60ª Reunião Anual da SBPC

INTRODUÇÃO:

The immobilized or structured water formation of interfacial water layer has been studied theoretically and experimentally in biology [1-3]. External electric fields may also affect the structure of water [4]. Recently, freezing of liquid interfacial water inside a gap of about 8Aº spacing under the control of external electric fields and gap distance has been observed [4]. Water was frozen within a junction gap at room temperature under a rather weak electric field of about 106 Vm-1. In this work we have observed the formation of structured water on the surface of the solid gold electrode of the QCM. References: [1]- R. L. Baldwin, Science 295, 1657 (2002), [2]- A. Fernandez, Biophys. J. 83,2475 (2002), [3] F. Despa, Phys. Rev. Letters, 93,228104 (2004)], [4] H. Kang et al. Phys. Rev. Letters, 95, 085701 (2005).

METODOLOGIA:

TheQCM sensor consisted of an optically polished 6 MHz AT-cut quartz crystal (KVG, Neckarbischofsheim, Germany) coated sequentially with layers (0.28 cm2 area) of chromium (20 nm thick) and gold (200 nm thick). The QCM gold electrode used as a solid substrate was polycrystalline, composed of mainly the (111) and the (110) orientations. The crystal sensitivity was S = 3,584 x 10-9 g/Hz. Only Milli-Q water (pH range 5-6, resistivity 18.2 MW cm, and temperature 25°C) was used in the experiments. In this work the quartz crystal microbalance (QCM) was initially immersed in milli-Q water. The experiment consisted by emerging and immersing the QCM in a sequence of beakers with water and 0.154M NaCl aqueous solutions, and recording the resultant changes on the frequency and resistance variations relatively to the water. The mainly theoretical approach utilized to match the relationship between the frequency changes and mass/viscoelastic changes is the Kanazawa’s formula.

RESULTADOS:

The results shows the measured resonant frequency and resistance variations relative to water (f and R´) during the QCM emersion and immersion in a sequence of beakers with water and 0.154M NaCl aqueous solutions. The frequency shifts observed can not be explained by Kanazawa´s formula [5]. More intriguing are the results by returning the QCM to water: they are compatible with the formation of a rigid layer of water on the electrode surface. References: [5]- K.K. Kanazawa, J. G. Gordon II, Anal. Chim. Acta,99, 175 (1985).

CONCLUSÕES:

Non-rigid structured films are formed by immersing the QCM sensor on the beaker filled with NaCl, and still remain after the re-immersing on water. The mass change and variation of resistance are compatible with the formation of a rigid layer of water on the electrode surface.

Instituição de fomento: The authors are grateful to FAPESP (03/12529-4), CNPq and CAPES.

Palavras-chave: QCM, structured water, NaCl

E-mail para contato: gomide@ifi.unicamp.br