Adhesive interactions, adsorption processes

The study of acid-base interaction is an important branch of interfacial science. These interactions are widely exploited in several practical applications such as adhesion and adsorption processes. Most of the current studies in this area are based on calorimetric studies or wetting measurements or peel test measurements. While these studies have been instrumental in the understanding of these interfacial interactions, to a certain extent the interpretation of the results of these studies has been largely empirical. The recent advances in the theory and experiments of contact mechanics could be potentially employed to better understand and measure the molecular level acid-base interactions. One of the following two experimental procedures could be utilized (1) Polymers with different levels of acidic and basic chemical constitution can be coated on to elastomeric caps, as described in Section 4.2.1, and the adhesion between these layers can be measured using the JKR technique and Eqs. 11 or 30 as appropriate. For example, poly(p-amino styrene) and poly(p-hydroxy carbonyl styrene) can be coated on to PDMS-ox, and be used as acidic and basic surfaces, respectively, to study the acid-base interactions. (2) Another approach is to graft acidic or basic macromers onto a weakly crosslinked polyisoprene or polybutadiene elastomeric networks, and use these elastomeric networks in the JKR studies as described in Section 4.2.1. 

Irrespective of the experiment to be done, sample preparation contains a number of necessary conditions. First, aggregation must be prevented if one wants to investigate structure and conformation of single molecules. Second, the adsorption process must be reversible, or at least, very slow in order to approach the equilibrium state and allow statistical analysis of the molecular assembly. Third, adhesion of the molecules to the substrate must be strong enough to sustain the mechanical and adhesive interactions with the tip. However, it should be relatively low to prevent the native structure from deformation. 

When molecules adsorb to a flat substrate, their conformation is modified due to the geometric confinement between the two interfaces and the direct interaction to the substrate. This state can be far from equilibrium if the adsorption process has been fast and irreversible. In this case, the molecules do not have time to sample the whole assembly of thermodynamic states and get trapped kinetically at contact sites. The reversibility is difficult to achieve because of the great size of the molecules and strong adhesion which might exceeds kBT by far. In order to approach an equilibrium state, the sample has to be pre-... 

The London dispersion forces are present and important in most adsorption processes and in adhesive interactions between dissimilar materials. The free energy of interaction per unit area between materials 1 and 2 in contact is where W 2 -s... 

Control of adhesion interaction by the addition to adhesives of surface-active substances (surfactants) is of great theoretical and practical interest. The particular effects of siu-factants lie in their ability to decrease the surface tension of the solution due to positive adsorption on the surface. Coating the surface of solid bodies and of liquids with the finest layer of a surfactant added to the system in very small quantities permits changes of the conditions of phase interaction and the progress of the physical-chemical processes. 

Bowen et al. [39] measured directly the adhesion (interaction) of cellobiose and cellulose with two polymeric UF membranes of similar MWCO, but of different materials. As probes, they used silica spheres (diameter 5-8 im) the surfaces of which were modified by static adsorption of cellobiose. They also used pure cellulose probes. Membrane ES 404 was made of poly(ether sulfone) alone, and EM 006 was made of a poly(ether sulfone)-polyacrylate blend, chosen specifically to increase the hydrophihc properties and decrease the fouling properties of the membrane. Study of ES 404 and EM 006 had shown that the interaction of cellobiose or of colloidal cellulose with the membranes was such that ES 404 always had the greater adhesion and greater fouling tendency. However, if the membrane was first fouled with cellobiose, the colloidal cellulose adhesion force was increased significantly, and the differences between the membranes diminished. Bowen et al. suggested that in the future, it would be possible to use the techniques developed to allow prior assessment of the fouUng propensity of process streams with different types of membranes. 

The increase in adhesive interaction with increasing contact time between particles and surface in air, by analogy with this sort of process in a liquid medium, is termed aging [89]. There may be several causes of aging an increase in contact area between particle and surface as a result of deformation or as a result of the influence of various contaminants adsorption processes and capillary condensation may take place in the contact zone, so that capillary forces are created. 

In summary, we may note that in evaluating the influence of temperature, adsorption processes, and vacuum on adhesive interaction, it is necessary to take into account the properties of the contiguous bodies, the conditions under which each particular factor is acting, and the methods used to evaluate these factors. Depending on the entire set of external conditions, adhesive interaction may change in different directions. 

Adsorption - adhesive interactions of polymer with a siuface of a filler limiting mobility of its kinetic fragments in a boundary layer results in increase of activation energy of relaxation process in this area and broadening of the spectrum of times of a structural relaxation [7]. 

The technique of using pre-formed PECs to enhance the adhesive interactions between fibres can be divided into three sub-processes (schematically illustrated in Fig. 8) formation of the complexes adsorption of the complexes onto the fibre surfaces and the performance of the PECs as a part of the fibre-fibre joint. The first... 

Proteins such as antibodies and enzymes can be deliberately anchored on microfiuidic device surfaces by covalent bonds or molecular recognition in order to fabricate aiTay biosensors. Nonspecific adsorption is the (usually) undesirable adsorption of molecules on the surface, and it ends up with loss of analyte. In nonspecific adsorption, the molecule-surface interactions are initially weaker, so the adsorption process is both slower and more difficult to control. Subsequent denaturation leads to stronger adhesion [7], as well as to changes in the surface hydrophilicity and roughness [8]. It is important to understand the mech-... 

The thin-film approach has been shown to be very informative when dilute solutions of commercial polymer formulations are applied to metallic substrates, as the segregation of minor components can be observed readily [42,80]. The adsorption of amine molecules, as analogs of an epoxy adhesive, has been studied by XPS and SIMS [125-128] and the complementary nature of the two techniques demonstrated once again. The existence of a strong donor-acceptor interaction with anodized aluminum was postulated in the later work, and the integrity of the adsorption process appeared to be related to the existence and density of BrOnsted sites on the AI2O3 surface [127]. 

Due to the development of advanced numerical methods in the last decades, quantum approaches are now able to accurately describe the chemical bonds formed between two reactants. Nevertheless, when a surface is involved, the actual systems met in practice, for example a dense polymeric layer adsorbed on a rough surface, cannot yet be simulated, because this would require too large a memory size or too long a computation time. Quantum calculations, thus, cannot compete with empirical models in the prediction of adhesion strengths. However, they may allow one to check their validity in model cases, for example small molecules adsorbed on a substrate, or large molecules adsorbed on a cluster of a few atoms which simulates the substrate. This has been done in a number of cases but, to the author s knowledge, mostly for adsorption processes on metallic surfaces. Numerical results for the adsorption of molecules on oxide surfaces may be found in the literature (Henrich and Cox, 1994), but there exists no systematic discussion in the framework of acid-base interactions. 

It should be emphasized that under these conditions the surface structure of Si(lOO) is substantially different from oxidized Si(lOO) which is polar and in effect hydrophilic [359]. This is nicely demonstrated in Fig, 14.7, where for SI and S3 the cPAC values are shown for bare and oxidized Si(lOO) samples. These data were obtained from AFM experiments of the peptides SI and S3 interacting with Si(lOO) and GaAs(lOO) substrates in solution [340]. The main result is that the binding of SI to oxidized GaAs(lOO) and Si(lOO) surfaces is virtually independent of the substrate type which is widely screened by the top oxygen layer. The different adhesion propensities to the bare (hydrated) substrates lead to the conclusion that oxidation does not strongly progress on timescales of the peptide adsorption process. 

Relaxation studies have shown that the attachment of an ion to a surface is very fast, but the establishment of equilibrium in wel1-dispersed suspensions of colloidal particles is much slower. Adsorption of cations by hydrous oxides may approach equilibrium within a matter of minutes in some systems (39-40). However, cation and anion sorption processes often exhibit a rapid initial stage of adsorption that is followed by a much slower rate of uptake (24,41-43). Several studies of short-term isotopic exchange of phosphate ions between aqueous solutions and oxide surfaces have demonstrated that the kinetics of phosphate desorption are very slow (43-45). Numerous hypotheses have been suggested for this slow attainment of equilibrium including 1) the formation of binuclear complexes on the surface (44) 2) dynamic particle-particle interactions in which an adsorbing ion enhances contact adhesion between particles (43,45-46) 3) diffusion of ions into adsorbents (47) and 4) surface precipitation (48-50). 

The adsorption theory states that the bioadhesive bond formed between an adhesive substrate and tissue or mucosae is due to van der Waals interactions, hydrogen bonds, and related forces. Alternatively, when mucus or saliva are interacting with a solid dosage form, the molecules of the liquid are adsorbed on the solid surface. This is an exothermic process. The free energy of adsorption is given by Eq. (1). 

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