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The study of bubbles has been frequently study by many chemists, physicists, and mathematicians. The soap bubble had a specific pattern, which noticed by Mysels (1960) that the surfactant monolayer formed the surface of the soap bubble are two-dimensional, and it can be easily sheared once it touch a hard surface like the floor (Mysels, 1960). The surfactant usually had the value of pH from 8-11 (Mysels, 1960). When the soapy bubble film were extended, the extended portion had less surfactant, and the extended portion will also had a higher surface tension than any other part of the soap bubble (Mysels, 1960). Surface tension could be restored when the extended portion of the surfactant spread to all parts of the monolayer (Mysels, 1960). Mysels (1960) indicated that the cohesion of detergent make outer and inner surface of bubble more stretchy and elastic. The surface tension is so great when pure water is used in the soapy concentration (Ward, 1990). Surface tension is what makes the bubble to become and look like bubble. The results of surface tension in the bubbles come from the force of attraction between the water molecules. The negative region of a polar molecule will attract the positive region of the other polar molecule nearby. The positive and negative dipoles of each molecule pull opposite against each other, canceling the charge; as in a result, the molecule would have no net charge. The forces of attraction between these molecules prevailed and form an elastic sheet, which will not be easily broken. Three known molecular forces help to explain why the bubble films can achieve equilibrium in forming an elastic sheet. As Isenberg (1992) proposed, "van der Waals attraction between molecules and ions, the electrostatic repulsion due to double layers of charge at each surface, and the Born repulsion due to hard cores of molecules and ions; in addition, there are many intermolecular forces act upon the film of soap bubbles that haven't been fully understood." According to Isenberg (1992), "the van der Waals attraction between two water molecules is results from the induced dipole moment in the molecules." Isenberg (1992) found out that the total van der Waals force of attraction of the thickness of the film, represented by the variable t, and V sub w to represent a constant value for a molecular system; thus, the attractive energy of van der Waals can be calculated by negative V sub w over t square. Another contribution of molecular forces in explaining the bubble film is the electrostatic repulsion due to the two layers, one layer made of negative surfactant and one layer made of positive surfactant, of the bubble overlapping (Isenberg, 1992). The potential energy of the electrostatic repulsion due to the two layers can be calculate by using the constant values of molecular system in the thermodynamic equilibrium and multiply by the exponential of the value t, representing the sum of the potential energy results in interaction between the molecules in the film of the bubble (Isenberg, 1992).
An experiment was conducted with the purpose of comparing the methods of making and keeping the soap bubble films. The comparison in terms of bubble duration, which is the length of time it takes for the bubbles to pop. According to (Guillermo, 2000), some of the factors that could affect the durability of the soap bubble are the internal air pressure temperature, presence of carbon dioxide, and ionic concentration in the soapy solution. The experiment is conducted by using five different methods to blow the bubble. These five different methods are blowing the bubble with a straw, blowing with a wire ring, using a straw, using a baster, using an inverted funnel, and using a syringe. In addition, the bubbles are blow and keep on a carpet or the soapy solution. Guillermo (2000) hypothesized that the thicker and denser the bubble solution would make the heavier bubble walls, making the bubbles more protective. However, Guillermo (2000) also hypothesized that the lightest bubble with less dense, and the ionic concentration are more soluble, thus the bubble would be more durable.
An experiment was conducted by Liguang Wang and Roe-Hoan Yoon to see the effect of pH and sodium chloride concentration on the stability of the surfactant-free foam films. There are many methods that can be used to test the effect of pH and sodium chloride concentration on the stability of the foam films. The method that Liguang Wang and Yoon used to test the stability of the foam films is what is called the thin film pressure balance technique, also known as the (TFPB). According to (Wang & Yoon, 2009), the thin film pressure balance technique was to make a special film holder, which made up of a porous glass disc and a glass tube with the radius around 1-2 mm. In addition, the method required the gastight cell placed with the film holder in order for the film to be exposed to the gas pressure, and the other end of the glass tube to be exposed to the reference pressure (Wang & Yoon, 2009). Although, the strong repulsive force double layer force was presence, but the foam films still cannot be produced in pure water, because of the foam films' weak elasticity and strong hydrophobic force (Wang & Yoon, 2009). But Wang & Yoon (2009) figured that the presence of small quantity of electrolyte could weaken the strong hydrophobic force of the foam films; thus, it allowed metastable foam film to form. Then Yang and Yoon collected all the needed information for the calculation of the thickness of the foam films. Yang & Yoon (2009) found out that as the sodium chloride concentration was raised above 10âˆ’6 M, and then the stability of the foam film will slowly diminishes as results of double layer compression. The TFPB technique measured the maximum of the thickness of foam film is around 130 nm when the pH is at 6.0-7.3, and the thickness of foam will decrease on either side of this pH range (Yang & Yoon, 2009).
Kanicky & Shah (2003) have used a method called acid-base titration to investigate the premicellar concentration region of the aqueous fatty acid solutions. "In the early studies of aqueous films of fatty acid salts at various solutions with different pH values, it was found that there is a value of pH where the solution could have the minimum water evaporation rate, maximum foamability, maximum foam stability, maximum bubble stability, and maximum surface viscosity of the foam (Kanicky & Shah, 2003). The earlier studies had indicated there is an optimum value of pH where the aqueous fatty acid solutions could have meets all the conditions listed above. According to Kanicky & Shah (2003), this optimum value of pH is approximately the value pKa, which is the constant value of acidity; the measure of the how strong the acid when suspense in solution. Kanicky & Shah (2003) performed the acid-base titration method to titrate a series of carbon chains, from C8 to C12 fatty acid chains to find out the value of pKa, the unknown concentration of the fatty acid solution. Kanicky & Shah (2003) saw an increase of the value pKa of the concentration, and they realized that the submicellar aggregation must be the reason why the value pKa increased. The result concludes that the premicellar aggregation and molecular association of the fatty acid salt affects the polar groups of the fatty acid chains from ionizing (Kanicky & Shah, 2003).
Sarma and Chattopadhyay (2001) had devised a method for observing an extraordinary phenomenon, which is to observe the Marangoni fluid flow in many different soap bubbles by using the ultraviolent-visible spectrophotometer that they built. They monitored and measured the thickness of the flowing fluid layer and the soap bubble film by based upon the results from an earlier study on the topic. As Sarma & Chatoopadhyay (2001) reports, "they observed the interference maxima and interference minima results in the ultraviolet region of the wavelength." Sarma and Chattopadhyay (2001) proposed, "The fluid layer thickness remains to be nearly constant for both of the vertical and horizontal bubbles even though the thickness of the soapy film will changes over time." They've concluded that the thickness of the fluid layer to be around 6.94 ± 0.15 µm for the vertical bubble, and for the thickness of the horizontal bubble, it is around 4.75 ± 0.09 µm (Sarma & Chatoopadhyay, 2001).
An experiment was conducted by Muthupandian Ashokkumar, Paul Mulvaney, and Franz Grieser to see the effect of pH on multibubble sonoluminescence from aqueous solutions containing weak acids and bases. Ashokkumar (1999) devised a way to figure out the sonoluminescence intensity by using the Rayleigh-Plesset equation to calculate the dynamics of the bubbles. According to the results that Ashokkumar found in his experiment, the sonoluminescence (SL) intensity of the aqueous solution containing weak acids and bases is proved to be dependent on how long the hydrocarbon chain of the solute and the pH of the aqueous solution (Ashokkumar et al., 1999). Ashokkumar concluded this experiment by resulting that the weak bases quench the (SL) intensity at the pH values above 9, the weak acids quench the (SL) intensity at the pH below 7 (Ashokkumar et al., 1999).
Sodium oleate produced bubble film that is short-lived, mainly because it couldn't withstand the instant stresses of other compounds such as: sodium stearate, and palmitate, in forming a solid layer on the surface of the bubble film (Kuehner, 1948). Usually, the addition of glycerol to the soapy solution could be used to strengthen the toughness of the soap film (Kuehner, 1948). Kuehner performed an experiment on testing the duration of bubble by preparing the solution using the commercial product, Aerosol O.T. Kuehner prepared many different solution with various concentration of Aerosol O.T. in it. He constructed a long and heavy platinum wire to measure the toughness of the soapy film formed from the Aerosol O.T. As the Kuehner (1948) indicated, "The solutions with Aerosol formed very weak bubble film; the average lifetime of the 8-cm bubble film formed from 2 percent of Aerosol only lasted for 2.2 seconds, while the bubble film with 5 percent Aerosol solution increased to 5.0 seconds."
There are many important concepts that had been extracted from the soap bubble and its film. For instance, the intermolecular forces played an important role in explaining the neutral layers of the bubble film. Another concept that could be extracted from the soap bubble will be the surface tension, the cohesion of the water molecules which ultimately form traps the air inside the spherical shape, and thus bubble was formed. There are many factors that could alter the duration of the bubble, which indicated in the experiment of Guillermo. The used of commercial product like Aerosol as shown in the experiment of Kuehner would tends to form weak bubble films, thus resulting in a short lifetime of the bubbles. Finally, the important concepts of bubble had been discovered after hundreds of experiments and investigations conducted by numerous scientists, mathematicians from all over the world.