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Yayın Photon transmission study on conformational ordering of iota-carrageenan in CaCl2 solution(Taylor & Francis Inc, 2005-06) Kara, Selim; Pekcan, Mehmet ÖnderCoil-to-double helix (c-h) and double helix-to-dimer (h-d) phase transitions of iota-carrageenan in CaCl2 solution upon cooling were studied using photon transmission technique. Photon transmission intensity, I-iota r was monitored against temperature to determine the (c-h) and (h-d) transition temperatures (T-ch and T-hd) and activation energies (Delta E-ch and Delta E-hd). An extra dimer-to-dimer (d-d) transition was also observed during cooling at low temperature region. However, upon heating dimers disappear to double helices by making dimer-to-double helix (d-h) transition. Further heating resulted double helix-to-coil (h-c) transition at high temperature region. T-dh and T-ch temperatures and Delta E-dh and Delta E-hc activation energies were also determined. It was observed that T-hc and T-ch temperatures and Delta E-dh and Delta E-hd activation energies do not effected by carrageenan content. However, T-hd, T-dh and T-dd temperatures and Delta E-ch and Delta E-hc activation energies were found to be strongly correlated to the carrageenan content in the system.Yayın Small molecule sorption and desorption in and out of iota-carrageenan gels(Taylor & Francis Group, 2007-08) Ataman, Evren; Pekcan, Mehmet ÖnderSmall molecule sorption and desorption in and out of Iota-Carrageenan was studied by using steady-state fluorescence (SSF) technique. Pyranine dissolved in water used as fluorescence probe. Fluorescence emission intensity, I-p from pyranine was monitored for studying sorption and desorption processes at various temperatures. The Fickian model was applied to produce sorption, D-s, early desorption, D-ed, and desorption, D-d, coefficients. Corresponding activation energies were obtained and found to be 20.5 kJ mol(-1), 7.0 kJ mol(-1) and 34.9 kJ mol(-1), respectively. The observed D-ed value is an order of magnitude smaller than the D-s and D-d coefficients. On the other hand, sorption processes were shown to be twice as fast as desorption processes.Yayın Monovalent and divalent cation effects on phase transitions of iota-carrageenan(Sage Publications Ltd, 2007-01) Kara, Selim; Arda, Ertan; Pekcan, Mehmet ÖnderPhase transitions Of L-carrageenan in a variety of monovalent (Li, Na, K) and divalent (Mg, Ca, Sr) cation solutions were studied during heating and cooling by using photon transmission technique. Photon transmission intensity (I-tr) was monitored against temperature to determine the transition temperatures and activation energies during the heating and cooling processes in the monovalent and divalent systems. Three distinct transition regions were observed during the heating and cooling cycles for the carrageenan-divalent salt system. At the first step of the heating process, dimer groups were transformed into dimers that presented (g-d) transitions, then these dimers were directly converted into a double helix by undergoing a (d-h) transition. In the higher temperature region, a double helix-to-coil (h-c) transition took place. During the cooling process, these transitions are arranged in the order of (c-h), (h-d), and (d-g). The carrageenan-monovalent salt system presented only coil-to-rod-like helix (c-r) and rod-like helix-to-coil (r-c) transitions during the cooling and heating processes, respectively. A hysteresis was observed between (r-c)-(c-r) and (g-d)-(d-g) transitions for the monovalent and divalent cations, respectively.Yayın Swelling of iota-carrageenan gels prepared with various CaCl2 content: A fluorescence study(European Polymer Federation, 2008-01-05) Tarı İlgin, Özlem; Pekcan, Mehmet ÖnderIota carrageenan gels prepared with various CaCl2 content were completely dried and then swelled in water vapor. Steady-state fluorescence ( SSF) technique was used to monitor the swelling process of each iota carrageenan gels at various temperatures. Pyranine was used as a fluorescence probe. Apparent fluorescence intensity, I increased as swelling time increased for all gel samples. The increase in I was modelled using Li-Tanaka equation from which the swelling time constants, tau(1) and cooperative diffusion coefficients, D-c were determined. It was observed that D-c increased as the swelling temperature was increased. On the other hand at each temperature, it was seen that D-c decreased as CaCl2 content was increased. Activation energies for swelling were obtained and found to be 60.5, 61.0, 61.5 and 62.8 kJmol(-1) for the gels prepared with increasing amount of CaCl2 content.Yayın Small molecule diffusion into swelling Iota-Carrageenan gels: A fluorescence study(Taylor & Francis Group, 2007-04) Ataman, Evren; Pekcan, Mehmet ÖnderSmall molecule diffusion into Iota-Carrageenan gel was studied by using steady-state fluorescence (SSF) technique. Pyranine, dissolved in water was used as fluorescence probe. Fluorescence emission intensity, I-p, and scattered light intensity, I-sc, were monitored to study diffusion and swelling processes at various temperatures respectively. Fickian and Li-Tanaka models were elaborated to produce diffusion, D, and collective diffusion, D-0, coefficients. Diffusion and swelling activation energies were also obtained and found to be 20.5 kj mol(-1) and 28.2 kj mol(-1). respectively.Yayın Study on critical behaviour in N-isopropyl acrylamide gels by using fluorescence technique(Taylor & Francis Group, 2006-11) Kaya Aktaş, Demet; Pekcan, Mehmet ÖnderThe steady state fluorescence (SSF) technique was used to study the sol-gel transition for the solution free radical crosslinking copolymerization of N-isopropyl acrylamide (NIPA), with N,N'-methylenebis (acrylamide) (BIS) as crosslinker in the presence of ammonium persulfate (APS) as an initiator. Pyranine (8-hydroxypyrene-1,3,6-trisulfonic acid, trisodium salt, HPTS) was used as a floroprobe for monitoring the polymerization. Pyranine molecules start to bind to NIPA polymer chains upon the initiation of the polymerization, thus the spectra of the bonded pyranines shift to the shorter wavelengths. The get fraction exponent beta and the weight average degree of polymerization exponent gamma' agree best with the mean-field (Flory-Stockmayer) results near the gel point for various crosslinker contents.Yayın Molecular recognition during sol-gel and gel-sol transition of kappa-iota carrageenan mixtures(Taylor & Francis ltd, 2005-12) Kara, Selim; Arda, Ertan; Pekcan, Mehmet ÖnderSol-gel and gel-sol phase transitions of kappa-iota (kappa-iota) carrageenan mixtures in two types of salts were studied using photon transmission technique. Here, KCl and CaCl2 were chosen as the proper salts for the carrageenan mixtures. Coil-to-double helix (c-h) and double helix-to-dimer (h-d) phase transitions of carrageenan mixtures in monovalent and bivalent cation solutions were studied upon heating and cooling by using photon transmission technique. Photon transmission intensity, It, was monitored against temperature to determine the (c-h) and (h-d) transition temperatures (T-ch and T-nd). Two distinct transition regions were observed in both heating and cooling of the carrageenan-bivatent CaCl2 salt system. At the first step of heating process dimers decompose into double helices by making (d-h) transition. At the high temperature region (h-c) transition takes place. Carrageenan-monovalent KCl salt system presented only (c-h) and (h-c) transitions upon cooling and heating processes. Transition temperatures at the heating and cooling processes were obtained. A hysteresis was observed between (h-c)-(c-h) and (d-h)-(h-d) transitions for the monovalent and bivalent cations respectively.Yayın Cation effects on phase transition of kappa-iota-carrageenan hybrids: a photon transmission study(Taylor & Francis Group, 2007) Pekcan, Mehmet Önder; Kara, Selim; Arda, ErtanPhase transitions of kappa-iota-carrageenan hybrid in various types of salts were studied using photon transmission technique: NaCl, KCI, MgCl2, CaCl2 and KCl+CaCl2 were chosen as the salt solutions for the carrageenan hybrid. Coil-to-helix (c-h), helix-to-dimer (h-d) and back to helix-to-coil (h-c), dimer-to-helix (d-h) phase transitions of carrageenan hybrid in these mono- and bivalent cation solutions were studied upon cooling and heating, respectively. Photon transmission intensity, It, was monitored against temperature to determine the (c-h), (h-d) and (h-c), (d-h) transition temperatures (T-ch, T-hd and T-hc, T-dh) and activation energies (Delta E-ch, Delta E-hd and Delta E-hc, Delta E-dh) of carrageenan hybrids. Two distinct transition regions were observed both during heating and cooling of the carrageenan-bivalent salt systems. During the heating process, at first dimers decompose into helices by making a (d-h) transition. Then at the high temperature region, a (h-c) transition takes place. During cooling, back transitions repeat themselves. However, the carrageenan-monovalent salt system presented only (c-h) and (h-c) transitions during the cooling and heating processes, respectively. A hysteresis was observed between (h-c)-(c-h) and (d-h)-(h-d) transitions for the monovalent and bivalent cation systems respectively.Yayın The liquid–glass transition – is it a fourth order phase transition?(Elsevier Science, 2005-09-01) Dimitrov, Ventzislav IvanovThe liquid-glass transition is analyzed using a theory of Brownian motion in liquids recently developed by the author. It is shown that if a liquid could be cooled in quasi-static process and still avoids crystallization it would transform into a stable non-crystalline solid, which would be a normal thermodynamic phase. This hypothetical phase transition is neither first nor second order. At equilibrium transition temperature the free energy of the system and its first, second and third derivatives are all continuous functions, but its fourth derivative with respect to temperature is discontinuous. Therefore, the equilibrium liquid to non-crystalline solid transition may be considered a fourth order phase transition. The temperature of this phase transition, T-K, which coincides approximately with the Kauzmann temperature, is below the standard glass transition temperature T, (When the temperature decreases below T-g, the viscosity increases above 10(13) dPa s.) When the temperature decreases below T-K, the system becomes an ideal solid because the molecular mobility becomes zero and the viscosity becomes infinite if we neglect vacancy-like mechanisms of mobility. This hypothetical quasi-static transition is physically unobservable because the real liquid-glass transition must be done at a cooling rate high enough to suppress the growth of nanocrystals, which makes the liquid-glass transformation a non-equilibrium complicated phenomenon. Understanding this ideal phase transition is a first step towards describing the real liquid-glass transition from first principles.
