The association between a highly branched polyelectrolyte with ionizable groups, polyethylene imine (PEI), and an anionic surfactant, sodium dodecyl sulfate (SDS), has been investigated at two pH values, using smallangle neutron and light scattering. The scattering data allow us to obtain a detailed picture of the association structures formed. Small-angle neutron scattering (SANS) measurements in solutions containing highly charged PEI at low pH and low SDS concentrations indicate the presence of disklike aggregates. The aggregates change to a more complex three-dimensional structure with increasing surfactant concentration. One pronounced feature in the scattering curves is the presence of a Bragg-like peak at high q-values observed at a surfactant concentration of 4.2 mM and above. This scattering feature is attributed to the formation of a common wellordered PEI/SDS structure, in analogue to what has been reported for other polyelectrolyte-surfactant systems. Precipitation occurred at the charge neutralization point, and X-ray diffraction measurements on the precipitate confirmed the existence of an ordered structure within the PEI/SDS aggregates, which was identified as a lamellar internal organization. Polyethylene imine has a low charge density in alkaline solutions. At pH 10.1 and under conditions where the surfactant was contrast matched, the SANS scattering curves showed only small changes with increasing surfactant concentration. This suggests that the polymer acts as a template onto which the surfactant molecules aggregate. Data from both static light scattering and SANS recorded under conditions where SDS and to a lower degree PEI contribute to the scattering were found to be consistent with a structure of stacked elliptic bilayers. These structures increased in size and became more compact as the surfactant concentration was increased up to the charge neutralization point.

Solvent isotope effects on the interaction between the hyperbranched cationic polyelectrolyte, polyethylene imine (PEI), and the anionic surfactant sodium dodecyl sulfate (SDS) were investigated using potentiometric titration and eletrophoretic mobility measurements. In the basic pH range a significantly higher fraction of the amine groups was found to be protonated when the PEI was dissolved in D2O compared to H2O at the same pH/pD. The difference in polymer charge in the two solvents gradually decreases with decreasing pH and it completely diminishes at around pH=4. Electrophoretic mobility measurements of PEI/SDS complexes at different pH values correlated very well with these observations. At pH/pD≈9 a much higher mobility of the PEI/SDS complexes was found at low surfactant concentrations in D2O than in H2O, and the charge neutralization point shifted to a considerably larger surfactant concentration in heavy water. These results can be explained by the significantly higher charge density of the PEI in D2O compared to H2O. On the other hand, at the natural pH/pD as well as at pH=4 and pD=4 conditions the PEI molecules have roughly equal charge density which results in very similar charged characteristics (mobilities) of the PEI/SDS complexes as well as the same charge neutralization SDS concentration. It can be concluded, that extreme care must be taken in the general analysis of those experiments where weak polyelectrolyte/surfactant aggregates are investigated in heavy water and then these observations are correlated with structures of the same system in water.

Certain ionic liquids are powerful cellulose solvents, but tend to be less effective when small-molecule hydrogen bond donors are present. This is generally attributed to competition with cellulose for hydrogen bonding opportunities to the anion of the ionic liquid. We show that the solubility of cellulose in dimethyl sulfoxide solutions of tetrabutylammonium acetate is less strongly affected by water than by ethanol on a molar basis, contrary to what can be expected based on hydrogen bond stoichiometry. Molecular dynamics simulations indicate that the higher tolerance to water is due to water-cellulose interactions that improves solvation of cellulose and, thereby, marginally favors dissolution. Through Kirkwood-Buff theory we show that water, but not ethanol, improves the solvent quality of DMSO and partly compensates for the loss of acetate-cellulose hydrogen bonds.

Solid-state nuclear magnetic resonance (CP/MAS 13C NMR) spectroscopy has often been used to study cellulose structure, but some features of the cellulose NMR spectrum are not yet fully understood. One such feature is a doublet around 84 ppm, a signal that has been proposed to originate from C4 atoms at cellulose fibril surfaces. The two peaks yield different T1, differing by approximately a factor of 2 at 75 MHz. In this study, we calculate T1 from C4-H4 vector dynamics obtained from molecular dynamics computer simulations of cellulose I β-water interfacial systems. Calculated and experimentally obtained T1 values for C4 atoms in surface chains fell within the same order of magnitude, 3-20 s. This means that the applied force field reproduces relevant surface dynamics for the cellulose-water interface sufficiently well. Furthermore, a difference in T1 of about a factor of 2 in the range of Larmor frequencies 25-150 MHz was found for C4 atoms in chains located on top of two different crystallographic planes, namely, (110) and (110). A previously proposed explanation that the C4 peak doublet could derive from surfaces parallel to different crystallographic planes is herewith strengthened by computationally obtained evidence. Another suggested basis for this difference is that the doublet originates from C4 atoms located in surface anhydro-glucose units with hydroxymethyl groups pointing either inward or outward. This was also tested within this study but was found to yield no difference in calculated T1.

The size and shape of micelles formed in aqueous mixtures of the anionic surfactant sodium dodecyl sulfate (SDS) and the nonionic sugar-based surfactant n-decyl â-D-glucopyranoside (C10G) at different concentrations of added salt have been investigated with small-angle neutron and static light scattering. Rather small prolate ellipsoidal micelles form in the absence of added salt and at [NaCl] ) 10 mM in D2O. The micelles grow considerably in length to large rods as the electrolyte concentration is raised to [NaCl] ) 0.1 M. In excess of nonionic surfactant ([SDS]/[C10G] ) 1:3) at [NaCl] ) 0.1 M in D2O, several thousands of Ångstroms long wormlike micelles are observed. Most interestingly, a conspicuously large isotope solvent effect was observed from static light scattering data according to which micelles formed at [SDS]/[C10G] ) 1:3 and [NaCl] ) 0.1 M in H2O are at least five times smaller than micelles formed in the corresponding samples in D2O.

We have studied mixtures of an anionic surfactant (deuterated sodium dodecyl sulfate, SDS-d) and cationic polyelectrolytes with different charge densities (10%, 30%, 60%, and 100%) using small-angle neutron scattering (SANS). Near compositions corresponding to charge neutralization, the solutions phase separate into a complex phase (precipitate) consisting of, in the cases of 30%, 60%, and 100% charge density, a two-dimensional (2D) hexagonal lattice of close-packed cylindrical micelles and a clear liquid. When either polyelectrolyte with charge density less than 100% or SDS-d is present in sufficient excess, the solution becomes clear and isotropic, and from the scattering data we may conclude that prolate or rod-shaped micelles are present. The micelles are seen to grow in length with increasing SDS-d concentration and polyelectrolyte charge density from about 80 Å to 550 Å, whereas the cross-sectional radius is 15 Å and approximately constant. The number of micelles per polyelectrolyte chain is found to be slightly larger than unity (1-6). In some of the (turbid) samples rod-shaped micelles are found to coexist with larger polyelectrolyte-surfactant complexes. Solutions consisting of 10% charged polyelectrolyte and SDS-d are very viscous and gellike, and the complex phase is much less defined with a much larger distance between adjacent aggregates in the complex phase.

The internal structure of the solid phase formed in mixtures of the anionic surfactant sodium dodecyl sulfate (SDS) and a range of oppositely charged polyelectrolytes with different side chains and charge density has been investigated using small-angle X-ray scattering. Polyelectrolytes with short side chains [3-(2-methylpropionamido)propyl]trimethylammonium chloride, MAPTAC, and poly{[(2-propionyloxy)ethyl]trimethylammonium chloride}, PCMA) form a 2-dimensional hexagonal structure with SDS, whereas a polyelectrolyte without side chains (poly(vinlyamine), PVAm) forms a lamellar structure. The hexagonal structure of MAPTAC is retained either when a neutral monomer (acrylamide, AM) is included in the polymer backbone to reduce the charge density or when a nonionic surfactant is admixed to the SDS/polyelctrolyte complex. The unit cell length of AM-MAPTAC increases with decreasing charge density from a = 47.7 Å (MAPTAC, 100% charge density) to 58.5 Å (AM-MAPTAC, 30% charge density). The unit cell length in the lamellar SDS/PVAm complex (a = 36.1 Å) is significantly smaller than for the different hexagonal structures. It is conjectured that the cylinders in the hexagonal structure and the bilayers in the lamellar structure are based on self-assembled surfactant aggregates with the polyelectrolyte mainly located in the aqueous region adjacent to the charged surfactant headgroups

The structure of aggregates formed in aqueous mixtures of a single-chain anionic surfactant, sodium dodecyl sulfate (SDS), and a double-chain cationic surfactant, didodecyldimethylammonium bromide (DDAB), has been investigated at 38° C using small-angle neutron scattering (SANS). Several overall surfactant concentrations [SDS] + [DDAB] between 0.1 and 5 wt % were measured at the two SDS-rich compositions [SDS]:[DDAB] = 90:10 and 95:5. Samples with a concentration above about [SDS] + [DDAB] = 1 wt % at [SDS]:[DDAB] = 95:5 contained only somewhat elongated tablet-shaped micelles (triaxial ellipsoids) with typical values of the half-axes a (related to the thickness) = 14 Å, b (related to the width) = 23 Å, and c (related to the length) = 27 Å. When a sample at [SDS]:[DDAB] = 95:5 is diluted below about [SDS] + [DDAB] = 1 wt %, an increasing amount of small unilamellar vesicles forms, and in the samples below about 0.2 wt %, only vesicles are observed. The average radius of the vesicles increases from about 90 Å at 0.3 wt % to 110 Å at 0.1 wt %. The transition from micelles to vesicles with decreasing surfactant concentration was also observed in the samples at [SDS]:[DDAB] = 90:10 in which, however, an additional amount of bilayer sheets was seen to be always present. Compared with the micelles at [SDS]:[DDAB] = 95:5, the micelles formed at [SDS]:[DDAB] = 90:10 were considerably longer (c ≈40 Å), but with similar cross section dimensions, and the vesicles formed were seen to be somewhat larger than the corresponding aggregates at 95:5. The relative standard deviation σR/ of the (number-weighted) vesicle size distributions was in the range 0.2 < σR/ < 0.3.

The complexation between one polyelectrolyte and one protein has been examined by employing a simple model system solved by Monte Carlo simulations. The polyelectrolyte was composed of a sequence of negatively charged hard spheres, and the protein was represented by a hard sphere with embedded pH-dependent discrete charges, the positions of which were taken from lysozyme. A short-range attractive interaction between the polyelectrolyte and the protein accounting for hydrophobic interactions completed the model. The complexation was found to depend decisively on the charge status of the protein model as well as on the presence of the short-range attractive interaction. In particular, the complexation weakens at decreasing ionic strength except for the highest positive protein net charge considered, and in the absence of the short-range attraction, a positively charged protein was required to obtain a complex. The distribution of the polyelectrolyte beads was inhomogeneous at the protein surface, and the polyelectrolyte contracted upon complexation. Finally, the protein model with discrete charges gave a stronger complex than the corresponding protein model with a homogeneous surface charge density.

The oligomerization of lysozyme in aqueous solution was investigated by Monte Carlo simulations as a function of protein concentration, pH, and electrolyte screening. Lysozyme was modeled as a hard sphere with embedded pH-dependent discrete charges and with an attractive 1/r6-potential representing nonspecific short-range attraction. The magnitude of the 1/r6-potential was adjusted to reproduce experimental second virial coefficients. Radial distribution functions, structure factors, cluster size distributions, and orientation correlations were determined at various conditions. It was observed that increasing protein concentration, or decreasing the electrostatic repulsion between protein molecules by either reducing the protein charge or increasing the ionic strength, promoted cluster formation. Structure factors and equilibrium constants obtained were compared to those obtained experimentally and were found to capture the experimentally obtained effects of pH and ionic strength. The influence of the location of the hydrophobic site was also examined

Small-angle neutron scattering (SANS) data for solutions containing a highly charged cationic polyelectrolyte and an anionic surfactant are presented. The scattering data were obtained in pure D2O, emphasizing the scattering from the polyelectrolyte, and in a H2O/D2O mixture that contrast matches the polyelectrolyte. In the absence of surfactant, a broad scattering peak due to the mesh size of the polyelectrolyte solution is the most characteristic feature. This peak moves to larger q-values (smaller distances) as the polyelectrolyte concentration is increased, as expected for a semidilute polyelectrolyte solution. Addition of a small amount of surfactant reduces and finally removes this peak. Instead a sharp diffraction peak appears at high q-values. This Bragg peak corresponds to a characteristic distance of 37-39 Å, and it is observed when either the polyelectrolyte or the surfactant is contrast matched by the solvent. Once this peak has appeared, its position does not change when the surfactant concentration is increased. The intensity of the peak grows, however, until a stoichiometric polyelectrolyte-surfactant complex has been formed. The Bragg peak remains in excess surfactant solution. These results are discussed in relation to the structure of the polyelectrolyte-surfactant aggregates and in connection with recent results from surface force and turbidity measurements using the same polyelectrolyte-surfactant pair.

The association between a 30% charged cationic polyelectrolyte and an anionic surfactant, sodium dodecyl sulfate (SDS), in 10 mM 1:1 electrolyte was investigated using surface force measurements and dynamic light scattering. The polyelectrolyte employed was a random copolymer of the neutral acrylamide and cationic [3-(2-methylpropionamide)propyl]trimethylammonium chloride (AM-MAPTAC-31). Light scattering measurements show that upon progressive addition of SDS to an AM-MAPTAC-31 solution the single coil size decreases until precipitation occurs at an SDS/MAPTAC ratio of just above 0.4. At SDS/MAPTAC ratios at or above 2, redispersion of the aggregates takes place. The interfacial behavior of AM-MAPTAC-31/SDS complexes was investigated in two ways. In one set of experiments a droplet containing a mixture of SDS and AM-MAPTAC-31 was placed between the surfaces and adsorption was allowed to occur from the aqueous mixture. It was found that the range of the steric force decreased when the SDS/MAPTAC ratio was increased from 0 to 0.4, indicating adsorption in a less extended conformation due to a decreased repulsion between the polyelectrolyte segments. At a ratio of 0.6 a compact interfacial complex was formed and the measured force was attractive over a small distance regime. A further increase in SDS/MAPTAC ratio resulted in precipitation of large aggregates at the surface, and reproducible force data could not be obtained. At an even higher SDS/AM-MAPTAC ratio of 4, individual aggregates were once again adsorbed at the surface. Hence, we find a good correspondence between association in bulk and at the solid surface. In another set of experiments the polyelectrolyte was first preadsorbed to mica surfaces and then SDS was added to the polyelectrolyte-free solution surrounding the surfaces. In this way precipitation of large SDS-polyelectrolyte aggregates onto the surfaces was avoided. Addition of SDS up to a concentration of 0.1 mM hardly affected the long-range interaction but gave an increased compressed layer thickness. A further increase in SDS concentrations to 1 mM results in a dramatic increase in the range of the force, suggesting formation of strongly negatively charged polyelectrolyte-surfactant complexes.

The effect of ionic strength on adsorption of chitosan on mica as well as the impact of addition of an anionic surfactant, SDS, on the adsorbed chitosan layers is explored. It is demonstrated by chemical surface analysis (ESCA) and surface force measurements (SFA) that an elevated salt concentration leads to larger adsorbed amounts and thicker adsorption layers of this cationic polyelectrolyte. It is also shown that in contrast to the bulk, the binding of oppositely charged surfactant to the polyelectrolyte adsorbed on a negatively charged surface is facilitated by elevated ionic strength. Thus, the association process in bulk and at solid-liquid interfaces is rather different. The main point of difference is that at the solid-liquid interface one also has to consider interactions between the polyelectrolyte and the surface as well as between the surfactant and the surface.

Hydrophobic surfaces prepared by adsorption of hexadecanethiol, 1,10-dithiodecane, octadecanoic acid, and hexadecanol onto gold have been used to study long-ranged "hydrophobic" interactions, and direct force measurements were performed in water, aqueous NaCl electrolytes, and water/ethanol mixtures, using a bimorph surface force instrument. Results confirm that for very stable hydrophobic surfaces the contact angle is sufficient to predict the presence of an attraction in excess of van der Waals forces, in which case the attraction is caused by the coalesence of microscopic bubbles on the surfaces. For the less stable hydrophobic films, the properties of the adsorbed layer is important for the qualitative nature of the interaction. For such surfaces different -- and as yet unknown -- mecanisms cause the attraction

Dynamic light scattering (DLS) and small-angle neutron scattering (SANS) were employed to study mixtures of xylitol and water. The results were also related to a previous dielectric relaxation study on the same system. In the temperature range of the DLS measurements the viscosity related structural (α) relaxation is too fast to be observed on the experimental time scale, but a considerably slower exponential and hydrodynamic relaxation process is clearly observable in the polarized light scattering data. A similar ultraslow process has been observed in many other types of binary liquids and commonly assigned to long-range concentration or density fluctuations. In some studies this interpretation has been supported by observations of substantial structural inhomogeneities in static light scattering or SANS experiments. However, in this study we observe such an ultraslow process without any indication of structural inhomogeneities on length-scales above 2 nm. Hence, we suggest that our observed ultraslow process is due to long-range diffusion of single xylitol molecules or small clusters of a few xylitol molecules (and perhaps some associated water molecules) which are randomly dispersed and sufficiently small to not be structurally detected in our SANS study. In the q-range of the DLS measurements this ultraslow relaxation process is around room temperature several orders of magnitude slower than the structural α-relaxation. However, if its 1/q2-dependent relaxation time is extrapolated to q-values where relaxation times from dielectric spectroscopy and quasielastic neutron scattering are compatible (about 10 nm-1), a relaxation time similar to that of the dielectric α-relaxation is obtained. Thus, the large difference in time scale between the two relaxation processes in the q-range of a DLS study is due to the fact that the α-relaxation is cooperative in nature, rather than caused by long-range single particle diffusion, and thus q-independent at low q-values.

To better understand the role of the interactions between surfactant, solvent, and a solid substrate on surface aggregation, we have studied the adsorption of a series of alkylpoly(ethylene oxide) CnEm surfactants on three different substrates: graphite, hydrophilic silica, and hydrophobic silica. Using atomic force microscopy (AFM), we find that adsorption to hydrophilic silica, with two exceptions, results in the formation of globular structures that are similar to bulk micelles. On silica that has been made hydrophobic by reaction with organosilane, adsorption results in a surface layer that is laterally homogeneous and is probably a monolayer with ethylene oxide groups in contact with the solution. A number of surfactants with ionic and zwitterionic headgroups were also observed to form monolayers on hydrophobic silica. This large perturbation from the solution-aggregate structure on the hydrophobic surface is driven by a minimization of the area of contact between water and the hydrophobic silica substrate. On graphite, the surface layer is either long, thin aggregates (consistent with a hemicylindrical structure) or a laterally homogeneous layer (consistent with a monolayer with the headgroups facing the solution). The nonionic C12 and C14 surfactants form hemicylinders, and the C10 surfactants with the same headgroups form a laterally homogeneous layer. This suggests that above a critical alkyl chain length the interaction between the graphite and the surfactant tail is sufficient to orient a layer of alkyl tail groups parallel to the graphite surface, which then templates further adsorption. Below the critical alkyl length, the arrangement on graphite is similar to that on hydrophobic silica and is probably driven by a minimization of the water-graphite interfacial area. The critical alkyl length for (zwitterionic) sulfobeteine surfactants is not the same as for the nonionic poly(ethylene oxide) surfactants. This shows that the headgroup also plays an important role in determining the adsorbed structure. All measurements were performed in equilibrium with bulk micelles at or above the critical micelle concentration and at approximately 25 °C.

Aqueous acetic acid solutions have been studied by vibrational sum frequency spectroscopy (VSFS) in order to acquire molecular information about the liquid-gas interface. The concentration range 0-100% acetic acid has been studied in the CH/OH and the C-O/C=O regions, and in order to clarify peak assignments, experiments with deuterated acetic acid and water have also been performed. Throughout the whole concentration range, the acetic acid is proven to be protonated. It is explicitly shown that the structure of a water surface becomes disrupted even at small additions of acetic acid. Furthermore, the spectral evolution upon increasing the concentration of acetic acid is explained in terms of the different complexes of aceticacid molecules, such as the hydrated monomer, linear dimer, and cyclic dimer. In the C=O region, the hydrated monomer is concluded to give rise to the sum frequency (SF) signal, and in the CH region, the cyclic dimer contributes to the signal as well. The combination of results from the CH/OH and the C-O/C=O regions allows a thorough characterization of the behavior of the acetic acid molecules at the interface to be obtained.

The interaction between a microbial lipase and an anionic and a cationic surfactant at the air-water interface has been studied by neutron reflectivity. Sodium dodecyl sulfate (SDS) and tetradecyltrimethylammonium bromide (TTAB) were used as anionic and cationic surfactant, respectively. The same enzyme-surfactant systems were also studied at the interface between a hydrophobic solid surface and water by ellipsometry, and the results from the two techniques were compared. Surface tension measurements were also performed in order to monitor complex formation in bulk. The data obtained from neutron reflectivity and from ellipsometry were in very good agreement with each other. Both techniques show that lipase adsorbs readily at the interfaces and that SDS at low concentration does not interact strongly with this protein layer. At higher SDS concentration, the protein is displaced from the surface. On the other hand, when TTAB is added at low concentration, a thick lipase-surfactant layer is formed at the surfaces. This compact layer is solubilized by further addition of the cationic surfactant.

Poly-NIPAm microgel particles with two different cross-linking densities were prepared with the classical batch polymerization process. These particles were adsorbed onto modified silica surfaces, and their nanomechanical properties were measured by means of atomic force microscopy. It was found that these particles have a hard core-soft shell structure both below and above the volume transition temperature. The core-shell-like structure appears due to a higher reaction rate of the cross-linker compared to that of the monomer, leading to depletion of cross-linker in the shell region. The microgel beads with lower average cross-linking density were found to be less stiff below the volume transition temperature than the microgel with higher cross-linking density. Increasing the temperature further to just above the volume transition temperature led to lower stiffness of the more highly cross-linked microgel compared to its less cross-linked counterpart. This effect is explained with the more gradual deswelling with temperature for the more cross-linked microgel particles. This phenomenon was confirmed by dynamic light scattering measurements in the bulk phase, which showed that the larger cross-linking density microgel showed a more gradual collapse in aqueous solution as the temperature was increased.

Three nonhalogenated ionic liquids (ILs) dissolved in 2-ethylhexyl laurate (2-EHL), a biodegradable oil, are investigated in terms of their bulk and electro-interfacial nanoscale structures using small-angle neutron scattering (SANS) and neutron reflectivity (NR). The ILs share the same trihexyl(tetradecyl)phosphonium ([P6,6,6,14]+) cation paired with different anions, bis(mandelato)borate ([BMB]−), bis(oxalato)borate ([BOB]−), and bis(salicylato)borate ([BScB]−). SANS shows a high aspect ratio tubular self-assembly structure characterized by an IL core of alternating cations and anions with a 2-EHL-rich shell or corona in the bulk, the geometry of which depends upon the anion structure and concentration. NR also reveals a solvent-rich interfacial corona layer. Their electro-responsive behavior, pertaining to the structuring and composition of the interfacial layers, is also influenced by the anion identity. [P6,6,6,14][BOB] exhibits distinct electroresponsiveness to applied potentials, suggesting an ion exchange behavior from cation-dominated to anion-rich. Conversely, [P6,6,6,14][BMB] and [P6,6,6,14][BScB] demonstrate minimal electroresponses across all studied potentials, related to their different dissociative and diffusive behavior. A mixed system is dominated by the least soluble IL but exhibits an increase in disorder. This work reveals the subtlety of anion architecture in tuning bulk and electro-interfacial properties, offering valuable molecular insights for deploying nonhalogenated ILs as additives in biodegradable lubricants and supercapacitors. 

X-ray photoelectron spectroscopy was used to estimate the absolute amount of cationic polyelectrolytes that adsorbs on mica and cellulose surfaces in aqueous media. The calculation takes advantage of the knowledge of the mica crystal composition at the basal plane and its ion-exchange properties in aqueous solution. The XPS was operated under monochromatic and unmonochromatic mode and good agreement was observed in the resulting adsorbed amount. The evaluation of the amount of cationic polyelectrolyte adsorbed on cellulose was achieved using calibration curves obtained from adsorption data for the same polyelectrolytes on bare mica surfaces. The adsorption isotherm for polyelectrolytes of low charge density adsorbed on cellulose reveals that their affinity toward cellulose is weaker compared to that observed for highly charged surfaces such as mica. The effect of the polyelectrolyte charge density on the adsorbed amount and the number density of charged segments adsorbed on cellulose were also investigated. From these results it can be concluded that nonelectrostatic interactions are the main contributors to the adsorption of polyelectrolytes on cellulose, but it cannot be ruled out that electrostatic effects also take part in the adsorption mechanism. Finally, it is demonstrated that it is not correct to use the adsorbed amount of polyelectrolytes to determine the surface charge on cellulose surfaces.

Surface light scattering (SLS) by capillary waves was used to investigate the adsorption behavior of non-ionic sugar surfactants at the air/liquid interface. SLS by the subphase (water) followed predictions from hydrodynamic theory. The viscoelastic properties (surface elasticity and surface viscosity) of monolayers formed by octyl β-glucoside, octyl α-glucoside, and 2-ethylhexyl α-glucoside surfactants were quantified at submicellar concentrations. It is further concluded that a diffusional relaxation model describes the observed trends in high-frequency, nonintrusive laser light scattering experiments. The interfacial diffusion coefficients that resulted from fitting this diffusional relaxation model to surface elasticity values obtained with SLS reflect the molecular dynamics of the subphase near the interface. However, differences from the theoretical predictions indicate the existence of effects not accounted for such as thermal convection, molecular rearrangements, and other relaxation mechanisms within the monolayer. Our results demonstrate important differences in molecular packing at the air-water interface for the studied isomeric surfactants

The interaction between molecules of a low molecular weight diblock copolymer of poly(ethylene oxide) (E) and poly(butylene oxide) (B), B8E41, at hydrophobic surfaces were investigated experimentally by using two surface force techniques and ellipsometry. Extended mean-field theory was employed to describe the adsorption of EB diblock copolymers at planar surfaces as well as the forces between surfaces with adsorbed diblock copolymers. It is the hydrophobic poly(butylene oxide) block that anchors the diblock copolymer at the hydrophobic surface with the water-soluble poly(ethylene oxide) block protruding in the aqueous solution in a "brushlike" or at least streched structure. The adsorption kinetics demonstrate that two adsorption regimes exist, one which is transport-limited and the other at higher adsorption where a slower branch due to crowding effects at the surface exists. Only monotonic repulsuve steric forces between the diblock copolymer-coated surfaces were observed in the surface force measurements. The range of the steric repulsion increased with increasing bulk copolymer concentration, whereas the concentration of an inert salt (KBr, up to 0.1 M) did not influence the measured steric interaction. Upon dilution the block copolymer slowly dissolved, which resulted in a less long-range steric force, and under a high force the layers were squeezed out from between the surfaces. The adsorbed layer thickness obtained in the experiments varied with solution volume-to-surface area ratio. This is interpreted as being caused by the polydispersity of the diblock copolymer. The interaction parameters entering the mean-field model were fitted to reproduce adsorption isotherms of the diblock copolymer and of two triblock copolymers of different architectures. Calculations were performed for mondisperse and polydisperse diblock copolymers. The agreement between theory and experiment was improved when the molecular polydispersity (Mm/Mn = 1.1) of the sample was taken into account. In particular, polydispersity led to predicted adsorption isotherms that are more of the high affinity type and more sensitive to low volume-to-surface area ratio and to the interaction between surfaces starting at a longer separation. Among the polymer components, it is those with the largest B block that adsorb preferentially, which leads to an increased amount adsorbed and forces the E chains to adopt more extended conformations.

The phase transition of the thermoreversible polymer PNIPAM, which shows a coil-to-globule transition due to the lower critical solution temperature (LCST) behavior in aqueous solution, is investigated in the restricted geometry of an adsorption layer. Furthermore, a charged copolymer is studied to investigate the influence of charges on the phase transition. Both polymers are adsorbed to colloidal silica and studied by 1H NMR and differential scanning calorimetric (DSC) experiments. In solid state 1H NMR relaxation experiments the signals of solid spins in trains and of liquid spins in tails and loops can be identified. 1H liquid state spectra detect the phase transition of the loops and tails into immobile segments with increasing temperature. The transition is broadened as compared to the polymer in solution, especially at low surface coverage. For the copolymer, the transition is incomplete, since mobile segments remain even at high temperature. They are attributed to electrostatic repulsion from the surface, leading to a mobile arrangement of the copolymer layer. Micro-DSC experiments confirm the finding of an increased width of the transition at the interface, which depends on the surface coverage. Furthermore, an increase of the transition temperature with decreasing polymer amount is observed, which is most pronounced for the copolymer, and is a further indication of an electrostatically hindered phase transition. In conclusion, despite a negligible influence of a low fraction of charges on the phase transition in solution, the phase transition in adsorption layers is very sensitive to charged segments. Combining NMR and DSC methods, local information on the transition behavior of different polymer segments at the interface, such as loops and tails, or charged monomers, can be obtained.

This paper features the interfacial behavior of nonionic surfactants and surfactant-decane microemulsions at the silica-water interface from micellar solutions and water-rich tricomponent CnEm-decane-water microemulsions. The adsorption of a nonionic surfactant (pentaethyleneglycol n-dodecyl ether, C12E5) and its decane microemulsions to silica and borosilicate glass was studied by ellipsometry and direct force measurements using a bimorph surface force apparatus. The ellipsometric measurements of the adsorbed layer properties provided evidence of an initial lateral swelling of adsorbed bilayer segments with increasing bulk oil fraction. At a weight fraction of about 0.12 w/w decane-to-surfactant + decane, the surface appeared to be fully covered by a continuous bilayer with a thickness of 42 Å, a refractive index of 1.448, and an mean area per surfactant of about 49 Å2. Further increase of the oil content results in the swelling of the bilayer in the direction normal to the surface plane. Force measurements between surfactant-covered surfaces showed a subtle dependence on the properties of the glass substrate. The height of the force barrier prior to jumping into hard-wall contact was found to increase with increasing lateral surface coverage up to a decane content of 0.12 w/w. However, further increase in the fraction of decane resulted in a marked decrease of the force barrier height. The steric force onset distance, however, was always found to be proportional to the thickness of the adsorbed layers. Hence, the adsorbed layer properties measured by ellipsometry and the interaction curves measured by direct force measurements were found to correlate well. Variations were sometimes seen in force profiles measured on different glass surfaces. In most cases, the force onset distance correlated well with the thickness of two adsorbed bilayers. However, in some cases, it agreed closely with the thickness of one bilayer. These variations were not easy to predict with regard to the pretreatment and measured properties of the glass surface. Our interpretation is that this difference is caused either by very small changes in the interaction strength between adsorbed surfactant headgroups and the glass surface or by defects of the adsorbed layer resulting from the "topochemical" heterogeneity of the glass surface

Vibrational sum frequency spectroscopy has been used to investigate the surface of aqueous acetic acid solutions. By studying the methyl and carbonyl vibrations with different polarization combinations, an orientation analysis of the acetic acid molecules has been performed in the concentration range 0-100%. The surface tension of acetic acid solutions was also measured in order to obtain the surface concentration. The orientation of the interfacial acetic acid molecules was found to remain essentially constant in an upright position with the methyl group directed toward the gas phase in the whole concentration range. The tilt angle (CH3) of the symmetry axis of the methyl group with respect to the surface normal was found to be lower than 15 when considering a distribution of angles or as narrow as 0 ± 11 when assuming a Gaussian distribution. Further investigations showed that the C=O bond tilt (C=O) of the acetic acid hydrated monomer was constant and close to 55 in the concentration range where it was detected. Finally, the orientation information is discussed in terms of different species of acetic acid, where the formation of a surface layer of acetic acid cyclic dimers is proposed at high acid concentrations.

The interaction forces between bovine serum albumin (BSA) layers adsorbed on silica surfaces have been measured using an atomic force microscope (AFM) in conjunction with the colloid probe technique. Measurements of force-distance curves were made at different pH values and electrolyte concentrations (NaCl and CaCl2). The interaction at long range is dominated by electrical double-layer forces, while at short surface separations an additional repulsion due to the compression of the adsorbed protein layers appears. However, prior to this steric interaction, when the pH is above the isoelectric point of the protein and at high salt concentration, a non-DLVO repulsive interaction is observed. This behavior is explained if the presence of hydration forces in the system is assumed. Theoretical predictions including a hydration term in the DLVO theory fit the experimental results satisfactorily. The results presented in this article provide a direct confirmation that the AFM colloid probe technique can provide a useful way of directly quantifying the interaction of biological macromolecules.

Atomistic simulations have been performed to investigate the microscopic structural organization of aqueous solutions of trihexyltetradecylphosphonium bis(oxalato)borate ([P6,6,6,14][BOB]) ionic liquid (IL). The evolution of the microscopic liquid structure and the local ionic organization of IL/water mixtures as a function of the water concentration is visualized and systematically analyzed via radial and spatial distribution functions, coordination numbers, hydrogen bond network, and water clustering analysis. The microscopic liquid structure in neat IL is characterized by a connected apolar network composed of the alkyl chains of [P6,6,6,14] cations and isolated polar domains consisting of the central segments of [P6,6,6,14] cations and [BOB] anions, and the corresponding local ionic environment is described by direct contact ion pairs. In IL/water mixtures with lower water mole fractions, the added water molecules are dispersed and embedded in cavities between neighboring ionic species and the local ionic structure is characterized by solvent-shared ion pairs through cation-water-anion triple complexes. With a gradual increase in the water concentration in IL/water mixtures, the added water molecules tend to aggregate and form small clusters, intermediate chain-like structures, large aggregates, and eventually a water network in water concentrated simulation systems. A further progressive dilution of IL/water mixtures leads to the formation of self-organized micelle-like aggregates characterized by a hydrophobic core and hydrophilic shell consisting of the central polar segments in [P6,6,6,14] cations and [BOB] anions in a highly branched water network. The striking structural evolution of the [P6,6,6,14][BOB] IL/water mixtures is rationalized by the competition between favorable hydrogen bonded interactions and strong electrostatic interactions between the polar segments in ionic species and the dispersion interactions between the hydrophobic alkyl chains in [P6,6,6,14] cations.