Soap solution surface tension

Qualitatively the equation shows that solutes which lower the surface tension have a positive surface concentration, e.g. soaps in water or amyl alcohol in water. Conversely solutes which increase the surface tension have a negative surface concentration. 

The type of behavior shown by the ethanol-water system reaches an extreme in the case of higher-molecular-weight solutes of the polar-nonpolar type, such as, soaps and detergents [91]. As illustrated in Fig. Ul-9e, the decrease in surface tension now takes place at very low concentrations sometimes showing a point of abrupt change in slope in a y/C plot [92]. The surface tension becomes essentially constant beyond a certain concentration identified with micelle formation (see Section XIII-5). The lines in Fig. III-9e are fits to Eq. III-57. The authors combined this analysis with the Gibbs equation (Section III-SB) to obtain the surface excess of surfactant and an alcohol cosurfactant. 

Soap. The reaction product of a fatty acid ester and a metal hydroxide, usually sodium hydroxide. Soap lowers the surface tension of water, permitting emulsification of soil-bearing fats if the soap is used for washing, of monomers in solution if the soap is used for emulsification in a polymerization process. 6 e saponification. 

The Gibbs adsorption theory (Birdi, 1989,1999, 2002, 2008 Defay et al., 1966 Chattoraj and Birdi, 1984) considers the surface of liquids to be monolayer. The surface tension of water decreases appreciably on the addition of very small quantities of soaps and detergents. The Gibbs adsorption theory relates the change in surface tension to the change in soap concentration. The experiments that analyze the spread monolayers are also based on one molecular layer. The latter data indeed conclusively verifies the Gibbs assumption (as described later). Detergents (soaps, etc.) and other similar kind of molecules are found to exhibit self-assembly characteristics. The subject related to self-assembly monolayer (SAM) structures will be treated extensively (Birdi, 1999). However, no procedure exists that can provide information by direct measurement. The composition of the surface of a solution with two components or more would require additional comments. 

The magnitude of surface tension change will depend on the concentration and the solute added. In some cases, the surface tension (y) of the solution (such as NaCl) increases. The change in y may be small (per mole added) (as in the case of inorganic salts) or large (as in the case of such molecules as ethanol or other soap-like molecules) with the addition of solute (equal gram per liter) ... 

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 ... 

Kamei and Oishi (K4), 1956 Flow of films of water, soap solution, millet-jelly solutions inside tubes of diameter 5.09-1.9 cm. X 100 cm., with zero and countercurrent air flow. Kinematic viscosities 1.1-40 cs. As = 1-4200. Surface tension found to affect holdup. 

There is also segregation of solutes to free surfaces. One example is soapy water. Soap segregates to the surface, lowering the surface energy (surface tension). 

As children, most of us were fascinated by soap bubbles. One cannot blow bubbles of pure water. We were told that soap made bubbles possible because soap lowered the surface tension by segregating to the surface of the water. However, it is not the surface tension, per se, that permits bubbles to be blown from soapy water. It is because the surface tension of soap solutions is variable. The surface tension at the top of a bubble is higher than that at the bottom because of the weight of the water it must support. This requirement can be met in soap solutions by different surface concentrations at different locations. 

This is an important stabilising effect in foams which are formed from solutions of soaps, detergents, etc. If a film is subjected to local stretching as a result of some external disturbance, the consequent increase in surface area will be accompanied by a decrease in the surface excess concentration of foaming agent and, therefore, a local increase in surface tension (Gibbs effect). Since a certain time is... 

An absence of the Gibbs-Marangoni effect is the main reason why pure liquids do not foam. It is also interesting, in this respect, to observe that foams from moderately concentrated solutions of soaps, detergents, etc., tend to be less stable than those formed from more dilute solutions. With the more concentrated solutions, the increase in surface tension which results from local thinning is more rapidly nullified by diffusion of surfactant from the bulk solution. The opposition to fluctuations in film thickness by corresponding fluctuations in surface tension is, therefore, less effective. 

Soaps and detergents make water wetter by lowering the surface tension of the water. In doing so, less energy is required to lift dirt off whatever it is on. Other agents within a cleaning solution may include materials that help emulsify oily matter, soften water, solubilize compounds, control pH, and perform other actions to assist the cleaning process. 

Solutions of soaps and other long-chain Colloidal electrolytes. The surface tension of soaps has been very extensively studied,1 but for the most part the results in the literature are discordant far beyond the usual error of measurement of surface tension. In general the surface tension diminishes rapidly with increasing concentration, reaching a steady, or nearly steady, low value after a certain concentration is reached this concentration is naturally lower the longer the hydrocarbon chain. The variation between the results obtained by different experimenters, and even by the same experimenter under different conditions, may... 

Addition of alkali suppresses the hydrolysis, and when sufficient alkali has been added for complete suppression of hydrolysis, the adsorbed layer consists of neutral soap. This is probably the state of affairs at the maxima of surface tension in Fig. 32. More alkali is required to reach the maximum with the stronger solutions, because more alkali is needed to suppress hydrolysis completely. The maximum surface tensions in Fig. 32 are probably very near to the surface tension of solutions of neutral soap only of the concentration indicated on each curve. The subsequent slow fall of tension, as more alkali is added, is probably due to a salting out of the soap by the alkali, an increase in escaping tendency caused by the presence of comparatively large amounts of another solute. It would be interesting to find whether addition of neutral salt, in addition to the small amount of alkali needed to reach the maximum, produces a fall in tension similar in amount to that given by additional alkali. 

So-called neutral soap solutions are more complex systems than at first appears, and the fairly small amounts of hydrolysis in the interior of the solution are very much magnified by the great difference in adsorbability between a neutral soap and an acid soap, or free fatty acid. A close approximation to the true surface tension-concentration curve of neutral soap can probably be obtained by plotting the maxima of tension shown in Fig. 32 against the concentration on each curve. It does not seem possible as yet to plot the true surface tension-concentration curve for acid soap, as the concentration of acid soap, and even the ratio of acid to soap molecules in any compound which may be formed in the interior, is unknown. 

According to Lottermoser and Baumgurtel,3 the final surface tension of soap solutions is reached in a few minutes Ekwall, however, records a fall lasting at least two hours in solutions more dilute than N/20,000. 

Paraffin-chain salts, similar in general constitution to the soaps but containing a strongly dissociated end group such as a sulphonic acid or a quaternary ammonium atom, are not subject to hydrolysis, and might be expected to behave in a simpler manner. The surface-tension measurements of Adam and Shute,1 R. C. Brown,2 and Lottermoser and others8 indicate, however, a curious, very slow attainment of the final surface tension in solutions so dilute that there are few, if any, ionic micelles present in the interior. The tension may take several days to reach the final value and when the final tension is reached it appears to be independent of the concentration, at least for solutions over 0-003 per cent., i.e. of the order N/10,000. The amount of this final tension depends somewhat on the nature of the end group it is usually about 30 dynes per cm. 

There are static and dynamic methods. The static methods measure the tension of practically stationary surfaces which have been formed for an appreciable time, and depend on one of two principles. The most accurate depend on the pressure difference set up on the two sides of a curved surface possessing surface tension (Chap. I, 10), and are often only devices for the determination of hydrostatic pressure at a prescribed curvature of the liquid these include the capillary height method, with its numerous variants, the maximum bubble pressure method, the drop-weight method, and the method of sessile drops. The second principle, less accurate, but very often convenient because of its rapidity, is the formation of a film of the liquid and its extension by means of a support caused to adhere to the liquid temporarily methods in this class include the detachment of a ring or plate from the surface of any liquid, and the measurement of the tension of soap solutions by extending a film. 

McBain1 has drawn attention to the existence of a number of cases in which strongly surface active substances first lower the tension of water considerably, but as the concentration increases further, the surface tension either becomes nearly constant or shows a slight increase. The cases for which the evidence is most conclusive are aqueous solutions of soaps or other paraffin chain salts and of some dyestuffs. The minimum surface tension often occurs in quite dilute solution, e.g. with dodecyl sodium sulphate2 it is about 0 006 N the exact concentration of the minimum, however, depends on the purity of the water used as solvent. 

It is very doubtful, however, if these solutions can be treated as two component systems they contain water and at least two ions, of which the adsorption is not the same and soaps, unless rather strongly alkaline, contain free fatty acid which is much more strongly adsorbed than the anion of the fatty acid. The course of a surface tension-concentration curve in a solution with several components may be complicated it seems possible that a rise of tension with increasing concentration may be due to a moderately surface active ion, present in large amount, displacing a more surface active component, present in smaller amount, from the surface. 

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