Micellization and adsorption

The micellization and adsorption properties of industrial sulfonate/ ethoxylated nonionic mixtures have been assessed in solution in contact with kaolinite. The related competitive equilibria were computed with a simple model based on the regular solution theory (RST). Starting from this analysis, the advantage of adding a hydrophilic additive or desorbing agent to reduce the overall adsorption is emphasized. 

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. 

Fig. 3.27 Schematic of the Monle Carlo moves in the lattice simulations of micellization and adsorption of block copolymers by Mattice and co-workers, (a) Brownian moton of a chain (b) end flip of an end bead (c) two types of kink jump (d) reptalion of a chain (Zhan et at. 1993d).

Although similar factors, such as the structure of the surfactant molecule and the microenvironmental conditions surrounding it, affect both micellization and adsorption, the effect of these factors on these two phenomena are generally not equal. 

Following the concept of Tanford (1980), we find interactions of surfactant molecules in aqueous solution encouraging the formation of micelles and adsorption layers at a liquid... 

Hancer, M., Patist, A., Kean, R. T., and Murahdhara, H. S. (2002) Micellization and adsorption of phospholipids and soybean oil onto hydrophilic and hydrophobic surfaces in non-aqueous media. Colloids and Surfaces 204, 31-41. 

In order to specify these effects, it will be useful to return to the simple case of non-ionic surfactants. We shall consider the various parameters governing competition between preferred solubilisation in one or other of the phases (possibly as micelles) and adsorption onto the interface. 

This chapter provides an introduction to the occurrence, properties and importance of surfactants as they relate to the petroleum industry. With an emphasis on the definition of important terms, the importance of surfactants, their micellization and adsorption behaviours, and their interfacial properties are demonstrated. It is shown how surfactants may be applied to alter interfacial properties, promote oil displacement, and stabilize or destabilize dispersions such as foams, emulsions, and suspensions. Understanding and controlling the properties of surfactant-containing solutions and dispersions has considerable practical importance since fluids that must be made to behave in a certain fashion to assist one stage of an oil production process, may require considerable modification in order to assist in another stage. 

Dantas, T. N. C., Moura, E. F., Scatena Junior, H., Dantas Neto, A. A., and GurgeL A. 2002. Micellization and adsorption thermodynamics of novel ionic surfactants at fluid interfaces. Coll. Surf. A, 207, 243-252. 

In surfactant textbooks, micellization is usually described as the consequence of smface saturation of adsorbed surfactant molecules. When the surfactant concentration in water is increased, more and more surfactant molecules adsorb at the water-air interface (usually referred to as surface ). At a certain concentration, the surface is saturated, and if the surfactant concentration is further increased, the excess surfactant forms aggregates called micelles. We do not believe this view is correct. In a very large spherical hypothetical container, the surface to volume ratio is very small. If we assume the wall of the container is made iu an hypothetical material having exactly the same characteristics as bulk water, no surfactant adsorption should occur at all, even at a fairly high concentration. It seems very difficult to admit that such a system would not form any micelle. We think that micellization is a bulk process, completely independent of the adsorption, which is a smface process. Monomer surfactant molecules can either form micelles or adsorb at various interfaces, and usually do both. Thus, micellization and adsorption are two processes that helps relieve the saturation of the solution. However, the adsorption process is active at a lower concentration than micellization, so that in a sequence of adding more surfactant, it usually takes place first. 

The simple fact that critical micelle concentration can be obtained from the surface tension-surfactant concentration isotherm is the best proof thit micellization prevents adsorption proceeding to completion and is not the result of surface saturation. Real situations are similar to the cases depicted by Figs. 7 and 8 (probably closer to Fig. 8). This is not too surprising, as both micellization and adsorption at the water-air surface or at a water-low-polarity solid or liquid result from the same driving force the exclusion of the fatty chains by water molecules ... 

Kronberg et al. have examined the thermodynamics of surfactant micellization and adsorption onto hydrophobic surfaces [47]. They considered two main contributions for the Gibbs free energy of transfer from the aqueous solution to the micelle or the surface ... 

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-... 

An expression for the number of particles formed during Stage I was developed, assuming micellar entry as the formation mechanism (13), where k is a constant varying from 0.37 to 0.53 depending on the relative rates of radical adsorption in micelles and polymer particles, r is the rate of radical generation, m is the rate of particle growth, is the surface area covered by one surfactant molecule, and S is the total concentration of soap molecules. 

Because of their preferential use as detergents, the main interest in the physicochemical properties of the salts of a-sulfo fatty acid esters is related to their behavior in aqueous solution and at interfaces. In principle these are surface-active properties of general interest like micelle formation, solubility, and adsorption, and those of interest for special applications like detergency, foaming, and stability in hard water. 

In addition, it is of interest to note that investigations of the microscopic processes leading to nucleation, growth, oriented growth by the surfactant monolayer, and growth inhibition of nanoparticles in reversed micelles and of confinement and adsorption effects on such phenomena represent an intriguing and quite unexplored research field [218]. 

Another method is based on the evaporation of a w/o microemulsion carrying a water-soluble solubilizate inside the micellar core [221,222], The contemporaneous evaporation of the volatile components (water and organic solvent) leads to an increase in the concentration of micelles and of the solubilizate in the micellar core. Above a threshold value of the solubilizate concentration, it starts to crystallize in confined space. Nanoparticle coalescence could be hindered by surfactant adsorption and nanoparticle dispersion within the surfactant matrix. 

The role of various surfactant association structures such as micelles and lyotropic liquid crystals (372), adsorption-desorption kinetics at liquid-gas interfaces (373) and interfacial rheology (373) and capillary pressure (374) on foam lamellae stability has been studied. Microvisual studies in model porous media indicate... 

Taking Simultaneous Micellizadon and Adsorption Phenomena into Consideration In the presence of an adsorbent in contact with the surfactant solution, monomers of each species will be adsorbed at the solid/ liquid interface until the dual monomer/micelle, monomer/adsorbed-phase equilibrium is reached. A simplified model for calculating these equilibria has been built for the pseudo-binary systems investigated, based on the RST theory and the following assumptions ... 

For pure nonionic EO adducts, increase in the number of oxyethylene groups in the molecule results in a decrease in the tendency to form micelles and an increase in the surface tension of the solution at the critical micelle concentration (1 ) (l. ) This change in surface activity is due to the greater surface area of the molecules in the adsorption layer and at the micellar surface as a result of the presence there of the highly hydrated polyoxyethylene chain. The reduction in the tendency to form micelles is due to the increase in the free energy of micelle formation as a result of partial dehydration of the polyoxyethylene chain during incorporation into the micelle ( 1 6) (17). 

Table III. Effects of POE Adsorption for from Rosen et Chain Length on Nonionics of n-al.(3) Micellization and Ci2H250(EO)mH, data...

From the solubility data of n-decane in water, the enthalpy for the process n-decane (H2O) - n-decane (pure) at 25°C has been estimated by Boddard et al. ( ) to be -5.85 kJ moT. Substracting this value from the calculated AH°(25°c) values for CioBMG and C12BMT, in Tables III and IV, the AH (-W) values for micellization and for adsorption at the aqueous solution/air interface at 25 C can be estimated. Values are shown in Table V. 

It is clear from equation (7) that the addition of a second surfactant results in further decrease in y the essential requirements being a not too small adsorption of the second surfactant. Whether it replaces the first surfactant or is adsorbed in addition to it is immaterial, just as it is not essential for the two surfactants to form a complex. If the two surfactants are of the same type e.g. both water soluble anionic surfactants, they will form mixed micelles and this will lower the activity of the second surfactant added and decrease both its Fand dp. However, if the two surfactants are different in nature, e.g. one predominantly water soluble and the other oil soluble, they will only slightly affect each other s activity and their combined effect on the interfactial tension may be large enough to bring y to zero at finite concentrations. 

It has been found that the 1,3-dioxolane ring corresponds to approximately two oxyethylene units with regard to effect on the critical micelle concentration and adsorption characteristics [42]. Thus, surfactant type I in Fig. 14 should resemble ether sulfates of the general formula R-(0CH2CH2)20S03Na. This is interesting since the commercial alkyl ether sulfates contain two to three oxyethylene units. 

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