Formation Of Mixed Monolayers Research Articles

Formation of two-dimensional mixed monolayers of integrated-cellulose nanofibers and starch nanoparticles using cellulose acetate and soluble starch

In this study, mixed monolayers comprising starch and cellulose nanofibers are prepared on the water surface and effectively controlled their surface morphologies. Through the spreading of cellulose acetate on the water surface, we achieve the preparation of integrated films with the spontaneous formation of nanocellulose. It is confirmed that starch, which is insoluble in the spreading solvent, is solubilized through a soluble treatment and spreads into a monolayer on the water surface. Upon mixing the nanocellulose integrated layer and starch particle layer, the morphology changes based on the mixed ratio of these components. In the monolayer film of a 1:1 mixture of cellulose acetate and soluble starch, the morphology predominantly consists of fibers, with particulate aggregates incorporated here and there. Furthermore, when the ratio changes to 1:5, the mixed monolayer transforms into a single particle layer, while the mixed monolayer evolves into a fibrous integrated film at a ratio of 5:1. Meanwhile, from the evaluation of the surface pressure-area isotherms, it is predicted that both components constitute an ideal mesoscopic miscible system. In addition, in the subphase temperature dependence of the cellulose acetate:soluble starch = 1:1 mixed monolayer, fibers are promoted under low-temperature conditions, and substantial domains are formed under high-temperature conditions. Notably, the mixed film remains amorphous at all ratios and exhibits low surface free energy, simultaneously suggesting hydrophobic and oil-repellent properties.

The Equilibria of Triterpene Sapogenins-Phosphatidylcholine in Monolayers at the Air/Water Interface.

Sapogenins are the non-sugar parts of saponins (aglycones), high-molecular-weight glycosides linked to one or more sugar side chains. This group of compounds presents many properties, e.g., the potent properties of reducing surface tension and foaming properties, as evidenced by the amphipathic nature of these substances. They are used in the cosmetics industry, the washing and detergent industry, and the food industry. In addition, they have many healing properties. They lower blood cholesterol but are also used to synthesize steroid drugs or hormones. As reported in the literature, saponins also show antitumor activity, leading to cell cycle inhibition and apoptosis of various neoplastic cells. In this study, the influence of two sapogenins: asiatic acid (AA) and oleanolic acid (OA), on the properties of monolayers made of phosphatidylcholine (DPPC) was investigated. The method used in these studies was the Langmuir method with Brewster angle microscopy. The interactions between the tested compounds in mixed monolayers were described. Using mathematical equations, we established that oleanolic acid and asiatic acid formed complexes with DPPC at 1:1 ratios, characterized by high stability constants. We derived the parameters characterizing the formed complexes and described the phase transitions that occur during the formation of pure and mixed monolayers.

Synergy in the Microbial Cyclic Lipopeptide/Petroleum-Derived Surfactant Mixture Can Reduce the Total Surfactant Consumption

We report the synergistic effect of the addition of a microbial cyclic lipopeptide, surfactin (SF), to the most common petroleum-derived surfactant, sodium dodecyl sulfate (SDS). The addition of a small amount of SF (0.1 mol %) to an aqueous solution of SDS reduces the critical micelle concentration (CMC) of SDS from 10–3 to 10–6 M order. The addition of SF to SDS at various ratios lowered the surface tension at the CMC of SDS, indicating CMC reduction and, thus, the minimization of total SDS consumption without loss of surface activity. Surface tension measurement and nuclear magnetic resonance spectroscopy were conducted to analyze the synergy between SDS and SF in aqueous solutions as a function of composition. The interaction parameters of the mixed monolayer and mixed micelles were negative, indicating the occurrence of attractive interactions between SDS and SF, which were stronger than those among the individual surfactants. Moreover, the surface tension at the CMC, area occupied per surfactant molecule, difference in the interaction parameter, and 1H NMR chemical shift values of the SDS/SF mixtures as a function of their composition exhibited an extremum at a mole fraction of 0.7. These results suggest that SF has the peculiarities of encapsulation of the anionic surfactant of SDS within the inner cavity and can screen the electrostatic repulsion of SDS via the formation of their inclusion complex at SDS:SF = 2:1, which facilitates the formation of mixed monolayers and micelles in a more stable state.

Nucleation of Surfactant-Alkane Mixed Solid Monolayer and Bilayer Domains at the Air-Water Interface.

We investigated the wetting transitions of tetradecane and hexadecane droplets in dodecyltrimethylammonium bromide (C12TAB), tetradecyltrimethylammonium bromide (C14TAB), and hexadecyltrimethylammonium bromide (C16TAB) aqueous solutions. By varying the surfactant concentration, the formation of mixed monolayers of a surfactant and an alkane was observed at the air–water interface. Depending on the combination of surfactant and alkane, these wetting monolayers underwent another thermal phase transition upon cooling either to a frozen mixed monolayer (S1) or a bilayer structure composed of a solid monolayer of a pure alkane rested on a liquid-like mixed monolayer (S2). Based on the phase diagrams determined by phase modulation ellipsometry, the difference in the morphology of the nucleated S1 and S2 phase domains was also investigated using Brewster angle microscopy. Domains of the S1 phase were relatively small and highly branched, whereas those of the S2 phase were large and circular. The difference in domain morphology was explained by the competition of the domain line tension and electrostatic dipole interactions between surfactant molecules in the domains.

The micelle thermodynamics and mixed properties of sulfobetaine-type zwitterionic Gemini surfactant with nonionic and anionic surfactants

The surface tension curves of three zwitterionic Gemini surfactants (GCS12, GCS14, and GCS16) were measured at different temperatures. According to the values of critical micelle concentration (cmc) and γcmc, the thermodynamic parameters of these surfactants were calculated. As temperature increases, the cmc values decrease, and the surface activities increase. The thermodynamic parameters indicate that the micellization and adsorption of these surfactants are spontaneous, endothermic and entropy-driven. The process of micellization and adsorption features an enthalpy-entropy compensation phenomenon. The compensation temperatures of micellization are in the range of 311.2~312.8 K, while those of adsorption are between 310.2 K and 312.2 K. Among these surfactants, GCS14 aggregates most easily into micelles, and the micelles of GCS14 are the most stable. Moreover, the mixed properties of GCS14 with anionic surfactant (SDS) and nonionic surfactant (CDA) were investigated, respectively. The GCS14/SDS mixtures exhibit excellent surface activities, and GCS14 plays an essential role in the mixtures. At αGCS14 = 0.8, the cmc and γcmc values reach a minimum. According to the regular solution theory, the great absolute values of βσ (for mixed monolayer formation at the aqueous solution/air interface) and βm (for mixed micelle formation in aqueous solution) indicate the strong attraction between GCS14 and SDS. The GCS14/SDS mixtures show synergistic effects in three properties, including the efficiency in surface tension reduction, the ability of micelle formation, and the ability of surface tension reduction. Also, the mixtures of GCS14 and CDA feature excellent surface activities. At αGCS14 = 0.8, the cmc and γcmc values reach a minimum. For the GCS14/CDA mixtures, the great absolute values of βσ and βm suggest these mixtures exhibit the synergistic effect in the surface tension reduction and the formation of the micelles. Zwitterionic Gemini surfactant GCS14 reveals stronger synergistic effects in the formation of mixed micelle and mixed monolayers with anionic surfactant (SDS) than with the nonionic surfactant (CDA). The molecule attraction between GCS14 and SDS or CDA in the mixed films is stronger than in the micelles.

Rationalizing the formation of binary mixed thiol self-assembled monolayers

Periodic DFT-D calculations are used to decipher the role of intermolecular forces on the stability of mixed linear thiol self-assembled monolayers (SAMs) on Au(111) and compared with experiment. The interaction energy is rationalized by quantifying its different contributions. The inter-chain interaction energy is shown to be in direct relation with the surface reconstruction and the formation of adatoms. The stability of the mixed SAM systems is predicted by calculations and validated with experiments. In order to describe predictively the segregation of binary thiol mixtures adsorption on Au surfaces a segregation descriptor is defined. This procedure is a promising step forward in the prediction of segregated SAMs leading to future functional nanomaterials, including Janus or patchy nanoparticles for optics, formulation and self-assembled patterns.

Tuning Oleophobicity of Silicon Oxide Surfaces with Mixed Monolayers of Aliphatic and Fluorinated Alcohols.

We demonstrate the formation of mixed monolayers derived from a microwave-assisted reaction of alcohols with silicon oxide surfaces in order to tune their surface oleophobicity. This simple, rapid method provides an opportunity to precisely tune the constituents of the monolayers. As a demonstration, we sought fluorinated alcohols and aliphatic alcohols as reagents to form monolayers from two distinct constituents for tuning the surface oleophobicity. The first aspect of this study sought to identify a fluorinated alcohol that formed monolayers with a relatively high surface coverage. It was determined that 1H,1H,2H,2H-perfluoro-1-octanol yielded high quality monolayers with a water contact angle (WCA) value of ∼110° and contact angle values of ∼80° with toluene and hexadecane exhibiting both an excellent hydrophobicity and oleophobicity. Tuning of the oleophobicity of the modified silicon oxide surfaces was achieved by controlling the molar ratio of 1H,1H,2H,2H-perfluoro-1-octanol within the reaction mixtures. Surface oleophobicity progressively decreased with a decrease in the fluorinated alcohol content while the monolayers maintained their hydrophobicity with WCA values of ∼110°. The simple and reliable approach to preparing monolayers of a tuned composition that is described in this article can be utilized to control the fluorocarbon content of the hydrophobic monolayers on silicon oxide surfaces.

Raman spectroscopy and surface wetting of self-assembled monolayer (SAM) of 1-octanethiol and 1,10-decanedithiol

We report the preparation of mixed self-assembled monolayer of 1-octanethiol and 1,10-decanedithiol on Au thin film with preferential Au(111) surface and their characterization using Raman spectroscopy of cysteine adsorbed on mixed self-assembled monolayer mediated isolated Ag nanoparticles. The self-assembled monolayer characterization has been also performed through water contact angle measurement. A significant enhancement in water contact angle from 24° to 103° has been observed on Au surface after self-assembled monolayer formation, primarily due to the hydrophobic nature of the methyl group at the terminal end of the 1-octanethiol, which confirms regular self-assembled monolayer formation. Availability of –SH group from 1,10-decanedithiol on the self-assembled monolayer surface and so, the formation of mixed self-assembled monolayer has been ascertained by immobilization of Ag nanoparticles probed via scanning electron microscopy and Raman spectroscopy of cysteine adsorbed on Ag nanoparticles. Raman spectrum of cysteine on self-assembled monolayer mediated Ag nanoparticles in the fingerprint region of 500–1800 cm−1 shows appreciable increase in the band intensity due to surface enhanced Raman scattering as compared to the band intensity on bare Au surface. These results clearly indicate that the mixed self-assembled monolayer with adequate proportion of component molecules can be utilized as a suitable and inexpensive host for surface enhanced Raman scattering substrates.

One-Pot Electrografting of Mixed Monolayers with Controlled Composition

Surface functionalization with ultrathin layers exhibiting a highly robust interface is of paramount importance for designing materials with tailored properties or operating functions, without modifying drastically the material’s bulk structures. A fine-tuning of the surface composition obtained, for instance from binary mixed layers, is also a key issue for developing high value-added applications like efficient sensors. Herein, binary mixtures of calix[4]arene-tetra-diazonium salts generated in situ from their corresponding calix[4]tetra-anilines are electrografted to form covalently bound monolayers onto substrates for yielding versatile functionalizable molecular platforms. Wettability studies, X-ray photoelectron spectroscopy analyses, and scanning electrochemical microscopy show the formation of homogeneous mixed monolayers. The distribution of the two calixarenes on the surface is directed by their relative molar fraction in the deposition solution. The strategy allows the control of the composition of mixed monolayers in a one-step approach. Postfunctionalization of the mixed layers with ferrocene centers is performed to exemplify the benefit of a dilution procedure when functional groups are introduced at the calix[4]arene small rim. This study highlights the potential of diazonium salt electrografting as a competitive alternative to chemisorption strategies such as self-assembled monolayers of alkyl thiols in the field of surface functionalization.

Properties of β-carotene and retinoic acid in mixed monolayers with dipalmitoylphosphatidylcholine (DPPC) and Solutol

The liposoluble vitamins and carotenoids are sensitive molecules and their antioxidant properties can be dramatically affected when included in membrane environment or in encapsulated carriers as liposomes or lipid nanocapsules (LNCs). The formation and state of mixed monolayers at air–water interface of β-carotene (βC) or retinoic acid (RA) with dipalmitoylphosphatidylcholine (DPPC) as simplest membrane model or with Solutol (Sol) which forms the soft layer covering the LNCs were studied by means of surface pressure (π) and surface potential (ΔV) measurements. The mutual miscibility and interfacial organization of components in studied mixed monolayers was found to strongly depend on their composition. It was suggested a better inclusion of βC small quantities in pure DPPC monolayer at low surface pressure than for films with more βC at higher surface pressure. The measurements of surface properties were complemented by Atomic Force Microscopy (AFM) as a suitable technique for direct visualization at nanoscale level of the monolayer microheterogeneity and the presence of interfacial structures. It was also found that the addition of bigger amounts of βC led to the appearance of stripe like phase which is more deeply formed at higher surface pressure.

Formation of Mixed Monolayers from 11-Mercaptoundecanoic Acid and Octanethiol on Au(111) Single Crystal Electrode under Electrochemical Control

The formation process and characterization of mixed layers of 11-mercaptoundecanoic acid (MUA) and octanethiol (OT) on Au(111) single crystal electrodes under electrochemical control from alkaline solutions are analyzed in this work by means of electrochemical techniques such as chronocoulometry, cyclic voltammetry, and electrochemical impedance spectroscopy. The surface composition of the layers differs from the component ratio of bulk solution and is supposed to be directed by both the adsorption potential of the individual molecules and its adsorption behavior. The relative competitive adsorption capacity of the two functional groups in the bifunctional molecules plays an important role in the electrochemical deposition of mixed layers as the second component can occupy the free space that the first molecule liberates during reorganization. Two different mixed layers can be obtained under the experimental conditions of this work: a macroscopically homogeneous layer and a homogeneously mixed layer at higher and lower MUA:OT ratios. The macroscopically mixed layer should be formed by small domains that keep the same energetic as the single-component SAM of the thiol constituents, and the homogeneously mixed should be formed by few MUA molecules diluted in an OT layer.

Autocatalytic effect of amine-terminated precursors in mixed self-assembled monolayers

We investigated the formation of mixed self-assembled monolayers (SAMs), comprising two silanes terminated with an amine and a non-reactive functional group, to demonstrate the autocatalysis of the mixed SAMs by the amine-terminated precursors. Measurements of surface energy and angle-resolved X-ray photoelectron spectroscopy on the mixed SAMs revealed that the final composition and the surface coverage of the mixed SAMs after deposition strongly depended on the presence and the concentration of the aminesilane precursor in solution. To explain the observed dependence, we further suggested a mechanism based on the autocatalytic effect of the aminesilane precursor on the mixed SAM. These results highlight the complex role of the aminesilane, which behaves simultaneously as a component of the mixed SAM and a catalyst. An important implication of this is that the compositions of mixed SAMs can be significantly influenced by the autocatalytic effect of the component carrying a reactive functional group such as amine.

Fluorescencing behavior of thin solid films based on polyelectrolyte‐surfactant complex and dye molecules

AbstractThis article is devoted to investigation of interaction between polycomplex and dye molecules assembled in fluorescent thin solid Langmuir‐Blodgett (LB) films. Preparation procedure of organosoluble stoichiometric polycomplex based on cationic polyelectrolyte and anionic surfactant is described. Formation of mixed monolayers consisting of polyelectrolyte‐surfactant complex (PSC) and dye molecules–Nile Red (NR) at water‐air interface is shown. Assembling conditions of LB films based on PSC and NR are defined and the spectral‐luminescent properties of obtained films are studied. It is shown that when nonamphiphilic NR and PSC are mixed the excellent monolayers formed, which can easily be transferred onto solid substrates by Langmuir‐Blodgett method with a high transfer coefficient. Absorption and fluorescence spectra of mixed LB films reveal the formation of dimers and existence of monomers in LB films. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013

Properties of langmuir monolayers based on organosoluble cationic polyelectrolyte-anionic surfactant complex and organic dyes

Phase states of monolayers of organosoluble stoichiometric polycomplex based on cationic polyelectrolyte and anionic surfactant has been studied. Formation of mixed monolayers consisting of polyelectrolyte-surfactant complex (PSC) and dye molecules at water-air interface was shown. Assembling conditions of fluorescent nanosized solid Langmuir-Blodgett (LB) films based on PSC and dyes were defined. It is ascertained that electrostatic interaction between polycomplex and dye molecules is responsible for formation of fluorescing LB films based on molecules of nonamphiphilic organic dyes.

In Situ Hydrolysis of Imine Derivatives on Au(111) for the Formation of Aromatic Mixed Self-Assembled Monolayers: Multitechnique Analysis of This Tunable Surface Modification

This paper presents a novel method for preparing aromatic, mixed self-assembled monolayers (SAMs) with a dilute surface fraction coverage of protonated amine via in situ hydrolysis of C═N double bond on gold surface. Two imine compounds, (4'-(4-(trifluoromethyl)benzylideneamino)biphenyl-4-yl)methanethiol (CF(3)-C(6)H(4)-CH═N-C(6)H(4)-C(6)H(4)-CH(2)-SH, TFBABPMT) and (4'-(4-cyanobenzylideneamino)biphenyl-4-yl)methanethiol (CN-C(6)H(4)-CH═N-C(6)H(4)-C(6)H(4)-CH(2)-SH, CBABPMT), self-assembled on Au(111) to form highly ordered monolayers, which was demonstrated by infrared reflection absorption spectroscopy (IRRAS) and X-ray photoelectron spectroscopy (XPS). A nearly upright molecular orientation for CF(3)- and CN-terminated SAM was detected by near edge X-ray absorption fine structure (NEXAFS) measurements. Afterward, the acidic catalyzed hydrolysis was carried out in chloroform or an aqueous solution of acetic acid (pH = 3). Systematic studies of this hydrolysis process for CN-terminated SAM in acetic acid at 25 °C were performed by NEXAFS measurements. It was found that about 30% of the imine double bonds gradually cleaved in the first 40 min. Subsequently, a larger hydrolysis rate was observed due to the freer penetration of acetic acid in the SAM and resultant more open molecular packing. Furthermore, the molecular orientation in mixed SAMs did not change during the whole hydrolysis process. This partially hydrolyzed surface contains a controlled amount of free amines/ammonium ions which can be used for further chemical modifications.

Surface‐enhanced Raman scattering study of monolayers formed from mixtures of 4–mercaptobenzoic acid and various aromatic mercapto‐derivative bases

AbstractMonolayers formed from aromatic compounds have many potential applications, for example in construction of bioelectronic elements having high efficiency of electron transfer. In this paper, the composition of monolayers formed on silver surfaces from mixtures of 4‐mercaptobenzoic acid (MBA) and four model (stable and easily available) aromatic thiols with strong base properties: 4,6‐diamino‐2‐mercaptopyrimidine (APY), 1H‐1,2,4‐triazole‐3‐thiol (HTR), 4‐methyl‐4H‐1,2,4‐triazole‐3‐thiol (MTR) and 3‐amino‐1,2,4‐triazole‐5‐thiol (ATR), were determined from surface‐enhanced Raman scattering (SERS) measurements. Our investigations showed that among studied aromatic bases, APY is the most promising candidate for the formation of mixed monolayers with MBA. In the whole pH range studied (2–12.5), mixed MBA + APY monolayers with similar surface concentration of both components are formed during the adsorption from the 0.5 mM MBA + 0.5 mM APY aqueous solution. Desorption of MBA and APY from the mixed monolayer is, however, significantly different. During immersion in water, surface concentration of APY decreases significantly faster than MBA (a significant part of the adsorbed MBA molecules is present on the silver surface even after 2 h of soaking in water). Presence of chlorides, bovine serum albumin or laccase in the surrounding solution does not observably influence the structure of MBA + APY monolayers. The properties of monolayers formed from MBA and substituted triazoles were found to be significantly different than those of MBA + APY monolayers. Copyright © 2009 John Wiley & Sons, Ltd.

Engineering Clustered Ligand Binding Into Nonviral Vectors: αvβ3 Targeting as an Example

The development of techniques to efficiently deliver genes using nonviral approaches can broaden the application of gene delivery in medical applications without the safety concerns associated with viral vectors. Here, we designed a clustered integrin-binding platform to enhance the efficiency and targetability of nonviral gene transfer to HeLa cells with low and high densities of alpha(v)beta(3) integrin receptors. Arg-Gly-Asp (RGD) nanoclusters were formed using gold nanoparticles functionalized with RGD peptides and used to modify the surface of DNA/poly(ethylene imine) (PEI) polyplexes. DNA/PEI polyplexes with attached RGD nanoclusters resulted in either 5.4- or 35-fold increase in gene transfer efficiency over unmodified polyplexes for HeLa cells with low- or high-integrin surface density, respectively. The transfection efficiency obtained with the commercially available vector jetPEI-RGD was used for comparison as a vector without clustered binding. JetPEI-RGD exhibited a 1.2-fold enhancement compared to unmodified jetPEI in cells with high densities of alpha(v)beta(3) integrin receptors. The data presented here emphasize the importance of the RGD conformational arrangement on the surface of the polyplex to achieve efficient targeting and gene transfer, and provide an approach to introduce clustering to a wide variety of nanoparticles for gene delivery.

Controlling structural properties of self-assembled oligonucleotide–mercaptohexanol monolayers

The achievement of high hybridization efficiency in DNA microarrays is largely affected by the surface density of immobilized functional DNA probes. We investigated the chemical conditions for the formation of mixed self-assembled monolayers of thiol-modified oligonucleotides and mercaptohexanol deposited onto gold surfaces. The surface density of DNA was studied by means of high-resolution X-ray photoemission spectroscopy. The measurements revealed that the spatial density of DNA strands can be controlled within a wide range by the concentration of MgCl 2 in the immobilization buffer. Moreover, improved preparation conditions for high-quality self-assembled hybrid monolayers are presented. Compared to our previous work, a reduction in unbound thiol was achieved by changing from ethanolic to aqueous solvent and lowering the MCH concentration.

Development of a myoglobin impedimetric immunosensor based on mixed self-assembled monolayer onto gold

Immunosensors rely on antibody-antigen binding with a range of possible detection methodologies. In this study, electrochemical impedance spectroscopy was used to monitor the sensor surface assembly and recognition of the analyte (myoglobin). Myoglobin is rapidly released into the circulatory system after an acute myocardial infarction and rapidly rising levels make it the first biochemical marker of myocardial damage. The immunosensor fabrication steps comprised the steps of (a) formation of mixed self-assembled monolayers (mSAM) on gold electrodes using a mixture of biotinyl-phospholipid and mer captohexadecanoic acid; (b) neutravidin functionalisation and (c) attachment of biotinyl anti-myoglobin antibodies. A range of analyte concentration (10−12–10−6 M) was successfully detected in phosphate buffered saline and in serum concentration ranging from 10% (v/v) to 100% (v/v) serum. Quartz crystal microbalance and atomic force microscopy studies were carried out to study each step of fabrication to elucidate binding characteristics and surface topography.

Monomolecular Films of Phthalocyanines: Formation, Characterization, and Expelling by Alkanethiols

Adsorption of aluminum-2,3-naphthalocyanine (Al-PC) onto gold (111) substrate from the pure and mixed (with alkanethiols) solutions of the target molecules in ethanol was studied. The resulting films were characterized by X-ray photoelectron spectroscopy and near-edge X-ray absorption fine structure spectroscopy. The adsorption from the pure solution resulted in formation of an Al-PC monolayer composed of the strongly inclined molecules. However, a presence of only one molar percent of a thiolated compound (alkanethiol) in the primary solution led to the complete expelling of the Al-PC molecules from the substrate and formation of a one-component alkylthiolate monolayer. The results indicate that an equilibrium formation of mixed monolayers of rodlike thiolated molecules and relatively large planar-geometry molecules, whose interaction with the metal surface is mainly provided by the pi-electron system, is difficult to achieve or is in most cases even impossible under equilibrium conditions and requires an introduction of additional anchor moieties (e.g., thiols) into these molecules, as has been demonstrated by successful coadsorption of hexadecanethiol and thiolated tetraphenylporphyrin.