Manipulating the hydrophilic / hydrophobic balance in novel cationic surfactants by ethoxylation: The impact on adsorption and self-assembly.

HYPOTHESIS: Cationic surfactants have a wide range of applications, often associated with their affinity for a range of solid surfaces and their anti-microbial properties. Manipulating their adsorption and self-assembly properties is key to most applications, and this is commonly achieved through surfactant mixtures or manipulating their headgroup or alkyl chain structure. Achieving this through adjustments to their headgroup structure is less common in cationic surfactants than in anionic surfactants. Ethoxylation provides the ability to adjust the hydrophilic / hydrophobic balance, as extensively demonstrated in a range of anionic surfactants. EXPERIMENTS: This same approach has been applied here to a range of ethoxylated cationic surfactants in the form of the quaternary ammonium salts, and their tertiary nonionic equivalents before quaternisation. Their adsorption and self-assembly properties are investigated using predominantly the neutron scattering techniques of neutron reflectivity, NR, and small angle neutron scattering, SANS. FINDINGS: The trends in the adsorption at the air-water interface and the self-assembly in aqueous solution demonstrate how the hydrophilic / hydrophobic balance can be adjusted by varying the degree of ethoxylation and the alkyl chain length, and illustrate the degree of interdependence of the different structural changes. The variation in the adsorption and the micelle structure shows how the surfactant conformation / packing changes as the degree of ethoxylation and alkyl chain length increases and how the introduction of charge induces further changes.

Saponin Adsorption at the Air-Water Interface-Neutron Reflectivity and Surface Tension Study.

Saponins are a large group of glycosides present in many plant species. They exhibit high surface activity, which arises from a hydrophobic scaffold of triterpenoid or steroid groups and attached hydrophilic saccharide chains. The diversity of molecular structures, present in various plants, gives rise to a rich variety of physicochemical properties and biological activity and results in a wide range of applications in foods, cosmetics, medicine, and several other industrial sectors. Saponin surface activity is a key property in such applications and here the adsorption of three triterpenoid saponins, escin, tea saponins, and Quillaja saponin, is studied at the air-water interface by neutron reflectivity and surface tension. All these saponins form adsorption layers with very high surface visco-elasticity. The structure of the adsorbed layers has been determined from the neutron reflectivity data and is related to the molecular structure of the saponins. The results indicate that the structure of the saturated adsorption layers is governed by densely packed hydrophilic saccharide groups. The tight molecular packing and the strong hydrogen bonds between the neighboring saccharide groups are the main reasons for the unusual rheological properties of the saponin adsorption layers.

The impact of electrolyte on the adsorption of the anionic surfactant methyl ester sulfonate at the air-solution interface: Surface multilayer formation.

The methyl ester sulfonates represent a promising group of anionic surfactants which have the potential for improved performance and biocompatibility in a range of applications. Their solution properties, in particular their tolerance to hard water, suggests that surface ordering may occur in the presence of multi-valent counterion. Understanding their adsorption properties in a range of different circumstances is key to the exploitation of their potential. Neutron reflectivity and surface tension have been used to characterise the adsorption at the air-aqueous solution interface of the anionic surfactant sodium tetradecanoic 2-sulfo 1-methyl ester, C14MES, in the absence of electrolyte and in the presence of mono, di, and tri-valent counterions, Na+, Ca2+, and Al3+. In particular the emphasis has been on exploring the tendency to form layered structures at the interface. In the absence of electrolyte and in the presence of NaCl and CaCl2 and AlCl3 at low concentrations monolayer adsorption is observed, and the addition of electrolyte results in enhanced adsorption. In the presence of NaCl and CaCl2 only monolayer adsorption is observed. However at higher AlCl3 concentrations surface multilayer formation is observed, in which the number of bilayers at the surface depends upon the surfactant and AlCl3 concentrations.

Self-assembly in dilute mixtures of non-ionic and anionic surfactants and rhamnolipd biosurfactants.

The self-assembly of dilute aqueous solutions of a ternary surfactant mixture and rhamnolipid biosurfactant/surfactant mixtures has been studied by small angle neutron scattering. In the ternary surfactant mixture of octaethylene glycol monododecyl ether, C12E8, sodium dodecyl 6-benzene sulfonate, LAS, and sodium dioxyethylene monododecyl sulfate, SLES, small globular interacting micelles are observed over the entire composition and concentration range studied. The modelling of the scattering data strongly supports the assumption that the micelle compositions are close to the solution compositions. In the 5-component rhamnolipid/surfactant mixture of the mono-rhamnose, R1, di-rhamnose, R2, rhamnolipids with C12E8/LAS/SLES, globular micelles are observed over much of the concentration and composition range studied. However, for solutions relatively rich in rhamnolipid and LAS, lamellar/micellar coexistence is observed. The transition from globular to more planar structures arises from a synergistic packing in the 5 component mixture. It is not observed in the individual components nor in the ternary C12E8/LAS/SLES mixture at these relatively low concentrations. The results provide an insight into how synergistic packing effects can occur in the solution self-assembly of complex multi-component surfactant mixtures, and give rise to an unexpected evolution in the phase behaviour.

Adsorption of hydrophobin/ß-casein mixtures at the solid-liquid interface.

The adsorption behaviour of mixtures of the proteins ß-casein and hydrophobin at the hydrophilic solid-liquid surface have been studied by neutron reflectivity. The results of measurements from sequential adsorption and co-adsorption from solution are contrasted. The adsorption properties of protein mixtures are important for a wide range of applications. Because of competing factors the adsorption behaviour of protein mixtures at interfaces is often difficult to predict. This is particularly true for mixtures containing hydrophobin as hydrophobin possesses some unusual surface properties. At ß-casein concentrations ⩾0.1wt% ß-casein largely displaces a pre-adsorbed layer of hydrophobin at the interface, similar to that observed in hydrophobin-surfactant mixtures. In the composition and concentration range studied here for the co-adsorption of ß-casein-hydrophobin mixtures the adsorption is dominated by the ß-casein adsorption. The results provide an important insight into how the competitive adsorption in protein mixtures of hydrophobin and ß-casein can impact upon the modification of solid surface properties and the potential for a wide range of colloid stabilisation applications.

Adsorption of Bovine Serum Albumin (BSA) at the Oil/Water Interface: A Neutron Reflection Study.

The structure of the adsorbed protein layer at the oil/water interface is essential to the understanding of the role of proteins in emulsion stabilization, and it is important to glean the mechanistic events of protein adsorption at such buried interfaces. This article reports on a novel experimental methodology for probing protein adsorption at the buried oil/water interface. Neutron reflectivity was used with a carefully selected set of isotopic contrasts to study the adsorption of bovine serum albumin (BSA) at the hexadecane/water interface, and the results were compared to those for the air/water interface. The adsorption isotherm was determined at the isoelectric point, and the results showed that a higher degree of adsorption could be achieved at the more hydrophobic interface. The adsorbed BSA molecules formed a monolayer on the aqueous side of the interface. The molecules in this layer were partially denatured by the presence of oil, and once released from the spatial constraint by the globular framework they were free to establish more favorable interactions with the hydrophobic medium. Thus, a loose layer extending toward the oil phase was clearly observed, resulting in an overall broader interface. By analogy to the air/water interface, as the concentration of BSA increased to 1.0 mg mL(-1) a secondary layer extending toward the aqueous phase was observed, possibly resulting from the steric repulsion upon the saturation of the primary monolayer. Results clearly indicate a more compact arrangement of molecules at the oil/water interface: this must be caused by the loss of the globular structure as a consequence of the denaturing action of the hexadecane.

Kinetics of surfactant desorption at an air-solution interface.

The kinetics of re-equilibration of the anionic surfactant sodium dodecylbenzene sulfonate at the air-solution interface have been studied using neutron reflectivity. The experimental arrangement incorporates a novel flow cell in which the subphase can be exchanged (diluted) using a laminar flow while the surface region remains unaltered. The rate of the re-equilibration is relatively slow and occurs over many tens of minutes, which is comparable with the dilution time scale of approximately 10-30 min. A detailed mathematical model, in which the rate of the desorption is determined by transport through a near-surface diffusion layer into a diluted bulk solution below, is developed and provides a good description of the time-dependent adsorption data. A key parameter of the model is the ratio of the depth of the diffusion layer, H(c), to the depth of the fluid, H(f), and we find that this is related to the reduced Péclet number, Pe*, for the system, via H(c)/H(f) = C/Pe*(1/2). Although from a highly idealized experimental arrangement, the results provide an important insight into the "rinse mechanism", which is applicable to a wide variety of domestic and industrial circumstances.

Effect of water hardness on surface tension and dilational visco-elasticity of sodium dodecyl sulphate solutions.

The complementary drop and bubble profile analysis and maximum bubble pressure tensiometry are used to measure the dynamic surface tension of aqueous SDS solutions in the presence of hardness salts (CaCl(2) and MgCl(2) in the ratio of 2:1 at concentrations of 6 and 40FH). The presence of hardness salts results in an essential increase of the SDS adsorption activity, which indicates the formation of Ca(DS)(2) and Mg(DS)(2) in the SDS solutions. The surface tension isotherms of SDS in presence of Ca(DS)(2) and Mg(DS)(2) are described using the generalised Frumkin model. The presence of hardness salts accelerates the ageing of SDS solutions as compared with the addition of 0.01 M NaCl due to a faster hydrolysis and hence formation of dodecanol. These results are used to estimate the possible concentration of dodecanol in the studied SDS solutions. The buoyant bubble profile method with harmonic surface oscillations is used to measure the dilational rheology of SDS solutions in presence of hardness salts in the frequency range between 0.005 Hz and 0.2 Hz. The visco-elasticity modulus in the presence of hardness salts is higher as compared with its values in the presence of 0.01 M NaCl additions. The ageing of SDS solutions leads to an essential increase of the visco-elastic modulus.

Adsorption layer characteristics of mixed oxyethylated surfactant solutions.

In the presented work, bubble profile analysis tensiometry is used to study the equilibrium surface tensions and the rheological behavior of solutions of mixed oxyethylated alcohols (C(12)EO(5)/C(14)EO(8)) and their mixtures with polyethylene glycol octylphenyl ethers (Triton X45/Triton X165). For the analysis of the experimental data, a new theoretical model for mixtures of two nonionic oxyethylated surfactants was employed which assumes two states of surfactant molecules with different molar areas in the surface layer and an intrinsic compressibility of the molecules in the state of closest packing. The theoretical models allow an accurate description of the experimental equilibrium surface tensions for all studied mixed solutions. For the analysis of the rheological behavior of the studied mixed surfactant solutions, the theory for a diffusional adsorption mechanism was applied, and a satisfactory agreement between the experimental data and the calculated viscoelasticity modulus and phase angle was achieved.

Adsorption layer characteristics of mixed SDS/C(n)EO(m) solutions. 3. Dynamics of adsorption and surface dilational rheology of micellar solutions.

The dynamic surface tensions of mixed SDS/C(12)EO(5) and SDS/C(14)EO(8) micellar solutions measured over a wide time range (0.1 ms to 10,000 s) at various mixing ratios are described satisfactorily by a theoretical model for the kinetics of adsorption of surfactant mixtures using the surfactant adsorption parameters obtained for premicellar mixed solutions. Additional relations used for the description of the adsorption kinetics from micellar solutions were expressions of the effective diffusion coefficient of monomers accounting for the disintegration of micelles. The modeled dynamic surface tensions agree well with the experimental data for all studied surfactant mixtures. The rheological behavior of the same mixtures--the dependencies of the viscoelasticity modulus and phase angle--were studied by using the bubble profile method at harmonic bubble surface area oscillations. The theoretical approach employed for data analysis was the same as for the dynamic surface tension behavior. Again, satisfactory agreement between the experimental data and theoretical calculations of the dilational rheological parameters was found.

Adsorption layer characteristics of mixed SDS/C(n)EO(m) solutions. II. Dilational viscoelasticity.

Bubble profile analysis tensiometry is used to study the surface rheological behavior of mixed SDS/C(12)EO(5) and SDS/C(14)EO(8) solutions. The experimental dependencies of the viscoelasticity modulus and phase angle are studied in a wide range of surfactant concentrations of the individual sodium dodecyl sulfate (SDS) and C(m)EO(n) solutions and SDS/C(n)EO(m) mixtures at various mixing ratios. By generating harmonic oscillations of the bubble area at low oscillation amplitudes, the relaxation behavior at oscillation frequencies between 0.005 and 0.2 Hz was studied. The applied theoretical approach to describe the dilational rheology of surfactant mixtures requires the specification of the equations of state of the mixed surface layer and the adsorption isotherm of the mixture's components. For the systems studied, the theoretical model considers different adsorption mechanisms for the different surfactants. In particular, the adsorption behavior of oxyethylated surfactants was described by the reorientation model (assumes two adsorption states of surfactant molecules with different molar areas), including an intrinsic compressibility of molecules in the state of minimal area. For the SDS component, the adsorption was assumed to be governed by the Frumkin model, which also accounts for the intrinsic compressibility. Satisfactory agreement between experimental data and theoretical calculations of the viscoelasticity modulus and the phase angle is obtained.

Adsorption layer characteristics of mixed sodium dodecyl sulfate/C(n)EO(m) solutions 1. Dynamic and equilibrium surface tension.

Bubble profile analysis tensiometry is used to study the dynamic and equilibrium surface tensions of mixed sodium dodecyl sulfate (SDS)/C(12)EO(5) and SDS/C(14)EO(8) solutions. For the data analysis, a new theoretical model was employed, which assumes different adsorption mechanisms for each type of surfactants. In particular, the adsorption behavior of oxyethylated surfactants was described by the so-called reorientation model, which assumes two states of surfactant molecules with different molar areas in the surface layer, and additionally an intrinsic compressibility of the adsorbed layer. For the anionic surfactant SDS, a modified Frumkin adsorption model was assumed, which also accounts for the intrinsic compressibility. For the theoretical analysis of the dynamic surface tensions, the theoretical model was based on the numerical solution of Fick's diffusion equation for a spherical geometry of the bubble. The proposed set of theoretical models describes accurately and consistently the experimental results of the equilibrium and dynamic surface tensions of the studied mixed solutions.

Surface dilational viscoelasticity of C14EO8 micellar solution studied by bubble profile analysis tensiometry.

The experimental dependences of viscoelasticity modulus and phase angle as a function of frequency for various C 14EO8 concentrations at the critical micelle concentration (cmc) of 7 micromol/L and far above the cmc (up to 70 x cmc) were studied using the buoyant bubble profile analysis method. With increasing C14EO8 concentration the viscoelasticity modulus decreases and the phase angle increases. At the highest surfactant concentrations, the phase angle was more than 45 degrees . For the theoretical description of the equilibrium surface tension isotherm and the limiting elasticity modulus, a combined theoretical model was used considering surface reorientation and molecular compression. To analyze the experimental dependencies of the viscoelasticity modulus and phase angle on frequency, a model proposed by Joos for fast micellar kinetics was applied. This theory agrees well with the experimental data of the viscoelasticity modulus obtained for all concentrations of the studied nonionic surfactant C14EO8.

Dynamic surface tension of micellar solutions in the millisecond and submillisecond time range.

The analysis of the available bubble life times and dead times for the bubble pressure tensiometer BPA-1S shows that dynamic surface tensions can be measured also for surfactant solutions at concentrations many times higher than the corresponding CMC. For the three nonionic surfactants Triton X-100, Triton X-45, and C14EO8 experiments are performed for solutions with a concentration of up to 200 times the CMC (C14EO8). Comparison of the experimental data with micelle kinetics models yields rate constants for the fast micelle dissolution process, which are in a good agreement with values obtained by other experimental methodologies.

Micellization and interfacial properties of alkyloxyethylene sulfate surfactants in the presence of multivalent counterions.

Alkyloxyethylene sulfates are a special class of surfactants that are unusually stable in the presence of multivalent counterions and are not as prone to precipitation as anionic surfactants without intermediate ethoxy groups in the molecule. However, formation of micelles, their structure, and the properties of monolayers of these surfactants exhibit very interesting and sometimes unexpected properties depending on the nature of the ions dissolved in the solution. This paper presents a brief overview of our recent efforts to reveal the nature of these properties, including some new results. We show that the strong binding of multivalent (and particularly trivalent counterions) triggers a sphere-to-cylinder shape transition of the micelles and facilitates their further growth, even at very low ionic strength. The properties of surfactant monolayers are coupled to those of the micelles in the bulk and are governed also by multivalent counterion binding. The effect of multivalent counterions on the aggregation and structure formation in anionic surfactant solutions has both fundamental and practical importance.