Browsing by Author "Ertekin, Umay Eren"

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Open Access
Greasy cations bind to neutral macromolecules in aqueous solution
(American Chemical Society, 2024-06-05) Ertekin, Umay Eren; Okur, Halil İbrahim
Ions influence the solution properties of macromolecules. Although much is known about anions, cationic effects are considered mostly in terms of weak interactions or exclusion from neutral interfaces. Herein, we have systematically studied the effect of quaternary tetraalkylammonium cations ($NH_4+, NMe_4+, NEt_4+, NPr_4+, NBu_4+$) on the phase transition of poly(N-isopropylacrylamide) (PNIPAM) in aqueous solution. Solubility measurements were coupled to H-1 NMR and ATR-FTIR spectroscopic measurements. The solubility and NMR measurements revealed a direct binding between the greasiest cations and the isopropyl group of the macromolecule, evidenced from the nonlinear, Langmuir-type chemical shift response only at the isopropyl NMR signals with increasing salt concentrations. The ATR-FTIR measurements focusing on the amide oxygen showed that it is not the main direct-binding site. Additionally, the salting-out effects of the greasier cations correlate with their hydration entropies. These results demonstrate that the most weakly hydrated cations can bind to macromolecules as strongly as the weakly hydrated Hofmeister anions.
Open Access
Molecular interactions and binding mechanisms of hydrophobic hofmeister cations to macromolecules
(2023-08) Ertekin, Umay Eren
It’s been known for over a century that ions specifically affect the bulk properties of solutions, behavior of macromolecules and a myriad of interfacial phenomena occurring in solution. Yet, the molecular mechanisms underlying these so-called Hofmeister effects are only recently being realized within the last few decades. In the resurgence in specific ion effects studies, the role attributed to cations has been relatively understated in comparison to the effects of anions. Whereas various molecular mechanisms have been elucidated for a diverse spectrum of anions, cationic effects have largely remained limited to common metal ions. Within the cationic Hofmeister series for cations, strongly-hydrated cations exhibit very weak binding to polar, electronegative groups, but weakly-hydrated cations in particular are classified as non-interacting. This thesis brings a much-needed expansion to specific cation effects by investigating the interactions between the weakly-hydrated tetraalkylammonium cations and model neutral thermoresponsive polymers, principally poly(N-isopropylacrylamide) (PNIPAM). The hydrophobicity of the cations is incrementally tuned by increasing the length of their alkyl chains, thus forming the series of salts investigated herein: NR4Cl where R = H, Me, Et, n-Pr, n-Bu, and the anion is kept constant as Cl-. By using a multi-instrumental approach, it is demonstrated that the largest of these cations exhibit a significant binding to the polymers and that the resulting salting-in effects are comparable in magnitude to those observed for sodium salts of weakly-hydrated anions. Thermodynamic phase transition measurements of the polymers are complemented by ATR-FTIR and quantitative 1H-NMR spectroscopic studies to systematically investigate the nature and molecular-level mechanism of the interaction. In stark contrast to the known behavior of the strongly-hydrated cations, through the temperature-controlled ATR-FTIR investigations it is found that carbonyl moieties are not the primary sites of interaction. Instead, it is found that these weakly-hydrated, ‘greasy’ cations preferentially interact with the most hydrophobic groups on the polymer: the isopropyl group on the PNIPAM side-chain, as revealed by a quantitative externally-referenced 1H-NMR methodology developed to elucidate ion-macromolecule interactions. The binding generally follows a Langmuir-type saturation behavior and exhibits site-specific dissociation constants as low as KD ≈ 0.2 M. This unprecedented, hydrophobically-mediated interaction between weakly-hydrated tetraalkylammonium cations and neutral macromolecules is then demonstrated to be a general mechanism and is shown to extend to polymers of vastly different molecular architectures. The results presented, thus, signify a new, more expansive view of cationic Hofmeister effects, where the far weakly-hydrated region of the series interacts with a novel mechanism entirely unlike that of other cations.
Open Access
Oil-in-water emulsions probed using fluorescence multivariate-curve-resolution spectroscopy
(American Chemical Society, 2024-06-11) Gündoğdu, Gülsüm; Topuzlu, Ezgi Yılmaz; Mutlu, Ferhat; Ertekin, Umay Eren; Okur, Halil İbrahim
Hydrophobic surfaces in contact with aqueous media are omnipresent in nature. A plethora of key biological and physiological processes occur at the interface of immiscible fluids. Besides its fundamental importance, probing such interfaces is rather challenging, especially when one medium is bathed in the other. Herein, we demonstrate a fluorescence-based method that probes the oil–water interface and interfacial processes through surface dielectric perturbations. The fluorescence response of Nile Red is measured in hexadecane in water nanoemulsions. Three major spectral components appear: two from the bulk liquid media (hexadecane and water) and a distinct band at around 640 nm due to the interfacial component. Such spectra are deconvoluted using the multivariate-curve-resolution algorithm, and interface-correlated fluorescence spectra are attained. The influence of anionic sodium dodecylbenzenesulfonate (SDBS) and cationic cetyltrimethylammonium bromide (CTAB) surfactants on the oil–water interface is elucidated with concentration-dependent measurements. A charge-dependent spectral shift is observed. The interface correlated band at 641 nm for bare hexadecane nanoemulsions red shifts in the presence of anionic surfactants, indicating an apparent dielectric increase. In contrast, the same band gradually blue shifts with increasing cationic surfactant concentration, indicating an apparent interface dielectric decrease. Such a method can be utilized to probe alterations at interfaces beyond the oil/water interface.
Restricted
Otomotiv ürünleri ve endüstrisinin ülkemizdeki gelişiminde geçmişten günümüze önemli bir yeri olan: Abdülkadir özcan A.ş.'nin kuruluşu ve tarihi
(Bilkent University, 2018) Şahin, Özgür; Özcan, Mustafa Can; Ertekin, Umay Eren; Aytek, Aziz; Yılmaz, Deniz
Bu yazıda Abdulkadir özcan A.ş.’nin kuruluşu ve gelişim dönemi anlatılmaktadır. şirketin ne tür zorluklarla karşılaştığı, dönemin ekonomik durumu ve sanayisi de,özellikle otomotiv sektöründeki, bu yazıda detaylı bir şekilde ele alınmıştır. Bu bölümler 1950-1970 arası yılları kapsamaktadır. Ardından kimya sektöründeki gelişimlerin otomotiv sektörü üzerindeki etkisi ve 80 darbesi ile 94 ekonomik krizinin Abdulkadir özcan A.ş. üzerindeki etkilerine de değinilmiştir. Abdulkadir özcan A.ş., kuruluşundan çok sonra da olsa, PETLAS’ı satın aldığı için PETLAS’ın kuruluşundan ve büyüme sürecinden de bahsedilmiştir. Bu bölümler ise 1970-1994 yılları arasını ele almaktadır.