How Electrolyte and Polyelectrolyte Affect the Adsorption of the Anionic Surfactant SDS onto the Surface of a Cellulose Thin Film and the Structure of the Cellulose Film. 1. Hydrophobic Cellulose

The nature of hydrophobic thin cellulose films, formed by Langmuir–Blodgett (LB) deposition on silica, has been studied using neutron reflectivity (NR). The impact of electrolyte and a polyelectrolyte, poly­(dimethyldiallylammonium chloride) (polydmdaac), on the adsorption of the anionic surfactant sodium dodecyl sulfate (SDS) onto the surface of the hydrophobic cellulose film and upon the structure of the cellulose film has been investigated. The results show how a combination of polyelectrolytes and electrolyte can be used to manipulate surfactant adsorption onto hydrophobic cellulose surfaces and modify the structure of the cellulose film by swelling and penetration. The results illustrate how polyelectrolytes can be used to reverse adsorption and swelling of cellulose films which are not reversible simply by dilution in solvent

Effect of Polymer Molecular Weight and Solution pH on the Surface Properties of Sodium Dodecylsulfate-Poly(Ethyleneimine) Mixtures

The effect of polymer molecular weight and solution pH on the surface properties of the anionic surfactant sodium dodecylsulfate, SDS, and a range of small linear poly­(ethyleneimine), PEI, polyelectrolytes of different molecular weights has been studied by surface tension, ST, and neutron reflectivity, NR, at the air–solution interface. The strong SDS–PEI interaction gives rise to a complex pattern of ST behavior which depends significantly on solution pH and PEI molecular weight. The ST data correlate broadly with the more direct determination of the surface adsorption and surface structure obtained using NR. At pH 3, 7, and 10, the strong SDS–PEI interaction results in a pronounced SDS adsorption at relatively low SDS and PEI concentrations, and is largely independent of pH and PEI molecular weight (for PEI molecular weights on the order of 320, 640, and 2000 Da). At pH 7 and 10, there are combinations of SDS and PEI concentrations for which surface multilayer structures form. For the PEI molecular weights of 320 and 640 Da, these surface multilayer structures are most well-developed at pH 10 and less so at pH 7. At the molecular weight of 2000 Da, they are poorly developed at both pH 7 and 10. This evolution in the surface structure with molecular weight is consistent with previous studies, where for a molecular weight of 25 000 Da no multilayer structures were observed for the linear PEI. The results show the importance with increasing polymer molecular weight of the entropic contribution due to the polymer flexibility in control of the surface multilayer formation

Effect of Architecture on the Formation of Surface Multilayer Structures at the Air–Solution Interface from Mixtures of Surfactant with Small Poly(ethyleneimine)s

The impact of ethyleneimine architecture on the adsorption behavior of mixtures of small poly­(ethyleneimines) and oligoethyleneimines (OEIs) with the anionic surfactant sodium dodecylsulfate (SDS) at the air–solution interface has been studied by surface tension (ST) and neutron reflectivity (NR). The strong surface interaction between OEI and SDS gives rise to complex surface tension behavior that has a pronounced pH dependence. The NR data provide more direct access to the surface structure and show that the patterns of ST behavior are correlated with substantial OEI/SDS adsorption and the spontaneous formation of surface multilayer structures. The regions of surface multilayer formation depend upon SDS and OEI concentrations, on the solution pH, and on the OEI architecture, linear or branched. For the linear OEIs (octaethyleneimine, linear poly­(ethyleneimine) or LPEI₈, and decaethyleneimine, LPEI₁₀) with SDS, surface multilayer formation occurs over a range of OEI and SDS concentrations at pH 7 and to a much lesser extent at pH 10, whereas at pH 3 only monolayer adsorption occurs. In contrast, for branched OEIs BPEI₈ and BPEI₁₀ surface multilayer formation occurs over a wide range of OEI and SDS concentrations at pH 3 and 7, and at pH 10, the adsorption is mainly in the form of a monolayer. The results provide important insight into how the OEI architecture and pH can be used to control and manipulate the nature of the OEI/surfactant adsorption

Adsorption of Model Perfumes at the Air–Solution Interface by Coadsorption with an Anionic Surfactant

The adsorption of the model perfumes phenyl ethanol, PE, and linalool, LL, at the air–solution interface by coadsorption with the anionic surfactant sodium dodecyl 6-benezene sulfonate, LAS-6, has been studied primarily by neutron reflectivity, NR. The variation in the mixed surface adsorption with solution composition is highly nonideal, and the more hydrophobic LL is more surface active. At a LAS-6 concentration of 0.5 mM the adsorption of PE and LL is broadly similar but with the LL systematically more surface active, and at 2 mM the LL completes more effectively for the surface than the PE. The variation in surface composition with solution composition and concentration reflect the greater hydrophobicity and hence surface activity of LL, and the greater solubility of PE in aqueous solution. Changing the geometry of the LAS isomer, from the symmetrical LAS-6 geometry to the more asymmetrical LAS-4, results in the LL competing more effectively for the surface due to changes in the packing constraints associated with the hydrophobic region. The results provide insights into the factors that affect coadsorption that can be more broadly applied to the surface delivery of a wide range of molecules other than perfumes

Unusual Adsorption at the Air–Water Interface of a Zwitterionic Carboxybetaine with a Large Charge Separation

The structures of layers of three different dodecylcarboxybetaine surfactants adsorbed at the air–water interface have been determined by neutron reflection. The zwitterionic compounds differed in the length of the spacer separating the quaternary ammonium and carboxylate groups, which was (CH₂)₁, (CH₂)₄, or (CH₂)₈. The limiting area per molecule was found to be 45, 52, or 84 Ų, respectively, and compared reasonably with results from surface tension showing that the Gibbs prefactor is 1 in each case. Isotopic labeling was used to distinguish between the position of the alkyl and spacer groups in the layer. The spacer was found to be well-immersed in water for the (CH₂)₁ and (CH₂)₄ spacers but significantly above water for the (CH₂)₈ spacer. The distribution of the (CH₂)₈ spacer along the surface normal was found to be similar to that of the dodecyl group; i.e., it projects out of the water, contrary to an earlier hypothesis that it forms a loop. Comparison of the overlap of water with dodecyl and spacer groups also indicates that the (CH₂)₈ spacer is well out of the water. This in turn suggests that the anionic carboxylic acid group, which is dissociated in solution, is not ionized in the adsorbed layer. A further observation is that the dodecylcarboxybetaine with the (CH₂)₈ spacer reaches surface saturation at one-tenth of the critical micelle concentration. This is highly unusual and is attributed to the long spacer destabilizing the micelle relative to the surface layer

Acclimation responses to temperature vary with vertical stratification: implications for vulnerability of soil-dwelling species to extreme temperature events

The occurrence of summer heat waves is predicted to increase in amplitude and frequency in the near future, but the consequences of such extreme events are largely unknown, especially for belowground organisms. Soil organisms usually exhibit strong vertical stratification, resulting in more frequent exposure to extreme temperatures for surface-dwelling species than for soil-dwelling species. Therefore soil-dwelling species are expected to have poor acclimation responses to cope with temperature changes. We used five species of surface-dwelling and four species of soil-dwelling Collembola that habituate different depths in the soil. We tested for differences in tolerance to extreme temperatures after acclimation to warm and cold conditions. We also tested for differences in acclimation of the underlying physiology by looking at changes in membrane lipid composition. Chill coma recovery time, heat knockdown time and fatty acid profiles were determined after 1 week of acclimation to either 5 or 20 °C. Our results showed that surface-dwelling Collembola better maintained increased heat tolerance across acclimation temperatures, but no such response was found for cold tolerance. Concordantly, four of the five surface-dwelling Collembola showed up to fourfold changes in relative abundance of fatty acids after 1 week of acclimation, whereas none of the soil-dwelling species showed a significant adjustment in fatty acid composition. Strong physiological responses to temperature fluctuations may have become redundant in soil-dwelling species due to the relative thermal stability of their subterranean habitat. Based on the results of the four species studied, we expect that unless soil-dwelling species can temporarily retreat to avoid extreme temperatures, the predicted increase in heat waves under climatic change renders these soil-dwelling species more vulnerable to extinction than species with better physiological capabilities. Being able to act under a larger thermal range is probably costly and could reduce maximum performance at the optimal temperatur

Adsorption of Hydrophobin–Protein Mixtures at the Air–Water Interface: The Impact of pH and Electrolyte

The adsorption of the proteins β-casein, β-lactoglobulin, and hydrophobin, and the protein mixtures of β-casein/hydrophobin and β-lactoglobulin/hydrophobin have been studied at the air–water interface by neutron reflectivity, NR. Changing the solution pH from 7 to 2.6 has relatively little impact on the adsorption of hydrophobin or β-lactoglobulin, but results in a substantial change in the structure of the adsorbed layer of β-casein. In β-lactoglobulin/hydrophobin mixtures, the adsorption is dominated by the hydrophobin adsorption, and is independent of the hydrophobin or β-lactoglobulin concentration and solution pH. At pH 2.6, the adsorption of the β-casein/hydrophobin mixtures is dominated by the hydrophobin adsorption over the range of β-casein concentrations studied. At pH 4 and 7, the adsorption of β-casein/hydrophobin mixtures is dominated by the hydrophobin adsorption at low β-casein concentrations. At higher β-casein concentrations, β-casein is adsorbed onto the surface monolayer of hydrophobin, and some interpenetration between the two proteins occurs. These results illustrate the importance of pH on the intermolecular interactions between the two proteins at the interface. This is further confirmed by the impact of PBS, phosphate buffered saline, buffer and CaCl₂ on the coadsorption and surface structure. The results provide an important insight into the adsorption properties of protein mixtures and their application in foam and emulsion stabilization

Multivalent-Counterion-Induced Surfactant Multilayer Formation at Hydrophobic and Hydrophilic Solid–Solution Interfaces

Surface multilayer formation from the anionic–nonionic surfactant mixture of sodium dodecyl dioxyethylene sulfate, SLES, and monododecyl dodecaethylene glycol, C₁₂E₁₂, by the addition of multivalent Al³⁺ counterions at the solid–solution interface is observed and characterized by neutron reflectivity, NR. The ability to form surface multilayer structures on hydrophobic and hydrophilic silica and cellulose surfaces is demonstrated. The surface multilayer formation is more pronounced and more well developed on the hydrophilic and hydrophobic silica surfaces than on the hydrophilic and hydrophobic cellulose surfaces. The less well developed multilayer formation on the cellulose surfaces is attributed to the greater surface inhomogeneities of the cellulose surface which partially inhibit lateral coherence and growth of the multilayer domains at the surface. The surface multilayer formation is associated with extreme wetting properties and offers the potential for the manipulation of the solid surfaces for enhanced adsorption and control of the wetting behavior

Impact of Electrolyte on Adsorption at the Air–Water Interface for Ternary Surfactant Mixtures above the Critical Micelle Concentration

The composition of the air–water adsorbed layer of the ternary surfactant mixture, octaethylene monododecyl ether, C₁₂E₈, sodium dodecyl 6-benzenesulfonate, LAS, and sodium dioxyethylene glycol monododecyl sulfate, SLES, and of each of the binary mixtures, with varying amounts of electrolyte, has been studied by neutron reflectivity. The measurements were made above the mixed critical micelle concentration. In the absence of electrolyte adsorption is dominated by the nonionic component C₁₂E₈ but addition of electrolyte gradually changes this so that SLES and LAS dominate at higher electrolyte concentrations. The composition of the adsorbed layer in both binary and ternary mixtures can be quantitatively described using the pseudo–phase approximation with quadratic and cubic interactions in the excess free energy of mixing (<i>G</i>_E) at both the surface and in the micelles. A single set of parameters fits all the experimental data. A similar analysis is effective for a mixture in which SDS replaces SLES. Addition of electrolyte weakens the synergistic SLES–C₁₂E₈ and LAS–C₁₂E₈ interactions, consistent with them being dominated by electrostatic interactions. The SLES–LAS (and SDS–LAS) interaction is moderately strong at the surface and is little affected by addition of electrolyte, suggesting that it is controlled by structural or packing factors. Most of the significant interactions in the mixtures are unsymmetrical with respect to composition, with the minimum in <i>G</i>_E at the 1:2 or 2:1 composition. There is a small structural contribution to the LAS-C₁₂E₈ interaction that leads to a minimum intermediate in composition between 1:2 and 1:1 (LAS:C₁₂E₈) and to a significant residual <i>G</i>_<i>E</i> in strong electrolyte

Multivalent-Counterion-Induced Surfactant Multilayer Formation at Hydrophobic and Hydrophilic Solid–Solution Interfaces

Surface multilayer formation from the anionic–nonionic surfactant mixture of sodium dodecyl dioxyethylene sulfate, SLES, and monododecyl dodecaethylene glycol, C₁₂E₁₂, by the addition of multivalent Al³⁺ counterions at the solid–solution interface is observed and characterized by neutron reflectivity, NR. The ability to form surface multilayer structures on hydrophobic and hydrophilic silica and cellulose surfaces is demonstrated. The surface multilayer formation is more pronounced and more well developed on the hydrophilic and hydrophobic silica surfaces than on the hydrophilic and hydrophobic cellulose surfaces. The less well developed multilayer formation on the cellulose surfaces is attributed to the greater surface inhomogeneities of the cellulose surface which partially inhibit lateral coherence and growth of the multilayer domains at the surface. The surface multilayer formation is associated with extreme wetting properties and offers the potential for the manipulation of the solid surfaces for enhanced adsorption and control of the wetting behavior