1. De-Gassed Water as a Cleaning Agent No Soap Needed ? 2005. 04. 27 Jihee Park

2. De-Gassed Water as a Cleaning Agent I. Introduction Water-based cleaning is effective because hydrophilic, polar solutes, such as sugar and salt (i.e., "dirt") dissolve readily in water, which is a good general-purpose solvent. However, hydrophobic dirt, such as charcoal, grease, and oil, is not water-soluble at normal temperatures. To overcome this problem, these materials are dispersed in water via a combination of mechanical agitation (during conventional washing) and by the use of added surface-active solutes, such as soaps or detergents (surfactants), which coat hydrophobic materials in water. Surfactant molecules adsorb onto hydrophobic surfaces in water, making them hydrophilic, which enhances dispersion (not solubility) in water, so that they can be carried away in the aqueous phase during cleaning. By this way, detergents help remove oily dirt from dishes and clothing, but in the environment, detergents can promote algal growth and destroy habitats. However, it seems that we face the time to toss out those bottles of laundry and dish detergent. New research suggests that clothes and dishes could be cleaned with water alone. According to a study carried by Pashley et al.[1-4], complete degassing of water improves its ability to disperse and remove hydrophobic dirt. It means that eliminating the dissolved nitrogen and oxygen bubbles in water lets the oil separate. See the Figure 1. What is the ultimate principle of the de-gassed water to act as a cleaning agent? In this research, we will study on the principle and how we can apply the science to our real life. 2005.04.27, Jihee Park Figure 1. Photograph of degassed and nitrogen gassed dodecane/water mixtures. Both samples were exposed to the same level of mild mechanical disturbance. [4] LHS : de-gassed and sealed mixture (no-shaking) NTU=53 RHS : nitrogen-equillibrated mixture (no-shaking) NTU=0.06

3. larger volume, the mechanical pump readily achieved pressures down to 20-40 Torr. Evacuation of the mixtures to lower than a mTorr corresponds to greater than 99.999% removal of dissolved gas. To directly compare the effect of degassing, nitrogen gas was bubbled through the control or gassed samples to remove dissolved carbon dioxide and increase the pH value of the aqueous phase to 7, in agreement with that of the degassed samples. Otherwise, differences in pH values between the gassed and degassed samples might affect the surface charge on the oil droplets. The device measured turbidity via detection of the amount of scattered light at right angles to an incident beam of white light. The turbidity of the aqueous phase of each sample was measured in Nephelometer Turbidity Units (NTU), where distilled water has a value of 0.02 and reservoir water typically 1-5. Electrical conductivity measurements were carried out on ordinary distilled water equilibrated with the atmosphere, in distilled water following bubbling with clean nitrogen gas and in fully de-gassed water. All measurements were performed in Pyrex vessels. The change in conductivity with time was also measured for de-gassed water and distilled water equilibrated with the atmosphere, when exposed to gentle bubbling of high-purity nitrogen gas. The temperature of the water was measured, and its pH was monitored. For distilled water, pH values were measured after the addition of a small amount of pure NaCl to stabilize the measuring current of the glass electrode. II. Experimental Methods In the researches by Pashley at al., distilled water was produced from tap water via a sequential process of coarse filtration, activated charcoal filtration, reverse osmosis filtration, and finally, distillation into a Pyrex glass storage vessel housed in a laminar-flow, clean-air cabinet. All the chemicals used were of the purest grade commercially available and were used as purchased. Any surfactant contaminants present in the oil samples would actually reduce the differences between the gassed and the de-gassed conditions. The removal of dissolved gas from an oil and water mixture can be achieved by repeated freezing (in liquid nitrogen) and pumping using an efficient, clean mechanical pump connected to the mixture via a liquid nitrogen trap, to maintain cleanliness. In the experiments reported here, the liquid mixture was always immersed slowly into the liquid nitrogen, to prevent ice expansion from cracking the glass tube. Once the space above the frozen liquid was outgassed, to typically better than a mTorr, a vessel tap was closed and the liquid warmed to room temperature, so that remaining dissolved gases were pulled into the space above the liquid. For each sample, this process was repeated four times to ensure almost complete removal of the dissolved gas. When connected to a separate vacuum line, of substantially

4. In some of the experiments, the degassing tubes were shaken vigorously by hand for 10 s, to mechanically disperse the emulsion or dispersion. However, some samples spontaneously dispersed during degassing, without the need for additional shaking. It is interesting to note that even after only two cycles of freezing and pumping with a mixture of 33 mL water and 2 mL dodecane, the water phase became noticeably cloudy, without any mechanical shaking other than the mixing caused by gentle bubbling as the frozen (pumped) sample was allowed to warm to room temperature. After five cycles, the glass tube was sealed tightly using the Teflon screw tap. Without any external shaking, the water phase, on final melting of the outgassed solid, had an NTU value of over 50 and was completely opaque. The change in turbidity with time after this sample was vigorously shaken for 10 s is shown in Figure 2. Also, in this figure, for comparison, is the corresponding curve for a nitrogen-saturated mixture (in the same type of glass tube and with the same liquid volumes) which was also vigorously shaken by hand for 10 s. Prior to shaking, the water phase had an NTU value of about 0.06. The time plotted in Figure 2, in this case, was time after shaking. The nitrogen equilibrated mixture returned to the clear, two phase mixture within about an hour, whereas the degassed mixture was always much more turbid and maintained turbidity for a much longer period of time. III. Results Figure 2. Comparison of the turbidity of dodecane and water mixtures with time after shaking, for degassed mixtures and after bubbling with nitrogen gas. In this case, the degassed mixture was sealed using a Teflon screw tap. Nitrogen gas was allowed into the degassed mixture halfway through the measurements. [2]

5. Figure 3. NTU values following degassing and spontaneous emulsification of the squalane droplets in water at pH 2, 7, and 11. [3] The degassed mixtures of the spontaneously formed squalane droplets in the solutions with pH values of ~2 and ~11 are also studied. Squalane was chosen for these detailed studies because of its long time effects. The effect of lower pH might be expected to induce a direct coalescence via the reduction of the double-layer repulsion (if any) between the negatively charged oil droplets. Changes in pH were made either prior to degassing or after the squalane- water mixture was degassed and then regassed. As shown in Figure 3, a lower pH substantially reduced the dispersion of squalane under the degassed conditions, although not to the same extent as for the not degassed (control) samples. This reduction of NTU occurs regardless of whether pH is lowered prior to degassing or after degassing and regassing. By comparison, increasing the pH to 11 appeared to have little effect; that is, the high pH case was similar to that at pH 7. After about 2 h, the degassed tube was opened and nitrogen gas let in. The results shown in Figure 2 clearly demonstrate that exposing the surfactant-free emulsion to dissolved gas after it has been formed has little or no effect on its stability. Forty-eight hours after shaking and exposure to gas, the aqueous phase still remained slightly turbid, with an NTU value of 2.3. Even after standing for 310 h, the NTU value of the aqueous phase was 0.35, significantly above the nitrogen blank value of 0.06.

6. Figure 4. Difference in turbidity of gassed and de-gassed cleaning water minutes after vigorous shaking with de-gassed perfluorohexane "dirt" on glassware. [4] The dramatic effects of de-gassing the solvent as well as the water are demonstrated by the results obtained with perfluorohexane cleaning from Pyrex glassware. The results of the comparison between gassed and de-gassed cleaning systems are shown in Figure 4. Clearly, the ability of rinsing water to disperse this oil and, hence, clean is significantly enhanced by de- gassing. Even within 1 min after shaking, the gassed liquids are almost completely phase- separated; that is, large droplets of residual oil are clearly visible on the walls of the glass vessel. See Figure 5. Removal of dissolved gas from the oil phase enhances this dispersion still further. It seems clear that the higher level fo dissolved gas in the oil phase acts as a reservoir, so its removal has a strong effect on dispersion. Figure 5. Photograph of the cleaning water 1 min after vigorous shaking with gassed (left-hand side) and de-gassed (right-hand side) cleaning water and perfluorohexane. [4]

7. Figure 6. Effect of nitrogen gas purging on the electrical conductivity of de-gassed and ordinary distilled water. [4] A second part of this work was focused on the issue of which other properties of water may be influenced by dissolved, nonpolar atmospheric gases, in the absence of dissolved carbon dioxide. The levels of these solutes (oxygen and nitrogen), typically at the level of 50 000 water molecules per dissolved gas molecule for water equilibrated with the atmosphere, are so low that for most properties we could expect little effect. However, some properties such as the electrical conductivity of water are extremely sensitive to solutes, especially ions. The natural conductance of "pure" water is based on the transport of protons and hydroxyl ions largely by water molecule linkages and sequential bond cleavage. It is known that bulk water contains a network of hydrogen-bonded rings of water molecules and that nonpolar solutes also nucleate surrounding rings, which are not connected to the bulk structures. It is possible, therefore, that the presence of millimolar concentrations of these disrupting solutes might reduce the conductivity of water. Conversely, complete de-gassing might increase the conductivity of water, as shown in the Figure 6. Table 1. Summary of values for the electrical conductivity of water under various conditions [4]

8. IV. Discussion Figure 7. Estimated DLVO interaction energies between the two charged oil droplets in the 1 mM NaCl solution, under conditions with and without a typical hydrophobic attraction. The numbers used here are r = 300 nm,  0 = -50 mV,  -1 = 10 nm, A 121 = 5 × 10 -21 J, and the rest are given in the text. [3] Oil and water can be formed by the dispersion of microscopic droplets or particles, often in water, to form a "colloidal solution or dispersion". However, hydrocarbon oils and finely divided hydrophobic particles will not readily disperse in water and will only remain stable for a short length of time, typically, for less than an hour, even after vigorous mechanical agitation. Thus, most industrial processes involving these mixtures require continuous agitation and fairly rapid reaction times. The efficiency of these processes is also reduced by the difficulty in maintaining small particle sizes and a high reaction surface area, because of the tendency to coalescence and coagulation. The stability of emulsions and dispersions can be much improved by the addition of surfactants and polymers, which can change the nature of the oil/water interface. However, these observations are, at first sight, not consistent with a simple application of the well-established DLVO theory of colloid stability. This is because oil droplets and fine, hydrophobic particles, even without additives, are known to develop significant surface electrostatic potentials in water and, in addition, generally have weak van der Waals attractive forces. These conditions usually indicate that the colloidal dispersion will be stable. The failure of the DLVO theory to predict the behavior of oil in water emulsions raises the issue of whether there might be additional forces involved. About 20 years ago, a new long- range attractive force, called the "hydrophobic interaction", [1] was discovered which acted over relatively large distances (>10 nm) between hydrophobic surfaces in water.  Hydrophobic force

9. To test this suggestion a simplified model to calculate the DLVO interaction energy between the 0.3 m radius oil droplets in water, both with and without a typical hydrophobic attraction. A typical hydrophobic interaction potential is given by the double exponential where F/R is the measured force F between the crossed cylinder surfaces of radius R and separated by the distance H. The experimentally determined values of the constants are C 1 = - 0.36 N/m, C 2 = -6.6 mN/m, 1 = 1.2 nm, and 2 = 5.5 nm. This relation was converted to the corresponding interaction energy between the spheres using the Derjaguin approximation, to give the following interaction energy: where V S is the total interaction energy between the spheres in kT units, r is the spherical droplet radius,  0 is the particle's electrostatic potential,  -1 is the Debye length, A 121 is the Hamaker constant, H is the distance between the droplet surfaces, and    is the permittivity of water. The (experimental) hydrophobic interaction constants A 1 and A 2 are 2.2 *10 -10 and 1.8 *10 -11 J m -1, respectively. Using above equation, it seems clear, from the results shown in Figure 7, that once the emulsion droplets have become charged, the reintroduction of dissolved gas, and thus a hydrophobic attraction, will not significantly alter the stability of the emulsion. This is consistent with the experimental observations. According to this calculation, only the droplets of a radius less than ~15 nm will have an energy barrier less than 20 kT and, therefore, will coagulate within a reasonable time scale (hours).  Gas cavitations The relatively low level of dissolved gas in water at STP, about 1 mM (or 20 mL of gas per liter), suggests that its influence should be limited to the processes involving relatively low (hydrophobic) surface areas. Thus, for the micellar solutions and microemulsions, the number of dissolved gas molecules is small relative to the interfacial area available per unit volume of the mixture. It is not surprising, therefore, that degassing does not appear to influence the properties of these systems. By comparison, the breakaway of the oil droplets from a macroscopic oil/water interface offers a low surface area process, which could be affected by this level of dissolved gas. This is even more so if the dissolved gas molecules tended to accumulate next to the oil/water interfaces. The presence of this layer of dissolved gas, in a thin film, which is placed under a negative (suction) pressure during droplet/particle breakaway, apparently causes the nucleation of gas cavities, which will create a bridge between the surfaces and oppose droplet release. Vapor cavitations is expected for the separation of hydrophobic surfaces in water. This situation is illustrated in Figure 8. The precise details of this process are not known, but cavity bridging between the surfaces and an enhanced vdw attraction between the surfaces will both act to hold the surfaces together.

10. Although the mechanism by which dissolved gas inhibits the droplet separation is not clear, it might be expected that some type of cavitation or vapor bridging occurs between the oil droplet and the macroscopic oil/water interface during the breakaway process. This is expected for surfaces with water contact angles greater than 90 o and stems from the thermodynamic drive to reduce the oil/water interfacial area by replacing it with the lower energy oil/air interface. That is, The unfavorable interaction of water layers adjacent to a hydrophobic surface might act to concentrate dissolved gas and this could lead to cavitation when two such surfaces approach. The proximity of two hydrophobic surfaces might also induce spontaneous water vapor cavitation, but presumably the presence of nonpolar dissolved gas solutes would make the formation of these cavities more favorable. Unfortunately, at present, there is no theoretical framework to progress these ideas. Figure 9. Schematic description of the air and vapor entrapment and attachment apparent from the results obtained with hydrophobic particles(or oil droplet) dispersions in water. [2] Figure 8. Schematic diagram of the detachment of an oil droplet from an oil/water interface. Dissolved atmospheric gas molecules will be drawn to the interface and assist the cavitation expected as two hydrophobic surfaces separate in water. [4]

11. Figure 10. Schematic description of the break off of fine oil droplets from the water/oil interface. [3]  Break-off of the fine oil droplets from the water/oil interface If dissolved gas is a central component of the hydrophobic interaction, the stability of oil droplets might be expected to change with the introduction of gas, into a degassed, oil in water, surfactant-free, emulsion. However, no effect was observed here. Instead, a strong effect was observed only at the formation stage of the emulsion. Thus, it appears that removal of dissolved gases enhances the ability of oil droplets to break away from the oil phase, even with modest mechanical disturbance, as illustrated in Figure 10. Once these droplets have been created, adsorption of hydroxyl ions will be expected to produce a substantial electrostatic potential of at least -50 mV, which when combined with the relatively weak van der Waals force, should lead to emulsion stability, with an expected barrier of nearly 800 kT. This phenomenon is shown by the results of the Figure 3.

12.  Application The results presented here do indicate that there may be some useful applications in cleaning where detergent residues need to be avoided, for example, in silicon wafer manufacture and surgical equipment cleaning. The slow re-gassing rate in water should enable cleaning applications where there is little time of exposure to the atmosphere prior to dispersion. A suggested cleaning system is illustrated in Figure 11. In such cleaning methods it will be important to both reduce exposure time and also supply mechanical action to aid dispersion. In the case illustrated this is supplied as liquid spray pressure and momentum. Because it appears that de-gassed oil is more effectively dispersed by de-gassed water than gassed oil, sequential spray cleaning based on de-gassed oil rapidly followed by de-gassed water may offer an effective detergent-free cleaning solution. The de-gassed oil could be a hydrocarbon, fluorocarbon, chlorohydrocarbon, or silicone liquid. This type of system offers effective detergent-free cleaning which should disperse and remove all hydrophobic and hydrophilic forms of dirt. In the current study de-gassing was achieved by the freeze/thaw/pump system, but industrially other methods may be more appropriate. For example, water can be de-gassed using a hydrophobic porous membrane. Mechanical action combined with the dispersive power of de- gassed water will disperse hydrophobic dirt (oils and grease) and hydrophilic, polar dirt (e.g., salts, sugars) will be dissolved in the normal manner. Long-term stability of the dispersion of dirt in water is not required in cleaning, and subsequent de-stabilization could actually be useful. Using de-gassed water to aid in the dispersive removal of hydrophobic (solid or liquid) dirt represents an entirely different approach compared to cleaning with detergents. Figure 11. Diagram of a system designed to clean by the sequential application of de- gassed solvent and de-gassed rinsing water [4]

13. V. Conclusion It is demonstrated that de-gassed water is more effective at dispersing hydrophobic "dirt", such as liquid hydrocarbons or oils. This effect appears to be due to the reduction of natural cavitation, which would otherwise oppose the dispersion of hydrophobic liquid droplets into water. If the dirt is a hydrophobic liquid, this dispersion is further enhanced by de-gassing of the liquid, as well as the water. This has led to the suggestion that detergent-free cleaning is possible using a sequential combination of de-gassed (hydrophobic) solvent followed by rinsing with de-gassed water. Use of different de-gassed, hydrophobic solvents could enhance the application of this cleaning process to a wide range of systems. In addition, completely de-gassed water has a significantly higher electrical conductivity. These results suggest that dissolved, nonpolar gases, even at relatively low levels, perturb the water structure and reduce its electrical conductivity. VI. References [1]R. M. Pashley, P. M. McGuiggan, B.W. Ninham, and D. F. Evans, Science, 1985, 229, 1088-1089 [2]R. M. Pashley, J. Phys. Chem. B, 2003, 107, 1714-1720 [3]N. Maeda, K. J. Rosenberg, J. N. Israelachvili, and R. M. Pashley, Langmuir, 2004, 20, 3129-3137 [4]R. M. Pashley, M. Rzechowicz, L. R. Pashley, and M. J. Francis, J. Phys. Chem. B, 2005, 109, 1231-1238