In this paper, the role of the different structural isomers of the anionic surfactant sodium para-dodecyl benzene sulfonate, LAS, on surface adsorption and solution self-assembly has been studied. Using a combination of neutron reflectivity, NR, and small angle neutron scattering, SANS, the effect of mixing an isomer with a short symmetric hydrocarbon chain with one which has an asymmetric hydrocarbon chain on both the equilibrium surface adsorption behavior and the solution microstructure of the mixtures, both in the presence and absence of a divalent cation (Ca(2+)), has been investigated. In the absence of electrolyte, the LAS isomer mixtures form small charged globular micelles throughout the composition range studied. The micelle aggregation number increases with the increase in the asymmetric isomer content, reflecting an increase in the packing efficiency within the micelle. The addition of calcium ions promotes the formation of planar aggregates, as multilamellar vesicles, but only when the symmetric LAS isomer is the major component of the mixture. At a surfactant concentration just above the critical micelle concentration, CMC, and in the absence of electrolyte, the variation in the surface composition is close to the solution composition. Regular solution theory, RST, calculations show that this variation is also close to what is expected for ideal mixing. The addition of Ca(2+) ions induces a different surface behavior, resulting in the formation of multilayer structures at the interface throughout the entire composition range.
The surface adsorption behavior and the solution microstructure of mixtures of the C(6) isomer of anionic surfactant sodium para-dodecyl benzene sulfonate, ABS, with nonionic surfactant monodecyl triethyleneglycol ether, C(10)E(3,) have been investigated using a combination of neutron reflectivity, NR, and small-angle neutron scattering, SANS. In solution, the mixing of C(10)E(3) and ABS results in the formation of small globular micelles over most of the composition range (100:0 to 20:80 ABS/C(10)E(3)). Planar aggregates (lamellar or unilamellar vesicles, ULV) are observed for solution compositions rich in the nonionic surfactant (>80 mol % nonionic). Prior to the transition to planar aggregates, the micelle aggregation number increases with increasing nonionic composition. The lamellar-phase region is preceded by a narrow range of composition over which mixtures of micelles and small unilamellar vesicles coexist. The variation in surface absorption behavior with solution composition shows a strong surface partitioning of the more surface-active component, C(10)E(3). This pronounced departure from ideal mixing is not readily explained by existing surfactant mixing theories. In the presence of Ca(2+) ions, a more complex evolution of solution phase behavior with solution composition is observed. The lamellar-phase region occurs over a broader range of solution compositions at the expense of the small-vesicle phase. The phase boundaries are shifted to lower nonionic compositions, and the extent to which the solution-phase diagrams are modified increases with increasing calcium ion concentration. The SANS data for the large planar aggregates are consistent with large polydisperse flexible unilamellar vesicles. In the presence of Ca(2+) ions, the surface adsorption patterns become more consistent with ideal mixing in the nonionic-rich region of the surface-phase diagram. However, in the ABS-rich regions the surface behavior is more complex because of the spontaneous formation of more complex surface microstructures (bilayers to multilayers). Both in water and in the presence of Ca(2+) ions the variations in the surface adsorption behavior and in the solution mesophase structure do not appear to be closely correlated.
Inspired by the carnivorous Nepenthes pitcher plant, a range of highly liquid repellent lubricant‐infused surfaces has been devised (low water droplet contact angle hysteresis and sliding angle values). This entails matching functional pulsed plasma polymer nanolayers with appropriate slippery lubricants. A molecular‐level structure–behavior relationship is developed, highlighting the importance of favorable aromatic–aliphatic intermolecular interactions between the coating and lubricant. Fluorinated lubricant‐infused pulsed plasma polymer nanocoatings resist wetting by liquids, spanning a wide range of surface tensions (including pentane, motor oil, and water, i.e., omniphobicity). In the case of natural antimicrobial compound‐infused functional plasma polymer surfaces (e.g., the essential oil cinnamaldehyde), multifunctional performance is attained combining high liquid repellency (self‐cleaning) with simultaneous strong antibacterial activity against both Gram‐positive Staphylococcus aureus and Gram‐negative Escherichia coli (log10 reduction > 7). In addition, these lubricant‐infused functional pulsed plasma polymer surfaces easily repel a variety of everyday liquids (including foodstuffs such as tomato ketchup and honey).
Water-soluble nonionic surfactant, pentaethylene glycol monododecyl ether, C12E5, spontaneously blooms to the surface of spin-cast hydrophobic polyisoprenes, generating hydrophilic surfaces. This system provides a simple model for hydrophilic chemical modification of rubbery polymers that demonstrates surprisingly rich, complex, and unexpected behaviour. The vertical depth profiles were quantified using neutron reflectometry (NR) using a novel procedure to account for undulations in the film thickness. Surface properties were characterized using contact angle analysis and atomic force microscopy (AFM). Despite the low surface tension of the toluene solvent used in film preparation and the low surface energy of the polyisoprene (PI) matrix, NR depth profiles revealed clear evidence of surfactant segregation. This surface layer was typically thicker than a monolayer, but incomplete, yet was remarkably stable with respect to dissolution, even when exposed to hundreds of thousands of times the volume of water required to dissolve all the surfactant on the surface. Despite the apparent resistance to removal from the surface, water exposure does alter the subsequent wettability of the surface, with a hydrophilic-to-hydrophobic transition occurring after rinsing. Complementary AFM images of these C12E5/cis-PI films showed unexpected strand-like features on the surface of the film, which we attribute to a non-uniform lateral distribution of some of the surfactant. This surface structure becomes more evident after rinsing, and it appears that there are two distinct populations of surfactant on the PI film surface. We conclude that some of the bloomed surfactant exists as layers, which are relatively inert with respect to rinsing or surface modification, and some is laterally inhomogeneous. This latter population is primarily responsible for surface wetting behaviour but is not detected by specular NR.
Water-soluble nonionic surfactant, pentaethylene glycol monododecyl ether, C12E5, spontaneously blooms to the surface of spin-cast hydrophobic polyiso-prenes, generating hydrophilic surfaces. This system represents a simple model for hydrophilic chemical modification of rubbery polymers yet demonstrates surprisingly rich, complex and unexpected behaviour. The ver-tical depth profiles were quantified using neutron reflectometry (NR) and deuterated surfactant, and the surface properties were characterized using con-tact angle analysis and atomic force microscopy (AFM). Despite the low surface tension of the toluene solvent used in film preparation and the low surface en-ergy of the PI matrix, NR depth profiles revealed clear evidence of surfactant segregation. This surface layer was typically thicker than a monolayer, but incomplete, yet was remarkably stable with respect to dissolution, even when exposed to hundreds of thousands of times the volume of water required to dissolve all the surfactant on the surface. Despite the apparent resistance to removal from the surface, water exposure does alter the subsequent wettabil-ity of the surface, with a hydrophilic-to-hydrophobic transition occurring after rinsing. Complementary AFM images of these C12E5 / cis-PI films showed unexpected strand-like features on the surface of the film, which we attribute to a non-uniform lateral distribution of some of the surfactant. This surface structure becomes more evident after rinsing, and it appears that there are two distinct populations of surfactant on the PI film surface. We conclude that some of the bloomed surfactant exists as layers, which are relatively inert with respect to rinsing or surface modification, and some is laterally inhomogene-ous. This latter population is primarily responsible for surface wetting be-havior, but is not detected by specular NR.
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