Phase separation nonionic micellar solutions

The phase separation of nonionic micellar solutions above the cloud point has been succesfully applied to the liquid-liquid extraction of some metal chelate complexes (5, 6J. In these systems the concentration of the analyte takes place in the micellar rich layer, which can be readily analyzed. 

A combination of SLS and DLS methods was used to investigate the behavior of nonionic micellar solutions in the vicinity of their cloud point. It had been known for many years that at a high temperature the micellar solutions of polyoxyethylene-alkyl ether surfactants (QEOm) separate into two isotropic phases. The solutions become opalescent with the approach of the cloud point, and several different explanations of this phenomenon were proposed. Corti and Degiorgio measured the temperature dependence of D pp and (Ig), and found that they can be described as a result of critical phase separation, connected with intermicellar attraction and long-range fluctuations in the local micellar concentration. Far from the cloud point, the micelles of nonionic surfactants with a large number of ethoxy-groups (m 30) may behave as hard spheres. ... 

Non-ionic Surfactants. These surfactants do not present the Krafft phenomenon. However, a nonionic micellar solution becomes turbid and separates in two phases when the temperature is raised. This is the clouding phenomenon. The polyoxyethylene chain, the polar part of most non-ionic surfactants, is progressively dehydrated as the temperature raises. Losing water molecules, the polyoxyethylene ehain becomes less polar and, at a particular temperature, a turbidity, the clouding, appears. This temperature is called the cloudpoint of the nonionic surfactant solution. Above the cloud point, the nonionic micellar solution separates in an aqueous phase saturated by the nonionic surfactant, and an organic phase saturated by water and containing the major part of the surfactant. 

Many different types of interaction can induce reversible phase transitions. For instance, weak flocculation has been observed in emulsions stabilized by nonionic surfactants by increasing the temperature. It is well known that many nonionic surfactants dissolved in water undergo aphase separation above a critical temperature, an initially homogeneous surfactant solution separates into two micellar phases of different composition. This demixtion is generally termed as cloud point transition. Identically, oil droplets covered by the same surfactants molecules become attractive within the same temperature range and undergo a reversible fluid-solid phase separation [9]. 

Provided that the temperature remains below Tc, where the micellar solution Li separates into and L" phases (see Fig. 1), the rate of dissolution of pure nonionic surfactants increases with increasing temperature. For example, dissolution time fo for a drop of Ci2(FO)6 with Ro = 73 pm was 11 s at 35 °C. As indicated above, to is proportional to Rq, so that to would be about 13 s at this temperature if Ro were 78 pm. As indicated in the preceding paragraph, a drop with Ro = 78 pm dissolved more slowly, taking 16 s, when the temperature was reduced to 30 °C. 

Aqueous micellar solutions of many nonionic surfactants, with an increase in temperature, become turbid over a narrow temperature range, which is referred to as their cloud-point [17,277]. Above the cloud-point temperature, such solutions separate into two isotropic phases. Cloud-point extraction (CPE) is also referred to as a particular case of ATPE [278,279] and more specifically as aqueous micellar two-phase systems [10,277]. Very recently, in an extensive review, Quina and Hinze [280] have discussed in detail the emergence of CPE as an environmentally benign separation process, highlighting the basic features, experimental protocols, recent applications, and future trends in this area. 

The purpose of this paper will be to develop a generalized treatment extending the earlier mixed micelle model (I4) to nonideal mixed surfactant monolayers in micellar systems. In this work, a thermodynamic model for nonionic surfactant mixtures is developed which can also be applied empirically to mixtures containing ionic surfactants. The form of the model is designed to allow for future generalization to multiple components, other interfaces and the treatment of contact angles. The use of the pseudo-phase separation approach and regular solution approximation are dictated by the requirement that the model be sufficiently tractable to be applied in realistic situations of interest. 

Assuming that different polymorphisms can be found in the extractant systems, a better understanding also comes from other phase-separation mechanisms studied in classical amphiphilic systems such as soaps and lipids. The first, largely described here, is the phase separation resulting from increased attractive interactions. The second occurs when a sphere-to-rod transition is observed for the shape of the aggregates. The attraction between cylinders is higher than between spheres when attraction is dominated by van der Walls (VdW) forces between polar cores (119). For micellar solutions (reverse or not), the liquid-liquid phase transition cannot be unambiguously attributed to either shape or attractive interactions only, as it appears that these two effects coexist in nonionic surfactants solutions (91, 120-123). 

A series of 4-alkylamido-2-hydroxybenzoic acids containing a different number of carbon atoms in the alkyl-amido group has been studied as model ligands for metal ion extraction in aqueous micellar solutions of nonionic surfactants. Their acid-base properties and reactivity towards metal ions in the presence of micelles were investigated. By operating at a proper temperature, the separation of the iron (III) chelate complexes into a micellar rich phase was achieved and the extraction efficiency was correlated with the ligand hydrophobicity. 

In aqueous solutions the micellar assembly structure allows sparingly soluble or water-insoluble chemical species to be solubilized, because they can associate and bind to the micelles. The interaction between surfactant and analyte can be electrostatic, hydrophobic, or a combination of both [76]. The solubilization site varies with the nature of the solubilized species and surfactant [77]. Micelles of nonionic surfactants demonstrate the greatest ability for solubilization of a wide group of various compounds for example, it is possible to solubilize hydrocarbons or metal complexes in aqueous solutions or polar compounds in nonpolar organic solutions. As the temperature of an aqueous nonionic surfactant solution is increased, the solution turns cloudy and phase separation occurs to give a surfactant-rich phase (SRP) of small volume containing the analyte trapped in micelle structures and a bulk diluted aqueous phase. The temperature at which phase separation occurs is known as the cloud point. Both CMC and cloud point depend on the structure of the surfactant and the presence of additives. Table 6.10 gives the values of CMC and cloud point for the surfactants most frequently applied in the CPE process. 

We have examined the stmcture of both ionic and nonionic micelles and some of the factors that affect their size and critical micelle concentration. An increase in hydrophobic chain length causes a decrease in the cmc and increase of size of ionic and nonionic micelles an increase of polyoxyethylene chain length has the opposite effect on these properties in nonionic micelles. About 70-80% of the counterions of an ionic surfactant are bound to the micelle and the nature of the counterion can influence the properties of these micelles. Electrolyte addition to micellar solutions of ionic surfactants reduces the cmc and increases the micellar size, sometimes causing a change of shape from spherical to ellipsoidal. Solutions of some nonionic surfactants become cloudy on heating and separate reversibly into two phases at the cloud point. 

Crystallization-induced phase separation can occur for concentrated solutions (gels) of diblocks [58,59]. SAXS/WAXS experiments on short PM-PEO [PM=poly(methylene) i.e. alkyl chain] diblocks revealed that crystallization of PEO occurs at low temperature in sufficiently concentrated gels (>ca. 50% copolymer). This led to a semicrystalline lamellar structure coexisting with the cubic micellar phase which can be supercooled from high temperatures where PEO is molten. These experiments on oligomeric amphiphilic diblocks establish a connection to the crystallization behaviour of related nonionic surfactants. 

In our discussion of the effect of temperature on solubilization capacity (Chapter 4, Section 1B7), we mentioned that when the temperature of an aqueous micellar solution Wo of a POE nonionic surfactant is increased, its solubilization of nonpolar material O increases due to the increased dehydration of the POE chains, which increases the lipophilic character of the surfactant. If this occurs for POE nonionics of the proper structure in the presence of excess nonpolar material, the volume of the aqueous phase Wo increases and that of the nonpolar phase oil decreases as the temperature increases (Figure 5-5a,b). This is accompanied by a decrease in the tension yow at the O/W interface. With further increase in temperature, the POE chains become more and more dehydrated, the surfactant becomes more lipophilic, and more and more nonpolar material oil is solubilized into the increasingly asymmetric micelles. When the vicinity of the cloud point of the nonionic is reached the surfactant micelles, together with solubilized material, will start to separate from Wo as a separate phase D. If excess oil is still present, the system now contains three phases (Shinoda, 1968) excess oil a phase D, the... 

Aqueous solutions of many nonionic amphiphiles at low concentration become cloudy (phase separation) upon heating at a well-defined temperature that depends on the surfactant concentration. In the temperature-concentration plane, the cloud point curve is a lower consolution curve above which the solution separates into two isotropic micellar solutions of different concentrations. The coexistence curve exhibits a minimum at a critical temperature T and a critical concentration C,. The value of Tc depends on the hydrophilic-lypophilic balance of the surfactant. A crucial point, however, is that near a cloud point transition, the properties of micellar solutions are similar to those of binary liquid mixtures in the vicinity of a critical consolution point, which are mainly governed by long-range concentration fluctuations [61]. 

Finally, it should be reminded that for micellar solutions of ionic surfactants, the temperature of the column should always be above the Krafft point (e.g., 15°C for SDS), in order to avoid clogging and possibly ruining the column. Nonionic surfactants, instead, have a cloud point temperature at which phase separation occurs (see Chapter 2). In this case, all chromatographic work should be conducted below this temperature. However, this depends on the concentration of surfactant and is usually very high (e.g., approximately 100°C for aqueous 1-6% Brij-35). 

The practical potential of nonionic MLC was demonstrated by the use of micellar solutions of Brij 35 in the analysis of tobacco [18], Samples of smoking tobacco were extracted with an ueous solution of 30% Brij 35, and an aliquot of the extract was chromatographically separated without further preparation, with a 6% Brij 35 mobile phase. Comparison with an aromatic aldehyde standard mixture enabled verification of vanillin and ethylvanillin as two of the extract components. Brij 35 was chosen for this study over other nonionic surfactants (such as Tritons , Spans , Igepals or Tweens ) on the basis of its commercial availability, high purity, low cost, low toxicity, high cloud temperature, and low background absorbance, compared to the other types of surfactants mentioned. Brij 35 does not possess a strong chromophore and its absorption is minimal. 

The solubility characteristics of surfactants are quite different from ordinary salts in water. For instance, the solubility of sodium n-dodecyl sulfate (NaDDS) in water is low, ca. 8 mM, at temperatures lower than 16 °C, while it abruptly increases at temperatures greater than 16 °C. This dependence of solubility on temperature as found for all ionic surfactants, is called the Krafft point. [5]. On the other hand, the solubility of nonionic surfactants (such as alkyl ethoxyethanol with varying number of ethyleneoxide units) exhibits negative solubility in water, that is they become insoluble at a temperature called the Cloud point. In the case of ionic surfactants, the solubility increases drastically at the Krafft point due to the formation of micelles. On the other hand, in the case of nonionic surfactant aqueous solutions, the micellar phase separates into an almost pure surfactant phase at temperatures greater than Cloud point [5,6]. 

Hill developed the thermodynamics of small systems and also applied it to the aggregation of solutes. This theory serves as a bridge between the mass-action and phase-separation models. Further development has been done by Hall. " Recently, Tanaka applied the theory to static light scattering data for aqueous solutions of nonionic surfactants, and proved its usefulness. This chapter introduces the fundamental concept of this thermodynamics as a basis for understanding micellar solutions. 

At low concentration the micellar solution is limited by a two-phase area (w+Lj) where a surfactant rich and a water rich phase coexist (coacervate). The area of coacervates is enlarged toward lower temperature and higher concentration if electrolytes are added. It is destroyed by addition of anionics. Similarly, the Krafft temperature decreases in the presence of ionic or other nonionic surfactants, so that neither phase separation by coacervate formation nor crystallization is observed at concentrations typically used in cleansing formulations. 

With C12E5 as the nonionic surfactant at a 1 wt% level in water, quite different phenomena were observed below, above, and well above the cloud point when tetradecane or hexadecane was carefully layered on top of the aqueous solution. Below the cloud point temperature of 31 °C, very slow solubilization of oil into the one-phase micellar solution occurred. At 35 C, which is just above the cloud point, a much different behavior was observed. The surfactant-rich L phase separated to the top of the aqueous phase prior to the addition of hexadecane. Upon addition of the oil, the L, phase rapidly solubilizes the hydrocarbon to form an oil-in-water microemulsion containing an appreciable amount of the nonpolar oil. After depletion of the larger surfactant-containing drops, a front developed as smaller drops were incorporated into the microemulsion phase. This behavior is shown schematically in Figure 12.16. Unlike the experiments carried out below the cloud point temperature, appreciable solubilization of oil was observed in the time frame of the study, as indicated by upward movement of the oil-microemulsion interface. Similar phenomena were observed with both tetradecane and hexadecane as the oil phases. 

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