Dirhamnolipid ester - formation of reverse wormlike micelles in a binary (primerless) system.

We report new dirhamnolipid ester forming reverse wormlike micelles in nonpolar solvents without the addition of any primer. Therefore, these compounds represent a rare case of a binary system showing this gel-like behavior. In this study, the influence of the concentration of the rhamnolipid ester and the ester alkyl chain length on the rheological properties of the reverse wormlike micelles in toluene was investigated in detail. Highly viscoelastic solutions were obtained even at a relatively low concentration of less than 1 wt %. The phase transition temperatures indicate that the formation of reverse wormlike micelles is favored for dirhamnolipid esters with shorter alkyl chain lengths. Oscillatory shear measurements for the viscoelastic samples reveal that the storage modulus (G') and the loss modulus (G'') cross each other and fit the Maxwell model very well in the low-ω region. As is typical for wormlike micelle systems, the normalized Cole-Cole plot of G''/G'' max against G'/G'' max was obtained as a semicircle centered at G'/G'' max = 1. The formation of network structures was also verified by polarized light microscopy. The sample was birefringent at ambient temperature and anisotropic at an elevated temperature. Differential scanning calorimetry analysis yielded a transition enthalpy of about ΔH SG/GS = ±7.2 kJ/mol. This value corresponds to a strong dispersion energy and explains the formation of the highly viscous gels by the entanglement of wormlike micelles through the interaction of the alkyl chains.

Effect of Geometry on Electrokinetic Characterization of Solid Surfaces.

An analytical approach is presented to describe pressure-driven streaming current (Istr) and streaming potential (Ustr) generation in geometrically complex samples, for which the classical Helmholtz-Smoluchowski (H-S) equation is known to be inaccurate. The new approach is valid under the same prerequisite conditions that are used for the development of the H-S equation, that is, the electrical double layers (EDLs) are sufficiently thin and surface conductivity and electroviscous effects are negligible. The analytical methodology is developed using linear velocity profiles to describe liquid flow inside of EDLs and using simplifying approximations to describe macroscopic flow. At first, a general expression is obtained to describe the Istr generated in different cross sections of an arbitrarily shaped sample. Thereafter, assuming that the generated Ustr varies only along the pressure-gradient direction, an expression describing the variation of generated Ustr along the sample length is obtained. These expressions describing Istr and Ustr generation constitute the theoretical foundation of this work, which is first applied to a set of three nonuniform cross-sectional capillaries and thereafter to a square array of cylindrical fibers (model porous media) for both parallel and transverse fiber orientation cases. Although analytical solutions cannot be obtained for real porous substrates because of their random structure, the new theory provides useful insights into the effect of important factors such as fiber orientation, sample porosity, and sample dimensions. The solutions obtained for the model porous media are used to device strategies for more accurate zeta potential determination of porous fiber plugs. The new approach could be thus useful in resolving the long-standing problem of sample geometry dependence of zeta potential measurements.

Streaming potential measurements to understand the rheological properties of surfactant formulations containing anionic and zwitterionic surfactant.

Surfactant formulations are often based on an anionic primary surfactant combined with an amphoteric secondary surfactant. One popular option is the combination of lauryl ether sulfate and cocamidopropyl betaine, because such formulations are not only mild but also easy to thicken. Changes in the molecular structure of the betaine in terms of alkyl chain length distribution and headgroup structure do have dramatic effects on the viscosity of these formulations, as can be explained in terms of properties of rod-like micelles and exchange kinetics by oscillatory rheological measurements. The root cause of the effect of the different betaine derivatives on the micellar structure, however, remains unclear when considering rheology only. Although the streaming potential of colloidal objects is typically determined to forecast the stability of dispersions, we have used the streaming potential to characterize micellar solutions of different betaine surfactant structures. It could be shown that (a) the hydrophilicity of the surfactants can be nicely probed by this method and (b) there is a good correlation of these values with the rheological properties of binary mixtures of the betaines with anionic surfactant. Also, the chemical structure of the headgroups has a significant influence on both the isoelectric point and the magnitude of the streaming potential of the zwitterionic surfactants. These effects have again a dramatic influence on the interaction with anionic surfactants, as becomes obvious when looking at the rheology of such mixtures. Therefore, the findings obtained can be utilized to better understand and design surfactant formulations of a desired viscosity profile.

Influence of architecture on the interaction of negatively charged multisensitive poly(N-isopropylacrylamide)-co-methacrylic acid microgels with oppositely charged polyelectrolyte: absorption vs adsorption.

Two sets of core-shell microgels composed of temperature-sensitive poly(N-isopropylacrylamide) (PNiPAM) with different spatial distribution of pH-sensitive methacrylic acid (MAA) groups were prepared. The cores consist of either PNiPAM (neutral core; nc) or PNiPAM-co-MAA (charged core; cc). A charged shell existing of PNiPAM-co-MAA was added to the neutral core (yielding neutral core-charged shell; nccs), on the charged core, on the other hand, a neutral shell of PNiPAM was added (charged core-neutral shell; ccns). Complexes of these microgels with positively charged poly(diallyldimethylammonium chloride) (PDADMAC) of different molar masses were prepared. The amount of bound polyelectrolyte was quantified, and the microgel-polyelectrolyte complexes were characterized with respect to electrophoretic mobility and hydrodynamic radius. The penetration of polyelectrolyte into the microgel was also monitored by means of lifetime analysis of a fluorescent dye covalently bound to poly(L-lysine) providing information on the probe's local environment. The architecture of the microgel has a significant influence on the interaction with oppositely charged polyelectrolyte. Complexes with microgel with the charged shell tend to flocculate at charge ratios of 1 and are thus similar to polyelectrolyte complexes with rigid colloidal particles. Complexes with microgels that consist of a charged core and a neutral shell show very different properties: They are still temperature sensitive and reveal an influence of the polyelectrolyte's chain length. Low molecular weight PDADMAC can penetrate through the neutral shell into the charged core, and thus nearly no charge reversal occurs. The high-MW polyelectrolyte does not penetrate fully and leads to charge reversal. The results demonstrate that microgels are able to absorb or adsorb polyelectrolytes depending on the polyelectrolyte's chain length and the microgels architecture. Complexes with different surface properties and different colloidal stability can be prepared, and polyelectrolytes can be encapsulated in the microgel core. Thus, multisensitive core-shell microgels combine permeability and compartmentalization on a nanometer length scale and provide unique opportunities for applications in controlled uptake and release.

Electrokinetic investigation of deposition of cationic fabric softener vesicles on anionic porous cotton fabrics.

HYPOTHESIS: Colloidal deposition on porous substrates is a complex process influenced by both, (i) characteristics of colloidal permeation into porous substrates, and (ii) mechanism of colloidal deposition on solid surfaces. Such processes are quintessential to action of products such as hair conditioners and fabric softeners where the substrates to be treated are porous. The performance of these formulations is linked with the distribution of deposited colloids across porous substrates i.e. whether deposition is localized near substrate periphery, or deposition is homogeneously distributed. EXPERIMENTS: In this work, we investigate the deposition of cationic vesicles, commonly used in fabric softeners, on anionic porous cotton yarns via spectrophotometric measurement of adsorption density of vesicles on yarns and electrokinetic measurement of cotton yarn apparent zeta potentials. Under the employed conditions, cotton yarn apparent zeta potentials are sensitive predominantly to external yarn surfaces. Therefore, these measurements can distinguish between deposition on external and internal yarn surfaces. FINDINGS: The phase behavior of lipid bilayers constituting the vesicles is identified as an important governing factor with solid-gel vesicles depositing more near yarn periphery, and liquid-crystalline vesicles depositing more uniformly throughout the yarns. Bulk electrical conductivity also influences the distribution of deposited vesicles. The results are explained with the help of a newly proposed theory.

Rheological characterization of the exopolysaccharide Paenan in surfactant systems.

Rheology-controlling agents are of importance for numerous products in a variety of industries. Replacement of synthetic chemicals with natural additives is desired in light of current environmental awareness and limited fossil resources. This study investigates the rheological features of Paenan, an exopolysaccharide produced by Paenibacillus polymyxa. Paenan exhibits highly shear-thinning flow behavior at concentrations ≥0.1% in 0.5% NaCl. Because of its pronounced intermolecular network, it forms stable, weak gels, thereby delivering elasticity as well as thixotropy. Application-relevant flow behavior is obtained with 60-65% less polymer as compared to the benchmark commercial products Xanthan and Gellan. In mixtures with surfactants (sodium lauryl ether sulfate, cetrimonium chloride, cocamidopropyl betaine, or lauryl glucoside), Paenan displays outstanding compatibility with every class of surfactant, making it superior to the partially incompatible Xanthan and Gellan. The weak-gel character of Paenan/surfactant systems is retained with three out of four surfactants, rendering Paenan highly interesting for various applications.

Intact deposition of cationic vesicles on anionic cellulose fibers: Role of vesicle size, polydispersity, and substrate roughness studied via streaming potential measurements.

HYPOTHESIS: Understanding the mechanism of intact vesicle deposition on solid surfaces is important for effective utilization of vesicles as active ingredient carriers in applications such as drug delivery and fabric softening. In this study, the deposition of large (davg=12µm) and small (davg=0.27µm) cationic vesicles of ditallowethylester dimethylammonium chloride (DEEDMAC) on smooth and rough anionic cellulose fibers is investigated. EXPERIMENTS: The deposition process is studied quantitatively using streaming potential measurements and spectrophotometric determination of DEEDMAC concentrations. Natural and regenerated cellulose fibers, namely cotton and viscose, having rough and smooth surfaces, respectively, are used as adsorbents. Equilibrium deposition data and profiles of substrate streaming potential variation with deposition are used to gain insights into the fate of vesicles upon deposition and the deposition mechanism. FINDINGS: Intact deposition of DEEDMAC vesicles is ascertained based on streaming potential variation with deposition in the form of characteristic saturating profiles which symbolize particle-like deposition. The same is also confirmed by confocal fluorescence microscopy. Substrate roughness is found to considerably influence the deposition mechanism which, in a novel application of electrokinetic methods, is elucidated via streaming potential measurements.

Rearrangements in and release from responsive microgel-polyelectrolyte complexes induced by temperature and time.

The binding of polyelectrolyte to a temperature- and pH-responsive microgel based on poly-N-isopropylacrylamide (PNiPAM) copolymerized with methacrylic acid (MAA) as a soft and porous substrate was investigated as a function of time and temperature in order to probe rearrangements in such complexes. Oppositely charged polyelectrolytes bind to the charged microgels, and the composition of the resulting complexes stays constant with time. The number of titrable COOH groups, the size, and the electrophoretic mobility of the complexes, however, change with time due to rearrangements of polyelectrolyte chains inside of the microgel. Polyelectrolytes can be used to modify the properties of microgels. The volume phase transition temperature (VPTT) of PNiPAM-co-MAA microgels depends on the pH value, while microgel polyelectrolyte complexes collapse above the VPTT of 32 °C independently of the pH value. The experiments reveal that polyelectrolytes can be partially released from microgel-polyelectrolyte complexes at T > VPTT. In addition, rearrangements are induced by the collapse. Rebinding of the polyelectrolyte occurs upon reswelling of the complex when the temperature is reduced below the VPTT. Such temperature cycles affect the size and electrophoretic mobility of complexes. The rearrangements can be used to increase the amount of polyelectrolyte that is bound to the microgel and are thus important for applications that rely on loading microgels with polymers. Interestingly, the colloidal stability of the complexes at T > VPTT depends on the preparation temperature; complexes prepared at T < VPTT remain colloidally stable when heated to T > VPTT; on the other hand, complexes prepared at T > VPTT display poor colloidal stability.