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The DLVO theory (named after Boris Derjaguin and Lev Landau, Evert Verwey and Theodoor Overbeek) explains the aggregation and kinetic stability of aqueous dispersions quantitatively and describes the force between charged surfaces interacting through a liquid medium. It combines the effects of the van der Waals attraction and the electrostatic repulsion due to the so-called double layer of counterions. The electrostatic part of the DLVO interaction is computed in the mean field approximation in the limit of low surface potentials - that is when the potential energy of an elementary charge on the surface is much smaller than the thermal energy scale, k_\text{B} T. For two spheres of radius a each having a charge Z (expressed in units of the elementary charge) separated by a center-to-center distance r in a fluid of dielectric constant \epsilon_r containing a concentration n of monovalent ions, the electrost

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Effective Charge of Adsorbed Poly(amidoamine) Dendrimers from Direct Force Measurements

Michal Borkovec, Ramon Pericet Camara

Interaction forces between silica surfaces with adsorbed poly (amidoamine) (PAMAM) dendrimers were measured with the colloidal probe technique based on the atomic force microscope (AFM). At small separations, attractive non-DLVO forces are observed. These forces likely originate from interactions between oppositely charged patches on the surfaces, resulting from the heterogeneous distribution of the adsorbed positively charged dendrimers on the negatively charged surface. At large separation distances, the interaction forces are repulsive and consistent with DLVO theory. These forces can be interpreted quantitatively as originating from the overlap of diffuse layers of positively charged surfaces in terms of the Poisson-Boltzmann theory. From the resulting diffuse layer potentials, one can extract the effective charge of the dendrimers in the adsorbed state. This effective charge is about half of the ion condensation value given by Poisson-Boltzmann theory for a charged spherical macromolecule in solution for the different dendrimer generations investigated. This reduction probably originates from the smaller volume available for the diffuse layer of adsorbed macromolecules, but charge neutralization may also play an essential role.

American Chemical Society2009

Specific Ion Effects at the Interface of Nanometer-Sized Droplets in Water: Structure and Stability

Halil Ibrahim Okur, Sylvie Roke, Evangelia Zdrali

The interaction of ions with interfaces influences numerous processes in chemistry, physics, and biology, while different ion species behave differently at hydrophobic or hydrophilic interfaces. Despite its importance, ion specificity and its relation to stability of such interfaces are not understood on the molecular interfacial level, in particular when the interfacial dimension approaches the submicron length scale. To study the molecular mechanism involved in interfacial nanoscale ion specificity, we use a hexadecane nanodroplet system stabilized with a dilute monolayer of positively charged (dodecyltrimethylammonium, DTA+) groups and bring it in contact with aqueous electrolyte solutions containing Na2SO4, NaCl, NaNO3, and NaSCN salts. Using vibrational sum frequency scattering, second harmonic scattering, and ζ-potential measurements, we determine the ionic structure, the interfacial water orientation, and the electrokinetic mobility of nanodroplets for each anion as a function of bulk electrolyte concentration. We observe diverse ion–water interface structuring behavior consisting of ion pairing, the reorientation of anions, and the adsorption to chemically different patches of the interface. Employing an adopted DLVO theory for our experimentally studied emulsion systems, we find that nanoemulsion stability is preserved far beyond the crucial coagulation concentrations estimated by DLVO. This discrepancy is due to the diverse interfacial structuring that occurs on the nanodroplets, which is not included in the averaged and the uniform ion–surface interaction employed in the models.

2019

On the stability and necessary electrophoretic mobility of bare oil nanodroplets in water

Hilton Barbosa De Aguiar, Sergey Kulik, Halil Ibrahim Okur, Saranya Pullanchery Sankara Narayanan, Sylvie Roke

Hydrophobic oil droplets, particles, and air bubbles can be dispersed in water as kinetically stabilized dispersions. It has been established since the 19th century that such objects harbor a negative electrostatic potential roughly twice larger than the thermal energy. The source of this charge continues to be one of the core observations in relation to hydrophobicity, and its molecular explanation is still debated. What is clear though is that the stabilizing interaction in these systems is understood in terms of electrostatic repulsion via Derjaguin, Landau, Verwey, and Overbeek theory. Recent work [A. P. Carpenter et al., Proc. Natl. Acad. Sci. U. S. A. 116, 9214 (2019)] has added another element into the discussion, reporting the creation of bare near-zero charged droplets of oil in neat water that are stable for several days. Key to the creation of the droplets is a rigorous glassware cleaning procedure. Here, we investigate these conclusions and show that the cleaning procedure of glassware has no influence on the electrophoretic mobility of the droplets and that oil droplets with near-zero charge are unstable. We provide an alternative possible explanation for the observations involving glass surface chemistry.

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