Micelles of nonionic surfactants

The comparison of CMC data in distilled vs. hard river water shows that the decrease in CMC with hardness has the order anionics cationics nonionics (Rosen et al., 1996). Hardness increases the dependence of the CMC on alkyl carbon chain length of CnE0mS04, indicating that in hard water the influence of additional carbon atoms is the same for CnE0mS04 as for CnEOm surfactants (Rosen et al., 1996). The influence of ionic strength on micellization of nonionic surfactants is due to a salting out effect of the hydro-phobic moiety of the surfactant molecule (Carala et al., 1994). 

Carale, T.R., Q.T. Phan, and D. Blankschtein. 1994. Salt effects on intramicellar interactions and micellization of nonionic surfactants in aqueous solutions. Langmuir, 10, 109-121. 

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. 

Nonionic micelles have a hydrophobic core surrounded by a shell of oxyethylene chains which is often termed the palisade layer (Fig. 6.28). This layer is capable of mechanically entrapping a considerable number of water molecules, as well as those that are hydrogen-bonded to the oxyethylene chains. Micelles of nonionic surfactants tend, as a consequence, to be highly hydrated. The outer surface of the palisade layer forms the shear surface that is, the hydrating molecules form part of the kinetic micelle. 

Figure 8. Variable polarities of microdomains in reverse micelles of nonionic surfactant (0.01 M Shell C11-14 EO5) in ethane (T = 35 C)...

Shin, M., Umebayashi, Y., Kanzaki, R., andlshiguro, S. I. (2000). Formation of copper(II) thiocyanato and cadmium(II) iodo complexes in micelles of nonionic surfactants with varying polyethylene chain length. J. Colloid Interface Sci. 225(1), 112-118. 

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. ... 

Dying of fabrics is one of the final operations. Solubilisation of dyes in micelles of nonionic surfactants enhances the penetration into the fibre [179]. This method can be used for dying wool, silk, nylon and polyacetate fibres. For cationic dyes, a surfactant is chosen which dissolves in the fibre and acts to mordant the dye in the fibre and to increase light-fastness. 

All four effects also appear when rodlike micelles of nonionic surfactants are rendered charged by the addition of a small amount of an ionic cosurfactant. On the other hand, the addition of excess salt eliminates the second process (12) by reducing the mutual repulsive interaction between micelles. 

Zheng, Y. and Davis, H.T. (2000) Mixed micelles of nonionic surfactants and uncharged block copolymers in aqueous solution microstructure seen by cryo-TEM. Langmuir, 16, 6453-6459. 

The micellization of nonionic surfactants in RTILs has been reported in the literature [4—24]. The combinations of RTILs and nonionic surfactants employed in these papers are summarized in Table 3.1. In this table, the RTILs are classified by their protic/aprotic nature that is, protic RTILs can form a hydrogen-bonding network structure between ion pairs [25] leading to a sponge-like phase in the bulk as a result of their self-assembly. In contrast, aprotic RTILs do not form such a network structure, and hence, their self-assembly in the bulk is absent or weak. One of the key conclusions regarding the micellization of nonionic surfactants in RTILs is that the micellization is only observed when surfactant molecules are associated with each other in appropriate RTILs. In general, nonionic surfactants with excellent solubility in RTILs cannot form micelles because of their insufficient intermolecular solvophobic interaction and those with poor solubility in RTILs cannot do that either [6]. 

However, other rheological studies reported the existence of two relaxation processes. Reference 112 presented an interpretation of the results that is very different from that in References 105-108. The slow process, which is that discussed in References 105-108, is now attributed to the network relaxation while the faster of the two processes, not seen in these references, is attributed to the exit of a hydrophobe from a junction. One of the difficrdties with this interpretation is that the lifetime of a hydrophobe in a junction would increase with temperature. The authors state that nonionic surfactants show such a behavior. Unfortunately, the references cited to back this point do not really refer to dynamic studies of micelles of nonionic surfactants. Such studies have been performed and show that the residence time/lifetime of nonionic surfactants in micelles decreases as the temperature is increased,just as for ionic surfactants. Thus at the present time there appears to be no good evidence for the assignment of the fast relaxation observed in Reference 112 to the exit of a hydrophobe from a junction. In contrast, the available experimental results seem to indicate that it is the slow relaxation that is associated with this process. 

Figure 4.7. Monomer fraction at CMC with aggregation number of monodisperse micelle of nonionic surfactant.

Bimolecular rate constants (k p = k bs/[R]x where [R]x represents the total concentration of one of the two reactants and whose concentration is larger than that of the second reactant, S, by a factor of more than 5) for 1,3-dipolar cycloaddition reactions of benzonitrile oxide (18) with a series of Af-substituted maleim-ides (19a-19c) in micelles of nonionic surfactants, C,2Eg, C,2E23, Cj Ejo, and CigE2o, fit reasonably well to a kinetic equation similar to Equation 3.61 (Chapter 3)." The calculated values of vary from 0.30 to 0.39 for all four nonionic micelles. Nearly 3-fold micellar deceleration effects have been shown to be similar to the effect of mixed water/1-propanol solvent with [H2O] about 15 M on k pp. However, a comparison of k with k pp in such water-organic solvents does not provide detailed information about the exact nature of the reaction environment in the micellar pseudophase. Instead, it provides information about the question insofar as the micellar reaction environment is satisfactorily mimicked by such mixed water-organic solvents. 

After reviewing various earlier explanations for an adsorption maximum, Trogus, Schechter, and Wade [244] proposed perhaps the most satisfactory one so far (see also Ref. 243). Qualitatively, an adsorption maximum can occur if the surfactant consists of at least two species (which can be closely related) what is necessary is that species 2 (say) preferentially forms micelles (has a lower CMC) relative to species 1 and also adsorbs more strongly. The adsorbed state may also consist of aggregates or hemi-micelles, and even for a pure component the situation can be complex (see Section XI-6 for recent AFM evidence of surface micelle formation and [246] for polymeric surface micelles). Similar adsorption maxima found in adsorption of nonionic surfactants can be attributed to polydispersity in the surfactant chain lengths [247], Surface-active impuri-... 

It is important to point out that, in general, the micelles composed of nonionic surfactants usually have a lower cmc and higher aggregation numbers than the analogous ionic micelles. This is partly due to the absence of electrostatic repulsion between the heads of the nonionic surfactants. However, in the ionic micelles these repulsion tend to limit the aggregation number and the cmc. 

In the present work, we have synthesized two betaines and three sulfobetaines in very pure form and have determined their surface and thermodynamic properties of micellization and adsorption. From these data on the two classes of zwitterionics, energetics of micellization and adsorption of the hydrophilic head groups have been estimated and compared to those of nonionic surfactants. 

Some surfactants aggregate at the solid-liquid interface to form micelle-like structures, which are popularly known as hemimicelles or in general solloids (surface colloids) [23-26]. There is evidence in favor of the formation of these two-dimensional surfactant aggregates of ionic surfactants at the alumina-water surface and that of nonionic surfactants at the silica-water interface [23-26]. 

In order to define a ionic/nonionic surfactant solution with high salinity/hardness tolerance, the following criterion should be followed. The mixed micelle should have as large of a negative deviation from ideality as possible. Surfactant mixture characteristics which result in this have already been discussed. The nonionic surfactant should have a high cloud point. Otherwise the amount of nonionic surfactant which can be added to the system is limited to low levels before phase separation occurs. If possible, a mixed ionic surfactant should be used for reasons Just discussed. There is no such benefit to using mixed nonionic surfactants, although this is not necessarily detrimental either. 

The cloud point of a mixture of nonionic surfactants is intermediate between the pure nonionic surfactants involved (95.99) The cloud point of a dilute nonionic surfactant solution increases upon addition of ionic surfactant (95.98—104). The coacervate phase forms because of attractive forces between the micelles in solution. The incorporation of ionic surfactant into the nonionic micelles introduces electrostatic repulsion between micelles, causing coacervate phase formation to be hindered, raising the cloud point. 

Consider the formation of a mixed micelle in aqueous solution from a binary surfactant solution consisting of a nonionic and an anionic surfactant. The process is depicted as the aggregation of ng molecules of nonionic surfactant B, of n molecules of anionic surfactant A", and in addition there will be counterions, C" ", of the anionic surfactant in the amount of an where a is the fraction of the counterions associated or bound with the surfactant anions in the micelle. The process as depicted is... 

In this paper, we report the solution properties of sodium dodecyl sulfate (SDS)-alkyl poly(oxyethylene) ether (CjjPOEjj) mixed systems with addition of azo oil dyes (4-NH2, 4-OH). The 4-NH2 dye interacts with anionic surfactants such as SDS (11,12), while 4-OH dye Interacts with nonionic surfactants such as C jPOEn (13). However, 4-NH2 is dependent on the molecular characteristics of the nonionic surfactant in the anlonlc-nonlonic mixed surfactant systems, while in the case of 4-OH, the fading phenomena of the dye is observed in the solubilized solution. This fading rate is dependent on the molecular characteristics of nonionic surfactant as well as mixed micelle formation. We discuss the differences in solution properles of azo oil dyes in the different mixed surfactant systems. 

The interactions between azo oil dye and mixed surfactant systems will be dependent on the difference in mixed surfactant micelle due to different molecular characteristic of nonionic surfactant. 

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