Adsorption, amphiphilic

This character, called amphiphilic, produces two characteristic sets of behavior, adsorption on the interfaces and auto-association in the form of micelles that extend into the oily surroundings as illustrated in Figure 9.8. 

Proteins, like other macromolecules, can be made into monolayers at the air-water interface either by spreading, adsorption, or specific binding. Proteins, while complex polymers, are interesting because of their inherent surface activity and amphiphilicity. There is an increasing body of literature on proteins at liquid interfaces, and here we only briefly discuss a few highlights. 

Most LB-forming amphiphiles have hydrophobic tails, leaving a very hydrophobic surface. In order to introduce polarity to the final surface, one needs to incorporate bipolar components that would not normally form LB films on their own. Berg and co-workers have partly surmounted this problem with two- and three-component mixtures of fatty acids, amines, and bipolar alcohols [175, 176]. Interestingly, the type of deposition depends on the contact angle of the substrate, and, thus, when relatively polar monolayers are formed, they are deposited as Z-type multilayers. Phase-separated LB films of hydrocarbon-fluorocarbon mixtures provide selective adsorption sites for macromolecules, due to the formation of a step site at the domain boundary [177]. 

Lattice models have been studied in mean field approximation, by transfer matrix methods and Monte Carlo simulations. Much interest has focused on the occurrence of a microemulsion. Its location in the phase diagram between the oil-rich and the water-rich phases, its structure and its wetting properties have been explored [76]. Lattice models reproduce the reduction of the surface tension upon adsorption of the amphiphiles and the progression of phase equilibria upon increasmg the amphiphile concentration. Spatially periodic (lamellar) phases are also describable by lattice models. Flowever, the structure of the lattice can interfere with the properties of the periodic structures. 

These poly(2-alkyl-2-oxazoline) silane coupling agents were copolycondensed with tetraethoxysilane by acid-catalyst to produce poly(2-alkyl-2-oxazoline)-modified silica gel. The composite gel from 2-ethyl-2-oxazoline was also homogeneous and transparent glass. Poly(2-alkyl-2-oxazoline)-modified silica gels, especially gels based on poly(2-ethyl-2-oxazoline) absorbed water and also organic solvents such as DMF or alcohols as shown in Table 7. This result means that the obtained composite gel shows the amphiphilic adsorption property. 

Fig. 3 a-c. Summary of data from different laboratories, obtained by surface force measurement, on the average layer thickness L as a function of tethered chain length for flat, tethered layers constructed by adsorption of amphiphilic polymers on mica. Adapted from Ref. 21. (a) Data of reference 20 on poly-tert-butylstyrene chains anchored by adsorbing blocks of poly-2-vinylpyridine. (b) Data of references 11 and 12 on polystyrene chains anchored by adsorbing blocks of poly-2-vinylpyridine. (c) Data of references 13 and 14 on polystyrene chains anchored by adsorbing zwitterionic groups [13] or by small adsorbing blocks of polyethyleneoxide [14]... 

Adsorption on solid matrices, which improves (at optimal protein/support ratios) enzyme dispersion, reduces diffusion limitations and favors substrate access to individual enzyme molecules. Immobilized lipases with excellent activity and stability were obtained by entrapping the enzymes in hydrophobic sol-gel materials [20]. Finally, in order to minimize substrate diffusion limitations and maximize enzyme dispersion, various approaches have been attempted to solubilize the biocatalysts in organic solvents. The most widespread method is the one based on the covalent linking of the amphiphilic polymer polyethylene glycol (PEG) to enzyme molecules [21]. 

Desulfurization processes are absolutely necessary for producing clean fuels. Possible strategies to realize ultradeep suffiirization currently include adsorption, extraction, oxidation, and bioprocesses. Oxidative desulfurization (ODS) combined with extraction is considered one of the most promising of these processes [13]. Ultradeep desulfurization of diesel by selective oxidation with amphiphilic catalyst assembled in emulsion droplets has given results where the sulfur level of desulfurized diesel can be lowered from 500 ppm to about 0.1 ppm without changing the properties of the diesel [12]. 

The process of adsorption of polyelectrolytes on solid surfaces has been intensively studied because of its importance in technology, including steric stabilization of colloid particles [3,4]. This process has attracted increasing attention because of the recently developed, sophisticated use of polyelectrolyte adsorption alternate layer-by-layer adsorption [7] and stabilization of surfactant monolayers at the air-water interface [26], Surface forces measurement has been performed to study the adsorption process of a negatively charged polymer, poly(styrene sulfonate) (PSS), on a cationic monolayer of fluorocarbon ammonium amphiphilic 1 (Fig. 7) [27],... 

FIG. 9 Schematic illustration of adsorption of poly(styrenesulfonate) on an oppositely charged surface. For an amphiphile surface in pure water or in simple electrolyte solutions, dissociation of charged groups leads to buildup of a classical double layer, (a) In the initial stage of adsorption, the polymer forms stoichiometric ion pairs and the layer becomes electroneutral, (b) At higher polyion concentrations, a process of restructuring of the adsorbed polymer builds a new double layer by additional binding of the polymer. 

One can further elaborate a model to have a concrete form of /(ft), depending on which aspect of the adsorption one wants to describe more precisely, e.g., a more rigorous treatment of intermolecular interactions between adsorbed species, the activity instead of the concentration of adsorbates, the competitive adsorption of multiple species, or the difference in the size of the molecule between the solvent and the adsorbate. An extension that may be particularly pertinent to liquid interfaces has been made by Markin and Volkov, who allowed for the replacement of solvent molecules and adsorbate molecules based on the surface solution model [33,34]. Their isotherm, the amphiphilic isotherm takes the form... 

Many kinds of nonbiodegradable vinyl-type hydrophilic polymers were also used in combination with aliphatic polyesters to prepare amphiphilic block copolymers. Two typical examples of the vinyl-polymers used are poly(/V-isopropylacrylamide) (PNIPAAm) [149-152] and poly(2-methacryloyloxyethyl phosphorylcholine) (PMPC) [153]. PNIPAAm is well known as a temperature-responsive polymer and has been used in biomedicine to provide smart materials. Temperature-responsive nanoparticles or polymer micelles could be prepared using PNIPAAm-6-PLA block copolymers [149-152]. PMPC is also a well-known biocompatible polymer that suppresses protein adsorption and platelet adhesion, and has been used as the hydrophilic outer shell of polymer micelles consisting of a block copolymer of PMPC -co-PLA [153]. Many other vinyl-type polymers used for PLA-based amphiphilic block copolymers were also introduced in a recent review [16]. 

The rheological properties of a fluid interface may be characterized by four parameters surface shear viscosity and elasticity, and surface dilational viscosity and elasticity. When polymer monolayers are present at such interfaces, viscoelastic behavior has been observed (1,2), but theoretical progress has been slow. The adsorption of amphiphilic polymers at the interface in liquid emulsions stabilizes the particles mainly through osmotic pressure developed upon close approach. This has become known as steric stabilization (3,4.5). In this paper, the dynamic behavior of amphiphilic, hydrophobically modified hydroxyethyl celluloses (HM-HEC), was studied. In previous studies HM-HEC s were found to greatly reduce liquid/liquid interfacial tensions even at very low polymer concentrations, and were extremely effective emulsifiers for organic liquids in water (6). 

In the preparation of the MOPPV LB films, the polyion complex monolayer were formed by the adsorption of the precursor onto the anionic amphiphile monolayer (route b-2 in Fig.5) [36,37]. The monolayer of 2C12SUC was spread on a aqueous solution of the MOPPV precursor polymer (repeating unit concentration about 10"4 M). The monolayer was allowed to stand for 4 hours to adsorb the precursor after being compressed to a surface pressure of 30 mNm 1. The monolayer was deposited on fused quartz substrate by the LB technique. At last, the deposited polyion complex was converted to MOPPV by the heat treatment at 200 C in vacuo (about 10 2 torr). 

Initial solid phase synthesis25 was carried out on Merrifield s resin (1 % crosslinked chloromethylated styrene/divinylbenzene copolymer, 200-400 mesh) because of its track record in solid-phase peptide synthesis.26 Unfortunately, the Merrifield resin has limitations as a carbohydrate carrier to study interactions between the carbohydrates and relevant binding proteins. The hydrophobic nature of the resin leads to nonspecific, irreversible protein adsorption.27 Later work utilized Rapp s TentaGel, an amphiphilic, polyethylene glycol resin.28... 

Many substances preferentially concentrate at interfaces, including liquid/liquid ones. Although TIRF is most easily adaptable to solid/liquid interfaces, Morrison and Weber(100) succeeded in observing the preferential adsorption of certain amphiphilic dyes at the interface between two immiscible and optically dissimilar liquids. Steady-state TIR fluorescence polarization in that system showed that the rotational diffusion of the interfa-cially adsorbed dye was restricted. 

Liu etal. [32] reported the characteristics and reactivity of highly ordered mesoporous carbon-titania hybrid materials synthesized via organic-inorganic-amphiphilic coassembly followed by in situ crystallization. In the degradation of Rhodamine B these materials also show enhanced properties due to the dispersion/stabilization of small titania nanocrystals and the adsorptive capacity of the nanocarbon. 

Similar types of electric double layer may also be formed at the phase boundary between a solid electrolyte and an aqueous electrolyte solution [7]. They are formed because one electrically-charged component of the solid electrolyte is more readily dissolved, for example the fluoride ion in solid LaFs, leading to excess charge in the solid phase, which, as a result of movement of the holes formed, diffuses into the soUd electrolyte. Another possible way a double layer may be formed is by adsorption of electrically-charged components from solution on the phase boundary, or by reactions of such components with some component of the solid electrolyte. For LaFa this could be the reaction of hydroxyl ions with the trivalent lanthanum ion. Characteristically, for the phase boundary between two immiscible electrolyte solutions, where neither solution contains an amphiphilic ion, the electric double layer consists of two diffuse electric double layers, with no compact double layer at the actual phase boundary, in contrast to the metal electrode/ electrolyte solution boundary [4,8, 35] (see fig. 2.1). Then, for the potential... 

Selected entries from Methods in Enzymology [vol, page(s)] Activation of lipolytic enzymes by interfaces, 64, 341 model for lipase action on insoluble lipids, 64, 345 interfacial enzyme inactivation, 64, 347 reversibility of the adsorption step, 64, 347 monolayer substrates, 64, 349 kinetic models applicable to partly soluble amphiphilic lipids, 64, 353 surface dilution model, 64, 355 and 364 practical aspects, 64, 357. 

Hetaeric chromatography, 230, 231 effect of charge on hetaeron, 233 retention model of, 231-238 Hetaeron. 191, 230, 231, 240, 243, 249, 280 see also Complexing agent adsorption on the stationary phase, 231, 249,230 amphiphilic, 243 cetrimide, 248 decylsulfonate, 230 dodecylbenzenesulfonate, 230 formation constant of complexes, 276 lauryl sulfate, 230 metal chelating, 262 micelle formation, 230 optically active, 262 surface concentration of, 232... 

If one adds an inorganic salt, such as NaCl, instead of detergent, then no foam is formed. Foam formation indicates that the surface-active agent adsorbs at the surface, and forms a TLF (consisting of two layers of amphiphile molecules and some water). This has led to many theoretical analyses of surfactant concentration (in the bulk phase) and surface tension (consequent on the presence of surfactant molecules at the surface). The thermodynamics of surface adsorption has been extensively described by the Gibbs adsorption theory (Chattoraj and Birdi, 1984). 

Decher G, Hong JD. Buildup of ultrathin multilayer films by a self-assembly process. 1. Consecutive adsorption of anionic and cationic bipolar amphiphiles on charged surfaces. Makromolekulare Chemie-Macromolecular Symposia 1991a 46 321-327. 

Subscript (ads) denotes adsorption via a thiolate linkage, while (ps) stands for a physisorbed and/or adsorbed state via different interactions. However, large dimensions of the studied molecules and their amphiphilic nature make the surface reaction mechanism more complex than in case of cystine/cysteine. Interfacial microstructure plays an important role in the determination of the surface behavior of the adsorbed molecules. From the study on the charge-transfer kinetics, the transfer coefficient a was calculated as slightly less than 0.50, while the rate constant (based on Laviron s derivations [105]) was of the order of 10 s k The same authors [106] have shown earlier that the adsorption rate constant of porcine pancreatic phospholipase A2 at mercury via one of its disulfide groups is of the order of 10 s h... 

---