Study of Thermal Phase Transitions in Iota Carrageenan Gels via Fluorescence Technique O ¨ zlem Tari, 1 Selim Kara, 2 O ¨ nder Pekcan 3 1 Department of Physics, Istanbul Technical University, Maslak, Istanbul 34469, Turkey 2 Department of Physics, Trakya University, Edirne 22030, Turkey 3 School of Arts and Sciences, Kadir Has University, Cibali, Istanbul 34083, Turkey Received 25 March 2009; accepted 3 August 2009 DOI 10.1002/app.31233 Published online 29 March 2011 in Wiley Online Library (wileyonlinelibrary.com). ABSTRACT: The effect of carrageenan concentration on thermal phase transitions of the iota carrageenan gels was investigated by using fluorescence technique. During heating and cooling processes, scattered light, I sc , and fluorescence intensity, I p , were monitored against temper- ature to investigate phase transitions. Transition tempera- tures from the derivative of the transition paths were determined. Two regions were observed during the heat- ing and cooling processes. At the first step of the heating, dimers were converted into double helix by undergoing dimer to double helix (d-h) transition. At the higher tem- perature region, double helix to coil (h-c) transition took place. During the cooling process, these transitions are arranged in the order of coil to double helix (c-h) and double helix to dimer (h-d). A hysteresis was observed between (h-d) and (d-h) transitions. The critical gel frac- tion exponents, b, were found to be independent of the system by indicating that they all fall into the same uni- versality class. V C 2011 Wiley Periodicals, Inc. J Appl Polym Sci 121: 2652–2661, 2011 Key words: fluorescence; gels; transitions INTRODUCTION Carrageenan is one of the key products in the sea- weed polysaccharide industry. 1 It is obtained from Rhodophyceas (red seaweed) by extraction. Carra- geenan is a large molecule made up of some 1000 galactose residues with three main types: Kappa, Iota, and Lambda according to the relative number and position of sulfate ester substituents. They are very important in many technological applications, especially in cosmetics, pharmaceuticals, and perso- nal care industries. 2 In cosmetic applications, pre- serving moisture in hand lotions is quite important for keeping skin softer. In pharmaceutical industry, they are used for the design of slow-release devices for oral drugs. i-carrageenan is the most highly sulfated of the helix-forming polysaccharides, which has a high mo- lecular weight linear polymer consisting principally of an alternating sequence of 3-linked b-D-galactose 4-sulfate and 4-linked 3,6-anhydro-b-D-galactose 2- sulfate. Thus, each monosaccharide unit in the ideal polysaccharide carries one sulfate group, and there- fore i-carrageenan behaves in aqueous solution as a highly charged polyanion in the extended confirma- tion. 3 It is known that the polysaccharide has a dou- ble helix conformation in the solid phase by X-ray Diffraction data while in calcium salt it is converted to a three-fold right-handed double helix with paral- lel strands. 4–6 In solution, i-carrageenan is reversibly transformed from an ordered to a disordered confor- mation. Naturally at high ionic strength and low temperature i-carrageenan forms an ordered state. Upon heating, the helices dissolve and the i-carra- geenan forms a random coil conformation. 7 Intermo- lecular double helix formation investigated by sev- eral groups should result in a doubling in the observed molecular weight of the i-carrageenan. 8,9 However, some authors have proposed monomolec- ular single-helix formations. 10 The kinetics and equilibrium processes of the sol– gel and gel–sol transitions of agar or agarose gels as well as the effect of gelation conditions on gel’s microstructure and rheological properties have been studied in last few years. 11–13 It was observed that gelation of agar molecules results in a large sigmoi- dal increase in the magnitude of the sol’s shear mod- ulus. 14,15 On reheating, the gel structure is destroyed and during the gel–sol transition, the shear modulus follows another sigmoidal path back to its initial value, forming a hysteresis loop. 16 The observed val- ues of the sol–gel and gel–sol temperatures found in this study are 36  C and 78  C, respectively. It was understood that the sol–gel and gel–sol temperatures can be affected by the agar concentration and the thermal history of the gel. Cation effect on sol–gel and gel–sol phase transitions of j-carrageenan was Correspondence to: O ¨ . Pekcan (pekcan@khas.edu.tr). Journal of Applied Polymer Science, Vol. 121, 2652–2661 (2011) V C 2011 Wiley Periodicals, Inc.