Data Files Accompanying: “Surface Tension Measurements of Aqueous Liquid-Air Interfaces Probed with Microscopic Indentation”

Authors:  Kaluarachchi, Chathuri P.; Lee, Hansol; Lan, Yiling; Lansakara, Thiranjeewa; Tivanski, Alexei 

Journal:  Langmuir

Contact: Tivanski, Alexei. V., alexei-tivanski@uiowa.edu, Department of Chemistry, University of Iowa, Iowa City, Iowa 52242

Corresponding author name, email, department, institution: Tivanski, Alexei. V., alexei-tivanski@uiowa.edu, Department of Chemistry, University of Iowa

Cite as:
Kaluarachchi, Chathuri P.; Lee, Hansol; Lan, Yiling; Lansakara, Thiranjeewa; Tivanski, Alexei V. (2021) Data from: Surface Tension Measurements of Aqueous Liquid-Air Interfaces Probed with Microscopic Indentation. In Center for Aerosol Impacts on Chemistry of the Environment (CAICE) Collection. UC San Diego Library Digital Collections.

DOI: https://doi.org/10.6075/J0ZW1JGC

Folder organization: AFM, Bulk tensiometer, and force plot analysis data separated into individual files.
       Method: Bulk solutions of hexanoic acid and sea surface microlayer samples. Hexanoic acid was purchased from Sigma Aldrich and used without additional purification. It was dissolved in an ultra-pure water (18 MΩ∙cm) to generate aqueous bulk solutions with molar concentrations ranging from 0.1 mM to 80 mM. Sea surface microlayer (SML) samples were collected from a wave-simulation channel facility which contained filtered seawater collected from the southern coast of California during the summer of 2019. A phytoplankton bloom in the wave-simulation channel was induced by adding nutrients following similar experimental approaches as in the previous wave-flume studies. Glass plate sampling method was utilized to collect the SML samples over course of the bloom lifetime. The glass plate was lowered carefully by hand at a rate of 5 - 6 cm/s and withdrawn at approximately the same rate. This withdrawal rate corresponds to a sampled SML thickness of around 50 µm. After removal from the flume, the glass plate was suspended for 20 seconds to allow excess seawater to drain off, and the glass plate was scrapped with a Teflon scraper to collect the remainder of the adsorbed liquid. Bulk surface tension measurements. Bulk surface tension measurements were performed on the aqueous hexanoic acid solutions and SML samples (ca 6 mL) using Kibron AquaPi force tensiometer (Kibron, Finland) with the Du Noüy-Padday method (macro-needle diameter of approximately 0.5 mm). The measurements have been previously described in detail and only briefly summarized here. The tensiometer was calibrated using ultra-pure water before and after each experiment, and the dyne probe was cleaned with ethanol, water and torched with a flame between each measuremnets. Surface tension of bulk solutions was measured as a function of increasing hexanoic acid solute concentrations. The surface tension of SML sample was quantified without further dilutions. At least three repeated measurements were performed for each liquid sample, with the bulk surface tension value reported as a mean value with one standard deviation.
       AFM surface tension measurements on the droplets. Molecular force probe 3D AFM (Asylum Research, Santa Barbara, CA) was used in contact mode for all force spectroscopy measurements at ambient temperature (25 °C) and pressure. A custom-made sealed humidity cell was used to provide a sealed environment to minimize the evaporation of the liquid droplet. High aspect ratio, constant diameter Ag2Ga nanoneedles (NN-HAR-FM60, Nauga Needles) with a nominal spring constant of 2.7 - 3.3 N/m and radius of 25 – 100 nm were used for surface tension measurements. The radius of the nanoneedle was calibrated by performing force measurements on a reference ultra-pure water droplet with known surface tension (72.0 mN/m at 25 °C). The resultant value was also compared with scanning electron microscopy image of each nanoneedle. The nanoneedle radius calibration was repeated both before and after each measurement to ensure no significant change in the probe radius. If the calibrated radius of the nanoneedle differed by more than 10% between before and after each AFM experiment, the data was discarded and the experiment was repeated with a new nanoneedle. 
       A droplet containing aqueous hexanoic acid solution at various concentrations and a droplet of SML was placed on a silicon wafer substrate inside the sealed humidity cell. Force measurements were collected over an approximate center of the droplet with a 1 Hz scan rate. Upon indenting the nanoneedle to several hundreds of nanometers into the air-liquid interface, the nanoneedle movement was paused within the droplet for 1-2 seconds of dwell time. The nanoneedle was then retracted away from the droplet with a constant pulling rate of 2 µm/s. For each force plot, the maximum retention force was measured and used to quantify the surface tension of the droplet using a previously reported method. At least five consecutive force plots were collected for each sample. AFM surface tension data is reported as the mean value and error bars correspond to one standard deviation.