Self-assembly in mixed dialkyl chain cationic-nonionic surfactant mixtures: dihexadecyldimethyl ammonium bromide-monododecyl hexaethylene glycol (monododecyl dodecaethylene glycol) mixtures

Neutron reflection and the thermodynamics of the air-water interface

By means of isotopic substitution, measurements of the neutron reflectivity (NR) from a flat water surface generally give model independent measurements of the amount of a chosen solute at the surface irrespective of whether the layer is a mixture or whether there is any aggregation in the bulk solution. Previously, adsorption at air-water interfaces has been determined by applying the Gibbs equation to surface tension (ST) measurements, which requires assumptions about the composition of the surface and about the activity of the solute in the bulk, which, in turn, means that in practice the surface is assumed to consist of the pure solute or of a mixture of pure solutes, and that the activity of the solute in the bulk solution is known. The use of NR in combination with ST-Gibbs measurements makes it possible to (i) avoid these assumptions and hence understand several patterns of ST behaviour previously considered to be anomalous and (ii) to start to analyse quantitatively the behaviour of mixed surfactants both below and above the critical micelle concentration. These two developments in our understanding of the thermodynamics of the air-water interface are described with recent examples.

The Influence of Co-Surfactants on Lamellar Liquid Crystal Structures Formed in Creams

It is well-established that oil-in-water creams can be stabilised through the formation of lamellar liquid crystal structures in the continuous phase, achieved by adding (emulsifier) mixtures comprising surfactant(s) combined (of necessity) with one or more co-surfactants. There is little molecular-level understanding, however, of how the microstructure of a cream is modulated by changes in co-surfactant and of the ramifications of such changes on cream properties. We investigate here the molecular architectures of oil-free, ternary formulations of water and emulsifiers comprising sodium dodecyl sulfate and one or both of the co-surfactants hexadecanol and octadecanol, using microscopy, small-angle and wide-angle X-ray scattering and small-angle neutron scattering. We then deploy these techniques to determine how the structures of the systems change when liquid paraffin oil is added to convert them to creams, and establish how the structure, rheology, and stability of the creams is modified by changing the co-surfactant. The ternary systems and their corresponding creams are shown to contain co-surfactant lamellae that are subtly different and exhibit different thermotropic behaviours. The lamellae within the creams and the layers surrounding their oil droplets are shown to vary with co-surfactant chain length. Those containing a single fatty alcohol co-surfactant are found to contain crystallites, and by comparison with the cream containing both alcohols suffer adverse changes in their rheology and stability.

Nature of the Intermicellar Interactions in Ethoxylated Polysorbate Surfactants with High Degrees of Ethoxylation

Ethoxylated polysorbate Tween nonionic surfactants are extensively used as foam and emulsion stabilizers and in aqueous solution form globular micelles. The ethoxylated polysorbate surfactants with higher degrees of ethoxylation than the Tween surfactants exhibit some interesting self-assembly properties. Small-angle neutron scattering, SANS, measurements have revealed intermicellar interactions which are more pronounced than the hard-sphere excluded volume interactions normally associated with nonionic surfactant micelles. The interactions are interpreted as arising from the partial charge on the ether oxygen of the ethylene oxide groups. This gives rise to an effective net negative charge on the micelles, which has been determined from the SANS data and zeta potential measurements. For degrees of ethoxylation of ⩽20, the effect is relatively small. The interaction increases with increasing ethoxylation such that for a degree of ethoxylation of 50 the interaction is comparable to that of ionic surfactant micelles. Unlike the intermicellar interaction in ionic surfactant micellar solutions, which results from the charge on the micelle arising from the partial binding of counterions, the interaction between ethoxthylated polysorbate surfactant micelles is unaffected by the addition of electrolyte.

Modeling of multiple equilibria in the self-aggregation of di-n-decyldimethylammonium chloride/octaethylene glycol monododecyl ether/cyclodextrin ternary systems

The surface tension equations of binary surfactant mixtures (di-n-decyldimethylammonium chloride and octaethylene glycol monododecyl ether) are established by combining the Szyszkowski equation of surfactant solutions, the ideal or nonideal mixing theory, and the phase separation model. For surfactant mixtures, the surface tension at the air-water interface is calculated using nonideal theory due to synergism between the two adsorbed surfactant types. The incorporation of cyclodextrin complexation model to the surface tension equations gives a robust model for the description of the surface tension isotherms of binary, ternary, and more complex systems involving numerous inclusion complexes. The surface tension data obtained experimentally shows excellent agreement with the theoretical model below and above the formation of micelles. The strong synergistic effect observed between the two surfactants is disrupted by the presence of CDs, leading to ideal behavior of ternary systems. Indeed, depending on the nature of the cyclodextrin (i.e., α, β, or γ), which allows a tuning of the cavity size, the binding constants with the surfactants are modified as well as the surface properties due to strong modification of equilibria involved in the ternary mixture.

Oligolamellar vesicles for covalent immobilization and stabilization of D-amino acid oxidase

Oligolamellar phospholipid vesicles incorporated with d-amino acid oxidase from porcine kidney (OV-DAO) were prepared by encapsulating pre-formed enzyme-bound unilamellar vesicles (UV-DAO) with bilayers of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC). The bilayer of UV-DAO was composed of POPC, 30 mol% of cholesterol and 15 mol% of 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine-N-(glutaryl) (NGPE) that was responsible for covalent linking to D-amino acid oxidase (DAO). OV-DAO and UV-DAO showed the activity to catalyze the oxidation of D-alanine as measured based on the hydrogen peroxide produced. The oligolamellar and unilamellar structure of OV-DAO and UV-DAO, respectively was elucidated based on the quenching characteristics of bilayers-incorporated fluorescent lipid 7-nitro-2,1,3-benzoxadiazol-4-yl-phosphoethanolamine (NBD-PE) and the size distribution of the vesicles measured with the dynamic light scattering method. The enzyme activity of OV-DAO and UV-DAO was significantly stabilized at 50°C compared to that of free DAO at the fixed enzyme concentration of 3.29 μg/mL. At the temperature, OV-DAO and UV-DAO showed the remaining activity of 52.7 and 29.6%, respectively at the incubation time of 20 min while free DAO was completely deactivated. Thus the dimeric form of DAO could be stabilized by its coupling to the surface of UV-DAO membrane being the inner bilayer of OV-DAO. Furthermore, the thermal denaturation of DAO and dissociation of flavin adenine dinucleotide (FAD) from the subunits of enzyme were prevented in the aqueous phase formed between the bilayers of OV-DAO.

Aqueous mixtures of di-n-decyldimethylammonium chloride/polyoxyethylene alkyl ether: dramatic influence of tail/tail and head/head interactions on co-micellization and biocidal activity

Mixed aggregate formation and synergistic interactions of binary surfactant mixtures of di-n-decyldimethylammonium chloride, [DiC(10)][Cl], with polyoxyethylene alkyl ethers, C(i)E(j) (i=10, 12, j=4, 6, 8), have been investigated for various [DiC(10)][Cl]/C(i)E(j) ratios. The critical aggregation concentration of the binary mixtures has been determined by tensiometry, and the aggregate characteristics (i.e., size and composition, free ammonium concentration) have been estimated using the pulsed field gradient NMR spectroscopy and a [DiC(10)]-selective electrode. Diffusion coefficient measurements of micelles confirmed the synergistic interaction between the surfactants. It is thus shown that the formation of surface monolayers and mixed aggregates from [DiC(10)][Cl]/C(10)E(j) mixtures is driven by both tail/tail and head/head interactions, whereas [DiC(10)][Cl]/C(12)E(j) co-aggregation is mainly driven by tail/tail interactions. As a consequence, the co-aggregation phenomenon notably influences the biocidal activity of [DiC(10)][Cl] on the Candida albicans fungi. In the presence of C(12)E(j), the biocidal activity of the ammonium salt is inhibited due to the trapping of the cationic surfactants in the mixed aggregates, whereas in the presence of C(10)E(j), the biocidal activity of the surfactant mixture is maintained. The mode of action is also confirmed by a faster increase in the zeta potential of a C. albicans suspension in the presence of [DiC(10)][Cl]/C(10)E(8) than in the presence of [DiC(10)][Cl]/C(12)E(8). Therefore, a judicious adjustment of the alkyl (i) and polyoxyethylene (j) chain lengths of C(i)E(j) avoids its antagonistic effect on the biocidal activity of [DiC(10)][Cl].

Self-assembly of mixed anionic and nonionic surfactants in aqueous solution

We present the phase diagram and the microstructure of the binary surfactant mixture of AOT and C(12)E(4) in D(2)O as characterized by surface tension and small angle neutron scattering. The micellar region is considerably extended in composition and concentration compared to that observed for the pure surfactant systems, and two types of aggregates are formed. Spherical micelles are present for AOT-rich composition, whereas cylindrical micelles with a mean length between 80 and 300 Å are present in the nonionic-rich region. The size of the micelles depends on both concentration and molar ratio of the surfactant mixtures. At higher concentration, a swollen lamellar phase is formed, where electrostatic repulsions dominate over the Helfrich interaction in the mixed bilayers. At intermediate concentrations, a mixed micellar/lamellar phase exists.

Solution self-assembly and adsorption at the air-water interface of the monorhamnose and dirhamnose rhamnolipids and their mixtures

The self-assembly in solution and adsorption at the air-water interface, measured by small-angle neutron scattering, SANS, and neutron reflectivity, NR, of the monorhamnose and dirhamnose rhamnolipids (R1, R2) and their mixtures, are discussed. The production of the deuterium-labeled rhamnolipids (required for the NR studies) from a Pseudomonas aeruginosa culture and their separation into the pure R1 and R2 components is described. At the air-water interface, R1 and R2 exhibit Langmuir-like adsorption isotherms, with saturated area/molecule values of about 60 and 75 Å(2), respectively. In R1/R2 mixtures, there is a strong partitioning of R1 to the surface and R2 competes less favorably because of the steric or packing constraints of the larger R2 dirhamnose headgroup. In dilute solution (<20 mM), R1 and R2 form small globular micelles, L(1), with aggregation numbers of about 50 and 30, respectively. At higher solution concentrations, R1 has a predominantly planar structure, L(α) (unilamellar, ULV, or bilamellar, BLV, vesicles) whereas R2 remains globular, with an aggregation number that increases with increasing surfactant concentration. For R1/R2 mixtures, solutions rich in R2 are predominantly micellar whereas solutions rich in R1 have a more planar structure. At an intermediate composition (60 to 80 mol % R1), there are mixed L(α)/L(1) and L(1)/L(α) regions. However, the higher preferred curvature associated with R2 tends to dominate the mixed R1/R2 microstructure and its associated phase behavior.

Surface and solution properties of anionic/nonionic surfactant mixtures of alkylbenzene sulfonate and triethyleneglycol decyl ether

The surface adsorption behavior and the solution microstructure of mixtures of the C(6) isomer of anionic surfactant sodium para-dodecyl benzene sulfonate, ABS, with nonionic surfactant monodecyl triethyleneglycol ether, C(10)E(3,) have been investigated using a combination of neutron reflectivity, NR, and small-angle neutron scattering, SANS. In solution, the mixing of C(10)E(3) and ABS results in the formation of small globular micelles over most of the composition range (100:0 to 20:80 ABS/C(10)E(3)). Planar aggregates (lamellar or unilamellar vesicles, ULV) are observed for solution compositions rich in the nonionic surfactant (>80 mol % nonionic). Prior to the transition to planar aggregates, the micelle aggregation number increases with increasing nonionic composition. The lamellar-phase region is preceded by a narrow range of composition over which mixtures of micelles and small unilamellar vesicles coexist. The variation in surface absorption behavior with solution composition shows a strong surface partitioning of the more surface-active component, C(10)E(3). This pronounced departure from ideal mixing is not readily explained by existing surfactant mixing theories. In the presence of Ca(2+) ions, a more complex evolution of solution phase behavior with solution composition is observed. The lamellar-phase region occurs over a broader range of solution compositions at the expense of the small-vesicle phase. The phase boundaries are shifted to lower nonionic compositions, and the extent to which the solution-phase diagrams are modified increases with increasing calcium ion concentration. The SANS data for the large planar aggregates are consistent with large polydisperse flexible unilamellar vesicles. In the presence of Ca(2+) ions, the surface adsorption patterns become more consistent with ideal mixing in the nonionic-rich region of the surface-phase diagram. However, in the ABS-rich regions the surface behavior is more complex because of the spontaneous formation of more complex surface microstructures (bilayers to multilayers). Both in water and in the presence of Ca(2+) ions the variations in the surface adsorption behavior and in the solution mesophase structure do not appear to be closely correlated.

Transition from vesicles to small nanometer scaled vesicles, arising from the manipulation of curvature in dialkyl chain cationic/nonionic surfactant mixed aggregates by the addition of straight chain alkanols

The addition of straight chain alkanols to the dialkyl chain cationic/nonionic surfactant mixtures of dihexadecyl dimethyl ammonium bromide, DHDAB, and dodecaethylene monododecyl ether, C(12)E(12), has been used to manipulate the mean curvature of the self-assembled aggregates. This induces some significant structural changes and notably the formation of small unilamellar vesicles, nanometer scaled vesicles, L(sv). These structural changes have been measured and quantified using small angle neutron scattering, SANS. At a solution concentration of 25 mM, the DHDAB/C(12)E(12) mixtures have a structural evolution, from C(12)E(12) rich to DHDAB rich solution compositions, of small globular micelles, L(1), to micellar/vesicle coexistence, L(1)/L(v) or L(v)/L(1), to vesicle structures, L(v), bilamellar or multilamellar vesicles, blv or mlv. The impact of the addition of straight chain alkanols (in the range octanol to hexadecanol) depends upon the alkyl chain length and the amount of alcohol added. Furthermore, the effect of the addition of octanol and decanol appears to be distinctly different from that of the larger straight chain alkanols of dodecanol and hexadecanol. For the addition of octanol and decanol to C(12)E(12) rich DHDAB/C(12)E(12) mixtures, the alcohol is solubilized into the micellar core, and as the amount of alcohol added increases, significant micellar growth is ultimately observed. However, notably at intermediate DHDAB/C(12)E(12) solution compositions, in the region of L(1)/L(v) or L(v)/L(1) coexistance in the absence of alcohol, the addition of octanol or decanol promotes the formation of relatively small unilamellar vesicles, L(sv), nanometer sized vesicles, with a mean diameter in the range 70-140 A. For solutions that are rich in DHDAB, the addition of octanol or decanol results in a transition to L(v)/L(sv) coexistence and ultimately to L(v) formation. In contrast, the addition of the larger straight chain length alkanols, dodecanol or hexadecanol, to DHDAB/C(12)E(12) mixtures results in a somewhat different behavior. In this case, the addition of dodecanol or hexadecanol results in the transition from L(1) to L(1)/L(v) to L(v) occurring for solutions less rich in DHDAB than is observed in the absence of alcohol. That is, there is an enhanced tendency toward the formation of structures with a lower net curvature, either blv or mlv. Notably, for these mixtures, the small unilamellar nanometer scaled vesicle phase, L(sv), is absent.

Interplay between the surface adsorption and solution-phase behavior in dialkyl chain cationic-nonionic surfactant mixtures

Neutron reflectivity, NR, and surface tension have been used to study the adsorption at the air-solution interface of mixtures of the dialkyl chain cationic surfactant dihexadecyl dimethyl ammonium bromide (DHDAB) and the nonionic surfactants monododecyl triethylene glycol (C12E3), monododecyl hexaethylene glycol (C12E6), and monododecyl dodecaethylene glycol (C12E12). The adsorption behavior of the surfactant mixtures with solution composition shows a marked departure from ideal mixing that is not consistent with current theories of nonideal mixing. For all three binary surfactant mixtures there is a critical composition below which the surface is totally dominated by the cationic surfactant. The onset of nonionic surfactant adsorption (expressed as a mole fraction of the nonionic surfactant) increases in composition as the ethylene oxide chain length of the nonionic cosurfactant increases from E3 to E12. Furthermore, the variation in the adsorption is strongly correlated with the variation in the phase behavior of the solution that is in equilibrium with the surface. The adsorbed amounts of DHDAB and the nonionic cosurfactants have been used to estimate the monomer concentration that is in equilibrium with the surface and are shown to be in reasonable qualitative agreement with the variation in the mixed critical aggregation concentration (cac).

Spontaneous formation of nanovesicles in mixtures of nonionic and dialkyl chain cationic surfactants studied by surface tension and SANS

Surface tension and small-angle neutron scattering have been used to characterize the surface properties and the structure of the aggregates formed in the dilute part of the ternary system didodecyldimethyl ammonium bromide, DDAB/tetraethylene glycol monododecyl ether, C12E4/D2O. For surfactant molar ratios, Rn, between 0.3 and 1 (pure DDAB), the surface tension measurements show the existence of two break points at concentrations of around 10(-5) and 10(-3) mol/L, respectively. The SANS measurements have shown that the first break point corresponds to a critical micellar concentration (cmc) and the second one corresponds to a critical vesicle concentration (cvc). In the intermediate composition range, Rn = 0.3-0.8, very small unilamellar vesicles (nanovesicles) are formed with the inner radius varying between 28 and 85 A and a bilayer thickness of approximately 23 A. At Rn = 0.8, we observed a transition from small vesicles (V) to large bilamellar or multilamellar vesicles (BLV, MLV) with a relatively large lamellar periodicity of around 1000 A. In the nonionic-rich region below Rn = 0.3, more classical surface tension behavior was observed, with only one break point corresponding to the onset of formation of small globular micelles.

Monomer-aggregate exchange rates in dialkyl chain cationic-nonionic surfactant mixtures

The monomer-aggregate exchange rate in self-assembled dialkyl chain cationic-nonionic mixed surfactant aggregates has been studied using small-angle neutron scattering, SANS, and a stopped-flow apparatus. SANS was used to follow the evolution of the structure with time of an equimolar mixture of the dialkyl chain cationic surfactant dihexadecyl dimethyl ammonium bromide, DHDAB, in D2O with the nonionic surfactant dodecaethylene monododecyl ether, C12E12, in D2O at a solution concentration of 1.5 mM. With increasing time, the bilamellar vesicle structure, blv, of DHDAB and the globular micellar structure, L1, of C12E12 evolved to a lamellar (Lbeta or Lalpha)/micellar (L1) coexistence. Measurements were made for the isotopically labeled combinations of hydrogeneous DHDAB (h-DHDAB) and alkyl chain deuterium-labeled C12E12 (d-C12E12) in D2O such that the lamellar contribution is the predominantly visible contribution to the scattering. From the variation (decrease) in the scattering intensity with time (measured at a scattering vector of approximately 0.014 angstroms(-1)), a characteristic time was measured at 32 degrees C (T < Lbeta/Lalpha transition temperature) and at 46 degrees C (T > Lbeta/Lalpha). The characteristic time was approximately 130 min and a few seconds respectively, indicating a dramatic change in the monomer/aggregate exchange rate between the solid-like Lbeta and fluid-like Lalpha phases. The characteristic time of approximately 130 min in the Lbeta phase is indicative of a slow monomer-aggregate exchange rate and is consistent with the slow kinetics of adsorption of DHDAB and DHDAB/nonionic surfactant mixtures observed at the air-water interface. This slow adsorption kinetics was assumed to arise from near-surface depletion effects associated with slow monomer/aggregate exchange rates, and these results support and reinforce that hypothesis.