Calcium ions, soap precipitation

The sodium soaps of fatty acid form calcium soaps of such low solubdity that they act as their own budders. Initial soap additions precipitate the calcium ion and the soap added thereafter functions in soft water. At high temperatures, the calcium soaps are relatively soluble compared to calcium tripolyphosphate. Thus sodium tripolyphosphate (STEP) can budd (revert) soaps in a hot water wash. However, at low temperatures the relative affinity of STEP for calcium decreases so that STEP cannot budd soaps in a cold water wash. 

Soaps are composed of sodium salts of various fatty acids. These acids include those with the general structure CH3-(CH2) -COOH where n = 6 (caprylic acid), 8 (capric acid), 10 (lauric acid), 12 (myristic acid), 14 (palmitic acid), and 16 (stearic acid). Oleic acid (CH3-(CH2)7-CH=CH-(CH2)7-COOH) and linoleic acid (CH3-(CH2)4-CH=CH- H2-CH=CH-(CH2)7-COOH) are also common soap ingredients. These sodium salts readily dissolve in water, but other metal ions such as Ca2+ and Mg2+ form precipitates with the fatty acid anions. For example, the dissolution of the sodium salt of lauric acid and the subsequent formation of a precipitate of the lauric acid anion with calcium ion is given by... 

Soaps react with the calcium and magnesium ions in hard water to produce soap curd that greatly reduces its effectiveness. The curds are actually insoluble calcium and magnesium salts. Synthetic laundry detergents have replaced soap for cleaning clothes in the last half century. Synthetic detergents are made from petroleum. They work like soap except they do not react with magnesium and calcium ions to form insoluble precipitates and salts. 

Sometimes people refer to water as being "hard" or "soft." If someone says water is hard, he means that there are a lot of calcium (Ca2+) or magnesium (Mg2+) ions dissolved in it. Hard water causes several problems. First, it can cause scales to form on the inside of pipes, water heaters, or teakettles. These scales occur when the calcium or magnesium precipitates out of solution and sticks to the insides of pipes. The scales build up and eventually the pipes are completely clogged. Hard water also prevents soap from lathering, and it reacts with soap to leave behind a sticky film commonly called soap scum. 

There are two solutions to soften water that is too hard. First, the water could be filtered to remove all of the calcium and magnesium ions. But this can be very expensive, so most people use a water softener instead. A water softener is a piece of equipment that can be attached to the water pipes that run into a house. This way, all the household water goes through the water softener before going into the other pipes in the house. Inside a water softener are small plastic beads. These beads have sodium ions (Na+) stuck on them. As the water flows over the bed of beads, the magnesium and calcium ions get replaced with the sodium ions. Since sodium is easily dissolved in water, it does not precipitate out in pipes like calcium and magnesium ions do. As a result, no scales form inside the pipes. Sodium ions do not react with soap to form soap scum, either, and they allow the soap to lather properly. 

Acylate ions are amphiphilic, and the hydrocarbon chains are able to penetrate fatty (hydrophobic) particles, leaving the surface of the particle ionic. (See Fig. 6-26.) Thus, the particle behaves as a micelle and is readily soluble in water. The sodium and potassium salts of fatty acids are soaps. Soaps have poor detergent properties in hard water because the calcium present in such water causes the micelles to aggregate and precipitate. The divalent calcium ion can act as a bridge between two micelles, but since a micelle is polyvalent, a small amount of calcium relative to the amount of the soap can cause all the micelles to aggregate. 

Soap scum is an insoluble precipitate that forms between the cations of minerals typically present in hard water and the anions of soaps and detergents. Divalent cations of calcium (Ca2+) and magnesium (Mg2+) from calcium carbonate and magnesium carbonate minerals are the primary components of hard water. Divalent cations of iron (Fe2+), manganese (Mn2+), and strontium (Sr2+) are also often present. An example of the dissolution (dissolving) process that releases calcium ions from calcium-containing minerals in contact with water with high acid levels is... 

Fatty acids, such as oleic and coco fatty acid (saturation level) can serve a multifunctional role when added to HDLDs. Although they primarily provide a foam suppression capability, they can also precipitate out some of the calcium ions in the wash by forming calcium soap. This could, however, pose a problem, since soap scum, commonly known as lime soap, is insoluble and may have an impact on the overall cleaning result. 

For soap to work, its anions must stay in solution. Unfortunately, they tend to precipitate from solution when the water is hard. FFard water is water that contains dissolved calcium ions, Ca, magnesium ions, Mg +, and often iron ions, Fe or Fe. These ions bind strongly to soap anions, causing the soap to precipitate from hard water solutions. 

Anionic Surfactants Early studies by Savins [1967] showed that sodium oleate soaps with potassium hydroxide and potassium chloride in aqueous solution had good DR effectiveness. Increasing the concentration of KCl from 5% to 10% gave better drag reduction results. Unfortunately, the soaps precipitate and are ineffective as DRAs in the presence of calcium ions, which are present in most aqueous systems. Solutions of anionic surfactants such as SDS and SDBS are not drag reducing. Little research on anionics as DRAs has been carried out. 

Soap has been used for over two millennia, as people discovered long ago that soap could be made by heating animal fat together with wood ashes, which contain alkaline substances. Nevertheless, the usefulness of soap is diminished in the presence of water that contains high concentrations of calcium ions (Ca ) or magnesium ions (Mg ). When soap is used with such water, called hard water, a precipitate is formed as a result of the following ion exchange reaction. 

These are one of the oldest toiletries products, having been used for over centuries. The earliest formulations were based on simply fatty add salts, such as sodium or potassium palmitate. However, these simple soaps suffer from the problem of calcium soap precipitation in hard water. For that reason, most soap bars contain other surfactants such as cocomonoglyceride sulphate or sodium cocoglyceryl ether sulphonate that prevent precipitation with caldura ions. Other surfadants used in soap bars indude sodium cocyl isethinate, sodium dodecyl benzene sulphonate and sodium stearyl sulphate. 

Some water-softening (conditioning) units which are sold for domestic use are advertized as no saltwater conditioners. These units are reported to remove CaCOs from hard water by using a catalyst, which by epitaxial nucleation, and the reduction of pressure by virtue of a change in water velocity, converts the Ca(HC03)2 into CaCOs and CO2. These units are very attractive and are advertised to work with detergents but are not intended for use with soap. This can be interpreted to mean that the calcium ions are not removed from the water system and that a precipitate will form from the calcium salt of the fatty acid from the soap (Eq. 15.14). 

The role of calcium soap particles as antifoam additives is surprisingly poorly understood. Available evidence (which is scant) suggests that such particles may function simply as hydrophobic particles. However, measurements of the relevant contact angles are few and never combined with imaging and sizing of the particles. Moreover, there is evidence of transient antifoam effects accompanying the precipitation of such particles by reaction of calcium ions with soaps [198]. There is as yet no explanation for such transient effects. 

Practical laundry detergent formulations usually include builders for control of polyvalent metal ion activities so that the relevant salts of the anionic surfactant, which is usually present, are not precipitated out of solution. The presence of builders also has implications for the effectiveness of antifoam action because they may prevent the precipitation of calcium soaps. We can illustrate the relevant issues if we consider, for example, the concentration of calcium harduess uecessary to just initiate calcium soap precipitation in the case of a solution built by sodium tripolyphosphate at a pH >10.5. If we iguore the effect of solubilizatiou of soaps and calcium binding in anionic surfactant micelles, then, according to Irani and Callis [4],... 

Reliance on the reaction of hardness ions with soap or alkyl phosphate ions to form calcium soap or calcium alkyl phosphate precipitates for foam control has an obvious limitation in the case of soft water. This factor has encouraged a search for alternative materials that can also form finely divided hydrophobic particles. Hydrocarbon waxes represent an obvious choice in this context. 

Calcium ion, along with magnesium and sometimes iron(II) ion, accounts for water hardness. The most common manifestation of water hardness is the eurdy precipitate formed by the reaction of soap, a soluble sodium salt of a long-chain fatty acid, with calcium ion in hard water ... 

The solubility of calcium carbonate is such that in a saturated solution the product of ion concentrations [Ca+2][C0 2] is 5 X I0 B. Though this may seem quite small, it is large enough to be important to man, especially if he lives in a region of the earth where there are extensive limestone deposits. Calcium carbonate can be dissolved in water, especially if it contains much dissolved C02. This is objectionable because soap added to water which contains even traces of Ca+2 forms a precipitate of calcium stearate. This is the ring that is so difficult to remove from the bathtub. 

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