Surface tension method

Alcohol sulfates commonly have free alcohol and electrolytes as impurities. Other hydrophobic impurities can also be present. A method suitable for the purification of surfactants has been proposed by Rosen [120]. Consequently, commercial products have CMCs that deviate from the accepted reference values. This was demonstrated by Vijayendran [121] who studied several commercial sodium lauryl sulfates of high purity. The CMC was determined both by the conductimetric method and by the surface tension method. The values found were similar for both methods but while three samples gave CMC values of 7.9, 7.8, and 7.4 mM, close to the standard range of 8.0-8.2 mM, three other samples gave values of 4.1, 3.1, and 1.7 mM. The sample with a CMC of 7.9 mM was found to have a CMC of 8.0 mM with no detectable surface tension minima after purification and recrystallization. This procedure failed in all other cases. 

CMCs of LAS-AOS mixtures in water that contained no calcium or magnesium ions were determined by Suri et al. [3] by means of the surface tension method. The purpose of this work was to study the advantage of LAS-AOS synergism in prototype phosphate-free carbonate-built formulations for use in the Indian market. The authors have used a commercial C10-C14 LAS sample with an average molecular weight of 343 and a commercial C16-C18 AOS sample with an average molecular weight of 350. 

Surface recombination, at semi conductors, 490 Surface reconstruction of gold, 83 and work of Kolb, 86 Surface tension and determination of the potential of zero charge, 32 Surface tension methods, and the potential of zero charge, 32 Surfaces,... 

None of these methods have been widely adopted because of their relative complexity or because they are not generally applicable to any arbitrary surfactant system. As examples, the surface tension method (4) relies on the pure surfactants having substantially different plateau surface tensions the ultrafi1tration method (1,) requires the micelles of the pure surfactants involved to have approximately the same size and the... 

The isolation of the microbubble surfactant mixture from forest soil has been described in detail in preceding chapters. Quantitative examination of the surface properties of mono-molecular films of the isolated microbubble surfactant complex, at an air/water interface, were carried out using a modified Langmuir trough apparatus incorporating the surface tension method of Padday et al. (ref. 380). 

Figure 3.8 Illustrations of the Wilhelmy plate (upper) and du Nouy ring (lower) surface tension methods. Both illustrations are for wetting contact angles.

The du Noiiy ring surface tension method involves slowly raising a platinum ring through a liquid until it detaches from the surface (Figure 3.8, Lower). The force at the point of detachment is measured using a balance or torsion wire. If F is the force on the ring,... 

Figure 3.9 Illustration of the drop weight/volume surface tension method (shown here for a wetting contact angle).

A number of methods are available for the measurement of surface and interfacial tension of liquid systems. Surface tension of liquids is determined by static and dynamic surface tension methods. Static surface tension characterises the surface tension of the liquid in equilibrium and the commonly used measurement methods are Du Notiy ring, Wilhelmy plate, spinning drop and pendant drop. Dynamic surface tension determines the surface tension as a function of time and the bubble pressure method is the most common method used for its determination. 

Dharmawardana, U.R. Christian, S.D. Tucker, E.E. Taylor, R.W. Scamehorn, J.E. A surface tension method for determining binding constants for cyclodextrin inclusion complexes of ionic surfactants. Langmuir 1993, 9 (9), 2258-2263. 

CMCs of these polyfluorinated surfactants are of the order of 10 5 m.47 Plots of the observed H chemical shifts versus surfactant concentration of cetyl trimethyl ammonium chloride, cetyl pyridinium chloride, cetyl dimethyl phenyl ammonium chloride, cetyl dimethyl benzyl ammonium chloride, cetyl dimethy 1-2-phenyl ethyl ammonium chloride, and cetyl dimethyl-3-phenyl propyl ammonium chloride, are sigmoidal and were fitted to a model based on the mass action. The H chemical shift-based CMC values are in excellent agreement with those determined by the surface tension method.48 The micellization processes of dodecyl trimethyl ammonium halides (chloride and bromide) studied by calorimetric titration show different behaviors at 298 K. However, these disappear at 313 K, while the results measured by the chemical shift versus surfactant concentration do not show this difference.49 The CMC of 3-aminopropyl triethoxy silane in toluene is ca. 0.47m, measured by H and l3C chemical shifts.50 The CMC of optically active potassium A -n-dodecanoyl alaminate measured by H and l3C chemical shifts is lower (11-15 him) in D20 than that in a mixed solvent of 1,4-dioxane and D20 (19mM).-51 The H chemical shift shows that the CMC of resorcinol-type calix[4] phosphoric esters having four alkyl side-chains, [4]Ar 5P-R-n, is insensitive to the length of the side-chains, n.52 The CMC values of a family of surfactants, the sodium cyclohexyl alkanoates, with different lengths of the alkanoate side-chains, were obtained from 13C chemical-shift measurements. The results show that these amphiphiles have high CMCs (0.12-1.02 m).-53... 

For dynamic adsorption studies several methods are suitable. The selection of a certain method depends on the experimental conditions, temperature and time interval, of the investigations to be performed. These experimental conditions are governed by the surface activity of the surfactant or polymer under study and its concentration. Some characteristics of dynamic surface tension methods are summarised in Table 5.2. 

The surface activity of surfactants varies over a broad concentration range and hence a broad time interval has to be studied. Therefore, complementary experiments are necessary to cover the extensive time range from less than a millisecond up to minutes, hours and sometime days. The following Table 4.2 summarises the most frequently used surface tension methods, their available time and temperature intervals and suitability for studying the liquid/air and liquid/liquid interface. The given values represent the interval available by standard instruments, while particular modifications certainly allow to go beyond these limits. 

Studies of the adsorption kinetics for a surface age comparable with the characteristic time X2 seem to be most interesting. For many surfactants the fast process is usually beyond the time interval accessible to dynamic surface tension methods. Besides, the fast process leads to a release or incorporation of only a small part of monomers in comparison to the slow process, and one can expect a noticeable influence on the adsorption only at high concentrations. If we assume that the micellar concentration is not too high and the size distribution of micelles is... 

It is not permissible to adopt data acquired from changes of the liquid resin s surface tension for solid state polymer. Zisman [72] does it supposing that the reversible adhesion work of the solid polymer must be close to that estimated for the liquid state. The conclusion follows from the assumption that the forces that act on the phase separation boundary spread out to a depth that does not exceed the size of some molecules. As a result, the interaction on the bound y cannot depend on the change of state of the substance. One must accept this because the determined v dues of the surface tension of solid polymers significantly exceed those for the liquid oligomers. If we deal with undercured products the v dues usually exceed those for the polymer surface tension acquired by wetting agent critic d surface tension methods. 

In the light of the critical aggregate concentration (CAC) of APC-(C2Lys2Cj 4)4 obtained by the surface tension method based on the Wilhel-my principle (1 x 10 mol-dm ), all the spectroscopic measurements were carried out below its CAC. APC(C2qN )4 was used as a reference host, and the following fluorescent probes were chosen as guest molecules ... 

NMR, up to 180 °C by Shinoda et al. [52]. Some work has been reported involving erne determination by calorimetry (measuring heats of dilntion or specific heats). Archer et al. [53] nsed flow calorimetry to determine the erne s of several sulfonate surfactants at up to 178 °C. NoU [5J] determined erne s for dodecyltrimethylammonium bromide and commercial surfactants in the temperature range 25—200 °C using flow calorimetry. Surface tension is the classical method for determining erne s but many surface tension methods are not suitable for use with aqueous solutions at elevated temperatures. Exceptions include the pendant, sessile, and captive drop methods which can be conducted with high-pressure cells [54, 55]. 

The concept of transitions, Ti and T2, was introduced by Jones (24) on the basis of his surface tension data on PEO/SDS mixed systems. His findings were confirmed and extended by Schwuger (26). Another system widely studied by the surface tension method... 

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