Effect of pH and Salt on Surface pK a of Phosphatidic Acid Monolayers

• Published in: Langmuir 2018-01, Vol.34 (1), p.530-539
• Main Authors: Zhang, Ting, Brantley, Shelby L, Verreault, Dominique, Dhankani, Raja, Corcelli, Steven A, Allen, Heather C
• Format: Article
• Language: English
• ISSN: 0743-7463; 1520-5827
• DOI: 10.1021/acs.langmuir.7b03579

The pH-induced surface speciation of organic surfactants such as fatty acids and phospholipids in monolayers and coatings is considered to be an important factor controlling their interfacial organization and properties. Yet, correctly predicting the surface speciation requires the determination of the surface dissociation constants (surface pK a) of the protic functional group(s) present. Here, we use three independent methodscompression isotherms, surface tension pH titration, and infrared reflection–absorption spectroscopy (IRRAS)to study the protonation state of dipalmitoylphosphatidic acid (DPPA) monolayers on water and NaCl solutions. By examining the molecular area expansion at basic pH, the pK a to remove the second proton of DPPA (surface pK a2) at the aqueous interface is estimated. In addition, utilizing IRRAS combined with density functional theory calculations, the vibrational modes of the phosphate headgroup were directly probed and assigned to understand DPPA charge speciation with increasing pH. We find that all three experimental techniques give consistent surface pK a2 values in good agreement with each other. Results show that a condensed DPPA monolayer has a surface pK a2 of 11.5, a value higher than previously reported (∼7.9–8.5). This surface pK a2 was further altered by the presence of Na+ cations in the aqueous subphase, which reduced the surface pK a2 from 11.5 to 10.5. It was also found that the surface pK a2 value of DPPA is modulated by the packing density (i.e., the surface charge density) of the monolayer, with a surface pK a2 as low as 9.2 for DPPA monolayers in the two-dimensional gaseous phase over NaCl solutions. The experimentally determined surface pK a2 values are also found to be in agreement with those predicted by Gouy–Chapman theory, validating these methods and proving that surface charge density is the driving factor behind changes to the surface pK a2.