Advances in Colloid and Interface Science (v.88, #1-2)

Cell model calculations for the electrophoretic mobility, electrical conductivity and sedimentation potential in concentrated suspensions of colloidal particles with low zeta potentials are reviewed with particular emphasis on an Onsager relation between sedimentation potential and electrophoretic mobility. A general Onsager relation is derived on the basis of the thermodynamics of irreversible processes. This relation, which involves the ratio of the electrical conductivity K* of the suspension to the conductivity K in the absence of the particles, reproduces the Onsager relation derived from cell model calculations at low zeta potentials, where K*/K becomes (1−φ)/(1+φ/2), φ being the particle volume fraction.
Keywords: Cell model; Electrokinetic phenomena; Concentrated suspensions; Onsager relation; Sedimentation potential; Electrophoretic mobility;

Characterisation by drop tensiometry and by ellipsometry of the adsorption layer formed at the air/champagne wine interface by N Péron; A Cagna; M Valade; R Marchal; A Maujean; B Robillard; V Aguié-Béghin; R Douillard (19-36).
A foam ring composed of small bubbles on the surface of a champagne glass is one of its hallmarks. The equilibrium state of that ring is linked with the rate of formation and of disappearance of bubbles. The stability of bubbles is usually ascribed to the occurrence and to the properties of an adsorption layer formed at the gas/liquid interface. Our goal is to characterise such an adsorption layer at the gas/wine interface in order to understand its role in bubble stability. Alcohol in wine lowers the surface tension to 49 mN/m. The adsorption of other molecules may cause a further decrease of 2 mN/m. Such a situation makes the study of adsorption by surface tension measurement inaccurate. To overcome this problem, we have diluted the wine four times with water before its surface tension measurement by pendant drop shape analysis. In these conditions, ethanol lowers the surface tension to 64 mN/m and the adsorption of other molecules of the wine can be monitored over 6–8 mN/m. The usual behaviour of such a diluted wine is a lowering of the surface tension during at least 20 min after drop formation. Since the role of macromolecules on the foaming properties of wine had been previously observed, we have chosen to evaluate the effect of this fraction of the wine molecules on its surface properties. Thus, wines were ultrafiltrated on a membrane with a 10 000 molecular mass cut-off. The ultrafiltrate (UF) does not show any decrease of its surface tension over a 20-min period while the ultraconcentrate (UC) has a kinetics similar to that of unfiltered wine. Mixtures of UF and UC have behaviours intermediate between those of these products. A technological treatment of the wine with bentonite, believed to lower the content of macromolecules, yields a wine similar to UF. The effect of ultrafiltration was also analysed by spectroscopic ellipsometry. UF has a spectrum similar to that of a water/alcohol mixture with the same ethanol content and its ellipticity is stable during at least 20 min. On the contrary, wine or UC show spectra with the features of an adsorption layer and those characteristics increase during more than 20 min. Two varieties of vine were compared: ‘Chardonnay’ and ‘Pinot noir’. The former is known to have better foaming properties than the latter. Its surface properties measured in this study are also more pronounced than those of Pinot noir. However, the representation of the dilational modulus against the surface pressure (which, in some instances, may be a mathematical transformation of the state equation) puts all the samples (wines, UF and UC of each) on the same master curve, a fact in favour of a common nature for all the adsorption layers. It can be concluded that surface properties of champagne wines are mostly determined by ethanol and by macromolecules with a molecular mass larger than 10 000. Moreover, the adsorption layers seem to have the same nature, irrespective of the vine variety and of the concentration ratio of the wine.
Keywords: Foaming behaviour; Air/champagne interface; Macromolecules; Pendant drop method; Ellipsometry;

The kinetics of growth of semiconductor nanocrystals in a hot amphiphile matrix by C.D Dushkin; S Saita; K Yoshie; Y Yamaguchi (37-78).
The first comprehensive study on the kinetics of nanocrystal growth in a hot amphiphile medium is presented. An example is given with CdSe semiconductor nanocrystals grown after the injection of precursor (a mixture of Cd- and Se-reagents) in concentrated tri-octylphosphine oxide matrix (heated to more than 300°C). The particle size distribution is reconstructed as a function of time from the absorption and photoluminescence spectra collected during the synthesis process. For this purpose a new expression is used relating the exciton energy due to quantum confinement with the nanocrystal radius. The growth kinetics is considered as a two-stage process in order to describe the time variation of nanoparticle size. During the first stage, called reaction-limited growth, the size of initial nucleus rapidly increases due to a sort of surface reaction exhausting the precursor in the nanoparticle vicinity. The growth in such conditions favors also a remarkable narrowing of the size distribution. The nanocrystal develops further on account of a slow precursor transfer from a distant space driven by the concentration gradient — classical diffusion-limited growth. The width of size distribution also increases proportional to the average particle size. Any growth will stop after the precursor concentration reaches a minimum value defining the limit for the final nanocrystal size in a batch. Solving the kinetic equations for the growth rate in each case of kinetics derives analytical expressions for the mean radius and variance of size distribution. Then the respective expressions are matched in a uniform solution valid during the entire synthesis. The theoretical model is in a good quantitative agreement with the experimental data for independent syntheses. Important characteristic scales of the processes (time-constant and length) and microscopic parameters of the reacting system (interfacial energy and reaction rate constant) are estimated from the data. It turns out that the fast reaction-limited growth is important to obtain well-defined nanocrystals of high optical quality by using less energy, time and consumable. However, to make them reproducibly uniform one should control also the ultra-fast nucleation process preceding the nanocrystal growth, which is still unknown. Nevertheless, our current findings allow the conceptual design of a new continuos-flow reactor for the manufacturing of a large amount of uniform nanocrystals.
Keywords: Nanocrystal growth; Semiconductor nanocrystal; Quantum confinement; CdSe; Tri-octylphosphine oxide;

The thermal properties of the dispersion of sodium salt of dimyristoylphosphatidylglycerol (NaDMPG) in water have been investigated as functions of incubation temperature and aging time by DSC, XRD, sodium ion activity, pH, ζ-potential, and IR measurements. The DSC charts for NaDMPG dispersions incubated below 30°C showed an endothermic peak at 31.7°C with a small shoulder peak at T m (gel–liquid crystal transition temperature: 23.5°C). The temperature of 31.7°C coincides with the T* temperature at which a high-order transition in the NaDMPG bilayer assembly has been found to occur in our previous studies. However, no peak was observed for the dispersions incubated above 32°C. These results indicate that thermal properties of NaDMPG bilayers definitely differ below and above the T* temperature. The dispersion which had been once incubated at 40°C for 24 h never showed the endothermic peak at T* even after the further aging at 3°C for 12-day. Namely, the NaDMPG bilayer assembly exhibits an intensive thermohysteresis. The XRD charts for the NaDMPG dispersions incubated at 25°C showed a sharp X-ray diffraction pattern corresponding to the repeat distance of d=4.75 nm regardless of their aging time, while the dispersions incubated at 40°C had no diffraction peak until 9-day elapsed. After 10-day aging at 40°C, however, a diffraction peak corresponding to d=5.55 nm clearly appeared. In the DSC measurements for the dispersion incubated at 40°C, a few endothermic peaks began to appear between T m and T* after approximately 7-day aging. Then, they shifted toward higher temperatures and finally converged into a single peak at 40–42°C after 14-day aging. These XRD and DSC peaks observed after a long period of aging time above T* suggest that conformations of the hydrophilic groups and the hydrocarbon chains in the NaDMPG bilayers take a more tight and closer arrangement very slowly via an intermediate state above T*, and a new gel phase of the bilayers is consequently formed, the transition temperature (T I temperature) of which is 40–42°C. A molecular interpretation for such transition processes in the bilayer assembly of NaDMPG dispersions has been proposed on the basis of pH, sodium ion activity, ζ-potential, IR data, etc.
Keywords: Dimyristoylphosphatidylglycerol; Bilayer assembly; Repeat distance; Thermal hysteresis; New phase transition;

Proteins are considered as surface active substances. On the basis of experimentally measured rheological parameters of interfacial layers, protein accumulation at an interface between two immiscible liquids, isotherms of interfacial tension, accounting theoretical ideas elaborated for multicomponent systems, the formation of interfacial layers was referred to phase transition. The property of proteins to stabilise emulsions supposedly is connected with the formation of middle phases of lamellar structure. The correlation between elastic properties of interfacial layers and a phase transition of the middle phase upon addition of salts or lipids has been shown. Lipids being added as cosurfactants lead to the transition from lamellar to other structures, which does not provide emulsion stabilisation.
Keywords: Proteins; Surface activity; Interfacial layers; Rheological parameters; Structure; Transitions;

Adsorption and partitioning of surfactants in liquid–liquid systems by Francesca Ravera; Michele Ferrari; Libero Liggieri (129-177).
The adsorption at liquid–liquid interfaces is a phenomenon with a remarkable impact on many scientific and technological fields concerning multiphasic systems. Though in principle similar to liquid–vapour, the study and the description of the dynamic aspects of the adsorption processes at liquid–liquid interfaces deserves some specific considerations. In fact, these systems are often characterised by the partitioning of the surfactant between the two liquid phases, which makes much more complex both their modelling and investigation. In some conditions, the partitioning can be the main process controlling the adsorption dynamics. This paper is aimed at reviewing the state-of-the-art of the theoretical modelling and experimental investigation of the adsorption dynamics of surfactants at liquid–liquid interfaces. After a brief introduction to the problem of adsorption dynamics, the principal models utilised to describe the process at liquid–liquid interfaces under different assumptions are critically presented, underlining the influence of the surfactant partitioning, with respect to the relative volumes of the liquid phases and of the initial partitioning conditions. The most important experimental methodologies for the measurement of the dynamic interfacial tension are also critically reviewed by pointing out the specific problems related with the investigation of the adsorption dynamics of surfactants at liquid–liquid interfaces. Moreover, the problem of the measurement of the thermodynamic quantities characterising the partitioning — mainly the partition coefficient — is also addressed, reporting some literature data. Finally, a review of the literature about the experimental work on the subject and an overview of the needs and of the open questions is given.
Keywords: Liquid–liquid interfaces; Adsorption dynamics; Surfactants; Dynamic interfacial tension; Partition coefficients;

Adsorption of surfactants and polymers at solid–liquid interfaces is used widely to modify interfacial properties in a variety of industrial processes such as flotation, ceramic processing, flocculation/dispersion, personal care product formulation and enhanced oil recovery. The behavior of surfactants and polymers at interfaces is determined by a number of forces, including electrostatic attraction, covalent bonding, hydrogen bonding, hydrophobic bonding, and solvation and desolvation of various species. The extent and type of the forces involved varies depending on the adsorbate and the adsorbent, and also the composition and other characteristics of the solvent and dissolved components in it. The influence of such forces on the adsorption behavior is reviewed here from a thermodynamics point of view. The experimental results from microcalorimetric and spectroscopic studies of adsorbed layers of different surfactant and polymer systems at solid–liquid interfaces are also presented. Calorimetric data from the adsorption of an anionic surfactant, sodium octylbenzenesulfonate, and a non-ionic surfactant, dodecyloxyheptaethoxyethylalcohol, and their mixtures on alumina, yielded important thermodynamic information. It was found that the adsorption of anionic surfactants alone on alumina was initially highly exothermic due to the electrostatic interaction with the substrate. Further adsorption leading to a solloid (hemimicelle) formation is proposed to be mainly an entropy-driven process. The entropy effect was found to be more pronounced for the adsorption of anionic–non-ionic surfactant mixtures than for the anionic surfactant alone. Fluorescence studies using a pyrene probe on an adsorbed surfactant and polymer layers, along with electron spin resonance (ESR) spectroscopy, reveal the role of surface aggregation and the conformation of the adsorbed molecules in controlling the dispersion and wettability of the system.
Keywords: Adsorption; Surfactant mixtures; Surface aggregation; Thermodynamics;

Foams and emulsions are stabilized by surfactant monolayers that adsorb at the air–water and oil–water interfaces, respectively. As a result of monolayer adsorption, the interfaces become viscoelastic. We will describe experiments showing that foaming, emulsification, foam and emulsion stability, are strongly dependent upon the value of compression elasticity and viscosity. This will include excited surface wave devices for the measurement of surface viscoelasticity and thin film videointerferometry for the study of model films between air bubbles and emulsion drops.
Keywords: Foams; Emulsions; Surfactant monolayers; Thin liquid films; Interfacial rheology;

Interactions between a lipase and charged surfactants — a comparison between bulk and interfaces by Krister Holmberg; Magnus Nydén; Lay-Theng Lee; Martin Malmsten; Brajesh K Jha (223-241).
The interaction between a charged surfactant and a lipase has been investigated by several methods. Interactions in aqueous bulk phase was studied by NMR and by microcalorimetry. Surface tension and neutron reflectivity were used for studies at the air–water interface. Interactions at the interface between a hydrophobic solid surface and water was investigated by ellipsometry. The results obtained are as follows. The cationic surfactant, tetradecyltrimethylammonium bromide (iodide in the NMR experiments), showed strong interaction at the air–water and the hydrophobic solid–water interfaces but no clear indication of an interaction in bulk phase was seen. The anionic surfactant showed no interaction with the lipase neither at the interfaces, nor in bulk. The difference in behavior of the system cationic surfactant–lipase in bulk and at the interfaces may be due to the change in enzyme conformation that is known to occur at interfaces between water and an apolar phase.

Structural investigations of phospholipid monolayers on aqueous subphases on the submolecular level using X-ray and neutron reflectivity measurements are reviewed. While such investigations have been limited in the past by a relatively restricted accessible momentum transfer range, recent developments in synchrotron technology — almost doubling this range — have considerably improved the capabilities of the technique. Until recently, data interpretation has entirely relied on ‘box models’ which describe the structures as molecularly homogeneous slabs — one hydrophobic and one hydrophilic. It is shown that box models of phospholipid monolayers are rather inadequate to model data at the high momentum transfer available nowadays in X-ray measurements. As an alternative, a hybrid data inversion strategy is proposed that treats the hydrophobic alkane phase as a homogeneous slab and describes the position of submolecular fragments of the lipid headgroups by means of distribution functions along the interface. Within this approach, composition-space refinement — enabling the coupling of data sets from various X-ray and neutron contrasts — in connection with volumetric constraints enables structural characterization of lipid monolayers in unprecedented detail. Extending a recent characterization of dimyristoylphosphatidic acid (DMPA) monolayers on pure water [Schalke et al., Biochim. Biophys. Acta 1464 (2000) 113–126] it is shown that stoichiometric binding of the divalent cations — DMPA:Cat2+=2:1 — occurs only at exceedingly low areas per molecule, A lipid. At low surface pressure π, both cations and anions are incorporated into the headgroup in significant amounts, ∼0.68 Ba2+ and ∼0.35 Cl per PA molecule at π=2 mN m−1. They are continuously squeezed out upon compression, until upon approaching A lipid=41 Å2, the stoichiometric ratio between bound cations and acidic headgroups is observed. The average inclination angle α of the headgroups as well as their water content is constant along the whole isotherm. The intrinsic contribution to the distribution width — i.e. the spread that is due to a distribution of the fragments within the headgroup without the action of capillary waves — increases with compression up to π∼30 mN m−1 and drops sharply thereafter in a regime of the isotherm where A lipid approaches its limiting value. The same general picture is observed for DMPA on subphases with 10 mM Ca2+, although the lower electron density of that cation limits the precision of the results.
Keywords: Lipid membrane models; Dimyristoylphosphatidic acid; Ion binding; Lipid headgroup structure; Lipid headgroup hydration;

The effects of protonation on alkyldimethyl amine oxide micelles are reviewed, mainly with regard to dodecyl and tetradecyl homologs. The topics discussed are hydrogen ion titration properties, critical micelle concentration (CMC), area per surfactant and micelle aggregation number. A hydrogen bond hypothesis is proposed to interpret the several characteristic results associated with protonation: between two cationic species as well as between the non-ionic–cationic pair. The dipole–dipole interaction of the non-ionic micelle is discussed in relation to both: (a) the unusually high CMC values of the non-ionic micelles compared with other non-ionic surfactants with the same hydrocarbon chain; and (b) the reversal of the stability of the non-ionic and the cationic micelles at high ionic strengths. Two different approaches of the salting out effect on the ionic micelles are compared, the Chan–Mukerjee approach and ours, in relation to the non-linear Corrin–Harkins relation. The obtained salting out constants of the surfactants carrying a dodecyl chain decreased as the head group becomes more polar. Infrared and 13C-NMR spectra data are examined from the point of the specific interaction claimed by the hydrogen bond model. Mixed surfactant systems including amine oxides and the solid state phase behavior of amine oxides are both briefly reviewed.
Keywords: Alkyldimethylamine oxide; Protonation of amine oxide; Critical micelle concentration; Surface excess; Hydrogen ion titration of micelles; Hydrogen bond; Dipole–dipole interaction; Non-linear Corrin–Harkins relation;