Polypeptoid Monomer Sequence and Chemical Composition as Independent Controls of Interfacial Tension and Elasticity at Air/Fluid Interfaces.

We use sequence-specific polypeptoids to characterize the impact of the monomer sequence on the adsorption of surface-active polymers at fluid/fluid interfaces. Sets of 36 repeat unit polypeptoids with identical chemical composition, but different sequences of hydrophobic moieties along the oligomer chain (taper, inverse taper, blocky, and evenly distributed), are designed and characterized at air/water interfaces. Polypeptoids are driven to the interfaces by decreasing the solvent quality of the aqueous solution. In situ processing of the adsorbed layers causes a collapse of polypeptoids and the formation of irreversibly adsorbed, solvent-avoiding layers at interfaces. Differences in thermodynamic properties, driven by solubility, between the collapsed structures at interfaces are studied with measurements of interfacial tension. The dilatational modulus of polypeptoid-coated interfaces is used as a proxy to probe the extent of the coil-globule collapse at the interface. The role of hydrophobicity is investigated by comparing four sequences of polypeptoids with an increased size of the hydrophilic side chains. In each set of polypeptoids, the composition of molecules, not the sequence, controls the surface concentration. The molecules are described in terms of the distribution of the hydrophobic monomers on the backbone of the polymer. Inverse taper (IT) and blocky (B) sequences of hydrophobic moieties favor the formation of highly elastic interfaces after processing, while taper (T) and distributed (D) showed lower elasticity after processing, which is achieved by replacing good solvent with poor solvent and then nonsolvent. These structures allow for the study of the impact of the chemical composition and sequence of monomers on the properties of polymer-coated interfaces.

Solvent Quality and Aggregation State of Asphaltenes on Interfacial Mechanics and Jamming Behavior at the Oil/Water Interface.

The formation of highly stable water-in-oil emulsions results in complications in both upstream and downstream processing. Emulsion stability in these systems has been connected to the adsorption of surface-active asphaltenes that are assumed to form a rigidified film at the oil/water (o/w) interface. Full characterization of this behavior is needed to allow for engineered solutions for enhanced oil recovery. Interfacial properties, such as surface pressure and interfacial elasticity, are implicated in the stabilizing mechanism for these observed films. Asphaltenes are known to be interfacially active in both good solvents (aromatics) and poor solvents (high ratio of aliphatic to aromatic). However, due to inherent complexities present in asphaltene studies, the details of the mechanical properties of the interface remain poorly understood. Despite the widely accepted perception that asphaltenes form persistent rigid films at fluid-fluid interfaces, the connection between bulk solution properties and interfacial mechanics has not been resolved. Here, the effects of solvent quality on the interfacial properties of asphaltene dispersions are determined by using a well-defined asphaltene/solvent system. Interfacial rigidity is observed only under poor solvent conditions, while the good solvent system remains fluid-like. The interfacial rheology under good and poor solvent conditions is measured simultaneously with surface pressure measurements to track interfacial development. It is shown that surface pressure and dilatational modulus measurements are not indicators of whether an interface demonstrates rigid behavior under large compressions. Finally, conditions required for asphaltene-coated interfaces to exhibit the mechanical behavior associated with a rigidified interface are defined. This work provides a framework for quantifying the impact of the aggregation state of asphaltenes on the stability and mechanics at the o/w interface.

Nonequilibrium structure formation in electrohydrodynamic emulsions.

Application of an electric field across the interface of two fluids with low, but non-zero, conductivities gives rise to a sustained electrohydrodynamic (EHD) fluid flow. In the presence of neighboring drops, drops interact via the EHD flows of their neighbors, as well as through a dielectrophoretic (DEP) force, a consequence of drops encountering disturbance electric fields around their neighbors. We explore the collective dynamics of emulsions with drops undergoing EHD and DEP interactions. The interplay between EHD and DEP results in a rich set of emergent behaviors. We simulate the collective behavior of large numbers of drops; in two dimensions, where drops are confined to a plane; and three dimensions. In monodisperse emulsions, drops in two dimensions cluster or crystallize depending on the relative strengths of EHD and DEP, and form spaced clusters when EHD and DEP balance. In three dimensions, chain formation observed under DEP alone is suppressed by EHD, and lost entirely when EHD dominates. When a second population of drops are introduced, such that the electrical conductivity, permittivity, or viscosity are different from the first population of drops, the interaction between the drops becomes non-reciprocal, an apparent violation of Newton's Third Law. The breadth of consequences due to these non-reciprocal interactions are vast: we show selected cases in two dimensions, where drops cluster into active dimers, trimers, and larger clusters that continue to translate and rotate over long timescales; and three dimensions, where drops form stratified chains, or combine into a single dynamic sheet.

Droplet-Based Microfluidic Tool to Quantify Viscosity of Concentrating Protein Solutions.

PURPOSE: Measurement of the viscosity of concentrated protein solutions is vital for the manufacture and delivery of protein therapeutics. Conventional methods for viscosity measurements require large solution volumes, creating a severe limitation during the early stage of protein development. The goal of this work is to develop a robust technique that requires minimal sample. METHODS: In this work, a droplet-based microfluidic device is developed to quantify the viscosity of protein solutions while concentrating in micrometer-scale droplets. The technique requires only microliters of sample. The corresponding viscosity is characterized by multiple particle tracking microrheology (MPT). RESULTS: We show that the viscosities quantified in the microfluidic device are consistent with macroscopic results measured by a conventional rheometer for poly(ethylene) glycol (PEG) solutions. The technique was further applied to quantify viscosities of well-studied lysozyme and bovine serum albumin (BSA) solutions. Comparison to both macroscopic measurements and models (Krieger-Dougherty model) demonstrate the validity of the approach. CONCLUSION: The droplet-based microfluidic device provides accurate quantitative values of viscosity over a range of concentrations for protein solutions with small sample volumes (~ µL) and high compositional resolution. This device will be extended to study the effect of different excipients and other additives on the viscosity of protein solutions.

Transport of Flexible, Oil-Soluble Diblock and BAB Triblock Copolymers to Oil/Water Interfaces.

The connection between block copolymer architecture and adsorption at fluid/fluid interfaces is poorly understood. We characterize the interfacial properties of a well-defined series of polyethylene oxide/polydimethyl siloxane (PDMS) diblock and BAB triblock copolymers at the dodecane/water interface. They are oil-soluble and quite flexible because of their hydrophobic PDMS block. Rather than relying on equilibrium interfacial measurements for which it is difficult to mitigate experimental uncertainty during adsorption, we combine measurements of steady-state adsorption, dilatational rheology, and adsorption/desorption dynamics. Steady-state interfacial pressure is insensitive to interfacial curvature and mostly agrees with theory. Adsorption does not occur in the diffusive limit as is the case for many aqueous, small-molecule surfactants. Dilatational rheology reveals differences in behavior between the diblocks and triblocks, and all interfaces possess elasticities below the thermodynamic limit. Desorption dynamics show that material exchange between the interface and the neighboring fluid occurs too slowly to relax dilatational stresses. The mechanism of relaxation occurs at the interface, likely from the reorientation of adsorbed chains.

Inflammation product effects on dilatational mechanics can trigger the Laplace instability and acute respiratory distress syndrome.

In the lungs, the Laplace pressure, ΔP = 2γ/R, would be higher in smaller alveoli than larger alveoli unless the surface tension, γ decreases with alveolar interfacial area, A, such that 2ε > γ in which ε = A(dγ/dA) is the dilatational modulus. In Acute Respiratory Distress Syndrome (ARDS), lipase activity due to the immune response to an underlying trauma or disease causes single chain lysolipid concentrations to increase in the alveolar fluids via hydrolysis of double-chain phospholpids in bacterial, viral, and normal cell membranes. Increasing lysolipid concentrations decrease the dilatational modulus dramatically at breathing frequencies if the soluble lysolipid has sufficient time to diffuse off the interface, causing 2ε < γ, thereby potentially inducing the "Laplace Instability", in which larger alveoli have a lower internal pressure than smaller alveoli. This can lead to uneven lung inflation, alveolar flooding, and poor gas exchange, typical symptoms of ARDS. While the ARDS lung contains a number of lipid and protein species in the alveolar fluid in addition to lysolipids, the surface activity and frequency dependent dilatational modulus of lysolipid suggest how inflammation may lead to the lung instabilities associated with ARDS. At high frequencies, even at high lysolipid concentrations, 2ε - γ > 0, which may explain the benefits ARDS patients receive from high frequency oscillatory ventilation.

Interfacial Properties of Polyelectrolyte-Surfactant Aggregates at Air/Water Interfaces.

The transport, equilibrium properties, and mechanics of stable, rodlike surfactant-polyelectrolyte aggregates, poly(cetyltrimethylammonium vinyl benzoate) or pCTVB, are characterized at air/water interfaces for bulk concentrations near and below the critical aggregation concentration. The surfactant drives the transport to air/water interfaces, while the polyelectrolyte provides elasticity to the coated interfaces and appears to remain adsorbed after the bulk solution is exchanged with water. The processing of interfaces is shown to allow the interfacial tension of the interface to be changed significantly while maintaining a high dilatational elasticity. The results of this work provide a tool to control interfacial properties through design of polyelectrolyte-surfactant complexes.

Colloidal stability dictates drop breakup under electric fields.

Electric fields can deform drops of fluid from their equilibrium shape, and induce breakup at sufficiently large field strengths. In this work, the electric field induced breakup of a squalane drop containing a colloidal suspension of carbon black particles with polyisobutylene succinimide (OLOA 11000) surfactant is studied. The drop is suspended in silicone oil. The breakup mode of a drop containing carbon black depends strongly on the suspension stability. It is observed that a drop of a stable suspension of carbon black has the same breakup mode as a drop with surfactant alone, i.e., without added carbon black. At lower electric fields, these drops break by the formation of lobes at the two ends of the drop; and at higher fields the homogeneous lobes break in a non-axisymmetric manner. However, a drop of an unstable suspension shows a drastically different breakup mode, and undergoes breakup much faster compared to a drop with surfactant alone. These drops elongate and form asymmetric lobes that develop into fingers and eventually disintegrate in an inhomogeneous, three-dimensional fashion. As a basis for comparison, the breakup of a pure squalane drop, and a squalane drop with equivalent surfactant concentrations but no carbon black particles is examined. Axisymmetric boundary integral computations are used to elucidate the mechanism of breakup. Our work demonstrates the impact of colloidal stability on the breakup of drops under an electric field. Colloidal stability on the time scale of drop deformation leads to rich and unexplored breakup phenomena.

Effect of surfactant tail length and ionic strength on the interfacial properties of nanoparticle-surfactant complexes.

Mixed nanoparticle-surfactant systems are effective foam stabilizing agents, but the lack of colloidal stability of the bulk dispersions makes interfacial characterization challenging. This study investigates the adsorption of CnTAB/SiO2 complexes at air/water interfaces through surface tension and interfacial rheology measurements. The effects of surfactant tail length, ionic strength, and interfacial processing on the surface properties are measured utilizing a bulk reservoir exchange methodology to avoid bulk destabilization. The surfactant structure controls the surface tension of the system, but has minimal impact on particle surface coverage or interfacial mechanics. Once adsorbed, nanoparticles remain pinned at the surface, while the surfactant is able to desorb upon bulk exchange with deionized water. Particle packing on the interface governs the interfacial mechanics, which can be modified by increasing the ionic strength of the bulk solution. Fully rigid interfaces can be generated at low particle coverages by controlling the ionic strength and interfacial processing. These findings contribute to the understanding of mixed particle-surfactant systems and inform formulation and process design to achieve the desired interfacial mechanical properties.

Formation of a Rigid Hydrophobin Film and Disruption by an Anionic Surfactant at an Air/Water Interface.

Hydrophobins are amphiphilic proteins produced by fungi. Cerato-ulmin (CU) is a hydrophobin that has been associated with Dutch elm disease. Like other hydrophobins, CU stabilizes air bubbles and oil droplets through the formation of a persistent protein film at the interface. The behavior of hydrophobins at surfaces has raised interest in their potential applications, including use in surface coatings, food foams, and emulsions and as dispersants. The practical use of hydrophobins requires an improved understanding of the interfacial behavior of these proteins, alone and in the presence of added surfactants. In this study, the adsorption behavior of CU at air/water interfaces is characterized by measuring the surface tension and interfacial rheology as a function of adsorption time. CU is found to adsorb irreversibly at air/water interfaces. The magnitude of the dilatational modulus increases with adsorption time and surface pressure until CU eventually forms a rigid film. The persistence of this film is tested through the sequential addition of strong surfactant sodium dodecyl sulfate (SDS) to the bulk liquid adjacent to the interface. SDS is found to coadsorb to interfaces precoated with a CU film. At high concentrations, the addition of SDS significantly decreases the dilatational modulus, indicating disruption and displacement of CU by SDS. Sequential adsorption results in mixed layers with properties not observed in interfaces generated from complexes formed in the bulk. These results lend insight to the complex interfacial interactions between hydrophobins and surfactants.

Measurements of Submicron Particle Adsorption and Particle Film Elasticity at Oil-Water Interfaces.

The influence of particle adsorption on liquid/liquid interfacial tension is not well understood, and much previous research has suggested conflicting behaviors. In this paper we investigate the surface activity and adsorption kinetics of charge stabilized and pH-responsive polymer stabilized colloids at oil/water interfaces using two tensiometry techniques: (i) pendant drop and (ii) microtensiometer. We found, using both techniques, that charge stabilized particles had little or no influence on the (dynamic) interfacial tension, although dense silica particles affected the "apparent" measured tension in the pendent drop, due to gravity driven elongation of the droplet profile. Nevertheless, this apparent change additionally allowed the study of adsorption kinetics, which was related qualitatively between particle systems by estimated diffusion coefficients. Significant and real interfacial tension responses were measured using ∼53 nm core-shell latex particles with a pH-responsive polymer stabilizer of poly(methyl methacrylate)-b-poly(2-(dimethylamino)ethyl methacrylate) (pMMA-b-pDMAEMA) diblock copolymer. At pH 2, where the polymer is strongly charged, behavior was similar to that of the bare charge-stabilized particles, showing little change in the interfacial tension. At pH 10, where the polymer is discharged and poorly soluble in water, a significant decrease in the measured interfacial tension commensurate with strong adsorption at the oil-water interface was seen, which was similar in magnitude to the surface activity of the free polymer. These results were both confirmed through droplet profile and microtensiometry experiments. Dilational elasticity measurements were also performed by oscillation of the droplet; again, changes in interfacial tension with droplet oscillation were only seen with the responsive particles at pH 10. Frequency sweeps were performed to ascertain the dilational elasticity modulus, with measured values being significantly higher than previously reported for nanoparticle and surfactant systems, and similar in magnitude to protein stabilized droplets.

Transport of nanoparticulate material in self-assembled block copolymer micelle solutions and crystals.

Water soluble poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) [PEO-PPO-PEO] triblock copolymers self-assemble into thermoreversible micellar crystals comprised of periodically spaced micelles. The micelles have PPO cores surrounded by hydrated PEO coronas and the dimensions of the unit cell of the organized micelles is on the order of several to tens of nanometers. Fluorescence recovery after photobleaching (FRAP) is used to quantify nanoparticle transport in these nanostructured polymer micelle systems. Diffusivity of bovine serum albumin (BSA, Dh ∼ 7 nm) is quantified across a wide range of polymer, or micelle, concentrations covering both the disordered fluid as well as the structured micellar crystal to understand the effects of nanoscale structure on particle transport. Measured particle diffusivity in these micellar systems is reduced by as much as four orders of magnitude when compared to diffusivity in free solution. Diffusivity in the disordered micellar fluid is best understood in terms of diffusion through a polymeric solution, while transport in the structured micellar phase is possibly due to hopping between interstitial sites. These results not only show that the nanoscale structures of the micelles have a measureable impact on particle diffusivity, but also demonstrate the ability to tune nanoscale transport in self-assembled materials.

Sequential adsorption of an irreversibly adsorbed nonionic surfactant and an anionic surfactant at an oil/aqueous interface.

Aerosol-OT (AOT) and Tween 80 are two of the main surfactants in commercial dispersants used in response to oil spills. Understanding how multicomponent surfactant systems interact at oil/aqueous interfaces is crucial for improving both dispersant design and application efficacy. This is true of many multicomponent formulations; a lack of understanding of competition for the oil/water interface hinders formulation optimization. In this study, we have characterized the sequential adsorption behavior of AOT on squalane/aqueous interfaces that have been precoated with Tween 80. A microtensiometer is used to measure the dynamic interfacial tension of the system. Tween 80 either partially or completely irreversibly adsorbs to squalane/aqueous interfaces when rinsed with deionized water. These Tween 80 coated interfaces are then exposed to AOT. AOT adsorption increases with AOT concentration for all Tween 80 coverages, and the resulting steady-state interfacial tension values are interpreted using a Langmuir isotherm model. In the presence of 0.5 M NaCl, AOT adsorption significantly increases due to counterion charge screening of the negatively charged head groups. The presence of Tween 80 on the interface inhibits AOT adsorption, reducing the maximum surface coverage as compared to a clean interface. Tween 80 persists on the interface even after exposure to high concentrations of AOT.

Interfacial tension dynamics, interfacial mechanics, and response to rapid dilution of bulk surfactant of a model oil-water-dispersant system.

In the 2010 Deepwater Horizon rig explosion and subsequent oil spill, five million barrels of oil were released into the Gulf over the course of several months. Part of the resulting emergency response was the unprecedented use of nearly two million gallons of surfactant dispersant at both the sea surface and well head, giving rise to previously untested conditions of high temperature gradients, high pressures, and flow conditions. To better understand the complex interfacial transport mechanisms that this dispersant poses, we develop a model surfactant-oil-aqueous system of Tween 80 (a primary component in the Corexit dispersant used in the Gulf), squalane, and both simulated seawater as well as deionized water. We measure surfactant adsorption dynamics to the oil-aqueous interface for a range of surfactant concentrations. Using techniques developed in our laboratory, we investigate the impact of convection, step changes in bulk concentration, and interfacial mechanics. We observe dynamic interfacial behavior that is consistent with a reorganization of surfactant at the interface. We demonstrate irreversible adsorption behavior of Tween 80 near a critical interfacial tension value, as well as measure the dilatational elasticity of equilibrium and irreversibly adsorbed layers of surfactant on the oil-aqueous interface. We report high values of the surface dilatational elasticity and surface dilatational viscosity, and discuss these results in terms of their impact regarding oil spill response measures.

Synergistic Effects of Multiple Excipients on Controlling Viscosity of Concentrated Protein Dispersions.

Viscosity control is essential for the manufacturing and delivery of concentrated therapeutic proteins. Limited availability of the precious protein-based drugs hinders the characterization and screening of the formulation conditions with new types or different combinations of excipients. In this work, a droplet-based microfluidic device with incorporated multiple particle tracking microrheology (MPT) is developed to quantify the effects of two excipients, arginine hydrochloride (ArgHCl) and caffeine, on the viscosity of concentrated bovine gamma globulin (BGG) dispersions at two different values of pH. The effectiveness of both ArgHCl and caffeine show dependence on the BGG concentration and solution pH. The data set with high compositional resolution provides useful information to guide formulation with multiple viscosity-reducing excipients and quantification appropriate to start elucidating the connection to protein-protein interaction mechanisms. Overall, this work has demonstrated that the developed microfluidic approach has the potential to effectively assess the impact of multiple excipients on the viscosity and provide data for computational methods to predict viscosity for high concentration protein formulations.

Highly Conductive Polyoxanorbornene-Based Polymer Electrolyte for Lithium-Metal Batteries.

This present study illustrates the synthesis and preparation of polyoxanorbornene-based bottlebrush polymers with poly(ethylene oxide) (PEO) side chains by ring-opening metathesis polymerization for solid polymer electrolytes (SPE). In addition to the conductive PEO side chains, the polyoxanorbornene backbones may act as another ion conductor to further promote Li-ion movement within the SPE matrix. These results suggest that these bottlebrush polymer electrolytes provide impressively high ionic conductivity of 7.12 × 10-4 S cm-1 at room temperature and excellent electrochemical performance, including high-rate capabilities and cycling stability when paired with a Li metal anode and a LiFePO4 cathode. The new design paradigm, which has dual ionic conductive pathways, provides an unexplored avenue for inventing new SPEs and emphasizes the importance of molecular engineering to develop highly stable and conductive polymer electrolytes for lithium-metal batteries (LMB).

Interfacial dynamics and rheology of polymer-grafted nanoparticles at air-water and xylene-water interfaces.

Particle-stabilized emulsions and foams offer a number of advantages over traditional surfactant-stabilized systems, most notably a greater stability against coalescence and coarsening. Nanoparticles are often less effective than micrometer-scale colloidal particles as stabilizers, but nanoparticles grafted with polymers can be particularly effective emulsifiers, stabilizing emulsions for long times at very low concentrations. In this work, we characterize the long-time and dynamic interfacial tension reduction by polymer-grafted nanoparticles adsorbing from suspension and the corresponding dilatational moduli for both xylene-water and air-water interfaces. The dilatational moduli at both types of interfaces are measured by a forced sinusoidal oscillation of the interface. Surface tension measurements at the air-water interface are interpreted with the aid of independent ellipsometry measurements of surface excess concentrations. The results suggest that the ability of polymer-grafted nanoparticles to produce significant surface and interfacial tension reductions and dilatational moduli at very low surface coverage is a key factor underlying their ability to stabilize Pickering emulsions at extremely low concentrations.

3D Printing of Liquid Metal Embedded Elastomers for Soft Thermal and Electrical Materials.

Liquid metal embedded elastomers (LMEEs) are composed of a soft polymer matrix embedded with droplets of metal alloys that are liquid at room temperature. These soft matter composites exhibit exceptional combinations of elastic, electrical, and thermal properties that make them uniquely suited for applications in flexible electronics, soft robotics, and thermal management. However, the fabrication of LMEE structures has primarily relied on rudimentary techniques that limit patterning to simple planar geometries. Here, we introduce an approach for direct ink write (DIW) printing of a printable LMEE ink to create three-dimensional shapes with various designs. We use eutectic gallium-indium (EGaIn) as the liquid metal, which reacts with oxygen to form an electrically insulating oxide skin that acts as a surfactant and stabilizes the droplets for 3D printing. To rupture the oxide skin and achieve electrical conductivity, we encase the LMEE in a viscoelastic polymer and apply acoustic shock. For printed composites with a 80% LM volume fraction, this activation method allows for a volumetric electrical conductivity of 5 × 104 S cm-1 (80% LM volume)─significantly higher than what had been previously reported with mechanically sintered EGaIn-silicone composites. Moreover, we demonstrate the ability to print 3D LMEE interfaces that provide enhanced charge transfer for a triboelectric nanogenerator (TENG) and improved thermal conductivity within a thermoelectric device (TED). The 3D printed LMEE can be integrated with a highly soft TED that is wearable and capable of providing cooling/heating to the skin through electrical stimulation.

Shear-Modulated Rates of Phase Transitions in Sphere-Forming Diblock Oligomer Lyotropic Liquid Crystals.

Hydration of the amphiphilic diblock oligomer C16H33(CH2CH2O)20OH (C16E20) leads to concentration-dependent formation of micellar body-centered cubic (BCC) and Frank-Kasper A15 lyotropic liquid crystals (LLCs). Quiescent thermal annealing of aqueous LLCs comprising 56-59 wt % C16E20 at 25 °C after quenching from high temperatures established their ability to form short-lived BCC phases, which transform into long-lived, transient Frank-Kasper σ phases en route to equilibrium A15 morphologies on a time scale of months. Here, the frequency and magnitude of applied oscillatory shear show the potential to either dynamically stabilize the metastable BCC phase at low frequencies or increase the rate of formation of the A15 to minutes at high frequencies. Time-resolved synchrotron small-angle X-ray scattering (TR-SAXS) provides in situ characterization of the structures during shear and thermal processing. This work shows that the LLC morphology and order-order phase transformation rates can be controlled by tuning the shear strain amplitude and frequency.

A pH-induced transition of surfactant-polyelectrolyte aggregates from cylindrical to string-of-pearls structure.

Aqueous solutions of polyelectrolyte and oppositely charged ionic surfactant are proposed to have a string-of-pearls structure under strongly acidic conditions. This differs from the cylindrical structure presented by this same system at neutral pH conditions. Experimental data are consistent with the conclusion that there is a cylinder-to-sphere transition of the adsorbed surfactant when pH is dropped below a critical value. At intermediate concentrations, this transition is manifested as a pH-induced gelation. At dilute conditions, the aggregate structure is characterized through light scattering, potentiometry, (1)H NMR, and solubility measurements. Protonation of the carboxylate groups on the polyelectrolyte and resulting restructuring of the micellar structure around the aggregate are argued to be the primary causes of the transition. Since these are completely reversible, the addition of NaOH, a strong base, is observed to reverse the structure from string-of-pearls back to cylindrical. This reversible pH-induced transition, which is caused by a shift in intermolecular forces within the aggregate, is likely to be common among self-assembled aggregates. These aggregates represent a region of the polyelectrolyte-surfactant aggregate formation (that of hydrophobic polymer and varying electrostatic attraction of opposite charged moieties) that exhibits distinctive aggregate structural behavior.