Aqueous Solution of Surfactants

1 Solubility Characteristics of Surfactants in Water (Dependence on Temperature) 

The solubility characteristics of surfactants (in water) is one of the most studied phenomena. Even though the molecular structures of surfactants are rather simple, their solubility in water is rather complex as compared to other amphiphiles such as long-chain alcohols, etc., in that it is dependent on the alkyl group. This is easily seen since the alkyl groups will behave mostly as alkanes. The hydrophobic alkyl part exhibits solubility in water, which has been related to a surface tension model of the cavity (see Appendix B). However, it is found additionally that the solubility 

FIGURE 3.6 Solubility (Krafft point KP) of ionic (anionic or cationic) surfactants in water (as a function of temperature). 

The linear dependence of KP on alkyl chain length is very clear. The KP for C12 sulfate is 21°C, and it is 34°C for C14 sulfate. It may be concluded that KP increases by approximately 10°C per CH2 group. It is also interesting to note that the KP of C8 is found from extrapolation to be -3.5°C. In fact, for C8 it is not possible to measure this KP from experiments. 

Since no micelles can be formed below the KP, it is important that one keep this information in mind when using any anionic detergent. Therefore, the effect of various parameters on KP needs to be considered in the case of ionic surfactants. The following are some of these  

---

The molar absorptivities for the two betaines and the three sulfobetaines in aqueous solution are listed in Table I. Before being used for surface tension measurements, aqueous solution of surfactants were further purified by repeated passage (12) through minicolumns (SEP-PAK Cjs Cartridge, Waters Assoc., Milford Mass.) of octadecylsilanized silica gel. The concentration of surfactant in the effluent from these columns was determined by ultraviolet absorbance, using the molar absorptivities listed in Table I. 

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... 

Consider a one dimensional oil droplet pressed against a solid surface and surrounded by an aqueous solution of surfactant (Fig. 8). The shape of the oil-water interface far away from the sohd surface is governed by the Laplace equation [18] and close to the sohd surface is augmented by an additional term relating to the film energy given by the structural disjoining pressure [12] ... 

The miscibility between hydrocarbon and fluorocarbon surfactants was studied by means of a steady-state fluorescence. Three mixed aqueous solutions of surfactants were employed sodium dodecyl sulfate (SDS)-p-t(CF )2CF]2C=C(CFj)0(CH CH O) -CH (NF), hexaoxyethylene-... 

Useful information regarding the adsorbed film and micelle in equilibrium with the aqueous solution of surfactant mixture at constant temperature and pressure is provided by adopting as the experimental variables the total molality m of surfactants and the mole fraction X of surfactant 2 in the total surfactant defined by... 

Heats of Immersion of Graphon in 0.60% Aqueous Solutions of Surfactants... 

Bibette has used this method to study the effect of osmotic pressure on the stability of thin films in concentrated o/w emulsions [96], by means of an osmotic stress technique. The emulsion is contained in a dialysis bag, which is immersed in an aqueous solution of surfactant and dextran, a water-soluble polymer. The bag is permeable to water and surfactant, but impermeable to oil and polymer. The presence of the polymer causes water to be drawn out of the emulsion, increasing the phase volume ratio and the deformation of the dispersed droplets (Fig. 10). 

Portalski (P4), 1963 Theories of film flow and methods of measuring film thickness are reviewed. Film thicknesses on vertical plate (zero gas flow) reported for glycerol solutions, methanol, isopropanol, water, and aqueous solutions of surfactants. Results compared with values calculated by Nusselt, Kapitsa, and corrected Dukler and Bergelin treatments. 

The state of the hydrocarbon chains in mesophases and micelles is reflected in the Krafft phenomena. In aqueous solutions of surfactants the Krafft point is defined as the temperature at which the solubility reaches the critical micelle concentration when the temperature is increased further, the solubility rises rapidly since the monomers form micelles (Figure 5) (10). Lipids that do not form micelles frequently start to swell by the uptake of water at a well-defined temperature they are transformed into a mesomorphous state (Figure 6) (11) The relation between these two Krafft phenomena is explained to some extent by the... 

Puvvada, S., and D. Blankschtein. 1992a. Thermodynamic description of micellization, phase behavior, and phase separation of aqueous solutions of surfactant mixtijr fe.ys. Chenr96 5567-5579. 

Aqueous solutions of surfactants (micellar and nonmicellar) without additives or with water soluble nonelectrolytes as additives... 

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. 

The interfacial energy data of aqueous solutions of surfactants synthesized according to Eq. 1 are summarized in Table 1 [10]. 

In addition to the application to pipeline transport of heavy crudes, there, is also considerable interest in downhole emulsification for heavy crude oil production (2, L3, 2J, 215). Here aqueous solutions of surfactants are added to the tubing-casing annulus of wells producing heavy oils and water. Oil-in-water emulsions of relatively low viscosity are formed resulting in increased production rates. 

Partition Coefficients of Phenols and Naphthols in Aqueous Solutions of Surfactants at 298.15 K... 

FIGURE 3-1 Plot of equivalent conductivity versus - /normality of solution for an aqueous solution of surfactant of type Na+R. ... 

---