dispersions. 

Description. These surfactants are formed by the reaction of sodium chloracetate with ethoxylated alcohols. Due to the addition of ethoxylated groups, ether carboxylates are more soluble in water and less sensitive to water hardness compared to conventional soaps. Also, keeping the best properties of nonionic surfactants, they do not exhibit any cloud point and show good wetting and foam stability. 

When nonionic surfactants, like those having polyoxiethylene moieties, are adsorbed at the film or particle surfaces, the formed polymer brushes give rise to a steric interaction between such two surfaces [4,284]. Its quantitative description is reviewed in Sec. VI.D. 

Full descriptions of the hydrophile-lipophile balance (HLB) concept are given by Becher and Becher and Schick. Griffin first defined the affinity of a nonionic surfactant in terms of an empirical quantity, the HLB. Surfactants are assigned an HLB number at 25 °C on a scale of 1 to 20, where low HLB numbers represent lipophilic surfactants and high HLB numbers represent hydrophilic surfactants. Generally, the application of a surfactant can be derived from its HLB number in accordance with Table 6.1. ... 

In contrast to the Detergent Laws, the general water legislation does not cover the anionic and nonionic surfactants specifically, but it covers surfactants in the same way as any other components present in effluent however, this is not done via product-specific analytical methods, but within the ambit of the particular requisite parameters, or at least via general parameters such as, for example COD [chemical oxygen demand] or DOC [dissolved organic carbon]. However, this is then no longer specific to surfactants, so that a further description is unnecessary. 

The rate constants for the reaction of a pyridinium Ion with cyanide have been measured in both a cationic and nonlonic oil in water microemulsion as a function of water content. There is no effect of added salt on the reaction rate in the cationic system, but a substantial effect of ionic strength on the rate as observed in the nonionic system. Estimates of the ionic strength in the "Stern layer" of the cationic microemulsion have been employed to correct the rate constants in the nonlonic system and calculate effective surface potentials. The ion-exchange (IE) model, which assumes that reaction occurs in the Stern layer and that the nucleophile concentration is determined by an ion-exchange equilibrium with the surfactant counterion, has been applied to the data. The results, although not definitive because of the ionic strength dependence, indicate that the IE model may not provide the best description of this reaction system. 

Equation (11) can be used to evaluate AG C from readily available CMC values. Note that setting m = 0 for ionic micelles is equivalent to reverting from Reaction (B) to (A) for a description of micellization. The AG c value calculated in this case describes the contribution of the surfactant alone without including the contribution of counterion binding. Since m = 0 for nonionics, the surfactant contribution alone is useful when comparisons between ionic and nonionic micelles are desired. 

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