Influence of Surfactant Structure on Polydisperse Formulations of Alkyl Ether Sulfates and Alkyl Amidopropyl Betaines.

Surfactants provide detergency, foaming, and texture in personal care formulations, yet the micellization of typical industrial primary and cosurfactants is not well understood, particularly in light of the polydisperse nature of commercial surfactants. Synergistic interactions are hypothesized to drive the formation of elongated wormlike self-assemblies in these mixed surfactant systems. Small-angle neutron scattering, rheology, and pendant drop tensiometry are used to examine surface adsorption, viscoelasticity, and self-assembly structure for wormlike micellar formulations comprising cocoamidopropyl betaine, and its two major components laurylamidopropyl betaine and oleylamidopropyl betaine, with sodium alkyl ethoxy sulfates. The tail length of sodium alkyl ethoxy sulfates was related to their ability to form wormlike micelles in electrolyte solutions, indicating that a tail length greater than 10 carbons is required to form wormlike micelles in NaCl solutions, with the decyl homologue unable to form elongated micelles and maintaining a low viscosity even at 20 wt % surfactant loading with 4 wt % NaCl present. For these systems, the incorporation of a disperse ethoxylate linker does not enable shorter chain surfactants to elongate into wormlike micelles for single-component systems; however, it could increase the interactions between surfactants in mixed surfactant systems. For synergy in surfactant mixing, the nonideal regular solution theory is used to study the sulfate/betaine mixtures. Tail mismatch appears to drive lower critical micelle concentrations, although tail matching improves synergy with larger relative reductions in critical micelle concentrations and greater micelle elongation, as seen by both tensiometric and scattering measurements.

Capillary Structured Suspensions from In Situ Hydrophobized Calcium Carbonate Particles Suspended in a Polar Liquid Media.

We demonstrate that capillary suspensions can be formed from hydrophilic calcium carbonate particles suspended in a polar continuous media and connected by capillary bridges formed of minute amounts of an immiscible secondary liquid phase. This was achieved in two different polar continuous phases, water and glycerol, and three different oils, oleic acid, isopropyl myristate, and peppermint oil as a secondary liquid phase. The capillary structuring of the suspension was made possible through local in situ hydrophobization of the calcium carbonate particles dispersed in the polar media by adding very small amounts of oleic acid to the secondary liquid phase. We observed a strong increase in the viscosity of the calcium carbonate suspension by several orders of magnitude upon addition of the secondary oil phase compared with the same suspension without secondary liquid phase or without oleic acid. The stability and the rheological properties of the obtained capillary structured materials were studied in relation to the physical properties of the system such as the particle size, interfacial tension between the primary and secondary liquid phases, as well as the particle contact angle at this liquid-liquid interface. We also determined the minimal concentrations of the secondary liquid phase at fixed particle concentration as well as the minimal particle concentration at fixed secondary phase concentration needed to form a capillary suspension. Capillary suspensions formed by this method can find application in structuring pharmaceutical and food formulations as well as a variety of home and personal care products.

Co-assembly and Structure of Sodium Dodecylsulfate and other n-Alkyl Sulfates in Glycerol: n-Alkyl Sulfate-Glycerol Crystal Phase.

HYPOTHESIS: Following the observation of a microfibrillar phase in sodium dodecylsulfate (SDS)-glycerol mixtures, it is hypothesized that this phase is a crystalline structure containing SDS and glycerol, where the interaction between sulfate and glycerol layers mediates the co-assembly, which also could be universal for similar systems formed by n-alkyl sulfate homologues. Experiment. n-alkyl sulfate glycerol solutions were studied using a combination of optical microscopy, small- and wide-angle X-ray scattering (SAXS/WAXS). Time-resolved SAXS was employed to determine the phase formation in SDS-glycerol-water mixtures. FINDINGS: The microfibrillar crystalline phase was reproduced in even-chained n-alkyl sulfates with a chain length between 12 and 18 carbon atoms, where the phase lamellar period increased uniformly with the alkyl chain length. Reconstruction of electron density profiles from the diffraction patterns allowed the lamellar structural motif of the phase, the glycerol location and stoichiometry to be determined. When SDS-glycerol-water mixtures with water concentration below 6 wt% are isothermally solidified at 20 °C, SDS-glycerol crystals and/or anhydrous SDS form, where the former is inhibited by the latter at higher water concentrations. The learnings from the SDS-glycerol phase formation allows new gels to be created, utilising the glycerol-sulfate motif generating microfibrils. This expands the knowledge of the applicable formulation space for SDS-water containing mixtures.

Thermally Responsive Capillary Suspensions.

We demonstrate that stimulus-responsive capillary-structured materials can be formed from hydrophobized calcium carbonate particles suspended in a non-polar phase (silicone oil) and bridged by very small amounts of a hydrogel as the secondary aqueous phase. Inclusion of thermally responsive polymers into the aqueous phase yielded a capillary-structured suspension whose rheology is controlled by a change in temperature and can increase its complex modulus by several orders of magnitude because of the gelation of the capillary bridges between the solid particles. We demonstrate that the rheology of the capillary suspension and its response upon temperature changes can be controlled by the gelling properties as little as 0.1 w/w % of the secondary aqueous phase containing 2 wt % of the gelling carbohydrate. Doping the secondary (aqueous) phase with methyl cellulose, which gels at elevated temperatures, gave capillary-structured materials whose viscosity and structural strength can increase by several orders of magnitude as the temperature is increased past the gelling temperature of the methyl cellulose solution. Increasing the methyl cellulose concentration from 0 to 2 w/w % in the secondary (aqueous) phase increases the complex modulus and the yield stress of the capillary suspension of 10 w/w % hydrophobized calcium carbonate in silicone oil by 2 orders of magnitude at a fixed temperature. By using an aqueous solution of a low melting point agarose as a secondary liquid phase, which melts as the temperature is raised, we produced capillary-structured materials whose viscosity and structural strength can decrease by several orders of magnitude as the temperature is increased past the melting temperature of the agarose solution. The development of thermally responsive capillary suspensions can find potential applications in structuring of smart home and personal care products as well as in temperature-triggered change in rheology and release of flavors in foods and actives in pharmaceutical formulations.

Simple method for controlled association of colloidal-particle mixtures using pH-dependent hydrogen bonding.

We describe a simple method for the controlled mixing of particles that could be used to produce materials with new properties. We demonstrate the procedure with sets of silica particles that have each been coated with one of two different organic thin films. One set of particles is functionalized with carboxylic acid groups and the other with ethylene oxide. Each of these sets of particles is stable in solution. When mixed, heteroaggregation can be induced reversibly and on demand simply by changing the pH. We provide evidence that control over aggregation is achieved by the ability to alter the number of hydrogen bonds between different types of particles and thus the strength of the attraction between different particles. We provide support for this mechanism by measuring the forces between a plate coated in a thin film of carboxylic groups and particles coated in ethylene oxide using colloid probe AFM. At pH 9, where we expect most of the acidic groups to be deprotonated, there is a strong repulsion between the particle and plate. However, at pH 3, the force is attractive, which we assign to the hydrogen bonding between the ether oxygen of the PEO and the hydrogen of the carboxylic acid group. Heteroflocculation occurred in the pH range of 3-4.5. At pH 5 and above, no flocculation was observed. Because the number of hydrogen bonds per surface area and therefore the strength of binding between dissimilar particles can be titrated through control of the pH, we can control the surface forces and avoid rapid coagulation that produces low density and disorganized particle arrangements. The control of interaction forces and therefore the approach of particles should allow the production of composite materials having mixture or product properties.

Surface chemistry and rheology of polysulfobetaine-coated silica.

We have measured the viscosity of suspensions of colloidal silica particles (d = 300 nm) and the properties of silica surfaces in solutions of a polymer consisting of zwitterionic monomer groups, poly(sulfobetaine methacrylate), polySBMA. This polymer has potential use in modifying surface properties because the polymer is net uncharged and therefore does not generate double-layer forces. The solubility of the polymer can be controlled and varies from poor to good by the addition of sodium chloride salt. Ellipsometry was used to demonstrate that polySBMA adsorbs to silica and exhibits an increase in surface excess at lower salt concentration, which is consistent with a smaller area per molecule at low salt concentration. Neutron reflectivity measurements show that the adsorbed polymer has a thickness of about 3.7 nm and is highly hydrated. The polymer can be used to exercise considerable control over suspension rheology. When silica particles are not completely covered in polymer, the suspension produces a highly viscous gel. Atomic force microscopy was used to show this is caused by bridging of polymer between the particles. At higher surface coverage, the polymer can produce either a high or very low viscosity slurry depending on the sodium chloride concentration. At high salt concentration, the suspension is stable, and the viscosity is lower. This is probably because the entrainment of many small ions renders the polymer film highly hydrophilic, producing repulsive surface forces and lubricating the flow of particles. At low salt concentrations, the polymer is barely soluble and more densely adsorbed. This produces less stable and more viscous solutions, which we attribute to attractive interactions between the adsorbed polymer layers.

Heteroflocculation of particle mixtures by a coacervation mechanism. A rheological study.

In most of the classical studies of heteroflocculation two sets of oppositely charged particles are mixed. In this current study, a somewhat different mechanism of heteroflocculation is described. Two sets of concentrated dispersions of polyacrylate latex particles (having the same surface-charge sign) have been mixed, where the surface of one set had been functionalized with methacrylic acid (MAAc) groups and the second set with poly(ethylene oxide) (PEO) chains. The resultant heteroflocculation has been investigated as a function of the number fraction (F) of MAAc-functionalized particles, the size ratio (R) of the two sets of particles (R = d(MMAc)/d(PEO)), the background electrolyte concentration (0-0.2 M KCl), the pH (3 or 9), and the order of mixing of the particles. The relative extent and strength of flocculation were assessed using two basic rheological techniques: (i) the plastic viscosity (eta(pl)) and the Bingham yield stress (sigmaB) were determined from steady-state shear experiments, (ii) the modulus (G) and the viscosity (eta) (both at a given applied stress) and the actual yield stress (sigmaY) were determined from creep-recovery experiments. Heteroflocculation was observed at a pH value of 3, where the carboxylic groups at the surface of the MAAc-functionalized particles remain largely undissociated. However, no flocculation was observed at pH 9, where the COOH groups dissociate to become COO-. The aggregation mechanism is, therefore, believed to be due to hydrogen-bonding between the hydrogens of the carboxylic acid groups and the ether oxygens present on the surface of the PEO-functionalized particles. To this extent, this mechanism of heteroflocculation resembles the coacervation of mixtures of solutions of two H-bonding polymers, for which aqueous mixtures of PMAAc and PEO (at low pH) are a well-known example. Because both sets of particles carried negative surface charge groups, arising from the polymerization initiator used in their preparation, a minimum concentration of added electrolyte (KCl) was needed before any heteroflocculation between the two sets of particles was observed. However, this minimum KCl concentration for the onset of heteroflocculation was significantly lower than the concentration of KCl required to induce homoflocculation of either set of latex particles separately. At an R value of 1.3, all the rheological parameters passed through a maximum value at F = 0.5, whereas when R was 6.2 the maximum occurred at a value of F = 0.018 or F = 0.025, depending on the order of mixing.