•Measurement of the surface dilatational modulus of aqueous SDeS solutions up to 500 Hz.•Crossover from surface elastic to surface visco-elastic behavior with increasing bulk concentration.•The stability of individual foam lamellae was assessed.•Foam stability correlates with crossover of the modulus to a surface visco-elastic state.•Demonstration of the key role of surface dilatational rheology in understanding foams.The anionic surfactant sodium decyl sulfate (SDeS) at concentrations below its critical micellar concentration (cmc) was studied in a surface chemically purified state at the air–water interface in terms of its static and dynamic interfacial properties. Equilibrium surface tension was determined using a pendant drop tensiometer. An oscillating bubble capillary pressure tensiometer allowed accessing the surface dilatational properties of the surfactant's adsorption layers in a frequency range from 2 to 500 Hz. The estimation of single foam lamella stability was obtained from visual observation of films formed within a rectangular glass frame in a saturated atmosphere. Upon increasing surfactant concentration, the foam lamellae were found to rupture at prolonged lifetimes. The dramatic difference in foam stability goes along with a pronounced transition from a surface elastic to a surface visco-elastic state of the adsorption layers. These findings were indicative for a correlation between foam lamella stability and surface viscoelasticity also for this model surfactant. These results may be of importance to further shed light on the processes governing foam and foam lamella stability.Download high-res image (68KB)Download full-size image

Highlights•The surface dilatational modulus of aqueous SDS solution was measured in the frequency range from 3 Hz to 500 Hz by a novel version of the oscillating bubble technique. The influence of highly charged Ce4+-ions on the surface dilatational modulus and on the foam stability is analyzed.•The experiments suggest a transition from surface visco-elastic behavior in case of solutions of pure SDS to an almost completely elastic behavior for mixed SDS/Ce4+-solutions.•The presence of Ce4+-ions introduced as Ce(SO4)2·4H2O modifies foam stability and surface activity, i.e., a decrease in surface tension, cmc and foam stability could be observed. The results illustrate the important role of surface dilatational rheology and foam stability.The surface dilatational modulus E is a fundamental system parameter of aqueous surfactant solutions which plays an important role in the physic of foams. In this contribution, we study the influence of highly charged Cerium-ions (Ce4+)-on the surface dilatational modulus E of aqueous sodium dodecyl sulfate (SDS) solutions. The E-modulus has been determined at two concentrations below the critical micellar concentration (cmc) in the frequency range from 3 Hz to 500 Hz using the oscillating bubble technique. The presence of Ce4+-ions increases the surface activity of the surfactant leading to a shift of the equilibrium surface tension isotherm to lower concentrations. The cmc is reduced to 2 × 10−3 mol L−1. Adsorption layers of aqueous SDS solutions exhibit a surface visco-elastic behavior. The magnitude and the phase of the complex dilatational modulus E increase in a continuous fashion with the frequency indicating a dissipative process within the adsorption layer. The presence of the electrolyte modifies the frequency characteristics of the complex modulus towards the features of a surface elastic system. Furthermore there are pronounced changes in the stability of the corresponding foams. The lifetime of wet foam lamella is significantly reduced by the presence of the Ce4+-ions. The results provide evidence for the decisive impact of surface visco-elastic properties on foam stability of aqueous surfactant solutions.Graphical abstract

Beetles of the species Stenus comma live and hunt close to ponds and rivers, where they occasionally fall on the water surface. To escape this jeopardized state, the beetle developed a strategy relying on the excretion of a secretion containing the substances stenusine and norstenusine. They reduce surface tension and propel the bug to the saving river bank. These substances were synthesized and analyzed with respect to their equilibrium and dynamic adsorption properties at the air–water interface (pH 7, 23 ± 1 °C). The surface dilatational rheological characteristics in a frequency range from 2 to 500 Hz at molar bulk concentrations of 20.6 mmol L–1 were studied using the oscillating bubble technique. Both alkaloids formed surface viscoelastic adsorption layers. The frequency dependence of the surface dilatational modulus E could successfully be described by the extended Lucassen–van den Tempel model accounting for a nonzero intrinsic surface viscosity κ. The findings confirmed a dual purpose of the spreading alkaloids in the escape mechanism of the Stenus beetle. Next to generating a surface pressure, a transition to surface viscoelastic behavior of the adsorbed layers was observed.

The relation between the complex surface dilatational modulus E of aqueous surfactant solutions and the splashing behavior of their drops on liquid surfaces was investigated. The surface dilatational modulus E of selected surfactant systems has been determined in the frequency range of 3 to 500 Hz by means of the oscillating bubble technique. According to the functional dependence of the phase ϕ of the complex modulus E(ω, c)exp[iϕ(ω, c)] at higher frequencies, adsorption layers can be classified as surface elastic or surface viscoelastic. Each behavior shows pronounced differences in drop splashing experiments. The impact of a drop on the liquid was monitored with a high-speed camera. The splash of a drop is a rather complex phenomenon, so the focus of this article is to establish a relationship between the imaginary part of the surface dilatational modulus E and the height of the drop rebound. These findings may be of importance for formulations in crop protection, introducing a chemical way to influence the impact of drops on solid and liquid interfaces.

In this Article, we investigate the impact of the fluid-air interface on the dissolved electrolyte potassium ferrocyanide K4[Fe(CN)6] by infrared-visible sum-frequency generation spectroscopy (SFG) and surface tension measurements. SFG is a surface specific nonlinear optical technique that records the vibrational spectrum of the adsorbed species with little to no contribution from the bulk phase. According to the selection rules, only modes which are simultaneously Raman and Infrared active can contribute to the SFG spectrum. Since [Fe(CN)6]4– belongs to the point group Oh, no vibrational mode is SFG active, unless a deformation takes place. Our spectra provide experimental evidence for a symmetry reduction of the ions at the interface to subgroups with no center of inversion. The distortion of the ions is the direct consequence of the asymmetric environment at the interface and is further discussed in this paper. The SFG spectra of the interfacial water were used to gain further insights in the distribution of the ions at the interface. Furthermore, the combined data of the surface tension equilibrium isotherm and the SFG spectra suggest a nonmonotonous interfacial concentration profile of the ion.

Natural nonionic amphiphiles forming monolayers, bilayers, micelles, or biomembranes create a positive dipole potential at the boundary with water. In a series of papers we have reported on Langmuir monolayers with CF3 terminals of the polar heads, which show a negative surface dipole potential ΔV (Petrov, J. G.; Andreeva, T. D.; Kurt, D. K.; Möhwald, H. J. Phys. Chem. B 2005, 109, 14102). Here we use vibrational sum frequency spectroscopy (SF) to study the origin of the opposite ΔV signs of Langmuir films of CH3(CH2)20COCH2CH3 (ethyl ether, EE) and CH3(CH2)20COCH2CF3 (fluorinated ethyl ether, FEE). The vibrational sum frequency spectra are recorded at the same film density of the S-phase of the EE and FEE monolayers and analyzed in the spectral regions of OH, COC, CH3, and CF3 stretching vibrations because these functional groups could be responsible for the different dipole potentials. We compare the rearrangement of the pure water surface by EE and FEE monolayers and the conformations of EE and FEE polar heads. The analysis is performed according to the three-capacitor model of the dipole potential of Langmuir monolayers (Demchak, R. T.; Fort, T., Jr. J. Colloid Interface Sci. 1974, 46, 191). The results show that reversal of the ΔV sign caused by fluorination of the polar heads originates from the upward-oriented CF3 terminals of the FEE heads, whose negative normal dipole moment component determines the negative dipole potential of the FEE monolayer.

Thiocyanate (SCN−) is a highly chaotropic anion of considerable biological significance, which interacts quite strongly with lipid interfaces. In most cases it is not exactly known if this interaction involves direct binding to lipid groups, or some type of indirect association or partitioning. Since thiocyanate is a linear ion, with a considerable dipole moment and nonspherical polarizability tensor, one should also consider its capability to adopt different or preferential orientations at lipid interfaces. In the present work, the interaction of thiocyanate anions with zwitterionic phospholipid monolayers in the liquid expanded (LE) phase is examined using surface pressure−area per molecule (π−AL) isotherms and vibrational sum frequency generation (VSFG) spectroscopy. Both dipalmitoyl phosphatidylcholine (DPPC) and dimyristoyl phosphatidylethanolamine (DMPE) lipids, which form stable monolayers, have been used in this investigation, since their headgroups may be expected to interact with the electrolyte solution in different ways. The π−ΑL isotherms of both lipids indicate a strong expansion of the monolayers when in contact with SCN− solutions. From the C−H stretch region of the VSFG spectra it can be deduced that the presence of the anion perturbs the conformation of the lipid chains significantly. The interfacial water structure is also perturbed in a complex way. Two distinct thiocyanate populations are detected in the CN stretch spectral region, proving that SCN− associates with zwitterionic phospholipids. Although this is a preliminary investigation of this complex system and more work is necessary to clarify certain points made in the discussion, a potential identification of the two SCN− populations and a molecular-level explanation for the observed effects of the SCN− on the VSFG spectra of the lipids is provided.