M. S. Romero-Cano

Nanoemulsion stability: experimental evaluation of the flocculation rate from turbidity measurements

The coalescence of liquid drops induces a higher level of complexity compared to the classical studies about the aggregation of solid spheres. Yet, it is commonly believed that most findings on solid dispersions are directly applicable to liquid mixtures. Here, the state of the art in the evaluation of the flocculation rate of these two systems is reviewed. Special emphasis is made on the differences between suspensions and emulsions. In the case of suspensions, the stability ratio is commonly evaluated from the initial slope of the absorbance as a function of time under diffusive and reactive conditions. Puertas and de las Nieves (1997) developed a theoretical approach that allows the determination of the flocculation rate from the variation of the turbidity of a sample as a function of time. Here, suitable modifications of the experimental procedure and the referred theoretical approach are implemented in order to calculate the values of the stability ratio and the flocculation rate corresponding to a dodecane-in-water nanoemulsion stabilized with sodium dodecyl sulfate. Four analytical expressions of the turbidity are tested, basically differing in the optical cross section of the aggregates formed. The first two models consider the processes of: a) aggregation (as described by Smoluchowski) and b) the instantaneous coalescence upon flocculation. The other two models account for the simultaneous occurrence of flocculation and coalescence. The latter reproduce the temporal variation of the turbidity in all cases studied (380≤[NaCl]≤600 mM), providing a method of appraisal of the flocculation rate in nanoemulsions.

Colloidal aggregation under steric interactions: Simulation and experiments

The influence of steric interactions in the initial stages of aggregation kinetics in colloidal systems has been studied by simulation and experiments. A simulation model has been proposed to study the initial stages of aggregation under short range repulsive potentials. A polystyrene latex was used as model colloid and the steric interaction was provided by adsorption of a nonionic surfactant. Depending on the range or strength of the interactions, sensitive or insensitive systems to electrolyte concentration can be observed. At low κa, the long-range electrostatic repulsion dominates the system behavior, stabilizing the colloidal system. In conditions of screened electrostatic potential, particle collision is the result of a competition between van der Waals attraction and steric repulsion, leading to a decrease in the dimer formation constant as the range or strength of the steric interaction increases. The steric interaction energy has been included in the theoretical calculation of the aggregation ra...

Electrophoretic mobility and swelling behaviour of 2-acrylamido-2-methylpropane sulphonic acid /poly(N-isopropylacrylamide) microgel particles(N-isopropylacrylamide) microgel particles

A microgel system has been characterized by measuring the size (by photon correlation spectroscopy) and the electrophoretic mobility in different conditions. The microgel particles are poly(N-iso-propylacrylamide) cross-linked with N,N’-methylene bisacrylamide, and the anionic charge of this system is increased by copolymerization with 2-acrylamido-2-methylpropane sulphonic acid. Both the electrophoretic mobility and the hydrodynamic diameter of the particles were measured as a function of pH and electrolyte concentration. Although a weak charge was detected by titration, no trend was observed for the size or mobility against pH variations. In order to apply Ohshi-ma’s electrophoretic theories for soft particles to fit the mobility data, the swelling behaviour of the particles was taken into account.

Calculation of the stability ratio of suspensions using Emulsion Stability Simulations

According to Emulsion Stability Simulations (ESS), the flocculation of two non-deformable drops in the primary minimum of the interaction potential, necessarily leads to their coalescence. This property is used here for the evaluation of the stability ratio (W ) of solid particles, interacting with the same interparticle potential as the one of non-deformable droplets. Two different methodologies are used. The first one consists on the repeated evaluation of the coalescence time between two particles. The second one consists on the estimation of the time required for a decrease in the number of aggregates of the dispersion equal to n0/ 2( wheren0 is the initial number of aggregates). The results of the simulations are contrasted with the stability ratio of an anionic latex suspension subject to several ionic strengths (400–1000 mM). The first methodology is far more efficient for the evaluation of W although it misses the development of the aggregates and their growth. Absolute coagulation rates (k f ) can also be obtained using one N-particle simulation for the calculation of the fast flocculation rate (k fast f ) ,a nd several twoparticle simulations for the evaluation of W (k slow f = k fast f /W ). This combined procedure is also more efficient than the N-particle evaluation of k slow f .

Colloidal stability of a cationic latex covered with Triton X-100

The stability behaviour of a cationic latex after adsorption of Triton X-100 has been studied with respect to the surfactant coverage, with pH being an important variable due to the weak character of the surface groups. It is possible to find an adequate coverage of surfactant that causes high stabilization.

Zeta potential study of a polystyrene latex with variable surface charge : influence on the electroviscous coefficient

Electrophoretic mobilities of a carboxyl polystyrene latex have been measured against electrolyte (NaCl) concentration for different pH values. The aim of this study is to know how the changes in the surface charge density affect the electrokinetic behaviour of this latex and, particularly, the presence and position of the maximum in the mobility versus electrolyte concentration curves. Mobilities become greater with increasing surface charge and the maximum mobility becomes greater and is reached at higher electrolyte concentration. This behaviour is discussed by analysing the classical explanations (hairy layer, coion adsorption, surface conductivity) proposed. Zeta potentials have also been calculated using different mobility—zeta potential conversion theories and the results have been compared. These mobility-zeta potential conversion theories have also been tested with an experimental study that is different from the usual electrokinetic techniques: the primary electroviscous effect.

Phase behaviour of a model colloid–polymer mixture at low colloid concentration

The phase behaviour of a colloid-polymer mixture is studied at very low colloid concentrations (below 0.5%). The size ratio between the polymer and the colloidal particles is around 0.09, so that the colloids experience short-range attractions. At these low volume fractions, fluid-crystal coexistence is found at moderate polymer concentrations and the kinetics of crystallization are analyzed by turbidimetry. At higher polymer concentrations, clustering of the particles occurs, but some of them remain in a diluted, gas, phase composed mainly of single particles. These states do not correspond to vapor-liquid coexistence, as shown by studying the density of the gas phase. Strong interactions induce flocculation of the system, producing fractal aggregates with dimension df ≈ 1.8, compatible with diffusion limited cluster aggregation (DLCA). These results are discussed in connection with the phase diagram of colloid-polymer mixtures, obtained at much higher colloid concentrations. For low colloid volume fractions, below ∼0.05%, no clustering of the particles is observed for any polymer concentration.

Carboxylated core–shell particles: II. Experimental and theoretical comparison of salt-induced swelling

In a previous work [1] we showed that the swelling behavior of carboxylated core-shell particles (PS-PC) can be modified using a specific sample preparation route or favoring the hydrophobic attractive interaction by other way, i.e. controlling the temperature. In that paper, we found that the swelling was promoted in those particles which were initially in a highly swollen state (pH⩾10) while it was hindered for those particles which were not previously in this trigger pH. In this work, we present a discussion of the salt-induced swelling of the same carboxylated core-shell system (PS-PC) with two tuned swelling behaviors: the former, called A-2, exhibits promoted swelling while in the latter, called B-1, the swelling is greatly suppressed because of a compact conformation of the polymer shell is induced [1]. Good agreement between experimental, numerical and theoretical results at all pH values is obtained for promoted particles (A-2). On the other hand, the salt-induced swelling behaviors shown by hindered particles (B-1) corroborate that polymer restructuring includes assembly among ionic groups which affect their ionization degree and also the electrosteric interaction between particles. Finally, the salt-induced swelling behavior shown by the B-1 system at pH 8.6 resembles the Pincus regime predicted by scaling theory.

Carboxylated core-shell particles: I. A system showing hindered swelling behavior

We study the swelling behavior of carboxylated core-shell particles. It is well-known that these particles swell with increasing pH due to the electrostatic repulsion between carboxylate groups. Our results reveal that the swelling behavior is affected by the preparation method. We find that the swelling is promoted in those particles which were initially in a highly swollen state (pH>or=10). However, the swelling is hindered for those particles which were not previously in this trigger pH. In the hindered systems, a compact conformation of the polymer shell is induced by hydrophobic attractions between the non-charged segments which compete against the swelling driving force. In addition, an interesting hysteresis behavior emerges when promoted systems are subjected to a heating-cooling cycle; a new stable system appears with a less extended polymer shell conformation. Furthermore, salt-induced swelling experiments corroborate not only polymer restructuring but also assembly among carboxylate groups which affects their ionization grade.