Surface active impurities

In 1961 Bretherton solved the problem of a long gas bubble, uncontaminated by surface-active impurities, flowing in a cylindrical tube at low capillary numbers, Ca /xU/a (/x is the... 

Problems that can arise in the process include the formation of emulsions during extraction from the presence of surface-active impurities in the filtered broth. The effect of these can be minimized by introducing appropriate surfactants that can also reduce the accumulation of solids in the extraction equipment. In addition, other organic impurities are present that can be coextracted with the penicillin. It has been found that a number of these can be removed by adsorption onto active carbon. 

As for other types of fluid particle, the internal circulation of water drops in air depends on the accumulation of surface-active impurities at the interface (H9). Observed internal velocities are of order 1% of the terminal velocity (G4, P5), too small to affect drag detectably. Ryan (R6) examined the effect of surface tension reduction by surface-active agents on falling water drops. 

Mass transfer rates for skirted bubbles in polyvinyl alcohol solutions have been measured by Guthrie and Bradshaw (G9) and Davenport et al (D4). When a skirt is present the transfer rate increases, but not in proportion to the increase in surface area. Davenport attributes this to the accumulation on the surface of the skirt of surface-active impurities which immobilize the interface and reduce the transfer rate. Presumably transfer rates from skirted bubbles or drops in very pure liquids would be appreciably higher than from fluid particles without skirts. 

There is considerable evidence (D3, G7, PI, P4, SI) that bubbles in liquid metals show the behavior expected from studies in more conventional liquids. Because of the large surface tension forces for liquid metals, Morton numbers tend to be low (typically of order 10 ) and these systems are prone to contamination by surface-active impurities. Figure 8.10a shows a two-dimensional nitrogen bubble in liquid mercury. For experimental convenience, the bubbles studied have generally been rather large, so that there are few data available for spherical or slightly deformed ellipsoidal bubbles in liquid metals. Data... 

In mixed surfactant systems, physical properties such as the critical micelle concentration (cmc) and interfacial tensions are often substantially lower than would be expected based on the properties of the pure components. Such nonideal behavior is of both theoretical interest and industrial importance. For example, mixtures of different classes of surfactants often exhibit synergism (1-3) and this behavior can be utilized in practical applications ( ).In addition, commercial surfactant preparations usually contain mixtures of various species (e.g. different isomers and chain lengths) and often include surface active impurities which affect the critical micelle concentration and other properties. 

The anionic surfactant, sodium dodecylsulfate, SDS, was obtained from Merck, Darmstadt, Federal Republic of Germany. It has a stated purity of 99.99% and was used without further purification. Surface tension measurements gave no minimum in the surface tension at the critical micelle concentration, indicating that the sample did not contain highly surface active impurities. 

For SDS, the reaction proceeded to a reproducible end point rapidly —viz., 1 to 2 minutes—when nonionic surface active impurities such as parent dodecyl alcohol, DOH, were removed by ethyl ether extractions. This impurity effect was verified by adding traces of alkyl alcohol—viz., 1 X 10 9 mole per liter—to purified SDS, whereupon the penetration reaction rate was halved. A possible explanation for this behavior is that formation of an SDS-DOH interfacial complex reduced the SDS activity in the interface and consequently its rate of reaction with the protein monolayer. The reasons for the somewhat slower rate of reaction of Cetab with the protein film are more obscure. The reaction rate did not increase after extracting the detergent repeatedly. Two possible reasons for the time dependence in this case may have been that (1) the ether extraction method was not effective in removing surface active impurities, or (2) because of the greater bulk of the Cetab hydrocarbon chain, Ci6 vs. Ci2 for SDS, more time was required for diffusion and appropriate orientation before complex formation. 

The following two items need to be considered from a practical perspective, especially for ionic surfactants, when measuring the CMC of surfactants (Constantinides and Steim, 1985) (a) surface-active impurities in commercial surfactants, such as SDS, give rise to a minimum in the surface tension-concentration plot, and unless a highly puriLed surfactant is used an approximate value ol the CMC is obtained, and (b) in the dye solubilization method, it is important that the dye and the surfactant are of the same charge, to avoid premicellar association, that is, salt formation between the dye and the surfactant below the true CMC of the surfactant. 

Materials. Sodium dodecylsulfate (SDS) and fully deuterated sodium dodecylsulfate (SDS-d ) were obtained from Sigma and Cambridge Isotope Laboratories respectively, and used as received. The cationic surfactants, dodecyltrimethylammonium chloride (DTAC), dodecyltrimethylammonium bromide (DTAB), and didodecyldimethylammonium bromide (DDAB) were purchased from Eastman Kodak, and purified by repeated recrystallization from an ethanol/acetone solvent pair. Even so, a small amount of surface active impurity was observed in surface tension plots for DTAC. The tetradecyldimethylamine oxide (C14AO) was a commercial sample (Ammonyx MO) obtained from Stepan (Control No. 533-30027). This sample is primarily C14AO, but also contains other chain length components. Sodium chloride (NaCl) was obtained from EM Science and used as received. Water was purified by a three stage Bamstead water purification system. 

Many methods have been developed for the quantitative determination of each class of surfactants. The analysis of commercial surfactants is much more complicated since they may be comprised of a range of compounds within a given structural class, may contain surface-active impurities, may be formulated to contain several different surfactant classes, and may be dissolved in mixed organic solvents or complex aqueous salt solutions. Each of these components has the potential to interfere with a given analytical method so surfactant assays are sometimes preceded by surfactant separation techniques. Both the separation and assay techniques can be highly specific to a given surfactant/solution system. Table 3.4 shows some typical kinds of analysis methods that are applied to the different surfactant classes. 

Because crystal growth is a surface phenomena, it is not surprising that impurities that concentrate at crystal faces will affect the growth rate of those faces and hence the crystal shape. With some surface active impurities, small traces, about 0.01%, are all lhat is required to change crystal habit during crystallization. These impurities can ... 

Each of these surface active impurities has a propensity to adsorb on a specific crystal surface. The change in the specific surface energy. 

Properties of Component Phases The composition and physicochemical properties of both the oil and aqueous phases influence the size of the droplets produced during homogenization (52). Variations in the type of oil or aqueous phase will alter the viscosity ratio, ri ,/ri(-, which determines the minimum size that can be produced under steady-state conditions. The interfacial tension of the oil-water interface depends on the chemical characteristics of the lipid phase, e.g., molecular structure or presence of surface-active impurities, such as free fatty acids, monoacylglycerols, or diacylglycerols. These surface-active hpid components tend to accumulate at the oil-water interface and lower the interfacial tension, thus lowering the amount of energy required to disrupt a droplet. 

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