Surfactant molecule distribution

Fig Schematic representation of surfactant molecules distributed in water (A) completely dissolved at low concentration and... 

The model of the micelle formation in which some surfactant molecules lie flat on the graphene SWNT surface along the tube axis is preferable than the model with surfactant molecules distributed uniformly around the tube with hydrophobic chains close to the SWNT surface. 

W. Gozdz, R. Holyst. Distribution functions for H nuclear magnetic resonance band shapes for polymerized surfactant molecules forming triply periodic surfaces. J Chem Phys 706 9305-9312, 1997. 

The expected surfactant distribution is also portrayed qualitatively in Figure 2. At low Ca, recirculation eddies in the liquid phase lead to two stagnation rings around the bubble, as shown by the two pairs of heavy black dots on the interface (18>19). Near the bubble front, surfactant molecules are swept along the interface and away from the stagnation perimeter. They are not instantaneously replenished from the bulk solution. Accordingly, a surface stress, rg, develops along the interface... 

When surfactant molecules contain more than one distribution, for example, a distribution of chain lengths in the hydrophobic and hydrophilic portions, two-dimensional liquid chromatography (2DLC) is a very powerful method for complete analysis. One can get the full quantitation of the distribution of molecular size by using the 2DLC technique. For example, take a surfactant molecule like alcohol ethoxylates (AE s) having a general structure of... 

The components of a technical cocamidopropyl betaine (CAPB Fig. 2.13.1) mixture were separated under reversed phase conditions in the order of increasing length of the alkyl chain (Fig. 2.13.2) [1]. Since the hydrophobic moiety of the surfactant molecule is derived from coconut oil, the two homologues Ci2 and C14 form the major constituents according to the distribution in the natural raw product containing approximately 49% of Ci2- and 19% of C14-fatty acid [2],... 

The diffusion coefficients obtained with another fhiorophore (NBD derivative) were slightly different. The values of the aggregations numbers were found to be often overestimated because incorporation of the fluorescent probe may require extra surfactant molecules. However, the relative size differences between the micelles are in good agreement with the values reported in the literature. In addition to the size of micelles, FCS can give information on the size distribution. 

These routes involve the formation of (usually) prereduced metal particles that are then adsorbed or deposited onto the support. They have the advantage that the particle size of the particles is predetermined by the chemistry of the colloids and that resulting catalysts have narrow particle size distributions. However, the colloidal particles often are surface stabilized by surfactant molecules, which can be difficult to remove once the particles are adsorbed onto the support. One further disadvantage is that the colloidal particles are prepared at high dilution (typically millimolar concentrations— for example, 0.2 g Ft 1 ), which is a disadvantage in terms of scale-up. 

Interfacial turbulence [60] Due to a nonuniform distribution of surfactant molecules at the interface or to local convection currents close to the interface, interfacial tension gradients lead to a mechanical instability of the interface and therefore to production of small drops. 

We commence with the adsorption of nonionic surfactants, which does not require the consideration of the effect of the electrical double layer on adsorption. The equilibrium distribution of the surfactant molecules and the solvent between the bulk solution (b) and at the surface (s) is determined by the respective chemical potentials. The chemical potential /zf of each component i in the surface layer can be expressed in terms of partial molar fraction, xf, partial molar area a>i, and surface tension y by the Butler equation as [14]... 

When a single surfactant species is introduced in a surfactant-oil-water (SOW) system, its molecules distribute at the interface and in the bulk liquid phases in different amounts, but since there is only a single species, the nature of the substance present at interface and in the phases is the same. 

Very recently, ESR techniques have been employed to study the packing of surfactant molecules at the oil/water interface in w/o HIPEs [102,103], By including an amphiphilic ESR probe, which is adsorbed at the oil/water interfaces, it is possible to determine the microstructure of the oil phase from the distribution of amphiphiles between the films surrounding the droplets and the reverse micelles. It was found that most of the surfactant is located in the micelles, over a wide range of water fraction values. However, when the water content is very high (water droplets of the emulsion, to stabilise the large interfacial area created. 

Although there are some aspects of micellization that we have not taken into account in this analysis —the fact that n actually has a distribution of values rather than a single value, for example —the above discussion shows that CMC values expressed as mole fractions provide an experimentally accessible way to determine the free energy change accompanying the aggregation of surfactant molecules in water. For computational purposes, remember Equation (3.24), which states that x2 n2/n, for dilute solutions. This means that CMC values expressed in molarity units, [CMC], can be converted to mole fractions by dividing [CMC] by the molar concentration of the solvent, [solvent] x2 [CMC]/[solvent] for water, [solvent] = 55.5 mole liter... 

Emulsion polymerization is applicable only to monomers that are relatively insoluble in water, such as styrene. A coarse emulsion of monomer in aqueous surfactant is prepared with a water-soluble initiator, say, H202 in the solution. The surfactant concentration is above the CMC, so surfactant molecules are present as monomers, micelles, and emulsifiers at the oil-water interface. Even an insoluble liquid like styrene dissolves in water to some extent. Therefore the monomer is present in coarse emulsion drops, solubilized in micelles, and as dissolved molecules in water. A schematic illustration of the distribution of surfactant, monomer, and polymer in an emulsion polymerization process is shown in Figure 8.14. 

Synthesis of solid state materials using surfactant molecules as template has been extensively used in this decade. Among the advantages of the use of amphiphilic molecules, the self-assembling property of the surfactants can provide an effective method for synthesising ceramic and composite materials with interesting characteristics, such as nanoscale control of morphology, and nano or mesopore structure with narrow and controllable size distribution [1-5]. 

Micelles are colloidal dispersions that form spontaneously, under certain concentrations, from amphiphilic or surface-active agents (surfactants), molecules of which consist of two distinct regions with opposite afL nities toward a given solvent such as water (Torchilin, 2007). Micelles form when the concentration of these amphiphiles is above the critical micelle concentration (CMC). They consist of an inner core of assembled hydrophobic segments and an outer hydrophilic shell serving as a stabilizing interface between the hydrophobic core and the external aqueous environment. Micelles solubilize molecules of poorly soluble nonpolar pharmaceuticals within the micelle core, while polar molecules could be adsorbed on the micelle surface, and substances with intermediate polarity distributed along surfactant molecules in intermediate positions. 

In micellar systems, nonpolar molecules are solubilized within the internal micelle hydro-phobic core, polar molecules are adsorbed on the micelle surface and substances with intermediate polarity are distributed along surfactant molecules in intermediate positions. 

In any evaluation of a remediation scheme utilizing surfactants, the effect of dose on HOC distribution coefficients must be quantified. Very often, only one coefficient value for HOC partitioning to sorbed surfactants has been reported in the literature, presumably because the experimental data covers only the sorption regions where the surfactant molecule interactions dominate at the surface (Nayyar et al., 1994 Park and Jaffe, 1993). However, all of the characteristic sorption regions will develop during an in-situ SEAR application as the surfactant front (i.e., mass transfer zone) advances through the porous medium. Therefore, the relative role ofregional HOC partition coefficients to sorbed surfactant should be considered in any remediation process. Finally, the porosity or solid volume fraction for the particular subsurface system must be taken into account when surfactant sorption is quantified. 

A potential limitation of surfactant-enhanced desorption is the observation that sorbed surfactant molecules can increase the sorption of hydrophobic organic contaminants (Edwards et al. 1994 Sun et al. 1995 Ko et al. 1998). Sun et al. (1995) reported that the nonionic surfactant Triton X-100 increased the sorption of p,p -DDT, 2,2 ,4,4 ,5,5 -PCB, and 1,2,4-trichlorobenzene to a soil (joc= 0.001) at concentrations below CMC. At concentrations above CMC, the distribution coefficients (Kp) of the DDT and PCB studied were reduced to levels below their respective values in pure water. However, at a surfactant concentration of five times CMC, the Kp of 1,2,4-trichlorobenzene was still a factor of three higher than Kp in pure water. Edwards et al. (1994) and Ko et al. (Ko et al. 1998) reported similar results for different groups of surfactants. 

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