Danièle Clausse

DSC and PVT measurements

The dissociation of gas and model hydrates was studied using a classical thermodynamic method and a calorimetric method, in various aqueous media including pure water, high concentration calcium chloride solutions and water-in-oil emulsions. Methane hydrate dissociation temperatures vs. pressure curves were determined using pressure vs. temperature measurements in a constant volume cell (PVT), and high pressure differential scanning calorimetry (DSC), at 5 to 10 MPa gas pressure and at temperatures ranging from -10 to +12°C. PVT and DSC results are in good agreement, and concordant with data available in literature. From a thermodynamic point of view, there are no measurable differences between bulk solutions and emulsions. From a kinetic point of view, due to the considerable surface of interface between the two phases, emulsions allow the formation of much greater amounts of hydrate than solutions, without any agitation. Model hydrate of trichlorofluoromethane was studied in 9 to 27 mass% calcium chloride solutions in emulsion in oil, using DSC under atmospheric pressure, at temperatures ranging from -20 to +5°C. A diagram of dissociation temperature vs. salt concentration is proposed.

THERMAL BEHAVIOUR OF EMULSIONS STUDIED BY DIFFERENTIAL SCANNING CALORIMETRY

This article is a review about the ways in which solidification and the melting may occur within emulsions submitted to steady cooling and heating performed in a differential scanning calorimeter. Simple, multiple and mixed emulsions are considered. Due to nucleation phenomena creating supercooled and supersaturated liquids, the DSC curves obtained during cooling and heating are quite different. The influence of a solute in the disperse phase is described in detail. Some implications about the instabilities of emulsions due to mass transfer phenomena are described.

Stability of W/O and W/O/W emulsions as a result of partial solidification

Abstract This communication reviews the drastic modifications that may occur within emulsions when they are submitted to temperature variations during manufacturing, storage, transport and use. Emphasis will be given to the modifications resulting from changes after solidification. Pathways of solidification of W/O and W/O/W emulsions will be presented. Due to the specific structure of the emulsions under study (simple or multiple) only partial solidification may be obtained. This situation, out of thermodynamic equilibrium, may lead to a water transfer across the oil phase. Resulting emulsion transformation and destabilization are examined.

Water transfer in mixed water-in-oil emulsions studied by differential scanning calorimetry

Abstract Water transfers are observed within complex systems containing aqueous phases separated by a membrane or an oil phase, such as biological cells or multiple emulsions. In order to better understand water transfer mechanism, a system made of a mixed water-in-oil (W/O) emulsion containing two kinds of aqueous droplets — pure water and a 30 % urea solution — was developed. Water transfer from pure water droplets to urea solution droplets was evidenced by Differential Scanning Calorimetry (DSC). Finally the mixed emulsion contains one kind of droplets made of a diluted urea solution which composition is in agreement with formulation and data obtained from experiments performed on single W/O emusions which dispersed phase is a diluted urea solution of the same composition. These mixed emulsions have been pictured as a three-fluid phases system containing two aqueous phases separated by a plane oil membrane. From a homogeneous solubility-diffusion model applied to a quasi-stationnary regime, the water intra-diffusion coefficient has been obtained and compared to the value calculated from the Stokes-Einstein equation. A factor ten makes the discrepancy between the two values, the value deduced from the model being the highest. A possible influence of the emulsifier molecules has been evoked.

The stability of methane hydrates in highly concentrated electrolyte solutions by differential scanning calorimetry and theoretical computation

Abstract The stability limits of methane hydrates have been investigated at pressures from 5 to 12 MPa by high-pressure differential scanning calorimetry, in sodium chloride and calcium chloride solutions, at concentrations ranging from pure water to saturated salt, in continuous solutions, in water-in-oil emulsions, as well as in complex dispersed media used as drilling fluids. Experimental results are in good agreement with available data, and concord well with predictions computed using the model of Van der Waals and Platteeuw (1959). DSC experiments revealed eutectic melting of solid mixtures of gas hydrate and crystallized salt. Corresponding invariant temperatures of melting and phase compositions were computed for various gas pressures. Complete phase diagrams are proposed for the systems (methane + water + sodium chloride) and (methane + water + calcium chloride) at 2 MPa and 10 MPa methane pressure.

Ripening Phenomena in Emulsions — A Calorimetry Investigation

ABSTRACT Ripening phenomena occurring within different kinds of emulsions have been studied. The emulsions concerned are simple water-in-oil (W/O) or oil-in-water (O/W) emulsions, mixed emulsions obtained by the mixture of two simple emulsions, and multiple emulsions water-in-oil-in-water (W/O/W) or oil-in-water-in-oil (O/W/O) emulsions. Composition ripening due to a mass transfer and solid ripening due to the formation of solid particles from the undercooled droplets or due to the formation of solid hydrate around the droplets have been pointed out on using a suitable calorimetric technique. For that purpose a non-diluted emulsion sample is submitted to a cooling and heating cycle during which solidification and melting temperatures and energies of the different phases are analyzed. It has been shown that correlations between these quantities and the properties of the dispersed phase permit one to get information about the ripening phenomena under study. The solution-diffusion model used for mass transfer is in good agreement with the experimental results. From the shell model used for the hydrate formation, it has been possible to deduce the formation energy and the influence of salt upon the temperature of formation.

WATER TRANSFER BETWEEN WATER AND WATER+NaCl DROPLETS IN EMULSIONS

Abstract Water transfer between water droplets and water + NaCl droplets dispersed within emulsions have been pointed out by differential scanning calorimetry. Using a solution- diffusion model, water fluxes have been calculated.

Composition Ripening in Mixed Water-in-Oil Emulsions Stabilized with Solid Particles

Water and oil transport in emulsified systems is far from being elucidated. Calorimetric analysis has proved to be an appropriate technique to study composition ripening in mixed water in oil emulsions. In this article, the role of the stabilizing agent is studied and particular attention is given to emulsions stabilized solely with solid particles. Mixed emulsions are prepared by mixing two simple water-in-oil (W/O) emulsions, one with pure water droplets and one with droplets containing an aqueous urea solution. At different time intervals, a sample is introduced in a calorimeter cell and submitted to successive cooling and heating cycles. During the cooling phase, the aqueous internal phase solidifies at a temperature which depends on its composition. Just after the mixed emulsion was prepared, the calorimetric experiment identified two solidification peaks, one corresponding to pure water droplets, and the other one to urea solutions. After a long enough stabilization time, just one peak was observed, showing that the systems evolved toward one type of droplets characterized by a unique composition, due to water transfer between the two aqueous phases. The effect of emulsion stabilizing agent (particles or nonionic emulsifier) on the kinetics of water transfer was investigated.

Determination of Cellulose Surface Energy by Imbibition Experiments in Relation to Bacterial Adhesion

Abstract Bacterial adhesion is an important initiating step of microbial contamination in the paper and board industry. The adhesion process results from interactions between the cell surface, the liquid, and the fibers. In this context, we determined the cellulose pulp surface energy to better understand these interactions. As contact angles cannot be directly measured on cellulose fibers, experiments involving liquid imbibition into cellulose pulp sheets were performed. The results were interpreted in terms of Lifshitz–van der Waals (γs LW), electron‐donor (γs −), and electron‐acceptor (γs +) components of the surface energy. Results evidenced that cellulose fiber surface is characterized by significant electron‐donor capacity, as is the cell surface of the bacteria. These results suggests that hydrophobic, non‐specific type of interactions between the cell surface and cellulose are involved in this case.

Stability of W/O Emulsions Encapsulating Polysaccharides

In the frame of formulation of W/O emulsions entrapping polysaccharides devoted to agricultural applications, the aim of this work was to study the stability over time of these emulsions, stabilized with either soybean lecithin or polyglycerol polyricinoleate (PGPR) as emulsifiers. Emulsifiers were dissolved in oil phase, and polysaccharides (carboxymethycellulose (CMC), guar, xanthan) in ultrapure water. Emulsions stability was studied through natural aging tests and accelerated aging tests, using bottle tests, microscopy and calorimetry. Experiments showed that PGPR was more efficient than lecithin to stabilize emulsions containing the polysaccharides studied, and that emulsions prepared with CMC showed the best stability.