Colloid Stability Influences on the Biological Organization and Functions

It is common to entities having sizes in the nano/micro-scale range be that, real or bio-intended systems, to undergo the action of many different forces, imparting them colloid stability. Ubiquitary electrostatic contributions, sometimes dominant, may overlap with steric stabilization ones; their combination effectively takes place in most cases. The two effects are jointly responsible, for instance, for the control of many phenomena such as: adhesion onto cells of alien agents, cellular separation during morpho-functional evolution, uptake of exogenous materials into cells and tissues. We evidence here, how the combination of these forces operates, and indicate the procedures leading to their effectiveness, when required for purposes inherent to biomimicry

Surfactant Mixtures: Performances vs. Aggregation States

Surfactants, Mixtures, interaction

Stabilization of Food Colloids: The Role of Electrostatic and Steric Forces

The role that some forces exert on food colloid stability is discussed. The focus is on the combination of different energy terms, determining particle-particle attraction or repulsion. The forces are relevant in dispersion stabilization and macroscopic phase separation. The observed features depend on the energies at work and colloid concentration. Examples deal with food manipulations giving cheese, yogurt, and mayonnaise. All products result from the overlapping of forces jointly leading to aggregation or phase separation in foods. The combination of attractive, van der Waals (vdW), and repulsive, double-layer (DL) forces results in the dominance of aggregation or dispersion modes, depending on the particle concentration, on the force amplitude, and on their decay length. DL and vdW forces are at the basis of Derjaguin-Landau-Verwey-Overbeek (DLVO) theory on colloid stability. That approach is modified when these forces, jointly operating in bio-based colloids, overlap with steric stabilization and depletion modes. Steric effects can be strongly dispersive even at high ionic strength, despite this is rather counterintuitive, when depletion ones favor the nucleation in a single phase

Characterization and stability of catanionic vesicles formed by pseudo-tetraalkyl surfactant mixtures

The phase behavior of an ad hoc synthesized surfactant, sodium 8-hexadecylsulfate (8-SHS), and its mixtures with didecyldimethylammonium bromide (DiDAB) in water is reported. We dealt with dilute concentration regimes, at a total surfactant content of <30 mmol kg-1 where vesicular aggregates may be formed. The high synergistic behavior of such catanionic mixtures is concomitant with strongly negative interaction parameters, β (≈-18 kBT), significant gain in the free energy of association, ΔGagg, and much lower association concentration compared to the pure surfactants. Vesicle size and ζ-potential depend on the mixture composition. Hydrodynamic diameters increase by progressive addition of oppositely charged surfactants to the one in excess. Counter-intuitively, the ζ-potential becomes more negative at DiDAB molar fractions close to 0.2. The same holds in the reverse case, the ζ-potential becomes more positive after small additions of 8-SHS; anyhow, the effect is more significant in anionic-rich mixtures. This phenomenon was explained by assuming a significant release of counterions and an asymmetric distribution of the two surfactants in the inner and outer vesicle leaflets. The equimolar mixtures form a cubic phase rather than the expected lamellar one. The effect of NaBr concentration on the stability of catanionic vesicles was also investigated. At high NaBr concentrations, all systems are destabilized. For DiDAB-rich vesicles, flocculation is observed, while for 8-SHS-rich ones, lamellar domains are formed at the bottom of the samples. The role played by NaBr depends on whether it is added before or after mixing the surfactants. In particular, preformed catanionic vesicles show a great kinetic stability towards addition of NaBr compared to those obtained by other procedures.We thank Jaume Caelles, in the SAXS-WAXS service at IQAC, for X-Ray measurements; Imma Carrera for technical assistance in the synthesis and surface tension measurements. Financial support from MINECO CTQ2013-41514-P and MAT2012- 38047-CO-02 is gratefully acknowledged. Financial support from Generalitat de Catalunya 2014SGR836 is gratefully acknowledged. Financial support from “La Sapienza” University of Rome (IT) is acknowledged too.Peer reviewe

Multi- to Unilamellar Transitions in Catanionic Vesicles

n/

BINDING OF SURFACTANTS ONTO POLYMERS. A KINETIC MODEL

The kinetics of surfactant binding onto polymers is discussed by taking into account recent investigation performed in some water-polymer-surfactant systems. The links between the obsd. kinetic parameters, i.e. relaxation times, and the thermodn. quantities controlling polymer-surfactant interactions are considered. A model for the kinetics of surfactant binding onto polymers is presented and discussed

PHASE EQUILIBRIA IN A WATER-BLOCK COPOLYMER SYSTEM

Mixts. contg. water and poly(oxyethylene-b-oxypropylene-b-oxyethylene) were studied and the phase boundaries detd. The phase diagram shows similarities with non-ionic surfactant systems of the n-alkyl-polyoxyethylene glycol family, with occurrence of different lyotropic liq. cryst. phases and of upper consolute boundaries. Added sodium salts have a pronounced effect on the crit. soln. boundaries, which can be shifted upwards (downwards), depending on the counterion. A qual. explanation of the above effects is given in terms of adsorption and/or depletion of the electrolytes at the polar-apolar interface of the aggregates formed by block copolymers

Hybrid Colloids Made with Polymers

Polymers adsorb onto nanoparticles, NPs, by different mechanisms. Thus, they reduce coagulation, avoid undesired phase separation or clustering, and give rise to hybrid colloids. These find uses in many applications. In cases of noncovalent interactions, polymers adsorb onto nanoparticles, which protrude from their surface; the polymer in excess remains in the medium. In covalent mode, conversely, polymers form permanent links with functional groups facing outward from the NPs’ surface. Polymers in contact with the solvent minimize attractive interactions among the NPs. Many contributions stabilize such adducts: the NP–polymer, polymer–polymer, and polymer–solvent interaction modes are the most relevant. Changes in the degrees of freedom of surface-bound polymer portions control the stability of the adducts they form with NPs. Wrapped, free, and protruding polymer parts favor depletion and control the adducts’ properties if surface adsorption is undesired. The binding of surfactants onto NPs takes place too, but their stabilizing effect is much less effective than the one due to polymers. The underlying reason for this is that surfactants easily adsorb onto surfaces, but they desorb if the resulting adducts are not properly stabilized. Polymers interact with surfactants, both when the latter are in molecular or associated forms. The interactions occur between polymers and ionic surfactants or amphiphiles associated with vesicular entities. Hybrids obtained in these ways differ each from each other. The mechanisms governing hybrid formation are manifold and span from being purely electrostatic to other modes. The adducts that do form are quite diverse in their sizes, shapes, and features, and depend significantly on composition and mole ratios. Simple approaches clarify the interactions among different particle types that yield hybrids

Supra-molecular association in bile salts.

A review on the supramolecular association modes of bile salts in solution