Surface tension hydrocarbon/water interface

Natural surfactants, such as soaps, are made by saponification of fats or triglycerides, such as tri-palmitin in palm oil. The main component of common soap is sodium stearate, C17H35COO" Na , which is made from the saponification of animal fats. When dissolved in water, the carboxylic headgroup ionizes and is strongly hydrophilic, whereas the hydrocarbon chain is hydrophobic. The hydrocarbon chain, alone, is almost completely insoluble in water. When dissolved into aqueous solution, the molecules can adsorb and orientate at the air/solution interface, as illustrated in Figure 4.1, to reduce the surface tension of water ... 

During the past few years, the determination of the interfacial properties of binary mixtures of surfactants has been an area in which there has been considerable activity on the part of a number of investigators, both in industry and in academia. The Interest in this area stems from the fact that mixtures of two different types of surfactants often have interfacial properties that are better than those of the individual surfactants by themselves. For example, mixtures of two different surface-active components sometimes reduce the interfacial tension at the hydrocarbon/water interface to values far lower than that obtained with the individual surfactants, and certain mixtures of surfactants are better foaming agents than the individual components. For the purpose of this discussion we define synergism as existing in a system when a given property of the mixture can reach a more desirable value than that attainable by either surface-active component of the mixture by itself. 

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. 

Emulsion polymerization is known to be a method of carrying out polymerization in a disperse system in which water is usually the dispersion medium. In order to ensure the stability of an emulsion containing 30-60% of the monomer, emulsifiers are used. They are compounds of diphilic type surfactants which decrease the surface tension at the hydrocarbon-water interface. This decrease facilitates the emulsification of the monomer in water and favours the stabilization of the emulsion. 

It is known that adsorption of a surface-active substance (surfactant) on the interface results in a formation at this surface of an oriented monolayer that lowers the surface tension. Typical water solutions of surfactants contain organic molecules with long hydrocarbon tails and polar heads [13]. Hydrocarbons are practically insoluble in water, and water is a highly polar liquid. Figure 17.3 shows how molecules of ideal surfactant are adsorbed on the water surface. The polar heads of molecules penetrate into water, while hydrocarbon tails remain in the gaseous medium. Formation of the monolayer requires a relatively small number of molecules. 

Surfactant efficiency can be expressed as the surfactant concentration needed to reduce the surface or interfacial tension by 20 mN m from the value of the pure solvent(s). Surfactant effectiveness, on the other hand, refers to the maximum reduction in surface or interfacial tension achievable by a surfactant (corresponding to saturation of the surface or interface). To give some sense of the extent to which surfactants can lower surface and interfacial tension, many hydrocarbon surfactants, at high concentrations (above the critical micelle concentration (cmc) see Section 3.5.3) can lower the surface tension of water at 20 "C from 72.8 to about 28 mN m . Polysiloxane surfactants can reduce it further, to about 20 mN m , and perfluoroalkyl surfactants can reduce it still further, to about 15 mN m . Similarly, hydrocarbon surfactants can reduce the interfacial tension of water - mineral oil from about 40 mN m down to about 3 mN m . ... 

These amphiphiles have some unique properties that can be advantageously used in a range of applications as compared to hydrocarbon surfactants in fact, they halve the surface tension of water and, because of the stability of the carbon-fluorine bond, are more stable and can survive in harsh conditions, although environmental issues can arise because of their very low biodegradation. Owing to the lipophobic nature of fluorocarbons, the interfacial behavior shows that fluorosurfactants tend to concentrate at the liquid-air interface with very low lipophilicity, because the electronegativity of fluorine reduces the polarizability of the fluorinated moiety of the surfactants. 

Schulman emphasized that micellar emulsions are systems in true equilibrium, it being proposed that the components of the surface films in these systems produce a negative interfacial tension at the hydrocarbon-water interface [172]. On mixing, a spontaneous interfacial area increase occurs until zero interfacial tension is attained. In Adamson s [173] model for micellar W/O emulsions, stability is accounted for by a balance of the Laplace pressure AP, related to the micellar radius r and interfacial tension y by... 

Replacement of gas by the nonpolar, e.g., hydrocarbon phase (or oil phase) is used to modify the interactions between molecules in a spread film of investigated long-chain substances [6,15,17,18]. The nonpolar solvent-water interface possesses the advantage over that between gas and water, that the cohesion (i.e., interactions between adsorbed molecules due to dipole and van der Waals forces) is negligible. Thus, at the oil-water interfaces behavior of adsorbates is much closer to ideal, but quantitative interpretation may be uncertain, in particular for the higher chains which are predominantly dissolved in the oil phase to an unknown activity. Adsorption of dipolar substances at the w/a and w/o interfaces changes surface tension and modifies the surface potential of water [15] ... 

It is thus seen that the II of a monolayer is the lowering of surface tension due to the presence of monomolecular film. This arises from the orientation of the amphiphile molecules at the air-water or oil-water interface, where the polar group would be oriented towards the water phase, while the nonpolar part (hydrocarbon) would be oriented away from the aqueous phase. This orientation produces a system with minimum free energy. 

One characteristic property of surfactants is that they spontaneously aggregate in water and form well-defined structures such as spherical micelles, cylinders, bilayers, etc. (review Ref. [524]). These structures are sometimes called association colloids. The simplest and best understood of these is the micelle. To illustrate this we take one example, sodium dode-cylsulfate (SDS), and see what happens when more and more SDS is added to water. At low concentration the anionic dodecylsulfate molecules are dissolved as individual ions. Due to their hydrocarbon chains they tend to adsorb at the air-water interface, with their hydrocarbon chains oriented towards the vapor phase. The surface tension decreases strongly with increasing concentration (Fig. 3.7). At a certain concentration, the critical micelle concentration or... 

Very small amounts of type 3 solutes produce a dramatic lowering of the surface tension of aqueous solutions. A substance with this property is called a surfactant. Usually, they consist of hydrocarbon chains, with n = 10-20 connected to polar groups (such as -OH, -CN, -COOH, -COOR, or -CONH2) or ionic groups (such as -SO,, -OSO,, or -NR3+). They exclusively concentrate at the water-air interface. A monolayer of this type of... 

The efficiency of the nonionic trisiloxane surfactants is comparable to nonionic hydrocarbon surfactants with a linear dodecyl hydrophobe. The surface properties of a homologous series of trisiloxane surfactants M(DE OH)M with n = 4—20 show that the CAC, the surface tension at the CAC and the area per molecule each vary with molecular structure in a way that is consistent with an umbrella model for the shape of the trisiloxane hydrophobe at the air/water interface [29]. The log(CAC) and the surface tension at the CAC both increased linearly with EO chain length. 

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