Surfactant aggregation, interface

Manne S 1997 Visualizing self-assembly Force microscopy of ionic surfactant aggregates at solid-liquid interfaces Prog. Colloid Polym. Sol. 103 226-33... 

Figure 12.2 Preferred orientation of diene and dienophile at a surfactant aggregate-water interface.

Surfactant solutions critical micelle concentration distribution of reactants among particles surfactant aggregation numbers interface properties and polarity dynamics of surfactant solutions partition coefficients phase transitions influence of additives... 

A Proposed Theory. In earlier publications (1-3), a theory was proposed to correlate solubilization rate, interfacial tension and size of the surfactant aggregate (1) the interfacial tension lowering between the oil-surfactant solution interface is a function of the rate of solubilization of oil, and (2) the rate of solubilization (AS/At) is a function of the effective volume for solubilization ... 

R. Aveyard, B. Binks, T.A. Lawless, and J. Mead Nature of the OilAVater Interface and Equilibrium Surfactant Aggregates in Systems Exhibiting Low Tensions. Can. J. Chem. 66, 3031 (1988). 

As mentioned earlier, surfactants aggregate to form micelles, which may vary in size (i.e., number of monomers per micelle) from a few to over a thousand monomers. However, surfactants can form, besides simple micellar aggregates (i.e., spherical or ellipsoidal), many other structures also when mixed with other substances. The curved micelle aggregates are known to change to planar interfaces when additives, the so-called cosurfactants, are added. A reported recipe consists of... 

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]. 

The settling rate of the dispersions in cyclohexane initially increases with AOT adsorption but later decreases. The initial increase is attributed to the formation of interparticle surfactant aggregates (Fig. 39A). At higher concentrations, the adsorbed molecules aggregate with excess of surfactant in solution rather than with molecules on the particle, so that flocculation ceases to occur and the dispersion is restabilized. The schematic representation of the surfactant assemblies at the interface is as shown in Fig. 39B. 

Reverse micelles are small (1-2 nm in diameter), spherical surfactant aggregates huilt in an apolar solvent (usually referred to as oil), whereby the polar heads form a polar core that can contain water - the so-called water pool. The connection with autopoiesis is historically important, because it was with the collaboration with Francisco Varela that the work started (in fact it began as a theoretical paper - see Luisi and Varela, 1990). The idea was this to induce a forced micro-compartmentalization of two reagents, A and B, which could react inside the boundary (and not outside) to yield as a product the very surfactant that builds the boundary (Figure 7.13). The product S would concentrate at the membrane interface, which increases its size. Since reverse micelles are usually thermodynamically stable in only one given dimension, this increase of the size-to-volume ratio would lead to more micelles. Thus the growth and multiplication would take place from within the structure of the spherically closed unit, be governed by the component production of the micellar structure itself, and therefore (as will be seen better in... 

Specific formulation strategies need to be employed for macromolecule compounds. An excellent review of protein stability in aqueous solutions has been published by Chi et al. (92). In addition to solution stability of proteins and peptides, aerosolization may result in significant surface interfacial destabilization of these compounds if no additional stabilization excipients are added. This is due to the fact that protein molecules are also surface active and adsorb at interfaces. The surface tension forces at interfaces perturb protein structure and often result in aggregation (92). Surfactants inhibit interface-induced aggregation by limiting the extent of protein adsorption (92). 

Figure 17.4 shows a surfactant sorption isotherm from low to high (>CMC) concentrations of the surfactant. It can be divided into three parts (Figure 17.5). In Region 1, individual surfactant molecules are in equilibrium with the surfactant molecules adsorbed to the solid sorbent. In Region 2, the surfactant concentration in the water has exceeded the CMC. That is equivalent to saturation of the air/water interface with surfactant molecules. Subsequent addition of surfactant molecules leads to increased sorption due to formation of sorbed surfactant aggregates (Region 2). In Region 3, the aggregates in solution (micelles) are in equilibrium with the sorbed aggregates, the so-called admicelles..

The many remarkable physico-chemical properties of aqueous surfactant systems, as well as their numerous practical applications, can be referred to the tendency of the nonpolar groups to avoid contact with water at the same time as the polar part tends to be strongly hydrated. The adsorption of surfactants at interfaces between aqueous solutions and air, another liquid phase or a solid is one consequence of this, the extensive aggregation into various types of large aggregates termed micelles — from lat. micella meaning small bit — and liquid crystalline phases is another. 

The spherical nature of the surfactant aggregates in reverse micelles is a response to a thermodynamically driven process. It basically represents a need for surfactants to reach an energetically favorable packing configuration at the interface, depending on the molecular geometry of the surfactant. The surfactant molecules can be represented as a truncated cone whose dimensions are determined by the hydrophilic and hydrophobic parts of the surfactant. Assuming that water-in-oil droplets are spherical, the radius of the sphere is expressed as... 

Aside from their ability to adsorb at interfaces, the most important aspect of surfactants is their ability to form colloidal-sized aggregates in solution. In dilute solution, the surfactant is present as individual molecules. Increasing the concentration promotes the formation of surfactant aggregates or micelles as shown in Fig. 36.17. The concentration at which micelles start to form is referred to as the critical micelle concentration (CMC). Micelle formation is an important... 

Equation (4.3) shows that the magnitude of the surfactant parameter fixes only the fimction of the interfadal curvatures, (1 + HZ + KZ2/3), rather than the curvatures of the interface themselves. In other words, the interfacial geometry - the structure of the surfactant aggregate - is not fixed by the surfactant parameter alone. Both the mean and Gaussian cur atures can be varied cooperatively without altering the value of the surfactant parameter. Nevertheless, the surfactant parameter does furnish a local constraint upon the curvatures of the interface. 

R.D. Void, M.J. Void, Colloid and Interface Chemistry, Addison-Wesley, London, 1983 D.M. Small, The Physical Chemistry of Lipids, Plenum Press, New York, 1986 J.H. Clint, Surfactant Aggregation, Blackie, Glasgow, 1992... 

Stubenrauch, C. (2001) Sugar surfactants-aggregation, interfacial and adsorption phenomena. Curr. Opin. Coll. Interface Sci., 6, 160-170. 

We now turn to the more complex situation where both polyelectrolytes and surfactant are present in solution and adsorption is allowed to occur from this mixture. Polyelectrolyte and surfactant mixtures are used in numerous applications such as pharmaceuticals, laundry, and cosmetics, just to mention a few [4], Sometimes polyelectrolytes and surfactants are unintentionally mixed and due to mutual interaction provide unexpected properties to the mixture. Sometimes they are purposefully added together to fill the function of changing the properties and feel of surfaces, e.g., hair or fabrics, or to act as deposition aids. It is thus important to understand how these mixtures act when they are first mixed in bulk and subsequently transferred to a surface, and how the properties of polyelectrolyte-surfactant aggregates formed in bulk correlate with the properties of such aggregates adsorbed at a solid-liquid interface. Further, it is necessary to learn what happens with the polyelectrolyte-surfactant mixture at the surface when it is diluted with water. 

Figure 17.4, in the absence of adsorbed ODTMA surfactant, the adsorption of 2-Naphthol does not exceed 20 pmol.g on the cellulose fibres. On the other hand, retention of 2-naphthol increased with the amount of ODTMA adsorbed up to a certain surfactant concentration close to the critical micellar concentration (CMC), then levelled off rapidly to a very small value. This behaviour has been correlated with the surfactant aggregation at the cellulose/water interface [26, 27, 29]. 

Figure 1 shows the aggregation behavior of AOT in liquid cyclohexane and supercritical fluid ethane. The systems are one-phase without added water. Surfactant aggregation is indicated by the solvatochromic probe pyridine A -oxide. Pyridine A -oxide was used because of its small size and large dipole moment (/x = 4.3 D), which allow it to partition to the center of reverse micelles instead of being trapped at the surfactant interface. This molecule is a blue shift indicator in that its U V absorption maximum shifts to lower... 

The phase behavior observed in surfactant systems can be viewed from several theoretical perspectives. One is the qualitative Winsor R theory. In its simplest form, the Winsor R parameter is the ratio of the forces acting on the oil side of the surfactant interface to those on the water side. This definition has been extended to include other interactions that tend to oppose surfactant aggregation [34]. When R>, the forces on the oil side of the interface are the strongest, so the interface curves about water, resulting in a 2 system. When i < 1, the water-side forces are dominant, which causes the formation of a 2 system. When R = U the forces at the interface are balanced, which leads to a bicontinuous microemulsion... 

To date the chemical trapping method has been used to estimate interfacial concentrations of water, alcohols, and counterions in cationic micelles and microemulsions [65,128] the affinity of cationic micelles toward Cl versus Br", expressed as an ion-exchange constant [Eq. (11)] [129], and interfacial alcohol/water molar ratios in microemulsions and distributions of 1-butanol and 1-hexanol between aggregate interfaces and bulk aqueous phases (O/W microemulsions) [130] and bulk oil phases (W/O microemulsions) over a range of alcohol and surfactant concentrations [131]. The focus here is on results in microemulsions. 

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