Physicochemical Properties of Pinic, Pinonic, Norpinic, and

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Physicochemical Properties of Pinic, Pinonic, Norpinic, and Norpinonic Acids as Relevant #-pinene Oxidation Products Agata Ko#odziejczyk, Patryk Pyrcz, Aneta Pobudkowska, Kacper Blaziak, and Rafal Szmigielski J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.9b05211 • Publication Date (Web): 27 Aug 2019 Downloaded from pubs.acs.org on August 30, 2019

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The Journal of Physical Chemistry

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Journal of Physical Chemistry B

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Physicochemical Properties of Pinic, Pinonic, Norpinic,

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and Norpinonic Acids as Relevant α-pinene Oxidation

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Products

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Agata Kołodziejczyka*, Patryk Pyrcza,b, Aneta Pobudkowskab, Kacper Błaziakc and Rafał

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Szmigielskia*

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a

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Warsaw, Poland

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b

Institute of Physical Chemistry, Polish Academy of Sciences, ul. Kasprzaka 44/52, 01-224

Department of Physical Chemistry, Faculty of Chemistry, Warsaw University of

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Technology, ul. Noakowskiego 3, 00-664 Warsaw, Poland

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c

University of Warsaw, Faculty of Chemistry, ul.Pasteura 1, 02-093 Warsaw, Poland

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ABSTRACT

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Here, the study is focused on the synthesis and determination of physicochemical properties of

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four α-pinene secondary organic aerosol (SOA) products: cis-pinic acid, cis-pinonic acid, cis-

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norpinic acid, cis-norpinonic acid. These encompasses their thermal properties, solid-liquid

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phase equilibria and dissociation constant (pKa). Thermal properties, including the melting

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temperature, enthalpy of fusion, temperature and enthalpy of the phase transitions were

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measured with differential scanning calorimetry technique (DSC). These SOA components

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exhibit relatively high melting temperatures from 364.32 K for cis-pinic acid to 440.68 K for

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cis-norpinic acid. The enthalpies of fusion vary from 14.75 kJ∙mol-1 for cis-norpinic acid to

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30.35 kJ∙mol-1 for cis-pinonic acid. The solubility in water was determined with the dynamic

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method (solid-liquid phase equilibria method) and then experimental results were interpreted

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and correlated using three different mathematical models: Wilson, NRTL and UNIQUAC

30

equations. The results of the correlation indicate that Wilson equation appears to work the best

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for all investigated compounds giving rise to the lowest value of a standard deviation. Cis-

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norpinic acid and cis-pinic acid (dicarboxylic acids) show better solubility in the aqueous

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solution, than cis-norpinonic acid and cis-pinonic acid (monocarboxylic acids), which affect

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the multiphase chemistry of α-pinene SOA processes. For cis-pinonic acid and cis-norpinonic

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acid also pH-profile solubility was determined. The intrinsic solubility (S0) for cis-norpinonic

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acid was measured to be 0.05 mmol · dm-3, while for cis-pinonic acid 0.043 mmol · dm-3. The

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acidity constants (pKa) at 298 K and 310 K using the Bates–Schwarzenbach spectrophotometric

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method were determined. The pKa values at 298.15 K for cis-norpinonic acid and cis-pinonic

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acid were found to be 4.56 and 5.19, respectively, whereas at 310.15 K – 4.76 and 5.25,

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respectively.

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1. Introduction

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The formation of particles in the atmosphere have been intensively studied owing to

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their impact on the human health, air quality and climate change.1-4 The major mass fraction of

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atmospheric fine particulate matter (PM2.5; particles with an aerodynamic diameter below 2.5

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µm) is secondary organic aerosol (SOA), which results from the chemical reactions of volatile

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organic compounds (VOCs) emitted from terrestrial ecosystems with different atmospheric

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oxidizing reagents, such as ozone (O3), OH-radicals and NO3-radicals followed by various

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multiphase processes.5-10

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α-Pinene (monoterpene, C10H16) is the second foremost released non-methane VOC with

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an estimated yearly emission rate of 70 Tg.11-12 Due to a high emission rate of α-pinene and its

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chemical reactivity, α-pinene SOA has been studied extensively over last decades.1, 5-9, 13-14 A

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number of reactions on α-pinene result in the formation of organic products with one or more

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functional groups added to α-pinene-derived skeleton, such as ketone (-C(=O)-), alcohol (-OH),

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aldehyde (-C(=O)H) or carboxylic acids (-COOH).1, 15 An increasing oxygen to carbon ratio

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renders α-pinene SOA products more hydrophilic and polar and thus prone to further aqueous

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processing (aging) and act as a cloud condensation nuclei (CCN) in the atmospheric

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hydrological cycle.14, 16-17 There is still a lot of uncertainties in these processes due to the lack

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of their fundamental physicochemical parameters.

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It has been well established that important α-pinene SOA components are cis-pinic acid

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(C9H14O4), cis-pinonic acid (C10H16O3) as well as cis-norpinic acid (C8H12O4), and cis-

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norpinonic acid (C9H14O3).18-19 These oxygenated products have been found as important

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tracers for α-pinene SOA source characterization.1, 20 As water-soluble compounds they are also

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involved in the formation of aqueous secondary organic aerosols (aqSOA).21 The experimental

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and theoretical modeling studies have shown that the contribution of aqueous reactions to the

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atmospheric SOA formation is likely significant.22-23 Therefore, in order to reduce the 3 ACS Paragon Plus Environment


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uncertainties in aqSOA formation mechanisms, in the present study we focus on the

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determination of properties of four important oxidation products of α-pinene, i.g., cis-pinic acid

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(PA), cis-pinonic acid (PNA), cis-norpinic acid (NPA) as well as cis-norpinonic acid (NPNA).

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Despite numerous reports showing atmospheric concentrations of α-pinene SOA products

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(Table 1), only few publications present experimentally derived physical properties of the

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describing compounds, such as: surface tension depletion and cloud activation potentials.24

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Surprisingly little is known about their solubility and thermal properties. Our work fills these

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gap and the solid-liquid phase equilibria (solubility in water) and thermal properties (melting

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temperature, enthalpy of fusion, temperature and enthalpy of the phase transitions) of

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investigated compounds are examined. The experimental data, such as the dissociation constant

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(pKa) and pH-profile solubility for PNA and NPNA is also reported.

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Table 1. Reported ranges of concentrations of the selected α-pinene SOA products in the

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atmosphere.

80 Compound

81

cis-pinic acid

82 83

cis-pinonic acid

84

cis-norpinic acid

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cis-norpinonic acid

Concentration in the atmosphere [ng · m-3] 0.35 - 1.19 25 0.39 - 82.72 26 0.8 - 16.1 27 1.23 - 3.1 25 0.9 - 5.5 27 7.05 - 97.7 26 0.2 - 1.6 27 0.2 28 0.14 - 24.3 26 0.4 - 2.2 27

86 87 88 89 90 91 92


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2. Experimental

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2.1. Materials and methods

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PA, PNA, NPA, NPNA were synthesized according to procedure reported by Moglioni

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et al.29 The investigated compounds structural formulas and the used abbreviations are shown

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in Table 2. For synthetic procedures, analytical data, as well as description of used chemicals

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please consult the Supporting Information.

99 100

Table 2. Investigated compounds: name, abbreviation, structure. Name of compound/ Abbreviation

cis-norpinonic acid NPNA

cis-norpinic acid NPA

cis-pinonic acid PNA

cis-pinic acid PA



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2.2. Differential scanning microcalorimetry (DSC)

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The differential scanning microcalorimetry technique was used to determined: the temperature of

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fusion (Tfus,1), the enthalpy of fusion (∆fusH1), temperature of transition (Ttr,1) and the enthalpy of

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transition (∆trH1). These measurements were performed with a DSC 1 STAR System (Mettler

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Toledo) calorimeter equipped with the liquid nitrogen cooling system and operating in a heat-flux

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mode. For each experiment, aluminum pans having mass of about 50 mg were used. The first pan

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contained the studied compound, while the second one served as a reference. The pans were

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hermetically sealed. Each sample (ca. 10 mg) was used throughout this study. The samples were

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heated in two separate furnaces. Experiments were carried out using the 10 K∙min-1 heating rate.

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The calibration was done with 0.999999 mol fraction purity indium sample. The uncertainty of the

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fusion temperature (Tfus,1) was ± 0.1 K and that of the enthalpy of fusion (∆fusH1 ) was ± 0.05

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kJ∙mol-1. The thermophysical characteristic was analyzed using STAR software. The molar

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volumes of each studied compound were calculated using the Barton’s method.30

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2.3. Phase equilibria in binary systems – apparatus and measurements

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In this work four binary systems: {i.e., PA(1) + water(2), PNA(1) + water(2), NPA(1) + water(2),

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NPNA(1) + water(2)} have been studied. Solid–liquid equilibrium temperatures were determined

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using a dynamic method for all systems described in detail by Domańska.42 For the solubility

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measurements dynamic method for all systems was used (Scheme 1). The mixtures of a studied

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compound and water were prepared by weighing synthesized (purity ≥ 99 %) components with the

120

uncertainty of 1∙10-4 g. The experimental points have been determined from 305.15 K to water

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boiling point. The sample prepared was heated gradually (5 K∙min-1) with continuous stirring

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inside a Pyrex glass cell placed in thermostated water bath. When the sample was in the vicinity

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of the phase transition temperature, the heating rate was decreased to about 0.2 K∙min-1. The

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temperature was taken as the temperature of the equilibrium in the saturated solution. The crystal

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disappearance temperature was detected visually and measured using an electronic thermometer P

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550 (DOSTMANN electronic GmbH) with a probe fully immersed in the thermostating liquid.

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The uncertainties of the temperature measurements were determined to be ± 0.1 K and of the mole

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fraction did not exceed ± 0.0005. The repeatability of the SLE experimental points was ± 0.1 K.

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Scheme 1.

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2. Glass cell, 3. Temperature sensor, 4. Mixing element I, 5. Mixing element II,

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6. Magnetic stirrer, 7. Water bath;

Static apparatus

for solid-liquid

equilibria (SLE);

133 134


1.

Electric heater,


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2.4. pH-Dependent solubility studies

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The solubility experiment was performed with a small-scale shake flask method at constant

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temperatures 298.1 K.31 The shake-flask method proposed by Higuchi and Connors is the most

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reliable and widely used solubility measurement method. This method determines thermodynamic

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solubility and could be carried out in several steps.32 Each compound was added in excess to about

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10 mL of dipotassium hydrogen phosphate (0.15 M) in a test tube. The test tubes were placed on

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a magnetic stirrer. The stirring time at certain pH have been examined, using pH-meter up to

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obtaining a stable pH (solubility equilibrium). The pH of each carboxylic acid suspension was

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measured and adjusted if necessary with either diluted 0.2 M H3PO4, or 0.2 M KOH to a selected

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pH value. The experiment was completed at least three pH measurements performed at an early,

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an intermediate and a late time-point of the 24 h period resulted in the same pH value. The

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equilibrium solubility was attained within 24 h. The pH values were also measured for the

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supernatants obtained after centrifugation, in order to certify that the carboxylic acid solutions

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analyzed had the same pH value as the suspensions. Measurements were made using pH-meter

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(CPC-401 ELMETRON) with an associated uncertainty of 0.01. The test-tubes were thermostated

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by the temperature control thermostat (Lauda A3, Germany) through the jacket of the vessel with

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uncertainty  0.1 K. Samples were withdrawn after 24 h. An excess of the solid was equilibrated,

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using a rotating-bottle apparatus, Hettich Zenrifugen, EBA 20, at 300 000 RPM for 30 min.

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Following centrifugation, the supernatant was collected and used for solubility and pH

154

determinations. The concentration of the carboxylic acid in the supernatant solution were

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determined by an HPLC procedure with a single-wavelength UV detection. All compounds were

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analysed four times at each investigated pH value, as it was shown in the previous works.30, 32-33

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150 mM K2HPO4 and 150 mM KH2PO4 were chosen as buffer, and this buffer was mixed with


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pure carboxylic acid to obtain the desired pH values. For pH values below the pKa value the H3PO4

159

was added and for the values above the pKa values, the KOH was added.33, 35

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2.4.1 HPLC analysis

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Each sample contained excess of studied compound and buffer solutions, potassium

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hydroxide or phosphoric acid solution. The concentration of the α-pinene-derived acid in each

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sample was measured with an HPLC-UV-vis apparatus delivered by Agilent Technologies,

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consisted of: 1200 Series Quat pump, 1200 vacuum degasser, 1200 DAD/MWD. A C18 analytical

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column (4.6 mm × 150 mm, Agilent Zorbax Eclipse XDB) with a mean particle size of 5 μm was

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used. As a mobile phase two solutions were used: methanol (A) and water (B). Injection volumes

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of 50 μL were used during the analysis. The chromatographic conditions for PNA and NPNA were

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as follows: mobile phase - 50% MeOH/50% H2O, flow rate – 0.4 ml/min, λPNA – 210 nm, , λNPNA

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– 200 nm. The calibration curves measured for NPNA and PNA are show in Supplementary

170

Information at Figure S11 and Figure S12.

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2.5. The pKa measurements

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2.5.1. The Bates-Schwarzenbach method

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The pKa measurements were performed with the Bates-Schwarzenbach method using an UV-vis

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spectrophotometer (Perkin – Elmer Life and Analytical Sciences Lambda 35, Shelton USA). The

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measurements were conducted at the two temperatures: 298.15 K and 310.15 K. Solutions in water

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of each compound were prepared with a mol concentration of 1∙10-2 mol∙dm-3. The buffer was

177

selected (mol concentration) i.e., acetic acid (0.010067), sodium acetate (0.009688) and sodium

178

chloride (0.010312).29 The buffer was chosen on the basis of the literature value of pKa of

179

investigated compounds. The values of pH, acidity function (𝑝(𝛼𝐻 𝛾𝐶𝑙 )) and the ionic strength (I) 9 Environment ACS Paragon Plus


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for the used buffer are presented in Supporting Information in Table S2. For each tested compound,

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three samples were prepared: in 0.2 M hydrochloric acid, in 0.2 M sodium hydroxide and in a

182

buffer solution. Samples were scanned with 0.2 M water - acid, 0.2 M water - base and water -

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buffer solutions as a reference, respectively, with scan step 1 nm from 320 nm to 190 nm. The pKa

184

values have been calculated by the following equation:

185

𝐷𝐻𝐴 − 𝐷𝐵𝑈𝐹 𝑝𝐾𝑎 = 𝑝(𝛼𝐻 𝛾𝑐𝑙 ) − 𝑙𝑜𝑔 ( ) 𝐷𝐵𝑈𝐹 − 𝐷𝐴−

186

where: pKa is the acidity constant, 𝑝(𝛼𝐻 𝛾𝐶𝑙 ) is the acidity function, 𝐷𝐻𝐴 , 𝐷𝐴− , 𝐷𝐵𝑈𝐹 are absorbance

187

values in acidic, basic and buffered solutions, respectively. The measurement error was calculated

188

as a standard deviation for the wavelength range.

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3. Results and discussion

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The basic thermal properties of selected α-pinene SOA aging products have been measured by the

191

differential scanning microcalorimetry technique (DSC). The calorimetric measurements were

192

also used to estimate the purity of synthesized compounds. The NMR analysis together with DSC

193

measurements has shown a high purity of investigated compounds (~99% for each compound).

194

The thermograms revealed the strong dependence of a heat flow on the temperature for each SOA

195

compound (Figure 1). The measured thermal properties included: temperatures of fusion (Tfus,1),

196

enthalpy of fusion (∆fusH1), temperatures of transition (Ttr,1) and enthalpy of transition (∆trH1) are

197

presented in Table 3. The thermographs recorded for each investigated SOA compound clearly

198

demonstrate that these substances exhibit a relatively high melting temperature from 364.32 K for

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PA to 440.68 K for NPA. The experimental value measured for PA and PNA are nearly identical

200

than these found in the literature.36 The enthalpies of fusion vary from 14.75 kJ∙mol-1 for NPA to


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30.35 kJ∙mol-1 for PNA. Interestingly, a solid – solid phase transition was observed (Ttr,1 = 338.15

202

K, ΔtrH1 = 15.63 kJ∙mol-1) only for NPA. The enthalpy of transition slightly exceeds the enthalpy

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of fusion for this compound. Moreover, the analysis of curves shape confirmed the high purity of

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the synthesized compounds (≥99%).

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Figure 1. DSC thermograms recorded for investigated compounds. a) cis-norpinonic acid

c) cis-pinic acid

b) cis-norpinic acid

d) cis-pinonic acid


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Table 3. Physicochemical characteristics of selected α-pinene SOA aging acids: the temperature

227

of fusion Tfus,1, the enthalpy of fusion ∆fusH1, heat capacity changes at the fusion temperature,

228

∆Cp(fus),1, the temperature of transition Ttr,1, the enthalpy of transition ∆trH1 and the molar volume

229

𝑉𝑚293.15 𝐾 .

Compound

Tfus,1/

ΔfusH1/

ΔCp(fus),1b/

Ttr,1/

ΔtrH1/

Vm293.15Kᵃ/

Tfus,1lit/

1

cm3·mol-1

K

kJ·mol-

K

kJ·mol-1

J·mol-1·K-1

K

NPNA

403.47

23.20

57.50

-

-

156.70

-

NPA

440.68

14.75

33.48

338.15

15.63

144.90

44840

PNA

376.68

30.35

80.58

-

-

172.80

375-37736

PA

364.32

16.60

45.57

-

-

161.00

376-37836

230 231 232

ᵃ Calculated according to the Barton’s group contribution method.30 b Calculated with Tfus,1 and ΔfusH1. Standard uncertainties u are as follows: u(Tfus,1) = ±0.1 K, u(ΔfusH1)= ±0.1 kJ∙mol-1.

233

The solubilities of NPNA, NPA, PNA and PA have been determined in water. We studied four

234

binary systems {i.e., PA(1) + water(2), PNA(1) + water(2), NPA(1) + water(2), NPNA(1) +

235

water(2)}. All systems have been tested using a dynamic method. Experimental solubilities for all

236

binary systems in mole fraction and calculated activity coefficients have been summarized in the

237

Table 4 and in Supplementary Information in Figure S5-S8. The combined experimental data in

238

Figure 2 shows better solubility of PA and NPA acids, which have in its structure two carboxylic

239

moiety. The arrangement of experimental points in the {NPA (1) + water (2)} system may be

240

caused by the presence of solid – solid phase transition in the vicinity of 338.15 K temperature.

241

The solubility of tested compounds in water increases in order: PA > NPA > PNA > NPNA. At a

242

given concentration each of these acids could likely modify the surface tension of aerosol particles

243

near a gas-particle interface, and consequently alter their cloud activation potentials. Since PA and

244

PNA were shown to weakly reduce cloud droplet activation at 1mM concentration one should

245

expect that other oxidation products studied here, i.e., NPA and NPNA would follow the same

246

trend. This behavior could be explained by the presence of highly polar carboxylic moieties in

247

their molecules, which triggers α-pinene-derived acids less surface active than those originated in

248

the atmosphere from the oxidation of sesquiterpenes, e.g., β-caryophyllene. The latter products 13 Environment ACS Paragon Plus


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have amphiphlic or surfractant-like behavior and recently have been proved to substantially lower

250

the surface tension of cloud-forming droplets containing aerosol nuclei. 24

251

Table 4. Experimental solubility for {α-pinene-derived acid (1) + water (2)} binary system in mole

252

fraction, x1 vs. equilibrium temperature T at saturated solution at p = 101.3 kPad and calculated

253

activity coefficients, γ1. x1b

Tc /K

γ1a

x1b

T c/K

γ1a

0.0032

318.61

49.99

0.0080

339.87

34.13

0.0038

323.87

48.72

0.0098

344.40

31.09

0.0044

327.45

45.83

0.0121

348.55

27.86

0.0053

332.40

43.33

1.0000

403.47

1.00

0.0065

336.55

39.19

0.0093

307.25

10.67

0.1023

336.37

2.72

0.0119

313.55

10.63

0.1259

339.75

2.40

0.0187

320.95

8.86

0.1500

343.01

2.12

0.0354

328.35

6.03

0.1823

345.55

1.81

0.0531

331.25

4.44

1.0000

440.68

1.00

0.0020

319.39

88.34

0.0075

341.27

48.84

0.0024

325.25

88.89

0.0118

344.45

34.37

0.0029

328.99

84.62

0.0162

346.77

26.76

0.0036

331.55

74.47

0.0183

347.73

24.36

0.0046

335.55

65.84

1.0000

376.68

1.00

0.0273

307.15

13.21

0.3683

336.86

1.74

0.1195

324.55

4.27

0.4050

338.35

1.62

0.1778

328.58

3.10

0.4804

341.11

1.43

0.2411

331.35

2.40

1.0000

364.32

1.00

0.3205

334.95

1.93

NPNA

NPA

PNA

PA

254

Standard uncertainties u are as follows:


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255

a

Calculated from the NRTL equation for PA and Wilson equation for PNA, NPA and NPNA.

256

b

u(x1water) = ±0.005

c

u(T/K) = ±0.1, d u(p/kPa) = ±2.

257 258

Figure 2. The experimental solubility of {α-pinene-derived acid (1) + water (2)} binary systems;

259

(♢) NPNA, (∆) NPA, (⧠) PNA and (◯) PA.

260 261

The phase diagrams of solid-liquid phase equilibria derived experimentally were subsequently

262

correlated with the use of the following thermodynamic models reported elsewhere: Wilson

263

equation, NRTL equation and UNIQUAC equation.37-39 For detailed parameters of the correlation 15 Environment ACS Paragon Plus


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and standard deviations please consult Supporting Information (Table S3). The low standard

265

deviation values δT suggest a good correlation between the experimental data and the

266

thermodynamic models. For each α-pinene SOA acid a lowest standard deviation is set below or

267

around 1. The ideal solubility calculated for three compounds: NPNA, PA and PNA is higher than

268

the experimental solubility in a measured concentration ranges, whereas the ideal solubility of

269

NPA is changing with the increasing temperature. In first stage it is initially higher, then it

270

decreases, which can be explained by a solid – solid phase transition observed at Ttr,1 = 338.15 K.

271

The pKa studies for NPNA and PNA were conducted by a spectrophotometric Bates-

272

Schwarzenbach method. The measurements were obtained at two temperatures: 298.15 K and

273

310.15 K. However, owing to the chromophore inactivity we could not pursue the measurements

274

of pKa for PA and NPA. The measurement of the absorbance of PNA and NPNA solutions as a

275

function of the wavelength in three solutions of 0.2 M NaOH, 0.2 M HCl and in buffer were

276

performed. The wavelengths were chosen for a maximum distance of the border absorbance. The

277

determination of the concentration ratio in the spectrophotometric measurements was possible by

278

assumption of the absorbance additivity law and the Bouguer-Lambert-Beer law for different form

279

of the acids existing in the in the solution. The comparison of the experimental results obtained for

280

both α-pinene SOA acids with these available in the literature are collected in Table 5. The UV-

281

vis spectra acquired for NPNA and for PNA at the two temperatures are presented in Supporting

282

Information in Figure S1–S4. The experimental value of pKa for NPNA obtained with a Bates-

283

Schwarzenbach method is 4.56 in 298.15 K, while at 310.15 K the value is 4.75. The experimental

284

value measured at the temperature T=298.15 K is nearly identical than these found in the literature

285

at the same temperature (pKalit = 4.50). In contrast, the pKa value at T=310.15 K was not earlier

286

reported. For PNA the experimental value of pKa at T=298.15 K is 5.19, while at T=310.15 K is


Page 16 of 22

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287

5.25. The comparison of the experimental value obtained at 298.15 K for PNA with that found in

288

literature shows that the latter is lower (pKalit = 4.72).39 It is worth noted that literature pKa values

289

for either compound was only predicted (not experimentally derived) based on the molecular

290

structure algorithm (ACD/Labs). For both α-pinene-derived acid the pKa value determined at

291

temperature T=310.15 K is higher than this at temperature T=298.15 K.

292

Table 5. The experimental and literature values of pKa and the intrinsic solubility (S0) determined

293

at 298.15 K and 310.15 K for NPNA and PNA.

Compound

𝐩𝑲𝐥𝐢𝐭 𝐚

𝐞𝐱𝐩

𝐞𝐱𝐩

𝐩𝑲𝐚

𝐩𝑲𝐚

𝐞𝐱𝐩

𝐩𝑲𝐚 (𝐩𝐇)

S0 / mmol · dm-3

298.15 K

298.15 K

310.15 K

298.15 K

298.15 K

NPNA

4.50±0.60a

4.56±0.23

4.75±0.25

4.64

0.05

PNA

4.72±0.10a

5.19±0.39

5.25±0.10

4.57

0.043

294 295

a

296

In this study we have also determine the pH-dependent solubility profiles for PNA and NPNA.

297

These were obtained by the application of a small-scale shake flask method at the constant

298

temperature 298.15 K. The pKaexp (pH) values were obtained from the crossing of the two lines

299

interpolated from the experimental points for the unionized and ionized form of the NPNA as well

300

as PNA. For both compounds pKaexp (pH) values are shown in Table 5. The experimental value of

301

pKa (pH) for NPNA is 4.64, whereas for PNA the pKa (pH) value is 4.57. The comparison of the

302

experimental values obtained at 298.15 K for both acids and values reported elsewhere shows that

303

literature value is lower (pKa it = 4.50) for NPNA and higher (pKa lit = 4.72) for PNA.40 The change

304

of pH values as a function of logS values determined by a shake-flask method and calculated with

Advanced Chemistry Development (ACD/Labs) Standard uncertainties u are as follows: u(pH) = ±0.1.



305

the Henderson-Hasselbalch equation is shown in Supporting Information, Figure S9/S10. For both

306

α-pinene SOA products the results prove that the solubility increases with an increase of pH.

307

Conslusions

308

In summary, this paper provides for the first time the determination of the physicochemical

309

properties i.e., thermal properties, solid-liquid phase equilibria and dissociation constant (pKa) of

310

key α-pinene SOA oxidation products: PA, PNA, NPA and NPNA along with their improved

311

synthesis. By the differential scanning microcalorimetry technique (DSC) thermal properties i.e.,

312

temperatures of fusion (Tfus,1), enthalpy of fusion (∆fusH1), temperatures of transition (Ttr,1) and

313

enthalpy of transition (∆trH1) were measured. The acquired thermograms revealed relatively high

314

melting temperatures of all studied compounds, ranging from 364.32 K for PA to 440.68 K for

315

NPA. The enthalpies of fusion varied from 14.75 kJ∙mol-1 for NPA to 30.35 kJ∙mol-1 for PNA. A

316

good correlation between the experimental data and the thermodynamic models was achieved as

317

proved by the low values of standard deviation parameters δT. The measured water solubility was

318

evidenced to increase in the following order: PA > NPA > PNA > NPNA, which affect their

319

multiphase aging chemistry in the atmosphere. For NPNA and PNA pH-profile solubility showed

320

that the solubility increases with an increase of pH. The experimental value of pKa (pH) revealed

321

that pKa (pH) for NPNA 4.64, whereas for PNA the pKa (pH) value was 4.57. However, the

322

experimental value of pKa for NPNA obtained with a Bates-Schwarzenbach method was 4.56 at

323

298.15 K and 4.75 at 310.15 K, whereas for PNA 5.19 at 298.15 K and 5.25 at 310.15 K. For both

324

α-pinene-derived acids the pKa value determined at 310.15 K was higher than that at 298.15 K. A

325

determined set of physicochemical parameters for four key α-pinene SOA products would serve

326

as a useful databank for the atmospheric community and is believed to be extended in the future

327

towards other important aerosol products, including isoprene SOA and sesquiterpene SOA.


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328 329

Supporting Information

330

Analytical data and copies of 1H and 13C NMR spectra of all compounds. The experimental of pH-

331

profile solubility points and calibration curves measured for PNA and NPNA. Phase diagrams of

332

PA, PNA, NPA, NPNA in water obtained by dynamic method.

333

Corresponding Authors

334

*Corresponding authors: Telephone: +48 22 343 34 02. e-mail: [email protected] and

335

[email protected]

336

Acknowledgments

337

Funding for this research was provided by the Polish National Science Centre grant ETIUDA6 (Nr

338

2018/28/T/ST4/00036) and by the Warsaw University of Technology, Warsaw, Poland.

339 340

References

341 342 343 344 345 346 347 348 349 350 351 352 353 354 355 356 357 358

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Physicochemical Properties of Pinic, Pinonic, Norpinic, and

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