Amplification of Salt-Induced Protein Diffusiophoresis by Varying Salt from Potassium to Sodium to Magnesium Chloride in Water

The Salt-Induced Diffusiophoresis of Nonionic Micelles-Does the Salt-Induced Growth of Micelles Influence Diffusiophoresis?

Salt-induced diffusiophoresis is the migration of a colloidal particle in water due to a directional salt concentration gradient. An important example of colloidal particles is represented by micelles, generated by surfactant self-assembly in water. For non-ionic surfactants containing polyethylene glycol (PEG) groups, PEG preferential hydration at the micelle-water interface is expected to drive micelle diffusiophoresis from high to low salt concentration. However, micelles are reversible supramolecular assemblies, with salts being able to promote a significant change in micelle size. This phenomenon complicates the description of diffusiophoresis. Specifically, it is not clear to what extent the salt-induced growth of micelles affects micelle diffusiophoresis. In this paper, a multiple-equilibrium model is developed for assessing the contribution of the micelle growth and preferential hydration mechanisms to the diffusiophoresis of non-ionic micelles. The available experimental data characterizing the effect of NaCl on Triton X-100 aggregation number are combined with data on diffusiophoresis and the preferential hydration of PEG chains to show that the contribution of the micelle growth mechanism to overall diffusiophoresis is small compared to that of preferential hydration.

Diffusiophoresis of a Weakly Charged Liquid Metal Droplet

Diffusiophoresis of a weakly charged liquid metal droplet (LMD) is investigated theoretically, motivated by its potential application in drug delivery. A general analytical formula valid for weakly charged condition is adopted to explore the droplet phoretic behavior. We determined that a liquid metal droplet, which is a special category of the conducting droplet in general, always moves up along the chemical gradient in sole chemiphoresis, contrary to a dielectric droplet where the droplet tends to move down the chemical gradient most of the time. This suggests a therapeutic nanomedicine such as a gallium LMD is inherently superior to a corresponding dielectric liposome droplet in drug delivery in terms of self-guiding to its desired destination. The droplet moving direction can still be manipulated via the polarity dependence; however, there should be an induced diffusion potential present in the electrolyte solution under consideration, which spontaneously generates an extra electrophoresis component. Moreover, the smaller the conducting liquid metal droplet is, the faster it moves in general, which means a smaller LMD nanomedicine is preferred. These findings demonstrate the superior features of an LMD nanomedicine in drug delivery.

Microscale Diffusiophoresis of Proteins

Living systems are characterized by their spatially highly inhomogeneous nature which is susceptible to modify fundamentally the behavior of biomolecular species, including the proteins that underpin biological functionality in cells. Spatial gradients in chemical potential are known to lead to strong transport effects for colloidal particles, but their effect on molecular scale species such as proteins has remained largely unexplored. Here, we improve on existing diffusiophoresis microfluidic technique to measure protein diffusiophoresis in real space. The measurement of proteins is made possible by two ameliorations. First, a label-free microscope is used to suppress label interference. Second, improvements in numerical methods are developed to meet the particular challenges posed by small molecules. We demonstrate that individual proteins can undergo strong diffusiophoretic motion in salt gradients in a manner which is sufficient to overcome diffusion and which leads to dramatic changes in their spatial organization on the scale of a cell. Moreover, we demonstrate that this phenomenon can be used to control the motion of proteins in microfluidic devices. These results open up a path towards a physical understanding of the role of gradients in living systems in the spatial organization of macromolecules and highlight novel routes towards protein sorting applications on device.

Diffusiophoresis of a Nonionic Micelle in Salt Gradients; Roles of Preferential Hydration and Salt-Induced Surfactant Aggregation

Diffusiophoresis is the migration of a colloidal particle in water driven by concentration gradients of cosolutes such as salts. We have experimentally characterized the diffusiophoresis of tyloxapol micelles in the presence of MgSO4, a strong salting-out agent. Specifically, we determined the multicomponent-diffusion coefficients using Rayleigh interferometry, cloud points, and dynamic-light-scattering diffusion coefficients on the ternary tyloxapol-MgSO4-water system at 25 °C. Our experimental results show that micelle diffusiophoresis occurs from a high to a low salt concentration (positive diffusiophoresis). Moreover, our data were used to characterize the effect of salt concentration on micelle size and salt osmotic diffusion, which occurs from a high to a low surfactant concentration. Although micelle diffusiophoresis can be attributed to the preferential hydration of the polyethylene glycol surface groups, salting-out salts also promote an increase in the size of micellar aggregates, ultimately leading to phase separation at high salt concentration. This complicates diffusiophoresis description, as it is not clear how salt-induced surfactant aggregation contributes to micelle diffusiophoresis. We, therefore, developed a two-state aggregation model that successfully describes the observed effect of salt concentration on the size of tyloxapol micelles, in the case of MgSO4 and the previously reported case of Na2SO4. Our model was then used to theoretically evaluate the contribution of salt-induced aggregation to diffusiophoresis. Our analysis indicates that salt-induced aggregation promotes micelle diffusiophoresis from a low to a high salt concentration (negative diffusiophoresis). However, we also determined that this mechanism marginally contributes to overall diffusiophoresis, implying that preferential hydration is the main mechanism causing micelle diffusiophoresis. Our results suggest that sulfate salts may be exploited to induce the diffusiophoresis of PEG-functionalized particles such as micelles, with potential applications to microfluidics, enhanced oil recovery, and controlled-release technologies.

Diffusiophoresis, Diffusioosmosis, and Microfluidics: Surface-Flow-Driven Phenomena in the Presence of Flow

Diffusiophoresis is the spontaneous motion of particles under a concentration gradient of solutes. Since the first recognition by Derjaguin and colleagues in 1947 in the form of capillary osmosis, the phenomenon has been broadly investigated theoretically and experimentally. Early studies were mostly theoretical and were largely interested in surface coating applications, which considered the directional transport of coating particles. In the past decade, advances in microfluidics enabled controlled demonstrations of diffusiophoresis of micro- and nanoparticles. The electrokinetic nature and the typical scales of interest of the phenomenon motivated various experimental studies using simple microfluidic configurations. In this review, I will discuss studies that report diffusiophoresis in microfluidic systems, with the focus on the fundamental aspects of the reported results. In particular, parameters and influences of diffusiophoresis and diffusioosmosis in microfluidic systems and their combinations are highlighted.

Directional migration propensity of calf thymus DNA in a gradient of metal ions

The migration propensity and spatially modulated surface deposition of calf thymus DNA have been reported in response to gradients of different monovalent and divalent ions.

Trade-off between carbohydrates and metal ions regulates the chemotactic directionality of alkaline phosphatase

The role of the Hofmeister interaction in governing the chemotactic behavior of alkaline phosphatase in the presence of carbohydrate and metal ion gradients has been established.

Effect of a Good buffer on the fate of metastable protein-rich droplets near physiological composition

Metastable protein-rich microdroplets are produced from liquid-liquid phase separation (LLPS) of protein aqueous solutions. These globules can be intermediates for the formation of other protein-rich phases. Lysozyme aqueous solutions undergo LLPS around 0 °C in the presence of NaCl near physiological conditions. Here, it is shown that insertion of small amounts of 4-(2-hydroxyethyl)-1-piperazineethanesulfonate (HEPES, 0.1 M) as a second additive to lysozyme-NaCl-water solutions near physiological ionic strength (0.2 M) is an essential step for triggering conversion of protein-rich droplets into another phase. Specifically, LLPS induced by cooling reproducibly leads to a rapid and high-yield formation of compact tetragonal crystalline microparticles only in the presence of HEPES. These microcrystals exhibit small size (1-3 μm), narrow size distribution and guest-binding properties. The temperature-concentration phase diagram shows a characteristic topology with LLPS boundary metastable with respect to tetragonal microcrystals, which in turn become less stable than rod-shaped orthorhombic crystals above 40 °C. Interestingly, dynamic light scattering, hydrogen-ion titrations and isothermal titration calorimetry reveal that lysozyme-HEPES interactions were found to be weakly attractive and exothermic. Our findings indicate that additives of salting-in type can represent an important factor controlling the fate of metastable protein-rich microdroplets relevant to drug formulations, femtosecond crystallography, and potential implications in protein-driven cytoplasmic compartmentalization.