Stearate ions

Detergents are made by, for example, treating petroleum hydrocarbons with sulphuric acid, yielding sulphonated products which are water soluble. These can also solubilise fats and oils since, like the stearate ion, they have an oil-miscible hydrocarbon chain and a water-soluble ionic end. The calcium salts of these substances, however, are soiu u-ic in water and, therefore, remove hardness without scum formation. 

Stearate ion contains two very different str-uctural units—a long nonpolar- hydrocarbon chain and a polar- carboxylate group. The electrostatic potential map of sodium stearate in Figure 19.5 illustrates how different most of the molecule is from its polar- carboxyl-ate end. 

In the case of stearate ions intercalated in Mg/Al LDHs, the electron density shows a pronounced minimiun in the center of the interlayer region, which is consistent with the presence of a bilayer of guest molecules [151,152], The electron density distribution for an anionic azobenzene derivative intercalated in an Mg/Al LDH has also been reported [ 153]. 

LB films prepared from stearate ions 20-and 70-A a-Fe203 particles were incorporated between the headgroups of LB films Molecular orientation and structure was investigated by FTIR absorption and linear dichroic measurements 124... 

Such a molecule can cause particles of grease or oil to mix somewhat with water and to be washed from a dirty article. Calcium and magnesium ions in hard water react with stearate ions to form a solid, yielding soap scum. To avoid this problem, chemists developed synthetic detergents that do not form insoluble salts with calcium and magnesium ions. Synthetic detergents are similar to soaps in that they have an ionic end and a large hydrocarbon-like end. 

The Stearate ion is typical of the anions in soaps. It has a polar carboxylate head,... 

C—O, and a long nonpolar tail, CH3(CH2)i5—. The head of the stearate ion is compatible with ( soluble in ) water, whereas the hydrocarbon tail is compatible with ( soluble in ) oil and grease. Groups of such ions can be dispersed in water because they form micelles (Figure 14-2 la). Their water-insoluble tails are in the interior of a micelle and their polar heads on the outside where they can interact with the polar water molecules. When sodium stearate is stirred into water, the result is not a true solution. Instead it contains negatively charged micelles of stearate ions, surrounded by the positively... 

One of the compounds in soap that helps produce lather is sodium stearate, NaCigH3502, which dissolves in water. In hard water, calcium ions react with the stearate ions to form calcium stearate, Ca(CigH3502)2. This material is insoluble and forms soap scum. Scum is often seen as a ring around sinks or tubs. If calcium ions are removed from the water by water softeners, soap wiU lather and no more scum wiU form. 

The formation of micelles results in a sharp drop in the electrical conductivity per mole of the electrolyte. Suppose 100 sodium and 100 stearate ions were present individually. If the stearate ions aggregate into a micelle and the micelle binds 70 Na as counter ions, then there will be 30 Na ions and 1 micellar ion having a charge of —30 units a total of 31 ions. The same quantity of sodium stearate would produce 200 ions as individuals but only 31 ions if the micelle is formed. This reduction in the number of ions sharply reduces the conductivity. The formation of micelles also reduces the osmotic pressure of the solution. The average molar mass, and thus an estimate of the average number of stearate ions in the micelle, can be obtained from the osmotic pressure. 

Correct choice (B) describes this process. Choice (D) is an incorrect variation. It is not the dirt in the hard water that is precipitating with the soap, it is previously dissolved ions. Choice (C) gives incorrect assumptions about the both the acidic nature of solutions required by soap and the basic property of hard water. Choice (A) uses a somewhat familiar term, hydrolysis, but it is used incorrectly in this case. Hydrolysis for the stearate ion describes the interaction of the ion with water to produce the conjugate acid and release hydroxide ions, not a factor in forming the observed precipitate. ... 

Water containing Ca2+, Mg2+, and Fe2+ ions is known as hard water because, when soap is first added, a lather cannot be obtained. Common soap made from animal fat or vegetable oil is a mixtures of sodium and potassium salts of palmitic, stearic, and oleic acids. These salts are soluble and are dissociated in water. They readily form a lather with pure water and are widely used for cleansing purposes. However, Ca2+, Mg2+, and Fe2+ ions react with soap and form insoluble salts that separate as slimy, sticky precipitates. For example, Ca2+ ions react with stearate ions as follows. 

Hydrophobic colloids can also be stabilized by hydrophilic groups on their surfaces. Oil drops are hydrophobic, for example, so they do not remain suspended in water. Instead, they abnegate, forming an oil slick on the water surface. Sodium stearate FIGURE 13,32), or any similar substance having one end that is hydrophilic (either polar or charged) and one end that is hydrophobic (nonpolar), will stabilize a suspension of oil in water. Stabilization results from the interaction of the hydrophobic ends of the stearate ions with the oil drops and the hydrophilic ends with the water. 

Which kind of intermoiecular force attracts the stearate ion to the oil drop ... 

A FIGURE 13.32 Stabilization of an emulsion of oil in water by stearate ions. 

Figure 13.32 Recall the rule that Ukes dissolve likes. The oil drop is composed of nonpolar molecules, which interact with the nonpolar part of the stearate ion with dispersion forces.

The stearate ion has both a hydrophilic—COj" head and a long organophilic CH3(CH2)i6— tail. As a result, stearate anions in water tend to form clusters consisting of as many as 100 anions clustered together, with their hydrocarbon tails on the inside of a spherical colloidal particle and their ionic heads on the surface in contact with water and with Na+ counter-ions. This results in the formation of micelles, as illustrated in Figure 7.12. 

Figure 7.12. Representation of colloidal soap nticelle particles. The stearate ions are shown...

Lime and Fatty Acids. At high pH (over 11) silica adsorbs calcium iohs which in turn adsorb stearate ions, making the silica hydrophobic enough to be removed from iron ore by flotation. Starch at this pH is apparently adsorbed on the iron oxide preventing interaction with stearate so that iron remains in suspension, according to Sorensen and Frommer (332). 

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