This paper describes several possible interactions among the different types of organic and inorganic aquatic colloids, based on our present knowledge of their size, electric charge, and conformation. The physicochemical properties of the different groups of colloids are described. Emphasis is placed on the various types of organic components, including fulvic compounds. Subsequently, the role of each colloid class is discussed with respect to homoaggregation (aggregation within a given colloid class) and heteroaggregation (aggregation among different colloid types). On the basis of a synthesis of literature reports, microscopic observations of natural colloids, experimental results obtained with model systems, and numerical simulations, it is concluded that the formation of aggregates in aquatic systems can be understood by mainly considering the roles of three types of colloids:  (i) compact inorganic colloids; (ii) large, rigid biopolymers; and (iii) either the soil-derived fulvic compounds or their equivalent in pelagic waters, aquagenic refractory organic matter. In most natural aquatic systems, the small (few nanometers) fulvic compounds will stabilize the inorganic colloids whereas the rigid biopolymers (0.1−1 μm) will destabilize them. The concentration of stable colloids in a particular aquatic system will depend on the relative proportions of these three components.

The complexation between a charged polymer and an oppositely charged spherical particle is investigated using Monte Carlo simulations. Electrostatic interactions are described in the Debye–Hückel approximation. The influence of particle size and ionic concentration on the adsorption/desorption limit, interfacial structure of the adsorbed layer, amount of adsorbed polymer, and the overcharging issue is investigated. Attention is focused on polyelectrolyte adsorption on small spherical particles whose surface curvature effects are expected to limit the amount of adsorbed monomers, large particles that allow the polyelectrolyte to spread to the same extent as on a flat surface, and particles whose radius is close to the polyelectrolyte radius of gyration so that the chain can completely wrap around it. The formation of a polyelectrolyte/particle complex and the conformations of the adsorbed polyelectrolyte are found to result from two competing effects: the electrostatic repulsions between the chain monomers which force the polyelectrolyte to adopt extended conformations and limit the number of monomers which may be attached in particular to small particles, and the electrostatic attractive interactions between the particle and the monomers forcing the charged polymer to undergo structural transition and collapse at the particle surface. It is shown that adsorption is favored by increasing particle size and decreasing ionic concentration. Trains are favored at low ionic concentrations while loops (prior desorption) are favored more when increasing the ionic strength. Below a critical particle size, by decreasing the ionic strength, electrostatic repulsions between the adsorbed monomers force the polyelectrolyte to form protuding tails in solution, hence decreasing the amount of polyelectrolyte adsorption. By decreasing the particle size still further, the low ionic concentration regime is dominated by monomer–monomer repulsions; the polymer partially wraps around or becomes tangential to the particle and two tails extend in opposite directions. The complex may or may not exhibit charge inversion depending on the particle size and ionic concentration. We find that charge reversal increases with salt concentration and reaches a maximum when the polyelectrolyte is able to wrap around the particle completely.

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This work describes an improved version of the original OPLS-all atom (OPLS-AA) force field for carbohydrates (Damm et al., J Comp Chem 1997, 18, 1955). The improvement is achieved by applying additional scaling factors for the electrostatic interactions between 1,5- and 1,6-interactions. This new model is tested first for improving the conformational energetics of 1,2-ethanediol, the smallest polyol. With a 1,5-scaling factor of 1.25 the force field calculated relative energies are in excellent agreement with the ab initio-derived data. Applying the new 1,5-scaling makes it also necessary to use a 1,6-scaling factor for the interactions between the C4 and C6 atoms in hexopyranoses. After torsional parameter fitting, this improves the conformational energetics in comparison to the OPLS-AA force field. The set of hexopyranoses included in the torsional parameter derivation consists of the two anomers of D-glucose, D-mannose, and D-galactose, as well as of the methyl-pyranosides of D-glucose, D-mannose. Rotational profiles for the rotation of the exocyclic group and of different hydroxyl groups are also compared for the two force fields and at the ab initio level of theory. The new force field reduces the overly high barriers calculated using the OPLS-AA force field. This leads to better sampling, which was shown to produce more realistic conformational behavior for hexopyranoses in liquid simulation. From 10-ns molecular dynamics (MD) simulations of alpha-D-glucose and alpha-D-galactose the ratios for the three different conformations of the hydroxymethylene group and the average (3)J(H,H) coupling constants are derived and compared to experimental values. The results obtained for OPLS-AA-SEI force field are in good agreement with experiment whereas the properties derived for the OPLS-AA force field suffer from sampling problems. The undertaken investigations show that the newly derived OPLS-AA-SEI force field will allow simulating larger carbohydrates or polysaccharides with improved sampling of the hydroxyl groups.

We used Monte Carlo simulations to study the formation of complexes between a flexible, semiflexible, and rigid polyelectrolyte and an oppositely charged spherical particle. Polyelectrolyte adsorption on a small particle, whose surface curvature effect is expected to limit the amount of adsorbed monomers, was considered. We focused on the effects of the intrinsic polyelectrolyte rigidity and ionic concentration of the solution and investigated the adsorption/desorption limit and conformation of the adsorbed polyelectrolyte. Polyelectrolyte adsorption is controlled by several competing effects such as the electrostatic confinement energy of the chain due to the electrostatic repulsions between the charged monomers, polyelectrolyte intrinsic flexibility, and electrostatic attractive interaction between the polyelectrolyte monomers and the particle. On one hand, rigidity and electrostatic repulsions force the polyelectrolyte to adopt extended conformations and limit the number of monomers that may be attached to the particle. On the other hand, electrostatic attractive interactions between the particle and the polyelectrolyte monomers force the chain to undergo a structural transition and collapse at the particle surface. In particular, by increasing the intrinsic rigidity, we observed a transition from a disordered and strongly bound complex to a situation where the polymer touches the particle over a finite length, while passing by the formation of a solenoid conformation. We found that the critical ionic concentration at which adsorption/desorption is observed is rapidly decreasing with the polyelectrolyte intrinsic rigidity, and the amount of adsorbed monomers has a maximum value for semiflexible chains. Adsorption is thus promoted by decreasing the chain stiffness or decreasing the salt concentration for a given chain length.