114 research outputs found
Surface Aggregate Structure of Nonionic Surfactants on Silica Nanoparticles
The self-assembly of two nonionic surfactants, pentaethylene glycol
monododecyl ether (C12E5) and n-dodecyl-{\ss}-maltoside ({\ss}-C12G2), in the
presence of a purpose-synthesized silica sol of uniform particle size (diameter
16 nm) has been studied by adsorption measurements, dynamic light scattering
and small-angle neutron scattering (SANS) using a H2O/D2O mixture matching the
silica, in order to highlight the structure of the surfactant aggregates. For
C12E5 strong aggregative adsorption onto the silica beads, with a high plateau
value of the adsorption isotherm above the CMC was found. SANS measurements
were made at a series of loadings, from zero surfactant up to maximum surface
coverage. It is found that the spherical core-shell model nicely reproduces the
SANS data up to and including the local maximum at q = 0.42 nm-1 but not in the
Porod region of high q, indicating that the surface area of the adsorbed
surfactant is underestimated by the model of a uniform adsorbed layer. A
satisfactory representation of the entire scattering profiles is obtained with
the model of micelle-decorated silica beads, indicating that C12E5 is adsorbed
as spherical micellar aggregates. This behaviour is attributed to the high
surface curvature of the silica which prevents an effective packing of the
hydrophobic chains of the amphiphile in a bilayer configuration. For the
maltoside surfactant {\ss}-C12G2 very weak adsorption on the silica beads was
found. The SANS profile indicates that this surfactant forms oblate ellipsoidal
micelles in the silica dispersion, as in the absence of the silica beads
Surfactant adsorption and aggregate structure at silica nanoparticles: Effects of particle size and surface modification
Polyelectrolyte Adsorption on Solid Surfaces: Theoretical Predictions and Experimental Measurements
This work utilizes a combination of theory and experiments to explore the adsorption of two different cationic polyelectrolytes onto oppositely charged silica surfaces at pH 9. Both polymers, poly(diallyldimethylammonium chloride), PDADMAC, and poly(4-vinyl N-methylpyridinium iodide), PVNP, are highly charged and highly soluble in water. Another important aspect is that a silica surface carries a relatively high surface charge density at this pH level. This means that we have specifically chosen to investigate adsorption under conditions where electrostatics can be expected to dominate the interactions. Of specific focus in this work is the response of the adsorption to the addition of simple salt (i.e., a process where electrostatics is gradually screened out). Theoretical predictions from a recently developed correlation-corrected classical density functional theory for polyelectrolytes are evaluated by direct quantitative comparisons with corresponding experimental data, as obtained by ellipsometry measurements. We find that, at low concentrations of simple salt, the adsorption increases with ionic strength, reaching a maximum at intermediate levels (about 200 mM). The adsorption then drops but retains a finite level even at very high salt concentrations, indicating the presence of nonelectrostatic contributions to the adsorption. In the theoretical treatment, the strength of this relatively modest but otherwise largely unknown nonelectrostatic surface affinity was estimated by matching predicted and experimental slopes of adsorption curves at high ionic strength. Given these estimates for the nonelectrostatic part, experimental adsorption data are essentially captured with quantitative accuracy by the classical density functional theory
On the formation of inclusion complexes at the solid/liquid interface of anchored temperature-responsive PNIPAAM diblock copolymers with γ-cyclodextrin
A review of wetting versus adsorption, complexions, and related phenomena: the rosetta stone of wetting
The Devil and the Deep Blue Sea, Liability for mismanagement and fraud and a call for a serious debate on truly fundamental issues
Phospholipase A(2) hydrolysis of supported phospholipid bilayers: A neutron reflectivity and ellipsornetry study
We have investigated the phospholipase A(2) catalyzed hydrolysis of supported phospholipid bilayers using neutron reflection and ellipsometry. At the hydrophilic silica-water interface, hydrolysis of phosphatidy1choline bilayers by phospholipase A(2) from Naja mossambica mossambica venom is accompanied by destruction of the bilayer at an initial rate, which is comparable for DOPC and DPPC but is doubled for POPC. The extent of bilayer destruction at 25 degreesC decreases from DOPC to POPC and is dramatically reduced for DPPC. Neutron reflectivity measurements indicate that the enzyme penetrates into the bilayers in increasing order for DOPC, POPC, and DPPC, while the amount of enzyme adsorbed at the interface is smallest for DPPC and exhibits a maximum for POPC. Penetration into the hydrophobic chain region in the bilayer is further supported by the fact that the enzyme adsorbs strongly and irreversibly to a hydrophobic monolayer of octadecyltrichlorosilane. These results are rationalized in terms of the properties of the reaction products and the effect of their accumulation in the membrane on the kinetics of enzyme catalysis
Composition of Supported Model Membranes Determined by Neutron Reflection
We have investigated the formation of supported bilayers by coadsorption of dipalmitoyl phosphatidylcholine (DPPC) with the nonionic surfactant P-D-dodecyl maltoside. The adsorption of mixed phospholipid-surfactant micelles on hydrophilic silica surfaces at 25 degrees C was followed as a function of bulk concentration by neutron reflection. Using chain-deuterated d(25)-beta-D-dodecyl maltoside and d(62)-DPPC, we demonstrate that it is possible to determine the composition of the bilayers at each stage of a sequential dilution process, which enriches the adsorbed layer in phospholipid and leads to complete elimination of the surfactant. The final supported bilayers have thicknesses of 51 +/- 3 angstrom and are stable to heating to 37 degrees C once all surfactant has been removed, and the structures agree well with other published data on DPPC supported bilayers. The coadsorption of cholesterol in a DPPC-surfactant mixture was also achieved, and the location and volume fraction of cholesterol in the DPPC bilayer was determined. Cholesterol is located in a 18 +/- 1 angstrom thick layer below the lipid headgroup region and leads to an increased bilayer thickness of 58 +/- 2 angstrom at 26 mol % of cholesterol
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