The overall goal of this procedure is to extract all the surfactants from environmental samples such as atmospheric aerosols and determine their concentrations and surface tension curves. This method can answer key question about surfactant concentration and surface tension behavior in all living things. The advantage of this technique in particular the extraction step is that it isolates surfactant in complex matrices for characterization.
The implications of this technique extend to all the understanding of cloud formation in the atmosphere because surfactants are expected to favor the transformation of atmospheric aerosols into cloud droplets. By this method can provide insight into surfactant in atmospheric aerosols it can also be applied to the system such as soil samples or microbial cultures. So the idea to develop the method came from the lack of selective extraction for surfactants in atmospheric samples.
Until now, only water extraction has been used. First, immerse a previously prepared filter sample in ultra-pure water for two hours at six degrees Celsius in a closed glass Petri dish. Every thirty minutes, shake the Petri dish while holding it flat.
After removing the sample from the refrigerator filter it with a clean syringe filter. And add the filtered solution to a pre-weighed 60 milliliter glass bottle. Following this, rinse the Petri dish with five milliliters of ultra-pure water.
Then filter the water with the syring filter and add it to the solution in the 60 milliliter glass bottle. Weigh the glass bottle containing the solution to determine the volume of filtered water and the surfactant concentration. For solid phase extraction attach an SPE silica based C18 cartridge onto an SPE vacuum manifold which is connected to a pump.
Wash the cartridge with six milliliters of acetonitrile and set the flow rate to one milliliter per minute by regulating the vacuum to the pump. Then wash the cartidge with six milliliters of ultra-pure water and stop the pump to maintain the water level just above the cartridge. Next flow the filtered sample through the SPE cartridge at a rate of less than one milliliter per minute.
Then flow one milliliter of ultra-pure water through the cartridge for cleaning. And dry the cartridge by applying a stronger vacuum on the SPE setup. Add a collector for the sample extract below the cartridge.
Elute the surfactant fraction absorbed on the column by flowing four milliliter of acetonitrile through it at a flow rate of less than one milliliter per minute. Transfer the collected extract into a four milliliter vial. Evaporate the acetonitrile solution by exposing it to a gentle flux of nitrogen until a dry extract is obtained, which typically takes about one hour.
Then redissolve the extract in 60 microliters of ultra-pure water. After diluting the 60 microliter extract into 10 milliliters of pure water add 200 microliters of acetate buffer solution, 100 microliters of E-D-T-A solution, 500 microliters of sodium sulfate solution, and 200 microliters of the Ethyl Violet solution to the bottle shaking the solution after each addition. Next, add 2.5 milliliters of toluene and a magnetic stir bar to the solution.
Stir the solution at 500 RPM for one hour. After stopping the stirring and allowing the two phases to separate remove the upper toluene phase with a Pasteur glass pipette, and transfer to a four milliliter light protected vial for UV-VIS analysis. To establish a calibration curve for anionic surfactants prepare 12 solutions of dioctyl sulfosuccinate sodium salt or AOT between zero and five micromolar in water and apply the previous steps to each of these solutions.
Place 1.5 to two milliliters of each of the 12 treated AOT solutions in a one centimeter spectrometer quartz cell. Then measure their absorbance at 612 nanometers using a UV-VIS spectrophotometer. To obtain a calibration curve for anionic surfactants plot the absorbance for each measured for each AOT solution as a function of its concentration.
To determine the absolute concentration of anionic surfactants in the atmospheric sample measure the absorbance of the extracted toluene solution at 612 nanometers. Next, place the absorbance obtained for the anionic fraction on the calibration curve for anionic surfactants to determine the concentration of anionic surfactants in the aerosol sample. Then calculate the total surfactant concentration in the aerosol sample by summing up the concentrations obtained for the anionic, cationic and non-ionic fractions.
Start the tensiometer camera and software. Equip a syringe with a 0.30 millimeter diameter needle fill it with the solution to be measured and place it on the tensiometer holder. Visually check that the needle tip is in the camera field.
Next, produce a droplet with a diameter between one and three millimeters by pushing the piston. Using the software take a picture or a video of the droplet before it falls. Then run the analysis function of the software to fit of the droplet shape to the Young-Laplace equation and obtain a surface tension value.
To plot the remainder of the curve, dilute the extract by a factor of two by adding ultra-pure water, and repeat the surface tension measurement with the diluted solution. The surfactant concentrations in fine aerosols collected at the coastal site of Rocosnietza Croatia in February 2015 show the distribution between anionic cationic and non-ionic surfactants. Combining surface tension measurements with concentration measurements resulted in the absolute surface tension curve for the surfactants in the samples shown here.
These curves indicate the surfactant concentrations in the aerosol samples, the surface tension of these samples, and the critical micelle concentration or CMC of the surfactants. Once mastered, this technique can be done in 10 hours per sample batch if performed properly. When attempting this procedure it is important not to use soap to clean the glassware and to make blanks so the concentration and surface tension will match.
Following the instruction procedure, this can be performed onto a surfactant extract such as enema or ACMS to attempt the elucidation of the chemical structure of the surfactants. The development of this method allow the comparison of surfactant concentrations on CMC with values and environmental parameters which can alert to for example, to determine their sources in general environment or their role on cloud formation. After watching this video you should have a good understanding on how to extract surfactants from environmental sample such as atmospheric aerosols and determine their concentration and their surface tension curve.
Don't forget that working with solvents such as acetonitrile and toluene is extremely hazardous. Precautions such as working in a fume hood and wearing personal protective equipment should always be taken while performing this procedure.
