Abstract:Using a confocal fluorescence microscope, we found a significant temporal fluctuation in the fluorescence spectrum of porphine molecules on the water surface. The spectra fluctuated when the accumulation time was 300 ms and the pH was within a certain range. No fluctuation was observed with a longer accumulation time (36 s) or on a solid surface. The present observation indicates that the distribution of porphine on the water surface is heterogeneous and that the Brownian motion of the surface species induces … Show more
“…Similar intensity fluctuation was observed for absorption spectra (20−40 spectra were recorded and accumulated). These intensity fluctuations should be caused by RhC 18 domains floating on the water surface, because the time-averaged absorption spectrum and the time-averaged fluorescence spectrum were linearly proportional to the surface density and revealed no peak shift; the domains probably were formed owing to the van der Waals interaction of long alkyl chains. ,,− The fluctuations disappeared at 210 Å 2 because the film became a continuous monolayer. A similar domain formation has been found for the long-chain fatty-acid films at the air−water interface below 0.1 mN/m. , …”
The absorption and fluorescence spectra and second harmonic generation (SHG) of the insoluble monolayer
of bis-(N-ethyl,N-octadecyl)rhodamine (RhC18) at the air−water interface have been measured. These
spectra were affected significantly by compression, and the observed changes were ascribed to the formation
and structural rearrangement of aggregated species on the water surface during compression. The
spectroscopic behavior of the monolayer was explained in accordance with its rheological properties, and
the transition from disordered monomers to dimers, from dimers to aggregates, and from aggregates to
two-dimensional arrays was proposed. SHG studies revealed that the RhC18 molecules in the expanded
film region are oriented with their C
2-axis tilted away from the surface normal on angle θ distributed in
the range of 31−39°. The rotational distribution around the C
2-axis was assumed to be 45−60° according
to preferable intermolecular interactions with the water subphase and surrounding molecules. The θ angle
distribution became slightly narrow because of the increase of molecular ordering caused by two-dimensional
external pressure. The sharp increase of SHG intensity and the phase shift observed at high compression
were ascribed to the formation of blue-shifted aggregates with their electronic transition being in resonance
with the incident laser frequency. The results of spectroscopic and SHG studies were jointly analyzed, and
the structural rearrangement within the monolayer during compression was described.
“…Similar intensity fluctuation was observed for absorption spectra (20−40 spectra were recorded and accumulated). These intensity fluctuations should be caused by RhC 18 domains floating on the water surface, because the time-averaged absorption spectrum and the time-averaged fluorescence spectrum were linearly proportional to the surface density and revealed no peak shift; the domains probably were formed owing to the van der Waals interaction of long alkyl chains. ,,− The fluctuations disappeared at 210 Å 2 because the film became a continuous monolayer. A similar domain formation has been found for the long-chain fatty-acid films at the air−water interface below 0.1 mN/m. , …”
The absorption and fluorescence spectra and second harmonic generation (SHG) of the insoluble monolayer
of bis-(N-ethyl,N-octadecyl)rhodamine (RhC18) at the air−water interface have been measured. These
spectra were affected significantly by compression, and the observed changes were ascribed to the formation
and structural rearrangement of aggregated species on the water surface during compression. The
spectroscopic behavior of the monolayer was explained in accordance with its rheological properties, and
the transition from disordered monomers to dimers, from dimers to aggregates, and from aggregates to
two-dimensional arrays was proposed. SHG studies revealed that the RhC18 molecules in the expanded
film region are oriented with their C
2-axis tilted away from the surface normal on angle θ distributed in
the range of 31−39°. The rotational distribution around the C
2-axis was assumed to be 45−60° according
to preferable intermolecular interactions with the water subphase and surrounding molecules. The θ angle
distribution became slightly narrow because of the increase of molecular ordering caused by two-dimensional
external pressure. The sharp increase of SHG intensity and the phase shift observed at high compression
were ascribed to the formation of blue-shifted aggregates with their electronic transition being in resonance
with the incident laser frequency. The results of spectroscopic and SHG studies were jointly analyzed, and
the structural rearrangement within the monolayer during compression was described.
“…The SH signal for each measurement was accumulated over 2 min, and several sets of films with specified compositions were repetitively characterized by this procedure. Special attention was given to the accuracy of the measurements in the low surface-pressure region because of the film floating on the water surface. ,, A stable SH signal during the accumulation was considered as a criterion for successful measurement.…”
Mixed insoluble monolayers of bis(N-ethyl,N-octadecyl)rhodamine perchlorate (RhC18) and arachidic
acid (ArAc) at the air−water interface were characterized by surface pressure−area isotherm, second
harmonic generation (SHG), and reflection spectroscopy studies. The analysis of surface pressure−area
isotherms of mixed RhC18/ArAc layers provided evidence that there was a high degree of miscibility of film
components and a strong interaction among them. A new band was found to appear on the blue side of
the reflection spectrum after an increase in both the concentration of ArAc in the layer and the applied
surface pressure. This new band was assigned to dye aggregates, where spontaneous formation in the
gaslike region was promoted by the presence of ArAc and induced by compression in the region of the
continuous monolayer. Both in gaslike and in compressed film, the formation of blue-shifted aggregates
coincided with the increase of the SHG response of the monolayer. Resonance SHG enhancement was
proven to be the main reason for this observation due to electronic resonance of aggregates with the
incident light frequency.
“…The signal was accumulated for 32−128 pulses of the laser, and an average of three to 10 sets of such data was taken as an observed result. Averaging and soft focus are essential to obtaining quantitative data because the molecule would produce domains on the water surface and the surface concentration may fluctuate at a high time resolution and over a small region of the surface. − 1 Experimental apparatus.…”
A new method has been proposed to determine the equilibrium between the bulk and the surface by directly
measuring surface concentrations using laser two-photon ionization. This method has been applied to
pyrenebutyric acid. The surface concentrations depended on the pH of the solution and were analyzed on the
basis of two equilibrium constants and two distribution coefficients. Most pyrenebutyric acid stays on the
surface at pH = 2.2. The equilibrium constant, pK
a, of pyrenebutyric acid on the water surface was determined
to be 7.85 ± 0.13, and this value is shifted to higher value than that in the bulk (4.76). The distribution
coefficient of the neutral pyrenebutyric acid was determined as (5.9 ± 2.8) × 10-2 m, and that of pyrenebutyric
anion as (4.82 ± 0.1) × 10-5 m. The ratio of the distribution coefficient of the neutral pyrenebutyric acid to
that of pyrenebutyric anion was determined to be (1.2 ± 0.6) × 103. These findings indicate that the equilibrium
shifts toward the neutral form on the water surface. Laser two-photon ionization was found to be a sensitive
and powerful technique to analyze equilibrium on the surface and that between the surface and the bulk.
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