Surfactant mixtures surface tension

Numerous studies results showed that there were significant interaction between polymer and surfactant. The surface tension of ordinary polymer/ surfactant mixture solution is often hi er than singles. The reason is that part of surfactants are adsorbed by polymer and the concentration of... 

This problem is circumvented by the presence of a surface active material composed of a complex mixture of lipids and proteins, the pulmonary surfactant, at the liquid-and-air interface within the alveolar lumen. The composition of the surfactant is such that it allows dense packing of the lipid film during expiration, thereby reducing the surface tension generated by the aqueous film. In the absence of surfactant increased surface tension along the alveolar lining results in the collapse... 

Figure 13.5 shows the variations of surface tension versus surfactant concentration for pure surfactant system and the mixture of polymer/surfactant where the polymer concentration is constant. In the case of pure surfactant solution, a sharp decrease in the surface tension occurs with the increase in surfactant concentration up to the critical micelle concentration (CMC). For surfactant concentrations higher than the CMC, the surface tension remains constant. In the mixture of polymer and surfactant, the surface tension plot shows two break points. The first point is the CAC point where the interaction between the polymer and the surfactant begins. The second point is the PSP point where the polymer chains become saturated with the surfactant. When the interaction between the polymer and surfactant is weak, CAC and PSP values are close to the CMC of pure surfactant (Mohsenipour 2011). 

The fluorinated surfactant-hydrocarbon surfactant mixtures have unique properties. In two-phase systems of water and a hydrocarbon solvent, the fluorinated surfactant reduces surface tension and the hydrocarbon surfactant decreases the interfacial tension. For example, an aqueous foam of mixed surfactants spreads on a hydrocarbon solvent because the fluorinated surfactant adsorbs preferentially at the air-water interface, whereas the hydrocarbon surfactant adsorbs at the water-oil interface (see Chapter 8, Fire-fighting Foams). 

Surface tension is usually predicted using group additivity methods for neat liquids. It is much more difficult to predict the surface tension of a mixture, especially when surfactants are involved. Very large molecular dynamics or Monte Carlo simulations can also be used. Often, it is easier to measure surface tension in the laboratory than to compute it. 

When the CMC determination is made by surface tension measurements, the resulting curve appears without minimum as a single surfactant. It is probable that an inversion takes place through the adsorption of the LSDA onto the surface of the Ca soap micelle, so that complete precipitation does not occur [23]. Zhang and Xiao [32] are of the opinion that the dispersion comes from the union of LSDA with the free ionic soap molecules. The particles from the soap-LSDA mixture are far larger than the corresponding soap molecules in soft water and therefore result in turbidity in hard water. 

Figure 1 gives the measurements of surface tension used for determining the CMCs of sulfonate/Genapol and nonylphenol 30 E.O. mixtures, with the last surfactant being called a desorbent (this term will be justified below). Minimum in surface tension was seen only for a few nonionic solutions (e.g. NP 50 E.O.). In this case, we used dyes that, once solubilized in the micelles, cause the solution to change color, which is another way of measuring the CMC. 

Surfactant mixtures were used as obtained and are listed with their properties in Table II. Sodium chloride and calcium chloride were Fisher reagent grade. Deuterium oxide was Aldrich Gold Label and had a surface tension of 70.4 mN/m at 23°C measured with a W iI heImy pi ate tens i ometer. 

The reaction was first conducted with success on sucrose [82], The degree of substitution (DS) obtained was controlled by the reaction time. Thus, under standard conditions (0.05% Pd(OAc)2/TPPTS, NaOH (1 M)/iPrOH (5/1), 50 °C) the DS was 0.5 and 5 after 14 and 64 h reaction time, respectively. The octadienyl chains were hydrogenated quantitatively in the presence of 0.8-wt.% [RhCl(TPPTS)3] catalyst in a HjO-EtOH (50/10) mixture, yielding a very good biodegradable surfactant (surface tension of 25 mN m-1 at 0.005% concentration in water) [84]. Telomerization reaction was also conducted with success on other soluble carbohydrates such as fructose, maltose, sorbitol and /i-cyclodextrin. 

So far only aqueous solutions have been considered however, mixtures of HF and ethanol or methanol are quite common, because this addition reduces the surface tension and thereby the sticking probability of hydrogen bubbles. While substantial quantities of ethanol or methanol are needed to reduce the surface tension, cationic or anionic surfactants fulfill the same purpose in concentrations as low as 0.01 M [So3, Chl6]. 

The situation is, however, different in the alveolar region of the lung where the respiratory gas exchange takes place. Its thin squamous epithelium is covered by the so-called alveolar surface liquid (ASL). Its outermost surface is covered by a mixture of phospholipids and proteins with a low surface tension, also often referred to as lung surfactant. For this surfactant layer only, Scarpelli et al. [74] reported a thickness between 7 and 70 nm in the human lung. For the thickness of an additional water layer in between the apical surface of alveolar epithelial cells and the surfactant film no conclusive data are available. Hence, the total thickness of the complete ASL layer is actually unknown, but is certainly thinner than 1 gm. 

Thorough separations of major synthesized products were performed leading to pure monooctadienyl-xyloside (18) or arabinoside isomers (23). The study of their surface activity showed their interesting capacity of lowering surface tension down to 30-35 mN/m at relatively low concentrations in the range of 1-4- mmol/L (Table 14). Submitting crude mixtures of telomers of xylose or bran syrup to the same examination established that the surface-activity was not improved after the separation process. Therefore, for an industrial application of these surfactants, the use of the lower cost, non-separated mixture can be recommended. 

Lung surfactant is a mixture of proteins and amphipathic lipids that acts like a detergent or soap to greatly decrease the surface tension forces at the alveolar fluid-air interface. 

The deviations from the Szyszkowski-Langmuir adsorption theory have led to the proposal of a munber of models for the equihbrium adsorption of surfactants at the gas-Uquid interface. The aim of this paper is to critically analyze the theories and assess their applicabihty to the adsorption of both ionic and nonionic surfactants at the gas-hquid interface. The thermodynamic approach of Butler [14] and the Lucassen-Reynders dividing surface [15] will be used to describe the adsorption layer state and adsorption isotherm as a function of partial molecular area for adsorbed nonionic surfactants. The traditional approach with the Gibbs dividing surface and Gibbs adsorption isotherm, and the Gouy-Chapman electrical double layer electrostatics will be used to describe the adsorption of ionic surfactants and ionic-nonionic surfactant mixtures. The fimdamental modeling of the adsorption processes and the molecular interactions in the adsorption layers will be developed to predict the parameters of the proposed models and improve the adsorption models for ionic surfactants. Finally, experimental data for surface tension will be used to validate the proposed adsorption models. 

Raney K, Benton W, Miller CA (1987) Optimum detergency conditions with nonionic siufactants II. Effect of hydrophobic additives. J Colloid Interface Sci 119 539-549 Rosen MJ, Wu Y (2001) Superspreading of trisiloxane surfactant mixtures on hydrophobic siufaces 1. Interfacial adsorption of aqueous trisiloxane surfactant -M-alkyl pyrrolidinone mixtures on polyethylene. Langmuir 17 7296-7305 Stevens PJG, Kimberely MO, Mimphy DS, Policello GA (1993) Adhesion of spray droplets to foliage - the role of dynamic surface tension and advantages of organosil-icone surfactants. Pesticide Sci 38 237-245... 

Below the CMC, the surfactant mixing in monolayers composed of similarly structured surfactants approximately obeys ideal solution theory. This means that the total surfactant concentration required to attain a specified surface tension for a mixture is intermediate between those concentrations for the pure surfactants involved. For mixtures of ionic/nonionic or anionic/cationic surfactants, below the CMC, the surfactant mixing in the monolayer exhibits negative deviation from ideality (i.e., the surfactant concentration required to attain a specified surface tension is less than that predicted from ideal solution theory). The same guidelines already discussed to select surfactant mixtures which have low monomer concentrations when micelles are present would also apply to the selection of surfactants which would reduce surface tension below the CMC. 

When an ionic/nonionic surfactant mixture adsorbs on a metal oxide surface, the admicelle exhibits negative deviation from ideality (74). This means that the adsorption level is higher than it would be if the admicelle were ideal, at a specific surfactant concentration below the CMC. Above the CMC, the adsorption level is dictated by the relative enhancement of micelle formation vs. admicelle formation. In this region, the level of adsorption can be viewed as the result of the competition between micelles and admicelles for surfactant. In analogy, the surface tension above the CMC can be viewed as competition between the monolayer and micelles for surfactant. 

This brief review has attempted to discuss some of the important phenomena in which surfactant mixtures can be involved. Mechanistic aspects of surfactant interactions and some mathematical models to describe the processes have been outlined. The application of these principles to practical problems has been considered. For example, enhancement of solubilization or surface tension depression using mixtures has been discussed. However, in many cases, the various processes in which surfactants interact generally cannot be considered by themselves, because they occur simultaneously. The surfactant technologist can use this to advantage to accomplish certain objectives. For example, the enhancement of mixed micelle formation can lead to a reduced tendency for surfactant precipitation, reduced adsorption, and a reduced tendency for coacervate formation. The solution to a particular practical problem involving surfactants is rarely obvious because often the surfactants are involved in multiple steps in a process and optimization of a number of simultaneous properties may be involved. An example of this is detergency, where adsorption, solubilization, foaming, emulsion formation, and other phenomena are all important. In enhanced oil recovery. 

A generalized nonideal mixed monolayer model based on the pseudo-phase separation approach is presented. This extends the model developed earlier for mixed micelles (J. Phys. Chem. 1983 87, 1984) to the treatment of nonideal surfactant mixtures at interfaces. The approach explicity takes surface pressures and molecular areas into account and results in a nonideal analog of Butler s equation applicable to micellar solutions. Measured values of the surface tension of nonideal mixed micellar solutions are also reported and compared with those predicted by the model. 

Results for the various binary mixed surfactant systems are shown in figures 1-7. Here, experimental results for the surface tension at the cmc (points) for the mixtures are compared with calculated results from the nonideal mixed monolayer model (solid line) and results for the ideal model (dashed line). Calculations of the surface tension are based on equation 17 with unit activity coefficients for the ideal case and activity coefficients determined using the net interaction 3 (from the mixed micelle model) and (equations 12 and 13) in the nonideal case. In these calculations the area per mole at the surface for each pure component, tOj, is obtained directly from the slope of the linear region in experimental surface tension data below the cmc (via equation 5) and the maximum surface pressure, from the linear best fit of... 

In a previous publication ( ), results were presented on the micellar properties of binary mixtures of surfactant solutions consisting of alkyldimethylamine oxide (C12 to Cig alkyl chains) and sodium dodecyl sulfate. It was reported that upon mixing, striking alteration in physical properties was observed, most notably in the viscosity, surface tension, and bulk pH values. These changes were attributed to 1) formation of elongated structures, 2) protonation of amine oxide molecules, and 3) adsorption of hydronium ions on the mixed micelle surface. In addition, possible solubilisation of a less soluble 1 1 complex, form between the protonated amine oxide and the long chain sulfate was also considered. 

The conditions for synergism in surface tension reduction efficiency, mixed micelle formation, and Surface tension reduction effectiveness in aqueous solution have been derived mathematically together with the properties of the surfactant mixture at the point of maximum synergism. This treatment has been extended to liquid-liquid (aqueous solution/hydrocarbon) systems at low surfactant concentrations.) The effect of chemical structure and molecular environment on the value of B is demonstrated and discussed. 

---