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Lipid Speciation and Ion Interactions at the Air-Aqueous Interface in Atmospheric Aerosol Model Systems

Zhang, Ting
http://orcid.org/0000-0001-6393-3543
http://rave.ohiolink.edu/etdc/view?acc_num=osu152416015716577

Abstract Details

2018, Doctor of Philosophy, Ohio State University, Chemistry.
Sea spray aerosols (SSA) are known to influence the climate directly by affecting the absorption and scattering of solar radiation and, indirectly, by acting as cloud condensation nuclei and ice nuclei. The chemical composition of SSA is driven by organic species and ions present in the sea surface microlayer (SSML), the organic film that coats the ocean surface. These organics are taken up into SSA and affect aerosol climate properties such as hygroscopicity and albedo. In addition, metal ions have been shown to alter the surface organization, speciation, and solubility of organic surfactants. Thus understanding the surface behavior of organics in SSA in response to factors including but not limited to surface area, pH and interactions with cations is necessary to fully realize the impact of SSA on climate. Langmuir monolayers of dipalmitoylphosphatidic acid (DPPA), a simple surfactant phospholipid, are utilized as proxies for studying the organic coating at the ocean surface and on marine aerosols. There are two themes of study in this work. The first theme addresses selectivity of metal cations binding to organics at the ocean surface resulting in enrichment in the sea surface microlayer (SSML) and sea spray aerosol (SSA), and their impact on surface properties upon binding. The second theme addresses the phenomena that speciation of organics is different at interfaces such as the SSML and surface of SSA. The speciation of organic surfactants on SSA is considered to be an important factor controlling the interfacial and climate properties of SSA. However, correctly predicting the surface speciation requires the determination of the surface dissociation constants (surface-pKa) of the protic functional group(s) present. Phase behavior, stability, and surface morphology of DPPA films with and without metal cations were studied using surface pressure compression isotherms and imaged with Brewster angle Microscopy (BAM). The surface speciation of DPPA was studied using surface tension pH titration. The binding interactions of DPPA with cations was further investigated using surface tension salt titration. Finally infrared reflection-absorption spectroscopy (IRRAS) was utilized to probe the molecular level structure and interaction of DPPA with and without metal cations. To explore metal enrichment selectivity, a systematic metal cation affinity study on surfactant lipids containing various functional headgroups (-OH, -H2PO4, -COOH, -N(CH3)2) was conducted. DPPA is a model lipid for studying a phosphate headgroup. The phosphate headgroup is observed to exhibit the strongest trace metal binding, followed by the carboxylate headgroup. Hydroxide and dimethylammonium headgroups does not exhibit significant trace metal binding. Thus DPPA is determined to have the greatest impact on trace metal enrichment. The impact of pH and cations on phase behavior, stability, and surface morphology of DPPA was further investigated. At pH < 10, DPPA monolayers on water are found to be predominantly neutral species and display the highest packing density. Cations are qualitatively found to expand and stabilize the monolayer in the following order of increasing magnitude at pH 5.6: Na+ > K+ ~ Mg2+ > Ca2+. Additionally, cation complexation is tied to the pH and protonation state of DPPA, which are the primary factors controlling the monolayer surface behavior. The binding affinity of cations to the headgroup and thus deprotonation capability of the cation, is ranked in the order of Ca2+ > Mg2+ > Na+ > K+. Nucleation of surface 3D lipid structures is observed from Ca2+, Mg2+, and Na+, but not from K+, consistent with the lowest binding affinity of K+. A more expansive and quantitative order of metal cation binding affinities to phosphatidic acid was investigated using the Langmuir- Szyszkowski model. The order of affinity strength is quantitatively determined to be: Al3+ > Fe3+ ~> Zn2+ > Mg2+ > Ni2+ > Mn2+ ~ Ca2+, an order not predicted from molecular area expansion nor bulk properties such as solid formation constants of metal-phosphonates. Finally, the pKa to remove the second proton of DPPA (surface-pKa2) at the aqueous interface was investigated by examining the molecular area expansion at basic pH, In addition, the vibrational modes of the phosphate headgroup were directly probed and the stretches assigned to understand DPPA charge speciation with increasing pH. Results show that a condensed DPPA monolayer has a surface-pKa2 of 11.5, a value higher than previously reported (¿7.9–8.5). This surface-pKa2 is further altered by the presence of Na+ cations in the aqueous subphase, which reduced the surface-pKa2 from 11.5 to 10.5. It is also found that the surface-pKa2 value of DPPA is modulated by the packing density (i.e., the surface charge density) of the monolayer, with a surface-pKa2 as low as 9.2 for DPPA monolayers in the 2D gaseous phase over NaCl solutions. The experimentally determined surface-pKa2 values are also in good agreement with those predicted by Gouy-Chapman theory, validating these methods and showing that surface charge density is the driving factor behind changes to the surface-pKa2. These studies signify that phospholipids such as DPPA are significant source of trace metal enrichment within the SSML and SSA. The environment dependent speciation of DPPA further shows that the interfacial properties of SSA are significantly impacted by the presence of an organic coating and ions. Thus all these factors must be taken into consideration when attempting to understand the impact of SSA on climate and the environment.
Heather Allen (Advisor)
Terry Gustafson (Committee Member)
Abraham Badu-Tawiah (Committee Member)
146 p.
Cellular Biology; Chemical Oceanography; Chemistry
Phospholipid; Lipid; pKa; metal binding; aerosol

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Citations

  • Zhang, T. (2018). Lipid Speciation and Ion Interactions at the Air-Aqueous Interface in Atmospheric Aerosol Model Systems [Doctoral dissertation, Ohio State University]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=osu152416015716577

    APA Style (7th edition)

  • Zhang, Ting. Lipid Speciation and Ion Interactions at the Air-Aqueous Interface in Atmospheric Aerosol Model Systems. 2018. Ohio State University, Doctoral dissertation. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=osu152416015716577.

    MLA Style (8th edition)

  • Zhang, Ting. "Lipid Speciation and Ion Interactions at the Air-Aqueous Interface in Atmospheric Aerosol Model Systems." Doctoral dissertation, Ohio State University, 2018. http://rave.ohiolink.edu/etdc/view?acc_num=osu152416015716577

    Chicago Manual of Style (17th edition)

osu152416015716577
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This open access ETD is published by The Ohio State University and OhioLINK.