Surfactant tail

Cosolvents ana Surfactants Many nonvolatile polar substances cannot be dissolved at moderate temperatures in nonpolar fluids such as CO9. Cosolvents (also called entrainers, modifiers, moderators) such as alcohols and acetone have been added to fluids to raise the solvent strength. The addition of only 2 mol % of the complexing agent tri-/i-butyl phosphate (TBP) to CO9 increases the solubility ofnydro-quinone by a factor of 250 due to Lewis acid-base interactions. Veiy recently, surfac tants have been used to form reverse micelles, microemulsions, and polymeric latexes in SCFs including CO9. These organized molecular assemblies can dissolve hydrophilic solutes and ionic species such as amino acids and even proteins. Examples of surfactant tails which interact favorably with CO9 include fluoroethers, fluoroacrylates, fluoroalkanes, propylene oxides, and siloxanes. 

Fig. 1. Representative TEM images of Ag nanoparticles synthesized by the AOT w/c RMs with W = 4.7 at 38.0 C and 34.50MPa. (Scale bar 10 nm). The arrows indicate the self-assembly of the adjacent nanoparticles through the surfactant tails. ...

P. Poulin and J. Bibette Influence of the Alkyl Surfactant Tail on the Adhesion Between Emulsion Drops. Langmuir 15, 4731 (1999). 

Assuming that the internal droplets experience a hard-sphere-like repulsion when surfactant layers come in contact, an estimation of the van der Waals interactions can be obtained from the average length of the surfactant tails (1 3 nm) [20]. The coalescence frequency is therefore the unique free pa-... 

For spherical micelles, one of the more commonly accepted models for the micellar structure is that proposed by Gruen (Fig. 1). This model features a rather sharp interface between a dry hydrophobic hydrocarbon core and a region filled with surfactant headgroups, part of the counterions (for ionic surfactants), backfolding surfactant tails, and water, namely, the Stern region. In the remainder of this chapter, intramicelle volumes with specific features such as the Stern region and the hydro-phobic core will be referred to as zones. ... 

Typical radii for spherical micelles (related to the length of a typical surfactant tail) are around 5 nm. Aggregation numbers N (surfactant monomers per micelle) are typically 40-100. The fractional counterion binding of micelles [3 generally lies... 

Free water 1 1 1 Bound water 1 1 1 Surfactant tails... 

A full discussion of water penetration into micelles is beyond the scope of this chapter. The results described above, and others employing longer chain keto -surfactants in other micelles and bilayers, indicated a trend toward less water penetration to the core of aggregates as the surfactant tail length increased, and as aggregate curvature decreased (bilayer formation). More data from FT-IR studies... 

Figure 17 presents the results of an FTIR spectroscopic study of the effect of salt concentration on the cmt of 70 mM SDS. As Mantsch et al. (4) have shown in similar work with alkali hexadecylsulfates, the cmt can be related to the sudden increase in frequency of the v9 CH2 bands as a function of temperature. The large increase in the gauche conformer content of the methylene chains of the surfactant tails as they "melt" at the cmt is responsible for this frequency shift. The effect of added salt is to raise the cmt of SDS, which is the cause of the "salting out" of this ionic surfactant at any given temperature. The cmt values, taken as the midpoint of the discontinuities in the frequency-temperature plots, agree well with those obtained by other means (14,54). 

Synonym for the dispersed phase in a water-in-oil type microemulsion. Here the surfactant heads, or polar groups, associate closely to minimize interaction with the oil phase. This close association can happen when they orient themselves inside water droplets, and it also allows the surfactant tails, or hydrocarbon groups, to stabilize the water droplets by orienting toward or into the oil. 

As discussed in Section 2.2, surfactant has a tendency to adsorb at interfaces since the polar head group has a strong preference for remaining in water while the hydrocarbon tail prefers to avoid water. The surfactant concentration affects the adsorption of surfactants at interfaces. Surfactant molecules lie flat on the surface at very low concentration. Surfactant molecules on the surface increase with increasing surfactant concentration in the bulk and surfactant tails start to orient towards gas or non-polar liquid since there is not enough space for the surfactant molecules to lie flat on the surface. Surfactant molecules adsorb at the interface and form monolayer until the surface is occupied at which point surfactant molecules start forming self-assembled structures in the liquid (Section 2.3). 

Surfactants adsorb on solid surfaces due to hydrophobic bonding, electrostatic interaction, acid-base interaction, polarisation of rr electrons and dispersion forces. Hydrophobic bonding occurs between the hydrophobic surfactant tail and the hydrophobic solid surface (tail down adsorption with monolayer structure) or between the hydrophobic tails of the surfactant adsorbed on the hydrophilic solid surface and the hydrophobic tails of the surfactant from the liquid phase (head down adsorption with bilayer structure) [54, 55]. 

The increase in the hydrophilic head group size reduces the amount of adsorbed surfactant at surface saturation. On the other hand, increasing the hydrophobic tail length may increase, decrease or maintain the surfactant adsorption. If the surfactant molecules are not closely packed, the increase in the chain length of the tail increases surfactant adsorption on solid surfaces. If the adsorption of surfactant on the solid surface is due to polarisation of tc electrons, the amount of surfactant adsorbed on the surface reduces at surface saturation. If the adsorbed surfactants are closely packed on the solid surface, increasing the chain length of the surfactant tail will have no effect on the surfactant adsorption. 

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