Surfactant Packing Density Research Articles

Ions Slow Water Dynamics at Nonionic Surfactant Interfaces.

Nonionic surfactant interfaces are often considered to be unaffected by aqueous ions. However, ions often localize to interfaces and, as such, can interact with surfactants either via direct contact or by affecting interfacial hydrogen bond structures in water. Characterizing these effects is essential to understanding how ions affect interfacial properties at the oil-water interface in the presence of nonionic surfactants. Here, we use two-dimensional infrared (2D IR) spectroscopy in combination with atomistic molecular dynamics (MD) simulations to study the effects of high-concentration Na+ and Ca2+ ions on interfacial hydrogen bond dynamics in heterogeneous sorbitan stearate reverse micelles. Experiments show only minor changes in interfacial hydrogen bond populations when salts are added but those interfacial water network dynamics are slowed by nearly 300%. Molecular dynamics simulations show the slowdown results from an increased disorder in surfactant headgroup orientation and packing density, which stabilizes hydrogen bonding interactions between surfactants and interfacial water.

Thermodynamics and Kinetics of Pretilachlor Adsorption on Organobentonites for Controlled Release.

Thermodynamics and kinetics of pretilachlor adsorption on organobentonites modified with hexadecyltrimethyl ammonium chloride were investigated to reveal the structural effects of organobentonites on the interaction with pretilachlor and the diffusion of the herbicide and were related to the controlled release from organobentonites. The adsorption of pretilachlor was entropically driven by hydrophobic interaction. The entropy change dropped with increasing surfactant loading from 0.4 to 1.50 times the cation exchange capacity (CEC) of the bentonite used, corresponding to a decrease in the degree of freedom of pretilachlor molecules due to the enhanced order of surfactant in the interlayer. The kinetics of pretilachlor adsorption was well fitted to the pseudo-second-order model and related to the structural features of organobentonites. The enhanced packing density of the surfactant in the interlayer generally resulted in a reduction of the rate constant of the pretilachlor adsorption onto organobentonites. However, the stepwise increase in the basal spacing due to the surfactant arrangement transition, from lateral-monolayer to lateral-bilayer at a loading level of more than 0.8 × CEC, benefited the diffusion of pretilachlor and diminished the influence of the increase in surfactant packing density. The release of pretilachlor from organobentonites was predominated by Fickian diffusion, which could be understood from the adsorption thermodynamics and kinetics. The time taken for the release of 50% of active ingredient was 16–23 times that for the control formulation and exhibited a linear increase with the relative value of the equilibrium constant to the rate constant of pretilachlor adsorption.

Amphoteric modified vermiculites as adsorbents for enhancing removal of organic pollutants: Bisphenol A and Tetrabromobisphenol A

Three novel organic vermiculites (VER) modified by amphoteric surfactants (BS, SB and PBS) with different negatively charged groups (carboxylate, sulfonate and phosphate) were demonstrated and used for removal of bisphenol A (BPA) and tetrabromobisphenol A (TBBPA). The difference in the structure and surface properties of modified vermiculites were investigated using a series of characterization methods. BS and SB surfactant mainly adsorbed on the surface and hard to intercalate into the interlayer of VER, while both adsorption and intercalation occurred in PBS modification. This difference resulted in different packing density of surfactant and hydrophobicity according to the results of contact angle, and affect the adsorption capacities ultimately. The adsorption of two pollutants onto these modified vermiculites were very fast and well fitted with pseudo-second-order kinetic model and Langmuir isotherm. PBS-VER exhibited the highest adsorption capacity (92.67 and 88.87 mg g−1 for BPA and TBBPA, respectively) than other two modified vermiculites in this order PBS-VER > BS-VER > SB-VER. The ionic strength (Na+, Ca2+) and coexisting compounds (Pb2+, humic acid) have different effects on the adsorption. PBS-VER had a good reusability and could remove ionic (methylene blue and orange G) and molecular (BPA) pollutants simultaneously and effectively due to the function of amphoteric hydrophilic groups and alkyl chains. The results might provide novel information for developing low-cost and effective adsorbents for removal of neutral and charged organic pollutants.

Effect of reaction temperature on intercalation of octyltrimethylammonium chloride into kaolinite

The structural property, thermal behavior, and morphology of octyltrimethylammonium chloride–kaolinite complexes prepared at different reaction temperatures were studied by X-ray diffraction, Fourier transform infrared spectroscopy, thermogravimetry–differential scanning calorimetry, and scanning electron microscope. The present study demonstrated that the arrangement model of octyltrimethylammonium cations (OTAC+) within the kaolinite interlayer space was independent of reaction temperature. The alkyl chains adopted a similar rigid paraffin-bilayer arrangement with different tilted angles. Although the intercalation led to an increased number of gauche conformers, the number of nonlinear conformers remained constant with increasing temperature. With increasing temperature, the number of trans conformers continuously augmented and resulted in decreased gauche/trans ratio. Therefore, the molecular environment remained solid like. Simultaneously, the surfactant packing density gradually increased, along with the decreasing water content in the organoclays. This effect improved thermal stability and hydrophobicity. The thermal decomposition processes of the kaolinite–OTAC+ complex can be divided into four steps. Furthermore, SEM images showed that the morphology of these complexes was strongly dependent on the given temperature. In general, increasing the temperature within the limited given temperature (≤70 °C) promoted the transformation from platy layers to nanoscrolls. Most of the transformed nanoscrolls were acquired in the products prepared at 70 °C, and further increasing in temperature decreased the nanoscrolls yield. Nevertheless, the packing density increased in the process, thereby demonstrating that the packing density not only promoted nanoscrolls transformation but also prevented the progress.

Remediation of distilleries wastewater using chitosan immobilized Bentonite and Bentonite based organoclays

Organic⿿inorganic nanocomposite, namely chitosan immobilized Bentonite (CIB) with chitosan content of 5% was synthesized in an acetic acid solution (2%). Organically modified CIB and Bentonite (mbent.) were prepared by intercalating cetyl trimethylammonium bromide (CTAB) as a cationic surfactant at doses equivalent to 1.5 and 3 times the cation exchange capacity (CEC) of clay. The prepared samples were characterized using FTIR, XRD and SEM to explore the interlayer structure and morphology of the resultant nanocomposites. The remediation of distilleries (vinasse) wastewater process was carried out using different adsorbents including CIB, modified CIB (mCIB), Bentonite (bent.), modified Bentonite (mbent.) and chitosan at different contact time. The results showed that the packing density of surfactant used in the synthesis of organoclays strongly affects the sorption capacity of the clay mineral and also showed that (mCIB)3 was found to be the most effective sorbent in the purification of distilleries wastewater with 83% chemical oxygen demand (COD) reduction and 78% color removal.

Adsorption of phenol and Cu(II) onto cationic and zwitterionic surfactant modified montmorillonite in single and binary systems

Organo-montmorillonites (OMts) modified by cationic surfactant (hexadecyltrimethylammonium bromide, C16) and zwitterionic surfactant (hexadecyldimethyl (3-sulphonatopropyl) ammonium, Z16) were used to remove phenol and Cu(II) from aqueous solution. The OMts were characterized by X-ray diffraction (XRD), thermogravimetric analysis (TG), Fourier-transform infrared spectroscopy (FTIR), X-ray fluorescence (XRF), and C, H, and N elemental analyses. Besides the independent adsorption of single contaminants, simultaneous and sequential adsorption of phenol and Cu(II) onto OMts were also investigated. The organic carbon contents of the two OMts were similar but the basal spacing of Z16 modified montmorillonite (Z16-Mt) was larger than that of C16 modified montmorillonite (C16-Mt) indicating a higher packing density of surfactant and hydrophobicity in C16-Mt with a slight higher adsorption capacity toward phenol. On the other hand, C16-Mt showed much lower capacity in adsorbing Cu(II) as compared with raw montmorillonite. On the contrary, the adsorption capacity of Z16-Mt toward Cu(II) was comparable with that of raw montmorillonite. The equilibrium data of phenol and Cu(II) were fitted satisfactorily with Linear and Langmuir models, respectively. In the sequential adsorption system, the adsorption of one contaminant was not affected by the other pre-adsorbed one; desorption of pre-adsorbed contaminant was also investigated. For both two contaminants, one did not affect the adsorption of the other one onto Z16-Mt and C16-Mt in the simultaneous system. Z16-Mt could rapidly and efficiently remove both phenol and Cu(II) simultaneously. The adsorption kinetics followed the Pseudo-second order model. The results of this work might provide novel information for developing new effective adsorbents toward organic contaminants and heavy metals.

Structural effects of organobentonites on controlled release of pretilachlor

To provide controlled release properties to the formulation containing pretilachlor, organobentonites with different loading levels of dodecyltrimethyl ammonium chloride (DTMA) and hexadecyltrimethyl ammonium chloride (HTMA) were prepared and used as carrier. The organobentonites were characterized by Fourier transform infrared (FTIR) spectroscopy and X-ray diffraction (XRD) analysis, and the adsorption towards pretilachlor was investigated, to reveal the structural effects of organobentonites on the release of pretilachlor. Organobentonites with quaternary alkylammonium surfactant showed excellent adsorption capacity for pretilachlor and greatly reduced its release in water. On the basis of the parameter of an empirical equation used to fit the release data, the release of pretilachlor from organobentonites could be deduced to be predominated by Fickian diffusion mechanism. The time taken for 50% of active ingredient to be released, T50, from DTMA-bentonite with surfactant loading of 60, 80, 100 and 120% of the cation exchange capacity was 12.9, 12.7, 18.5 and 26.7 times of that for the formulation without organobentonites and exhibited good linear relationship with the adsorption constant Kf from the Freundlich equation. Similar results were obtained for HTMA-bentonite. Hydrophobic interaction played a key roll in the adsorption and sustained release of pretilachlor. An increasing loading level of quaternary alkylammonium surfactant on organobentonites generally led to an enhancement of the sustained release. The permeability of organobentonite also had an important impact on the diffusion of the herbicide. The extension of the alkyl chain of the quaternary ammonium surfactant resulted in an increased permeability of organobentonites and release of pretilachlor, owing to the increased basal spacing and lower surfactant packing density.

Morphology, Structure, Thermal Stability, XR-Diffraction, and Infrared Study of Hexadecyltrimethylammonium Bromide–Modified Smectite

We characterized the interlayer structure and morphology of modified natural clays. Organoclay-based materials are increasingly used for multiple applications. We modified a purified Na-bentonite using cation exchange with hexadecyltrimethyl ammonium bromide (HDTMA) and characterized it using; UV spectroscopy, X-ray diffraction, photoacoustic FTIR spectroscopy (PA-FTIR), thermogravimetric analysis (TGA) techniques, laser scattering distribution (LSD), atomic force microscopy (AFM) and transmission electron microscopy (TEM). The adsorption of HDTMA on interlayer sites of Na-smectite fraction is nearly total until an adsorption equivalent to 1.85 times the cation exchange capacity (CEC) of the clay. The formation of the HDTMA-smectite changes the properties of the clay from hydrophilic to hydrophobic. The XRD patterns show an increase in the basal spacings. TGA characterization and analysis were shown to be useful to estimate the amount of surfactant within the organoclay. LSD analyses have shown a decrease of the effective particle diameter with increasing sonication times. AFM images showed that these Na-smectite surfaces appeared foliated with irregular aggregate structures at high HDTMA loading. The present study shows that not only the basal spacing but the morphology and the particle size of the organoclay strongly depend on the packing density of surfactant within the smectite interlayer space.

Investigation of the Orientation of CTA+ Ions in the Interlayer of CTAB Pillared Montmorillonite

The orientation of CTA+ in the interlayer of organic pillared montmorillonite prepared by adding different amounts of surfactant corresponding to the CEC of the pristine clay mineral has been studied using X‐ray diffraction (XRD), Fourier Transform Infrared (FTIR) spectra. Morphology of the samples was examined by scanning electron microscopy (SEM). The results are supported by the measurements of zeta potentials and contact angles of pristine clay and organoclay samples. From the XRD results, a series of arrangement models of CTA+ in the interlayer of montmorillonite have been proposed as lateral‐bilayer, pseudotrilayer, paraffin‐type‐monolayer and pseudotrilayer, paraffin‐type‐bilayer and pseudotrilayer for 0.5, 1, 1.5, and 2.0 CEC, respectively. FTIR spectrum and contact angle measurements of pristine montmorillonite and organoclays indicated the incorporation of surfactant and the changing of hydrophility in the different OMts. This study demonstrates that not only the arrangement model of surfactant, but also the morphology of organoclay strongly depends on the surfactant packing density within the montmorillonite interlayer space. In addition, it can be also proposed that, the magnitude of surface charge or its distribution on clay mineral might be an important factor for expansion characteristics of organoclay.

Synthesis of multiple platinum-attached iron oxide nanoparticles

Multiple, small platinum (Pt, ∼2 nm) attached iron oxide nanoparticles were successfully synthesized using two different approaches. One approach involves growing Pt nanoparticles onto iron oxide seeds in an organic solvent followed by a phase transfer process. We discovered that the surfactant packing density on the seed NPs played a critical role in multiple Pt nucleation. The presence of Pt nanoparticles on the seeds was confirmed using energy dispersive X-ray spectroscopy and high angle annular dark-field imaging. Alternatively, multiple Pt nanoparticles were deposited onto polyacrylic acid coated iron oxide nanoparticles in aqueous solution where the Pt nanoparticles were linked to the polymer coatings. The capability of growing multiple Pt nanoparticles on oxide seeds will be directly beneficial to both catalytic research and nanomedicine.

Structure of surfactant–clay complexes and their sorptive characteristics toward HOCs

The influence of structural characteristics of series of surfactant–clay complexes on their sorption capacities toward naphthalene were examined, which presented some novel information about how to improve the sorption capacities of the complexes. It was shown that sorption of naphthalene on these complexes strongly depended on the conformations of the adsorbed surfactants, and the organic carbon normalized sorption coefficient ( K oc) value varied in a wide range (5000–20,000 L/kg) with surfactant conformation. That was, K oc value first increased gradually until the maximum and then decreased sharply as the surfactant conformation developed progressively from liquidlike to solidlike (i.e., increasing surfactant packing density and ordering). At low packing density stage the surfactant aggregates have high surface/volume ratio and the attractive forces between surfactant and solute are relatively weak; whereas at high packing density stage interactions between surfactant alkyl chains are strong and more free energy is needed for creating cavities in the surfactant mediums. In both cases the sorption capacities of the complexes are weak. Thereby, the surfactant aggregates can be considered as different partition phases as their conformations changed, which then lead to the variation of K oc value. Controlling the surfactant aggregates within intermediate packing density range can optimize the sorption capacities of the complexes.

Thermodynamics of naphthalene sorption to organoclays: Role of surfactant packing density

Temperature dependence of naphthalene sorption to four organoclays with different surfactant (CTMA +) packing densities was examined. The results showed that both Δ H o and Δ S o increase generally with CTMA + packing density. For organoclays with a low CTMA + packing density, the sorption process is driven by both the enthalpy term ( Δ H o ) and the entropy term ( − T Δ S o ) , with values ranging from −4.7 to − 7.5 kJ mol −1 and −15.9 to − 20.8 kJ mol −1 , respectively. As the CTMA + packing density increases, the sorption process is driven by the entropy term (from −29.2 to − 65.0 kJ mol −1 ) while it is opposed by the enthalpy term (from 7.9 to 40.5 kJ mol −1 ). These results indicate that the enthalpy demand for cavity formation within the surfactant aggregates and the mixing entropy of solute with surfactant aggregates both increase with the surfactant packing density. This means that the surfactant aggregates will form various organic phases as their packing density varies. Controlling the surfactant aggregates within an intermediate packing density range can improve the sorption capacities of the organoclays.

Sorption characteristics of CTMA–bentonite complexes as controlled by surfactant packing density

This work was to examine the relationship between the configuration and sorption characteristics of surfactant–clay complexes. Various amounts of cetyltrimethylammonium bromide (CTMAB) were intercalated into the bentonite matrixes with different layer charges. Packing densities of the adsorbed surfactants and sorption characteristics of the obtained CTMA–bentonite complexes towards phenol and naphthalene were examined. Experimental results indicated that packing density of the adsorbed surfactant was proportional to the surfactant loading amount and layer charge of the bentonite, and sorption capacities of these complexes had a close relationship with the surfactant packing density. That was, with the increase of surfactant packing density, the organic-carbon normalized sorption coefficient ( K oc) first rose till the maximum, and then began to decrease as the packing density further increased. This could be interpreted that increase of surfactant packing density would render the surfactant phases more hydrophobic environment, and the hydrophobic affinity of the surfactant phases towards the solutes thus increased accordingly. However, in the high surfactant packing density region, the densely packed surfactants reduced the available free space for the solutes, resulting in decrease of sorption capacity for these complexes. Hence, with the increase of surfactant packing density, the adsorbed surfactants would form a series of partition phases showing different affinity to the solutes.

Probing the Surface of Polymer Colloids by Conductometric Surfactant Titration

This work revisits the use of surfactant titrations for the characterization of latex particle surfaces. Experiments were performed to study the effect of comonomer composition and the effect of acid comonomers, and the technique is applied to the characterization of particle morphology in composite latices for several different systems. It is confirmed that the packing density of surfactant on a polymer surface is a linear function of copolymer composition. Inclusion of acid comonomers has the expected effect of decreasing the amount of surfactant adsorbed on the polymer surfaces. The usefulness of the technique in the determination of particle morphology is demonstrated, in particular toward the detection of thin layers of either seed or second-stage polymer on the particle surface which are not easily detected by other techniques such as transmission electron microscopy (TEM). Finally, it is shown that the use of acid comonomers in composite particles greatly reduces the usefulness of the surfactant titration technique for morphology characterization. A possible explanation for this effect is proposed.

Changes in the morphology of organoclays with HDTMA + surfactant loading

The detailed understanding of the interlayer structure of organoclays is of importance in the design of organoclay based materials and their industrial applications. In this study, transmission electron microscopy (TEM), scanning electron microscopy (SEM) and X-ray diffraction (XRD) have been used to provide new insights into the interlayer structure and morphology of HDTMA +/montmorillonite organoclays. XRD patterns show that thermal treatment has an important effect on the stability of organoclays, reflected by significant changes in the basal spacing. TEM and SEM micrographs demonstrate that the organoclays with lower surfactant packing density are mainly composed of irregular layer stacking with a number of curved organoclay layers, while those with higher surfactant packing density are mainly composed of regularly intercalated and flat layers. Variations of the interlayer distances exist in all organoclays and are more pronounced in the organoclays with lower surfactant packing density. This study demonstrates that not only the arrangement model of surfactant but also the morphology of organoclay strongly depend on the surfactant packing density within the montmorillonite interlayer space.

Molecular Simulation of the Interlayer Structure and the Mobility of Alkyl Chains in HDTMA+/Montmorillonite Hybrids

Molecular simulation techniques are used to find the basal spacing of organoclay on the basis of the energy minimum, using the canonical NVT ensemble. Then, the interlayer structure and mobility of alkyl chains are explored based on the interlayer atomic density profiles. Besides the basic lateral-monolayer arrangement, lateral-bilayer accompanied by partial a pseudo-trilayer and a transition structure between the two basic lateral models are observed. The later provides an excellent explanation about the reflection at 16 angstroms on XRD patterns in the literature. The atomic density profiles reveal that nitrogen atoms show stronger layering behavior than carbon atoms do. Our simulation demonstrates that the molecular mobility of the confined alkyl chains decreases from lateral-monolayer to lateral-bilayer with the increase of the intercalated surfactant. This is in accordance with the suggestion deduced from experiments. Furthermore, our simulation indicates that the mobility of the alkyl chains strongly depends on the surfactant arrangement rather than the surfactant packing density.

Thermal Characterization of Surfactant-Modified Montmorillonites

AbstractThe thermal stability of surfactant-modified clays plays a key role in the synthesis and processing of organoclay-based nanocomposites. Differential thermal analysis (DTA), thermogravimetric measurement and differential scanning calorimetry (DSC) were used in this study to characterize the thermal stability of hexadecyltrimethylammonium bromide-modified montmorillonites prepared at different surfactant concentrations. Analysis by DSC shows that the molecular environment of the surfactant within the montmorillonite galleries is different from that in the bulk state. The endothermic peak at 70–100°C in the DTA curves of the modified montmorillonites is attributed to both the surfactant phase transformation and the loss of free and interlayer water. With an increase of surfactant-packing density, the amount of water residing in the modified montmorillonite decreases gradually, reflecting the improvement of the hydrophobic property for the organoclay. However, the increase in the surfactant packing density within the galleries leads to a decrease in the thermal stability of the organoclays.With an increase of initial surfactant concentration for the preparation of organoclays, the surfactant- packing density increases gradually to a ‘saturated’ state. It was found that the cationic surfactant was introduced into the montmorillonite interlayer not only by cation exchange but also by physical adsorption.

Silicone Antifoam Performance: Correlation with Spreading and Surfactant Monolayer Packing

The relationship between the spreading of antifoam oils and their performance is much discussed in the literature, but a demonstrated connection between antifoam spreading and performance has been lacking. This paper reports the performance of a poly(dimethylsiloxane) (PDMS)-based antifoam on foam produced by 12 surfactant solutions. These include single or mixed surfactant systems, including impure surfactant mixtures to model fabric washing detergents. The oil film spreading pressure, πo/w, is presented as a simple and relevant measurement of the thermodynamics of antifoam oil spreading. Antifoaming efficacy was measured as the relative reduction in the initial foam height, ΔHrel, using cylinder shake tests at a fixed antifoam dosage. ΔHrel is shown to increase with πo/w, demonstrating a strong statistical correlation between antifoam oil spreading and its performance. Antifoam effectiveness varies with surfactant concentration, surfactant type, and surfactant hydrophobe size and also with increased density of surfactant packing. Surface shear viscosity, μs, was used to quantify surfactant packing. Antifoam effectiveness decreases with increasing surface shear viscosity. This finding provides a potentially useful link between antifoam efficacy and surfactant selection based on well-established surfactant molecular packing parameters. The role of spreading of antifoam oil at the air/surfactant solution interface is investigated. Oil film spreading pressure is shown to decrease by a power law function with increasing surface shear viscosity of the surfactant film. A new fluorescence technique was used to measure the extent of PDMS spreading. Initial results suggest a correlation between the spreading distance and antifoaming performance. An antifoam mechanism is proposed that features antifoam spreading as a direct contributor to bubble film rupture and incorporates surfactant type and concentration, surfactant packing density, and antifoam oil film spreading pressure as factors contributing to antifoam efficacy.

Adsorption of polyoxyethylated nonyl phenols at silica/aqueous solution interfaces

Adsorption isotherms for a range of polyoxyethylated nonyl phenols on silica and methylated silica have been determined. It is shown that the length of the ethylene oxide chain has greater effect on the maximum packing density of surfactant at the interface the more hydrophilic the substrate. Normalisation of adsorption isotherms with respect to the critical micelle concentration of the surfactant reveals a clear difference in the mechanism of adsorption on silica (hydrophilic) and methylated silica (hydrophobic) surfaces. Results are consistent with the attachment of surfactant species via the ethylene oxide chain onto silica and via the nonyl phenol hydrophobe onto methylated silica.