Cationic liposomes in mixed didodecyldimethylammonium bromide and dioctadecyldimethylammonium bromide aqueous dispersions studied by differential scanning calorimetry, Nile Red fluorescence, and turbidity

The thermotropic phase behavior of cationic liposomes in mixtures of two of the most investigated liposome-forming double-chain lipids, dioctadecyldimethylammonium bromide (DODAB) and didodecyldimethylammonium bromide (DDAB), was investigated by differential scanning calorimetry (DSC), turbidity, and Nile Red fluorescence. The dispersions were investigated at 1.0 mM total surfactant concentration and varying DODAB and DDAB concentrations. The gel to liquid-crystalline phase transition temperatures (Tm) of neat DDAB and DODAB in aqueous dispersions are around 16 and 43 degrees C, respectively, and we aim to investigate the Tm behavior for mixtures of these cationic lipids. Overall, DDAB reduces the Tm of DODAB, the transition temperature depending on the DDAB content, but the Tm of DDAB is roughly independent of the DODAB concentration. Both DSC and fluorescence measurements show that, within the mixture, at room temperature (ca. 22 degrees C), the DDAB-rich liposomes are in the liquid-crystalline state, whereas the DODAB-rich liposomes are in the gel state. DSC results point to a higher affinity of DDAB for DODAB liposomes than the reverse, resulting in two populations of mixed DDAB/DODAB liposomes with distinctive phase behavior. Fluorescence measurements also show that the presence of a small amount of DODAB in DDAB-rich liposomes causes a pronounced effect in Nile Red emission, due to the increase in liposome size, as inferred from turbidity results.     

Vesicle-micelle transition in mixtures of dioctadecyldimethylammonium chloride and bromide with nonionic and zwitterionic surfactants

We have investigated the effect of mixing spontaneously formed dispersions of the cationic vesicle-forming dioctadecyldimethylammonium chloride and bromide (DODAX, with X being anions Cl- (C) or Br- (B)) with solutions of the micelle-forming nonionic ethylene oxide surfactants penta-, hepta-, and octaethyleneglycol mono-n-dodecyl ether, C12E(n) (n = 5, 7, and 8), and the zwitterionic 3-(N-hexadecyl-N,N-dimethylammonio)propane sulfonate (HPS). We used for this purpose differential scanning calorimetry (DSC), turbidity, and steady-state fluorescence spectroscopy to investigate the vesicle-micelle (V-M) transition yielded by adding C12E(n) and HPS to 1.0 mM vesicle dispersions of DODAC and DODAB. The addition of these surfactants lowers the gel-to-liquid crystalline phase transition temperature (T(m)) of DODAC and DODAB, and the transition becomes less cooperative, that is, the thermogram transition peak shifts to lower temperature and broadens to disappear when the V-M transition is complete, the vesicle bilayer becomes less organized, and the T(m) decreases, in agreement with measurements of the fluorescence quantum yield of trans-diphenylpolyene (t-DPO) fluorescence molecules incorporated in the vesicle bilayer. Turbidity data indicate that the V-M transition comes about in three stages: first surfactants are solubilized into the vesicle bilayer; after saturation, the vesicles are ruptured, and, finally, the vesicles are completely solubilized and only mixed micelles are formed. The critical points of bilayer saturation and vesicle solubilization were obtained from the turbidity and fluorescence curves, and are reported in this communication. The solubility of DODAX is stronger for C12E(n) than it is for HPS, meaning that C12E(n) solubilizes DODAX more efficiently than does HPS. The surfactant solubilization depends slightly on the counterion, and varies according to the sequence C12E5 > C12E7 > C12E8 > HPS.     

Vesicle-micelle transition in aqueous mixtures of the cationic dioctadecyldimethylammonium and octadecyltrimethylammonium bromide surfactants

The vesicle-micelle transition in aqueous mixtures of dioctadecyldimethylammonium and octadecyltrimethylammonium bromide (DODAB and C(18)TAB) cationic surfactants, having respectively double and single chain, was investigated by differential scanning calorimetry (DSC), steady-state fluorescence, dynamic light scattering (DLS) and surface tension. The experiments performed at constant total surfactant concentration, up to 1.0 mM, reveal that these homologous surfactants mix together to form mixed vesicles and/or micelles, depending on the relative amount of the surfactants. The melting temperature T(m) of the mixed DODAB-C(18)TAB vesicles is larger than that for the neat DODAB in water owing to the incorporation of C(18)TAB in the vesicle bilayer. The surface tension decreases sigmoidally with C(18)TAB concentration and the inflection point lies around x(DODAB) approximately 0.4, indicating the onset of micelle formation owing to saturation of DODAB vesicles by C(18)TAB molecules. When x(DODAB)>0.5 C(18)TAB molecules are mainly solubilised by the vesicles, but when x(DODAB)<0.25 micelles are dominant. Fluorescence data of the Nile Red probe incorporated in the system at different surfactant molar fractions indicate the formation of micelle and vesicle structures. These structures have apparent hydrodynamic radius R(H) of about 180 and 500-800 nm, respectively, as obtained by DLS measurements.     

Interaction between Pluronic F127 and dioctadecyldimethylammonium bromide (DODAB) vesicles studied by differential scanning calorimetry

A number of fundamental studies on the interactions between lipid bilayers and (ethylene oxide)-b-(propylene oxide)-b-(ethylene oxide) copolymers (PEO-PPO-PEO, Pluronics) have been carried out recently as model systems for the complex behavior of cell membranes with this class of polymers often employed in pharmaceutical formulations. We report here a study by differential scanning calorimetry (DSC) of the interactions in water between Pluronic F127 (F127), and the cationic vesicles of di-n-octadecyldimethylammonium bromide (DODAB), as a function of concentration of the two components (DODAB 0.1 and 1.0 mM; F127 0.1 to 5.0 mM) and of the sample preparation protocol. The DSC studies follow the critical micellization temperature (cmt ≈ 27 °C at 1.0 mM) of F127 and the gel-liquid crystal transition (T(m) ≈ 45 °C) of the DODAB bilayer and of F127/DODAB mixtures. Upon heating past T(m), vesicle/polymer mixtures undergo an irreversible conversion into mixed DODAB/F127 micelles and/or F127-bearing vesicles, depending on the relative amount of each component, together with, in some cases, residual intact F127 micelles or DODAB vesicles. Sample preparation protocol is shown to have little impact on the composition of mixed systems once they are heated above T(m).     

Unusually high chemical compressibility of normally rigid type-I clathrate framework: synthesis and structural study of Sn(24)P(19.3)Br(x)I(8)(-)(x) solid solution, the prospective thermoelectric material

A novel tin phosphide bromide, Sn(24)P(19.3(2))Br(8), and Sn(24)P(19.3(2))Br(x)()I(8)(-)(x) (x = 0-8) solid solution have been prepared and structurally characterized. All compounds crystallize with the type-I clathrate structure in the cubic space group Pmn (No. 223). The clathrate framework of the title solid solution shows a remarkable chemical compressibility: the unit cell parameter drops from 10.954(1) to 10.820(1) A on going from x = 0 to x = 8, a feature that has never been observed for normally rigid clathrate frameworks. The chemical compressibility as well as non-Vegard dependence of the unit cell parameter upon the bromine content is attributed to the nonuniform distribution of the guest halogen atoms in the polyhedral cavities of the clathrate framework. The temperature-dependent structural study performed on Sn(24)P(19.3(2))Br(8) has shown that, in contrast to the chemical compressibility, the thermal compressibility (linear contraction) of the phase is similar to that observed for the Group 14 anionic clathrates. The tin phosphide bromide does not undergo phase transition down to 90 K, and the atomic displacement parameters for all atoms decrease linearly upon lowering the temperature. These linear dependencies have been used to assess such physical constants as Debye temperature, 220 K, and the lattice part of thermal conductivity, 0.7 W/(m K). Principal differences between the title compounds and the group 14 anionic clathrates are highlighted, and the prospects of creating new thermoelectric materials based on cationic clathrates are briefly discussed.     

Giant vesicles of a single-tailed chiral cationic surfactant, (1R,2S)-(-)-N-dodecyl-N-methylephedrinium bromide, in water

Self-assembly properties of a single-tailed chiral cationic surfactant, (1R,2S)-(-)-N-dodecyl-N-methylephedrinium bromide (DMEB), have been studied in water. The molecular self-assemblies of the amphiphile have been characterized by surface tension, fluorescence probes, light scattering, and microscopic techniques. The results have been compared with those of dodecyltrimethylammonium bromide (DTAB) surfactant. The critical aggregation concentration of DMEB was found to be much less than that of DTAB. Surface tension and fluorescence probe studies have suggested formation of micellar structures at low temperature (<28 degrees C) and spontaneous formation of giant vesicles in water above 28 degrees C. The mean size of the aggregates has been measured by a dynamic light scattering method. The micropolarity and microviscosity of the self-assemblies were determined by fluorescence probe technique. The (1)H NMR and FTIR spectra were recorded to elucidate the role of the hydrophobic head group towards the formation of bilayer structures. The phase transition temperatures of the vesicular aggregates were determined by measurement of fluorescence anisotropy at various temperatures.     

A COMPARISON OF THERMALLY INDUCED CHANGES IN SONICATED PHOSPHOLIPID AND SURFACTANT VESICLES BY MEANS OF INTRA-MOLECULAR EXCIMER-FORMING PROBE

Abstract The molecule (1,l'-dipyrenyl)-methyl ether (dipyme) was used for monitoring the bilayer fluidity of surfactant and sonicated phospholipid vesicles. In the latter systems, the observed transition temperatures ( Tc ) are identical with those found by different methods. Surfactant vesicles prepared from dioctadecyldimethylammonium bromide (DODAB) and dihexadecylphosphate (DHP) molecules manifest a similar fluidity of their bilayers as those of sonicated phospholipid vesicles below their Tc. However, unlike in phospholipid vesicles, there was no significant change of the bilayer structure above Tc observed in surfactant vesicles. DHP vesicles formed in pure water provide a different solubilization site for dipyme than those prepared in a buffer solution. Such sites are characterized by a relatively high local concentration of the probe and the appearance of the blue shifted spectrum of the excimer.     

Solid State Dynamics of Tricarbonyl(eta-1,5-cyclohexadienylium)iron Tetrafluoroborate and Tricarbonyl(eta-1,5-cycloheptadienylium)iron Tetrafluoroborate

The dynamic behavior of [(C(6)H(7))Fe(CO)(3)]BF(4) (I) and [(C(7)H(9))Fe(CO)(3)]BF(4) (II) in the solid state has been investigated principally by NMR spectroscopy. High-resolution variable-temperature (1)H and (13)C NMR spectra indicate that both complexes have a solid state phase transition above which there is rapid reorientation of the cyclodienylium rings and fast exchange of the carbonyl groups. The transition occurs between 253 and 263 K for I and between 329 and 341 K for II. The presence of the phase transition is confirmed by differential scanning calorimetry (DSC). (57)Fe M?ssbauer spectroscopy supports the notion that complex I is highly mobile at room temperature, while II is relatively static. The activation energy for the cyclodienylium group rotation in the high-temperature phase of I is estimated from (1)H spin-lattice relaxation time measurements to be 17.5 kJ mol(-)(1). Static (13)C NMR measurements of the solid complexes in the high-temperature phase indicate that the (13)C chemical shift anisotropies are only 20-30 ppm. This is significantly less than that expected to result from motion of individual groups and thus suggests that rotation of the whole molecule is involved. A single-crystal X-ray structural determination of complex II, at 295 K, showed that the complex is tetragonal (space group P4(1), a = 10.610(1) ?, c = 21.761(3) ?, V = 2449.7(5) ?(3), rho(calc) = 1.734 g cm(-)(3)), with eight cycloheptadienyl cations and eight tetrafluoroborate anions per unit cell. In addition, powder X-ray diffraction studies of both I and II confirm that at low temperatures both complexes have a tetragonal unit cell, which transforms to a cubic unit cell above the phase transition. The powder patterns, recorded above the phase transition, support the proposal that the complexes are undergoing whole-molecule tumbling in their dynamic regimes.     

Aggregation behavior of aqueous dioctadecyldimethylammonium bromide/monoolein mixtures: a multitechnique investigation on the influence of composition and temperature

A recently described non-viral gene delivery system [dioctadecyldimethylammonium bromide (DODAB)/monoolein (MO)] has been studied in detail to improve knowledge on the interactions between lamellar (DODAB) and non-lamellar-forming (MO) lipids, as a means to enhance their final cell transfection efficiency. Indeed, the morphology, fluidity, and size of these cationic surfactant/neutral lipid mixtures play an important role in the ability of these systems to complex nucleic acids. The different techniques used in this work, namely dynamic light scattering (DLS), fluorescence spectroscopy, differential scanning calorimetry (DSC), cryogenic transmission electron microscopy (cryo-TEM), light microscopy (LM), and surface pressure-area isotherms, allowed fully characterization of the phase behavior and aggregate morphology of DODAB/MO mixtures at different molar ratios. Overall, the results indicate that the final morphology of DODAB/MO aggregates depends on the balance between the tendency of DODAB to form zero-curvature bilayer structures and the propensity of MO to form non-bilayer structures with negative curvature. These results also show that in the MO-rich region, an increase in temperature has a similar effect on aggregate morphology as an increase in MO concentration.     

Vibrational energy relaxation of naphthalene in the S(1) state in various gases

Time-resolved fluorescence spectra of naphthalene in the S(1) state have been measured in various gases below 10(2) kPa. The band shape of the fluorescence changed in an earlier time region after the photoexcitation when an excess energy (3300 cm(-1)) above the 0-0 transition energy was given. The excitation energy dependence of the fluorescence band shape of an isolated naphthalene molecule was measured separately, and the time dependence of the fluorescence band shape in gases was found to be due to the vibrational energy relaxation in the S(1) state. We have succeeded in determining the transient excess vibrational energy by comparing the time-resolved fluorescence band shape with the excitation energy dependence of the fluorescence band shape. The excess vibrational energy decayed almost exponentially. From the slope of the decay rate against the buffer gas pressure, we have determined the collisional decay rate of the excess vibrational energy in various gases. The dependence of the vibrational energy relaxation rate on the buffer gas species was similar to the case of azulene. The comparisons with the results in the low temperature argon and the energy relaxation rate in the S(0) state in nitrogen were also discussed.     

Equilibrium behavior and dilational rheology of polyelectrolyte/insoluble surfactant adsorption films: didodecyldimethylammonium bromide and sodium poly(styrenesulfonate)

The surface pressure of monolayers of an insoluble surfactant, didodecyldimethylammonium bromide (DODAB), has been measured onto subphases with different concentrations of poly(styrenesulfonate) (PSS) and at different temperatures. The presence of PSS in the subphase shifts the surface-pressure (Pi) curves to larger areas per DODAB molecule, A, and shifts the surface phase transition to higher Pi's. The presence of PSS chains decreases the surface electric potential; the decrease is higher than expected from the formation of a double layer between the DODAB molecules and the PSS segments. Increasing the temperature shifts the surface-pressure curves to higher areas and also increases the values of Pi of the surface phase transition. The effect of the PSS chains on the Pi versus A curves is contrary to the one induced by the presence of inert electrolytes in the subphase. The behavior is consistent with the existence of a dense layer of PSS segments beneath the DODAB monolayer at low PSS concentrations, c. Two PSS layers exist at higher concentrations, a dense layer immediately below the DODAB and a less-dense layer, below the first one, that protrudes deep into the subphase. The surface-pressure relaxation curves have been found to be bimodal through the whole range of surface pressures and at all the values of polymer concentration studied. These results point out that the adsorption layers behave mainly as elastic bodies, with zero-frequency elasticity, epsilon(omega = 0), which agrees with the equilibrium compressibility modulus. The increase [epsilon(omega = 1) - epsilon(omega = 0)] has been found to be independent of both polymer concentration and molecular weight. The zero-frequency-dilational viscosity, kappa(omega = 0), strongly increases with Pi in the two-dimensional condensed-liquid region. The surface viscosity strongly decreases with increasing frequency; the decreasing rate is higher than the one found for the monolayers of nonionic insoluble polymers. kappa(omega = 0) has also been found to be independent of both polymer concentration and molecular weight. These results seem to indicate that it is the film formed by the DODAB molecules and the first dense polymer layer that determines the surface viscoelastic moduli of this system.     

Rotational mobility of guest molecules in o-terphenyl below T(g)

An optical anisotropy decay technique for measuring probe rotational times in glassy materials is presented. Rotational times from 10(1.4) to 10(5) s have been obtained for a molecule of 1-naphthyl-azomethoxybenzene (NAMB) in o-terphenyl (OTP) over a temperature range from T(g) +3.5 to T(g) -16.5 K. The rotational diffusion follows the temperature dependence of Debye-Stokes-Einstein down to T(g) -4 K with an activation energy of 320 +/- 30 kJ/mol. Below T(g) -9 K, the temperature dependence of rotation mobility was found to be much weaker with an activation energy of 70 +/- 15 kJ/mol.     

Kinetic study of the reaction of OH with CH2I2

Flash photolysis (FP) coupled to resonance fluorescence (RF) was used to measure the absolute rate coefficients (k(1)) for the reaction of OH(X(2)Π) radicals with diiodomethane (CH(2)I(2)) over the temperature range 295-374 K. The experiments involved time-resolved RF detection of the OH (A(2)Σ(+)→X(2)Π transition at λ = 308 nm) following FP of the H(2)O/CH(2)I(2)/He mixtures. The OH(X(2)Π) radicals were produced by FP of H(2)O in the vacuum-UV at wavelengths λ > 120 nm. Decays of OH radicals in the presence of CH(2)I(2) are observed to be exponential, and the decay rates are found to be linearly dependent on the CH(2)I(2) concentration. The results are described by the Arrhenius expression k(1)(T) = (4.2 ± 0.5) × 10(-11) exp[-(670 ± 20)K/T] cm(3) molecule(-1) s(-1). The implications of the reported kinetic results for understanding the atmospheric chemistry of CH(2)I(2) are discussed.     

Dioctadecyldimethylammonium bromide vesicles prepared by the surfactant removal method

Dioctadecyldimethylammonium bromide (DODAB) is a double chain vesicle-forming cationic surfactant, whereas octa-ethyleneglycol mono-n-dodecyl ether (C12E8) is a single chain micelle-forming nonionic surfactant. At room temperature (ca. 22 degrees C) C12E8 molecules self-assemble in water as micelles while DODAB is insoluble. A mixture of DODAB and C12E8, however, can be soluble in water at room temperature depending on the relative amount of the compounds. We report the formation of small unilamellar vesicles (SUVs) by dialyzing at room temperature a mixture of 1.0 mM DODAB with 10 mM C12E8 in water. Extended bilayers are formed as well in equilibrium with vesicles. Such structures are viewed by a cryogenic transmission electron microscopy (cryo-TEM) image.     

FLUIDITY AND OXYGEN PENETRATION OF LIPID VESICLES STUDIED BY FLUORESCENCE PROBES

Abstract. –An analysis of the temperature dependence of trans -stilbene fluorescence yield in dipalmitoyl lecithin vesicles is used to obtain activation energies. The results are interpreted in terms of bilayer fluidity through and above the phase transition. Oxygen quenching of the fluorescence of pyrenebutyric acid (incorporated in dipalmitoyl lecithin and egg lecithin vesicles) is reported as a function of temperature and bulk oxygen concentration. Above the bilayer phase transition, quenching rates (determined by oxygen quenching) decrease with decreasing temperature. A reduction in oxygen quenching is observed through the dipalmitoyl lecithin phase transition.     

Polymerisation in lyotropic liquid-crystalline phases of dioctadecyldimethylammonium bromide

The polymerisation of styrene in lyotropic liquid-crystalline (LC) phases of dioctadecyldimethylammonium bromide (DODAB) in water is explored. Amphiphile concentrations between 20 and 50 wt % are employed. The study is set out as a model study for polymerisation reactions in nonstabilised, nonfunctional bilayer systems. X-ray characterisation was used to assess the phase behaviour of the lyotropic mesophases before, during and after polymerisation. The DODAB/water system forms the lamellar phase within the concentration range considered. Addition of styrene to the lamellar phase of DODAB at an equimolar ratio induces a phase shift to a bicontinuous cubic phase at elevated temperatures near the phase-transition temperature. Upon polymerisation within this cubic phase, the phase structure is maintained if the system is kept at constant temperature; however, if the polymer/amphiphile phase is cooled, the lamellar phase, being typical of the DODAB/water system, is restored. It is concluded that, as a result of phase separation between the polymer and the amphiphile phase, the polymerisation in lyotropic LC phases does not provide a stable copy of the templating amphiphile phase. This is in analogy to the observations for polymerisations in other lyotropic phases. Received: 16 March 2000 Accepted: 1 July 2000     

Spectroscopic properties and hydration of the Cm(III) aqua ion from 10 to 85 °C

The optical absorption, fluorescence excitation, and emission spectra of the Cm(III) aqua ion in 0.001 M perchloric acid were studied in pure H(2)O, pure D(2)O, and in mixtures of H(2)O-D(2)O at temperatures from 10 to 85 °C. The quantum yield of the fluorescence of the Cm(III) aqua ion in pure H(2)O and D(2)O was also measured in this temperature range and the radiative decay rate constant was obtained from these data. The results indicate that, from 10 to 85 °C, the effect of temperature on the absorption, excitation, and emission spectra is very small. By correcting the observed decay rate constant for the radiative rate constant, a set of correlations between the observed fluorescence decay rate constant and the hydration number of Cm(3+) in H(2)O at temperatures from 10 to 85 °C was developed. A weak temperature dependence was observed for the nonradiative decay rate constant for the (6)D'(7/2)-(8)S'(7/2) transition and described by the Arrhenius equation. The activation energy of the nonradiative decay was measured to be 0.9 kJ mol(-1), approximately matching the energy gap between the first and the second (A(1) and A(2)) levels of the metastable (6)D'(7/2) multiplet of the Cm(III) aqua ion. On the basis of these observations, it is postulated that the slight increase in the observed fluorescence decay rate constant as the temperature increases from 10 to 85 °C is due to the effect of thermal population of the A(2) level.     

Temperature-dependent energy gap of the primary charge separation in photosystem I: study of delayed fluorescence at 77-268 K

The dynamics of fluorescence decay and charge recombination were studied in the ether-extracted photosystem I reaction center isolated from spinach with picosecond resolution over a wide time range up to 100 ns. At all temperatures from 268 to 77 K, a slow fluorescence decay component with a 30-40 ns lifetime was detected. This component was interpreted as a delayed fluorescence emitted from the singlet excited state of the primary donor P700*, which is repopulated through charge recombination that was increased by the lack of secondary acceptor phylloquinone in the sample. Analysis of the fluorescence kinetics allowed estimation of the standard free-energy difference -DeltaG between P700* and the primary radical pair (P700(+)A0(-)) state over a wide temperature range. The values of -DeltaG were estimated to be 160/36 meV at 268/77 K, indicating its high sensitivity to temperature. A temperature-dependent -DeltaG value was also estimated in the delayed fluorescence of the isolated photosystem I in which the secondary acceptor quinone was partially prereduced by preillumination in the presence of dithionite. The results revealed that the temperature-dependent -DeltaG is a universal phenomenon common with the purple bacterial reaction centers, photosystem II and photosystem I reaction centers.     

Phase behaviour of the thermotropic cubic mesogen 1,2-bis-(4- n -octyloxybenzoyl)hydrazine under pressure

The phase transition behaviour of an optically isotropic, thermotropic cubic mesogen 1,2-bis-(4- n -octyloxybenzoyl)hydrazine, BABH(8), was investigated under pressures up to 200 MPa using a high pressure differential thermal analyser, wide-angle X-ray diffraction and a polarizing optical microscope equipped with a high pressure optical cell. The phase transition sequence, low temperature crystal (Cr 2 )-high temperature crystal (Cr 1 ) - cubic (Cub)-smectic C (SmC)-isotropic liquid (I) observed at atmospheric pressure, is seen in the low pressure region below about 30 MPa. The cubic phase disappears at high pressures above 30-40 MPa, in conjunction with the disappearance of the Cr 1 phase. The transition sequence changes to Cr 2 -SmC-I in the high pressure region. Since only the Cub-SmC transition line among all the phase boundaries has a negative slope (d T /d P ) in the temperature-pressure phase diagram, the temperature range for the cubic phase decreases rapidly with increasing pressure. As a result, a triple point was estimated approximately as 31.6 ±2.0 MPa, 147.0 ±1.0°C for the SmC, Cub and Cr 1 phases, indicating the upper limit of pressure for the observation of the cubic phase. Reversible changes in structure and optical texture between the Cub and SmC phases were observed from a spot-like X-ray pattern and dark field for the cubic phase to the Debye-Sherrer pattern and sand-like texture for the SmC phase both in isobaric and isothermal experiments.     

Self-termination rate of para-substituted benzyl radicals in aqueous solutions: a time-resolved study

The self-termination rates of the benzyl radical (C₆H₅---CH₂^√) and para-substituted benzyl radicals (X---C₆H₄---CH₂^√) were studied in aqueous solutions. The Arrhenius parameters and activation energies were determined in the temperature range 275.5–328 K. The kinetic activation energies of these radicals were close to the dynamic activation energy of the solvent, indicating that the termination rate is controlled by diffusion. The values for the rate constants (2k_t (10⁹ dm³ mol⁻¹ s⁻¹)) and the activation energies (E (kJ mol⁻¹)) were 5.94±0.52 and 14.69±0.61 for CH₃O---C₆H₄---CH₂^√, 4.52±0.2 and 17.65±1.16 for CH₃7z.sbnd;C₆H₄---CH₂^√, 3.07±0.45 and 17.58±0.97 for H---C₆H₄---CH₂^√, 4.13±0.81 and 19.10±1.20 for Cl---C₆H₄---CH₂^√ and 4.17±0.44 and 14.62±0.52 for NO₂---C₆H₄---CH₂^√.