Polymeric Surfactants: Novel Agents with Exceptional Properties

This article presents recent progress in the field of polymeric surfactants made of permanently amphiphilic block copolymers or of stimulus-sensitive ones. We highlight key points in the design of amphiphilic macromolecules, to yield polymer surfactants with tailor-made properties, as well as recently developed and still challenging application fields for this new class of surfactants. The efficiency boosting of amphiphilic block copolymers as co-surfactants in microemulsions is discussed, as are surface modification by polymer surfactants, and stabilization of dispersions. Moreover, the use of block copolymers in nanosciences is presented, for instance as a tool for nanomaterial fabrication, or for biomedical and cosmetic applications in bio-nanotechnology. Finally, self-assembly and applications of some newly developed “exotic” amphiphilic block copolymer structures as new surface-active materials will be highlighted.

Plastic and elastic properties of the systems interstratified clay–water–electrolyte–xanthan

The present study concerns the rheological behavior of the Jebel Shemsi clay dispersions (Shp). Shp is an interstratified illite/smectite clay from southern Tunisia. The influences of clay concentration, NaCl, and xanthan-a semirigid polymer-on the yield stress, the elastic modulus, and the xanthan adsorption were investigated. The sol-gel transition and the scale laws of rheometric properties are established. Progressive addition of NaCl to the clay dispersions decreases the thickness of the diffuse double layer, which makes the system rigid, increasing the yield and the elastic modulus. In the presence of xanthan, the negative surface charges become higher and the repulsive interparticle interactions increase; consequently the yield and the elastic modulus increase. The xanthan adsorption on the clay particle surface increases slightly with the NaCl concentration. The particle aggregation due to the salt and the particle dispersion due to the polymer are observed. The behavior of this interstratified clay is compared to that found for pure smectite. The 15% illite stratified with smectite in the Shp clay does not change the gels' rheological properties significantly. Meanwhile the amount of Shp clay needed to obtain a gel is more important than in the case of a pure smectite.

Scaling Behavior of the Rheological Properties of Montmorillonite Suspensions: Correlation between Interparticle Interaction and Degree of Flocculation

In this work we investigate some aspects of the rheological behavior of sodium montmorillonite (NaMt) suspensions in the pH range 3 to 9, of NaCl concentrations between 10(-3) and 10(-1) M, and of solid concentrations between 5 and 11% w/v. Three kinds of experiments were performed: steady-state viscometry, oscillatory test, and creep recovery. The physical quantities of interest were the yield stress sigma(y) of the suspensions, the elastic rigidity modulus G', and the instantaneous elastic compliance. Furthermore, G' was obtained from oscillatory tests in three different experiments: determination of the viscoelastic linear region, oscillograms, and the gelation process. All quantities were found to scale with the concentration of solids, C, according to a power law of the form Y=k(y)C(n). The exponents n were found to change from approximately 3 to approximately 6 when the pH was increased from 3 to 9 (at constant ionic strength 10(-2) M), although values as high as 10 were estimated when the NaCl concentration was reduced to 1 mM. Such values of n correlate well with the characteristics of the edge-to-face (E-F), edge-to-edge (E-E), and face-to-face (F-F) interparticle interactions. The minimum values of n correspond to maximum E-F attractions, whereas the largest n are associated with strong F-F repulsions. Copyright 2001 Academic Press.

Yield Stress of Concentrated Zirconia Suspensions: Correlation with Particle Interactions

The presence of a sufficient concentration of solid particles in a solution gives rise to a large increase in its viscosity and, more importantly, to significant deviations with respect to its original Newtonian behavior. Different rheological techniques are available to characterize such deviations, but the simplest one, obtention of steady-state rheograms, is already extremely useful with that purpose. In this work, this technique is applied to suspensions of zirconia particles, both synthesized with spherical geometry and commercial. The sigma(shear stress)-gamma;(shear rate) curves show that the suspensions are nonideal plastic, thus exhibiting a finite yield stress, sigma(0), and a shear-thinning flow. It is through sigma(0) that a connection can be established between steady-state rheological behavior and interaction energy between particles, since sigma(0) can be estimated as the maximum attractive force between particles multiplied by the number of bonds per unit area between a given particle and its neighbors. Having an experimental determination of sigma(0), the verification of its relation with the attractive forces requires estimation of the potential energy of interaction between any pair of particles. Two approaches will be considered: one is the classical DLVO model, in which the potential energy, V, is the sum of the van der Waals (V(LW)) and electrostatic (V(EL)) contributions. The second approach is the so-called extended DLVO theory, in which the acid-base interaction V(AB) (related to the hydrophilic repulsion or hydrophobic attraction between the particles) is considered in addition to V(LW) and V(EL). The three contributions can be calculated as a function of the interparticle distance if the particle-solution interface is characterized from both the electric and the thermodynamic points of view. The former is carried out by means of electrophoretic mobility measurements and the latter by contact angle determinations for three probe liquids on zirconia powder layers. Comparison between measured and calculated sigma(0) values was carried out for suspensions of spherical, monodisperse ZrO(2) particles, with volume fraction of solids, straight phi, ranging between 4.6 and 21.7%, in 10(-3) M NaCl solutions. In the case of commercial particles, the effects of both NaCl concentration (10(-5) to 10(-1) M) and volume fraction (3.5 to 21%) were investigated. It is found that the classical DLVO theory cannot be used to predict the yield stress when [NaCl]=10(-5) M, since the high zeta potentials and thick double layers never yield partial differential V/ partial differential R>0 (the interaction is repulsive for all distances) in such a case. A similar problem was encountered in 10(-1) M solutions, but now because V is always attractive, and no maximum force can be found. On the contrary, the extended DLVO model always yield physically reasonable sigma(0) values (coincident with those deduced from the classical approach when calculation is possible in the latter case). The comparison with experimental data shows that theory clearly underestimates sigma(0) by one order of magnitude or even more. The possible role of particle aggregation in this underestimation is discussed in terms of the scaling behavior of sigma(0) as a function of straight phi. Copyright 2000 Academic Press.

Rheological and Electrokinetic Properties of Sodium Montmorillonite Suspensions

In this article, we describe the rheology of Na montmorillonite suspensions as a function of pH, at constant ionic strength. The observed behavior is discussed quantitatively in terms of the potential energy of interaction between particles, keeping in mind the anisotropic nature of clay particles. The extended DLVO model that includes electrostatic, van der Waals, and polar acid-base contributions to the total energy is used. It is found that face-to-face interactions are virtually independent of pH, whereas edge-to-edge interactions are most attractive at the isoelectric point of edges (pH approximately 7). The most significant variations occur in face-to-edge potential energy, with strong attractions at pH<7. Steady-state viscometry showed that the yield stress decreases up to an order of magnitude between pH 3 and pH 7, with a much slower rate of decrease in the 7-11 pH interval. Concerning oscillatory measurements, it is found that both the elastic (G') and viscous (G") moduli are practically independent of frequency. It is also demonstrated that G'>G", the difference being larger at acid pH values. These results, in addition to potential energy calculations, suggest the existence of an elastic, coagulated structure up to pH 7, whereas as the pH is increased such structure is more relaxed because of electrostatic repulsions. Similar conclusions are reached when creep-recovery data are analyzed. Copyright 2000 Academic Press.