9 research outputs found
pH effects on the molecular structure and charging state of b-Escin biosurfactants at the air-water interface
Saponins like b-escin exhibit an unusually high surface activity paired with a remarkable surface rheology which makes them as biosurfactants highly interesting for applications in soft matter colloids and at
interfaces. We have applied vibrational sum-frequency generation (SFG) to study b-escin adsorption layers at the air-water interface as a function of electrolyte pH and compare the results from SFG spectroscopy to complementary experiments that have addressed the surface tension and the surface
dilational rheology. SFG spectra of b-escin modified air-water interfaces demonstrate that the SFG intensity of OAH stretching vibrations from interfacial water molecules is a function of pH and dramatically
increases when the pH is increased from acidic to basic conditions and reaches a plateau at a solution
pH of > 6. These changes are attributable to the interfacial charging state and to the deprotonation of
the carboxylic acid group of b-escin. Thus, the change in OAH intensity provides qualitative information
on the degree of protonation of this group at the air-water interface. At pH < 4 the air-water interface is
dominated by the charge neutral form of b-escin, while at pH > 6 its carboxylic acid group is fully deprotonated and, consequently, the interface is highly charged. These observations are corroborated by the
change in equilibrium surface tension which is qualitatively similar to the change in OAH intensity as
seen in the SFG spectra. Further, once the surface layer is charge neutral, the surface elasticity drastically
increases. This can be attributed to a change in prevailing intermolecular interactions that change from dominating repulsive electrostatic interactions at high pH, to dominating attractive interactions, such as
hydrophobic and dispersive interactions, as well as, hydrogen bonding at low pH values. In addition to
the clear changes in OAH intensity from interfacial H2O, the SFG spectra exhibit drastic changes in the
CAH bands from interfacial b-escin which we relate to differences in the net molecular orientation.
This orientation change is driven by tighter packing of b-escin adsorption layers when the b-escin moiety
is in its charge neutral form (pH < 4).The authors gratefully acknowledge funding from the European
Research Council (ERC) under the European Union’s Horizon 2020
research and innovation program (Grant Agreement 638278).Ye
Photo-Switchable Surfactants for Responsive Air–Water Interfaces: Azo versus 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 of the surfactants changed drastically between E/Z isomers for
AAP-TB (maximum change of : 2.15 mol/m); for Azo-TB the change
was only moderate (maximum change of : 1.02 mol/m). Analysis
of SFG spectra revealed that strong non-resonant contributions that heterodyned
the resonant vibrational bands were proportional to , enabling the
aromatic C-H band to be interpreted as an indicator for changes in interfacial
molecular order. Close comparison of 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 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.Comment: Pre-edited final version, 36 manuscript pages, 2 tables, 6 figures.
Supporting Information available up request to the corresponding autho
Photoresponsive Arylazopyrazole Surfactant/PDADMAC Mixtures: Reversible Control of Bulk and Interfacial Properties
In many applications of polyelectrolyte/surfactant (P/S) mixtures, it is difficult to fine-tune them after mixing the components without changing the sample composition, e.g. pH or the ionic strength. Here we report on a new approach where we use photoswitchable surfactants to enable drastic changes in both the bulk and interfacial properties. Poly(diallyldimethylammonium chloride) (PDADMAC) mixtures with three alkyl-arylazopyrazole butyl sulfonates (CnAAP) with -H, -butyl and -octyl tails are applied and E/Z photoisomerization of the surfactants is used to cause substantially different hydrophobic interactions between the surfactants and PDADMAC. This affects significantly the P/S binding and allows for tuning both the bulk and interfacial properties of PDADMAC/CnAAP mixtures through light irradiation. For that, we have fixed the surfactant concentrations at values where they exhibit pronounced surface tension changes upon E/Z photoisomerization with 365 nm UV light (Z) and 520 nm green (E) light and have varied the PDADMAC concentration. The electrophoretic mobility can be largely tuned by photoisomerisation of CnAAP surfactants and P/S aggregates, which can even exhibit a charge reversal from negative to positive values or vice versa. In addition, low colloidal stability at equimolar concentrations of PDADMAC with CnAAP surfactants in the E configuration lead to the formation of large aggregates in the bulk which can be redissolved by irradiation with UV light when the surfactant‘s alkyl chain is short enough (C0AAP). Vibrational sum-frequency generation (SFG) reveals changes at the interface similar to the bulk, where the charging state at air-water interfaces can be modified with light irradiation. Using SFG spectroscopy, we interrogated the O-H stretching modes of interfacial H2O and provide qualitative information on surface charging that is complemented by neutron reflectometry, from which we resolved the surface excess of PDADMAC and CnAAP at the air-water interface, independently
Photoresponsive arylazopyrazole surfactant/PDADMAC mixtures: reversible control of bulk and interfacial properties
In many applications of polyelectrolyte/surfactant (P/S) mixtures, it is difficult to fine-tune them after mixing the components without changing the sample composition, e.g. pH or the ionic strength. Here we report on a new approach where we use photoswitchable surfactants to enable drastic changes in both the bulk and interfacial properties. Poly(diallyldimethylammonium chloride) (PDADMAC) mixtures with three alkyl-arylazopyrazole butyl sulfonates (CnAAP) with -H, -butyl and -octyl tails are applied and E/Z photoisomerization of the surfactants is used to cause substantially different hydrophobic interactions between the surfactants and PDADMAC. These remotely controlled changes affect significantly the P/S binding and allows for tuning both the bulk and interfacial properties of PDADMAC/CnAAP mixtures through light irradiation. For that, we have fixed the surfactant concentrations at values where they exhibit pronounced surface tension changes upon E/Z photoisomerization with 365 nm UV light (Z) and 520 nm green (E) light and have varied the PDADMAC concentration. The electrophoretic mobility can be largely tuned by photoisomerisation of CnAAP surfactants and P/S aggregates, which can even exhibit a charge reversal from negative to positive values or vice versa. In addition, low colloidal stability at equimolar concentrations of PDADMAC with CnAAP surfactants in the E configuration lead to the formation of large aggregates in the bulk which can be broken up by irradiation with UV light when the surfactant's alkyl chain is short enough (C0AAP). Vibrational sum-frequency generation (SFG) spectroscopy reveals changes at the interface similar to the bulk, where the charging state at air–water interfaces can be modified with light irradiation. Using SFG spectroscopy, we interrogated the O–H stretching modes of interfacial H2O and provide qualitative information on surface charging that is complemented by neutron reflectometry, from which we resolved the surface excesses of PDADMAC and CnAAP at the air–water interface, independently.<br/
Neogene sharks and rays from the Brazilian ‘Blue Amazon’
The lower Miocene Pirabas Formation in the North of Brazil was deposited under influence of the proto-Amazon River and is characterized by large changes in the ecological niches from the early Miocene onwards. To evaluate these ecological changes, the elasmobranch fauna of the fully marine, carbonate-rich beds was investigated. A diverse fauna with 24 taxa of sharks and rays was identified with the dominant groups being carcharhiniforms and myliobatiforms. This faunal composition is similar to other early Miocene assemblages from the proto-Carribbean bioprovince. However, the Pirabas Formation has unique features compared to the other localities; being the only Neogene fossil fish assemblage described from the Atlantic coast of Tropical Americas. Phosphate oxygen isotope composition of elasmobranch teeth served as proxies for paleotemperatures and paleoecology. The data are compatible with a predominantly tropical marine setting with recognized inshore and offshore habitats with some probable depth preferences (e.g., Aetomylaeus groups). Paleohabitat of taxa particularly found in the Neogene of the Americas (†Carcharhinus ackermannii, †Aetomylaeus cubensis) are estimated to have been principally coastal and shallow waters. Larger variation among the few analyzed modern selachians reflects a larger range for the isotopic composition of recent seawater compared to the early Miocene. This probably links to an increased influence of the Amazon River in the coastal regions during the Holocene