Surfaces activity

The surface activity of polysoaps has rarely been studied. This is surprising because surface activity is one of the key features in soap performance, as 

In the existing studies on the surface activity of polysoaps, three cases can be distinguished  

There are some additional reports stating considerable surface activity of polysoaps, but without specifying the concentration dependence [152,194, 301], 

The appearance of surface activity and an apparent CMC could well be caused by low molecular weight contaminants, originating e.g. from insufficient removal of educts or as by-products of the synthesis, or from partial decomposition [143, 144, 213]. The strong effects of hydrophobic counterions bound to polyelectrolytes on the surface tension are well known [350] and traces many suffice to provoke the effect. Alternatively, apparent CMC s of polysoaps have been reported when low oligomers are involved [241, 242, 251, 353]. Thus dimers or trimers etc. might be responsible for the effect, exhibiting inter-molecular micellization (see Sect. 6). 

The appearance of CMC s for polysoaps bearing particularly short hydro-phobic tails [193] is rather a semantic problem as these examples do not match the original definition of polysoaps anymore. If the hydrophobe tails are too short, no intramolecular aggregation can take place as evident from viscosity measurements [24, 133, 193], and intermolecular aggregation is needed to reduce hydrophobic interactions. 

The defining characteristic of surfactants is their ability to lower surface tension at the air-water interface. Surface tension results from an imbalance in intermolecular forces at the surface of a liquid. There are fewer molecules on the vapour side than on the liquid side of molecules near the surface, leading to a net repulsive force and hence a gradual decrease in density (Fig. 4.5). Surface tension, y, can be defined in two equivalent ways. First, in terms of 

Alternatively, surface tension is given by the force per unit length associated with this process. The equivalence of these two definitions can be readily confirmed by a simple example. Consider a liquid film (such as soap film) suspended on a wire frame, which is stretched by moving a slider (Fig. 4.6). The surface tension is the force per unit length, y = F/(2/), where the factor of two arises because the film has two sides. Then the work done for an infinitesimal extension dx is 

These equivalent definitions imply that surface tension can be expressed either in units of mj m , or more commonly in mN m . The latter convention will be adopted in the remainder of this chapter. 

Surface tension can be measured in many ways. One of the most accurate and conceptually simple methods is to measure the rise of a liquid in a capillary (Fig. 4.7a). The surface tension is related to the height of liquid supported by gravity, the tube radius, the contact angle of the liquid meniscus and the density difference between liquid and vapour. The determination of surface tension using this capillary rise method is easiest when the liquid completely wets the capillary wall, i.e. when the contact angle (Section 4.5.1) is near zero. 

Another conceptually simple method is to weigh falling drops of a liquid. The surface tension of drops at the point of detachment from a vertically mounted tube is proportional to their weight. Care has to be taken to make 

Traditional surfactant molar masses range from a few hundreds up to several thousands. In addition to traditional surfactant structures, some block and graft copolymers can be surface active as well, having much higher molar masses. As there will be a balance between adsorption and desorption (due to thermal motions) the interfacial condition requires some time to establish. Because of this, surface activity should be considered a dynamic phenomenon. This can be seen by measuring surface tension versus time for a freshly formed surface. 

A consequence of surfactant adsorption at an interface is that it provides an expanding force acting against the normal interfadal tension. If % is this expanding pressure (surface pressure), then y = /solvent Thus, surfactants tend to lower interfadal tension. If a low enough value of y is reached, emulsification can take place because only a small increase in surface free energy is required, for example, when Jt /solvent- solute-solvent forces are greater than solvent-solvent forces. 

1) Synonyms for surfectants include amphiphiles, surface-active agents, tensides or, in the very old literature, paraffin-chain salts. 

Spherioai miceiie of an anionic Surfactant with its counter ions 

From thermodynamics, the lowering of surface free energy due to surfactant adsorption is given by the Gibbs adsorption equation for a binary, isothermal system containing excess electrolyte  

Spherical micelle of an anionic surfactant with its counter ions 

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 see Section 3.5.3), can lower the surface tension of water at 20 °C from 72.8 mN/m 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 down to about 3 mN/m. 

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Soaps of heavy metals have been used but cationic surface-active agents have proved more suitable, notably organic amines of relatively high molecular weight. 

Dimeihylamine, C2H7N, (CH3)2NH. Colourless, inflammable liquid with an ammoniacal odour, mp -96" C, b.p. 7°C. Occurs naturally in herring brine. Prepared in the laboratory by treating nitrosodimetbyl-aniline with a hot solution of sodium hydroxide. Dimethylamine is largely used in the manufacture of other chemicals. These include the solvents dimethylacetamide and dimethyl-formamide, the rocket propellant unsym-metrical dimethylhydrazine, surface-active agents, herbicides, fungicides and rubber accelerators. 

Sorbitol is manufactured by the reduction of glucose in aqueous solution using hydrogen with a nickel catalyst. It is used in the manufacture of ascorbic acid (vitamin C), various surface active agents, foodstuffs, pharmaceuticals, cosmetics, dentifrices, adhesives, polyurethane foams, etc. 

These surface active agents have weaker intermoiecular attractive forces than the solvent, and therefore tend to concentrate in the surface at the expense of the water molecules. The accumulation of adsorbed surface active agent is related to the change in surface tension according to the Gibbs adsorption equation... 

Results can sometimes be unexpected. The first study of this type made use of labeled Aerosol OTN [111], an anionic surfactant, also known as di-n-octylsodium sulfosuccinate. The measured F was twice that in Eq. III-93 and it was realized that hydrolysis had occurred, that is, X + H2O = HX + OH , and that it was the undissociated acid HX that was surface-active. Since pH was essentially constant, the activity of HX was just proportional to C. A similar behavior was found for aqueous sodium stearate [112]. 

Derive the equation of state, that is, the relationship between t and a, of the adsorbed film for the case of a surface active electrolyte. Assume that the activity coefficient for the electrolyte is unity, that the solution is dilute enough so that surface tension is a linear function of the concentration of the electrolyte, and that the electrolyte itself (and not some hydrolyzed form) is the surface-adsorbed species. Do this for the case of a strong 1 1 electrolyte and a strong 1 3 electrolyte. 

As mentioned in Section IX-2A, binary systems are more complicated since the composition of the nuclei differ from that of the bulk. In the case of sulfuric acid and water vapor mixtures only some 10 ° molecules of sulfuric acid are needed for water oplet nucleation that may occur at less than 100% relative humidity [38]. A rather different effect is that of passivation of water nuclei by long-chain alcohols [66] (which would inhibit condensation note Section IV-6). A recent theoretical treatment by Bar-Ziv and Safran [67] of the effect of surface active monolayers, such as alcohols, on surface nucleation of ice shows the link between the inhibition of subcooling (enhanced nucleation) and the strength of the interaction between the monolayer and water. 

When a surface-active agent is present in a liquid droplet, it can adsorb to the surface, lower the surface energy, and cause the liquid contact angle to increase. This phenomenon, known as autophobicity, was postulated by Zisman many years ago [78, 79]. Autophobicity is quite striking in wetting films on clean... 

It is not necessary to limit the model to idealized sites Everett [5] has extended the treatment by incorporating surface activity coefficients as corrections to N and N2. The adsorption enthalpy can be calculated from the temperature dependence of the adsorption isotherm [6]. If the solution is taken to be ideal, then... 

The surface-active agents (surfactants) responsible for wetting, flotation and detergency exhibit rather special and interesting properties characteristic of what are called association colloids or, in the older literature, colloidal electrolytes. These properties play an important role in determining, at least indirectly, the detergency of a given surfactant and are therefore considered here... 

Surface active electrolytes produce charged micelles whose effective charge can be measured by electrophoretic mobility [117,156]. The net charge is lower than the degree of aggregation, however, since some of the counterions remain associated with the micelle, presumably as part of a Stem layer (see Section V-3) [157]. Combination of self-diffusion with electrophoretic mobility measurements indicates that a typical micelle of a univalent surfactant contains about 1(X) monomer units and carries a net charge of 50-70. Additional colloidal characterization techniques are applicable to micelles such as ultrafiltration [158]. 

In addition to lowering the interfacial tension between a soil and water, a surfactant can play an equally important role by partitioning into the oily phase carrying water with it [232]. This reverse solubilization process aids hydrody-namically controlled removal mechanisms. The partitioning of surface-active agents between oil and water has been the subject of fundamental studies by Grieser and co-workers [197, 233]. 

After reviewing various earlier explanations for an adsorption maximum, Trogus, Schechter, and Wade [244] proposed perhaps the most satisfactory one so far (see also Ref. 243). Qualitatively, an adsorption maximum can occur if the surfactant consists of at least two species (which can be closely related) what is necessary is that species 2 (say) preferentially forms micelles (has a lower CMC) relative to species 1 and also adsorbs more strongly. The adsorbed state may also consist of aggregates or hemi-micelles, and even for a pure component the situation can be complex (see Section XI-6 for recent AFM evidence of surface micelle formation and [246] for polymeric surface micelles). Similar adsorption maxima found in adsorption of nonionic surfactants can be attributed to polydispersity in the surfactant chain lengths [247], Surface-active impuri-... 

M. J. Rosen and H. A. Goldsmith, Systematic Analysis of Surface-Active Agents, Wiley-Interscience, New York, 1972. 

K. Durham, Ed., Surface Activity and Detergency, Macmillan, New York, 1961. 

An important industrial example of W/O emulsions arises in water-in-crude-oil emulsions that form during production. These emulsions must be broken to aid transportation and refining [43]. These suspensions have been extensively studied by Sjoblom and co-workers [10, 13, 14] and Wasan and co-workers [44]. Stabilization arises from combinations of surface-active components, asphaltenes, polymers, and particles the composition depends on the source of the crude oil. Certain copolymers can mimic the emulsion stabilizing fractions of crude oil and have been studied in terms of their pressure-area behavior [45]. 

J. L. Moilliet, B. Collie, and W. Black, Surface Activity, E. F. N. Spon, London, 1961. D. H. Napper, Polymeric Stabilization of Colloidal Dispersions, Academic, New York,... 

W. D. Harkins, TTie Physical Chemistry of Surface Active Films, Reinhold, New York, 1952, p. 90. 

Proteins, like other macromolecules, can be made into monolayers at the air-water interface either by spreading, adsorption, or specific binding. Proteins, while complex polymers, are interesting because of their inherent surface activity and amphiphilicity. There is an increasing body of literature on proteins at liquid interfaces, and here we only briefly discuss a few highlights. 

Adsorptive stripping analysis involves pre-concentration of the analyte, or a derivative of it, by adsorption onto the working electrode, followed by voltanmietric iiieasurement of the surface species. Many species with surface-active properties are measurable at Hg electrodes down to nanoniolar levels and below, with detection limits comparable to those for trace metal detemiination with ASV. 

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