Dilute micellar solutions

The structure of the hydrophobic tail is usually limited to either hydrocarbon or fluorocarbon chains. Compounds possessing a single such tail are the most common, although molecules with several linear or branched tails are known. Dialkyl surfactants such as lecithin (below) are particularly important naturally occurring materials. 

if not all, surfactants are soluble in water, but compounds with an alkyl chain length of C16 or greater become increasingly insoluble. The physicochemical properties of aqueous surfactant systems are a consequence of the tendency of the non-polar groups to avoid contact with 

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In dynamic light scattering (DLS), or photon correlation spectroscopy, temporal fluctuations of the intensity of scattered light are measured and this is related to the dynamics of the solution. In dilute micellar solutions, DLS provides the z-average of the translational diffusion coefficient. The hydrodynamic radius, Rh, of the scattering particles can then be obtained from the Stokes-Einstein equation (eqn 1.2).The intensity fraction as a function of apparent hydrodynamic radius is shown for a triblock solution in Fig. 3.4. The peak with the smaller value of apparent hydrodynamic radius, RH.aPP corresponds to molecules and that at large / Hs,Pp to micelles. 

The phase behaviour established for concentrated aqueous solutions of PEO-PPO-PEO copolymers has its counterpart in PEO/PBO copolymer solutions. A phase diagram for PE058PB0i7PE0M based on tube inversion experiments is shown in Fig. 4.14 (Luo et al. 1992). The hard gel is isotropic under the polarizing microscope and can be characterized as a cubic phase formed from spherical micelles of a similar size to those in the dilute micellar solution. 

T. Imae and S. Ikeda, Intermicellar correlation in light-scattering from dilute micellar solutions of dimethyloleylamine oxide, J. Colloid Interface Sci. 98 (1984) 363-372. 

By careful measurement of line widths, second moments, and the R(8/2) parameter (the ratio of the line width at eighth-height to that at half-height) the neat and middle lyotropic mesophases can be distinguished from each other. The R(8/2) parameter is shown to be sensitive to changes in both phase structure and molecular structure. The NMR parameters of the neat and middle phases are consistent with the structural pictures proposed for these phases (4, 22) but do not define the details of the middle structure. The NMR spectra observed in viscous isotropic mesophases are surprising since they are essentially the same as those obtained from dilute, micellar solutions. This type of spectrum does not appear to be consistent with the proposed structure of this phase. 

Kunieda s group reported numerous viscoelastic worm-like micellar systems in the salt-free condition when a lipophilic nonionic surfactant such as short hydrophilic chain poly(oxyethylene) alkyl ether, C EOni, or N-hydroxyethyl-N-methylaUcanolamide, NMEA-n, was added to the dilute micellar solution of hydrophilic cationic (dodecyltrimethylammonium bromide, DTAB and hexade-cyltrimethylammonium bromide, CTAB) [12-14], anionic (sodium dodecyl sulfate, SDS [15, 16], sodium dodecyl trioxyethylene sulfate, SDES [17], and Gemini-type [18]) or nonionic (sucrose alkanoates, C SE [9, 19], polyoxyethylene cholesteryl ethers, ChEO [10, 20], polyoxyethylene phytosterol, PhyEO [11, 21] and polyoxyethylene sorbitan monooleate, Tween-80 [22]) surfactants. The mechanism of formation of these worm-Hke stmctures and the resulting rheological behavior of micellar solutions is discussed in this section based in some actual published and unpublished results, but conclusions can qualitatively be extended to aU the systems studied by Kunieda s group. 

It is not possible to use viscous solutions as mobile phases in liquid chromatography. Only relatively diluted micellar solutions with spherical micelles will be useful. 

The effect of alcohols on dilute micellar solutions is to decrease the cmc and micelle molecular weight and to increase the micelle ionization. Also, the exchange of detergent ions between micelles and surrounding solution and the micelle formation-dissolution are strongly accelerated upon addition of alcohol (micelles become more labile in presence of alcohol). All these variations can be explained on the basis of the effects associated with the dissolution of alcohol into micelles, leading to mixed alcohol + detergent micelles. 

The effect of alcohol on surfactant mass transfer from bulk solution to the oil/dilute micellar solution interface was studied Various interfacial properties of the surfactant solutions and their ability for displacing oil were determined. For the surfactant-oil-brine systems studied, the interfacial tension (IFT) and surfactant partition coefficient did not change when isobutanol was added to the following systems 0.1% TRS 10-410 in 1.5% NaCl vs. n-dodecane and 0.05% TRS 10-80 in 1.0% NaCl vs. n-octane. On the other hand, the interfacial viscosity, oil drop flattening time (i.e. the time required for an oil droplet to flatten out after being deposited on the underside of a polished quartz plate submerged in the micellar solution) and oil displacement efficiency were influenced markedly by the addition of alcohol. 

In the presence of isobutanol, the oil/dilute micellar solution interface became more fluid and the flattening time decreased from 90 seconds to less than a second or 420 seconds to less than a second, and the final oil saturation decreased from 30% to 5.36% and 11.73% to 1.28% respectively for the two systems mentioned above. Furthermore, it was observed that after the arrival of the oil bank, the AP leveled off for the isobutanol containing systems, whereas it continuously increased for the systems without isobutanol. This observation is consistent with the proposed role of alcohol in lowering the interfacial viscosity and promoting coalescence of oil ganglia in porous media. 

Both sandpacks and Berea cores gave similar results. The results of this study demonstrate the importance of transient phenomena at oil/dilute micellar solution interface for oil displacement process with emphasis on the effect of alcohol and salinity. 

Shear-Thickening in Dilute Micellar Solutions 2.1 Introduction... 

Self-diffusion measurements in dilute micellar solutions... 

In contradiction of this expectation, Denkov and cowoikers [34-36] have shown that p for entry of oil drops into the air-water surface of surfactant solutions is usually essentially independent of the equatorial radii of those drops for submicellar and relatively dilute micellar solutions (where concentrations are <10 x CMC). Systems included dodecane (and other oils) in aqueous salt solutions of sodium dodecylben-zene sulfonate and polydimethylsiloxane oil in sodium dodecyl polyoxyethylene sulfate solutions. Experimental results [36] for the critical applied capillary pressures, p , as a function of equatorial drop radius for the latter system, are presented in Figure 3.7 to exemplify typical behavior. For relatively low surfactant concentrations, pf" is seen to be essentially constant. An exception concerns an extremely high surfactant concentration of 200 x CMC where p became strongly dependent... 

The authors start from the experimentally weU-estabhshed fact that relatively dilute micellar solutions are characterized by two well-separated relaxation processes. They attribute the fast process to the exchange of a surfactant A between aggregates (micelles) Ag and Ag.i as in reaction (3.4), with the rate constants of association (entry), k, and dissociation (exit), kg ... 

Criticism concerning the validity of assumption (2) was shown to be unfounded. Nevertheless, it is noteworthy that Equation 3.9 strictly applies only to nonionic surfactants and dilute micellar solutions. 

The last contribution of Aniansson to micellar d3mamics before his untimely death in 1984 was the derivation of the relationship between the micelle lifetime T and x, in the case of dilute micellar solutions ... 

A micellar solution may be at its most efficient in terms of extraction just above the cmc of the surfactant, so it is possible to work with very dilute micellar solutions (<1% v/v). 

Capillary flow viscometry has been extensively used to investigate micellar solutions of block copolymers. Values of sp/c, where is the specific viscosity, have been reported for dilute micellar solutions of various block and graft copolymers. The effects of temperature and... 

Mesophases also form when the concentration of a surfactant in its micellar solution is increased. When the concentration of the surfactant above cmc is increased, the number of micelles and their size increase, in accord with the mass action model. Dilute micellar solutions are isotropic, but at higher surfactant concentrations, intermicellar interactions produce mesophases which are anisotropic and have a one- or two-dimensional ordering. 

Dilute micellar solutions of surfactants are characterized by two well-separated relaxation times. The molecular origin of the fast relaxation time has been related to a monomer-micelle exchange [181-184]. It was realized later that the relaxation spectra of micellar solutions really exhibit two relaxation times. The theory of Aniansson and Wall [167,185] assumes a stepwise aggregation of surfactant monomers to form micelles [186]. The fast relaxation time is attributed to the exchange of monomeric surfactants between the micelles and the intermicel-lar solution. The slow relaxation time is attributed to micelle formation and breakdown. The theory and its modifications by Kahlweit and co-workers [170-174]... 

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