Surface Rheological Data for a Polymeric Surfactant Using a Pulsed Drop Rheometer

Oscillatory and Relaxation Study of the Interfacial Rheology of Star Polymers with Low-Grafting-Density PEO Arms and Hydrophobic Poly(divinylbenzene) Cores

Star polymers have been gaining interest due to their tunable properties. They have been used as effective stabilizers for Pickering emulsions. Herein, star polymers were synthesized via activators regenerated by electron transfer (ARGET) atom transfer radical polymerization (ATRP). Poly(ethylene oxide) (PEO) with terminal α-bromoisobutyrate ATRP functionality was used as a macroinitiator and divinylbenzene as a crosslinker for the arm-first star synthesis. Stars with PEO arms with a molar mass of either 2 or 5 kDa had a relatively low density of grafted chains, i.e., ca. 0.25 chain/nm². The properties of PEO stars adsorbed at oil-water interfaces were investigated using interfacial tension and interfacial rheology. The magnitude of interfacial tensions at oil-water interfaces depends on the nature of the oil phase, being lower at the m-xylene/water interface than at the n-dodecane/water interface. Small differences were observed for stars with different molecular weights of PEO arms. The overall behavior of PEO stars adsorbed at an interface can be considered as an intermediate between a particle and a linear/branched polymer. Obtained results offer an important insight into the interfacial rheology of PEO star polymers in the context of their application as stabilizers for Pickering emulsions.

Adsorption of sterically-stabilized diblock copolymer nanoparticles at the oil-water interface: effect of charged end-groups on interfacial rheology

The RAFT aqueous emulsion polymerization of either methyl methacrylate (MMA) or benzyl methacrylate (BzMA) is conducted at 70 °C using poly(glycerol monomethacrylate) (PGMA) as a water-soluble precursor to produce sterically-stabilized diblock copolymer nanoparticles of approximately 30 nm diameter. Carboxylic acid- or morpholine-functional RAFT agents are employed to confer anionic or cationic functionality at the ends of the PGMA stabilizer chains, with a neutral RAFT agent being used as a control. Thus the electrophoretic footprint of such minimally-charged model nanoparticles can be adjusted simply by varying the solution pH. Giant (mm-sized) aqueous droplets containing such nanoparticles are then grown within a continuous phase of n-dodecane and a series of interfacial rheology measurements are conducted. The interfacial tension between the aqueous phase and n-dodecane is strongly dependent on the charge of the terminal group on the stabilizer chains. More specifically, neutral nanoparticles produce a significantly lower interfacial tension than either cationic or anionic nanoparticles. Moreover, adsorption of neutral nanoparticles at the n-dodecane-water interface produces higher interfacial elastic moduli than that observed for charged nanoparticles. This is because neutral nanoparticles can adsorb at much higher surface packing densities owing to the absence of electrostatic repulsive forces in this case.

Dilational properties of anionic gemini surfactants with polyoxyethylene spacers at water-air and water-decane interfaces

The dilational properties of anionic gemini surfactants (oligooxa)-alpha,omega-bis(m-octylbenzene sulfonate) (C(8)E(x)C(8)) with polyoxyethylene spacers at the water-air and water-decane interfaces were investigated via the oscillating barriers method. The influences of oscillating frequency and bulk concentration on dilational properties were explored. The interfacial tension relaxation method was employed to obtain dilational parameters in a reasonably broad frequency range. The experimental results show that the number of ethylene oxide groups is one of the principal factors to control the nature of the interfacial film. With an increase of ethylene oxide groups, the dilational modulus of C(8)E(8)C(8) shows two maxima with the increasing concentration. Furthermore, the dilational moduli at the water-decane interface are remarkably lower than those at the water-air interface for C(8)E(1)C(8) and C(8)E(4)C(8), while the dilational modulus at the water-decane interface is close to that at the water-air interface for C(8)E(8)C(8), which indicates that the structure of the adsorption sublayer plays a more important role. Possible schematic diagrams of adsorbed molecules with different polyoxyethylene spacers at the water-air and water-decane interfaces are proposed. The results of relaxation experiments and Cole-Cole plots can support our provided mechanism strongly.

Relaxation processes of PGPR at the water/oil interface inferred by oscillatory or transient viscoelasticity measurements

The rheological properties of PolyGlycerol PolyRicinoleate (PGPR) at the oil/water interface were studied using a drop-shaped tensiometer. Small deformation oscillations of the drop area allow the measurement of the interfacial viscoelasticity spectrum, that is, the elastic and viscous moduli as a function of frequency. Another way to obtain such a spectrum is to perform a transient relaxation measurement from which the relaxation modulus as a function of time is deduced and interpreted. Several models containing one or more relaxation times were considered, and their resulting spectra were compared to the oscillatory ones. Similar results suggest that one could in principle use oscillatory or transient relaxations indifferently. However, the transient relaxation technique proved to be more adapted for the determination of the relaxation times. At low PGPR concentrations in oil, the behavior is controlled by long relaxation times, whereas short ones take over when approaching and exceeding the saturation interfacial concentration. This was understood as a shift from a diffusion-dominated regime to a rearrangements-dominated regime.

Dynamic Interfacial Dilational Properties of Hydroxy-Substituted Alkyl Benzenesulfonates

Synthesis, characterization, and interfacial properties of hydroxy-substituted alkyl benzenesulfonates, sodium 2-hydroxy-3-decyl-5-octylbenzenesulfonate (C10C8OHphSO3Na) and 2-hydroxy-3-octyl-5-decylbenzenesulfonate (C8C10OHphSO3Na), are reported. The dynamic dilational properties of the surfactants are expounded by means of oscillating the bubble/drop method at both water-air and water-decane interfaces. The distinct maxima appear in dilational modulus vs time curves in some cases, which is believed to be attributed to the change of surfactant conformation and the arrangement of surface layer. Our results show that the measurement of dynamic interfacial dilational properties is a powerful tool to probe the structure of the surfactant adsorption film.

Interfacial rheological properties of adsorbed protein layers and surfactants: a review

Proteins and low molecular weight (LMW) surfactants are widely used for the physical stabilisation of many emulsions and foam based food products. The formation and stabilisation of these emulsions and foams depend strongly on the interfacial properties of the proteins and the LMW surfactants. Therefore these properties have been studied extensively. In this review an overview is given of interfacial properties of proteins at a mesoscopic scale and the effect of LMW surfactants (emulsifiers) on them. Properties addressed are the adsorbed amount, surface tension (reduction), network formation at interfaces and possible conformational changes during and after adsorption. Special attention is given to interfacial rheological behaviour of proteins at expanding and compressing interfaces, which simulate the behaviour in real emulsions and foams. It will be illustrated that information on interfacial rheological properties, protein conformation and interactions between adsorbed molecules can be obtained by changing experimental conditions. The relation between interfacial rheology and emulsion and foam stabilisation is subsequently addressed. It is concluded that there is a need for new measuring devices that monitor several interfacial properties on a mesoscopic and microscopic scale at the same time, and for physical models describing the various processes of importance for proteins. Real progress will only be possible if both are combined in an innovative way.

Determination of Surface Dilational Viscosity Using the Oscillating Bubble Method

A new version of the oscillating bubble method enables us to determine the dilational modulus of fluid surfaces in the frequency range 1 Hz</=f</=500 Hz. The frequency behavior of this complex modulus exhibits the influence of various physical effects which requires explanation in the framework of an appropriate theoretical model. For the evaluation of our measurements a theoretical modulus is used which includes compositional and intrinsic viscous effects. The compositional effects are described by the established model of dynamic surface tension. However, a modified interpretation is required to explain the values of the involved parameters which deviate strongly from the values calculated using equilibrium data. In addition, several experiments demonstrate the influence of an intrinsic surface dilational viscosity. This can be interpreted as the effect of molecular exchange at the surface in a nonequilibrium state. For the investigation of such processes we have compared the surface dilational moduli of solutions of alkyldimethylphosphine oxides with various chain lengths. The theoretical model was tested using these experimental results. It was possible to determine the parameters of the model. The parameters characterize the elastic and viscous properties of the surface as well as molecular exchange processes between surface and bulk. Their values mark scales for the influence of compositional effects, intrinsic effects, and molecular kinetics on the dilational properties of the surface. Copyright 2001 Academic Press.

Analysis of dynamic surface properties of therapeutic surfactants and lung phospholipids

Exogenous surfactant is a specialized biomaterial used for substitution of the lipoprotein mixture normally present in lungs--pulmonary surfactant. Respiratory Distress Syndrome is a disease of preterm infants mainly caused by a deficiency of mature lung surfactant. Pulmonary surfactant is known to stabilize small alveoli and prevent them from collapsing during expiration due to its unique surface properties. A pulsating bubble surfactometer was used for in vitro analysis of surface parameters of therapeutic surfactants and of test formulations to be used for exogenous therapy in Respiratory Distress Syndrome. Surface parameters that were considered for comparison were minimum surface tension (gamma(min)) at three different frequencies (20, 40 and 60 cpm), adsorption at two extreme bubble radii (Rmin and Rmax), stability index at the three frequencies, recruitment index and the surface viscoelastic parameters. Survanta, ALEC and Exosurf were compared with formulations consisting of the main phospholipids of pulmonary surfactant, namely dipalmitoyl phosphatidylcholine (PC), phosphatidylethanolamine (PE) and phosphatidylglycerol (PG) as well as binary mixtures of these phospholipids in the ratio 2:3. Survanta performed much better than the non-protein therapeutic surfactants in all parameters and at all three frequencies. Exosurf had a very low stability index and a very low modulus of surface dilatational elasticity at all three frequencies. The test compounds showed a frequency dependence in their performance. At 20 cpm, PC:PG (2:3) was the best test combination. It achieved a gamma(min) and stability index equivalent to Survanta at this frequency. None of the test compounds were comparable to Survanta at 40 and 60 cpm. These findings may have important therapeutic implications for exogenous surfactants.

Measurements of the Surface Elasticity in Medium Frequency Range Using the Oscillating Bubble Method

Various experimental techniques are available for the investigation of dynamic surface tension, and a generally accepted theoretical model of these dynamics has been established. However, reliable rheological parameters of a fluid surface are very scarce. Therefore, comparisons of rheological parameters resulting from slow and faster processes or from theoretical calculations are required. In particular, a comprehensive experimental verification of the complex surface elasticity modulus which characterizes the dynamic behavior of a fluid surface in an appropriate manner is desirable. For this reason a new version of oscillating bubble method was developed which allows exact measurements of the complex elasticity modulus in the frequency range 3-500 Hz. With this method the assumptions of the theory of dynamic surface tension can be verified for medium frequencies. The new experimental results, in particular the experimental determination of the Gibbs elasticity, reveal that these assumptions are only approximately valid for faster processes. However, with a slight modification of the established model the experimental results can be explained. These experiments were carried out with solutions of tridecyl dimethyl phosphine oxide, fatty acids, n-alkanols, and triton X-100 at different surfactant concentrations. Copyright 1998 Academic Press.