Surfactants diluted solutions

In this model, a gaseous film is considered to be a dilute surface solution of surfactant in water and Eq. in-108 can be put in the form... 

In a detersive system containing a dilute surfactant solution and a substrate bearing a soHd polar sod, the first effect is adsorption of surfactant at the sod—bath interface. This adsorption is equivalent to the formation of a thin layer of relatively concentrated surfactant solution at the interface, which is continuously renewable and can penetrate the sod phase. Osmotic flow of water and the extmsion of myelin forms foHows the penetration, with ultimate formation of an equdibrium phase. This equdibrium phase may be microemulsion rather than Hquid crystalline, but in any event it is fluid and flushable... 

In highly diluted solutions the surfactants are monodispersed and are enriched by hydrophil-hydrophobe-oriented adsorption at the surface. If a certain concentration which is characteristic for each surfactant is exceeded, the surfactant molecules congregate to micelles. The inside of a micelle consists of hydrophobic groups whereas its surface consists of hydrophilic groups. Micelles are dynamic entities that are in equilibrium with their surrounded concentration. If the solution is diluted and remains under the characteristic concentration, micelles dissociate to single molecules. The concentration at which micelle formation starts is called critical micelle concentration (cmc). Its value is characteristic for each surfactant and depends on several parameters [189-191] ... 

Two other general ways of treating micellar kinetic data should be noted. Piszkiewicz (1977) used equations similar to the Hill equation of enzyme kinetics to fit variations of rate constants and surfactant concentration. This treatment differs from that of Menger and Portnoy (1967) in that it emphasizes cooperative effects due to substrate-micelle interactions. These interactions are probably very important at surfactant concentrations close to the cmc because solutes may promote micellization or bind to submicellar aggregates. Thus, eqn (1) and others like it do not fit the data for dilute surfactant, especially when reactants are hydrophobic and can promote micellization. 

Reactions of 2,4-dinitrochloro-benzene and -naphthalene are speeded by DDDAOH and the corresponding chloride -I- NaOH (Cipiciani et at., 1984). The rate/surfactant concentration profiles and the rate constants are very similar to those for reactions in solutions of the corresponding C16 single chain surfactants which form normal micelles. The spontaneous hydrolysis of 2,4-dinitrophenyl phosphate dianion is also speeded by DDDAC1 and rates reach plateau values in very dilute surfactant (Savelli and Si, 1985). 

The value of TK is best determined by warming a dilute solution of surfactant, and noting the temperature at which it becomes clear. Table 10.4 lists the Krafft points for a series of colloidal systems based on aqueous solutions of sodium alkyl sulphate (cf. structure III). 

In dilute solutions of surfactants adsorption processes are controlled by transport of the surfactant from the bulk solution towards the surface as a result of the concentration gradient formed in the diffusion layer the inherent rate of adsorption usually is rapid. For non-equilibrium adsorption the apparent (non-equilibrium) isotherm can be constructed for different time periods that are shifted with respect to the true adsorption isotherm in the direction of higher concentration (Cosovic, 1990) (see Fig. 4.10). 

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... 

In the case of adsorption from solution, the surfactant layers are in equilibrium with the solution and will de-sorb on dilution. However, it would be very useful to produce adsorbed layers in both air and water, which will remain adsorbed. This can be achieved using the Langmuir-Blodgett deposition technique. The technique is based on the observation that if a surfactant, which is insoluble in water, is dissolved in a volatile, non-aqueous solvent and then spread on water, an insoluble monolayer of orientated surfactant molecules will remain at the air/solution interface. The effect of the spreading surfactant and its surface film pressure can be dramatically demonstrated by spreading hydrophobic talc powder on a clean water surface and then placing a... 

In this equation the standard state corresponds to the state that results from letting fw - 1 and xw - 1, in which case = n°s w. Letting/ - 1 is equivalent to saying that the surfactant behaves ideally, and letting xw - 1 is equivalent to having pure surfactant possessing the kind of interactions it has when surrounded by water. Physically, this corresponds to an infinitely dilute solution of surfactant in water. Using the primed symbol to represent the chemical potential of surfactant in micelles per mole of micelles, we write... 

Block copolymers are widely used industrially. In the solid and rubbery states they are used as thermoplastic elastomers, with applications such as impact modification, compatibilization and pressure-sensitive adhesion. In solution, their surfactant properties are exploited in foams, oil additives, solubilizers, thickeners and dispersion agents to name a few. Particularly useful reviews of applications of block copolymers in the solid state are contained in the two books edited by Goodman (1982,1985) and the review article by Riess etal. (1985). The applications of block copolymers in solution have been summarized by Schmolka (1991) and Nace (1996). This book is concerned with the physics underlying the practical applications of block copolymers. Both structural and dynamical properties are considered for melts, solids, dilute solutions and concentrated solutions. The book is organized such that each of these states is considered in a separate chapter. 

Solutions of highly surface-active materials exhibit unusual physical properties. In dilute solution the surfactant acts as a normal solute (and in the case of ionic surfactants, normal electrolyte behaviour is observed). At fairly well defined concentrations, however, abrupt changes in several physical properties, such as osmotic pressure, turbidity, electrical conductance and surface tension, take place (see Figure 4.13). The rate at which osmotic pressure increases with concentration becomes abnormally low and the rate of increase of turbidity with concentration is much enhanced, which suggests that considerable association is taking place. The conductance of ionic surfactant solutions, however, remains relatively high, which shows that ionic dissociation is still in force. 

HALL, D.G. and TIDDY, G.J.T., Surfactant solutions dilute and concentrated , in LUCASSEN-REYNDERS, E.H. (editor), Anionic Surfactants, 11, 55-108, Dekker (1981)... 

Adsorption isotherms represent a relationship between the adsorbed amount at an interface and the equilibrium activity of an adsorbed particle (also the concentration of a dissolved substance or partial gas pressure) at a constant temperature. The analysis of adsorption isotherms can yield thermodynamic data for the given adsorption system. Theoretical adsorption isotherms derived from statistical and kinetic data, and using the described assumptions (see 3.1), are known only for the gas-solid interface or for dilute solutions of surfactants (Gibbs). Those for the system gas-solid are of a few basic types that can be thermodynamically predicted81. From temperature relations it is possible to calculate adsorption and activation energies or rate constants for individual isotherms. Since there are no theoretically founded equations of adsorption isotherms for dissolved surfactants on solids, the adsorption of gases on solides can be used as a starting point for an interpretation. 

Traditionally, components of the NMF are measured following extraction of comeocytes recovered from superficial tape-strippings, or from direct extraction of the skin surface by attaching open-ended chambers to the skin and eluting with small volumes of aqueous buffers or dilute surfactant solutions. By analysing sequential tape strips recovered from the same site profiles of how NMF levels change with depth can be constructed. These profiles indicate that the levels of NMF decline markedly toward the surface of the skin. This is typical of normal skin exposed to routine soap washing where much of the readily soluble NMF is washed out from the superficial SC.83... 

Adsorption of surfactant on solid surfaces is generally described by adsorption isotherms. For this purpose, a simple adsorption experiment can be performed at a constant temperature by dispersing known amounts of solid adsorbent into a constant volume of dilute surfactant solution at which the initial surfactant concentrations are varied and shaking the mixture until equilibrium is reached. The moles of surfactant adsorbed per unit mass of the solid (Ns) for each solution can be determined from ... 

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... 

Reliable Ao(C) isotherms of diluted surfactant solution can be monitored using the very precise (accuracy 0.01 mN m 1) spherotensiometric technique [363]. It is based on determination of the forces emerging when a well wetted sphere is drawn out of a solution. Thus the surface tension of various surfactants such as NaDoS, sodium octyl sulphate [364,365], low molecular fatty acids and normal alcohols [366,367] and acetals, a special kind of nonionic surfactants [368] has been measured. These measurements were performed within a large surfactant concentration range in the presence of different electrolytes and at various temperatures. 

Gas-liquid systems are classified as coalescing or noncoalescing, depending on the behavor exhibited by the bubbles. Water is classed as coalescing by Konig (7). Examples of systems exhibiting lower coalescence rates are 0.5% propanol solutions, dilute solutions of surfactants and many of the media used in fermentation reactions. 

In another interesting development, Yei et al. [124] prepared POSS-polystyrene/clay nanocomposites using an emulsion polymerization technique. The emulsion polymerization for both the virgin polystyrene and the nano composite started with stirring a suspension of clay in deionized water for 4h at room temperature. A solution of surfactant ammonium salt of cetylpyridinium chloride or POSS was added and the mixture was stirred for another 4 h. Potassium hydroxide and sodium dodecyl sulphate were added into the solution and the temperature was then raised to 50 °C. Styrene monomer and potassium persulfate were later on added slowly to the flask. Polymerization was performed at 50 °C for 8 h. After cooling, 2.5% aqueous aluminium sulphate was added to the polymerized emulsion, followed by dilute hydrochloric acid, with stirring. Finally, acetone was added to break down the emulsion completely. The polymer was washed several times with methanol and distilled water and then dried overnight in a vacuum oven at 80 °C. The obtained nanocomposite was reported to be exfoliated at up to a 3 wt % content of pristine clay relative to the amount of polystyrene. 

In aqueous solution, dilute concentrations of surfactant act much as normal electrolytes, but at higher concentrations very different behavior results. This behavior (illustrated in Figures 8 and 9) was explained by McBain in terms of organized aggregates called micelles in which the lipophilic parts of the surfactants associate in the interior of the aggregate and leave hydrophilic parts to face the aqueous medium. (Details are given in refs. 16 and 31.) The concentration at which micelle formation becomes... 

Gibbs monolayers are widespread. The simplest system is that of the surface of a fully miscible binary liquid. More complex ones are monolayers of uncharged molecules adsorbed from dilute solutions (example aliphatic alcohols from aqueous solution) electrolytes surfactants (non-ionic or ionic) polymers and polyelectrolytes and yet more. On the other hand, the methods for characterizing... 

An outline of the ideas is as follows [62-65]. It suffices to assert that a dilute solution of surfactant molecules can be considered to consist of water plus monomers, dimers, trimers, and larger allowed aggregates (micelles, vesicles, liposomes,. ..). The concentration is assumed to be so low that aggregates can be considered to be non-interacting. The probability distribution of aggregates, is then determined from the law of mass action... 

The relationship between hydrocarbon chain length and surface activity is expressed by Traube s rule, which states that in dilute aqueous solutions of surfactants belonging to any one homologous series, the molar concentrations required to produce equal lowering of the surface tension of water decreases threefold for each additional CH2 group in the hydrocarbon chain of the solute. Traube s mle also applies to the interfacial tension at oil/water interfaces. 

Stable o/w creams prepared with ionic or nonionic emulsifying waxes are composed of (at least) four phases (Fig. 7.20) (f) dispersed oil phase, (2) crystalline gel phase, (3) crystalline hydrate phase, and (4) bulk aqueous phase containing a dilute solution of surfactant. The interaction of the surfactant and fatty alcohol components of emulsifying mixtures to form these stmctures (body) is critical. It is also time-dependent, giving the name self-bodying to these emulsions. The overall stability of a cream is dependent on the stability of the crystalline gel phase. 

The Influence of Temperature on the Viscoelastic Properties. The viscoelastic properties of the dilute surfactant systems depend on the temperature of the solutions strongly. Figure 8 shows the values for the storage modulus G as a function of the angular frequency at different temperatures for a 20 mM solution of CPySal. The elastic properties of the surfactant solutions decrease with increasing temperature. The solution equilibrated at 35 C shows only little elasticity in the frequency range below 1 Hz and at temperatures of 50 C the solutions behave as Newtonian fluids. The supermolecular structures which are present in these solutions and which are responsible for the viscoelastic properties seem to be completely destroyed under these experimental conditions. 

In contrast with inorganic salts whose adsorption at water-air interface is negative, surfactants are strongly adsorbed at water-air interface, and this results in depression of the surface tension. In very dilute solutions of surfactants, adsorption at water air interface can lead to substantial depletion of the solution. Orientation of surfactant molecules at air-water interface is illustrated in Fig. 4.64. 

---