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Slavka Tcholakova

Publications and source records attributed to Slavka Tcholakova.

15 recordsLinked to original sources
arXiv2025

Sucrose ester surfactants: current understanding and emerging perspectives

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, incl. emulsions, foams, oleogels, and others, is provided. Novel concepts are discussed alongside future research directions, emphasizing the SEs potential as sustainable, functional ingredients.

cond-mat.soft↗
arXiv2025

Salt-induced gelation of nonionic sucrose ester dispersions

The dispersions of nonionic sucrose ester surfactants in water exhibit a highly negative zeta-potential, though its origin remains controversial. The addition of electrolytes to these dispersions may influence their zeta-potential, thus potentially affecting their physicochemical properties. The electrolyte- and pH- driven gelation of aqueous dispersions of commercial sucrose stearate (S970) containing ca. 1:1 monoesters and diesters was studied using optical microscopy, rheological and zeta-potential measurements, and small-angle X-ray scattering techniques. At low electrolyte concentrations and pH $\gtrapprox$ 5, 0.5-5 wt. % S970 dispersions exhibited low viscosities and behaved as freely flowing liquids. The addition of electrolytes of low concentrations, e.g. 9 mM NaCl or 1.5 mM MgCl$_2$, induced the formation of a non-flowing gels. This sol-gel transition occurred due to the partial screening of the diesters particles charge, allowing the formation of an attractive gel network, spanning across the dispersion volume. Complete charge screening, however, led to a gel-sol transition and phase separation. Gel formation was observed also by pH variation without electrolyte addition, whereas the addition of free fatty acids had negligible impact on dispersion properties. These findings support the hypothesis that the negative charge in sucrose ester dispersions arises from hydroxyl anions adsorption on particles surfaces. Gels were formed using just 1.3 wt. % surfactant, and the critical electrolyte concentration for gelation was found to scale approximately with the square of the cation charge, in agreement with the low surface charge density theory. The biodegradable sucrose esters gels offer a sustainable alternative for structuring personal and home care products, replacing the wormlike micelles of synthetic surfactants typically used at much higher surfactant and salt concentrations.

cond-mat.soft↗
arXiv2023

Structure of rotator phases formed in C$_{13}$-C$_{21}$ alkanes and their mixtures: in bulk and in emulsion drops

Crystallization of alkane mixtures has been studied extensively for decades. However, majority of the available data consider the behaviour of alkanes with chain length of 21 C-atoms or more. Furthermore, important information about the changes of the unit cell structure with temperature is practically absent. In this work, the phase behavior of several pure alkanes $C_{n}H_{2n+2}$, with $n$ ranging between 13 and 21, and their binary, ternary or multi-component 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 formation of mixed rotator phases for all systems with chain length difference between the neighbouring alkanes of $Δn \leqslant 3$. Partial demixing is observed when $Δn = 4 $, yet the main fraction of the alkane molecules arranges in a mixed rotator phase in these samples. This demixing is suppressed if an alkane with intermediate chain length is added to the mixture. Interestingly, a steep temperature dependence of the interlamellar spacing in mixed rotator phases was observed upon cooling at temperatures down to 10°C below the melting temperature of the mixture. The volumetric coefficient of thermal expansion of the rotator phases of mixed alkanes ($α_{V} \approx 2$ x $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.

cond-mat.mtrl-sci↗
arXiv2023

Polymorphic phase transitions in bulk triglyceride mixtures

Triacylglycerols (TAGs) are among the most important ingredients in food, cosmetic and pharmaceutical products. Many physical properties of such products, incl. morphology, texture and rheology, are determined by the phase behaviour of the included TAGs. Triglycerides are also of special interest for the production of solid lipid nanoparticles, applied for controlled drug delivery and for encapsulation of bioactive ingredients. In this paper, we study the polymorphic behaviour of complex TAG mixtures, composed of 2 to 6 mixed TAGs, by differential scanning calorimetry and X-ray scattering techniques, aiming to reveal the general rules for their phase behaviour upon cooling and heating. The results show that two or more coexisting phases form upon solidification $(α$, $β'$ and/or $β)$, the number of which depends strongly on the cooling rate and on the number of components in the mixture. No completely miscible $α$- or $β'$-phases were observed. The structure of the most stable $β$ polymorphs, formed upon subsequent heating of the solidified samples, does not depend on the thermal history of the samples. For all mixtures studied, we observed one-component $β$ domains, coexisting with binary mixed $β$ domains with composition and structure which do not depend on the specific TAG ratio in the mixture. In other words, for a mixture with $k$ saturated TAGs we observed $(2k-1)$ different $β$ phases. These conclusions provide some predictive power when analysing the phase transition properties of TAG mixtures.

cond-mat.soft↗
arXiv2022

Minimum Surfactant Concentration Required for Inducing Self-shaping of Oil Droplets and Competitive Adsorption Effects

Surfactant choice is key in starting the phenomena of artificial morphogenesis, the bottom-up growth of geometric particles from cooled emulsion droplets, as well as the bottom-up self-assembly of rechargeable microswimmer robots from similar droplets. The choice of surfactant is crucial for the formation of a plastic phase at the oil-water interface, for the kinetics, and for the onset temperature of these processes. But further details are needed to control these processes for bottom-up manufacturing and understand their molecular mechanisms. Still unknown are the minimum concentration of the surfactant necessary to induce the processes, or competing effects in a mixture of surfactants when only one is capable of inducing shapes. Here we systematically study the effect of surfactant nature and concentration on the shape-inducing behaviour of hexadecane-in-water emulsions with both cationic (CTAB) and non-ionic (Tween, Brij) surfactants over up to five orders of magnitude of concentration. The minimum effective concentration is found approximately equal to the critical micelle concentration (CMC), or the solubility limit below the Krafft point of the surfactant. However, the emulsions show low stability at the vicinity of CMC. In a mixed surfactant experiment (Tween 60 and Tween 20), where only one (Tween 60) can induce shapes we elucidate the role of competition at the interface during mixed surfactant adsorption by varying the composition. We find that a lower bound of ~ 75% surface coverage of the shape-inducing surfactant with C14 or longer chain length is necessary for self-shaping to occur. The resulting technique produces a clear visual readout of otherwise difficult to investigate molecular events and establish basic requirements for minimum concentration and protocols to find % surface coverage to induce oil self-shaping by surfactant mixtures.

cond-mat.soft↗
arXiv2021

Rheological properties of rotator and crystalline phases of alkanes

Linear long-chain organic molecules are known to form lamellar intermediate phases (called also rotator phases) between their fully ordered crystalline phases and their isotropic liquid phases. The properties of intermediate rotator phases are crucially important for various industrial and living nature processes, but the data for their rheological properties are almost missing, due to the specific difficulties in the respective experiments. In the current study we describe a methodology for measuring and comparing the shear rheological properties of rotator (R) and crystalline (C) phases formed in bulk hydrocarbons at temperatures below the melting temperature. We apply this approach to characterize the rheological properties of R and C phases formed upon cooling of alkanes with chain length varied between 17 and 30 carbon atoms. For comparison, we study also several alkane mixtures and one alkene with double bond at the end of its chain. The obtained results show that the storage and loss moduli of the rotator phases are ca. 10-times lower than those of the respective crystalline phases. We found also that the rheological properties of the crystal phases depend mainly on the subcooling temperature below the crystallization temperature, while the R phases become softer with the increase of the molecular length. We explain these results by assuming that the rheological properties of the crystal phases are determined mainly by the sliding of the ordered crystal domains with respect to each other, while in the rotator phases we have multiple defects in the molecular packing which increase with the alkane length. The proposed methodology and the obtained results serve as a solid basis for further rheological studies of this important class of technological systems.

cond-mat.soft↗
arXiv2021

Rechargeable self-assembled droplet microswimmers driven by surface phase transitions

The design of artificial microswimmers is often inspired by the strategies of natural microorganisms. Many of these creatures exploit the fact that elasticity breaks the time-reversal symmetry of motion at low Reynolds numbers, but this principle has been notably absent from model systems of active, self-propelled microswimmers. Here we introduce a class of microswimmer that spontaneously self-assembles and swims without using external forces, driven instead by surface phase transitions induced by temperature variations. The swimmers are made from alkane droplets dispersed in aqueous surfactant solution, which start to self-propel upon cooling, pushed by rapidly growing thin elastic tails. When heated, the same droplets recharge by retracting their tails, swimming for up to tens of minutes in each cycle. Thermal oscillations of approximately 5 degrees Celsius induce the swimmers to harness heat from the environment and recharge multiple times. We develop a detailed elastohydrodynamic model of these processes and highlight the molecular mechanisms involved. The system offers a convenient platform for examining symmetry breaking in the motion of swimmers exploiting flagellar elasticity. The mild conditions and biocompatible media render these microswimmers potential probes for studying biological propulsion and interactions between artificial and biological swimmers.

cond-mat.soft↗
arXiv2021

Cold-burst method for nanoparticle formation with natural triglyceride oils

Preparation of nanoemulsions of triglyceride oils in water usually requires high mechanical energy and sophisticated equipment. Recently, we showed that alpha-to-beta (viz. gel-to-crystal) phase transition, observed with most lipid substances (triglycerides, diglycerides, phospholipids, alkanes, etc.), may cause spontaneous disintegration of micro-particles of these lipids, dispersed in aqueous solutions of appropriate surfactants, into nanometer particles/drops using a simple cooling/heating cycle of the lipid dispersion (Cholakova et al. ACS Nano 14 (2020) 8594). In the current study we show that this "cold-burst process" is observed also with natural oils of high practical interest, incl. coconut oil, palm kernel oil and cocoa butter. Mean drop diameters of ca. 50 to 100 nm were achieved with some of the studied oils. From the results of dedicated model experiments we conclude that intensive nano-fragmentation is observed when the following requirements are met: (1) The three phase contact angle at the air-water-solid lipid interface is below ca. 30 degrees; (2) The equilibrium surface tension of the surfactant solution is below ca. 30 mN/m and the dynamic surface tension decreases rapidly. (3) The surfactant solution contains non-spherical surfactant micelles. e.g. ellipsoidal micelles or bigger supramolecular aggregates; (4) The three phase contact angle measured at the contact line (frozen oil-melted oil-surfactant solution) is also relatively low. The mechanism(s) of the particle bursting process is revealed and, on this basis, the role of all these factors is clarified and discussed. We explain all main effects observed experimentally and define guiding principles for optimization of the cold-burst process in various, practically relevant lipid-surfactant systems.

cond-mat.soft↗
arXiv2021

Rotator phases in hexadecane emulsion drops revealed by X-ray synchrotron techniques

Micrometer sized alkane-in-water emulsion drops, stabilized by appropriate long-chain surfactants, spontaneously break symmetry upon cooling and transform consecutively into series of regular shapes (Denkov et al., Nature 2015, 528, 392). Two mechanisms were proposed to explain this phenomenon of drop "self-shaping". One of these mechanisms assumes that thin layers of plastic rotator phase form at the drop surface around the freezing temperature of the oil. This mechanism has been supported by several indirect experimental findings but direct structural characterization has not been reported so far. We combine small- and wide-angle X-ray scattering (SAXS/WAXS) with optical microscopy and DSC measurements of self-shaping drops in emulsions. In the emulsions exhibiting drop self-shaping, the scattering spectra reveal the formation of intermediate, metastable rotator phases in the alkane drops before their crystallization. In addition, shells of rotator phase were observed to form in hexadecane drops, stabilized by C16EO10 surfactant. This rotator phase melts at ca. 16.6 °C which is significantly lower than the melting temperature of crystalline hexadecane, 18 °C. The scattering results are in a very good agreement with the complementary optical observations and DSC measurements.

cond-mat.soft↗
arXiv2020

Nanopore and nanoparticle formation with lipids, undergoing polymorphic phase transitions

We describe several unexpected phenomena, caused by a solid-solid phase transition (gel-to-crystal) typical for all main classes of lipid substances - phospholipids, triglycerides, diglycerides, alkanes, etc. We discovered that this transition leads to spontaneous formation of a network of nanopores, spreading across the entire lipid structure. These nanopores are spontaneously impregnated (flooded) by water when appropriate surfactants are present, thus fracturing the lipid structure at a nano-scale. As a result, spontaneous disintegration of the lipid into nanoparticles or formation of double emulsions is observed, just by cooling and heating of an initial coarse lipid-in-water dispersion around the lipid melting temperature. The process of nanoparticle formation is effective even after incorporation of medical drugs of high load, up to 50 % in the lipid phase. The role of the main governing factors is clarified, the procedure is optimized, and the possibility for its scaling-up to industrially relevant amounts is demonstrated.

cond-mat.mtrl-sci↗
arXiv2020

Origin of the extremely high elasticity of bulk emulsions, stabilized by Yucca Schidigera saponins

We found experimentally that the elasticity of sunflower oil-in-water emulsions (SFO-in-W) stabilized by Yucca Schidigera Roezl saponin extract, is by more than 50 times higher as compared to the elasticity of common emulsions. We revealed that strong specific interactions between the phytosterols from the non-purified oil and the saponins from the Yucca extract lead to the formation of nanostructured adsorption layers which are responsible for the very high elasticity of the oil-water interface and of the respective bulk emulsions. Remarkably, this extra high emulsion elasticity inhibits the emulsion syneresis even at 65 vol % of the oil drops. These emulsions remain homogeneous and stable even after 30 days of shelf-storage. These results demonstrate that the combination of saponin and phytosterols is a powerful new approach to structure oil-in-water emulsions with potential applications for formulating healthier functional food.

cond-mat.soft↗
arXiv2019

Role of interfacial elasticity for the rheological properties of saponin-stabilized emulsions

Hypothesis Saponins are natural surfactants which can provide highly viscoelastic interfaces. This property can be used to quantify precisely the effect of interfacial dilatational elasticity on the various rheological properties of bulk emulsions. Experiments We measured the interfacial dilatational elasticity of adsorption layers from four saponins (Quillaja, Escin, Berry, Tea) adsorbed on hexadecane-water and sunflower oil-water interfaces. In parallel, the rheological properties under steady and oscillatory shear deformations were measured for bulk emulsions, stabilized by the same saponins (oil volume fraction between 75 and 85 %). Findings Quillaja saponin and Berry saponin formed solid adsorption layers (shells) on the SFO-water interface. As a consequence, the respective emulsions contained non-spherical drops. For the other systems, the interfacial elasticities varied between 2 mN/m and 500 mN/m. We found that this interfacial elasticity has very significant impact on the emulsion shear elasticity, moderate effect on the dynamic yield stress, and no effect on the viscous stress of the respective steadily sheared emulsions. The last conclusion is not trivial, because the dilatational surface elasticity is known to have strong impact on the viscous stress of steadily sheared foams. Mechanistic explanations of all observed effects are described.

cond-mat.soft↗
arXiv2019

Food grade nanoemulsions preparation by rotor-stator homogenization

High-pressure homogenizers, typically used for producing nanoemulsions at the industrial scale, are energy and maintenance intensive, and limited to produce only dilute, low viscosity nanoemulsions. We propose an alternative approach to produce dilute to concentrated food-grade nanoemulsions with droplet size ranging between 100 and 500 nm using rotor-stator homogenization. Gum Arabic (GA) or modified starch (MS) was used as both viscosity modifier and emulsion stabilizer. GA and MS have relatively low surface activity compared to the common low-molecular-mass surfactants used typically for nanoemulsion preparation. The main differences between GA and MS are the lower viscosity of the GA solutions, compared to MS solutions, and the faster adsorption of MS, as compared to GA. The obtained results show that stable nanoemulsions are formed by rotor-stator homogenization when the rapidly adsorbing MS is used as emulsifier. Much larger drops are formed during emulsification with GA, which is due to significant drop-drop coalescence in the respective emulsions. The experimental results for the nanoemulsions prepared with MS are well-described by the theoretical expression for emulsification in turbulent viscous regime, after proper account for the effects of temperature and drop-drop interactions in the sheared emulsions.

cond-mat.soft↗
arXiv2019

Spontaneous particle desorption and "Gorgon" drop formation from particle-armored oil drops upon cooling

Drop "self-shaping" is a phenomenon in which cooled oily emulsion drops undergo a spectacular series of shape transformations (Denkov et al., Nature 528, 2015, 392). Solid particles adsorbed on the oil-water interface could affect this drop self-shaping process in multiple ways which have not been studied. We prepared Pickering emulsions stabilized by spherical latex particles and afterwards added surfactant of low concentration which enabled drop self-shaping. Next we observed by optical microscopy the processes which occur upon emulsion cooling. Several new processes were observed: (1) Adsorbed latex particles rearranged into regular hexagonal lattices upon freezing of the surfactant adsorption layer. (2) Spontaneous particle desorption from the drop surface was observed at a certain temperature - this phenomenon is rather remarkable, as the solid particles are known to irreversibly adsorb on fluid interfaces. (3) Very strongly adhered particles to drop surfaces acted as a template to enable the formation of tens to hundreds of semi-liquid fibers, growing outwards from the drop surface, thus creating a shape resembling the Gorgon head from Greek mythology. We provide mechanistic explanations of all observed phenomena using our understanding of the rotator phase formation on the surface of cooled drops.

cond-mat.soft↗
arXiv2019

Surface phase transitions in foams and emulsions

Surface phase transitions in surfactant adsorption layers are known to affect the dynamic properties of foams and to induce surface nucleation in freezing emulsion drops. Recently, these transitions were found to play a role in several other phenomena, opening new opportunities for controlling foam and emulsion properties. This review presents a brief outlook of the emerging opportunities in this area. Three topics are emphasized: (1) The use of surfactant mixtures for inducing phase transitions on bubble surfaces in foams; (2) The peculiar properties of natural surfactants saponins which form extremely viscoelastic surface layers; and (3) The main phenomena in emulsions, for which the surface phase transitions are important. The overall conclusion from the reviewed literature is that surface phase transitions could be used as a powerful tool to control many foam and emulsion properties, but we need deeper understanding of the underlying phenomena to explore fully these opportunities.

cond-mat.soft↗