Trends in Colloid and Interface Science XV

With the classical “thin-film balance” apparatus, one can study the properties of thin soap films (air/water/air films). Here, we present a new version of that apparatus allowing us to build a single thin oil film horizontally held on a frame and completely immersed in water. The frame used here is a glass frit: to make it suitable for holding oil films in water and to overcome wetting problems, we have developed a special surface treatment by silanization of the frit. With that device, we can directly and simultaneously control, change and measure both the film thickness and the disjoining pressure in these water/oil/water films. Related to structural, dynamical and stability issues, the range of studies and applications is wide with the new experimental configuration. We present experimental tests on the validity of the setup, also showing typical thickness instabilities which appears to be important in these kind of films. We also discuss information provided on the stability of thin liquid films in liquid-liquid extraction problems.

A series of 5′-phenyl-m-terphenyl carboxylic acid derivatives with methyl, phenyl, chloro, p- chlorophenyl and fluoro substituents have been characterised as Langmuir monolayers at the air/water interface by measuring the surface pressure and the electric surface potential upon monolayer compression. The three-layer capacitor model proposed by Demchak and Fort [(1974) J Colloid Interface Sci 46:191] is employed to relate the experimental surface potentials of the monolayers investigated to the molecular dipole moment calculated using semiempirical quantum methods. The local dielectric permittivity in the vicinity of hydrophobic groups was calculated to be 4.8. By adopting a dielectric constant of 7.6 in the vicinity of the hydrophilic groups, the contribution from the water reorientation was found to be 0.15 D, very close to that estimated for small aromatic molecules forming Gibbs monolayers at the air/ water interface.

The mechanism of the rupture process of liquid films is not fully understood yet, particularly in the case of an asymmetric film between a solid surface and a gas bubble. There are two theoretical approaches describing this problem: -Growing fluctuation waves (spin-odal dewetting) on fluid interfaces under the influence of any kind of attractive force [electrostatic, van der Waals, and maybe a so-called long-range hydrophobic force (LRHF)]. This mechanism was first developed by Scheludko.-Nucleation inside the film first proposed by Derjaguin.Metastable wetting films on glass surfaces either hydrophobized by methylation (negatively charged) or with Al3+ ions positively charged and hydrophilic, are analyzed by film thinning according to the Reynolds law.These experiments demonstrate that -Both mechanisms can be responsible for thin wetting film rupture: in the case of hydrophobic surfaces, the nucleation mechanism; in the case of oppositely charged silica surfaces, the capillary waves mechanism due to the attractive electrostatic double layer force between silica and the air bubble.-The existence of a LRHF on a hydrophobic surface can be excluded. The apparent interaction can be explained by the presence of gas nuclei formed on heterogeneous sites.The results provide deeper insight into the mechanisms of wetting film stability, the adhesion process in flotation and droplet coalescence.

High-surface-area spinels of the general formula MA1₂O₄, where M = Mg, Co, Ni, Cu and Zn have been successfully prepared at low temperature (600 °C) from precursor solutions containing the nitrate salts and the surfactant cetyltrimethylammonium bromide as the gelating agent. Thermal analysis (thermogravimetry/differential thermogravimetry/differential thermal analysis) of the precursors dried at 100 °C showed an exothemal decomposition around 250–260 °C and no mass loss above 600 °C under airflow. The solids were heated at 600, 800 and 1000 °C and at each step the X-ray diffraction spectra were obtained in order to check the development of the spinel MAl₂O₄ crystal phase. For M = Mg, Co and Zn samples treated at 600 °C, the MgAl₂O₄, CoAl₂O₄ and ZnAl₂O₄ crystal phases are formed. Nitrogen porosimetry for the MAl₂O₄ samples, heated at 600 °C, revealed mesoporous solids of specific surface areas from 106 to 190 m2 g−1 depending on the M cation. At 1000 °C the MgAl₂O₄ spinel possesses a specific surface area of 57 m2 g−1 and ZnAl₂O₄ has a specific surface area of 75 m2 g-1.

A study of monolayer mixing behaviour in binary d,l-dipalmitoyl phosphatidylcholine/ 3-monopalmitoyl glycerol mixtures was undertaken. For this purpose, the isotherms of surface pressure versus molecular area were acquired at four different tempera-tures and a surface thermodynamic analysis was applied to these isotherms.

Europium ions were dispersed in reverse micellar solutions of bis(2-ethylhexyl) sulfosuccinate (AOT) in chloroform and then thin films were obtained by the dipcoating method. Stabilization of the films was achieved in the presence of poly(methyl methacrylate) (PMMA). The photophysical behavior of Eu3+ in these composite organic films was studied. It was found that the combination of AOT with PMMA results in efficient dispersion of Eu3+, a decrease of concentration quenching and enhancement of weak radiative transitions, particularly, of the emission corresponding to the 5D₁→7F₁ transition at 538 nm. The importance of the relative polymer and surfactant concentrations as well as some other factors affecting Eu3+ emission are discussed.

A short review is given on recent progress which we achieved in the characterization of condensed monolayer phases induced by the adsorption of amphiphilic species (surfactants, proteins). The combination of surface pressure adsorption kinetics, Brewster-angle microscopy and X-ray diffraction at grazing incidence is the highly effective experimental basis of these investigations. At the beginning, a tailored amphiphile was used to be sure that artefacts caused by highly surface active trace components were avoided. So far, a first-order phase transition has been found in adsorbed monolayers of numerous other surfactants and systems. Various types of 2D modifications have been identified. A first-order phase transition can also be induced by the coadsorption of two surfactants and by the penetration of soluble surfactants into gaseous Langmuir monolayers.

Amphiphilic “host—guest” assemblies are formed between a “host” monolayer and “guest” molecules dissolved in the aqueous subphase by acid—base interactions. The specific features of the surface films of amphiphilic benzamidinium—benzoate complexes are determined by surface pressure-area isotherms, Brewster-angle microscopy and atomic force microscopy studies. Molecular recognition of the dissolved component by the amphiphilic monolayer causes drastic changes in the properties of the surface film. Not only the area per molecule is considerably increased but also condensed-phase domains of special texture and topography can be formed. Details of a specific substructure reveal that a second layer grows over the primary structures at further compression.

Equilibrium penetration and penetration kinetics of the dissolved homologue N- decyl-γ-hydroxybutyric acid amide into Langmuir monolayers of N-tetrade-cyl-γ-hydroxybutyric acid amide were studied by surface pressure measurements and Brewster-angle microscopy. If penetration of the dissolved component takes place into the fluid monolayer a first-order phase transition is induced. Under all conditions the condensed phase formed consists only of the longer-chain homologue. The single components were also characterised.

We report the immobilization of glucose oxidase into a polypyrrole layer owing to multilamellar vesicles. Multilamellar vesicles consist of a mixture of lipids and surfactants and are used to encapsulate glucose oxidase and vectorize it towards an electrode where polypyrrole is electrosynthesized. We show by cyclic voltammetry experiments performed on vesicles free of glucose oxidase that they are likely to interact with pyrrole oligomers during synthesis, leading to their incorporation into the film. Their engulfment into the film is evidenced by scanning electron microscopy and Auger analysis. Vesicles in which glucose oxidase is encapsulated do not seem to be destroyed during film synthesis since the enzymatic activity of the mixed film can be triggered by adding a surfactant able to dissolve the vesicles. The advantage of using vesicles for inserting an enzyme into a polymer film is demonstrated in the special case where a pulsed field is applied for film synthesis. We indeed measure a higher enzymatic activity when glucose oxidase is incorporated in the polypyrrole film using multilamellar vesicles.

We have studied the behaviour of aqueous mixtures of a hydrophobically modified poly(sodium acrylate) derivative (PA-Na3C18, containing 3 mol% octadecyl groups) with a cationic copolymer of N-isopropylacrylamide (PNIPAM10, containing 10 mol% cationic groups). Rheology and pyrene fluorescence probing were used to this end. The PA-Na3C18/PNIPAM10 mixtures were compared to mixtures of PANa3C18 with PNIPAM. The combination of both electrostatic attractions and hydrophobic interactions makes the PANa3C18/PNIPAM10 mixtures more effective thickeners than the PANa3C18/PNIPAM ones. In the latter case, only hydrophobic interactions are effective.

The phase behavior of a hydrophilic A—B-type silicone surfactant, (CH₃)₃SiO-[(CH₃)₂SiO]_3.8-(CH₃) ₂SiCH₂CH₂CH₂-O-(CH₂CH₂O)_51.6H, Si_5.8C₃EO_51.6, was investigated by phase study and small-angle X-ray scattering Si_5.8C₃EO_51.6 forms a micellar cubic phase and a hexagonal phase in aqueous mixtures. The structure of the cubic phase seems to be face-centered type. When lipophilic surfactant (Si₁₄C₃EO_7.8 or Si₂₅C₃EO_7.8) is added to Si_5.8C₃EO_51.6/water systems, a transition from the hexagonal phase to the lamellar phase takes place, owing to a change in the hydrophile—lipophile balance of the system. The change in the surface area per surfactant molecule is larger as the polydimethylsiloxane chain is longer, even if the EO number remains constant. This fact is attributed to the coiling of the long lipophilic chain in order to reduce the entropy loss.

Polymer—surfactant solutions, in which the surfactant molecules interact strongly with the polymer chains forming polymerbound micelles above the critical association concentration, are used as media for inorganic precipitation reactions. The formation of PbS is used as a prototype reaction with unexpected results. Under a wide range of conditions, the PbS crystallites initially produced evolve into a range of metastable structures composed of PbS and lead dodecyl sulfate. X-ray diffraction and transmission electron microscopy serve as valuable tools to study the evolution of the crystallizing system in detail. The coexistence of three different colloidal particles (polymers, surfactant micelles and inorganic nanocrystals) leads to extremely complex behavior. The present work highlights the significance of coupling colloidal aggregation to ionic equilibria, and introduces polymer-surfactant solutions as a new medium for the study of inorganic crystallization reactions and the production of organic—inorganic nanocomposite materials.

Acrylic copolymers have been widely used in the past for the surface coating of porous materials of artistic/architectonic interest. Information on the alteration of the physicochemical properties of the porous materials is quite scarce. In this study we showed that the surface area, the contact angle, the water vapour permeability, and the capillary rise profiles of aerial mortar specimens were strongly modified by coating the surface sample with these copolymers. Therefore, a crucial topic is the development of a suitable method to remove these copolymers from the surfaces of the work of art/architecture. Moreover, fluorescence spectra collected from these acrylates photochemically aged by UV radiation indicated an alteration of the chemical structure enhancing the difficulty in removing the coating layers. We succeeded in developing two different four-component oil-in-water (o/w) micro-emulsions where the oil phase was p-xylene: the first using Tween 80 as surfactant and 1,2-propandiol as cosurfactant and the second with sodium dodecyl sulfate and 1-pentanol. These o/w microemulsions were shown to be able to solubilize and remove acrylic copolymers from the surface of porous materials constituted of aerial mortar. The microemulsions were tested during the restoration of the wall paintings by Spinello Aretino in the Cappella Guasconi in the San Francesco church, Arezzo. Scanning electron microscopy images, Fourier transform IR spectra and energy-dispersive X-ray data indicated that the removal of the hydrophobic polymeric resins from the painted surface was very satisfying without any negative side effects.

The kinetic stability of dispersions of Ca(OH)₂ particles (1-2 µm) in short-chain aliphatic alcohols was investigated. The alcohols were shown to strongly enhance the kinetic stability with respect to water. Ca(OH)2 crystalline nanoparticles were also synthesised at 60 °C from aqueous supersaturated solutions. The nanoparticles were characterised by scanning electron microscopy and atomic force microscopy techniques. The kinetic stability of the Ca(OH)₂ nanoparticle dispersions was higher than that of the micron-sized particles. Dispersions of the nanoparticles in 1-propanol were successfully tested as consolidating on aerial mortar and carbonatic stone specimens. Applications of these dispersions on carbonatic stones presenting flaking and powdering from architectonic sites in Rome and near Padua gave positive results.

The properties of 2D colloidal crystals have been widely investigated over the last 20 years. Recently, it has been recognized that colloids are also useful for the investigation of systems comprised of only a few particles. We study the melting behavior of a finite number (N < 30) of paramagnetic colloidal spheres (σ = 4.5 μm) in 2D circular cavities. By applying a magnetic field, B, the interaction strength between the particles is varied. At high B, i.e. strong dipole interaction, the particles are arranged in a highly ordered shell-like structure. With decreasing B we observe a loss of angular order between adjacent shells. Upon further reduction of the external field, however, different scenarios are observed. For cavities with hard-wall confinement and commensurate particle numbers angular order is restored again. In contrast, the latter effect is absent for soft-confinement potentials. In both cases the system melts completely for small magnetic fields.

We study the phase behavior of a 2D colloidal system in the presence of an external periodic 1D potential. As colloidal suspension we use an aqueous suspension of superparamagnetic spheres with a diameter of 4.5 μm. The periodic 1D potential is fabricated by evaporation of thin magnetic nickel lines onto a glass substrate, which is afterwards covered with a protective poly(methyl methacrylate) layer. When the phase behavior of the colloidal system is investigated as a function of the applied external magnetic field, we observe a promoting effect of the underlying periodic 1D potential to the crystallization of the 2D colloidal system.

We investigated the solidification behaviour of thoroughly deionised aqueous suspensions of polystyrene latex spheres by various optical scattering methods. We found a dramatic increase in the nucleation rate densities with increasing particle number density. Crystalline and nanocrystalline samples showed two relaxation processes on widely separated time scales. For an index-matched suspension of perfluorinated particles an amorphous state was accessible with the glass-typical signature of frozen long-time relaxation. From our results we propose a route into the amorphous state different to that observed in hard-sphere suspensions. It seems that in charged-sphere systems the increased nucleation rate density triggers the appearance of a Bernal-type glass.

A sticking probability model for irreversible aggregation processes is developed. It allows a kernel capable of describing not only the diffusion-limited and reaction-limited aggregation regimes but also the whole transition region to be deduced. According to the definition given by van Dongen and Ernst, the kernel establishes λ = 0 for the entire range of sticking probabilities. The model presented gives further insight into the detailed aggregation mechanism for slow aggregation processes.

We report first measurements of the crystal growth velocity in deionised two-component aqueous suspensions of charged polystyrene latex spheres. The size ratio was 1:1.3 and mixing ratios up to 18% of the larger particle were investigated. For the pure components limiting growth velocities of (V_∞,120 = 4.8 µm s−1 and V_∞,156 = 2.9 µm s−1) were observed. In the mixture v drops with increasing mixing ratio even below (V_∞,156) of the large minority component. Careful monitoring of the deionisation procedure excludes the explanation of enhanced ionic contamination. Alternatives based on the kinetics of particle attachment and the position of the phase boundary are discussed.

TíO₂ mesostructured films have been made on glass slides by a dip-coating process, using ethanolic solutions containing titanium iso-propoxide and poly(ethylene glycol) oligomers of various chain lengths. This convenient and easy procedure provides a means of controlling the size of titania nanoparticles, as well as the fractality and the roughness of the film surface by simply choosing the size of the oligomer chain length.

The adsorption of Ni2+ on zeolites is presented. Using the techniques of thermal analysis and sorptometry information such as the surface capacity, wetting and the nature of the centres was obtained and good correlations were obtained.

The properties of water solubiliséd by cetyltripropylammonium bromide (CTPABr) and sodium bis(2-ethylhexyl)sulfosuccinate (AOT) in CCl₄ were studied as a function of the number of water molecules per surfactant molecule(W) by the kinetics of decarboxylation of the 6-nitrobenzisoxazole-3-carboxylate ion (6NBIC) and IR spectroscopy in the O—H stretching region in the region of low water (0 < W < 10) where the properties depend strongly upon W. Decarboxylation of 6NBIC in CTPABr reverse micelles is strongly inhibited by an increase in W and the first-order constants level off at W > 1. The rate effects are ascribed to increases in polarity and hydrogen bonding in the reaction region as water is added and are related to changes in the IR spectrum. For the AOT system, the kinetics is independent of W and this difference from the behaviour of the CTPABr system is ascribed to the difference in the interactions of 6NBIC with surfactant headgroups.

Synthetic silica-bearing organic groups derived from amino acids at their surface have been prepared via the sol—gel technique. The particle size of the colloidal silica obtained in this way has been assessed by different methods. The complexation of divalent metal cations, such as Cu2+, Cd2+, and Pb2+, has been studied in aqueous media by using ion-selective electrodes.

The adsorption at the solid—xylene interface of poly(isobutenyl-succinimides) (PIBSI) has been studied on carbon black by means of adsorption isotherms and small-angle neutron scattering. Simple diblock PIBSI having various chemical structures and poly(PIBSI) with a comblike structure were compared. The adsorption is due to the hydrophilic polyamine part. It was related to the chemical structure of the dispersants (length of the polyamine part, simple diblock structure versus comblike). The adsorption phenomenon was irreversible at low concentrations; the adsorbed macromolecules are fully stretched and form a monolayer of 30-Å thickness. The consequences for the colloidal stability of carbon black dispersions in xylene were analyzed by means of quasielastic light scattering and rheology measurements.

We show how single-cluster light scattering may be used to study the aggregation kinetics of surface-modified colloidal particles. The surface characteristics of the particles were modified by adsorbing different amounts of bovine serum albumin (BSA). All aggregation measurements were performed at high salt concentration and at the protein’s isoelectric point. Single-cluster light scattering was employed to determine how different amounts of BSA adhered on the particle surface affect the aggregation mechanism. The results obtained show that the cluster size distributions scale independently of the degree of surface coverage. It was found that the BSA molecules drastically change the time evolution of the number-average cluster size. Nevertheless, the homogeneity parameter remains practically zero at all degrees of surface coverage and, hence, the aggregation regime is diffusion-like.

The constant composition method (pF-stat method) was employed to prepare submicrometer-sized (few tens of nanometers to several hundred nanometers), very stable, positively charged LaF₃ particles of narrow size distribution in the acidic region from pH 4 to 4.5. Polarizing microscope pictures (an-isotropic clusters) and dynamic light scattering (DLS) data demonstrate that the submicrometer structure is self-similar and a fractal with the dimension going from D_f = 1.4 to 2.1 (±0.05), i.e. the aggregation process proceeds from diffusion-limited to reaction-limited with increasing supersaturation of the solution. The pF-stat rate in acidic solution obeys first-order kinetics (p = 1.09 ± 0.08). Also the rate of growth from the DLS data, i.e. the slope of the change in the size (normalized median hydrodynamic diameter) with mass increment (which is proportional to time), is about 1; more precisely it increases from 0.85 to 1.05 with increasing supersaturation.

We present some results obtained in 2D aggregation experiments. Experiments were made using a salt subphase of 2.0 M KBr and pH values ranging from 1.5 to 6.0. We used two latex polystyrene samples with different contact angles (i.e. different hydrophobic properties) and similar particle sizes. Digitised images were obtained at different aggregation times. These images were analysed to obtain the aggregates from the whole image. The aggregation results were compared using the kinetic exponents obtained from the asymptotic behaviour of the total number of aggregates as a function of time. N ∝ tz. In contrast to the 3D case, the aggregation at the air—solution interface was faster for the hydrophilic particles than for the hydrophobic ones. These experimental findings were analysed using a model of the interactions between colloidal particles at the air—liquid interface that takes an electrostatic dipolar repulsive term into account between the air-exposed parts of the particles.

κ-Carrageenans are known to form strong but turbid gels in the presence of calcium ions. Nevertheless, the resulting gels show no syneresis and a relatively low melting temperature, which makes them of great interest in food applications. To better understand the mechanism of gelation of these polysaccharides, the effect of chemical modification of the remaining free secondary hydroxyl groups was studied. The formation of hydroxyethyl derivatives increases the hydrophilicity, resulting in less turbid but thermally and physically weaker gels. Alkylation of the κ-carrageenan polymer resulted in increased hydrophobicity, turbidity and brittleness of the gels. Differences in thermal stability and physical strength of both types of modified κ-carrageenan gels were studied by rheology. Turbidity was assessed by UV—v is spectroscopy.

Ultrasonic velocity and attenuation measurements were performed for a series of emulsions containing sodium caseinate and Tween 20. The kinetics of creaming were followed in order to establish relationships between depletion flocculant concentration and the onset of creaming. An interesting phenomenon was noticed where attenuation throughout the bulk of the emulsion dropped from very high values to lower ones with time. This was more intense at higher concentrations of Tween 20 (more flocculated emulsions). The lower attenuation values corresponded approximately to the theoretically predicted attenuation spectra for flocculated emulsions. The explanation for the higher attenuation at the beginning of the lifetime of the emulsions seems to be related to the formation of a transient gel spanning throughout the structure, eventually breaking up to produce discretely creaming flocs.

We report an investigation of the microscopic structure and dynamics of biopolymer gels and relate them to the macroscopic viscoelastic properties of such systems. Biopolymer solutions and gels represent one of the most interesting class of gelling systems since they are of major industrial and scientific interest. We performed a systematic study using concentrated solutions of casein micelles which we destabilized and investigated during the process of gelation using diffusing wave spectroscopy (DWS) and rheological measurements. An analysis of the light scattering data shows a significant increase in the characteristic decay time of the correlation functions during the sol—gel transition. For the analysis of the DWS data we developed an algorithm which, based on the so-called microrheology approach, determines the viscoelastic properties G′(ω) and G′'(ω) of the gel. A comparison of the results obtained with DWS and measurements with a rheometer shows excellent agreement of both approaches. We demonstrate that we can clearly link the changes observed in the microscopic dynamics to the formation of a macroscopic gel with drastically modified viscoelastic properties.

The influence of temperature on the kinetics of aggregation of poly(ethylene oxide) coated latex particles induced by adding appropriate amount of simple electrolyte has been investigated by means of dynamic light scattering measurements. The hydrodynamic radius of the resulting aggregates markedly depends on the molecular weight of the polymer adsorbed on the polystyrene particle surface. We investigated the influence of poly(ethylene oxide) of three different molecular weights (from 8 to 5 × 103 kD) in the temperature interval from 10 to 50 °C. A bridging mechanism has been suggested as responsible, at an intermediate surface coverage, for enhancing the aggregation rate. The influence of the polymer solubility in this process is briefly discussed.

We report the synthesis and the properties of a new class of monobactams containing a triazole ring, that are both bio- and surface-active materials. A standard procedure for β-lactam synthesis from fatty amines, aminoacid, and 2,2-bis (hydroxymethylpropionic acid), involves an intramolecular reaction. After introduction of an azido group, the reaction with acetylenic derivatives leads to a triazole ring. The critical micelle concentrations of aqueous solutions have been determined; antibiotic and antiviral activities have been evaluated.

Hydrophilic matrices or hydrocolloids are polymers which swell on contact with aqueous solutions and dissolve slowly from the surface forming a gel mass. Several studies have been carried out in the past few years on the use of hydro-colloids in controlled release formulations. The present study used three modified celluloses, carboxymethyl cellulose sodium, hydroxyethyl cellulose (HEC), and hydroxypro-pylmethyl cellulose (HPMC) in systems using the dihydropyridine felodipine, which is slightly soluble in water, as the active ingredient. This study was concerned with solid dispersions, which were prepared following the dissolution method using a common solvent. The drug—polymer interactions were studied using differential scanning calorimetry and IR techniques, as well as high-performance liquid Chromatographic purity after storage in strength conditions. Neither significant interactions nor degradation of the active ingredient was observed after storage at 40 °C for 3 months. In addition, felodipine release from the solid dispersion systems was studied and the factors influencing release, such as the drug—polymer ratio, interactions, and polymer properties were investigated. HPMC was observed to promote a more significant retard and a more linear release of the active ingredient than HEC. Finally, the natural mixtures presented a larger variation and high relative standard deviation values.

We investigated the behaviour of cell cultures on the surface of thermoresponsive polymers. We synthesised a series of copolymers of N-isopropylacrylamide and N-tert -butylacrylamide (NtBA) with ratios of 85:15, 65:35 and 50:50. Increasing the amount of NtBA results in a reduction in the lower critical solution temperature as determined by microcalorimetric methods and an increase in surface hydrophobicity. Experiments determined that human epithelial cell growth was almost identical on surfaces with a higher degree of hydrophobicity. Preconditioning of surfaces with cell culture medium containing serum promoted cell adhesion on more hydrophilic polymers.

The shrinking kinetics of terpolymer gels composed of 10:20:70 N-tert-butylacryl-amide: N,N’-dimethylacrylamide: N-isopropylacrylamide were found to be sensitive to the initiator concentration in the pregel solution. The gels were synthesised by redox polymerisation using ammonium peroxydisulphate and N,N,N’,N’-tetraethylenediamine, resulting in SO₄ groups at the free end of the forming chains. Increasing the initiator concentration resulted in significantly faster shrinking times. The transition temperature and the continuity of the transition were not affected by the initiator concentration, and the degree of swelling at low temperatures was only slightly increased. Thus, there was no evidence of increased osmotic pressure owing to increased charge density. However, the shrinking process was dramatically different for the gel at higher initiator concentration, being 2 orders of magnitude faster, and preventing the formation of a surface skin layer at high temperatures. It is postulated that this resulted from an increased number of growing chains being formed at higher initiator concentration, resulting in an increased number of elastically ineffective chains. It is further postulated that these free chains have increased mobility compared to the network chains, resulting in faster shrinking of gels formed at higher initiator concentration.

The interaction between DNA and alkyltrimethylammonium bromides of different chain lengths is studied, both on a macroscopic and on a single-molecule level. The phase maps for aqueous DNA—surfactant systems as well as some interesting salt effects are presented. Some preliminary results on the structure of DNA—surfactant complexes are also given. Studies involving DNA and surfactant mixtures are also conducted.

Cationic liposomes are efficient carriers of genetic material and because of this they have been studied by different biophysical and biochemical approaches. In the present article dielectric spectroscopy (DS) and circular dichroism measurements on cationic liposome—DNA mixtures are reported. These techniques are suitable to characterise the formation of a DNA-lipo-some complex, which is the first step in the transfection process. In particular, the estimation of a significant structure parameter of DNA, the so-called “persistence length”, is possible by DS. Our results confirm that the link between cationic liposomes and DNA is direct and of an electrostatic nature. The maximum complexation is at electroneutrality as reported in the literature. Finally, an interesting structural effect on DNA was evidenced by our measurements. When the biopolymer is partially bonded to liposomes, its free stretches exposed to solvent appear rigider with an increase in the persistence length.

A number of N-alkylaldo-namides, i.e., nonionic saccharide-based surfactants such as N-alkyl-N-methylgluconamides, N-alkyl-N-methyllactobionamides and N-alkyl-N,N’-bis[(3-D-aldonamidopro-pyl]amines were evaluated for environmental responses. All the N-alkylaldonamides were screened for antimicrobial activity against selected gram-positive (Staphylococcus aureus PCM 1944, Sarcina lutea PCM 1947, Bacillus subtilis PCM 1949, gram-negative (Escherichia coli PCM 2057, Serratia marcescens PCM 549, Pseudomonas putida PCM 2124) bacterial strains and some fungi (Saccharomyces cerevisiae, Penicillum citrinum, Aspergillus niger). It was generally recognized that the compounds tested were completely inactive towards fungi. N-Alkyl-N-methylgluconamides and N-alkyl-N-methyllactobionamides were practically inactive towards the bacteria studied. On the other hand, dicephalic saccharide-based surfactants were active against gram-positive cocci, much more strongly than the N-alkyl-N-methylaldonamides. Using the closed-bottle test (OECD guideline 301D), the biodegradability of these surfactants was determined. N-Alkyl-N-methyl-aldonamides may be considered to be readily biodegradable by activated sludge. The dicephalic surfactants studied showed slower biodegradation rates and they may be considered to be inherently biodegradable. Members of this class of saccharide surfactants exibit biological properties needed for environmental acceptance.

The gelatin content in graphite—gelatin composites was determined from the difference in the mass of graphite particles before and after the treatment with gelatin and by thermogravimetric analysis. It was found that the content of gelatine has a maximum at those pH values of gelatin solution which correspond to the isoelectric point of gelatin. The result is correlated with previous findings concerning gelatin adsorption on various substrates as a function of pH. The “hairy” structure of gelatin allows deposition of different types of small particles (e.g. carbon black) and, under appropriate conditions, may lead to the formation of thick (0.3 μm) and dense layers. On the basis of the good adsorption of gelatin on graphite surfaces, we prepared anodes for lithium accumulators. In graphitic anodes, gelatin serves both as a binder between particles and as a surface modifier, which leads to lower irreversible losses of charge owing to anode passivation.

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.

The formation of nano-emulsions has been studied in water/ mixed nonionic surfactant/oil systems using two emulsification methods. In one method, the composition was kept constant and the temperature was changed (phase-inversion temperature, PIT, method), while in the other method, water was added dropwise to a solution of the mixed surfactants in oil at constant temperature (method B). The droplet size and stability were determined as a function of surfactant mixing ratio, W₁, at 25 °C. The droplet size of nano-emulsions obtained by the PIT method is practically independent of W₁ and falls in the range 60-80 nm. In contrast, the droplet size of nano-emulsions prepared by method B, is highly dependent on W₁ and varies between 60 and 300 nm. At W₁ values where the PIT or the hydrophile—lipophile balance temperature (Thlb) of the system is close to 25 °C, the droplet sizes of the nano-emulsions are similar for both emulsification methods. There are three equilibrium phases of the latter compositions: an aqueous micellar solution or oil-in-water microemulsion (W_m), a lamellar liquid-crystalline phase and an oil phase (O) in addition, these nano-emulsions showed higher kinetic stability than those with lower W₁ values (higher Thlb) and consisting of two liquid phases (W_m + O).

The initial period of CdS film growth appears to strongly influence the quality of the final film. Scanning electron microscopy pictures of film grown by the flow of a supersaturated (in CdS) solution suggest that nuclei are continuously generated on the substrate and grow as discrete “surface” particles. With time, these growing particles grow and “coalesce” with neighbouring ones to create a continuous film. Similar growth patterns are also observed in the chemical bath deposition process. A simple model of the process is developed employing a “unit cell” approach, which is capable of addressing issues such as the existence of an “induction period” in film growth and its relation to supersaturation, in addition to predicting film thickness growth.

A study was made of the effect of addition of chitosan on the rheology of acidified milk gels. The milk gels were made from skimmed milk powder and were acidified at 43 °C until pH 4.5 was reached. Glucono-δ-lactone or two starter cultures, differing in the viscosity of the final product, one culture producing exopolysaccharide were used. Set- and stirred-type milk gels were studied, both with and without chitosan. Dynamic low amplitude oscillatory rheological mea surements were made during acidification, following gelation, and after 2-days storage at 5 °C. Dynamic rheology showed interesting differences between the elastic moduli of those milk gels with chitosan added and those prepared without. The syneresis and the effective viscosity after 2 days were compared for gels with or without chitosan.

Microscopic imaging of single particles is a powerful tool to investigate the local structure of colloidal suspensions. For single-particle identification with high-resolution microscopy the resolution power is limited by refraction to roughly the wavelength of light. In this case the depth of sharpness is on a scale of less than this limit. For this reason the simultaneous observation of particles in two or more layers of a colloidal crystal seems to be impossible. We report a method with which we can image more than one particle layer in dilute colloidal suspensions with preserved resolution. The analysis of the images obtained, in particular for the investigation of crystal layer motion in sheared colloidal crystals, is discussed.

The coalescence efficiency of two Newtonian droplets submerged in a Newtonian fluid subjected to a simple shear flow was investigated experimentally and theoretically. The experimental investigation was based on observing collisions between two droplets under a microscope. The theoretical investigation considered three drainage models: immobile, partially mobile and mobile interfaces. Both the experimental results and the theoretical analysis showed that a critical approach angle exists below which the colliding droplets separate. Above this critical angle the collision leads to coalescence. Knowledge of the critical angle permits calculation of the coalescence efficiency. The dependence of the coalescence efficiency on various dimensionless groups such as the flow number, the capillary number and the viscosity ratio was studied. The theoretical analysis indicated that the coalescence efficiency decreases as the capillary number and the flow number increase. The experimental results showed that the coalescence efficiency goes through a minimum as the value of the flow number increases. The discrepancy between the experimental and the theoretical results was attributed to some mechanism that enhances coalescence and that is not accounted for in the equation used for the critical thickness for film rupture. Both the experimental and the theoretical results indicated that the coalescence efficiency decreases as the viscosity ratio decreases.

A new rheological model for highly concentrated emulsions containing 77–98% of the dispersion phase is suggested which relates the macroscopic functional properties of these systems (elasticity modulus, yield stress and yield strain) to the microscopic physicochemical parameters (droplet size, interfacial tension, surface forces acting in thin liquid films, specific surface of these films, adhesion force between the droplets, their deformability, etc.). Whereas Princen’s model describes only the effect of the capillary pressure and the volume fraction of the internal phase on the elasticity modulus of such emulsions, the new model also predicts the effect of the adhesion free energy between the droplets (or the contact angle between the droplets) on the elasticity modulus and the yield stress and strain of these emulsions. The model proposed is applied to explain and systematize the effect of physico-chemical parameters on the rheological properties of highly concentrated fluorinated water-in-oil emulsions.

We propose an analogy between a new type of glass, recently found within the mode coupling theory framework, and a particle gel, experimentally observed in colloidal suspensions where the particles have attractive interactions. We report the study of a colloidal system model, made of particles with hard core interacting via an attractive square-well potential. The well-width has a range much shorter than the particle diameter. We find new phenomena in the temperature-composition plane related to the width of the attractive interactions, namely a re-entrant behaviour in the ‘phase’ diagram and a coexistence line between two types of glasses. One has been identified as the commonly studied colloidal glass and the other as a new type, the ‘attractive’ glass, that can be viewed as a particle gel. The coexistence line terminates at an end-point, named A₃, after which the gel and the colloidal glass become indistinguishable. We also show characteristic features of the normalised density correlators, for the gel at a relatively low density and close to this singularity point, where the gel and the colloidal glass start to coexist. For the latter it is remarkable to note that the density correlators show a logarithmic time decay.

In previous work the binding of the cationic surfactant dodecyltrimethylammonium bromide to DNA was studied. The original work has been extended to include data for the decyltrimethy-lammonium ion. Additionally, while previously considerations of the complex structure were done within the framework of the dynamics of rigid rods, in the present case we have extended this analysis to include a wide range of three-dimensional configurations. Furthermore, the secondary structure of the DNA within the complex is taken into account. Examination of secondary structural changes on surfactant binding indicates that there are no significant changes in the DNA secondary structure. Consideration of the hydrodynamic properties of the surfactant—DNA complex and extension of the experimental data to include decyltrimethylammonium along with application of hydrody-namic modelling allowed us to exclude highly bent or folded complex conformations. The magnitude of the DNA diffusion coefficient decrease on surfactant binding was measured for surfactant molecules of two different tail lengths. The data showed that a rod covered in a single surfactant layer can provide a simple explanation for the difference in the magnitude of the decrease between the two surfactants. In order to account for the observed ratio of 0.8 surfactants per DNA phosphate observed on completion of the first-stage binding, the surfactant headgroups should be located close to the DNA surface, within the condensation volume. This would leave the tail groups projecting outwards, with lateral hydrophobic association between the tails.

We study the aggregation behavior of two highly water-soluble fullerene derivatives, hexa(sulfobutyl) fullerenes (FC₄S) and polyhydroxylated fullerenes (fullerenols) [C₆₀(OH)₁₈] using small-angle x-ray scattering. We found that FC₄S forms spheroidal aggregates having a similar radius of gyration R_g ≈ 19 A, in a wide concentration range from 0.4 to 26 mM in water solutions, whereas the mean sizes observed for C₆₀(OH)₁₈ aggregates in water solutions grow nearly two fold from R_g = 20 A to R_g = 40 A as the concentration increases from 0.7 to 50 mM. The implication of the structural differences between the two fullerene derivatives on their free-radical scavenging activity is discussed.

Calix[n]arenes and their amphiphilic derivatives act as strong ligands, by selectively binding cations and small organic molecules (such as chloroform, toluene, fullerene). The presence of a cavity and of specific electron donor sites results in their ability to form stable host—guest systems with electron-acceptor molecules. We report our studies on the interfacial properties of amphiphilic calixarenes, both at the liquid/liquid and at the air/liquid interfaces, and their selective complexation of alkaline cations and small organic molecules, by performing Langmuir isotherms and small-angle neutron scattering experiments. Changing the guest species, the functional groups, and the experimental conditions (such as temperature, concentration, and ionic strength) can efficiently modulate the interfacial properties of these amphiphilic ligands.

We show that unilamellar vesicles composed of zwitterionic lipids such as dipalmitoylphosph-atidylcholine can fuse in the presence of DNA. Ca2+ ions are required to induce fusion. Nonradiative energy transfer was used to monitor the fusion process. The dependence of the extent and kinetics of fusion on the mode of preparation, the size of the vesicles, calcium concentration and lipid concentration was investigated. Small unilamellar vesicles of size less than 50 nm fuse rapidly, whereas vesicles larger than 100 nm will not fuse.

We describe the use of supramolecular assemblies of phos-pholipidic molecules as a support for the surface-mediated synthesis of inorganic nanoparticles. Diacetylenic phospholipids such as DC_8,9PC are known to form hollow tubular microstructures reffered to as tubules. Mineralization of DC_8,9PC tubules is obtained by nucleation and growth of an iron oxyhydroxide on their surface. This leads to tubular organic—inorganic hybrid microstructures.

A new anionic surfactant lauryl amidomethylsulfate (LAMS-) was investigated. The aggregation behaviour was determined by small-angle neutron scattering (SANS) measurements. The Na-salt micelles are highly charged, while the Casalt micelles are almost uncharged. Na-LAMS (100 mM) solutions in the presence of 100 mM CaCl₂ undergo several phase transformations with increasing n-hexanol concentration. We found the expected micellar L₁ phase and a lamellar phase, but also a novel phase: a white precipitate is formed at the bottom of the sample. With increasing n-hexanol concentration, the precipitate dissolves into a liquid-crystalline L_ga phase. Investigation by freeze—fracture transmission electron microscopy, light micros-copy and SANS shows that the precipitate consists of agglomerated polydisperse multilamellar vesicles. The bilayer thickness is about 20 A and is independent of the composition, whereas the interlamellar distance is strikingly linked to the concentrations of cosurfactant (sur-factant/cosurfactant ratio) and electrolyte. With increasing cosurfactant content, the bilayers become less rigid and resulting thermal undulations force the membranes apart until a common L_ga phase is formed. This transition is an example of a bonding—nonbonding transition of membranes.

The existence of two lyotropic liquid-crystal phases (micelles and lamellar) of the Triton X-114-water binary system has already been determined by several different methods [Walsh MF (1980) PhD thesis. Salford University; Heusch R (1984) Ber Bunsenges Phys Chem 88:1094]. We observed changes in the phase transitions of the system studied upon addition of octanol, butanol, glucose or fructose. A semiquantitative explanation of the experimental results is given based on the hydrophile—lipophile balance theory.

We prepared colloidal crystals from aqueous suspensions of spherical, charged polystyrene spheres under deionised conditions. Using a home-built multipurpose light scattering apparatus we measured the static structure factor, the static shear modulus and the intermediate scattering function. In addition we also monitored also the conductivity as a function of the composition and the particle number density. For the mixture investigated the data are well described assuming the formation of randomly substituted body-centered-cubic crystals.

We investigate the solidification dynamics of hard-sphere colloidal suspensions applying simultaneously small-angle and Bragg light scattering. These experiments allow a consistent picture of nucleation and crystal growth on the level of large-scale density fluctuations and of density fluctuations on the level of individual crystallites. We observe a temporally almost constant nucleation rate after an induction time that decreases with supersaturation. The classical expectation for the nucleation rate density as a function of supersaturation is in accordance with our data. We investigate the validity of the Wilson—Frenkel growth law for hard-sphere systems, which also fits our data satisfactorily. The kinetic prefactors are found to be only 2 orders of magnitude smaller (for nucleation) and 1 order of magnitude larger (for crystal growth) than expected from classical considerations. We discuss the influences of particle size poly-dispersity on our data.

A new computer simulation methodology is developed which determines the structure factor, S(q), of concentrated suspensions accurately and subsequently calculates S(q) from the wavelength dependence of the “transport mean free path”, l*(λ), for the same systems. Therefore our method can test directly the validity of approximations involved in the analysis of multiple scattering data. The simulation results for l* agree closely with experimental data and explain the substantial overestimation of l* by the “photon diffusion” formula [1, 2] observed in some experiments.

The aggregative phenomena taking place in gel-forming and not gel-forming reverse micelles are investigated and compared. In particular, the different growth processes of the micelles (spherical in Aerosol OT based systems and cylindrical in lecithin-based systems) are monitored by the different diffusional properties of the inner-core water molecules, as seen by the neutronic probe in a quasielastic neutron scattering experiment. The different intermicellar aggregation processes taking place in the two kinds of systems have been evidenced through the study of the concentration dependence of the electric conductivity. The results seem to support the proposed idea that the structure of the lecithin gel consists of a percolated network of branched cylinders instead of (as previously proposed) a random entangled network of (not interconnected) polymer-like micelles.

A study of fast aggregating surface modified colloidal particles is presented. The surface characteristics of the particles were modified by adsorbing different amounts of bovine serum albumin. Since the protein charge depends on the electrochemical properties of the suspension medium, the influence of the protein net charge on both the cluster structure and the aggregation mechanisms could be studied by changing the pH of the aqueous phase. Physical parameters, such as the fractal dimension, the scaling exponent and the aggregation rate, were determined by laser light scattering. The experimental results show that the adsorbed protein molecules modify the aggregation mechanism and allow structural rearrangement within the clusters.These effects are more pronounced when the protein molecules bear net charge. Weak flocculation could not be detected, which means that steric stabilization is totally effective. Near the isoelectric point, coagulation and bridging flocculation are the predominant aggregation mechanisms and are diffusion-controlled. Bridging flocculation, however, becomes less effective at pH 9.

Several suspensions of disklike colloids undergo puzzling liquid—solid transitions. The case of laponite, a synthetic clay, is a demonstrative example which has recently generated a good deal of experimental and theoretical study. At relatively high ionic strength, above 10-4 M, a transition from a liquid to a soft solid appears. Typical nematic defects are then observed. The mechanism and the status of this transition is currently actively debated. In this regime, the Debye screening length, defining the range of electrostatic repulsion, is always smaller than the particle size, which controls short-range anisotropic interactions. These two contributions are then mixed in a nontrivial way. One possible way to uncouple these interactions is to lower the ionic strength. In the first part of this article, we show that a liquid—solid transition exhibiting a correlation peak is observed at very low ionic strength. Such a structural transition involves long-range electrostatic stabilization and/or jamming which is compatible with a Wigner glass transition. However, 3D off-lattice reconstructions of these soft solids from small-angle scattering spectra suggest that the particle distribution is not homogeneous in space. In the second part of this work, the interaction potential of different pair configurations is computed using a basic charge distribution model. Such a stability analysis raises interesting questions about long-term stabilization and/or coagulation, (very) slow structural relaxation and nematic defects generally observed for these colloidal suspensions at relatively high ionic strengths.