J. Phys. Chem. B 2010, 114, 11039–11045
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Transformation Relationships among Monomers, Micelles, Metastable Solid, and Stable Solid in Aqueous Cetylpyridinium Chloride Solution Shigeo Sasaki* Department of Chemistry, Faculty of Sciences, Kyushu UniVersity, 33 Hakozaki, Higashi ku, Fukuoka 812, Japan ReceiVed: April 13, 2010; ReVised Manuscript ReceiVed: July 5, 2010
Kinetic studies on metamorphoses among monomer, micelle, metastable solid, and stable solid during the Krafft transition of cetylpyridinium chloride (CPC) in the aqueous solution were carried out. The Krafft transition accompanied a decrease of the electric conductance with a large increment of the dielectric permittivity ∆ε, which vanished after finishing the transition. The observing duration of large ∆ε in transforming to the metastable solid was much shorter than the duration in transforming to the stable solid. From the observing duration of ∆ε, a formation boundary between the metastable and the stable solids was determined as a function of the temperature and the CPC concentration. It was found from the boundary that the metastable solid forms in the micelle solution. A change in the electric conductivity κ of metastable solid suspensions during metastable-stable transformation suggests the solid dissolution-limited metamorphosis of metastable solid into the stable solid in the solution. Arrhenius plots of the metastable-stable transformation durations obtained from the time courses of κ changes gave the high activation energy of 290 kJ/mol for the metastable solid dissolution-limited metastable-stable transition. 1. Introduction Many surfactant molecules are insolubilized and solidified with decreasing temperatures of the aqueous surfactant solutions. The insolubilization with the solidification of alkyl chains of the surfactant molecules inducing the formation of hydrated solids in water is termed the Krafft transition.1 Recently dynamical polymorphisms of surfactant-hydrated solid have been found in the transition from the metastable bilayer lamella to the stable interdigitated lamella during the Krafft transition of cetylpyridinium chloride (CPC)2 in aqueous solution. The dynamical polymorphism of hydrated solid has been also found in the changing periodic length of undulated lamella solid of octadecyltrimethylammonium chloride (OTAC)3 in the aqueous solid suspension. The polymorphic modification of crystal taking place in the solution often follows the Ostwald’s rule,4 which describes the initial appearance of metastable crystal form followed by its transformation to the stable form via whatever intermediate structures may be accessible. The polymorphic transformation kinetics of crystal in the solution sometimes involves three processes: (a) dissolution of metastable solid, (b) self-recognition of the molecular units to nucleate a more stable solid phase, and (c) growth of the stable phase. This is termed the solution-mediated transformation. Cardew and Davey5 have first reported a semiquantitative analysis of the process. The kinetics of such a process is dominated by the relative growth and dissolution rate constants for the transforming polymorphs.6,7 The solution, which is supersaturated for the stable crystal but not saturated for the metastable crystal, sometimes provides a low activation energy pathway for the necessary molecular rearrangement to take place by dissolution of the metastable crystal and concomitant crystallization of the stable form. The dynamical polymorphisms observed in the Krafft transitions of CPC2 and OTAC3 are the solution-mediated transformations. * Corresponding author. Phone and Fax: +81-92-642-2609. E-mail:
[email protected] or
[email protected].
Little has not been recognized about the solution-mediated transformation of polymorphic hydrated solids of surfactants. Our interests in the present study are kinetic pathways to form the metastable and the stable solids. In the present experiments we elucidated the metastable solid formation from micelles and the metastable solid dissociation-limited transformation to the stable solid. We can obtain the concentration of ionic monomers and micelles from the electric conductance of the solid suspension, κ.8 When the solution temperature is held lower than the melting temperature of the hydrated solid, a large temporal increase in the dielectric increment, ∆ε, with a decrease in the κ was observed in the Krafft transition process. The ∆ε enlarged at the early stage of solid formation was diminished after the end stage of transition. The decrease in ∆ε after the formation of CPC solid was different from the result3 observed for OTAC. The large ∆ε had been observed even after forming the solids. It was also found that the observing duration of ∆ε in forming the stable solid was much longer than that in forming the metastable solid. A formation boundary between the metastable solid and the stable solid was revealed as a function of the temperature and the CPC concentration from the observing duration of ∆ε. The boundary demonstrates that the metastable solid forms in the micelle solution and does not form in the micelle-free solution. The transformation period from the metastable solid to the stable solid as a function of the incubation temperature of the metastable solid was obtained from the κ change behavior of the solution. The κ value was not so much changed for some time after starting the incubation and decreased to attain a final value as the metastable solid metamorphosed into the stable solid. The fact that the plateau κ value is close to κ at the solubility of stable solid indicates the metastable solid dissolution-limited transformation to the stable solid. It was found that the transformation time decreased with an increase in the incubation temperature. Arrhenius plots of the transformation times gave an activation energy of 290 kJ/mol for the
10.1021/jp103293x 2010 American Chemical Society Published on Web 08/10/2010
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J. Phys. Chem. B, Vol. 114, No. 34, 2010
metastable-stable metamorphosis. The transformation periods in the 5 mM CPC solutions were found to be longer than those in the 50 and 100 mM CPC solutions. This also indicates the metastable solid dissolution-limited transformation. 2. Experimental Section The CPC purchased from Tokyo-kasei Co. Ltd. was recrystallized from methanol/acetone and used. All chemicals were of reagent grade, and double-distilled water was used in the experiments. Measurements of electric conductance, κ, of the solution phase suspending the hydrated solids were carried out with using a TOA CM-60 V conductivity meter at an ac frequency of 80 Hz for the solution standing still. Measurements of the dielectric dispersion in the frequency range between 50 Hz and 5 MHz were made using computer-aided LCR meters (4285A precision LCR meter, Hewlett-Packard Inc., and 3522 LCR HiTester, Hioki Inc.) for the solution, a volume of which was about 2 cm3. It took about 40 s to obtain one dielectric dispersion spectrum in the frequency region from 50 Hz to 5 MHz. The temperature of solution in the cell, T, was controlled within 0.1 °C by a circulation of the liquid (a mixture of water and methanol) from a constant-temperature bath. In the kinetic experiment of Krafft transition, the solution incubated at 25 °C for a time longer than 1 h was cooled down to a given T lower than the temperature at which the solid was precipitated in the solution at equilibrium. A temperature change of the solution was achieved within 100 s by changing the temperature of the thermobath providing the circulation liquid. The metastable solid formed on dropping T of the CPC solution to 2 °C from 25 °C within 100 s. For revealing the transformation kinetics from the metastable solid to the stable solid, the κ measurement was made for the solution suspending the metastable solid obtained by the procedure mentioned above. The differential scanning calorimetry (DSC) measurements were carried out at a heating rate of 0.5 °C/min for a 100 mM CPC aqueous solution using a DSC calorimeter (DSC120 Seiko inc.). About 60 mg of the CPC solution was encapsulated in an aluminum cell at a room temperature, incubated at a given temperature during a given period, and measured from 2 °C. 3. Kinetic Equations for the Transformation from the Metastable Solid to the Stable Solid The dissociation of metastable solid and the growth of the stable solid involved in the solution-mediated transformation at the CPC concentration C can be described by the kinetic equations as follows:
dMMet ) KMet(C - CMet*) < 0 and dt dMSta ) KSta(C - CSta*) > 0; CSta*
