Hypothesis
Polyglycerol esters of fatty acids are generated via the esterification of a polydisperse mixture of polyglycerol with naturally derived fatty acids. The polymerization process of polyglycerol results in the production of various oligomers, ranging from di-, tri-, and higher-order forms, which contribute to the complexity of final products. The combination of complementary experimental techniques and adequate theoretical interpretations can reveal the wide variety of their physicochemical properties.

Experiments
The colloid and interface properties of polyglyceryl mono-laurate, mono-stearate, mono-oleate, and a mixture of mono-caprylate and mono-caprate esters solutions were characterized by measurements of the electrolytic conductivity, static and dynamic surface tension, aggregate and micelle sizes and distributions, thin liquid film stability and stratification, and solubility in aqueous and in oil phases. The formation, stability, and bubble size distribution of foams generated from polyglycerol esters aqueous solutions were systematically investigated.

Findings
The low concentrations of double-tail molecules and fatty acids in polyglycerol esters affect considerably their micellar, aggregation, and vesicle formations in aqueous solutions. The theoretical data interpretation of polyglycerol esters isotherms and thin liquid films data provide information on the adsorption energies, excluded areas per molecule, interaction parameters of molecules at interfaces, surface electrostatic potential, and the size of micelles. Polyglyceryl mono-oleate exhibits spontaneous emulsification properties. Short chain length polyglycerol esters have excellent foaming ability but relatively low foam stability. The optimal weight fractions of the short-chain polyglyceryl esters and polyglyceryl mono-stearate mixtures with respect to good foaminess and foam stability upon Ostwald ripening are obtained. The reported physicochemical characterization of the water-soluble polyglycerol esters could be of interest to increase the range of their applicability in practice.

Sucrose esters (SEs), derived from sucrose and fatty acids, are biodegradable and non-toxic surfactants increasingly favored as substitutes for petrochemically synthesized ones in food, cosmetics, and pharmaceuticals. SEs provide versatile hydrophilic–lipophilic properties, determined by the degree of sucrose esterification ranging from one to eight. The length of the fatty acid residues further influences the phase behavior of SEs, allowing creation of tailored formulations for specific applications. This review provides insights about our current understanding of the SEs phase behavior, their aggregation in aqueous and oily solutions, and its correlation with formulation outcomes. Furthermore, an overview of recent studies investigating SEs in various colloidal systems, including emulsions, foams, oleogels, and others, is provided. Novel concepts are discussed alongside future research directions, emphasizing the SEs potential as sustainable, functional ingredients.

Medium- to long-chain alkanes can form upon cooling intermediate phases between isotropic liquid and solid crystalline, called rotator phases, where relative freedom of the molecules to rotate about their long axis is combined with long range translational order. Rotator phases are well documented experimentally but the mechanism of their formation at the molecular level is still not fully explained. In a previous work, we have shown that molecular dynamics simulations can produce rotator phases upon cooling of hexadecane [S. Iliev et al., J. Col. Int. Sci., 2023, 638, 743]. The aim of the current work is to develop a procedure to identify the specific ordered phase obtained in the simulations. The influence of the cooling rate on the freezing process of hexadecane (bulk and surfactant-interfaced to water) is tested as well. Several parameters are combined to quantify the degree of ordering and the type of phase in the studied systems. These are the tilt angle of the molecules with respect to the crystallite plane, the radial distribution function of the centre of mass of the molecules in the crystallite, the percentage of the gauche torsion angles in the molecules, the angle of the second principal axis of each molecule with respect to the x axis of the coordinate system, and estimates from Voronoi analysis. The results show that the systems form a rotator phase, which transitions gradually towards the thermodynamically most stable triclinic crystal, and the transformation progresses to different extent depending on the system. The influence of the cooling rate is related only to the size of the largest crystallite formed, the other parameters of the freezing process remain unaffected. The work also presents a robust procedure for obtaining and identifying different types of ordered phases in alkane-containing systems with thoroughly tested computational protocol and a comprehensive set of structural analyses. Several key characteristics are advanced, compared to previous research [Ryckaert et al., Mol. Phys., 1989, 67, 957; Wentzel et al., J. Chem. Phys. 2011, 134, 224504], namely, a new methodology is proposed to compute the unit cell deformation parameter and azimuthal angle from MD simulation trajectories of the freezing process in alkane-containing systems. The suggested structural analysis, which is independent of the coordinate system, is applicable to any linear-chain system with polycrystalline structure.

Hypothesis: Aqueous solutions of long-chain water-soluble sucrose ester surfactants exhibit non-trivial response to temperature variations, revealing a peak in viscosity around 40–50 ◦C. While previous investigations have explored the structures within sucrose stearate systems at various constant temperatures, a comprehensive understanding of the entire temperature dependence and the underlying molecular factors, contributing to this phenomenon is currently missing. Experiments: Temperature dependent properties and supramolecular structures formed in aqueous solutions of commercial sucrose palmitate were examined using SAXS/WAXS, DSC, optical microscopy, rheological measurements, NMR, and cryo-TEM. Findings: The underlying mechanism governing this unusual behavior is revealed and is shown to relate to the mono- to di-esters ratio in the solutions. Solutions primarily containing sucrose monoesters (monoesters molecules ≳ 98% of all surfactant molecules) exhibit behavior typical of nonionic surfactants, with minimal changes with temperature. In contrast, the coexistence of mono- and di-esters results in the formation of discrete monodisperse diester particles and a network of partially fused diester particles at low temperature. As the temperature approaches the diesters’ melting point, wormlike mixed micelles form, causing a viscosity peak. The height of this peak increases significantly with the diester concentration. Further temperature increase leads to fluidization of surfactant tails and formation of branched micelles, while excess diester molecules phase separate into distinct droplets.

Air bubbles in pure water appear to coalesce much faster compared to oil emulsion droplets at the same water solution conditions. The main factors explaining this difference in coalescence times could be interface mobility and/or pHdependent surface charge at the water interface. To quantify the relative importance of these effects, we use high-speed imaging to monitor the coalescence of free-rising air bubbles with the water− air interface as well as free-falling fluorocarbon-oil emulsion droplets with a water−oil interface. We measure the coalescence times of such bubbles and droplets over a range of different water pH values (3.0, 5.6, 11.0). In the case of bubbles, a very fast coalescence (milliseconds) is observed for the entire pH range in pure water, consistent with the hydrodynamics of fully mobile interfaces. However, when the water−air interface is immobilized by the deposition of a monolayer of arachidic acid, the coalescence is significantly delayed. Furthermore, the coalescence times increase with increasing pH. In the case of fluorocarbon-oil droplets, the coalescence is always much slower (seconds) and consistent with immobile interface coalescence. The fluorocarbon droplet’s coalescence time is also pH-dependent, with a complete stabilization (no coalescence) observed at pH 11. In the high electrolyte concentration, a 0.6 M NaCl water solution, bubbles, and droplets have similar coalescence times, which could be related to the bubble interface immobilization at the late stage of the coalescence process. Numerical simulations are used to evaluate the time scale of mobile and immobile interface film drainage.

The primary objective of this review is to consolidate our current understanding of the factors controlling the foamability of surfactant solutions under hydrodynamic conditions realized in various laboratory tests. In particular, two regimes of foam generation are considered: at low surfactant concentrations where the coalescence between the bubbles plays a crucial role, and a high surfactant concentration range where the hydrodynamic conditions are much more important for the final outcome of foaming. The review discusses the role of surfactant concentration, dynamic surface coverage, and surface forces acting between film surfaces for the foam generated in the surfactant-poor regime. Additionally, the interplay between the hydrodynamic conditions and the viscosity of the formed foams in the surfactant-rich regime is also discussed.

The aim of the current study was (1) to develop an automation-based protocol for in vitro assessment of enzymatic drug stability at fasted- and fed-state intestinal conditions, (2) to characterize the inter-individual variability of drug degradation in fasted- and fed-state human intestinal fluids, and (3) to compare the obtained in vitro results to drug degradation in human intestinal fluids by taking variability into account. In human intestinal fluids, drug degradation displayed large inter-individual variability, with coefficients of variance generally ranging between 30 and 70 %. The effect of food on the inter-individual variability was highly dependent on the type of drug. The increase of pH in the range between 5.0 and 7.0 significantly accelerated the degradation rate of the studied drugs both in the in vitro and ex vivo experiments. In contrast, the increase of bile salt and phospholipid concentrations in the in vitro screen decreased strongly the degradation rate of the hydrophobic drugs. The developed automated in vitro screen mimicked relatively well the ex vivo degradation of all drugs in the fasted state, whereas in the fed state the degradation of only one of the drugs was adequately reproduced.

Hypothesis: Methyl ester sulfonates (MES) show limited water solubility at lower temperatures (Krafft point). One way to increase their solubility below their Krafft points is to incorporate them in anionic surfactant micelles. The electrostatic interactions between the ionic surfactant molecules and charged micelles play an important role for the degree of MES solubility. Experiments: The solubility and electrolytic conductivity for binary and ternary surfactant mixtures of MES with anionic sodium alpha olefin sulfonate (AOS) and sodium lauryl ether sulfate with two ethylene oxide groups (SLES-2EO) at 5 °C during long-term storage were measured. Phase diagrams were established; a general phase separation theoretical model for their explanation was developed and checked experimentally. Findings: The binary and ternary phase diagrams for studied surfactant mixtures include phase domains: mixed micelles; micelles + crystallites; crystallites, and molecular solution. The proposed general phase separation model for ionic surfactant mixtures is convenient for construction of such complex phase diagrams and provides information on the concentrations of all components of the complex solution and on the micellar electrostatic potential. The obtained maximal MES mole fraction of transparent micellar solutions could be of interest to increase the range of applicability of MES–surfactants.

Triacylglycerols (TAGs) exhibit a monotropic polymorphism, forming three main polymorphic forms upon crystallization: α, β’ and β. The distinct physicochemical properties of these polymorphs, such as melting temperature, subcell lattice structure, mass density, etc., significantly impact the appearance, texture, and long-term stability of a wide range products in the food and cosmetics industries. Additionally, TAGs are also of special interest in the field of controlled drug delivery and sustained release in pharmaceuticals, being a key material in the preparation of solid lipid nanoparticles. The present article outlines our current understanding of TAG phase behavior in both bulk and emulsified systems. While our primary focus are investigations involving monoacid TAGs and their mixtures, we also include illustrative examples with natural TAG oils, highlighting the knowledge transfer from simple to intricate systems. Special attention is given to recent discoveries via X-ray scattering techniques. The main factors influencing TAG polymorphism are discussed, revealing that a higher occurrence of structural defects in the TAG structure always accelerates the rate of the α → β polymorphic transformation. Diverse approaches can be employed based on the specific system: incorporating foreign molecules or solid particles into bulk TAGs, reducing drop size in dispersed systems, or using surfactants that remain fluid during TAG particle crystallization, ensuring the necessary molecular mobility for the polymorphic transformation. Furthermore, we showcase the role of TAG polymorphism on a recently discovered phenomenon: the creation of nanoparticles as small as 20 nm from initial coarse emulsions without any mechanical energy input. This analysis underscores how the broader understanding of the TAG polymorphism can be effectively applied to comprehend and control previously unexplored processes of notable practical importance.

The effects of antifoam and surfactant concentration on the foamability of solutions of an anionic (SLES) and nonionic (Brij 35) surfactants and a series of polyvinyl alcohols with 88% and 98% degree of hydrolysis and molecular masses between 31 and 205 kDa, were studied. Three methods which differ in the way of air incorporation were used for foaming – Bartsch test, shake test and Ultra Turrax. Mixed silicone oil-silica particles antifoam was studied. The antifoam was introduced in the foaming solution as pre-dispersed in organic solvent or as antifoam-in-water emulsion. It was shown that the antifoam is very active in the fast foaming methods (Bartsch and shake tests) for the slow adsorbing polymers PVA and has no any activity in the slow foaming method (Ultra Turrax) for the fast adsorbing surfactants with electrostatic stabilization (SLES). The efficiency of pre-dispersed in organic solvent antifoam is much higher as compared to that of emulsified antifoam, due to the faster segregation of the silica particles and silicone oil in the emulsified antifoam. The antifoam efficiency increases with antifoam concentration and with lowering the surfactant concentration. In a given foaming method, the antifoam efficiency is the highest in PVA solutions with 98% DH, intermediate for PVA with 88% DH and Brij 35, and the lowest for SLES solutions. At a certain degree of hydrolysis, the molecular mass of PVA has no significant effect on the antifoam activity. Good correlation between the antifoam efficiency and the stability of the pseudo emulsion film formed between the antifoam globule and the bubble surface is established, showing that the electrostatic repulsion is more efficient to prevent the entering of the antifoam globules on the air-water interface, as compared to the steric repulsion.

The surface, film and foam properties of two nonionic surfactants, dodecyl sucrose ester (L1695) and dodecyl polyoxyethylene ether (Brij L23), were studied at four different temperatures between 25 and 60 °C and three surfactant concentrations (0.01%, 0.1%, and 1% wt.). The impact of 6 M urea was also assessed to determine the role of hydrogen bonds for the observed trends. The foams were generated using two methods: the fast foaming method (Bartsch test), producing foams with smaller bubbles, and the slow foaming method (foam rise method), yielding foams with bigger bubbles. For Brij L23, an increase in temperature resulted in a decrease in the critical micellar concentration (CMC), reduced surfactant adsorption on the air-water interface, weakened electrostatic repulsion between the foam film surfaces and significantly decreased foam stability. For L1695, an increase in temperature increased surfactant adsorption, maximized the foamability at 40 °C, and did not affect the foam stability for foams with small bubbles. However, the temperature increase leads to decreased stability at low concentrations for foams with bigger bubbles. The addition of 6 M urea, resulted in increased adsorption without any effect on foam stability for L1695, whereas it decreased the adsorption and foam stability for Brij L23. The comparison of relative foamability vs. dynamic surface coverage revealed that a lower threshold surface coverage is required to increase the foamability of L1695 (≈ 80%) compared to Brij L23 (≈ 95%). This difference is explained by the action of weak electrostatic repulsion and the adsorption of sucrose diesters on the bubble surfaces when L1695 surfactant is used. The higher stability of L1695 foams under all studied conditions is attributed to the formation of a denser adsorption layer due to the adsorption of sucrose diesters. The diesters prevent the formation of weak spots within the foam films, even at high temperatures. This work contributes to the advancement of the foam field by demonstrating that a mixture of sucrose mono and diester surfactants can be highly effective in forming stable foams at higher temperatures and in the presence of urea. Both factors (higher temperature and 6 M urea) have negative effect on Brij L23 foams, while they have no significant effect on L1695 foams with smaller bubbles.

Different oils can be homogeneously dispersed in the network junctions of the separated bicontinuous micellar phases. Upon dilution, these dispersions spontaneously form nanoemulsions. The possibility of a micellar sponge phase formation in the case of mixtures with three anionic and two zwitterionic surfactants in the presence of divalent and monovalent salts is studied. The best results are obtained using sodium lauryl ether sulfate with 1 ethylene oxide group (SLES-1EO) and both cocamidopropyl betaine (CAPB) or N,N-dimethyldodecylamine N-oxide (DDAO) in the presence of an appropriate small amount of MgCl2 and CaCl2. Bicontinuous micellar phases can be produced also in high-salinity NaCl solutions. The bulk properties of these phases are independent of the concentration of the initial solutions from which they are separated, and their Newtonian viscosities are in the range from 0.3 Pa·s to 0.8 Pa·s. Both 8 wt% CAPB- and DDAO-containing sponge phases engulf up to 10 wt% limonene and spontaneously form nanoemulsion upon dilution with droplet sizes of 110–120 nm. Vitamin E can be homogeneously dispersed only in CAPB-containing saturated micellar network, and upon dilution, these dispersions spontaneously form nanoemulsions with smaller droplet sizes of 66 nm for both 8 diastereomers and 2 diastereomers mixtures of vitamin E.

Small emulsion drops typically exhibit spherical shape at positive interfacial tension due to the energy minimization principle. However, in a series of studies (Denkov et al., Nature, 2015, 528, 392–395; Cholakova et al., Nature Phys., 2021, 17, 1050–1055) we showed that alkane droplets stabilized by appropriate saturated long-chain surfactants may spontaneously change their shape upon cooling, morphing into various polyhedra; hexagonal, tetragonal and triangular platelets; rod-like particles and even synthetic swimmers. These deformations are governed by the formation of thin plastic rotator phases adjacent to the drop surface. Although alkanes have numerous industrial applications, they cannot be used in food and pharma related products, in which most often triglyceride molecules are employed. The possibility for self-shaping of triglyceride drops has been demonstrated, but the detailed understanding of the process is currently missing. In the present study, we performed model experiments aimed to reveal the conditions under which the triglyceride emulsion drops may change their shape upon cooling. We show that most of the various non-spherical shapes known for alkanes can be reproduced with triglyceride droplets providing that the surfactant adsorption layer freezes before the nucleation of the oily molecules inside the drops. By comparing the behavior of triglyceride and alkane droplets, we draw unified picture and provide guiding principles which can be used for selection of appropriate surfactants enabling the spontaneous shape deformations upon cooling of oily drops of different chemical compositions.

Crystallization of alkane mixtures has been studied extensively for decades. However, the majority of the available data consider the behavior of alkanes with chain length of 21 C atoms or more. Furthermore, important information about the changes of the unit cell structure with the temperature is practically absent. In this work, the phase behavior of several pure alkanes C_nH_2n+2, with n ranging between 13 and 21, and their binary, ternary, or multicomponent equimolar mixtures are investigated by X-ray scattering techniques. Both bulk alkanes and oil-in-water emulsions of the same alkanes were studied. The obtained results show the formation of mixed rotator phases for all systems with chain length difference between the neighboring alkanes of Δn ≤ 3. Partial demixing is observed when Δn = 4, yet the main fraction of the alkane molecules arranges in a mixed rotator phase in these samples. This demixing is suppressed if an alkane with an intermediate chain length is added to the mixture. Interestingly, a steep temperature dependence of the interlamellar spacing in mixed rotator phases was observed upon cooling to temperatures down to 10 °C below the melting temperature of the mixture. The volumetric coefficient of thermal expansion of the rotator phases of mixed alkanes (α_V ≈ 2 × 10^-3 °C^-1) is around 10 times bigger compared to that of the rotator phases of pure alkanes. The experiments performed with emulsion drops containing the same alkane mixture while stabilized by different surfactants showed that the surfactant template also affects the final lattice spacing which is observed at low temperatures. In contrast, no such dependence was observed for drops stabilized by the same surfactant while having different initial diameters; in this case, only the initial temperature of the crystallization onset was affected.

The surface, film, and foam properties of six polyvinyl alcohols (PVA) with different degrees of hydrolysis (DH) and molecular weights were studied and compared with the properties of nonionic Brij 35 and anionic SLES. Four different foaming methods were employed: the fast foaming method (Bartsch test), intermediate tests (shake test and Ultra Turrax), and slow foaming method (foam rise method) to assess the foamability at various bulk concentrations. The foamability data obtained from different foaming tests, utilizing various surfactant and polymeric concentrations, and differing foaming times, were shown to follow a universal master curve when plotted as relative foamability vs. scaled concentration. A new simple theoretical equation was derived to describe this universal curve, allowing for foamability prediction. The threshold surfactant concentration required to achieve 50% of the maximal foam volume under given conditions (used for scaling the bulk concentration) was found to decrease with foaming time and increase from slow foaming methods to fast foaming methods. When the experimental data are plotted against surface coverage, the results for PVA solutions exhibit intermediate behavior between nonionic surfactants, where a threshold surface coverage of 95% is required to achieve 50% of maximal foamability and anionic surfactants, where 30% surface coverage is sufficient to reach 50% of maximal foamability due to the action of electrostatic repulsion. This intermediate behavior observed in PVA solutions is attributed to the presence of a long-range steric repulsion arising from the adsorption of PVA molecules onto the bubble surfaces. This work advances the foam field by showing that the approach developed in Petkova et al. 2020 can be used for polymeric molecules and by deriving a new equation for foamability which is expected to be applicable for wide range of systems.

Previous research revealed marked differences in the composition of intestinal fluids between infants and adults. To explore the impact on the solubilization of orally administered drugs, the present study assessed the solubility of five poorly water-soluble, lipophilic drugs in intestinal fluid pools from 19 infant enterostomy patients (infant HIF). For some but not all drugs, the average solubilizing capacity of infant HIF was similar to that of HIF obtained from adults (adult HIF) in fed conditions. Commonly used fed state simulated intestinal fluids (FeSSIF(-V2)) predicted fairly well drug solubility in the aqueous fraction of infant HIF, but did not account for the substantial solubilization by the lipid phase of infant HIF. Despite similarities in the average solubilities of some drugs in infant HIF and adult HIF or SIF, the underlying solubilization mechanisms likely differ, considering important compositional differences (e.g., low bile salt levels). Finally, the huge variability in composition of infant HIF pools resulted in a highly variable solubilizing capacity, potentially causing variations in drug bioavailability. The current study warrants future research focusing on (i) understanding the mechanisms underlying drug solubilization in infant HIF and (ii) evaluating the sensitivity of oral drug products to interpatient variations in drug solubilization.

The concentrated surfactant solutions have a wide application in industry, oil recovery, drug delivery, turbulent drag reduction, etc. The competition between the companies-producers has led to use of new kind of formulations to improve: washing action; skin and eye irritation; stability and durability; biodegradability; tolerance to hard water. Here, we present a review on the state of the art and our contributions to the molecular thermodynamic theory and experiment on the growth of giant micellar aggregates. Despite the considerable advances in theory and computer simulations, agreement with experimental data has been achieved only in isolated cases. Our predictive molecular thermodynamic approach accounts for the different contributions to the micellar scission energy in the case of nonionic, zwitterionic and ionic surfactant solutions and their mixtures. Excellent agreement was achieved between the theoretical model and experimental data for wormlike surfactant micelles at various concentrations of salt and temperatures. At high salt concentrations, the model also predicts loss of chemical equilibrium, which implies a transition to self-assemblies of other morphology or the onset of crystallization and phase separation. The results have applications for the design of new products and nanostructured materials.

The research paper explores the adsorption properties of cationic surfactants on silicon wafers through imaging ellipsometry. The objective of this research is to shed light on the layer structures formed by cationic surfactants, specifically those based on dimethyl ammonium chloride, on silicon wafers. The study involved the deposition of three distinct cationic surfactants on the wafer’s surface, followed by the measurement of the adsorption layers formed. The findings reveal the creation of thin, smooth, and irregular adsorption layers. Interestingly, no correlation was found between the thickness of the adsorption layer and the surfactant tail’s chain length. The research underlines the significant role which the imaging ellipsometry can have for studying surfactants’ adsorption properties on surfaces, contributing to their optimal usage in various fields.

We present an experimental study of the hydrodynamics of a buoyant sphere accelerated horizontally along an air–water interface. At low speeds, the sphere floats at the surface, while at higher speeds, the sphere starts oscillating, moving below and toward the free surface akin to underwater skipping. The sphere often breaches and forms an air cavity during its subsequent dive. These underwater air cavities become horizontal and are attached to the sphere surface near the laminar flow separation point ⁠. High-speed imaging is used to investigate the effects of changing the pulling angle and counterweight-induced velocity on the hydrodynamics. We examine the transition from underwater skipping oscillations to water exit, particularly above the critical Froude number of 1.2, where buoyant spheres experience complex fluid–solid interactions revealing the influence of the air cavity on drag and lift coefficients and overall sphere hydrodynamics. Finally, we analyze the novel phenomenon of the steady motion of the horizontally pulled sphere with an attached inverted-wing-shaped air cavity.

Rotator phases are rotationally disordered plastic crystals, some of which can form upon freezing of alkane at alkane–water interfaces. Existing X-ray diffraction studies show only partial unit cell information for rotator phases of some alkanes. This includes the rotator phase of n-hexadecane, which is a transient metastable phase in pure alkane systems, but shows remarkable stability at interfaces when mediated by a surfactant. Here, we combine synchrotron X-ray diffraction data and molecular dynamics (MD) simulations, reporting clear evidence of the face-centered orthorhombic RI rotator phase from spectra for two hexadecane emulsions, one stabilized by Brij C10 and another by Tween 40 surfactants. MD simulations of pure hexadecane use the recently developed Williams 7B force field, which is capable of reproducing crystal-to-rotator phase transitions, and it also predicts the crystal structure of the RI phase. Full unit cell information is obtained by combining unit cell dimensions from synchrotron data and molecular orientations from MD simulations. A unit cell model of the RI phase is produced in the crystallographic information file (CIF) format, with each molecule represented by a superposition of four rotational positions, each with 25% occupancy. Powder diffraction spectra computed using this model are in good agreement with the experimental spectra.