Name: extraction and characterization of surfactants from atmospheric aerosols
Uploaded: 2017-04-21T13:00:00
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This work was funded by SONATA, a joint project between the French Agence Nationale de la Recherche (ANR-13-IS08-0001) and the US National Science Foundation. Christine Baduel is funded by the French National Research Agency (ANR) through the ANR-16-ACHN-0026 project. The authors also warmly thank Marija Margu&#353;, Ana Cvite&#353;i&#263;, Sanja Frka Milosavljevi&#263; and Irena Ciglenecki, from Rudjer Boskovic Institute of Zagreb, Croatia for the help with the aerosol sampling at Marina Frapa, Rogoznica, Croatia.

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The authors have nothing to disclose.

In the protocol, all the critical steps have been detailed. They include the collection of the aerosols on filters, the extraction of surfactants from them (using a double extraction: a water extraction followed by a SPE extraction) and the analysis of the extracts (surface tension and concentration measurements).
For the whole method, a quality control has been made 1) by the application of the extraction and analysis method on blank filters (deviation &#60;5 mN m-1 compared with ultrapure water on the surface tension and absorbance under the detection limit for the colorimetric method), 2) by determining the extraction efficiency and their uncertainties including the reproducibility/repeatability, the % of extracted surfactants in a given range of concentration, 3) by checking the potential interferents on the colorimetric method, i.e. by checking that the method detects only the targeted type of surfactant (anionic, cationic and non-ionic) and do not see the others as fully detailed in references4,6) by checking potential interferents from the aerosol extracts (inorganic salts, small acids) on the colorimetric method as fully detailed in reference6.
To our knowledge, the extraction method for surfactants from atmospheric samples presented in this article is currently the most selective one in atmospheric chemistry. In particular it is much more selective than the simple water extractions performed in the past for the investigation of these compounds.11,23,24 The second extraction step is important as it has been shown to remove ionic components, such as inorganic salts and small organic acids, that are in large concentrations in the aerosol samples and interfere with the concentration measurements.6 This extraction method has also been shown to remove all the surfactants present in the samples, at the surface and in the bulk. The resulting extracts are thus concentrated enough to allow for accurate characterizations of these compounds.
However, in addition to surfactants, it is possible that other non-polar or mildly polar compounds are extracted from the atmospheric aerosols. For instance, &#34;Humic-like Substances&#34; (HULIS), that are usually extracted by similar methods25 and, depending on the sampling region, could be present in the extracts. These compounds are only mildly surfactant compared to the surfactants characterized in our samples,26,27,28 thus should not contribute significantly to the surface tension or CMC measured. However, they are polyacids and could interfere with the anionic concentration measurements. In the future, their contribution to the surfactant concentrations (i.e. whether or not they react with ethyl violet, the dye used to titrate anionic surfactants) will need to be determined. If their contribution is significant, extra steps could be added to the extraction method, to eliminate for instance all the compounds that are active in the UV-Vis or by fluorescence, which would include HULIS but not surfactants.
So far, no other method for the measurement of the surface tension of aerosols and of the surface tension curve for aerosol surfactants than the one presented in this manuscript is available. The hanging droplet technique is recommended for these measurements as it is the only one requiring sample volumes consistent with atmospheric samples. Optical techniques, measuring directly the surface tension on micron-size particles without any extraction, are being developed.10,20,29 So far, they are only applicable to laboratory-produced particles but could potentially someday be applied to atmospheric ones.
The colorimetric method presented in this work for the measurement of surfactant concentration has been applied previously to atmospheric aerosol samples11,13,14,30 but only to water extracts and not to double extracts, as in our method. This is an important difference as, as underlined above, the second extraction step removes compounds such as inorganic salts and small organic acids, which interfere with the concentration measurements.6
An electrochemical technique, initially developed for seawater and larger aqueous samples, has also been used to measure the concentration of surfactants in atmospheric aerosols.31,32 This method is relative, i.e. the surfactant concentrations obtained depend on the reference compounds chosen and assume that the detection sensitivity of all surfactants is identical. The detection limit reported for this technique is 0.02 mg L-1 when using tetra-octylphenolethoxylate as reference, thus 0.03 &#956;M, and comparable to the detection limit of about 0.05 &#956;M for anionic and cationic surfactants by the colorimetric method. But because of the uncertainties in the determination of the non-ionic and total surfactant concentrations with the colorimetric method, it would be interesting to compare both methods (inter-calibration).
A few points in the presented methods could be further improved.
Another dye than cobalt thiocyanate that would detect all non-ionic surfactants and wit the same sensitivity would be very useful and reduce the main source of uncertainties in the current concentration measurements.
The extraction efficiency for cationic surfactants, currently estimated to 20%, could also be improved, as these compounds are often at the detection limit in atmospheric samples. This could be done, for instance, by using a specific SPE column.
The extractions and titration conditions could be further improved. For instance, using in parallel three different SPE set-ups, each optimized for a class of surfactants, could improve the extraction efficiency, and improve the quality of the procedure (less contamination risks). The optimum sorbent density of the SPE cartridge for the sample mass to be analyzed could also been determined. The conditions for the titration reactions (pH, additives) could also be further optimized, to further improve the sensitivity of the concentration measurements, i.e. lower the detection limits.
Additional tests or steps could be added to the extraction protocol to exclude the non-surfactant compounds that might have been extracted. For instance, the potential presence of HULIS in the samples could be investigated by optical techniques (UV-Vis or fluorescence).
Further modifications, while not improving the quality of the analysis itself, would bring more information on atmospheric surfactants, such as applying the present method to different size-fractions (i.e. sub-populations) of the aerosol rather than on all the particles collected, as presented here. Other types of analyses could also be applied to the extracts such as, LC/HR MS, tandem MS, or NMR to determine the chemical structure of the surfactants or UV-Vis absorbance, fluorescence, or polarimetry, to indicate the presence of highly-conjugated or chiral compounds in the extracts.

Clouds are essential in the Earth&#39;s atmosphere, for the hydrology of most environments and ecosystems, and for the climate system. But some aspects of their formation mechanisms are still not understood, in particular the contributions of the chemical compounds present in the aerosol particles that act as condensation nuclei. Theory1 predicts that surface-active compounds, or surfactants, present in aerosol particles should strongly enhance cloud droplet formation by lowering their surface tension, thus their formation energy. But these effects have remained elusive to observation for decades and the role of surfactants on cloud formation is currently denied by a large part of the atmospheric community and ignored in all cloud investigations and atmospheric and climate models.
One reason for the lack of understanding of the role of aerosol surfactants in cloud formation has been the absence of method to isolate and characterize them. Unlike samples from other environments, the analysis of atmospheric samples faces recurring challenges2 such as very small sample volume and mass (here, typically between 10 and 100 &#956;g) and chemical complexity (mixtures of salts, minerals, and numerous organics). To overcome these challenges and improve the understanding of aerosol surfactants some methods have been recently developed by our group to 1) extract specifically these compounds from atmospheric aerosol samples, 2) determine their absolute concentrations in the aerosol phase and 3) determine their surface tension curves in water, including their Critical Micelle Concentration (CMC), the concentration at which the surfactants are saturated at the surface and start to form micelles in the bulk. The latest versions of these methods are presented in this paper.
Further improvements and other types of characterizations, that could be used in complement to those presented, will be discussed. Recent applications of these methods have already shown how such analyses can improve the understanding of the role of surfactants in cloud formation, by evidencing this role itself,3 determining the surfactant concentrations in atmospheric aerosols3,4,5,6 and mode of action in cloud droplet formation,3,6 evidencing their biogenic origin,3,4,7 and explaining their lack of observation by classical instruments.8,9,10

<table border="0" cellpadding="0" cellspacing="0" width="1448">
	<tbody>
		<tr height="24">
			<td height="24" style="height:24px;width:828px;">Quartz filters</td>
			<td style="width:219px;">Fioroni</td>
			<td style="width:235px;"></td>
			<td style="width:167px;">for example &#216;47mm or &#216;150mm, Grammage 85g/m2</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:828px;">Aluminium foils or glass Petri dishes</td>
			<td style="width:219px;"></td>
			<td style="width:235px;"></td>
			<td>backed in oven (773 K, 6h)</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:828px;">Tweezers, scissors</td>
			<td style="width:219px;"></td>
			<td style="width:235px;"></td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:828px;">Desiccator</td>
			<td style="width:219px;"></td>
			<td style="width:235px;"></td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:828px;">SPE (Solid Phase Extraction) set-up</td>
			<td style="width:219px;"></td>
			<td style="width:235px;"></td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:828px;">SPE vacuum manifold Ac-Elut</td>
			<td style="width:219px;">Varian&#160;</td>
			<td></td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:828px;">Pump Laboxat</td>
			<td style="width:219px;">Knf LAB</td>
			<td></td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:828px;">Nitrogen dryer set-up</td>
			<td style="width:219px;">hand-made</td>
			<td></td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:828px;">Compressed Nirogen 4.5 in bottle B50, 200 bar at 15&#176;C</td>
			<td style="width:219px;">Linde</td>
			<td></td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:828px;">Tensiometer</td>
			<td>Dataphysics&#160;</td>
			<td style="width:235px;">&#160;OCA 15EC</td>
			<td></td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:828px;">Software</td>
			<td style="width:219px;"></td>
			<td style="width:235px;">SCA software for OCA version 4-4.1</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:828px;">UV-Vis spectrometer</td>
			<td>Agilent</td>
			<td style="width:235px;">8453</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:828px;">Stir-plates</td>
			<td style="width:219px;"></td>
			<td style="width:235px;"></td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:828px;">Glassware</td>
			<td style="width:219px;"></td>
			<td style="width:235px;"></td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:828px;">Glass Petri dishes</td>
			<td style="width:219px;"></td>
			<td style="width:235px;"></td>
			<td>for the water extraction step</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Beakers</td>
			<td style="width:219px;"></td>
			<td style="width:235px;"></td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">15 mL, 30 mL, 60 mL glass bottles with corks</td>
			<td style="width:219px;"></td>
			<td style="width:235px;"></td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Tubes for SPE</td>
			<td style="width:219px;"></td>
			<td style="width:235px;"></td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Magnetic stirring bars</td>
			<td style="width:219px;"></td>
			<td style="width:235px;"></td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:828px;">Ultrasound bath</td>
			<td style="width:219px;"></td>
			<td style="width:235px;"></td>
			<td>for glassware washing</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:828px;">Micropipettes (0.5 - 5 mL, 0.100 - 1mL, 10 - 100 &#956;L)</td>
			<td style="width:219px;">Rainin Pipette-Life XLS</td>
			<td style="width:235px;"></td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:828px;">Disposable small&#160; equipment</td>
			<td style="width:219px;"></td>
			<td style="width:235px;"></td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:828px;">Syringe filters 0.40&#956;m PVDF</td>
			<td>Fisherbrand</td>
			<td style="width:235px;"></td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:828px;">SPE C18 cartridges Strata C18-E cartridges 500 mg / 3 mL&#160;</td>
			<td style="width:219px;">Phenomenex</td>
			<td style="width:235px;"></td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Plastic syringes</td>
			<td style="width:219px;"></td>
			<td style="width:235px;"></td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Needles</td>
			<td style="width:219px;"></td>
			<td style="width:235px;"></td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">4 mL-vials</td>
			<td style="width:219px;"></td>
			<td style="width:235px;"></td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Pasteur glass pipettes</td>
			<td style="width:219px;"></td>
			<td style="width:235px;"></td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Micropipette tips</td>
			<td style="width:219px;"></td>
			<td style="width:235px;"></td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:828px;">Chemicals</td>
			<td style="width:219px;"></td>
			<td style="width:235px;"></td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Sodium dodecyl sulfate (SDS) &#8805; 98.5 % Bioreagent</td>
			<td>Sigma- Aldrich</td>
			<td style="width:235px;">L3771</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Dioctyl sulfosuccinate sodium salt (AOT) &#8805; 97%</td>
			<td>Sigma- Aldrich</td>
			<td style="width:235px;">323586</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Benzyltetradecyldimethylammonium (zephiramine)&#160; &#8805; 99.0 % anhydrous Fluka</td>
			<td>Sigma- Aldrich</td>
			<td style="width:235px;">13401</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Cetyltrimethylammonium chloride solution (CTAC) 25 wt % in H2O</td>
			<td>Sigma- Aldrich</td>
			<td style="width:235px;">292737</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">(1,1,3,3-tetramethylbutyl)phenyl-polyethylene glycol (Triton X114) laboratory grade</td>
			<td>Sigma- Aldrich</td>
			<td style="width:235px;">X114</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Polyethylene glycol dodecyl ether (Brij35) Fluka Bio Chemika</td>
			<td>Sigma- Aldrich</td>
			<td style="width:235px;">858366</td>
			<td style="width:167px;"></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">L-&#945;-phosphatidylcholine from egg yolk type XVI-E&#160; lyophilized powder &#8805; 99 %</td>
			<td>Sigma- Aldrich</td>
			<td style="width:235px;">P3556</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Surfactin from Bacillus subtilis &#8805; 98 %</td>
			<td>Sigma- Aldrich</td>
			<td style="width:235px;">S3523</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">R-95Dd rhamnolipid (95 % dirhamnolipid, 5 % monorhamnolipid)&#160;</td>
			<td>Sigma- Aldrich</td>
			<td style="width:235px;">L510025</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Ethyl violet cationic triarylmethane dye</td>
			<td>Sigma- Aldrich</td>
			<td style="width:235px;">228842</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Patent Blue VF dye content 50 %</td>
			<td>Sigma-Aldrich</td>
			<td style="width:235px;">198218</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Ammonium thiocyanate &#8805; 99 % puriss. p.a.,&#160; ACS reagent</td>
			<td>Sigma-Aldrich</td>
			<td style="width:235px;">31120</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Cobalt(II) nitrate hexahydrate &#8805; 98 % ACS reagent</td>
			<td>Sigma-Aldrich</td>
			<td style="width:235px;">239267</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Acetic anhydride &#8805; 99 % ReagentPlus</td>
			<td>Sigma-Aldrich</td>
			<td style="width:235px;">320102</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Sodium acetate &#8805; 99.0 % anhydrous Reagent Plus</td>
			<td>Sigma-Aldrich</td>
			<td style="width:235px;">S8750</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Ethylenediaminetetraacetic acid 99.4&#8722; 100.6 % ACS reagent powder</td>
			<td>Sigma-Aldrich</td>
			<td style="width:235px;">E9884</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Sodium sulfate anhydrous &#8805; 99.0 % granulated puriss. p.a. ACS reagent Fluka</td>
			<td>Sigma-Aldrich</td>
			<td style="width:235px;">71960</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Ethanol puriss. p.a. ACS Reagent&#160; reag. Ph. Eur. 96% (v/v)</td>
			<td>Sigma-Aldrich</td>
			<td style="width:235px;">32294</td>
			<td></td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;">Acetonitrile&#160; &#8805; 99.9 % HiPerSolv CHROMANORM Reag. Ph. Eur. (European Pharmacopoeia Reagent) grade gradient for HPLC</td>
			<td>VWR BDH Prolabo</td>
			<td>20060.32</td>
			<td style="width:167px;">to be manipulated under hood</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;">Chloroform 99 % stable with 0.8&#8722;1 % ethanol</td>
			<td>Alfa Aesar</td>
			<td style="width:235px;">L13200-0F</td>
			<td style="width:167px;">to be manipulated under hood</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;">Toluene &#62; 99 %</td>
			<td>Chimie Plus</td>
			<td style="width:235px;">24053</td>
			<td style="width:167px;">to be manipulated under hood</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Denatured ethanol for washing</td>
			<td style="width:219px;"></td>
			<td style="width:235px;"></td>
			<td style="width:167px;"></td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:828px;">Ultra-Pure water</td>
			<td style="width:219px;">Ultrapure water system Purelab Classic, Elga</td>
			<td style="width:235px;"></td>
			<td></td>
		</tr>
	</tbody>
</table>

extraction and characterization of surfactants from atmospheric aerosols

Surface-active compounds, or surfactants, present in atmospheric aerosols are expected to play important roles in the formation of liquid water clouds in the Earth&#39;s atmosphere, a central process in meteorology, hydrology, and for the climate system. But because specific extraction and characterization of these compounds have been lacking for decades, very little is known on their identity, properties, mode of action and origins, thus preventing the full understanding of cloud formation and its potential links with the Earth&#39;s ecosystems.
In this paper we present recently developed methods for 1) the targeted extraction of all the surfactants from atmospheric aerosol samples and for the determination of 2) their absolute concentrations in the aerosol phase and 3) their static surface tension curves in water, including their Critical Micelle Concentration (CMC). These methods have been validated with 9 references surfactants, including anionic, cationic and non-ionic ones. Examples of results are presented for surfactants found in fine aerosol particles (diameter &#60;1 &#956;m) collected at a coastal site in Croatia and suggestions for future improvements and other characterizations than those presented are discussed.

Note: Before being applied to atmospheric samples, all the protocols presented in this section have been tested with 9 reference surfactants and the surface tension curves, minimum surface tensions, and CMCs obtained were in excellent agreement with the literature.21,22
1. Concentrations
Fine aerosols (&#60;1 &#956;m in diameter, or &#34;PM1&#34;) samples were collected on Quartz fiber filters at the coastal site of Rogoznica, Croatia in February 2015. These samples were handled and extracted as described in Sections 2 and 3, respectively, of this manuscript. The concentrations for anionic, cationic and non-ionic surfactants and the total surfactant concentration in the aerosol sample volume, Csurf,p (M), were measured according to Section 4. The results are presented in Figure 1, and evidence the dominance of anionic and non-ionic surfactants among the surfactants measured.
2. Surface Tension of Sample and Surface Tension Curve for Extracted Surfactants
Combining surface tension measurements as described in Section 5, and the concentration measurements, resulted in absolute surface tension curves for the same samples, as shown in Figure 2. These curves indicate the surfactant concentration in the aerosol sample and the surface tension of these samples (&#34;&#963;min&#34;) and allow to determine graphically the CMC values (Figure 2).
<img alt="Figure 1" class="xfigimg" src="/files/ftp_upload/55622/55622fig1.jpg" /> 
Figure 1: Surfactant concentrations in fine aerosols (PM1) collected in Rogoznica, Croatia. Concentrations for anionic (in blue), cationic (in red), non-ionic surfactants (in green) and total surfactant concentration in the aerosol phase (sum of each surfactant concentrations), Csurf,p (M), measured with the colorimetric method in fine (&#60;1 &#956;m) atmospheric aerosols collected at the coastal site of Rogoznica, Croatia in February 2015. The results clearly show the dominance of anionic and non-ionic surfactants. <a href="http://cloudflare.jove.com/files/ftp_upload/55622/55622fig1large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 2" class="xfigimg" src="/files/ftp_upload/55622/55622fig2.jpg" /> 
Figure 2: Typical surface tension curve and CMC for the surfactants in aerosols from Rogoznica, Croatia. Absolute surface tension curve for the surfactants in the sample of 03/02/2015 obtained by combining the surface tension and concentration measurements. The black dots represent the measured surface tension of the surfactant extract. The orange dot at the end of the curves represents the calculated concentration in the aerosol sample (step 4.4.6), and &#34;&#963;min&#34; its surface tension. The graphical determination of the CMC is illustrated. <a href="http://cloudflare.jove.com/files/ftp_upload/55622/55622fig2large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>

Watch this Scientific Journal Video about Extraction and Characterization of Surfactants from Atmospheric Aerosols at JoVE.com

Extraction and Characterization of Surfactants from Atmospheric Aerosols

Methods are presented for the targeted extraction of surfactants present in atmospheric aerosols and the determination of their absolute concentrations and surface tension curves in water, including their Critical Micelle Concentration (CMC).

Surface-active compounds, or surfactants, present in atmospheric aerosols are expected to play important roles in the formation of liquid water clouds in ...

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