Data Files Accompanying: “Let There be Light: Stability of Palmitic Acid Monolayers at the Air/Salt Water Interface in the Presence and Absence of Solar Light and a Photosensitizer”
Authors:  Shrestha, Mona.| Luo, Man.| Yingmin,Li.| Bo, Xiang.|Grassian, Vicki, H. 
Journal: Chemical Science
Contact: Grassian, Vicki, H., vhgrassian@ucsd.edu, Department of Chemistry & Biochemistry,Scripps Institution of Oceanography, Department of Nanoengineering, University of California, San Diego, La Jolla, CA 92093
Cite as: Shrestha, Mona; Luo, Man; Yingmin, Li; Bo, Xiang; Grassian, Vicki H. (2018). Data From: Let There be Light: Stability of Palmitic Acid Monolayers at the Air/Salt Water Interface in the Presence and Absence of Solar Light and a Photosensitizer. In Center for Aerosol Impacts on Chemistry of the Environment (CAICE). UC San Diego Library Digital Collections. https://doi.org/10.6075/J0ZS2TPQ

File Format: All data is held in comma delimited text files.
Folder Organization: In the data packet, there are two folders, labeled as “main text” and SI, for data from the main text and data in supporting information, respectively. Additionally, data from each figure have their own folder. In each of these folders, there are files labeled "HOMMA which means "holding mean molecular area" and files labeled "HOSP" which means "holding surface pressure". There are also "NOR" and "RAD" which mean "no radiation" and "radiation" respectively. In figure 2 (b) folder, there are "NACL_IRRAS_SP0", "NACL_IRRAS_SP10" and "NACL_IRRAS_SP25" files, which mean the IRRAS data taken for palmitic acid monolayer at these specific surface pressure as noted in the main text figure. In figure 4 (c) and figure S2 folder, there are also IRRAS data. The "HOMMA", "HOSP", "NOR" and "RAD" in those names are explained above. The labels “Before” and "After" refer to IRRAS data taken before and after 3 hour experiments. 
The "Relative Area Decrease", value, in the paper is calculated from the difference of the relative area (MMA/MMA0) at time, t=200 mins, between irradiation experiments and non-irradiation experiments. The k2 prime values were calculated from plotting the ln(MMA/MMA0) with time and take the slope of the portion between time=30 mins and time=200 mins.  
Replicate experiments for these measurements are provided.

Method: Palmitic acid (PA) (>99 %) and humic acid (HA) were purchased from Sigma Aldrich and were used without further purification. Chloroform (>99.9%) was purchased from Fisher Scientific. The chloroform was used to prepare 1 mg/mL palmitic acid solution. Milli-Q water with electric resistance of 18.2 MΩ was used. Salts, CaCl2 and NaCl, were purchased from Fisher Scientific and were purified by baking at 500°C overnight and filtering twice with Whatman Carbon-Cap activated carbon filter (Whatman Carbon Cap 75, Fisher Scientific) to remove organic contaminants that are known to be surface active. The final concentrations of the salt solutions were obtained by Mohr titration. 
A computer controlled film balance with a Langmuir trough (KSV NIMA LB, S/N AAA100505) enclosed in a dry air purged plexiglass chamber was used for these experiments. Irradiation experiments were performed using a solar simulator (Newport Corporation, Oriel LCS-100). A water filter was placed after the lamp to cut off infrared radiation. The spectral output of the lamp is comparable to that of the solar spectrum and provides irradiation in an area   approximately 4 cm by 7 cm. The output power and spectrum of the simulator are shown in Figure S1 (https://www.newport.com/f/small-area-solar-simulators). The temperature of the solution in the trough was controlled by a water circulator (Beckman Geneline Cooler). The pH of the subphases ranged from 5 to 6.
Palmitic acid in chloroform was injected onto the subphase in the trough using a microsyringe (10 μL). After the injection of palmitic acid solution, the monolayer was left for 20 minutes to allow for the evaporation of the solvent. Monolayer stability was investigated in different ways including area and pressure relaxation measurements. For area relaxation measurements, the palmitic acid monolayer is held at a constant surface pressure and the change in molecular area is recorded over time. In particular, for these experiments, the monolayer was held at 25 mN/m for ~3 hours. This surface pressure was chosen since the monolayer is in a condensed phase with fatty acids concentrated at the interface, and it allows full utilization of the irradiation area. The irradiation was started 10 minutes after reaching the surface pressure of 25 mN/m. For pressure relaxation experiments, the monolayer is held at a particular molecular area/ barrier position corresponding to a certain surface pressure and the change in surface pressure with time is measured. For these experiments, an isotherm was collected up to 25 mN/m and the monolayer was then expanded after obtaining the isotherm. After 10 minutes, the second isotherm was held at a mean molecular area corresponding to the surface pressure of 25 mN/m for ~3 hours. The irradiation was started 10 minutes after holding the monolayer.
Each IRRAS spectrum is a collection of 300 scans taken at a spectral resolution of 8 cm-1 and the IR spectral range used is from 400 to 4000 cm-1. IRRAS spectra were collected at two time points: as soon as irradiation started (10 minutes after holding for non-irradiation experiments), and after 3 hours of irradiation (3 hours 10 minutes after holding for non-irradiation experiments).
Same sample preparation protocol was used for SFG measurements. Salt solution was loaded in the petri dish, the palmitic acid solution was injected onto the subphase using a microsyringe. Before beginning the SFG measurements, the sample was equilibrated for 20 mins for the solvent to evaporate out. The power of 800 nm and mid-IR beams were 0.5 µJ and 1 µJ, respectively, and the exposure time was 5 mins.