Skip to main content

Adsorbed Layers of Natural Organic Macromolecules on Solid Substrates: Structure, Interactions, and Mechanisms of Growth

Final Report Summary - ADSORPLAYERS (Adsorbed Layers of Natural Organic Macromolecules on Solid Substrates: Structure, Interactions, and Mechanisms of Growth)

In the natural environment, surfaces and colloids are typically modified by adsorption of natural organic matter, which governs their fate and transport. In engineered processes, adsorption of polymers greatly influences colloidal stability, but it can also negatively affects the properties of surfaces: for example, it induces fouling in membrane-based water purification, thus impairing the process efficiency. Therefore, while better understanding of the mechanism of polymer adsorption onto surfaces is an important standalone scientific task, it also has direct impact on many environmental and engineering problems.
The project “Adsorbed Layers of Natural Organic Macromolecules on Solid Substrates: Structure, Interactions, and Mechanisms of Growth” studied the formation and structure of adsorption layers of macromolecules and polymers onto surfaces in aqueous solution. A variety of polymer-substrate systems was investigated under relevant environmental conditions, allowing elucidation of the physicochemical determinants governing the mechanism of adsorption, as well as its effect on the behavior of solid surfaces. The research briefly summarized here provides a coherent understanding of these phenomena and represents a consequential tool to control the behavior of aqueous environmental systems.
Adsorption of polyelectrolytes on an oppositely charged surface is a well studied process. We provided a systematic review of this system in “I. Szilagyi; G. Trefalt; A. Tiraferri; P. Maroni and M. Borkovec (2014) Polyelectrolyte adsorption, interparticle forces, and colloidal aggregation, Soft Matter, 10, 2479-2502”. Here, we underlined how this particular adsorption mechanism is fast, irreversible, and quantitative until saturation, due to electrostatic attraction. In this case, the adsorbed mass normally increases with increasing solution ionic strength. However, the adsorbed mass of weak polyelectrolytes goes through a maximum with salt concentration, and decreases in high salt.
We further investigated the case of weak polyelectrolytes, by focusing on chitosan and its interaction with silica. Chitosan is a natural cationic polysaccharide with a wide range of applications, for example, to produce effective drug delivery systems and to clean contaminated waters. In the article “A. Tiraferri; P. Maroni; D. Caro Rodriguez and M. Borkovec (2014) Mechanism of chitosan adsorption on silica from aqueous solutions, Langmuir, 30, 4980-4988”, we applied the same silica substrates in reflectometry and in quartz crystal microbalance experiments, to allow the univocal determination of the properties of the chitosan adsorption layer. In mildly acidic solutions, electrostatic attraction is the main driving force for adsorption, and chitosan forms rigid and thin monolayers. In neutral solutions, chitosan exists in aggregates, thus forming thick adlayers that are also viscoelastic and contain a large amount of water. The possibility to produce tailored adsorption layers simply by adjusting the solution pH, along with the availability and compatibility of chitosan, represents a remarkable opportunity to use this polyelectrolyte in a variety of systems.
While the mechanisms underlying layer formation in oppositely charged systems is well understood, limited information is available about adsorption in the absence of electrostatic attraction. Therefore, we took advantage of the flexibility of self-assembled monolayers bearing different functionalities, to systematically investigate the role of surface chemistry and, in particular, that of charge type and hydrophobicity. We also employed polymers with different charge properties to understand the role of polymer charge on adsorbed amounts. The combination of all these systems allowed an understanding of the relative importance of various polymer-surface and polymer-polymer interactions driving adsorption. In “P. Maroni; F. J. Montes Ruiz-Cabello and A. Tiraferri (2014) Studying the role of surface chemistry on polyelectrolyte adsorption using gold-thiol self-assembled monolayer with optical reflectivity, Soft Matter, 10, 9220-9225”, we demonstrated the potential of this effective system in an optical reflectivity setup. Subsequently, we provided a comprehensive discussion of the results obtained with all the polymer-surface combinations in “P. Maroni; F. J. Montes Ruiz-Cabello; C. Cardoso and A. Tiraferri (2015) Adsorbed Mass of Polymers on Self-Assembled Monolayers: Effect of Surface Chemistry and Polymer Charge, submitted to Langmuir”. Results suggested that adsorption of polymers is the result of a balance between chain-surface and chain-chain interactions. For polyelectrolytes with a high density of ionized charges, electrostatic forces are dominant. In the presence of electrostatic attraction, the adsorbed mass and the kinetics are the largest. On the other hand, if polyelectrolytes adsorb on substrates with the same sign of potential, the kinetic barrier to deposition can be reduced by higher ionic strengths, thus increasing the rate and amount of adsorption significantly. Surface hydrophobicity also promotes adsorption. When electrostatic interactions are weak, e.g. for small densities of charged groups, low degree of ionization, or in high salt, forces of non-electrostatic nature become important. The irreversibility of adsorption observed in all cases and the behavior of an uncharged polymer confirmed that these interactions are operative at short range and are responsible to drive polymer adsorption even in some cases whereby a low-energy electrostatic barrier is present.
We applied all this knowledge in “A. Tiraferri and M. Borkovec (2015) Probing effects of polymer adsorption in colloidal particle suspensions by light scattering as relevant for the aquatic environment: An overview, Science of the Total Environment (in press)”. In this paper, we discussed the usefulness of light scattering methods to probe adsorbed layers of polymers on particle surfaces, as well as to study the resulting properties of the particle suspensions. We elucidated the behavior of two environmentally relevant processes, namely, the modification of particles by natural adsorption of humic substances and the design of iron-based nanoparticles to remediate contaminated aquifers. In particular, we highlighted how the behavior of these two environmental systems is intimately linked to the fundamental mechanisms described above and we drew a systematic picture of how the adlayer properties affects interparticle interactions and colloidal stability.
Finally, we investigated an additional mechanism of adsorption of polyelectrolytes on like-charged surfaces in “A. Tiraferri; P. Maroni and M. Borkovec (2015) Adsorption of polyelectrolytes to like-charged substrates induced by multivalent counterions as exemplified by poly(styrene sulfonate) and silica, submitted to Physical Chemistry Chemical Physics”. While adsorption in the presence of monovalent ions is negligible, adsorbed amounts are substantial with multivalent counterions. We discussed the case of negative poly(styrene sulfonate) on negative silica, evidencing the formation of an initially electrically neutral complex between the anionic polyelectrolyte chain and multivalent cations, and its subsequent charge reversal at increasing cation concentration. When the complex is neutral, the adsorbed layer forms slowly, but continues to grow due to ripening and multilayer formation. At higher ionic levels, the adsorption saturates in a monolayer resembling systems characterized by polymer-surface electrostatic attraction.
Substantial impact of this work is expected in basic sciences as well as in applied disciplines. Alongside with advancing the understanding of interactions between macromolecules and surfaces, the acquired knowledge can be directly applied to engineering systems, for example to prevent fouling in membrane-based water and wastewater treatment, thus increasing the efficiency of technologies that promise to increase the availability of safe water.