Intermediate Rotator Phase in Lead(II) Alkanoates F. J. Martı ´nez Casado, ² M. V. Garcı ´a Pe ´ rez, ² M. I. Redondo Ye ´ lamos, ² J. A. Rodrı ´guez Cheda,* ,² A. Sa ´ nchez Arenas, ‡ S. Lo ´ pez-Andre ´ s, § J. Garcı ´a-Barriocanal, | A. Rivera, | C. Leo ´ n, | and J. Santamarı ´a | Departamento de Quı ´mica-Fı ´sica I, Facultad de C. Quı ´micas, Seccio ´ n Departamental de Fı ´sica Aplicada I, Facultad de Veterinaria, Departamento de Cristalografı ´a, Facultad de C. Geolo ´ gicas, and Departamento de Fı ´sica Aplicada III, Facultad de C. Fı ´sicas, UniVersidad Complutense, Madrid 28040, Spain ReceiVed: December 21, 2006; In Final Form: February 19, 2007 Lead(II) alkanoates, from hexanoate to dodecanoate, have been analyzed by means of XRD, optical microscopy, DSC, FTIR, and electric spectroscopy. Four different phases have been identified, corresponding to the three thermal transitions measured by DSC: two of them solid (crystal and “intermediate” phases), and another two fluid (neat phase and isotropic liquid). Powder crystal XRD data indicate that the samples present a bilayered structure. The analysis of the (00l) spacing dependence with temperature in the three ordered phases strongly points to the intermediate phase to be a rotator phase. Optical microscopy and FTIR versus temperature also confirm a structural change from the crystal to the intermediate phase and its solid-state nature. Electrical conductivity maps the thermal transitions of the samples and shows a high ionic conductivity in the intermediate phase, which does not depend much on the carbon chain length. The high conductivity values (3 orders of magnitude higher in comparison with that of the ordered crystal at room temperature) obtained for the intermediate phase gave a further support to the existence of a rotator mesophase in the lead(II) alkanoate series. 1. Introduction Ionic transport in the organic-inorganic hybrid materials is a recent topic receiving increasing attention in recent years due to their application in batteries and other devices. 1-4 In particular, some organic salts show a very high ionic conductiv- ity in the liquid state due to the dynamics characteristic of the melt, which would normally drop sharply when going into the solid state. Yet, these materials show a rich variety of meso- phases (in solid or fluid states), and part of the high ionic conductivity is retained not only by the liquid crystal phase but also by the solid mesophase as the so-called rotator phase. 3 In this respect, fast ion conduction has been recently observed in lithium-doped organic salts (alkylmethylpyrrolidinium imide), 2,4 where the 3 orders of magnitude increase of long-range ionic conductivity reported is related to the onset of rotational motions of the matrix. Among organic salts, alkanoates have one of the simplest structures. The anion is formed by a hydrocarbon chain and the negative charged carboxylic group, CH 3 -(CH 2 ) n-2 -COO - . Different cations can be incorporated to form the salt. While previous studies focused 5 mainly on light cations, their substitu- tion by heavy elements has been less explored. In this work, we analyze a large and heavy bivalent cation: Pb 2+ . There have been some studies on the lead(II) alkanoates series (Pb(Cn) 2 , where n is the number of carbon atoms per chain, hereafter) of medium and large chain length members, 6-12 because of their thermotropic ionic liquid crystal phases, and also of the short chain members, which are not mesogens but easily quench into different glass states. 13 Another interesting aspect of the lead- (II) alkanoates series is that some of its members have been identified in some oil painting masterpieces (e.g., Rembrandt’s Lesson of Anatomy), in the form of protrusions, which deteriorate the painting. 14 These lead(II) soaps appeared due to the reaction between the “white lead” pigment used in oil painting with the oleic or palmitic acids as a result of the hydrolysis of their three glyceride esters. We analyze here all members with n from 6 to 12. 10 Three different transitions have been observed in the temperature range studied (above room temperature) for these compounds: the first one from the ordered crystal at room temperature to an intermediate solid phase, followed by a second one from this phase to a liquid crystal phase (“smectic A like” or neat phase), which transforms finally into an isotropic liquid at the clearing point. All of the ordered phases have a bilayered structure: one ionic layer (formed by the ionic interaction between the Pb 2+ cations and the -COO - anions), and a second lipidic one (van der Waals interactions between the -CH 3 chain ends). 10 Amorim da Costa et al. 9 and Adeosun et al. 11 proposed an ordered liquid crystal structure for the intermediate phase, while Ellis 15 reported a “lamellar” crystalline nature. On the other hand, this intermediate phase was identified by us in a previous work 10 as a condis phase, following the Wunderlich nomen- clature. 16 This identification was based on a DSC study (Δ trs H of the crystal-to-intermediate phase transition increases with n, see Figure 1B, due to the formation of “gauche” defects, whose amount would increase with increasing the chain length) and FTIR experiments carried out on medium length lead(II) alkanoates (evolution of the wagging and rocking progression bands of the CH 2 groups, which disappear as the chain “melts”, losing its all-trans conformational order). 10 In the present work, * Corresponding author. Telephone: +91-3944306. Fax: +91-3944135. E-mail: cheda@quim.ucm.es. ² Departamento de Quı ´mica-Fı ´sica I. ‡ Seccio ´n Departamental de Fı ´sica Aplicada I. § Departamento de Cristalografı ´a. | Departamento de Fı ´sica Aplicada III. 6826 J. Phys. Chem. C 2007, 111, 6826-6831 10.1021/jp068823j CCC: $37.00 © 2007 American Chemical Society Published on Web 04/13/2007