Adsorption of hexyl-alpha,omega-bis(dodecyldimethylammonium bromide) gemini surfactant on silica and its effect on wettability

The adsorption of hexyl-alpha,omega-bis(dodecyldimethylammonium bromide) (C(12)C(6)C(12)Br(2)) gemini surfactant on silica and its effect on wettability have been studied. The structure of the adsorbed aggregates experiences an evolving course with the increase of C(12)C(6)C(12)Br(2) concentration. It is from no aggregates to circular islands, then to semicontinuous islands, and at last to the two-bilayer structure. No matter what kind of aggregates are in existence, their thickness values are to be the same 3.3+/-0.3 nm. The fraction of silica surface covered by the surfactant aggregates also varies from nearly zero at 0.05 mM to approximately 0.92 at 5.0 mM. The variation of contact angle against C(12)C(6)C(12)Br(2) concentration shows two distinct regions. The upward shift indicates that the surfactant molecules are adsorbed with their hydrophobic tails facing air upon increasing concentration, while the downward shift reveals that the surfactant aggregates are in existence with the hydrophilic headgroups facing air. IR spectra suggest that two different courses are involved with the increase of the surfactant concentration. One possible course is that the surfactant tails pack more closely and orderly, and the other may be that the spacer changes from stretched profile to bended conformation upon increasing the surfactant concentration.     

AFM study on the adsorption and aggregation behavior of dissolved humic substances on mica

Humic substances (HS) are a category of naturally occurring, biogenic, heterogeneous organic materials found in or extracted from soils, sediments, and natu- ral waters that can generally be characterized as being yellow-to-black in color, of highly variable relative molecular masses, and refractory[1,2]. Derived from a variety of organic precursors (plant biopolymers such as lignin etc.), plant residues and animal debris via both transformation and synthesis processes[3] under the profound ge…     

Wettability of ionic surfactants SDS and DTAB on wheat (Triticum aestivum) leaf surfaces

Abstract

Due to the important use of pesticide formulation, it is necessary to make it clear how ionic surfactant effect the wettability at leaf surface. In this work, we used the sessile drop method to study the wettability of SDS and DTAB on wheat leaf surfaces at different leaf stages, and reveal the relationship between surfactants structures and leaf stages of wheat leaf surfaces on wettability behavior. Results showed that few surfactant molecules adsorbed at the interface at low concentrations. With the concentration increased, the surfactant replaced the air layer partially within the nano/micro structure of leaf surfaces. When the concentration exceeded to CMC, the adsorption of surfactant molecules was saturated at both air-liquid interface and solid-liquid interface, the wetting state was still the transitional state between Cassie-Baxter’s and Wenzel’s state. In all concentrations, the adhesional tension and surface tension showed the linear relationship and the slope values were all below ?1, suggesting there were more surfactant molecules adsorbed at the solid-liquid interface than the liquid-air interface. As SDS is a common wetting agent and DTAB is a common fungicide in agrochemical, this study will provide potential guidance in practical application of pesticide solutions in leaf surface wetting.     

Dissimilar pH-dependent adsorption features of bovine serum albumin and α-chymotrypsin on mica probed by AFM

We studied bovine serum albumin (BSA) and α-chymotrypsin adsorption onto mica surfaces over a large pH range by atomic force microscopy (AFM) measurements in liquid. Data analyses (height, roughness and roughness factor) brought new insights on the conformation of proteins in soil environments, with mica as a model of soil phyllosilicates and non-hydrophobic surfaces. Validation of AFM approach was performed on BSA, whose behavior was previously described by nuclear magnetic resonance and infra-red spectroscopic methods. Maximum adsorption was observed near the isoelectric point (IEP). A stronger interaction and a lower amount of adsorbed proteins were observed below the IEP, which contrasted with the progressive decrease of adsorption above the IEP. We then studied the adsorption of α-chymotrypsin, a proteolytic enzyme commonly found in soils. AFM pictures demonstrated a complete coverage of the mica surface at the IEP in contrast to the BSA case. Comparison of the AFM data with other indirect methods broadened the understanding of α-chymotrypsin adsorption process through the direct display of the protein adsorption patterns as a function of pH.     

The AFM observation of linear chain and crystalline conformations of ultrahigh molecular weight polyethylene molecules on mica and graphite

Atomic force microscopy (AFM) has been applied to visualize expanded linear chain and compact crystalline conformations of ultrahigh molecular weight polyethylene (PE) molecules deposited on mica and graphite from diluted solutions at elevated temperatures. Isolated PE chains are visualized on mica with the apparent negative AFM height and the contour length much shorter than the molecular length. The chain conformations have both the kinked random‐coil sites and the sites of the unexpectedly large two‐dimensional expansion. The crystalline conformations on mica are small single‐molecule rod‐like nanocrystallites and the isolated block‐type “edge‐on” nanolamellae comprising several PE molecules. Noticeable fluctuations of the fold length in the range of approximately 10–20 nm around the averaged value of about 15 nm are observed for nanocrystallites and on tips of some nanolamellae. The explanation of the experimentally observed features of chain surface conformations on mica is proposed. It implies the immobilization of PE molecules in the nm‐thickness salt layer formed on mica surface at ambient conditions after PE deposition and the presence along the chain of multiple expanded chain folds. Only isolated lamellae and lamellar domains of a monolayer height are observed on graphite samples. The substrate/polymer epitaxial incommensurability important for the observation of the PE linear chain surface conformations is discussed from the comparison of the results obtained for mica and graphite, the coil‐to‐crystal intramolecular transformation is assumed to be inhibited on mica surface. The slow disintegration of the original gel structure of PE stock‐solution used for the high‐temperature depositions was found to result in the characteristic large‐scale morphological heterogeneity of the samples. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 766–777, 2010     

Study on the adsorption and aggregation of dilute gemini surfactant solutions

As a function of temperature in aqueous solutions, the adsorption and aggregation of N,N’-bis (tetradecyl dimethyl)-1,2-dibromide-ethanediyl ammonium salt (GS14-2-14) and N,N’-bis (hexadecyl dimethyl)-1,2-dibromide-ethanediyl ammonium salt (GS16-2-16), were researched with drop-volume technique and conductometry, respectively. The results of surface tension measurements, which were analyzed by originally developed thermodynamic equations, illustrate that GS14-2-14 has a better surface activity and arranges more tightly in the adsorbed film than GS16-2-16. The data of conductivity were used to find critical micelle concentration (cmc) and counterion binding degree of micelle (β). Thermodynamic parameters of micellization were also obtained from the temperature dependence of cmc values. From the study, it is discovered that the micellization process is spontaneous and exothermic in nature and it is mainly driven by entropy.     

Adsorption of anionic dye by anionic surfactant modified chitosan beads: Influence of hydrophobic tail and ionic head-group

In this paper, how chitosan hydrogel beads were modified by anionic surfactants (SDS, SDOS, SDBS, AOT, and DTM-12) and then used for the adsorption and removal of an anionic dye (congo red) from aqueous solutions were described. The effect of surfactant concentration, surfactant ionic head-group, and surfactant hydrophobic tail were investigated in detail. The result revealed the modified CS beads all had the obviously higher adsorption capacity than CS beads. Compared to the ionic head-group, the hydrophobic tail of the surfactant plays more important role in the adsorption, and a high adsorption capacity was observed for CS/AOT beads and CS/DTM-12 beads (both with two hydrophobic tails). The Sips isotherm model showed a good fit with the equilibrium experimental data, and the values of the heterogeneity factor (n) indicated heterogeneous adsorption. The adsorption kinetics analysis indicated that the pseudo-second-order rate model could better describe the adsorption process than the pseudo-first-order rate model.     

The effect of electrolyte on ionic surfactant adsorption

For submicellar solutions of ionic surfactants rigorous thermodynamic expressions are presented which show that the surfactant adsorption will not depend on electrolyte concentration c₃ at constant surfactant concentration c₁, if there is no corresponding dependence on c₁ at constant c₃. Results which indicate otherwise are inconsistent with thermodynamics.     

AFM imaging of adsorbed Nafion polymer on mica and graphite at molecular level

Perfluorosulfonic acid ionomer (PFSA, specifically Nafion at EW = 975 g/mol) was visualized at the single molecule level using atomic force microscopy (AFM) in liquid. The diluted commercial Nafion dispersion shows an apparent M(w) = 1430 kg/mol and M(w)/M(n) = 3.81, which is assigned to chain aggregation. PFSA aggregates, imaged on mica and HOPG during adsorption from EtOH-H(2)O solvent at pH(e) 3.0 (below isoelectric point), showed a stable, segmented rod-like conformation. This structure is consistent with earlier NMR, SAXS/SANS, and TEM results that support a stiff helical Nafion conformation with long persistence length, a sharp solvent-polymer interface, and an extension of the sulfonated side chain into solution. Adsorption of Nafion structures on HOPG was observed at even higher pH(e) from EtOH due to screening of the repulsive electrostatic interaction in lower dielectric constant solvent, while the chain adopted an expanded coil conformation. These measurements provided direct evidence of the chain aggregation in EtOH-H(2)O solution and revealed their equilibrium conformations for adsorption on two model surfaces, highly ordered pyrolitic graphite (HOPG) and mica. The commercial Nafion dispersion was autoclaved at 0.10% w/w in nPrOH/H(2)O = 4:1 v/v solvent at 230 °C for 6 h to give a single-chain dispersion with M(w) = 310 kg/mol and M(w)/M(n) = 1.60. The autoclaved chains adopt an electrostatically stabilized compact globule conformation as observed by AFM imaging of the single PFSA molecules after rapid deposition on mica and HOPG at a low surface coverage.     

Tailoring and investigation of surface chemical nature of virgin and ion beam modified muscovite mica

We report that the surface chemical properties of muscovite mica [KAl₂(Si₃Al)O₁₀(OH)₂] like important multi-elemental layered substrate can be precisely tailored by ion bombardment. The detailed X-ray photoelectron spectroscopic studies of a freshly cleaved as well as 12-keV Ar⁺ and N⁺ ion bombarded muscovite mica surfaces show immense changes of the surface composition due to preferential sputtering of different elements and the chemical reaction of implanted ions with the surface. We observe that the K atoms on the upper layer of mica surface are sputtered most during the N⁺ or Ar⁺ ions sputtering, and the negative aluminosilicate layer is exposed. Inactive Ar atoms are trapped, whereas chemically reactive N atoms form silicon nitride (Si₃N₄) and aluminum nitride (AlN) during implantation. On exposure to air after ion bombardment, the mica surface becomes more active to adsorb C than the virgin surface. The adsorbed C reacts with Si in the aluminosilicate layer and forms silicon carbide (SiC) for both Ar and N bombarded mica surfaces. Besides the surface chemical change, prolonged ion bombardment develops a periodic ripple like regular pattern on the surface.     

缓蚀剂吸附行为的电化学及AFM力曲线研究

结合极化曲线,微分电容曲线测试和AFM力曲线技术研究了直链十二胺对氯化钠溶液中铜镍合金的缓蚀行为以及吸附机理。结果表明:十二胺在合金表面形成单分子层吸附膜而起到缓蚀作用。十二胺浓度越大,吸附膜越致密,缓蚀率越高,力曲线上测得的粘附力值也越大。质子化的十二胺在荷负电的合金表面的吸附使电极零电荷电位正移,电荷屏蔽作用使得AFM力曲线上探针与试样之间的长程静电斥力减小。     

Atomic force microscopy characterization of β-casein nanoparticles on mica and graphite

1. Download : Download high-res image (193KB)
2. Download : Download full-size image

     

Microcontact printing of multiproteins on the modified mica substrate and study of immunoassays

Microcontact printing (µCP) is an easy and efficient way of producing patterns of self‐assembled monolayers (SAM) containing different functional groups. We have developed a simple and convenient µCP‐based technique for the modification of a mica substrate with 3‐aminopropyltriethoxysilane (APTES) and the micropatterning of proteins (chicken IgG and rabbit IgG) on the modified mica surface. Our approach provides a quick and easy way to produce protein patterns on solid surfaces. The printed immunoglobulin patterns were detected by exposing the substrate to solutions containing fluorescently labeled complementary anti‐IgGs, and the formed immunoassays were studied using fluorescence microscopy. Copyright © 2005 John Wiley & Sons, Ltd.     

光催化作用下丙酮改性对神府煤煤岩组分表面性质影响的研究

研究了光催化作用下丙酮对不同煤岩组分表面改性的作用,分析了改性时间对神府丝炭和神府镜煤表面性质的影响,为建立增大煤岩组分表面性质差异的新方法提供理论依据。光催化作用下丙酮改性可以明显改变煤岩组分表面性质,改性时间为3min时,神府丝炭和神府镜煤润湿性差异最大。改性后煤岩组分表面C－O、C=O和COO－等含氧官能团含量发生了变化,且神府丝炭表面C=O和COO－含量的变化较神府镜煤更为明显。
     

Auto-organized nano-structure of collagen on Gemini surfactant monolayer

Two kinds of Gemini surfactant monolayer, which showed different hydrophobic property, were selected as adsorption substrates for collagen. The topographic images of collagen were investigated by using an atomic force microscopy (AFM). Their auto-organized nano-structures were influenced by the property of substrate and the process of sample preparation, such as concentration of collagen solution, adsorption time and drying condition. Network-like structures formed on the both Gemini monolayers. With increasing concentration of collagen solution and adsorption time, the density of the network-like structure increased and their strands became wider and the mesh sizes decreased apparently. Contrary to the reference, the network-like structures of collagen also formed on the less hydrophobic Gemini surfactant monolayer even after very short adsorption time, which was considered to result from the more hydrophobic patch on it.     

AFM morphological study of electropolymerised polyaniline films modified by surfactant and large anions

Atomic force microscopy (AFM) ex situ experiments were carried out in order to study morphologies of polyaniline films grown in the presence of an anionic surfactant, sodium dodecylsulphate (SDS), and also using different functionalised organic acids. Polyaniline film morphologies obtained in the presence of SDS were analysed in terms of the formation of a complex between the rodlike micelles and the anilinium cation in solution. For the different acids, the influence of anion size and the solubility of the oligomeric salt were considered to explain the different sizes of globular morphologies.     

Molecular morphology of modified partially hydrolyzed polyacrylamide (MHPAM) on mica substrates and Langmuir-Blodgett films of MHPAM/CTAB complexes as observed by AFM

Direct observation of molecular morphology of modified partially hydrolyzed polyacrylamide (MHPAM) has been studied in this paper by atomic force microscopy (AFM). The MHPAM molecule chains are stretched forming worm-like coils due to the electrostatic repulsion between randomly distributed positive charges along the chain at pH 3. The further data analysis demonstrates single MHPAM molecules are being imaged. In addition, morphology of Langmuir-Blodgett films of MHPAM/CTAB complexes on mica surfaces also investigated as a function of the subphase pH. At pH 6, MHPAM/CTAB molecules covered on mica surfaces adopted the worm-like nanostructures, and at pH 7, conformational transition of polymer complexes appeared, showing the morphology of bump-like to some extent owing to aggregation of intrachains and interchains. Eventually, more compact globule morphology was formed at pH 12.     

Surface film formation on a graphite electrode in Li‐ion batteries: AFM and XPS study

The formation of a passivation film (solid electrolyte interphase, SEI) at the surface of the negative electrode of full LiCoO₂/graphite lithium‐ion cells using LiPF₆ (1M ) in carbonate solvents as electrolyte was investigated by means of x‐ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). The analyses were carried out at different potentials of the first and the fifth cycles, showing the potential‐dependent character of the surface‐film species formation. These species were mainly identified as Li₂CO₃ up to 3.8 V and LiF up to 4.2 V. This study shows the formation of the SEI during charging and its partial dissolution during discharge. Copyright © 2005 John Wiley & Sons, Ltd.     

In situ AFM study of sorbed humic acid colloids at different pH

Humic acid colloids adsorbed on the basal plane of cleaved muscovite are investigated under in situ conditions by non-contact mode atomic force microscopy (AFM) in liquid (also called fluid tapping-mode AFM). Structures are found to be of nanometer scale, consisting of flat particles (8–13 nm in diameter), aggregates of particles (20–100 nm), chain-like assemblies, networks and torus-like structures. In contrast to former investigations colloids are investigated in aquatic solution and structures are not influenced by sample preparation. Nanostructure, surface coverage and particle sizes are found to depend on solution pH. Humic colloids can be distinguished from surface roughness and background noise by image processing. Furthermore, an approach to quantify the surface coverage is discussed. Therefore, non-contact mode AFM in liquid is shown to be a powerful method to study the interaction of colloids at solid–liquid interfaces.