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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 $(\alpha$, $\beta'$ and/or $\beta)$, the number of which depends strongly on the cooling rate and on the number of components in the mixture. No completely miscible $\alpha$- or $\beta'$-phases were observed. The structure of the most stable $\beta$ 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 $\beta$ domains, coexisting with binary mixed $\beta$ 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 $\beta$ phases. These conclusions provide some predictive power when analysing the phase transition properties of TAG mixtures.

Understanding how the surfactant molecular structure affects foam stability is important in various applications, such as soil remediation, the food industry, enhanced oil recovery, and hydraulic fracturing. In this study, we conduct a systematic series of experiments in a Hele-Shaw cell to assess and explain the effect of surfactants on foamability and foam stability in the absence and presence of oil and asphaltene aggregates. Four surfactants with different molecular weights were studied, including three anionic surfactants, sodium 1,5-bis[(1H,1H,2H,2H-perfluorohexyl)oxy]-1,5-dioxopentane-2-sulfonate (FG4), C18H37SO4Na (LSES), and sodium dodecyl sulfate (SDS), and one cationic surfactant (CTAB, (C16H33NMe3)Br). We observed that the higher electrostatic repulsion between the foam film surfaces for the anionic surfactants (FG4 > LSES > SDS) strongly influences the process of foam generation and stability when compared with the cationic surfactant (CTAB). For example, the foam coarsening rate for CTAB was 700% higher than that for FG4 when we performed experiments in an empty Hele-Shaw cell. Furthermore, the foam experiments performed in the presence of oil revealed maximum long-term stability and minimum bubble coarsening for the partially fluorinated FG4 surfactant. The experiments in the presence of asphaltene-oil mixtures revealed that the latter has a detrimental effect on foam stability, except for FG4. The foam experiments clearly show the significance of the subtle interactions between the surfactant headgroup charge, chain length, and branching. The obtained results could be useful in designing appropriate surfactants for foam stabilization in various applications.

Correction for ‘Spontaneous particle desorption and “Gorgon” drop formation from particle-armored oil drops upon cooling’ by Diana Cholakova et al., Soft Matter, 2020, 16, 2480–2496, DOI: 10.1039/C9SM02354B.

Настоящата дисертация разглежда процес на спонтанни деформации на формата на диспергирани емулсионни капки от алкани и други неполярни вещества. Показано е, че капките се деформират повече, ако са по-малки по размер, стабилизирани са с повърхностно-активни вещества с наситена опашка с дължина близка или по-голяма от тази на молекулите в капките и/или емулсиите се охлаждат с ниска скорост. Тези тенденции са наблюдавани с емулсии от тип масло-във-вода от различни вещества, съставени от молекули, съдържащи права наситена верига. За изследваните вещества е известно, че обемната им кристализация се предхожда от появата на междинни ротаторни (пластични) фази. Механизмът, обясняващ наблюдаваните деформации включва замръзване на адсорбционния слой от ПАВ по повърхността на капките, което индуцира подреждане на прилежащи слоеве от неполярни молекули. Чрез директни експерименти с DSC и SAXS е определен делът на подредените молекули в капките и вида на подредената структура, съответстваща на пластична ротаторна фаза. С помощта на теоретичен модел са интерпретирани експерименталните резултати като е показано, че за някои от системите тя достига дебелина до 24 ± 6 слоя от подредени молекули, а за други е от порядъка на няколко молекулни дължини. Определено е, че структурата на ротаторните фази в капките наподобява тази на твърдите тела, при които има захващане на слоевете един спрямо друг, водещо до значително съпротивление при деформация на прехлъзване. The current thesis describes a spontaneous self-shaping phenomenon observed upon cooling of dispersed emulsion drops of alkanes or other non-polar chemical compounds with saturated long alkyl chains. Plastic rotator phases are known to form for all these substances, prior to their bulk crystallization. It is shown that the drops in oil-in-water emulsions deform further along the universal evolutionary scheme when they have smaller initial size, emulsions are stabilized by surfactants with saturated linear tail whose length is similar or longer than that of the dispersed oil molecules and/or the samples are cooled at lower rates. The mechanism of drop shape deformations includes freezing of the surfactant adsorption layer on the drop surface, which further induces ordering of the adjacent layers of hydrophobic molecules. Via DSC and SAXS experiments we determined the percentage of alkane molecules undergoing phase transition prior to drop freezing and the type of molecular arrangement for several of the systems. Using theoretical model we interpreted the obtained data and estimated that the rotator layer thickness varyies between few molecules to 24 ± 6 stacked monolayers or ordered molecules. The obtained results indicate that the interactions inside the formed plastic multilayers are closer to those in solid crystals in which significant resistance exists for shearing of the neighboring layers with respect to each other.

International audience; The generation of liquid foams is at the heart of numerous natural, technical or scientific processes. Even though the subject of foam generation has a long-standing history, many recent progresses have been made in an attempt to elucidate the fundamental processes at play. We review the subject by providing an overview of the relevant key mechanisms of bubble generation within a coherent hydrodynamic context; and we discuss different foaming techniques which exploit these mechanisms.