Quantifying the Hydrophobic Effect per CF2 Moiety from Adsorption of Fluorinated Alcohols at the Water/Oil Interface.

Amphiphilic fluorocarbon substances are a trending topic of research due to their wide range of applications accompanied by an alarming environmental and health impact. In order to predict their fate in the environment, use them more economically, develop new water treatment methods, etc., a better understanding of their physicochemical behavior is required. Their hydrophobicity in water/oil systems is particularly sensitive to one key thermodynamic parameter: the free energy of transfer of a perfluoromethylene group from oil to water. However, for the -CF2- moiety, the transfer energy values reported in the literature vary by more than ±25%. Due to the exponential relationship between this energy and the adsorption constants or the partition coefficients, such an uncertainty can lead to orders of magnitude error in the predicted distribution of fluorinated species. We address this problem by presenting an experimental determination of the hydrophobic effect of a -CF2- moiety with a greater certainty than currently available. The transfer energy is determined by measuring the interfacial tension of water|hexane for aqueous solutions of short-chained fluorotelomer alcohols. The obtained results for the free energy of transfer of a -CF2- moiety from oil to water are 1.68±0.02×RT0, 1.75±0.02×RT0, and 1.88±0.02×RT0 at 288.15 K, 293.15 K, and 303.15 K, respectively.

Measuring the Equilibrium Spreading Pressure-A Tale of Three Amphiphiles.

A surfactant's equilibrium spreading pressure (ESP) is the maximum decrease in surface tension achievable at equilibrium below the Krafft point. Difficulties in measuring the ESP have been noted previously but no well-established experimental protocols to overcome them exist. We present a case study of three solid amphiphiles with different propensities to spread on the air-water interface. Starting with the partially water soluble n-dodecanol (C12H25OH), which spreads instantaneously. The strong Marangoni flows associated with the spreading result in the dislocating of the Wilhelmy plate or crystals attaching to it. A temporary mechanical barrier in front of the spreading crystals mitigates the flows disturbing the plate. Presaturating the subphase with the amphiphile prevents the establishment of dynamic steady states, reduces the standard error by a factor of three and causes faster equilibration. The perfluoroalkylated analog of dodecanol (11:1 fluorotelomer alcohol, C11F23CH2OH) is slow spreading. With surfactant crystals on the interface, the surface pressure reaches a pre-equilibrium plateau within an hour, followed by equilibration on day-long timescales. We show that it is better to estimate the ESP by averaging the values of multiple pre-equilibrium plateaus rather than waiting for equilibrium to be established. Finally, the nonspreading amphiphile DPPC exhibits a large barrier for the mass transfer from the DPPC crystal to the aqueous surface. This was overcome by introducing a volatile, water-immiscible solvent deposited on the surface next to the crystals to facilitate the spreading process and leave behind a monolayer.

Barrier kinetics of adsorption-desorption of alcohol monolayers on water under constant surface tension.

The desorption of spread decanol and dodecanol monolayers at controlled constant surface tension is shown to proceed under mixed barrier-diffusion control; the role of the convective diffusion is also discussed. The desorption rate is measured as a function of the density of the monolayer and the temperature. The rate of barrier desorption increases as the monolayer approaches the collapse point, reaching an infinite value. The average desorption time of an adsorbed dodecanol molecule increases linearly with the area per molecule, and is phase-specific - it is higher for the liquid condensed state of the monolayer than for the liquid expanded. The desorption rate increases with temperature; the activation energy for desorption is independent of the compression and the surface phase. The increase of the intensity of convection is shown to produce a vanishingly thin diffusion layer and causes the desorption to proceed under pure barrier control. A schematic map of the adsorption-desorption regimes acting as a function of time and intensity of the convection is constructed. General expressions for the rate of adsorption and desorption of alcohols are formulated.

Correction: Barrier kinetics of adsorption-desorption of alcohol monolayers on water under constant surface tension.

Correction for 'Barrier kinetics of adsorption-desorption of alcohol monolayers on water under constant surface tension' by Ivan L. Minkov et al., Soft Matter, 2019, DOI: 10.1039/c8sm02076k.

Combined Sum Frequency Generation and Thin Liquid Film Study of the Specific Effect of Monovalent Cations on the Interfacial Water Structure.

Some salts have been recently shown to decrease the sum frequency generation (SFG) intensity of the hydrogen-bonded water molecules, but a quantitative explanation is still awaited. Here, we report a similar trend for the chloride salts of monovalent cations, that is, LiCl, NaCl, and CsCl, at low concentrations. Specifically, we revealed not only the specific adsorption of cations at the water surface but also the concentration-dependent effect of ions on the SFG response of the interfacial water molecules. Our thin-film pressure balance (TFPB) measurements (stabilized by 10 mM of methyl isobutyl carbinol) enabled the determination of the surface potential that governs the surface electric field affecting interfacial water dipoles. The use of the special alcohol also enabled us to identify a remarkable specific screening effect of cations on the surface potential. We explained the concentration dependency by considering the direct ion-water interactions and water reorientation under the influence of surface electric field as the two main contributors to the overall SFG signal of the hydrogen-bonded water molecules. Although the former was dominant only at the low-concentration range, the effect of the latter intensified with increasing salt concentration, leading to the recovery of the band intensity at medium concentrations. We discussed the likelihood of a correlation between the effect of ions on reorientation dynamics of water molecules and the broad-band intensity drop in the SFG spectra of salt solutions. We proposed a mechanism for the cation-specific effect through the formation of an ionic capacitance at the solution surface. It explains how cations could impart the ion specificity while they are traditionally believed to be repelled from the interfacial region. The electrical potential of this capacitance varies with the charge separation and ion density at the interface. The charge separation being controlled by the polarizability difference between anions and cations was identified using the SFG response of the interfacial water molecules as an indirect probe. The ion density being affected by the absolute polarizability of ions was tracked through the measurement of the surface potentials and Debye-Hückel lengths using the TFPB technique.

Specific effects of Ca(2+) ions and molecular structure of ß-lactoglobulin interfacial layers that drive macroscopic foam stability.

ß-Lactoglobulin (BLG) adsorption layers at air-water interfaces were studied in situ with vibrational sum-frequency generation (SFG), tensiometry, surface dilatational rheology and ellipsometry as a function of bulk Ca(2+) concentration. The relation between the interfacial molecular structure of adsorbed BLG and the interactions with the supporting electrolyte is additionally addressed on higher length scales along the foam hierarchy - from the ubiquitous air-water interface through thin foam films to macroscopic foam. For concentrations <1 mM, a strong decrease in SFG intensity from O-H stretching bands and a slight increase in layer thickness and surface pressure are observed. A further increase in Ca(2+) concentrations above 1 mM causes an apparent change in the polarity of aromatic C-H stretching vibrations from interfacial BLG which we associate to a charge reversal at the interface. Foam film measurements show formation of common black films at Ca(2+) concentrations above 1 mM due to considerable decrease of the stabilizing electrostatic disjoining pressure. These observations also correlate with a minimum in macroscopic foam stability. For concentrations >30 mM Ca(2+), micrographs of foam films show clear signatures of aggregates which tend to increase the stability of foam films. Here, the interfacial layers have a higher surface dilatational elasticity. In fact, macroscopic foams formed from BLG dilutions with high Ca(2+) concentrations where aggregates and interfacial layers with higher elasticity are found, showed the highest stability with much smaller bubble sizes.

Measuring the Adsorption of Electrolytes on Lipid Monolayers.

The interactions between ions and lipid monolayers have captivated the attention of biologists and chemists alike for almost a century. In the absence of experimentally accessible concentration profiles, the electrolyte adsorption remains the most informative quantitative characteristic of the ion-lipid interactions. However, there is no established procedure to obtain the electrolyte adsorption on spread lipid monolayers. As a result, in the literature, the ion-lipid monolayer interactions are discussed qualitatively, based on the electrolyte effect on more easily accessible variables, e.g., surface tension. In this letter, we demonstrate how the electrolyte adsorption on lipid monolayers can be obtained experimentally. The procedure requires combining surface pressure versus molecular area compression isotherms with spreading pressure data. For the first time, we report an adsorption isotherm of NaCl on a lipid monolayer as a function of the density of the monolayer. The leading interactions seem to be the osmotic effect from the lipid head groups in the surface layer and ion-lipid association.

Synergy in Aqueous Systems Containing Bioactive Ingredients of Natural Origin: Saponin/Pectin Mixtures.

Biocompatible and biodegradable ingredients of natural origin are widely used in the design of foam and emulsion systems with various technological applications in the food, cosmetics and pharmaceutical industries. The determination of the precise composition of aqueous solution formulations is a key issue for the achievement of environmentally-friendly disperse systems with controllable properties and reasonable stability. The present work is focused on the investigation of synergistic interactions in aqueous systems containing Quillaja saponins and Apple pectins. Profile analysis tensiometer (PAT-1) is applied to study the surface tension and surface dilational rheology of the adsorption layers at the air/solution interface. The properties and the foam films (drainage kinetics, film thickness, disjoining pressure isotherm, critical pressure of rupture) are investigated using the thin-liquid-film (TLF) microinterferometric method of Scheludko-Exerowa and the TLF-pressure-balance technique (TLF-PBT). The results demonstrate that the structure and stability performance of the complex aqueous solutions can be finely tuned by changing the ratio of the bioactive ingredients. The attained experimental data evidence that the most pronounced synergy effect is registered at a specific saponin:pectin ratio. The obtained information is essential for the further development of aqueous solution formulations intended to achieve stable foams based on mixtures of Quillaja saponins and Apple pectins in view of future industrial, pharmaceutical and biomedical applications.

Hydrophobisation of Silica Nanoparticles Using Lauroyl Ethyl Arginate and Chitosan Mixtures to Induce the Foaming Process.

We studied silica suspensions with chitosan and biodegradable synthetic surfactant lauroyl ethyl arginate (LAE). Hydrophilic and negatively charged silica nanoparticles were neutralised due to the coating with chitosan. That presence of LAE led to the partial hydrophobisation of their surface, which favoured their attachment to the surface of a thin foam film. It was found that the presence of small and medium-sized (6-9 nm) hydrophobic particles in the interfacial layer of lamella foam film inhibited the coalescence and coarsening processes, which prolonged the life of the foam. Furthermore, hydrophobising of 30 nm particles allowed the formation of large aggregates precipitating from the mixture under steady-state conditions. These aggregates, however, under the conditions of the dynamic froth flotation process in the foam column, were floated into the foam layer. As a result, they were trapped in the foam film and Plateau borders, effectively preventing liquid leakage out of the foam. These results demonstrate the efficiency of using chitosan-LAE mixtures to remove silica nanoparticles from aqueous phase by foaming and flotation.

Effect of pH-Regulation on the Capture of Lipopolysaccharides from E. coli EH100 by Four-Antennary Oligoglycines in Aqueous Medium.

Bacterial lipopolysaccharides (LPS) are designated as endotoxins, because they cause fever and a wide range of pathologies in humans. It is important to develop effective methodologies to detect trace quantities of LPS in aqueous systems. The present study develops a fine-tuning procedure for the entrapment of trace quantities of LPS from E. coli EH100. The capture agents are self-assemblies (tectomers) formed by synthetic four-antennary oligoglycine (C-(CH2-NH-Gly7)4, T4). Based on previously performed investigations of bulk and adsorption-layer properties of aqueous solutions containing T4 and LPS, the optimal conditions for the entrapment interactions are further fine-tuned by the pH regulation of aqueous systems. A combined investigation protocol is developed, including dynamic light scattering, profile analysis tensiometry, microscopic thin-liquid-film techniques, and transmission electron microscopy. The key results are: (1) two types of complexes between T4 and LPS are generated-amphiphilic species and "sandwich-like" hydrophilic entities; the complexes are smaller at lower pH, and larger at higher pH; (2) an optimum range of pH values is established within which the whole quantity of the LPS is entrapped by the tectomers, namely pH = 5.04-6.30. The obtained data substantiate the notion that T4 may be used for an effective capture and the removal of traces of endotoxins in aqueous systems.

Smart Complex Fluids Based on Two-Antennary Oligoglycines.

Antennary oligoglycines are synthetic products, obtained as a result of preliminary molecular design. Equal-length antennae are built of glycine residues joined through the C end to an oligoamine branching core with an amine group at the N terminus exposed outside. The results of systematic research on the properties of aqueous solutions containing two-antennary oligoglycine with four glycine portions are reported. The central feature is the competition between amphiphilic self-assembly and formation of polyglycine II motifs. A combined procedure is developed to characterize bulk and interfacial structures and coatings. It includes registration of bulk aggregates, examination of interfacial layers at solution/air and solution/solid boundaries, drainage, and stability of liquid films. The obtained results provide new insight into the structure-property relationships in these smart fluids and give essential hints about key factors allied to possible applications in medicine, pharmaceuticals, and environmental protection.

Self-Assembly of a Thermally Responsive Double-Hydrophilic Copolymer in Ethanol-Water Mixtures: The Effect of Preferential Adsorption and Co-Nonsolvency.

Lower alcohols can induce a combined collapse-swelling de-mixing transition (lower critical solution temperature (LCST)-type co-nonsolvency) in aqueous solutions of poly( N-isopropylacrylamide) (PNIPAM) by interacting with the polymer's amide groups. This interaction results in an increase of the total surface area of hydrophobic sites and destabilizes the chains. Here, we make use of this phenomenon to drive the counterintuitive self-assembly of a PNIPAM-containing double-hydrophilic graft copolymer in water-ethanol mixtures at T ≪ LCST. Rheological frequency sweeps are used to quantify the distinct solvation states of PNIPAM at various temperatures and ethanol concentrations. The energy stored through elastic deformation at the de-mixing transition is simply related to the solvent binding. We find that the storage modulus decreases progressively, but nonlinearly with ethanol concentration, which evidences a preferential solvation pattern. Analogously, through a combination of dynamic light scattering and transmission electron microscope analyses, we demonstrate that a low-temperature structure variation takes place by adding ethanol following a similar solvent-content morphology dependent model.

Amphiphilic nanostructures in thin liquid films.

A survey on recent experimental investigations of microscopic foam films containing self-assembled amphiphilic nanostructures is presented. The film characteristics are investigated via microinterferometric method, which operates with the measuring cell of Scheludko-Exerowa. The results show the following: (1) Unstable black patterns (dots and spots) are observed; they have very short lifetimes and the films which contain them rupture quickly. (2) Drainage times of the films display sharp changes within the studied surfactant concentration range. (3) The peculiarities in the film drainage properties are in accordance with the specific run of the adsorption isotherms of the initial surfactant solutions. The data are interpreted based on the assumption that a series of smaller self-assembled aggregates (premicelles) exist in amphiphilic solutions. The results show that the microscopic foam film technique has a serious potential as a prospective instrumentation for the study of amphiphilic self-assemblies in surfactant solutions.

Entrapment efficiencies of hydrodynamic boundary layers on rising bubbles.

Flotation and separation practice shows that fine hydrophilic solids are often drawn into the froth product. The occurrence of this unwanted event in the classical froth flotation has led to the idea of using it for the separation by size of ground materials. Thus, a method for the extraction of hydrophilic fines by foaming of a suspension was proposed. The aim of the present study is to relate this phenomenon to the residence time of the particles in the vicinities of the rising bubbles. Dynamic interactions of fine solids with rising bubbles cause perturbations in the background flow field. A procedure for the mathematical modeling of these disturbance effects is proposed. The initial idea is that the particles lag behind the background bubble-driven flows. A key point is the possibility of classifying fine entities according to a general criterion, containing only parameters of the outer flow. The basic result is that there exists a range of particle and bubble dimensions for which this entrapment is optimal. The proposed model investigation gives a concise explanation for the observed capture of fine solids in many flotation and separation processes.