Surfactants dilute micellar solutions

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resulting rheological behavior of micellar solutions is discussed in this section based in some actual published and unpublished results, but conclusions can qualitatively be extended to aU the systems studied by Kunieda s group. 

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 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. 

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). 

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]... 

On the other hand, micelle formation has sometimes been considered to be a phase separation of the surfactant-rich phase from the dilute aqueous solution of surfactant. The micellar phase and the monomer in solution are regarded to be in phase equilibrium and cmc can be considered to be the solubility of the surfactant. When the activity coefficient of the monomer is assumed to be unity, the free energy of micelle formation, Ag, is calculated by an equation... 

Of the possible types of measurements, heats of micellar mixing obtained from the mixing of pure surfactant solutions are perhaps of the greatest interest. Also of interest is the titration (dilution) of mixed micellar solutions to obtain mixed erne s. While calorimetric measurements have been applied in studies of pure surfactants (6,7) and their interaction with polymers ( ), to our knowledge, applications of calorimetry to problems of nonideal mixed micellization have not been previously reported in the literature. 

This phenomenon can be exploited for separation and concentration of solutes. If one solute has certain affinity for the micellar entity in solution then, by altering the conditions of the solution to ensure separation of the micellar solution into two phases, it is possible to separate and concentrate the solute in the surfactant-rich phase. This technique is known as cloud point extraction (CPE) or micelle-mediated extraction (ME). The ratio of the concentrations of the solute in the surfactant-rich phase to that in the dilute phase can exceed 500 with phase volume ratios exceeding 20, which indicates the high efficiency of this technique. Moreover, the surfactant-rich phase is compatible with the micellar and aqueous-organic mobile phases in liquid chromatography and thus facilitates the determination of chemical species by different analytical methods [104]. 

Since their effective diffusivities are of the same magnitude as those of micellar solutions, the hquid crystalUne phases, though viscous, do not significantly hinder surfactant dissolution for these rather hydrophihc surfactants. Indeed, a drop of Ci2(EO)6 having Ro = 78 pm dissolved completely in only 16 s at 30 °C. Rapid dissolution is favored because free energy decreases as the surfactant is transferred from the Hquid surfactant phase L2 to liquid crystals) to aqueous micellar solution and the aggregate shape approaches that of a dilute Li phase, where its free energy is minimized at this temperature. 

Due to the relatively high viscosity of surfactant vesicle and microemulsion systems (refer to data on DODAB and CTAB/50J BuOH in Table X), their use in HPLC will be limited since lower flow rates would be required which would lengthen the required time for a separation. Additionally, most surfactant vesicular (112) as well as some micellar solutions are optically opaque which limits the wavelength range available for spectroscopic detection unless a postcolumn dilution step is employed (219). 

As discussed in Section 12.3.3, unusual time- and shear-rate-dependencies have been reported for some wormy micellar solutions at dilute concentrations—for example, 1-5 mTAB/NaSal. At higher concentrations, 7-250 mM, of a similar surfactant, tetrade-cyltrimethyammonium bromide in NaSal, the extensional viscosity increases with in-creasing extension rate until a maximum is reached, and extension thinning then follows Thomme and Warr 1994). Prud homme and Warr interpret the maximum as the critical... 

Figure 5. Concentration dependence of equivalent conductivity, at 25°C, of SDS, an ordinary micellar solution, and aged aqueous surfactant (S). One mmol/L of surfactant (S) corresponds to 0.0405 wt %. The critical micelle concentration of SDS is 8 mmol/L. For comparison, equivalent conductivities of sodium chloride and sodium iony at infinite dilution, are shown.

One may also consider a reverse transition from macroscopic heterogeneous system (surfactant crystals in water) to micellar solutions existing above the Krafft point, via gel formation and its spontaneous dispersion stages. In this case, the swelling of soap upon the penetration of water between the lamella formed with polar ( strongly hydrated) groups occurs prior to the formation of colloidal solution. At sufficient dilutions separation of individual particles, e.g. lamella, from crystals occurs due to the... 

Ionic surfactants are strong electrolytes in dilute aqueous solution, and non-ionic surfactants are monomers, but above the so-called critical micelle concentration (cmc) they spontaneously self-associate to form micelles [15]. Micellization in water is an example of the hydrophobic effect at work [18]. The phenomenon is more properly called the solvophobic effect, because it is important in associated solvents which have three-dimensional structure, and normal micelles form in 1,2-diols, or formamide [19] and micelles with a carbocationic head group form in 100% sulfuric acid [20], for example. However, we live in an aqueous world, and most normal micellar systems are studied in water, so we can reasonably retain the term hydrophobic with the hydrophobic bond dictated by water association. 

Most of the traditional adsorption studies of surfactants correspond to dilute systems without aggregation in the bulk phase. At the same time micellar solutions are much more important from a practical point of view. To estimate the equilibrium adsorption, neglecting the effect of micelles can usually lead to reasonable results. However, the situation changes for nonequilibrium systems when the adsorption rate can increase by orders of magnitude when the of surfactant concentration is increased beyond the CMC. Current interest in the adsorption from micellar solutions is mainly caused by recent observations that the stability of foams and emulsions depends strongly on the concentration in the micellar region [1]. This effect can be explained by the influence of the micellisation rate on the adsorption kinetics. 

It is well-established now that the concentration of surfactant ions in micellar solutions changes when the total surfactant concentration c is increased. This leads to changes in the adsorption value and, consequently, to changes in the surface tension. These alterations, however, are small, even for ionic surfactants. For relatively dilute solutions, i.e. c< 10 CMC, as a first approximation one can consider that the monomer concentration ci is constant (ci CMC). Actually, for c > CMC surface tension changes are usually low and in the range of accuracy of conventional methods. This fact evidences an approximate constancy of the adsorption. 

C. Note that the pure surfactant forms a liquid, which is miscible with water over a certain composition/temperature range. The clouding region arises from the partial miscibility of the Ci2EOg micellar solution with water above about 49 °C. This is caused by a net intermicellar attraction arising from EO-EO interactions between adjacent micelles. It is discussed further in [103] and the references therein. Whilst the clouding phenomenon is important for many micellar properties of these surfactants, its only influence on the mesophases is to determine the limit to which they can swell in water, that is it fixes the boundary between mesophases and the dilute aqueous solution (Fig. 14). 

Di-(C7-Glu) also has a biphasic H/Li phase forming at 62 wt% surfactant existing to =60 °C. The H] phase melts over the range 62-80°C. Between the H and 1 phases is another biphasic region between 71 and 75 wt%. This, and the L( phase exist to >90 °C. A two phase loop occurs at low surfactant concentrations and temperatures (<8 wt% and <50 °C). The authors believe this to be the first observation of an upper critical solution temperature for a nonionic surfactant. The shape of the upper consolute loop is extremely unusual suggesting remarkable changes in micellar interactions. .. or the presence of impurities. One would be pleased to see these observations verified by other groups. Di-(Cg-Glu) shows only a L phase but with a wide miscibility gap to a dilute aqueous solution. This is not remarkable. 

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