Properties of CHAPS micelles modulated by different polyelectrolytes

[EN]Fluorescence probing, and -potential measurements have been used to study the interactions between the zwitterionic surfactant 3-[(3-cholamidopropyl)-dimethyl-ammonium]-1-propanesulphonate, CHAPS and two polyelectrolytes with different electric charge. The polyelectrolytes chosen are poly (diallyl-dimethyl-ammonium chloride), PDADMAC, and poly (sodium styrene sulphonate), PSS. Results show that the zwitterionic surfactant CHAPS interacts with both, polyanion and polycation molecules, resulting in polymer–surfactant micelles. The dominating factor in this interaction is the electrostatic attraction. We found that the -potential of polymer–surfactant micelles can be modulated by the addition of different type of polyelectrolytes or modifying the polymer concentration. Using dynamic surface tension measurements we estimate the micelle dissociation rate constants. Results show that the addition of polymers increases the micelle lifetime

Effect of the Addition of Polyelectrolytes on Monolayers of Carboxybetaines

[EN]We have studied the effect of the addition of sodium poly(styrene sulfonate) polymers on the properties of monolayers formed by the adsorption of two carboxybetaines with different number of separation methylenes between their charged groups. Fluorescence and surface tension measurements indicate that above the critical aggregation concentration a surfactant-polymer complex of electrostatic origin is formed in bulk. The complexes have a negative charge that is repelled by the negative charge of the surfactant adsorbed at the interface; consequently, the monolayer seems to be exclusively formed by surfactant carboxybetaines. The high-frequency surface viscoelasticity of the monolayers was studied by surface dynamic light-scattering measurements. The behavior of the dilational elasticity and viscosity is explained by relaxation involving molecular reorientation within the adsorbed layer

Adsorption of Quantum Dots onto Polymer and Gemini Surfactant Films: A Quartz Crystal Microbalance Study

[EN]We used quartz crystal microbalance with dissipation to study the mechanical properties, the kinetics of adsorption, and the amount of CdSe quantum dots (QDs) adsorbed onto a SiO2 sensor, referred as bare sensor, onto the sensor modified with a film of the polymer poly(maleic anhydride-alt-1-octadecene), PMAO, or with a film of the Gemini surfactant ethyl-bis(dimethyl octadecyl ammonium bromide), abbreviated as 18-2-18. Results showed that when the sensor is coated with polymer or surfactant molecules, the coverage increases compared with that obtained for the bare sensor. On the other hand, rheological properties and kinetics of adsorption of QDs are driven by QD nanoparticles. Thus, the QD films present elastic behavior, and the elasticity values are independent of the molecule used as coating and similar to the elasticity value obtained for QDs films on the bare sensor. The QD adsorption is a two-step mechanism in which the fastest process is attributed to the QD adsorption onto the solid substrate and the slowest one is ascribed to rearrangement movements of the nanoparticles adsorbed at the surface

Specific ion effects on the properties of cationic Gemini surfactant monolayers

[EN]The effects of some anions of the Hofmeister series and different divalent cations of alkaline earth metals on the properties of Langmuir monolayers of the cationic Gemini surfactant ethyl-bis (dimethyl octadecylammonium bromide) have been investigated. Surface pressure and potential isotherms at the air–water interface were obtained on aqueous subphases containing sodium salts with several anions of the Hofmeister series (Cl−, NO3 −,Br−, I−,ClO4 −,and SCN−). The influence of the investigated anions on the monolayer properties can be ordered according to the Hofmeister series with a change in the order between bromide and nitrate anions. On the other hand, for a given anion, the cation of the salt also influences the surface properties of the Langmuir films. The monolayers can be transferred onto mica by the Langmuir–Blodgett technique and then the Langmuir–Blodgett films were characterized by atomic force microscopy (AFM). The AFM images show that the molecules become more closely packed and nearly vertical to the surface when anions screen the electric charge of the surfactant molecules

The behavior of graphene oxide trapped at the air water interface

[EN]Graphene oxide is a derivate of graphene obtained by oxidation of graphite and other carbonaceous materials. The more accepted structure consists in carbonyl and carboxyl groups located at the edge of the graphene network and hydroxyl and epoxy groups attached to the basal plane. The percentage of O-groups depends on the synthesis route and the material used as carbon source. In addition, highly oxidized fragments, called oxidative debris, OD, are produced during the oxidation process. These fragments are adsorbed onto the graphene oxide network and can be removed by alkaline washing. The purified material has lower O/C ratio than graphene oxide and its properties are also quite different. Due to its structure, graphene oxide can be adsorbed at the air-water interface of the aqueous solution by diffusion, Gibbs monolayers, or by spreading on a clean water subphase resulting in a Langmuir film. This review is intended to provide information on the importance of controlling the chemical composition, structure, size, and oxidative debris, on the manufacture of graphene oxide films. To this end the review shows the influence of the synthesis route and the starting material on the structure of graphene oxide and analyzes several examples of the behavior and properties of films prepared with different types of graphene oxides. The great variability of behaviors of graphene oxide films caused by the different structure of this material provides a great opportunity to fine-tune the properties of films according to the needs of different applications

Correlations between structure and photoluminescence properties in N-doped carbon nanoparticles

[EN]Carbon nanoparticles (CNPs) have received great attention because of their unique optical and electronic properties. To modulate their photoluminescence properties, nitrogen doping has been previously reported, but it was difficult to find correlations between the photoluminescence properties and the nanoparticle structure. For this purpose, we analyzed the effect of the chemical composition and the particle size on the photoluminescence of N-doped carbon nanoparticles (NCNPs) obtained by the acidic treatment of different starting materials. However, the use of different precursors introduced a high degree of heterogeneity in the nanoparticle structure, which hindered the interpretation of the photoluminescence results. To minimize these effects, in this work we have selected the hydrothermal treatment of the graphene oxide (GO) and urea by modifying the urea: GO mass ratio. Our results show the existence of four emissive centers assigned to p/p* transition for aromatic domains in zigzag and armchair configurations, charge-transfer, and n/ p* transitions. We also report linear correlations between the charge-transfer and n/p* emissions and the percentage of pyrrolic and pyridinic N-groups. Besides, we show that the charge-transfer emission is quenched by the carbonyl groups at the basal plane of the nanoparticles

Interaction Between Polyethyleneimine and Zwitterionic Surfactant: Effect on Association Processes in Bulk and Deposition onto Solid Wafers

[EN]Fluorescence probing and -potential measurements have been used to characterize the interactions in bulk between the zwitterionic surfactant 3-[(3-cholamidopropyl)-dimethyl-ammonium]- 1-propanesulphonate, CHAPS and the polymer polyethylene imine, PEI. Results show that the strongest attractive interactions are produced between the surfactant and the neutral form of the polymer PEI. Because the zwitterionic surfactant CHAPS has been proposed as component of biosensors, we are interested to study the effect of the polymer on CHAPS films, therefore we transfer the surfactant and PEI/CHAPS mixtures from the solution onto mica by using the Langmuir- Schaefer and dipping methodologies. The morphology of films is analyzed by means of Atomic force microscopy, AFM, Optical microscopy and Micro-Raman spectroscopy. The hydrodynamic flow of water after its evaporation induces the formation of ring-based structures not available to fabricate devices. This process predominates when materials are transferred from the solution onto mica by dipping. In contrast, the Langmuir-Schaefer films consist of spherical aggregates and the densest and the most ordered films were obtained by transferring the PEI/CHAPS mixture of surfactant concentration above the CMC

Modulating the Optoelectronic Properties of Silver Nanowires Films: Effect of Capping Agent and Deposition Technique

[EN]Silver nanowires 90 nm in diameter and 9 m in length have been synthesized using different capping agents: polyvinyl pyrrolidone (PVP) and alkyl thiol of different chain lengths. The nanowire structure is not influenced by the displacement of PVP by alkyl thiols, although alkyl thiols modify the lateral aggregation of nanowires. We examined the effect of the capping agent and the deposition method on the optical and electrical properties of films prepared by Spray and the Langmuir-Schaefer methodologies. Our results revealed that nanowires capped with PVP and C8-thiol present the best optoelectronic properties. By using different deposition techniques and by modifying the nanowire surface density, we can modulate the optoelectronic properties of films. This strategy allows obtaining films with the optoelectronic properties required to manufacture touch screens and electromagnetic shielding

Understanding the Role of Oxidative Debris on the Structure of Graphene Oxide Films at the Air–Water Interface: A Neutron Reflectivity Study

[EN]We address here the role of oxidation impurities on the structure of graphene oxide films at the air−water interface by specular neutron reflectivity (SNR). We study films of purified graphene oxide (PGO) and nonpurified graphene oxide in the close-packed state. Nonpurified graphene oxide is constituted by graphene oxide (GO) layers with oxidation impurities adsorbed on the basal plane, while in PGO sheets, impurities are eliminated. SNR measurements show that GO films are formed by welldefined bilayers constituted by 2−3 layers of GO stacked in contact with air and a second layer of impurities submerged in the aqueous subphase. In contrast, PGO films are formed by a single layer in contact with air. We show for the first time that impurities constitute a layer submerged in the aqueous subphase, decrease the elasticity, and favor the collapse of graphene oxide films. Our results allow designing the surface properties of GO trapped at fluid interfaces

Effect of Micelles on the Dynamic Surface Tension of Zwitterionic Surfactants

[EN]The dynamic surface tension of micellar surfactant solutions was measured using the maximum bubble pressure method. The chosen surfactants were carboxy- and sulfo-betaines. At the beginning, surface tension decay curves are consistent with a diffusion-controlled adsorption. However, at long times these curves were analyzed by using the Fainerman model. From these experiments, the rate constants for demicellization were obtained. The micelle dissociation rate constant depends of the surfactant concentration, the concentration dependence is interpreted by Aniansson's theory, and the micelle dissociation constant independent of surfactant concentration was calculated. The rate constants found in this work are of the same order of magnitude as those obtained using classical techniques for fast reactions