Surfactants in aqueous systems

Surfactants form the basis of procedures for the extraction of proteins from biological membranes and for the preparation of liposomes. The exclusion or removal of low concentrations of surfactants from solutions following their use can, however, be tedious and difficult (e.g. dialysis). Low concentrations of surfactants can also denature proteins when extracting biological membranes. By combining ferrocenyl surfactants and electrochemical methods, we have established principles for the creation of surfactant molecules with lifetimes of 10 " -10 s in localized regions of solutions, thereby providing new principles for the use of surfactants in aqueous systems in which their sustained or widespread presence is detrimental [3]. 

One of the main characteristics of the behavior of surfactants in aqueous systems is their tendency to form larger aggregates (micelles) when a certain concentration is reached (critical micelle concentration, CMC). The CMC is specific for each given tenside at a given temperature, but the values given in the literature should be viewed with caution as it is also known that the size of the surfactant aggregates increases with concentration. When different species of surface-active substances are present in a solution, mixed micelles are formed and the CMC of the blend is different from that of each individual surfactant. 

Sabo, M., J. Gross, I. E. Rosenberg, Anionic surfactants in aqueous systems via Fourier transform IR spectroscopy,/. Soc. Cosmet. Chem., 1984,35,207-220. 

Sander, S., G. Henze, AC-voltammetric determination of the total concentration of nonionic and anionic surfactants in aqueous systems. Electroanalysis, 1997, 9, 243-246. 

However, coalescence of the foam may occur. In aqueous systems, this may be prevented by adding surfactants to lower the surface tension. With organic solvents, this is not as facile. Hence there may be limits to applicability. For unstable gas/liquid dispersions, the micro devices described here may only be used for shortterm contacting. 

Surfactants employed for w/o-ME formation, listed in Table 1, are more lipophilic than those employed in aqueous systems, e.g., for micelles or oil-in-water emulsions, having a hydrophilic-lipophilic balance (HLB) value of around 8-11 [4-40]. The most commonly employed surfactant for w/o-ME formation is Aerosol-OT, or AOT [sodium bis(2-ethylhexyl) sulfosuccinate], containing an anionic sulfonate headgroup and two hydrocarbon tails. Common cationic surfactants, such as cetyl trimethyl ammonium bromide (CTAB) and trioctylmethyl ammonium bromide (TOMAC), have also fulfilled this purpose however, cosurfactants (e.g., fatty alcohols, such as 1-butanol or 1-octanol) must be added for a monophasic w/o-ME (Winsor IV) system to occur. Nonionic and mixed ionic-nonionic surfactant systems have received a great deal of attention recently because they are more biocompatible and they promote less inactivation of biomolecules compared to ionic surfactants. Surfactants with two or more hydrophobic tail groups of different lengths frequently form w/o-MEs more readily than one-tailed surfactants without the requirement of cosurfactant, perhaps because of their wedge-shaped molecular structure [17,41]. 

When the variation of any colligative property of a surfactant in aqueous solution is examined, two types of behavior are apparent. At low concentrations, properties approximate those to be expected from ideal behavior. However, at a concentration value that is characteristic for a given surfactant system (critical micelle concentration, CMC), an abrupt deviation from such behavior is observed. At concentrations above the CMC, molecular aggregates called micelles are formed. By increasing the concentration of the surfactant, depending on the chemical and physical nature of the molecule, structural changes to a more... 

The effect of surfactant on enantioselective hydrogenation has been thoroughly investigated. Rhodium complexes of phosphinated glucopyranosides were used for hydrogenation of prochiral dehydroaminoacid derivatives in aqueous systems in the presence of sodium dodecylsulfate (SDS)... 

Figure 9 CL response curves from the oxidation of H202 with sodium hypochlorite in the presence of fluorescein and surfactants. (1) Aqueous system without fluorescein (2) aqueous system (3) CSDS = 2.0 X 1(L2 M (4) CBrij.35 = 4.2 X 1CT3 M (5) Chtac = 3.0 X lO"3 M CH2o2 = 1.5 X 10-4 M CNa0a = 2.0 X KT3 M Cfluolescdn = 2.7 X KT4 M Cnsoh — 0.05 M. (From Ref. 39 with permission.)...

Surfactants are produced on very large or medium technical scales. Their analysis by manufacturers in products and their formulations sometimes may be complicated because of the great variety of surfactants [5]. After use as directed in aqueous systems they were discharged mainly with wastewaters. Their analysis in environmental samples then becomes quite difficult because analysis must be performed at trace concentrations with limited sample amounts after essential matrix-dependent pre-concentrations steps. In addition, homologues and isomers that exist for many surfactants, besides metabolites which are generated in biochemical processes, complicate their specific determination [6]. 

These types of surfactants have the largest share of world-wide total production [14] and the highest application rates in household, trade and industrial processes. As these compounds were handled mainly in aqueous systems, they were consequently discharged with the waste-water. After a more or less efficient wastewater treatment (WWT) process, which results in an elimination, i.e. degradation by wastewater biocoenosis and/or adsorption of the surfactants to the sludge, these polar compounds and their metabolites either reach the environment by wastewater discharges or are adsorbed to wastewater sludge,... 

As shown in Table 5.5.1,15% of the silicone surfactants annually used were disposed of via wastewater treatment plants [6], but no studies have addressed their fate or persistence in this environmental compartment. Due to the hydrolytic instability and tendency for sorption to surfaces, it is generally thought that limited persistence of the parent molecule in aqueous systems should occur. Consequently more attention has been focused on interactions with solid media such as that resulting from direct application as agricultural adjuvants, and in re-use of sludge. Increased water solubility for the degradation products of trisiloxane surfactants has, however, been observed [10,12,15], demonstrating the need to also monitor the... 

Over the last years the utilisation of supramolecular arrays of surfactant molecules as structure-directing templates [1] has been applied to the synthesis of numerous mesostructured aluminophosphates [2-11]. In most cases the preparations were carried out in aqueous systems under hydrothermal conditions, but tetraethylene glycol and/or unbranched primary alcohols were also used [2,4]. Several discussions have been made on the reaction mechanisms that are involved in the syntheses of mesostructured materials [1,12-15] and recently a number of in-situ investigations on the formation processes of mesostructured silica phases in aqueous media have been reported these studies employed small angle X-ray diffraction [16-19] as well as 2H, 13C, 29Si, and 8lBr NMR spectroscopy and polarised light optical microscopy [17]. 

Figure 9 CL response curves from the oxidation of H202 with sodium hypochlorite in the presence of fluorescein and surfactants. (1) Aqueous system without fluorescein (2)...

As with normal hydrocarbon-based surfactants, polymeric micelles have a core-shell structure in aqueous systems (Jones and Leroux, 1999). The shell is responsible for micelle stabilization and interactions with plasma proteins and cell membranes. It usually consists of chains of hydrophilic nonbiodegradable, biocompatible polymers such as PEO. The biodistribution of the carrier is mainly dictated by the nature of the hydrophilic shell (Yokoyama, 1998). PEO forms a dense brush around the micelle core preventing interaction between the micelle and proteins, for example, opsonins, which promote rapid circulatory clearance by the mononuclear phagocyte system (MPS) (Papisov, 1995). Other polymers such as pdty(sopropylacrylamide) (PNIPA) (Cammas etal., 1997 Chung etal., 1999) and poly(alkylacrylicacid) (Chen etal., 1995 Kwon and Kataoka, 1995 Kohorietal., 1998) can impart additional temperature or pH-sensitivity to the micelles, and may eventually be used to confer bioadhesive properties (Inoue et al., 1998). 

A number of experimental techniques by measurements of physical properties (interfacial tension, surface tension, osmotic pressure, conductivity, density change) applicable in aqueous systems suffer frequently from insufficient sensitivity at low CMC values in hydrocarbon solvents. Some surfactants in hydrocarbon solvents do not give an identifiable CMC the conventional properties of the hydrocarbon solvent solutions of surfactant compounds can be interpreted as a continuous aggregation from which the apparent aggregation number can be calculated. Other, quite successful, techniques (light scattering, solubilization, fluorescence indicator) were applied to a number of CMCs, e.g., alkylammonium salts, carboxylates, sulfonates and sodium bis(2-ethylhexyl)succinate (AOT) in hydrocarbon solvents, see Table 3.1 (Eicke, 1980 Kertes, 1977 Kertes and Gutman, 1976 Luisi and Straub, 1984 Preston, 1948). 

As the use of cocamidopropyl betaine increased as a secondary surfactant in anionic systems, the relatively low concentration of about 35% nonvolatiles at which it is normally sold became an issue. At this concentration, betaines are somewhat susceptible to bacterial growth so a preservative is often needed and the low concentration also increases freight costs so that several patented technologies were developed to address this [7]. Typically, the inclusion of about 2% of one of the patented additives allows the producers to prepare an aqueous solution of 45% nonvolatiles which is hostile to microbial growth without... 

The most important property of micelles in aqueous or nonaqueous solvents is their ability to dissolve substances that are insoluble in the pure solvent. In aqueous systems, nonpolar substances are solubilized in the interior of the micelles, whereas polar substances are solubilized in the micellar core in nonaqueous systems. This process is called solubilization. It can be defined as the formation of a thermodynamically stable isotropic solution with reduced activity of the solubilized material (8). It is useful to further differentiate between primary and secondary solubilization. The solubilization of water in tetrachloroethylene containing a surfactant is an example of primary solubilization. Secondary solubilization can be considered as an extension of primary solubilization because it refers to the solution of a substance in the primary solubilizate. 

Whether or not this is the true explanation for the retardation of initiator decomposition by surfactant, the fact that the surfactant behaves in this way explains why the rate of the aqueous-phase polymerisation of butadiene passes through a maximum as the level of surfactant in the system is Increased. 

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