Surface Density of Pyrenesulfonic Acid at the Air-Water Interface as Determined by Laser Two-Photon Ionization: Effect of pH and Salt

Abstract

The properties of the interfacial region between aqueous solution and the air are dissimilar to those of the bulk water. The importance of the interfacial region to many fields of chemistry, biology, and physics has long been recognized, and has demanded an intimate knowledge of the nature of such interfacial microenvironments.Traditionally, the interfacial layer has been investigated using surface tension and calorimetric measurements. It is, however, difficult to determine the surface concentration from these methods, because the surface can not be defined on a molecular level. New spectroscopic methods have recently appeared for studies on the air/water interface.1 Laser two-photon ionization is one of them, and is a very sensitive and effective probe for air-water interface.2-5 When the laser irradiates the water surface, a photoabsorbing molecule on the thin surface layer absorbs a photon and is excited into a real excited state. Then it absorbs a second photon and is photoionized. The water is transparent and is not ionized in a two photon process. The positive electrode, which is located above the water surface, accepts ejected electrons from ionized molecules on the surface. The escape depth of each electron from water can be estimated to be about 1 nm from the escape depth of electrons from D 2 O ice 6 and the Onsager length 7 for water. Thus, a surface layer of water about 1 nm thick can be probed by the laser photoionization method.The solubility of a molecule in water depends on pH. Thus, the surface concentration may depend on the pH of the solution.8 In this paper, the effect of pH and coexisting salt (NaCl) on the surface concentration of pyrenesulfonic acid on water surface was studied using a two-photon ionization technique.

Welding of Gold Nanoparticles on Graphitic Templates for Chemical Sensing

“…However, the dissociation of pyrene acid in the aqueous system was a factor that should be considered in this case. Unlike the strong acid PSA (with p K a ≈ 0.7), PCA is a weak acid with p K a = 4.0 that mostly remained in a neutral state . The nondissociated PCA molecule indeed demonstrated weaker binding to Au compared to the PSA anion (Figure S7i,j).…”

Do hydrophobic interactions exist in the gas phase?

“…. .À OOC-C 17 Because of higher masses, decomposition of these dimers occurs with more difficulty compared to that of the dimers presented in Fig. 3.…”

“…This is important for our study since the difference in decomposition of the dimers studied may be affected by the pK a of the monomers. Butyric acid (member of the fatty acid subgroup called shortchain fatty acids) has a pK a of 4.83, 17 octanoic acid has a pK a of 4.89. However, as the chain length increases the solubility of the fatty acids in water decreases very rapidly, so that the longer-chain fatty acids have very little effect on the pH of a solution.…”

Characterization of Langmuir Monolayers of the Amphiphilic Ru Complex at the Air/Water Interface by Ultraviolet Photoelectron Yield Spectroscopy

“…The only analytical technique that can be applied to the characterization of chemical species on the air/water interface employing a photoelectron as a probe is the laser two-photon ionization method. This technique has been used to determine the concentration of chemical species on the water surface in ambient air. − The laser two-photon ionization method is very sensitive to the surface and actually has been applied to the characterization of the Langmuir layer of pyrene hexadecanoic acid on water which gives information on the structure of the surface species, because the pulsed laser is an essential light source for two-photon excitation.…”

“…This technique has been used to determine the concentration of chemical species on the water surface in ambient air. [18][19][20] The laser twophoton ionization method is very sensitive to the surface and actually has been applied to the characterization of the Langmuir layer of pyrene hexadecanoic acid on water. 21 With this technique, however, it is difficult to obtain a continuous photoexcitation spectrum, 22 which gives information on the structure of the surface species, because the pulsed laser is an essential light source for two-photon excitation.…”