Critical Temperature and LE/G Phase Transitions in Monolayer Films of the Amphiphilic TEMPO Derivatives at the Air/Water Interface

Johnson, Melissa; Majmudar, Chinmay Y.; Skolimowski, and Janusz J.; Majda, Marcin

doi:10.1021/jp0122878

Cited by 9 publications

(19 citation statements)

References 24 publications

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“…We note that cyclic voltammograms obtained with line electrodes controlled by linear diffusion of redox species in 2D are equivalent to those obtained with planar electrodes and controlled by linear diffusion of redox species in 3D. 19,25 The comparison of the DigiSim 34 and our FEMLAB simulations in Figure 4 shows excellent agreement between the two for the solution processes. The peak currents in the two cases are 27.6 nA (DigiSim) and 27.4 nA (FEMLAB).…”

Section: ∂C(xyt) ∂Tmentioning

confidence: 55%

Electrochemical Studies of the Lateral Diffusion of TEMPO in the Aqueous Liquid/Vapor Interfacial Region

et al. 2006

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Surface partitioning and lateral mobility of TEMPO (2,2,6,6-tetramethyl-1-piperidynyloxy free radical) in the aqueous liquid/gas interfacial region were investigated electrochemically with 100 nm wide, 1.0 cm long microband electrodes positioned at the air/water interface. For redox active amphiphiles such as TEMPO, the electrochemical current is the sum of the surface and solution components representing the diffusive transport of TEMPO in both domains as well as the dynamics of equilibration at the air/water interface. Interpretation of the recorded current-voltage curves was aided by a FEMLAB simulation code developed to analyze transport processes in this class of systems. TEMPO and TEMPO(+) partition constants (K(T), K(T)+) and solution diffusivities (D(sol), equal for the two species) were obtained experimentally yielding K(T) = 5.0 +/- 0.7 x 10(2) M(-1), K(T+) = 41 +/- 3 M(-1), and D(sol) = 7.7 +/- 0.35 x 10(-6) cm(2)/s. In view of the low value of K(T)+, TEMPO(+) was assumed not to partition to the air/water interface. We further assumed that the desorption rate constants (k(des)) of both TEMPO and TEMPO(+) were the same. Good fits between the recorded and simulated cyclic voltammograms were obtained using two correlated, adjustable parameters, k(des) and the TEMPO lateral, surface diffusion constant (D(surf)). Detailed analysis of the behavior of this class of systems was obtained for a broad range of D(surf) and k(des) values. In addition, a calibration curve of k(des) versus D(surf) was obtained. Assuming that TEMPO k(des) is in a likely range of 10-100 s(-1), its lateral diffusion constant is in the range of 7.9-3.6 x 10(-5) cm(2)/s. In view of our earlier work (Wu, D. G.; Malec, A. D.; Head-Gordon, M.; Majda, M. J. Am. Chem. Soc. 2005, 127, 4490-4496) showing that at the air/water interface TEMPO is unimmersed, and that its interactions with water are limited to hydrogen bonding with one or two water molecules, D(surf) can be related to the viscosity of the aqueous interfacial region.

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Section: ∂C(xyt) ∂Tmentioning

confidence: 55%

Electrochemical Studies of the Lateral Diffusion of TEMPO in the Aqueous Liquid/Vapor Interfacial Region

et al. 2006

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“…Materials. Synthesis of 4-octaneamido-2,2,6,6-tetramethyl-1-piperidinyloxy free radical (C 8 TEMPO) followed the same procedure as that used in the synthesis of the octadecane derivative (C 18 TEMPO) described earlier . Briefly, octanoyl chloride was reacted with 4-amino-TEMPO in methylene chloride and the resulting carboxamide was oxidized with hydrogen peroxide in the presence of sodium tungstate/ethylenediaminetetraacetic acid (EDTA) catalyst in a solution of 1:6 acetone/water.…”

Section: Methodsmentioning

confidence: 99%

“…Line Electrode Fabrication. Two-dimensional electrochemical measurements at the air/water interface required specially designed “line” microelectrodes that are placed to touch the water surface and which self-position to become aligned exactly with the plane of the air/water interface. − They were produced by creating a sharp gradient of wettability along a fracture line of ∼80 nm thick gold films, vapor-deposited on microscope glass slides (ca. 8 × 20 mm 2 ).…”

Section: Methodsmentioning

confidence: 99%

Gibbs Monolayers at the Air/Water Interface: Surface Partitioning and Lateral Mobility of an Electrochemically Active Surfactant

Malec

Skolimowski

et al. 2004

Langmuir

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Monolayer films of a water-soluble surfactant, 4-octaneamido-2,2,6,6-tetramethyl-1-piperidinyloxy (C8-TEMPO) were investigated at the air/water interface. An electrochemical, horizontal touch method was developed to measure the equilibrium surface concentrations (gamma) of C8TEMPO. The dependence of gamma on the solution concentration followed a Langmuir isotherm and yielded the partition constant K = (2.3+/-0.2) x 10(4) M(-1). These results were verified by surface tension measurements and Brewster angle microscopy. Within experimental error, the same K values were obtained. The lateral diffusion constants vs surface concentration of this molecule were measured by 2D voltammetry. In these experiments, the component of the oxidation current due to C8TEMPO in the bulk of the solution was subtracted from the total measured current to obtain the component due to the lateral surface diffusion. In the ange of mean molecular areas from 120 to 400 A2/molecule, the lateral diffusion constant of C8TEMPO increased from 1.0 x 10(-6) to 1.0 x 10(-5) cm2/s. The latter value is about 2.5 times larger than the C8TEMPO diffusion constant in bulk water. Comparison of the lateral mobilities of C8TEMPO and two longer alkane chain, water-insoluble homologues, C14TEMPO and C18TEMPO, showed no statistically significant differences.

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“…At the beginning of compression, a gradual increase in surface pressure can be seen in all the isotherms, which is typical to liquid expanded films [ 23 ]. The phase transition between gaseous and liquid expanded states occurs at very low—near zero—surface pressure [ 51 ]. In our BA films, the surface pressure at spreading was near or above 1 mN/m and, consequently, the gaseous state was not observed.…”

Section: Resultsmentioning

confidence: 99%

“…The intermediate hydrophobic section of the molecules can be responsible for hydrophobic aggregation. The three distinct parts in its compression isotherm ( Figure 3 ) are the liquid expanded phase region at large molecular area values, the inflection point in the liquid expanded region at 119 Å 2 /molecule, and the phase transition region from liquid expanded to liquid condensed phase [ 51 ] starting at 97 Å 2 /molecule. The film collapses 33 Å 2 /molecule without formation of a stable continuous liquid condensed phase.…”

Section: Resultsmentioning

confidence: 99%

The Effect of Hydroxyl Moieties and Their Oxosubstitution on Bile Acid Association Studied in Floating Monolayers

Szekeres

Viskolcz

Poša

et al. 2014

The Scientific World Journal

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Bile salt aggregates are promising candidates for drug delivery vehicles due to their unique fat-solubilizing ability. However, the toxicity of bile salts increases with improving fat-solubilizing capability and so an optimal combination of efficient solubilization and low toxicity is necessary. To improve hydrophilicity (and decrease toxicity), we substituted hydroxyl groups of several natural bile acid (BA) molecules for oxogroups and studied their intrinsic molecular association behavior. Here we present the comparative Langmuir trough study of the two-dimensional (2D) association behavior of eight natural BAs and four oxoderivatives (traditionally called keto-derivatives) floated on an aqueous subphase. The series of BAs and derivatives showed systematic changes in the shape of the compression isotherms. Two types of association could be distinguished: the first transition was assigned to the formation of dimers through H-bonding and the second to the hydrophobic aggregation of BA dimers. Hydrophobic association of BA molecules in the films is linked to the ability of forming H-bonded dimers. Both H-bond formation and hydrophobic association weakened with increasing number of hydroxyl groups, decreasing distance between hydroxyl groups, and increasing oxosubstitution. The results also show that the Langmuir trough method is extremely useful in selecting appropriate BA molecules to design drug delivery systems.

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Critical Temperature and LE/G Phase Transitions in Monolayer Films of the Amphiphilic TEMPO Derivatives at the Air/Water Interface

Cited by 9 publications

References 24 publications

Electrochemical Studies of the Lateral Diffusion of TEMPO in the Aqueous Liquid/Vapor Interfacial Region

Electrochemical Studies of the Lateral Diffusion of TEMPO in the Aqueous Liquid/Vapor Interfacial Region

Gibbs Monolayers at the Air/Water Interface: Surface Partitioning and Lateral Mobility of an Electrochemically Active Surfactant

The Effect of Hydroxyl Moieties and Their Oxosubstitution on Bile Acid Association Studied in Floating Monolayers

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