Addition Of Non-adsorbing Polymer Research Articles

Effects of polymer nonideality on depletion-induced phase behaviour of colloidal disks and rods

Colloidal dispersions composed of either platelets or rods exhibit liquid crystalline phase behaviour that is strongly influenced by the addition of nonadsorbing polymers. In this work we examined how polymer segment–segment interactions affect this phase behaviour as compared to using either penetrable hard spheres (PHS) or ideal (‘ghost’) chains as depletants. We find that the simplified polymer description predicts the same phase diagram topologies as the more involved polymer descriptions. Therefore the PHS description is still adequate for qualitative predictions. For sufficiently large polymer sizes we find however that the precise polymer description significantly alters the locations of the phase coexistence regions. Especially the stability region of isotropic–isotropic coexistence is affected by the polymer interactions. To illustrate the quantitative effects some examples are presented.

Polymer-induced recovery of nanoparticles from microemulsions

A new isothermal approach to the recovery of inorganic nanoparticles (NPs) is demonstrated. The NPs can be incorporated into a background microemulsion (ME) supporting fluid, and they can be recovered by addition of non-adsorbing polymer. A clean liquid-liquid (L-L) phase transition can be readily induced by addition of polymer to the MEs. Furthermore, the L-L transitions are also observed in the presence of added NPs, but now the nanoparticles concentrate in the lower co-existing ME phases. Once recovered, the NPs can be redispersed by adding extra ME as a solvent.

Direct Measurement of Double-Layer, van der Waals, and Polymer Depletion Attraction Forces between Supported Cationic Bilayers

The interactions of supported cationic surfactant bilayers and the effects of nonadsorbing cationic polyelectrolytes on these interactions were studied using the surface forces apparatus (SFA) technique. Bilayers of the cationic surfactant di(tallow ethyl ester) dimethyl ammonium chloride (DEEDMAC) were deposited on mica surfaces using the Langmuir-Blodgett technique, and the interactions between the bilayers were measured in various salt, nonionic polymer (PEG), and cationic polyelectrolyte solutions at different polymer molecular weights and concentrations. The forces between the bilayers in CaCl(2) solution are purely repulsive and follow the DLVO theory quantitatively down to bilayer separations of ∼2 nm. Addition of nonadsorbing polymer or polyelectrolyte has a number of effects on the interactions including the induction of a depletion-attraction between the bilayers and screening of the double-layer repulsion due to the added ions in the solution from the polyelectrolyte. The experimental results are shown to agree well with standard theories of depletion attraction and double-layer screening associated with dissolved polyelectrolyte. We also observed significant time and rate effects on measuring the equilibrium bilayer-bilayer interactions possibly due to the unexpectedly long times (>1 min) associated with the charge regulation of the bilayer surfaces. Implications for the interactions and stability of vesicle dispersions, i.e., of free rather than supported bilayers, in polymer solutions are discussed.

Polymer-Induced Membrane Contraction, Phase Separation, and Fusion via Marangoni Flow

Experiments have shown that the depletion of polymer in the region between two apposed (contacting or nearly contacting) bilayer membranes leads to fusion. In this paper we show theoretically that the addition of nonadsorbing polymer in solution can promote lateral contraction and phase separation of the lipids in the outer monolayers of the membranes exposed to the polymer solution, i.e., outside the contact zone. This initial phase coexistence of higher- and lower-density lipid domains in the outer monolayer results in surface tension gradients in the outer monolayer. Initially, the inner layer lipids are not exposed to the polymer solution and remain in their original “unstressed” state. The differential stresses on the bilayers give rise to a Marangoni flow of lipid from the outer monolayers in the “contact zone” (where there is little polymer and hence a uniform phase) to the outer monolayers in the “reservoir” (where initially the surface tension gradients are large due to the polymer-induced phase separation). As a result, the low-density domains of the outer monolayers in the contact zone expose their hydrophobic chains, and those of the inner monolayers, to the solvent and to each other across the narrow water gap, allowing fusion to occur via a hydrophobic interaction. More generally, this type of mechanism suggests that fusion and other intermembrane interactions may be triggered by Marangoni flows induced by surface tension gradients that provide “action at a distance” far from the fusion or interaction zone.

Phase behavior of a mixture of platelike colloids and nonadsorbing polymer

Suspensions of sterically stabilized colloidal gibbsite platelets have recently been found to exhibit both an isotropic-nematic (I-N) and a nematic-columnar (N-C) phase transition. In the present paper we show that depletion attraction, brought about by the addition of nonadsorbing polymer, enriches the phase behavior of these platelet suspensions even further. Pronounced broadening of the isotropic-nematic gap occurs, with purely nematic samples re-entering the biphasic state by the addition of nonadsorbing polymer. At the same time, depletion attraction enhances size fractionation between coexisting phases, which actually provides an effective means for reducing the polydispersity of the suspensions. An additional isotropic phase emerges, which leads to the appearance of several three-phase equilibria and even a four-phase equilibrium. We explain the observed topology of the phase diagram by the interplay between depletion attraction and the platelets' polydispersity.

Tunable Attractions Directing Nonequilibrium States in Dispersions of Hard Rods

The effect of attractive interactions on the phase behavior of colloidal hard rods has been studied by the addition of nonadsorbing polymer. In these rod−polymer mixtures, four pathways of phase separation can be distinguished: nucleation and growth, spinodal decomposition, aggregation, and gelation. The first three mechanisms give rise to macroscopic phase separation, whereas for the latter mechanism this is not necessarily the case. The observed phenomena are explained on the basis of an interplay between the position of both the spinodal line and the glass line, the overall composition of the sample, and the range of the potential.

Numerical study of the phase behavior of rodlike colloids with attractive interactions

We examine the influence of attractive interactions on the phase behavior of rodlike colloids. We model the rodlike particles by spherocylinders, for which the phase diagram, in the absence of attraction, is known for arbitrary aspect ratio. We consider the case that the attraction is due to depletion forces caused by the addition of nonadsorbing polymer. The range of this attraction is determined by the size of the polymer. If the radius of gyration of the polymer is small compared to the diameter of the rods, we can model the polymer-induced attraction by a suitable generalization of the square-well model for spherical particles. However, for longer ranged attractions, pairwise additive attractions lead to phase behavior that is very different from what is found when the nonadditivity of depletion forces is taken into account. In our simulations, we find evidence for demixing transitions in the isotropic, nematic and solid phases. We compare our simulation results with predictions based on the perturbation theory of Lekkerkerker and Stroobants [Nuovo Cimento D 16, 949 (1994)]. A crucial input in this theory is the so-called free-volume fraction of the hard spherocylinder reference system. In the work of Lekkerkerker and Stroobants, this quantity is estimated using scaled-particle theory. We test the validity of this approach by comparing it to numerical results for the free-volume fraction.

Interactions, phase transitions and metastable states in concentrated colloidal dispersions

We describe three situations in which strong, short-ranged attractive interactions between colloidal particles arise. These attractions can be induced by the quality of the solvent, addition of non-adsorbing polymer or addition of small colloidal particles. In all three cases phase transitions arise. On further increasing the strength of the attractions, the attainment of thermodynamic equilibrium is severely slowed down, or even suppressed.

Rheology of weakly interacting colloidal particles at high concentration

The paper describes an experimental study of the rheology of concentrated latex under conditions where a secondary miminum in the interparticle pair potential is expected. Particular attention is given to the Bingham stress σB derived from steady-shear-flow data, which is taken as a measure of the additional or extra stress arising from the interparticle attraction. The data are used to deduce a correlation describing the dependence of the Bingham stress on the strength of attraction, S, the latter being taken a proportional to the calculated depth of the secondary minimum, with the result σB=kf(ϕ)S1.9d–2, where k is a constant and d is the particle diameter. A weak attraction between colloidal particles can also be induced by the addition of non-adsorbing polymer. Data for systems of this type are analysed similarly and shown to support this dependence. Earlier data illustrating the effect of particle size and concentration on the storage modulus are also discussed. The dependence upon volume fraction is shown to be very similar to that of the Bingham stress and the variation of both properties is described well by an equation deduced from liquid-state theory on the assumption that the attractive forces do not greatly perturb the structure, this reads G∞, σB∝(4ϕ2+ 2ϕ3–ϕ4)/(1 –ϕ)3. It is also shown, however, that for volume fractions ϕ > 0.2 or so this is virtually indistinguishable from the concentration dependence of the modulus observed for particulate gels formed by coagulation in the primary minimum. It would thus appear that the concentration dependence of the modulus is very insensitive to local structure and the strength of the attraction. In contrast, this is not the case for the particle-size dependence. The effect of attractive forces on the low-shear and high-shear viscosity coefficients is also described.

Phase separation in dispersions of weakly-interacting particles in solutions of non-adsorbing polymer

Phase separation may be induced in dispersions of hydrophobic silica particles in cyclohexane upon the addition of non-adsorbing polymer (polydimethylsiloxane). The origin of this effect is ascribed to the depletion interaction. Depending on the particle size and molecular weight of the polymer, the analogues of the molecular gas/solid, gas/liquid and liquid/solid co-existing phase regions may each be observed. It is demonstrated that using relatively simple, mean-field theories for both the fluid state (random phase approximation or adhesive sphere model), and also the solid state (cell model), all these transitions may be predicted. Although the quantitative agreement between experiment and theory is not exact, qualitatively all the observed trends in behaviour are accounted for.