arXiv subjects

Ahmet Uysal

Publications and source records attributed to Ahmet Uysal.

7 recordsLinked to original sources

arXiv2021

Trivalent ion overcharging on electrified graphene

The structure of the electrical double layer (EDL) formed near graphene in aqueous environments strongly impacts its performance for a plethora of applications, including capacitive deionization. In particular, adsorption and organization of multivalent counterions near the graphene interface can promote nonclassical behaviors of EDL including overcharging followed by co-ion adsorption. In this paper, we characterize the EDL formed near an electrified graphene interface in dilute aqueous $YCl_3$ solution using in situ high resolution x-ray reflectivity (also known as crystal truncation rod (CTR)) and resonant anomalous x-ray reflectivity (RAXR). These interfacial-specific techniques reveal the electron density profiles with molecular-scale resolution. We find that yttrium ions ($Y^{3+}$) readily adsorb to the negatively charged graphene surface to form an extended ion profile. This ion distribution resembles a classical diffuse layer but with a significantly high ion coverage, i.e., 1 $Y^{3+}$ per 11.4 $\pm$ 1.6 A$^2$, compared to the value calculated from the capacitance measured by cyclic voltammetry (1 $Y^{3+}$ per ~240 A$^2$). Such overcharging can be explained by co-adsorption of chloride that effectively screens the excess positive charge. The adsorbed $Y^{3+}$ profile also shows a molecular-scale gap ($\geq$5 A) from the top graphene surfaces, which is attributed to the presence of intervening water molecules between the adsorbents and adsorbates as well as the lack of inner-sphere surface complexation on chemically inert graphene. We also demonstrate controlled adsorption by varying the applied potential and reveal consistent $Y^{3+}$ ion position with respect to the surface and increasing cation coverage with decreasing applied potential.

cond-mat.soft↗

arXiv2020

LogDos: A Novel Logging-based DDoS Prevention Mechanism in Path Identifier-Based Information Centric Networks

Information Centric Networks (ICNs) have emerged in recent years as a new networking paradigm for the next-generation Internet. The primary goal of these networks is to provide effective mechanisms for content distribution and retrieval based on in-network content caching. The design of different ICN architectures addressed many of the security issues found in the traditional Internet. Therefore, allowing for a secure, reliable, and scalable communication over the Internet. However, recent research studies showed that these architectures are vulnerable to different types of DDoS attacks. In this paper, we propose a defense mechanism against distributed denial of service attacks (DDoS) in path-identifier based information centric networks. The proposed mechanism, called LogDos, performs GET Message logging based filtering and employs Bloom filter based logging to store incoming GET messages such that corresponding content messages are verified, while filtering packets originating from malicious hosts. We develop three versions of LogDos with varying levels of storage overhead at LogDos-enabled router. Extensive simulation experiments show that LogDos is very effective against DDoS attacks as it can filter more than 99.98 % of attack traffic in different attack scenarios while incurring acceptable storage overhead.

cs.NI↗

arXiv2020

The Role of Specific Ion Effects in Ion Transport: The Case of Nitrate and Thiocyanate

The selective transport of trivalent rare earth metals from aqueous to organic environments with the help of amphiphilic "extractants" is an industrially important process. When the amphiphilic extractant is positively charged or neutral, the coextracted background anions are not only necessary for charge balance but also have a large impact on extraction efficiency and selectivity. In particular, the opposite selectivity trends observed throughout the lanthanide series in the presence of nitrate and thiocyanate ions have not been explained. To understand the role of background anions in the phase transfer of lanthanide cations, we use a positively charged long-chain aliphatic molecule, modeling a common extractant, and gain molecular level insight into interfacial headgroup-anion interactions. By combining surface sensitive sum frequency generation spectroscopy with X-ray reflectivity and grazing incidence X-ray diffraction, we observed qualitative differences in the orientational and overall interfacial structure of nitrate and thiocyanate solutions at a positively charged Langmuir monolayer. Though nitrate adsorbs without dramatic changes to the solvation structure at the interface or the monolayer ordering, thiocyanate significantly alters the water structure and reduces monolayer ordering. We suggest that these qualitatively different adsorption trends help explain a reversal in system selectivity toward lighter or heavier lanthanides in solvent extraction systems in the presence of nitrate or thiocyanate anions.

cond-mat.soft↗

arXiv2018

Heavy Anionic Complex Creates a Unique Water Structure at a Soft Charged Interface

Ion hydration and interfacial water play crucial roles in numerous phenomena ranging from biological to industrial systems. Although biologically relevant (and mostly smaller) ions have been studied extensively in this context, very little experimental data exist about molecular scale behavior of heavy ions and their complexes at interfaces, especially under technologically significant conditions. It has recently been shown that PtCl62- complexes adsorb at positively charged interfaces in a two-step process that cannot fit into well-known empirical trends, such as Hofmeister series. Here, a combined vibrational sum frequency generation and molecular dynamics study reveals that a unique interfacial water structure is connected to this peculiar adsorption behavior. A novel sub-ensemble analysis of MD simulation results show that after adsorption, PtCl62- complexes partially retain their first and second hydration spheres, and it is possible to identify three different types of water molecules around them based on their orientational structures and hydrogen bonding strengths. These results have important implications for relating interfacial water structure and hydration enthalpy to the general understanding of specific ion effects. This in turn influences interpretation of heavy metal ion distribution across and reactivity within, liquid interfaces.

cond-mat.soft↗

arXiv2017

Two-Step Adsorption of $PtCl_6^{2-}$ Complexes at a Charged Langmuir Monolayer: Role of Hydration and Ion Correlations

Anion exchange at positively charged interfaces plays an important role in a variety of physical and chemical processes. However, the molecular scale details of these processes, especially with heavy and large anionic complexes, are not well-understood. We studied the adsorption of $PtCl_6^{2-}$ anionic complexes to floating DPTAP monolayers in the presence of excess $Cl^-$ as a function of the bulk chlorometalate concentration. In situ x-ray scattering and fluorescence measurements, which are element and depth sensitive, show that the chlorometalate ions only adsorb in the diffuse layer at lower concentrations, while they adsorb predominantly in the Stern layer at higher concentrations. The response of DPTAP molecules to the adsorbed ions is determined independently by grazing incidence x-ray diffraction, and supports this picture. Molecular dynamics simulations further elucidate the nanoscale structure of the interfacial complexes. The results suggest that ion hydration and ion-ion correlations play a key role in the competitive adsorption process.

cond-mat.soft↗

arXiv2014

Interfacial Ionic Liquids: Connecting Static and Dynamic Structures

It is well-known that room temperature ionic liquids (RTILs) often adopt a charge-separated layered structure, i.e., with alternating cation- and anion-rich layers, at electrified interfaces. However, the dynamic response of the layered structure to temporal variations in applied potential is not well understood. We used in situ, real-time X-ray reflectivity (XR) to study the potential-dependent electric double layer (EDL) structure of an imidazolium-based RTIL on charged epitaxial graphene during potential cycling as a function of temperature. The results suggest that the graphene-RTIL interfacial structure is bistable in which the EDL structure at any intermediate potential can be described by the combination of two extreme-potential structures whose proportions vary depending on the polarity and magnitude of the applied potential. This picture is supported by the EDL structures obtained by fully atomistic molecular dynamics (MD) simulations at various static potentials. The potential-driven transition between the two structures is characterized by an increasing width but with an approximately fixed hysteresis magnitude as a function of temperature. The results are consistent with the coexistence of distinct anion and cation adsorbed structures separated by an energy barrier (~0.15 eV).

cond-mat.soft↗

arXiv2010

How water meets a very hydrophobic surface

Is there a low-density region ('gap') between water and a hydrophobic surface? Previous X-ray/neutron reflectivity results have been inconsistent because the effect (if any) is sub-resolution for the surfaces studied. We have used X-ray reflectivity to probe the interface between water and more hydrophobic smooth surfaces. The depleted region width increases with contact angle and becomes larger than the resolution, allowing definitive measurements. Large fluctuations are predicted at this interface; however, we find that their contribution to the interface roughness is too small to measure.

cond-mat.soft↗