Surface tension and CMC

In [280], dynamic surface tension and CMC values were determined for one nonionic surfactant NP-lOO and four anionic surfactants containing sulfo-groups. The measurements were made in suspensions containing between 40 to 50% coal particles with a mean size of 55pm. These data may be useful in developing surfactant formulations for HWCS stabilisation. 

The effect of surfactant structure on cmc is discussed in Section 6.6. The complex relationship between surface tension and cmc depends on the hydrophobe and the hydrophile, including the counterion, of the surfactant. An increase in the chain length of the hydrophobe decreases cmc branching of the carbon chain increases cmc. Fluorination of the hydrophobe lowers cmc considerably. In addition to the chemical structure of the surfactant, cmc depends on external factors, including electrolyte effects, temperature, and other dissolved or solubilized organic components. 

Solutions of fluorinated surfactants have been investigated and their micellar nature has been confirmed [58,59]. The substitution of the larger and highly electronegative fluorine atom for the smaller hydrogen increases the amphiphilic nature of the surfactant and lowers the surface tension and cmc. The alkali and ammonium salts of perfluoroalkanoic acids exhibit surfactant properties and form micelles for a chain length of four carbon atoms, whereas eight carbon atoms are needed for the nonfluorinated alkanoates. 

In general, organic contaminants induce foaming and inorganics increase surface tension, although clearly there are exceptions. For example, sugar increases surface tension, while tannins, lignosulfonates, car-boxymethyl cellulose (CMC), phosphinocarboxylic acids (PCAs), and other dispersants reduce surface tension and help destabilize foams. 

The influence of the presence of alcohols on the CMC is also well known. In 1943 Miles and Shedlovsky [117] studied the effect of dodecanol on the surface tension of solutions of sodium dodecyl sulfate detecting a significant decrease of the surface tension and a displacement of the CMC toward lower surfactant concentrations. Schwuger studied the influence of different alcohols, such as hexanol, octanol, and decanol, on the surface tension of sodium hexa-decyl sulfate [118]. The effect of dodecyl alcohol on the surface tension, CMC, and adsorption behavior of sodium dodecyl sulfate was studied in detail by Batina et al. [119]. 

Schulze [51] described an extensive study on C12-C14 ether carboxylic acid sodium salt (4.5 mol EO) in terms of surface tension, critical micelle concentration (CMC), wetting, detergency, foam, hardness stability, and lime soap dispersing properties. He found good detergent effect compared to the etho-xylated C16-C18 fatty alcohol (25 mol EO) independent of CaCl2 concentration, there was excellent soil suspending power, low surface tension, and fewer Ca deposits than with alkylbenzenesulfonate. 

Surface Activity. It is obvious from the J -log C plots (F ig.1 and 2) that the addition of alkyl alcohol results in lowering both the surface tension and, the cmc. In particular, it is worth noting that the surface tension of the mixed solution at cmc,7cme, is significantly lower than the surface tension at the pure surfactant cmc. Cmc and 7cwic of 111 (molar ratio) RDH-Surfactant mixed systems were shown in Table 1. Tcmc RDH-CyFNa solution reaches... 

A large number of experimental techniques can be utilized to determine CMC values, the most popular ones being investigations of electrical conductance, surface tension and the solubilization of a compound having a low solubility in water. The... 

The characteristic effect of surfactants is their ability to adsorb onto surfaces and to modify the surface properties. Both at gas/liquid and at liquid/liquid interfaces, this leads to a reduction of the surface tension and the interfacial tension, respectively. Generally, nonionic surfactants have a lower surface tension than ionic surfactants for the same alkyl chain length and concentration. The reason for this is the repulsive interaction of ionic surfactants within the charged adsorption layer which leads to a lower surface coverage than for the non-ionic surfactants. In detergent formulations, this repulsive interaction can be reduced by the presence of electrolytes which compress the electrical double layer and therefore increase the adsorption density of the anionic surfactants. Beyond a certain concentration, termed the critical micelle concentration (cmc), the formation of thermodynamically stable micellar aggregates can be observed in the bulk phase. These micelles are thermodynamically stable and in equilibrium with the monomers in the solution. They are characteristic of the ability of surfactants to solubilise hydrophobic substances. 

Figure 1. Effect of increasing humic acid concentration on the surface tension and apparent solubility of DDT and pyrene. The solid line indicates the position of the HA CMC. Standard deviations are less than the height of the symbols.

Figure 4-12. Relationship between detergency, surface tension, interfacial tension and CMC [9].

Critical Micelle Concentration and Kraff Point Another important characteristic of a surfactant is critical micelle concentration (CMC). CMC is defined as the concentration of surfactants above which micelles are spontaneously formed. Upon introduction of surfactants (or any surface active materials) into the system, they will iifitiaUy partition into the interface, reducing the system free energy by (a) lowering the energy of the interface (calculated as area times surface tension) and (b) removing the hydro-phobic parts of the surfactant from contact with water. Subsequently, when the surface coverage by the surfactants increases and the surface free energy... 

According to this model a clear discontinuity in the physical property of a surfactant solution, such as surface tension and turbidity, should be observed at the cmc. This is not always found experimentally and the cmc is not a sharp break point. 

The results of the CMC determinations are given in Table I which also includes the data of Miyamoto (10). As is generally found, the CMC values obtained from the k vs. N plots are slightly, but definitely, higher than those from the A- N plots which show satisfactory agreement with the y - In M derived values at the two temperatures where comparison is possible. Furthermore, agreement with the values of Miyamoto obtained by surface tension and dye solubilization is satisfactory for CaDS but not so for MgDS. 

FIGURE 2.8 The effect of micelle formation on some solution properties, (a) Schematic picture of micelle formation, (b) Osmotic pressure, surface tension, and turbidity of solutions of sodium dodecyl sulfate (SDS) as a function of concentration (approximate). CMC = critical micellization concentration. 

Over 50 methods have been employed in the literature to determine CMC values of bile salt solutions (reviewed in [6]). These can be divided into two broad categories (a) methods requiring no physical or chemical additive in the bulk solution and (b) methods involving the use of an additive in the bulk solution. The former methods, also called non-invasive, include surface tension and the measurements of a variety of colligative bulk properties (conductivity, turbidimetry, osmometry, self-diffusion, refractive index, modal volumes, electrometric force) or electromagnetic bulk properties (NMR, sound velocity and adsorption, etc.), all as functions of bile salt concentration. The second set of methods, also called invasive, depends upon a change in some physical or chemical property of an additive which occurs with the formation of micelles. These include the spectral change of a water-soluble dye, micellar solubilization of a water-insoluble dye, interfacial tension at liquid-liquid interfaces, and partition coefficients between aqueous and immiscible non-polar phases. Whereas a detailed discussion of the merits and demerits of both approaches can be found elsewhere [6], non-invasive methods which are correctly utilized provide the most reliable CMC values. 

The position of point A in the curve shown in Fig. 1.7 is characteristic for a sufficient purity of the surfactant and solvent. The point is located at a definite surface tension and concentration (CMC). As a rule, a minimum in the y(c) or y(log c) plot is evidence of traces of highly surface-active impurities which affect the results [147]. In chapters 2 to 4 the effect of lateral molecular interaction with increasing alkyl chain length, the effects of changes in orientation and conformation, essentially shown by polymeric surfactants such as proteins, may also lead to significant changes in the adsorption behaviour. 

The situation changes for non-equilibrium systems. The dynamic surface properties of micellar solutions depend strongly on the concentration in a broad range of surface life time and/or of the frequency of surface compression and dilation. First of all this is related to the fact that the adsorption rate of surfactants increases with concentration for both sub-micellar and micellar solutions. As an example, dynamic surface tensions of SDS in 0.1 M NaCl measured by Fainerman and Lylyk [77] are shown in Fig. 7. As one can see entirely different values of the dynamic surface tension and of the adsorption can correspond to the same surface age at c > CMC. 

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