Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Photo-Switchable Surfactants for Responsive Air-Water Interfaces: Azo
  vs. Arylazopyrazole Amphiphiles   Arylazopyrazoles (AAPs) as substitutes for azo derivatives have gained
considerable attention due to their superior properties offering E/Z
photo-isomerization with high yield. In order to compare and quantify their
performance, azobenzene tetraethylammonium (Azo-TB) and arylazopyrazole
tetraethylammonium (AAP-TB) bromides were synthesized and characterized in the
bulk (water) using NMR spectroscopy. At the air-water interface complementary
information from vibrational sum-frequency generation (SFG) spectroscopy and
neutron reflectometry (NR) has revealed the effects of E/Z isomerization in
great detail. In bulk water the photostationary states of >89% for E/Z
switching in both directions were very similar for the surfactants, while their
interfacial behavior was substantially different. In particular, the surface
excess $\Gamma$ of the surfactants changed drastically between E/Z isomers for
AAP-TB (maximum change of $\Gamma$: 2.15 $\mu$mol/m$^2$); for Azo-TB the change
was only moderate (maximum change of $\Gamma$: 1.02 $\mu$mol/m$^2$). Analysis
of SFG spectra revealed that strong non-resonant contributions that heterodyned
the resonant vibrational bands were proportional to $\Gamma$, enabling the
aromatic C-H band to be interpreted as an indicator for changes in interfacial
molecular order. Close comparison of $\Gamma$ from NR with the SFG amplitude
from the aromatic C-H stretch as a function of concentrations and E/Z
conformation revealed substantial molecular order changes for AAP-TB. In
contrast, only $\Gamma$ and not the molecular order varied for Azo-TB. These
differences in interfacial properties are attributed to the molecular structure
of the AAP center that enables favorable lateral interactions at the air-water
interface, causing closed-packed interfacial layers and substantial changes
during E/Z photo-isomerization.

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The categories of this paper are: hep-ex
31
The categories of this paper are: ao-sci
34
The categories of this paper are: chem-ph
32
The categories of this paper are: atom-ph
37
The categories of this paper are: supr-con
9
The categories of this paper are: nlin
6
The categories of this paper are: astro-ph
2
The categories of this paper are: math
24
The categories of this paper are: dg-ga
28
The categories of this paper are: alg-geom
26
The categories of this paper are: bayes-an
22
The categories of this paper are: chao-dyn
33
The categories of this paper are: plasm-ph
20
The categories of this paper are: acc-phys
25
The categories of this paper are: patt-sol
0
The categories of this paper are: hep-ph