M.E. Wickham St. Germain

Since the burning of the Cuyahoga River in Cleveland, manufacturing processes have been regulated in order to prevent another environmental disaster. Manufacturers are fine-tuning their processes to minimize the environmental impact while they maximize their production efficiency. There are up to nine regulations that manufacturers may need to follow. Two regulations affect all manufacturers. The Toxic Substances Control Act (TSCA) requires manufacturers to provide data on health and environmental effects of chemicals and mixtures, and gives EPA comprehensive authority to regulate the manufacture, use, distribution, transportation, and disposal of chemicals. The RCRA provides "cradle-to-grave" controls by imposing management requirements on generators, transporters, owners, and operators of treatment, storage, and disposal (TSD) facilities. It includes waste minimization, a national land disposal ban program, and a national hazardous waste management program (Subtitle C).

Because of these regulations, manufacturers have minimized their hazardous waste. In fact, "regulators are encouraging customer industries to turn to aqueous systems for such applications as metal degreasing, plastics cleaning, and fabrics and electronic cleaning"(Layman, 1995). By reducing their volume of spent solvents, an F-listed hazardous waste, the manufacturers can reduce the required paperwork for hazardous waste disposal and reduce the cost of waste disposal. Manufacturers have used two methods to reduce their solvent waste. The first method recycles the solvents in closed systems. The second method returns to an old cleaning technique using aqueous solvents with alkaline detergents or polyglymes. This technique is an effective cleaning and degreasing process. It is this approach that poses a problem for the analyst in the laboratory.

It is well known that aqueous solutions with either alkaline detergents or polyglymes tend to foam. The foam can be severe enough to prolong the extraction process or even contaminate the analytical system. This paper presents a summary of manufacturers’ solutions for foaming, a brief summary of foam theory, a description of the problems encountered with the analysis of waste stream samples that foam, an extrapolation of manufacturers’ solutions to the laboratory, a summary of some preliminary data, and some recommended future directions. Because my expertise is in the area of volatile organic compound (VOC) analysis, I will focus on this technique. However, the problems I encountered with this method could be extended to most other analytical methods.

When the manufacturers returned to using alkaline detergents, they returned to an old problem of foam. However, they have access to over 50 years of published solutions for the foaming problems. They have advanced technology to assist in the solutions including physical size solutions, mechanical solutions, and a variety of chemical solutions.

For small manufacturing processes, companies have resorted to physical size solutions. When the process is designed, they simply use storage vats for their recycled rinsing solutions that would hold the volume of rinsing solution and a daily volume of foam. The aqueous solution is pumped from the bottom of the vat, and foam is not a factor in the liquid carrier lines. Because detergent foams are not stable, no foam is present at the beginning of each process day. By the end of the process day, the storage vat is full with the solution and foam. By projecting the volume of foam storage, the small manufacturers have solved their foam problem.

For other manufacturing processes, foam presents a bigger problem because the presence of foam in the liquid lines could interfere or interrupt the manufacturing process. By using mechanical means, the problem is reduced or eliminated. Some mechanical solutions include spraying the foam interface, placing a beater that whips the foam down, using a vacuum deaerator, adding heat, centrifuging the resulting solutions, and using ultrasonic vibration. Thus, smaller storage vats are needed for the recycled or disposed waste streams.

Finally, for some manufacturing processes, foam is not acceptable. The companies have resorted to using chemical defoamers and chemical antifoams. A chemical defoamer is a compound that knocks down a foam already present in the mixture. A chemical antifoam is a compound that prevents the solution from having a stable liquid-gas interface, (i.e., foam). Defoaming agents/antifoams include soaps (carboxylates), nitrogenous antifoams such as monoamides, phosphoric acid esters, mineral oil blends, long chain alcohols, fluorosurfactants, and hydrophobed silicon/hydrophilic oil mixtures. For lists of patented antifoams, review Kerner, 1976; Cikbertm 1981; and McGregor, 1954. Of special interest to manufacturers are the silica/silicone containing antifoams. This class of compounds is important because it lacks hazardous activity even in food products. This class of antifoams is flexible because it efficiently destroys foam or creates foam depending on the conditions.

Many industries have combated the difficulties arising from the presence of foams and the lack of stable foams. The various industries include food preparation (Pszcsola, 1991), textiles and dyestuff (Kouloheris, 1989; Awad and Hauser, 1987), ink and coatings(Heilen, et al., 1994), pulp and paper (McGregor, 1954 and Owen, 1981), lubricants (Centers, 1993), and even semiconductors (McGhee, 1985). Since 1940, several publications have been printed describing the properties of satisfactory foams and non-foams. All papers included the discussions of surface activity and free surface energy, solubility of the active ingredients, surface tension, adsorption, cohesion and adhesion, wetting ability, and gas hold-up theory. I will describe a foam and briefly discuss why they exist. I will also discuss the three key theories that affect the analysis of waste streams. For further details, I recommend a reference describing the theory of foams found in Garret, 1972. Other references may be found in the references section.

What is a foam? A foam is a collection of bubbles, ideally forming a structure of contiguous dodecahedra to which real foams approximate. The stable foam films are flat between neighboring gas cells of the same size and are a minimum film thickness. The gas is dispersed in the liquid such that the mixture’s bulk density approaches the gas density rather than the liquid density. Foam is governed by the properties of interfaces. Therefore, a foam does not occur with a pure liquid. Foams are generally unstable; they are self-destroying. Foams increase the total energy at the interfacial region, the area between the solvent and surface active agent. Foams can be stable when the surface tension of the solution is much less than the surface tension of the solvent. Persistent foams need surface active agents. Foams have a strong surface elasticity and resist external and internal stresses.

The three most important theories of foam are solubility of the surfactant in the solution, the formation of foam due to surface tension as it related to the concentration of surfactant in water, and a newly discussed theory of gas hold-up.

There are three terms describing the dissolution properties of compounds in a liquid. They are solubility, miscibility, and solubilization.

Solubility and miscibility are both the ability of the solute to be dissolved in a liquid. The rule of thumb is "like dissolve like" and simply describes the rules of dissolution. When dealing with aqueous systems, only ionic compounds are normally dissolved. The oils do not mix in water because the oils are squeezed out by the powerful forces between water molecules. However, the creation of surface active agents, surfactants, has modified the rule of thumb, because a surfactant has a dissolving portion and a non-dissolving portion. Therefore, dilute aqueous solutions of surfactants aid in the spontaneous dissolution of normally insoluble compounds, the process is known as solubilization.

The dissolution of surfactant in water involves three steps. The first step is the separation of the surfactant from bulk liquid. Second, a cavity is created. Finally, the cavity is filled with ions or surfactant, and is called a micelle.

Surfactants, due to their chemical nature, can form micelles. Critical micelle concentration (CMC) is the concentration in which micelles just begin to form. If the liquid is below the CMC, then the solute behaves like the ideal solvent/solute mixture. The solute behaves the same as if in pure water. If the liquid is above the CMC, then large differences between ideal properties and actual properties occur. Mainly, the solute can have increased solubility in the solvent, increased osmotic pressure, and increased formation of micelles. The solubility of the solute is greater and the thermodynamic activity of the solute is smaller than in pure water. The CMC depends on the size of the non-polar tail--the CMC is lower for surfactants with larger, hydrocarbon tails. When the surfactants are ionic tensides, the charged end drags the hydrophobic tail into the water, and at high concentrations, the tension causes solubility. The insoluble substance is captured within the micelles.

There are three types of tensile forces acting on the molecules in the liquid: interfacial, adhesive, and surface. Interfacial tension is the equivalent to surface tension at the boundary of two liquid or solid phases. Adhesive tension is the tension between liquid and solid. It is the equivalent to surface tension at the boundary of a liquid and a solid. Surface tension is the force between the liquid and air, and is the work required to extend the surface by unit area. It is the consequence of an unsymmetrical force field acting on the molecules in the surface which results in a net inward attraction on them, perpendicular to the surface

The molecules near the surface of a pure liquid have a different environment than those in the interior of the fluid. A molecule in the bulk of the fluid will experience forces in all directions due to the surrounding molecules. The resultant force on the molecule will be zero over a long time period. Molecules near the surface of the liquid will experience a weaker, directed force. Consequently, surface molecules will experience a force, a tension, pulling them back into the bulk of the fluid.

Tension varies with temperature and types of liquids. As surface tension decreases, foaming will occur. If liquids are associative (like water), then tension is stronger because of intermolecular forces. Tension in mixtures with surfactant changes because the surfactant aggregates at the surface with the hydrophilic heads in the liquid and the hydrophobic tails extending into the air. A surfactant film is comparable to two surfactant surfaces back-to-back. Films tend towards a minimum surface area and a minimum film thickness. The minimum film thickness is two molecules deep. Even then, the films are easily broken due to their dynamic nature. A surfactant micelle is the formation of a "bubble" within the liquid-- a "bubble" with a liquid-liquid interface.

Takesano, et.al, 1995 described a new theory about the reabsorption of gases into the liquid at the foam interface. This may be good for bioreactors in fermentation, but it is not good for purging efficiency. The aromatic compounds like benzene would be caught in the two- dimensional interface of the films and dragged back into the liquid when the foam decomposes. The lighter gases would tend to return to the liquid phase because the concentration of the gas in the film would be lower than in the purged gas phase. This theory explains why the surrogate recoveries are lower in foaming samples.

The samples from waste streams containing polyglymes have a pH of 7 or a pH of 3 with normal acid preservation. These samples foam such that a 1:10 dilution solves the problem of foaming. On the other hand, waste streams containing alkaline detergents have a pH of 10 or a pH of 7-8 with excess acid preservation. These samples foam severely, and even a 1:1000 dilution is insufficient to solve the problem of foaming. At this dilution, the method detection limits for these samples are larger than the regulatory action limits. If a chemist were to supply data to the manufacturer or to the regulatory agency, the data could not be used by either because it is not known if the manufacturer was within compliance of the regulations.

Let’s review the process for one analytical technique, and note the potential difficulties caused by foaming samples. The approved VOC methods for waste streams include EPA Method 624 and SW-846 Method 8260. The analysis of VOCs involves placing a flow of inert gas (purge) through the sample, transferring the gas to a sorbent tube (analytical trap), heating and transferring the gas (desorb) to a gas chromatograph for separation, and finally detecting the compounds with a mass spectrometer. As you can see, the minute we place gas flow through a sample, foam will be generated. If the foam stability is sufficient, it will proceed through the transfer lines to the analytical trap, and possibly proceed onto the chromatographic column. As the foam travels through the system, the foam will contaminate common lines which would prevent successful analysis of subsequent samples.

Three types of contamination can occur with the presence of foam that would interfere with further analyses. The first type is the deposition of contaminants on the lines which would be leached off with subsequent analyses. The second type is the effective stripping of the coating on the transfer lines which would make active site available. Because many of the volatile compounds are reactive, the active sites would allow the volatile compounds to react, and not be purged to the column. The third type is the filling of the adsorbent sites on the sorbent trap. By filling the adsorbent sites, the volatile compounds would not be efficiently trapped for the analysis. All three methods of contamination effectively reduce the ability of the analytical system to trap the VOCs. Therefore, the recovery of all VOCs or selected VOCs would be reduced or eliminated. This impact is not accepted by either of the approved methods.

Therefore, if we solve the foaming problem, we would eliminate our analytical difficulties.

After performing a literature search, I discovered that all publications on foaming were for manufacturing processes and none were for analytical processes. The only analytical procedures described in the literature were for the quantitation of foam for manufacturing purposes (Kouloheris, 1987) and the relative effectiveness of the techniques used for defoaming (Middleditch).

However, by extrapolating the manufacturing solutions to foam, we can review the potential solutions for the laboratory. We can also review the VOC analysis and identify the most likely applications. I will review the physical, mechanical, and chemical solutions as they apply to the VOC methods.

Physical methods for solving foam problems include larger volume of containers. For some of the extraction procedures, these methods may be possible. Unfortunately, for VOC analyses there are only disadvantages to this technique. First, the methods are very specific about the allowable headspace (volume of air above the liquid sample) of the sample vessels. Second, the recoveries of the target analytes drop in the presence of foam. By reviewing the theory of the liquid-gas interface of foam, especially a reference on gas holdup (Takesano, et al., 1995), this effect is not surprising. The volatile compounds in the presence of both liquid and air will reside more in equilibrium, and prefer to be in the air with purge gas flow. If, however, the liquid has a very low concentration of target compounds, the equilibrium is shifted towards the liquid, and the volatile compounds return to the liquid phase. Third, the process of purging gas through the sample is a requirement to obtain the target analytes. The process cannot be changed. Last, dilution of the sample causes the results to become invalid. It is a situation where the data are not usable because the method detection limits are above the action limits of the regulation.

Mechanical solutions for solving foam problems include spraying the foam, beaters, vacuum deaerators, heat, centrifugation, and ultrasonic vibration. For some extraction procedures, one or more of these techniques may be possible. However, for VOC analysis there are only disadvantages. Spraying the samples introduces the potential for additional laboratory contamination. Manufacturers and regulatory agencies do not accept data with high laboratory contamination. Beater rings, vacuum deaerators, and ultrasonic vibrators are costly and difficult to design for small glassware. An alternative to beater rings are glass wool plugs in the air space above the samples. The foam for polyglyme waste streams can be stopped by this technique. However, the alkaline detergent waste stream samples wet the glass wool plug sufficiently that the liquid becomes trapped above the plug, and the problem begins again. Heating or cooling the samples is acceptable if all the samples and standards are heated or cooled respectively. However, heating or cooling the sample vessels does not solve the severe foaming problem caused by alkaline detergents. Centrifuging the samples is not possible because the process is ongoing and cannot be physically done.

Chemical solutions for solving foam problems include the use of solvents, the use of salt, or the use of antifoams. Solvents can change the surface tension such that foaming is reduced. However, many of the solvents are target analytes in the VOC listing. The only possible solvent is methanol. Methanol performs well for polyglyme waste samples, but not well for alkaline detergent samples. The addition of sodium chloride to samples has been known to improve the recoveries of semi-volatile target analytes during extraction. The process is known as "salting out" because the addition of salt to the water forces the target compounds to migrate to the extraction solvent. Unfortunately, the "salting out" process is not as successful for volatile compounds and could introduce impurities that would interfere with the analysis.

The chemical solution using the antifoam had some possibilities. There are eight qualities needed for successful analysis of the samples if a chemical solution is possible. First, the solution should not introduce any new impurities, especially the impurities that are target analytes. Second, the solution should not introduce any interferences that preclude the analysis of any target compounds. Third, the sample should not be diluted outside of the effective working range, i.e., not above the action limits. Therefore, the addition of a chemical should be less than 0.1% by volume. Fourth, the solution should be easy to use for all samples. Fifth, the solution should be stable, especially if it is a chemical solution. Sixth, the solution should prevent the foam from polyglyme waste streams as well as alkaline detergent waste streams. Seventh, the solution should be effective over a wide range of pH. Finally, the solution should minimize the need for dilutions. By finding a solution that satisfies all these qualities, the analysis in the laboratory is made more efficient without the loss of analytical data. After the literature review, many of the compounds were eliminated because these compounds did not meet the criteria and would interfere with the analysis for VOC. The remaining choices included a class of compounds known as silica-containing antifoams, including polydimethylsiloxane polymer with silica.

To recap the potential solutions: there are no feasible solutions of a physical or mechanical nature. There might be a solution if silica-containing antifoams are used.

Antifoam prevents the formation of foam. Defoamer eliminates already present foam. Both are substances which spread on water to films with no surface elasticity and are strong foam breakers.

Antifoams consist of three main components 1) a solvent, like water, is used to carry a hydrophobic active substance uniformly into a hydrophilic medium; 2) a surfactant-- a surface active agent; and 3) active substances, incompatible spreading/adsorptive compounds for foam destruction such as silica. Factors affecting the effectiveness of antifoams include the type of solvent and surfactant, the pH range, and the active substance. For example, polydiethylenemethyl silicone (PDMS) can cause foam, prevent foam, or be non-reactive under different conditions for lubricants, depending on the viscosity of the lubricants.

Several types of antifoams exist, but two antifoams would be possible in the VOC analysis. The antifoams are diethylene glycol because it has a low vapor pressure and silicone because it is so versatile. Silicone antifoams had several advantages including flexible backbone, organic--intrinsic surface activity, thermally and oxidatively stable in closed systems, nontoxic and non-irritating, and easily degradable under natural weathering conditions with soil. Therefore, we began collecting and testing silica antifoams.

To measure the amount of foam generated by samples and standards, we used a foameter. The foameter is made using a five milliliter (ml) purge vessel connected to a two-foot-long column with a diameter of 1.5 inches. A nitrogen gas source was connected to the inlet of the purge vessel. Nitrogen flow was regulated at 40 ml/min. The heights of the foam were recorded after 1 minute, 5 minutes, and 12 minutes.

Antifoams were diluted in water to 10 ppm using serial dilutions in volumetric glassware. The solutions were stored at room temperature throughout the study.

An alkaline detergent was prepared by adding 2 grams (g) of detergent to 20 ml water. The sample was shaken vigorously. Because the pH of the sample was greater than 12, aliquots of this sample were diluted with water until the pH was 10. This sample simulated alkaline detergent waste stream samples and foamed to the top of the foameter.

A neutral detergent was prepared by adding 2 g of detergent to 20 ml water.

Limited testing has occurred using one silicone antifoam on two waste streams from metal degreasing processes. The initial preparation of the antifoam mix contained two impurities. The first compound eluted at 23.7 minutes, and was a silicone-containing compound (Figure 1). The second compound eluted at 27.6 minutes, and was identified as decamethylcyclopentasiloxane (Figure 2). Neither of these compounds were target compounds nor did they interfere with the analysis. After three months of room temperature storage, these impurities were no longer present, and methylene chloride, a normal laboratory contaminant, was not observed.

Foam measured in the laboratory surfactant samples and two waste stream samples had a height of 45 cm after 5 min in the foameter. By using 50 uL of antifoam solution, foam was reduced to the normal water level of less than 0.5 cm. The surrogate recoveries were within acceptable criteria.

With the increase of waste stream samples containing surfactant and polyglymes, the samples have become a challenge when trying to obtain action level data. A feasible solution has been researched and preliminary tests have been performed. The solution includes the use of chemical antifoams containing silicone.

The silicon antifoam used in this preliminary test met the eight criteria. It did not have any interferences which were target compounds and which coeluted with target compounds. The addition of diluted antifoam (50 uL) was less than 0.1% of the total sample volume of 5 mL, was simple, and did not require further sample dilutions. The diluted antifoam was stable over a three-month period during the intermittant testing and worked well over a wide pH range for all sample types. By using volumes up to 50 uL, the diluted antifoam could be adjusted for the amount of foaming from the sample.

Future directions should include publishing laboratory application of antifoams, testing additional chemical antifoams, testing additional waste streams, evaluating effects upon the matrices, and determining the limits of antifoam use in the laboratory.

This work has been funded by the U.S. Environmental Protection Agency. The opinions of the author are not necessarily the opinions of the agency. This document has been peer reviewed by the EPA Agency. Special thanks are given to Joseph Arello, Dr. Terry Crone, Dr. Don Miller, Les Vahshultz, and Helen Bennett for their advice and support.

This paper does not establish agency-wide policies or procedures. This paper is not intended to and cannot be relied upon to create any rights, substantive or procedural, enforceable by any party in litigation with the United States. EPA reserves the right to act at variance with the policies and procedures in this paper and to change them at any time without public notice. Mention of trade names or commercial products does not constitute endorsement or recommendation for use.

Figure 1. Mass spectra of the first impurity in the antifoam.