Effect of Divalent Cations on the Interaction of Carboxylate Self-Assembled Monolayers

Surface charge regulation using classical density functional theory: the effect of divalent potential determining ions

The charge regulation approach has been used to describe the charge of surfaces susceptible to the presence of protons and other ions. Conventionally, this model is used with the Poisson-Boltzmann equation, which generally neglects the finite size of the ions and the electrostatic correlations. Recently, progress has been made by coupling charge regulation with classical density functional theory (DFT), which explicitly includes these correlations. However, little is known about charge regulation at surfaces with both acid-base equilibria and complexation with multivalent ions. The main purpose of this work is to investigate the role divalent ions play in charge regulation. Using DFT, we show that the size of the divalent ion has significant consequences on the surface charge density and it should not be neglected. For the surface reactions investigated, the larger the size of the divalent cation, the greater the charge on the surface due to higher divalent concentration there. At low divalent concentration, the ion correlations play a second-order but non-negligible role; using Poisson-Boltzmann theory with point ions cannot recover the DFT surface charge. At high concentrations, ion correlations play a dominant role by creating charge inversion.

Formation of Organic Fouling during Membrane Desalination: The Effect of Divalent Cations and the Use of an Online Visual Monitoring Method

Reverse osmosis (RO) is the most popular technology for brackish, seawater and wastewater desalination. An important drawback of RO is membrane fouling, which reduces filtration effectiveness and increase the cost of produced water. This study addresses two important topics of membrane fouling: (i) the impact of different divalent ions on the formation of organic fouling and (ii) online monitoring and prediction of fouling formation. In the absence of divalent ions, dissolved organic matter had little effect on fouling formation, even at 3.5 mgC/L, in the upper range of groundwater concentration. Calcium, strontium and iron enhanced (organic) fouling formation, whereas barium had negligible effect. However, while iron affected fouling throughout the entire tested range (0-0.5 mg/L), calcium and strontium enhanced organic fouling only at high concentrations: more than 140 mg/L and 10 mg/L for calcium and strontium, respectively. An online system was developed for monitoring the formation of organic fouling, consisting of (i) an ex-situ RO cell with a transparent cover, (ii) a video camera continually monitoring the surface of the membrane and (iii) an algorithm which automatically identified changes in the color of the membrane caused by fouling, using a specially designed membrane spacer with colored reference dots. Changes in the color of the membrane surface were normalized to the reference colors, to eliminate all non-fouling related interference. The system was used to record and analyze changes in membrane color during numerous filtration tests. The data was successfully correlated to changes in specific flux (and subsequently to fouling formation rate) and can be applied to monitor and predict the formation of membrane fouling during desalination.

Hydration and Charge-Transfer Effects of Alkaline Earth Metal Ions Binding to a Carboxylate Anion, Phosphate Anion, and Guanine Nucleobase

To investigate the ability of alkaline earth metal ions to tune ion-mediated DNA adsorption, hydrated Mg²⁺, Ca²⁺, Sr²⁺, and Ba²⁺ ions bound to a carboxylate anion, phosphate anion, and guanine nucleobase were modeled using density functional theory (DFT) and a combined explicit and continuum solvent model. The large first solvation shell of Ba²⁺ requires a larger solute cavity defined by a solvent-accessible surface, which is used to model all hydrated ions. Alkaline earth metal ions bind indirectly or directly to each binding site. DFT binding energies decrease with increasing ion size, which is likely due to ion size and hydration structure, rather than quantum effects such as charge transfer. However, charge transfer explains weaker ion binding to guanine compared to phosphate or carboxylate. Overall, carboxylate and phosphate anions are expected to compete equally for hydrated Mg²⁺, Ca²⁺, Sr²⁺, and Ba²⁺ ions and larger alkaline earth metal ions may induce weaker ion-mediated adsorption. The ion size and hydration structure of alkaline earth metal ions may effectively tune ion-mediated adsorption processes, such as DNA adsorption to functionalized surfaces.

Poly(acrylic acid) Hydrogel Microspheres for a Metal-Ion Sensor

Monodispersed cross-linked poly(acrylic acid) (PAA) droplets (PAA X-droplets), prepared using the microfluidic method with in situ ultraviolet curing, were used as small spherical sensors to simultaneously detect both Ca²⁺ and Mg²⁺ in human saliva and serum. The PAA X-droplet treated with KOH (PAA_KOH X-droplet) was used as a reference droplet because of its highly swollen state. The PAA_KOH X-droplets shrunk in response to the presence of divalent metal ions (Ms) by forming a bridged structure of COO-M-OOC. The sizes of the PAA_KOH X-droplets were precisely and dynamically monitored in the poly(dimethylsiloxane) (PDMS) channel with passing time when the aqueous metal-ion solutions were flowing at a controlled flow rate. The sizes of the PAA_KOH X-droplets continuously decreased to the saturated constant size. The saturated size of the PAA_KOH X-droplet did not change; however, the speed of size reduction increased with an increase in the concentration of the divalent metal ion. The saturated size was studied using the saturated diameter ratio (R_sat-dia) with respect to the initial diameter of the PAA_KOH X-droplet before the metal-ion treatment, and the speed of the size reduction was investigated using the inverse time to reach half the saturated diameter reduction (T_1/2^-1). Ca²⁺ and Mg²⁺ exhibited R_sat-dia values of 75.9 and 83.6%, respectively, when the flow rate was 5 μL min^-1, regardless of the metal concentration. The T_1/2^-1s for the Ca²⁺ and Mg²⁺ linearly increased with an increase in their concentrations. The R_sat-dia of the aqueous Ca²⁺/Mg²⁺ mixture solution had a linear relationship with φ [= C_Ca/(C_Ca + C_Mg), where C_Ca and C_Mg are the molar concentrations of Ca²⁺ and Mg²⁺, respectively]. The T_1/2^-1 of the aqueous Ca²⁺, Mg²⁺ mixture solution was calculated by adding the individual T_1/2^-1s of pure aqueous Ca²⁺ and Mg²⁺ solutions. Using the R_sat-dia and T_1/2^-1 of the Ca²⁺/Mg²⁺ mixture aqueous solution, the individual C_Ca and C_Mg in the mixture solution were successfully calculated. This method was applied to the human saliva and serum in which the major metal ions are Ca²⁺ and Mg²⁺, and other metal ions existed in undetectable amounts by the PAA_KOH X-droplets. This method is simple, cost-effective, and highly accurate and solves the hurdles of separating the interference effect of a Mg²⁺ ion when a Ca²⁺ ion is measured in biofluids.