For this project, according to the lab manual1, research was needed to be done for a local environmental group. The environmental group needed help developing safe soap and detergent options to use on birds involved in an oil spill accident. The group was mainly interested in an alternative soap to one made from lard. A few goals of the investigation were to be accomplished to determine which starting material would be best in making the most environmentally friendly soap. The first goal was to test the solubility of the starting materials used in the making of the soaps, the solubility of the soaps themselves, and the solubility of the detergents. The next goal was to compare the desirable properties of the soaps and detergents. Then, an examination of the impact of each of the soaps and detergents on the environment was to be conducted. After that, an investigation of the cause of soap scum and how to prevent it was carried out. The final goal was to make a decision concerning which soap or detergent would be the best in terms of what the environmental group was expecting and the qualifications determined from the previous goals.
A soap, when mixed with water, is used for cleaning and washing the skin. It is a surfactant, meaning it reduces surface tension of water allowing soap molecules to spread out and cover the surface being cleaned. Soaps are made from a natural oil or fat and a strong alkali. Some examples of natural oils and fats are olive oil, vegetable oil, lard (animal fat), and shortening (a fat or butter using in baking). An alkali is a base, which has a high concentration of hydroxide (OH-) ions and gives a pH greater than the neutral pH, 7. A strong alkali, or a strong base, is one with a pH closer to 14 than 7 on the pH scale. A strong base would completely dissociate, or dissolve, in water into its ions, including hydroxide ions.
A detergent is a cleaning agent that is soluble in water. Detergents are made from similar mixtures of chemical compounds as soaps, but that are less reactive to hard water. In the making of detergents, phenolphthalein is used. According to Soaps, Detergents, and Disinfectants2, phenolphthalein is solid crystal structure dissolved in alcohol. It is an acid-base indicator. Phenolphthalein was used to synthesize detergents so that one can observe as the solution becomes acidic, a color change from pink to colorless. Phenolphthalein is not used in the synthesis of soaps because a color change would not occur. Adding the basic NaOH solution to the sodium chloride solution would yield a less basic solution.
The structures of soaps and detergents are very similar in that they have a hydrophilic end, which is polar, and a hydrophobic end, which is nonpolar. The hydrophilic end contains the spectator ions O- and Na+. Spectator ions are ions that do not combine to form a solid precipitate in an aqueous solution, but rather a water soluble salt. Therefore, they make the hydrophilic end water soluble, or polar. The hydrophobic end is a hydrocarbon chain. Because it contains carbon, it is organic. Therefore the hydrophobic end is nonpolar. It will not dissolve in water. The difference in structure between a soap and a detergent is the functional group which connects the end of the hydrocarbon chain to the spectator ions. A soap molecule contains a carbonyl group and a detergent contains a sulfur atom double bonded to two oxygen atoms and single bonded to another oxygen atom.
The effective of soaps and especially detergents as cleaning agents depends on their structures. As mentioned previously, soaps and detergents have a hydrophobic end and a hydrophilic end. Once soap or detergent comes in contact with water and a particle of dirt or another impurity, the molecules they are made up of arrange into what is called a micelle. The hydrophobic tails surround the dirt particle and the hydrophilic heads face the water molecules in the opposite direction. This orientation is due to polarity. The hydrophilic polar heads react with the water while the hydrophilic tails work to bond to the dirt particle to remove it from the article of clothing or from skin.
The starting materials used in the synthesis of the soaps are organic materials, which make them nonpolar. Nonpolar compounds do not dissolve in water. To illustrate, the bonds which hold water molecules together are hydrogen bonds. The bonds which hold molecules of fats and oils together are simple van der waals forces. Therefore, the familiar statement “like dissolves like†does not hold for this case. The starting materials, olive oil, vegetable oil, lard, and shortening, were not soluble in water.
The soaps synthesized during experimentation all had a pH of 11. The soaps were basic because they were synthesized by adding a strong base (NaOH) to the weak acid structure of glycerol, the carboxylic group. On the pH scale, a strong base is closer to 14 than to 7, but a weak base is closer to 7 than to 1. Combining a strong base with a strong acid or a weak base with a weak acid would yield a soap of neutral pH. However, the strong base overwhelms the weak acid, making the soap more basic than acidic. On the pOH scale, the soaps would have a pOH of 2 because pH + pOH = 14. Soaps used in everyday life would have a pH of or close to 5 as this is the neutral pH of skin. A reason for the error in pH of the soaps made for the project would be a result of possibly adding the wrong volumes of reactants, specifically NaOH which is a base and would yield a very basic soap. A way to correct this would be to titrate the soap with an acid. This method is discussed later.
The detergents synthesized during experimentation both had a pH of 1, which is very acidic. The detergents were synthesized by adding strong base (NaOH) to a strong acid (H2SO4). Therefore, the detergents should have had a close to neutral pH. Similar to the soaps, an error occurred at some point during synthesis because a detergent with a pH of 1 would be very hazardous to the environment and people, not to mention it would ruin clothing. This could also be corrected through titration using a base as the titrant. On the pOH scale, a pH of 1 would be a pOH of 13.
One way to examine the effectiveness of the soaps and detergents was to conduct a latherability test on the solid residue left on the filter paper after vacuum filtration. The results for the latherability test can be seen in Table 1 of the group report. The soap made from vegetable oil had the best lathering ability and produces the largest bubbles. This is due to the significant reduction of surface tension caused by the soap. The decrease in surface tension allows for many bubbles to form and grow very large and thick without bursting3.
Another way to examine the effectiveness of the soaps and detergents was to test the solubility of the solid residues in water. The results for the solubility test can be seen in Table 2 of the group report. None of the residues were completely soluble in water because just like the starting materials used in the synthesis of the soaps, they are nonpolar and water is polar, and; therefore, “like dissolves like†does not hold. The soap with the best solubility in water was made from olive oil.
Hard water contains the metal ions calcium and magnesium. Soaps react to these cations, and as a result, soap scum forms which reduces the lathering ability of the soaps. This is due to the solubility of soaps as mentioned previously. When in contact with hard water molecules, the soap molecules partially dissociate while most react with the magnesium and calcium ions to form insoluble salts, hence scum. Soaps only partially dissociate because they are weakly basic. The best way to prevent soap scum would be accomplished by a process of ion exchange, or water softening. Ion exchange4 is a process in which a resin, or water insoluble substance, that is coated in sodium and potassium ions. When in contact with hard water, these positively charged ions exchange places with the also positively charged calcium and magnesium ions in the hard water as the sodium and potassium cations are not tightly held by the resin. This process “softens†the water. As a result, soap scum does not form because when soap molecules bond to the sodium and potassium ions in the water, the salts that are yielded are water soluble. Detergents do not react so readily with hard water because they have a close to neutral pH and better solubility in hard water.
Titration5 is a laboratory technique in which a solution of a known volume and concentration, the titrant, is slowly added to another known volume of solution of unknown concentration using a buret. The process proceeds until the solution being titrated reaches the pH of the titrant. If the solution being titrated is an acid, a base should be used for the titrant and vice versa. This process yields a neutralization reaction.
A neutralization reaction occurs when a base and an acid are combined to form a salt and water. The soaps synthesized in this investigation were made using a strong base, NaOH, and the pH of the soap wastewaters was 11. Therefore, a strong acid, HCl, was used to titrate the four soap wastewater samples. In this situation, the salt made was sodium chloride, which is water soluble. The neutralization reaction for the titration of the soap wastewaters is as follows:
NaOH (aq) + HCl (aq) ïƒ H2O + NaCl (aq)
The detergents synthesized in this investigation were made using a strong acid, sulfuric acid, and the pH of the detergent wastewaters was 1. Therefore, a strong base, NaOH, was used to titrate the two detergent wastewater samples. In this situation, the salt made was sodium sulfate, which is also a water soluble salt. The neutralization reaction for the titration of the detergent wastewaters is:
H2¬SO4 (aq) + NaOH ïƒ H2O + Na2SO4 (aq)
The titration curves, seen in Figures 1 through 6 in the group report are shaped in such a way because they are logarithmic. The titration process begins slowly, as not enough titrant has been added yet to make a difference in the pH of the wastewater. The rapid positive (for detergents) or negative (for soaps) slope of the curve occurs as the wastewater and titrant solution reaches neutralization. The reactions slows again and levels off as the solution reaches the pH of the titrant and cannot be titrated further. The equilibrium point of the curves is the point at which the wastewater was neutralized, or reached a neutral pH. Each of the graphs were different as each wastewater took different amounts of drops of titrant to neutralize.
Phenolphthalein is commonly used as a pH indicator during titration. It does not affect the reaction which takes place, but simply helps to observe pH. Phenolphthalein would look pink in a basic solution and colorless in an acidic solution. Phenolphthalein was not used during the titration process for this project because a pH probe was placed into the solution and the Pasco computer program displayed the pH that was recorded by the probe.
For this project, titration was used for calculating the concentrations of the wastewaters. Because the concentration and volume of the titrants were known as well as the volume of the wastewaters, the equation M1V1 = M2V2 allowed for easy calculation of the unknown concentrations. These calculations can be seen in the group report. M1 represents the unknown concentration. V1 represents the volume of wastewater in a beaker to be titrated. For each of the wastewaters, 5 mL was used and each beaker was filled to 75 mL so that the pH probe could read the pH. For the calculations, the 70 mL of water was not included as it only had a minor effect because water has a neutral pH. M2 represents the concentration of the titrants. To titrate the soap wastewaters, 1 M HCl was used, and to titrate the detergents, 6 M NaOH was used. Finally, V2 represents the volume of the titrant. This varied for each calculation; it was based on the number of drops it took to neutralize each wastewater and reach the pH of the titrant.
When determining the best soap to recommend to the environmental group, a few qualifications were considered, including pH, latherability, solubility in water, and the volume of titrant required to neutral the soap. The overall best soap was the one made from olive oil. It was the most soluble in water and it took the least amount of titrant to neutralize, which occurred around 5, the neutral pH of skin and the ideal pH of a soap. The olive oil soap did not produce the best bubbles, but they were decent and its lathering ability would effectively remove the oil from the birds because the bubbles produced indicate it would act as a surfactant to reduce surface tension and pick up the oil molecules. The qualification that was most important was its environmental friendliness. It was not made out of an animal fat and it would be the least hazardous to the environment, the birds, the individuals using the soaps on the birds, and other aquatic life.
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