19446
J. Phys. Chem. 1996, 100, 19446-19454
Volumetric Behavior of ((Dodecyloxy)methyl)-18-Crown-6 in Water: Micellar Interactions and Phase Separation Sumio Ozeki* Department of Chemistry, Faculty of Science, Chiba UniVersity, 1-33 Yayoi-cho, Inage-ku, Chiba 263, Japan
Akira Kojima Laboratory of Chemistry, Tokyo Dental College, Chiba 263, Japan
Shigeharu Harada Hamamatsu College of Shizuoka Prefectural UniVersity, Hamamatsu 432, Japan ReceiVed: March 22, 1996; In Final Form: October 3, 1996X
The densities of aqueous solutions of ((dodecyloxy)methyl)-18-crown-6 (C12-OM-crown) were measured at various temperatures (5-45 °C). The partial molar volume of the micelles at infinite dilution (Vm°) in water increased linearly with the temperature below the cloud point (tc ) 30.5 °C) of aqueous C12-OM-crown solutions from 443.7 cm3/mol at 5 °C to 452.9 cm3/mol at 25 °C. C12-OM-crown micelles seemed to be more bulky than micelles of conventional nonionic surfactants having an oligo(oxyethylene) chain, because of a cyclization effect of the chain. Vm° increased upon the addition of KCl. In 0.5 mol/dm3 KCl systems, the Vm°-temperature (T) plot comprised two linear sections with a break point at around 25 °C, one of which at over 25 °C nearly agreed with the extrapolated line of the Vm°-T relation for a no-salt system. The agreement seems to imply a certain compensation effect on the molar volume of partially chelated mixed micelles, since the difference (4.1 cm3/mol) in Vm° at 5 °C between water and 0.5 mol/dm3 KCl systems may be fairly well interpreted by K+ chelation of the head group of the C12-OM-crown. The electrostatic interactions between partially chelated mixed micelles may be one of the causes of the concentration-dependent molar volume in KCl systems. Wormlike cylinder micelles of C12-OM-crown in water at 25 °C have a persistence length of about 20 nm, and the interaction between the flexible cylindrical micelles may lead to phase separation. The attractive contribution to the micelle-water interaction may predict the temperature of the phase transition.
Introduction The micellar growth in oligo(oxyethylene)glycol monoalkyl ether (CnEm) is associated with a dehydration and conformation change of the head group.1 The rigid, bulky, and asymmetric crown head group of C12-OM-crown may play an important role in their aggregation process in a solution, which may refer to the cyclization effect of an oxyethylene chain.2-7 The molecular assemblies of ((dodecyloxy)methyl)-18-crown-6 (C12OM-crown; 1°) changed with the surfactant concentration: premicelles, polymer-like aggregates, and a few kinds of micelles.2-7 Approaching a cloud point (tc ) 30.5 °C), the aggregation number of the C12-OM-crown micelle increased extremely from 125 at 15 °C to 2300 at 25 °C.4 This growth seemed to arise from a conformation change from the parallel mode to the vertical mode of the head groups at micellar surfaces, depending on the hydration of the head groups.3,6 The addition of KCl led to a drastic decrease in the micellar size of C12-OM-crown, e.g., from 40 nm in hydrodynamic diameter for water to 7 nm for 0.1 mol/dm3, by K+ complexation of the crown surfactant, which may be regarded as one of the cyclization effects.4 The mixed C12-OM-crown micelles comprising an uncomplexed surfactant (D) and a K+-complexed surfactant (DK+) had rugose surfaces, different from the smooth surfaces of uncharged micelles, and included Cl- counterions among the crown head groups at the micellar surfaces, which * To whom all correspondence should be addressed. FAX: 81-43-2902783. E-mail:
[email protected] X Abstract published in AdVance ACS Abstracts, November 15, 1996.
S0022-3654(96)00877-5 CCC: $12.00
can efficiently shield the positive charges of micellar surfaces.5 Since the shielding efficiency of the mixed micelles due to Clcounterions strongly depended on the temperature, the aggregation mode of the mixed micelles also changed with the temperature. At high temperature above 25 °C, it was inferred that a microphase separation or a distribution change of D and DK+ in a micelle occurred with dehydration from the head groups.5,6 Despite investigations using various techniques, the dissolution states of C12-OM-crown are still unclear, especially near tc; thus the phase-separation behavior of C12-OM-crown solutions remains puzzling. The density reflects the solute-solute and solvent-solute interactions as well as conformational changes of the molecules and assemblies. Therefore, the densities of C12-OM-crown solutions or partial molar volumes of C12-OM-crown micelles should change with the association mode of their assemblies inferred so far. Also, the interaction between K+ ions and the crown head groups of C12-OM-crown at the micellar surfaces could lead to changes in the apparent volume of C12-OM-crown by dehydration from the K+ ions and the head groups and their association mode. In this study, we measured the density of aqueous C12-OM-crown solutions containing KCl; we also considered the volumetric behavior and phase separation from the view point of the cyclization effects of the head group (regarding an 18-crown-6 group of C12-OMcrown as the result of cyclization of a hexakis(oxyethylene) chain) on molecular assemblies, such as the effects of the cation complexation (charge) and the rigidity and assymmetry of the head group. © 1996 American Chemical Society
Volumetric Behavior of a Crown Surfactant
J. Phys. Chem., Vol. 100, No. 50, 1996 19447
Experimental Section C12-OM-crown (tc ) 30.5 °C for 1% surfactant solution) was prepared and purified as previously reported.2 Special-grade KCl (Wako Junyaku Co., Ltd.) was heated strongly and stored over P2O5 in a desiccator. The water for density measurements was purified using MilliQII, and the water for other measurements was redistilled from distilled, deionized water containing alkaline KMnO4 in a Pyrex glass still. The solution densities were determined at (5.0-45.0) ( 0.002 °C with an oscillating-tube densimeter (Anton Paar, DMA 60) operated in a phase-locked loop mode using two measuring cells (DMA 602). The precision of the measurements was found to be better than 2 × 10-6 g/cm3. The apparent molar volumes (φv) of C12-OM-crown in water and aqueous KCl solutions were calculated from eqs 1a and 1b, respectively:
φv )
M1 1000 (d0 - d) + m1dd0 d
Results
(in water) (1a)
and
φv )
1000 + m2M2 M1 (d2 - d) + m1dd2 d (in aqueous KCl solutions) (1b)
The partial molar volume (V1) of the surfactant was calculated from the following equation:
V1 )
∂(m1φv) ∂m1
(2)
where Mi is the solute molecular weight, mi is the molal concentration of solute i, and d0, d2, and d are the densities of water, aqueous KCl solution (as a solvent), and the solution, respectively. Subscripts 1 and 2 respectively denote C12-OMcrown and KCl. When densities were measured under the condition that the molar ratio (Xi) of solute i to the mixed solutes (C12-OM-crown and KCl, whose molality is m) is maintained constant, the mean apparent molar volume (Φv) and the mean partial molar volume (Vt) of the mixed solutes are obtainable:
Φv )
M1X1 + M2X2 1000 (d0 - d) + mdd0 d
(3a)
∂(mΦv) ∂m
(3b)
and
Vt )
The 1H NMR spectra were measured using a JEOL GSX400 Fourier-transform spectrometer (400 MHz for the proton resonance) at (0-70) ( 0.5 °C. C12-OM-crown was dissolved in D2O (99.75%; Wako Jyunyaku Co., Ltd., Japan), and the solution was transferred to a 5 mm diameter NMR tube. The diffusion coefficient and the weight-averaged micellar weight of the micelles were measured using an Otsuka ELS800 spectrophotometer at (15-46) ( 0.1 °C and a Beckman ultracentrifuge Spinco L8-80 at 25 ( 0.5 °C, respectively, as previously described.3 The hydrodynamic radius (Rh) as spherical micelles was calculated from the diffusion coefficient by the Einstein-Stokes equation. The relative kinematic viscosity was measured using an Ubbelode-type capillary viscometer having four bulbs of successive heights at 25.0 ( 0.02 °C, as previously reported.3
In no-salt systems, the densities of C12-OM-crown solutions increased along with the surfactant concentration, as shown in Figure 1A. At 25 °C, the three linear regions separated by arrows in the figure seem to stand approximately in parallel. These subtle changes were also observed at lower temperatures. Such changes in physical properties of aqueous C12-OM-crown solutions also were observed by other methods, and the transition regions were referred to as Cii and Ciii in order from lower concentration.3,4 In the presence of KCl (Figure 1B), each density curve for C12-OM-crown solutions has a maximum at Cii and a subtle break point at Ciii, irrespective of the temperature. Ci, which was observed by viscosity and dynamic lightscattering measurements,3,4 was not detected because of the very low concentration. These critical concentrations are listed in Table 1. The density of aqueous C12-OM-crown solutions monotonically decreased along with an increase in the temperature up to 25 °C, irrespective of the KCl concentrations, as shown in Figure 2. On the other hand, beyond around 25 °C, the density in 0.5 mol/dm3 KCl systems increased with an increase in the temperature. The temperature at the minimum density depends on the surfactant concentration; the lower the surfactant concentration, the higher the temperature (35 °C at m1 ) 5 × 10-3 mol/dm3 to 20 °C at m1 ) 3 × 10-2 mol/dm3). In the absence of KCl, φv was independent of the surfactant concentration only at 25 °C, and the concentration dependence became larger along with a decrease in the temperature (Figure 3A). In the 0.5 mol/dm3 KCl system, the concentration dependence of φv at 45 °C, especially below 1 × 10-2 mol/kg, was distinguished from those at other temperatures (Figure 3B). φv and V1, which were calculated by eqs 1 and 2, were fitted by the following equations:
m1φv ) am12 + bm1 + c
(4)
19448 J. Phys. Chem., Vol. 100, No. 50, 1996
Ozeki et al. TABLE 1: Critical Concentrations of C12-OM-Crown Micellar Solutions critical conc/10-3 mol/dm3 C2
(mol/dm3) 0 0.5
t (°C) 5 15 25 5 15 25 35 45
cmca
Ci
Cii
Ciii
0.067
