Effect Of Surfactant Structure Research Articles

Dispersing Carbon Nanotubes in Aqueous Solutions of Trisiloxane‐Based Surfactants Modified by Ethoxy and Propoxy Groups

AbstractSilicone surfactants favor spreading at interfaces and siloxane has strong interaction with carbon nanotubes (CNT), thus silicone surfactant may be a good dispersant of CNT. Here, four silicone surfactants (named S1E19, S2E38, S2E16 and S1E16P8) were used to disperse CNT in aqueous solutions. The effects of surfactant structure and concentration on the ability at dispersing CNT were considered. All of the four silicone surfactants can disperse CNT in aqueous solution and the sample with 1,000 mg L−1 S1E16P8 was the best one. The hydrophilic group polyoxyethylene (PEO) and the hydrophobic groups siloxane and polypropylene (PPO) are crucial factors in the ability of dispersing CNT. S2E38 with more ethylene oxide (EO) groups has a stronger ability to disperse CNT than S2E16. The dispersion system provided by S1E19 which contains fewer siloxane and EO groups is relatively unstable and disperses less CNT. These experimental results are explained by molecular dynamics simulation. S2E38 compared with S1E19 and S2E16 has stronger interactions with CNT. The interaction energy of CNT with S1E16P8 which has a PPO moiety but fewer siloxane groups is close to that of S2E16. Furthermore, it can be concluded that these four surfactants are adsorbed on CNT mainly by van der Waals forces and the Si–O–Si chain of silicon surfactant was flexible due to the long Si–C bond and it could easily wrap onto the surface of CNT through hydrophobic and other intermolecular interactions. The hydrophilic part of PEO helped the CNT dispersed in the aqueous solution and prevented CNT from aggregating in water through steric stabilization.

Effects of chemical structure on the dynamic and static surface tensions of short-chain, multi-arm nonionic fluorosurfactants

Fluorinated surfactants with short perfluoroalkyl chains (RF) as potential substitutes for the environmentally questionable, long RF systems are presented. Three types of nonionic hydrophilic-fluorophilic amphiphiles are synthesized and evaluated based on surface activity in equilibrated (static) and non-equilibrated (dynamic) states. Furthermore, several mono- and disaccharide-based fluorosurfactants are also examined as potential non-bioaccumulative alternatives. A correlation between the chemical structure and resulting surface properties is made by comparing RF length, number and size, alkyl-spacer, and hydrophilic moieties. Based on dynamic and static surface tension experiments, the effects of surfactant structure are summarized to provide a basis for the future design of fluorosurfactants. We have found that surfactants with more perfluorinated chains tend to have a higher surface tension reduction, but typically result in slower dynamic behaviors. Using the presented structural characteristics, surfactants with RF<4 can be prepared with static surface tensions as low as 18.1mN/m or reduce surface tension within milliseconds.

Aggregation of alcohols ethoxylates (CiEOj) in dibutoxymethane and surface activity at the water/dibutoxymethane interface

The aggregation of a series of alcohol ethoxylates, CiEOj (j=3–8), in dibutoxymethane (DBM) has been studied by fluorescence probing measurements using sodium pyrenesulfonate (Py-S) as fluorescent probe and by measuring the surface activity at the interface formed by DBM and water. Changes of the ratio of intensities of the fluorescence bands with maximum at 374 and 378nm, I374/I378, and interfacial tension γint plotted against surfactant concentration show discontinuities that have been related to the critical micelle concentration (CMC). Similar values of CMC were obtained by both methods. The effect of surfactant structure on the effectiveness in decreasing γint at the water/DBM interface has been evaluated, i.e. ΠCMC increases by increasing i and j. The dependence of ΠCMC with the effectiveness of adsorption, Γm, the efficiency in reducing γint, pC12, and CMC is discussed. The area per molecule in the saturated interface, amint, is three times higher for non-linear CiEOj surfactants. This difference is attributed to the –iso group which avoids the parallel orientation of the chains into the DBM phase. For C10EOj the free energies of adsorption and micellization have been obtained as a function of j. The results indicate that adsorption is favored in front of the aggregation process.

Effect of the spacer of gemini surfactants on reverse micellar extraction of bovine serum albumin

Reverse micellar extraction has the potential to overcome the hurdles associated with the conventional separation and purification procedures for proteins and hence has been considered as an alternative to conventional techniques. The surfactants used in protein extraction so far have been almost exclusively di-2-ethylhexyl sodium sulfosuccinate and cetyl trimethyl ammonium bromide. In the present paper, we extracted bovine serum albumin (BSA) by using a kind of novel surfactant – gemini surfactant Cm-s-Cm·2Br (m = 12 and 16, and s = 2, 5, 8, and 12). Compared with conventional surfactant reverse micelles, all the gemini surfactant reverse micelles show superior performance. The length and flexibility of spacer s of Cm-s-Cm·2Br contribute to the advantages of gemini surfactant reverse micelles, and increasing hydrophobic alkyl chain length weakens the effect of the spacer. As a result, the minimum content ([surfactant]min) of the surfactant needed for transferring protein into an organic phase is increased in the order of s being 5, 8, 12 (2) sequentially and [surfactant]min for C16-s-C16·2Br is higher than that for C12-s-C12·2Br. The loading capacity of C16-s-C16·2Br reverse micelle is lower than that of C12-s-C12·2Br reverse micelle and C12-12-C12·2Br reverse micelle can load the maximum amount of protein. In the backward extraction process, gemini surfactants with s being 8 and 12 are much more efficient than the other gemini surfactants at pH 4.3. With C12-8-C12·2Br and C12-12-C12·2Br reverse micelles, BSA can be recovered under neutral or basic conditions given a sufficient amount of salt present (while a pH of ca. 4.3 is prerequisite for the recovery of BSA from conventional surfactant reverse micelles) and the efficiency with C12-12-C12·2Br reverse micelle is higher. Since so far the reports about the effect of surfactant structure on protein extraction have been limited, this study should help us know more about how to optimize the surfactant structure.

The critical role of surfactants towards CdS nanoparticles: synthesis, stability, optical and PL emission properties

Cadmium sulfide (CdS) nanoparticles (NPs), prepared by a convenient chemical precipitation method, have been characterized using techniques such as TEM, XRD, zeta potential, absorption and photoluminescence (PL) emission spectroscopy to establish the structure directing role of different cationic and anionic surfactants and their impact on the nanoparticles stabilization. In the synthesis of the CdS NPs, cadmium acetate and sodium sulfide, employed as starting reagents, were dissolved in aqueous solutions of different surfactants to study the effect of their structures on the nucleation, growth, optical and PL emission properties of the NPs. By varying the surfactant species, the CdS NPs have significantly different optical and PL emission properties despite being produced under similar reaction conditions. Depending on the surfactant structure, the growing CdS NPs were stabilized by the surfactants to different extents. For example, in the surfactant with the longest chain length (e.g. cetyltrimethylammonium bromide; CTAB), the CdS NPs were most stable, whereas using a surfactant with a smaller chain length i.e. DTAB, the NPs were unstable for even 1 h. On the other hand, anionic surfactants of even smaller chain lengths were able to stabilize the CdS NPs for quite long times. The generalized study of growth of spherical CdS NPs involves monitoring the kinetics during the progress of the reaction. Additionally, an interesting prominent effect of surfactant structure on the PL emission properties of the NPs has been established under identical reaction conditions.

Reverse micellar extraction of bovine serum albumin – A comparison between the effects of gemini surfactant and its corresponding monomeric surfactant

Gemini surfactant displayed distinct advantages over monomeric surfactant in the liquid–liquid reverse micellar extraction process. First, less amount of gemini surfactant than monomeric surfactant was needed for transferring almost complete bovine serum albumin (BSA) into organic phase from aqueous phase. Second, the loading capacity of gemini surfactant reverse micelle phase was much higher than that of the corresponding monomeric surfactant reverse micelle. Third, efficient backward extraction (75–92%) of BSA could be effected in a wide pH range from 4 to 9 with gemini surfactant reverse micelle while a pH of ca. 4.3 is prerequisite to the recovery of BSA from monomeric surfactant reverse micelle. So far, the reports about the effect of surfactant structure on protein extraction have been limited. This study indicates the important role of the spacer of gemini surfactant in protein extraction process and may provide more knowledge on how to optimise surfactant structure.

Molecular dynamics simulations of the interactions between SWNT and surfactants

To explore the effect of surfactant structure and concentration on the dispersibility of SWNTs in aqueous solutions, the molecular dynamics (MD) simulation was employed to investigate the interactions between the single-walled carbon nanotubes (SWNTs) and four surfactants with and without conjugated structure in water. It is confirmed that surfactants with conjugated structure have a stronger interaction with the SWNT. Aggregate morphologies of surfactants with conjugated structure are different from those without conjugated structure. Multi-layers are prone to form in the former system and the latter one appears semi-micelles on the surface of SWNT. Furthermore, the structures of water and counterion layers around the headgroups are also affected by surfactant structure and concentration. All these changes could present the dispersing ability of surfactants for SWNTs and instruct their applications.

Study of interactions between single-wall carbon nanotubes and surfactant using molecular simulations

Carbon nanotubes (CNT) exhibit interesting electrical and mechanical properties. However, the insolubility of CNT in either water or organic solvents, poses serious obstacles to their future applications. The main problems are strong van der Waals attractive interactions and CNT tendency to form bundles which are very difficult to disrupt. In this study, molecular dynamics and quantum mechanics simulations were conducted to investigate the interactions between a carbonaceous nanoparticle and surfactants. It was found that a benzoic ring in the surfactant molecule improves its binding to the graphitic surface. It was shown that a structure of two stacked graphene layers causes a significant straightening of the aliphatic tail of the surfactant molecule adsorbed on the outer graphene layer. Binding energy calculations showed the effect of surfactant structure and CNT diameter on their interaction intensity.

Influence of Surface Treatment of Ferromagnetic Nanoparticles on Properties of Thermotropic Nematic Liquid Crystals

We observe a strong effect of surfactant structure on the aggregation of ferromagnetic nanoparticles in nematic liquid crystal (LC) hosts, and the colloidal stability and ordering of the resulting nematic matrix. We find that ferromagnetic nanoparticles coated with surfactants that possess terminal mesogenic groups induce minimal aggregation of the nanoparticles, resulting in a 1.5 times reduction in the value of the Frederikscz transition field in comparison with the parent LC host.

Influence of Surfactant Structure on Reverse Micelle Size and Charge for Nonpolar Electrophoretic Inks

Electrophoretic inks, which are suspensions of colorant particles that are controllably concentrated and dispersed by applied electric fields, are the leading commercial technology for high-quality reflective displays. Extending the state of the art for high-fidelity color in these displays requires improved understanding and control of the colloidal systems. In these inks, reverse micelles in nonpolar media play key roles in media and particle charging. Here we investigate the effect of surfactant structure on reverse micelle size and charging properties by synthesizing different surfactants with variations in polyamine polar head groups. Small-angle X-ray scattering (SAXS) and dynamic light scattering (DLS) were used to determine the micelle core plus shell size and micelle hydrodynamic radius, respectively. The results from SAXS agreed with DLS and showed that increasing polyamines in the surfactant head increased the micelle size. The hydrodynamic radius was also calculated on the basis of transient current measurements and agreed well with the DLS results. The transient current technique further determined that increasing polyamines increased the charge stabilization capability of the micelles and that an analogous commercial surfactant OLOA 11000 made for a lower concentration of charge-generating ions in solution. Formulating magenta inks with the various surfactants showed that the absence of amine in the surfactant head was detrimental to particle stabilization and device performance.

Some effects of surfactant structure on the solubilization of salicylic acid

Some effects of surfactant structure on the solubilization of salicylic acid

Effects of Surfactant Structure on the Phase Inversion of Emulsions Stabilized by Mixtures of Silica Nanoparticles and Cationic Surfactant

Silica nanoparticles without any surface modification are not surface active at the toluene-water interface due to their extreme hydrophilicity but can be surface activated in situ by adsorbing cationic surfactant from water. This work investigates the effects of the molecular structure of water-soluble cationic surfactant on the surface activation of the nanoparticles by emulsion characterization, adsorption and zeta potential measurements, dispersion stability experiments, and determination of relevant contact angles. The results show that an adsorbed cationic surfactant monolayer on particle surfaces is responsible for the wettability modification of the particles. In the presence of a trace amount of cationic surfactant, the hydrophobicity of the particles increases, leading to the formation of stable oil-in-water O/W(1) emulsions. At high surfactant concentration (>cmc) the particle surface is retransformed to hydrophilic due to double-layer or hemimicelle formation, and the concentration of the free surfactant in the aqueous phase is high enough to stabilize emulsions alone. O/W(2) emulsions, probably costabilized by free surfactant and particles, are then formed. The monolayer adsorption seems to be charge-site dependent. Thus, using single-chain trimethylammonium bromide surfactants or a double-head gemini cationic surfactant, the hydrophobicity of the particles achieved is not sufficient to stabilize water-in-oil (W/O) emulsions, and no phase inversion is induced. However, using a double-chain cationic surfactant, the chain density on the particle surfaces endows them with a hydrophobicity high enough to stabilize W/O emulsions, and double phase inversion, O/W(1) --> W/O --> O/W(2), can then be achieved by increasing the surfactant concentration.

Retention of CO2-Foaming Agents on Chalk: Effects of Surfactant Structure, Temperature, and Residual-Oil Saturation

Summary Injection of carbon dioxide (CO2) is a well-known enhanced-oil-recovery (EOR) technique. Formation of stable foam inside the reservoir can improve macroscopic sweep efficiency. On the other hand, retention of surfactants decreases the cost-efficiency of the EOR process. This paper presents flow-through retention experiments with CO2-foaming agents on outcrop Liege chalk plugs at two different temperatures: 55 and 70°C. Two branched ethoxylated (EO) sulfonates with different ethoxylation degree, S1 (EO = 7) and S2 (EO = 12), were used. The aim was to investigate the effect of ethoxylation degree on surfactant retention. Furthermore, the effects of temperature and residual oil on surfactant retention were studied. The effect of waterflooding followed by CO2 flooding on surfactant retention at reservoir conditions was also examined. Partitioning of the foaming agents between water and oil phases was studied. Results show that increasing the ethoxylation degree of the surfactant decreases the retention on chalk cores saturated with formation water at 55°C. S2, which was found to give the lowest retention at 55°C, was found to have a higher retention at 70°C. The presence of residual-oil saturation after waterflooding (Sorw) decreased the retention of S1 and increased the retention of S2 in comparison to the absence of residual oil. The retention of S2 after waterflooding followed by CO2 flooding at 340 bar and 55°C was in the same range as retention on 100%-water-saturated core, but significantly lower than retention in residual-oil-saturated cores. The experiments have shown that not only are surfactant structure and temperature important for the retention of surfactants, but also the presence of oil.

Effect of nonionic surfactant molecular structure on cloud point extraction of phenol from wastewater

Above a temperature known as the cloud point, aqueous solutions of nonionic ethoxylated surfactants separate into two phases: a coacervate phase concentrated in surfactant and a dilute phase with low surfactant concentration. The coacervate phase contains surfactant aggregates which are micelles or micelle-like and will solubilize any organic solutes originally present in the water, resulting in a liquid–liquid extraction known as a cloud point extraction (CPE). Removal of pollutants from water is an important potential application of CPE and we use phenol as a model organic pollutant in this work. In this study, we investigate the effect of surfactant structure on important equilibrium CPE parameters like cloud point, fractional coacervate volume, and phenol and surfactant partition ratios for alcohol ethoxylates (AEs), a major class of nonionic surfactants. Pure, homogeneous surfactants with linear hydrophobes are studied as well as commercial heterogeneous AEs with both linear and branched hydrophobes. The effects of degree of polymerization in the polyethoxylate group (EO number) as well as hydrophobe size (alkyl carbon number) are systematically investigated as well as the effect of hydrophobe branching. The solubilization equilibrium constant is shown to increase linearly with EO number and is unaffected by alkyl carbon number or hydrophobe branching from which we deduce that the phenol is solubilized with the benzene ring at the surface of the micelle core and the hydroxyl group having attractive interactions with the polyethoxylate chains in the palisade layer of the micelle. The effects of surfactant structural features on net intermicellar attractive forces, micellar excluded volume, and solubilization capacity are discussed. A model is developed which can predict the phenol partition ratio at a given temperature for any AE surfactant dependent on only one simple measured parameter: fractional coacervate volume. This universal model is somewhat unique to phenolics or short chain alcohols because of their solubilization loci in the micelle and mechanism of solubilization. Guidelines for surfactant selection for CPE of phenol or similar solutes are outlined.

Effect of Surfactant Structure on the Stability of Carbon Nanotubes in Aqueous Solution

The suspending behaviors of multiple-wall carbon nanotubes (MWNTs), including pristine MWNTs (p-MWNTs) and acid-mixture-treated MWNTs (MWNTCOOH), stabilized by cationic single-chain surfactant, dodecyltrimethylammonium bromide (DTAB), and cationic gemini surfactant hexyl-alpha,beta-bis(dodecyldimethylammonium bromide) (C 12C 6C 12Br 2) were studied systematically. The surfactant structure influences the suspendability of MWNTs dramatically as well as the surfactant adsorption behavior on the nanotubes. Although both the surfactants can disperse the MWNTs effectively, they actually show different stabilizing ability. DTAB is not capable of stabilizing these two MWNTs below critical micelle concentration (CMC). However, C 12C 6C 12Br 2 can suspend both the nanotubes effectively even well below its CMC. Moreover, the adsorption of these two surfactants reaches equilibrium at twice the CMC with the original MWNT concentration of 2 mg/mL, 2 mM for C 12C 6C 12Br 2, and 30 mM for DTAB. After the adsorption equilibrium, the maximum amounts of the two suspended MWNTs in C 12C 6C 12Br 2 solution are about twice as much as those in DTAB solution. The strong hydrophobic interaction among the C 12C 6C 12Br 2 molecules and between the C 12C 6C 12Br 2 molecules and the nanotubes as well as the high charge capacity of C 12C 6C 12Br 2 lead to its much stronger adsorption ability on the MWNTs and result in its superior stabilizing ability for the MWNTs in aqueous phase. The gemini surfactant provides a possibility to effectively stabilize the MWNTs in aqueous solutions even at very low surfactant concentration well below its CMC.

Effect of surfactant structure on catalysis of microemulsion for photoisomerization of trans-stilbene

In this study, the photoisomerization of trans-stilbene was carried out in water in oil (W/O) microemulsions by using sodium dodecyl sulfate (SDS), dodecyl trimethyl ammonium bromide (DTAB) and cetyl trimethyl ammonium bromide (CTAB) as surfactant, respectively. The catalytic effect of microemulsion on this reaction is closely related to the structure of surfactant. When there is no photosensitizer 9,10-dicyanoanthracene (DCA), the surfactant with shorter hydrophobic chain is preferred, while in the presence of DCA, the surfactant with anionic polar group is preferred.

Effect of surfactant structure on the residual fluorescence of micelle-based fluorescent probes

In the present paper we have investigated some photo-physical characteristics of different micellar-based fluorescent probes containing a fluorophore (pyrene) and a quencher unit (dodecyl-dioxo 2,3,2). The fluorescent response of the probe in the presence of Cu(II) ions was studied using different micellar substrates, and it was found that the pH at which the On–Off jump occurs is not influenced by the chemical structure of surfactant. In addition, the experimental residual fluorescence is not proportionally affected by microviscosity or by the size of the micellar aggregates. The signal of the native fluorescence of pyrene was observed even when the quencher's occupancy number was greater than one. Moreover, we observed discrepancies between experimental values and calculated residual fluorescence using Laplace data. These results were interpreted suggesting that the residual fluorescence has two main components, one that seems to be independent on micellar properties, while the other is directly related to location of molecules inside the surfactant aggregates that serve as substrate.

Hydrocarbon Surfactants for CO2: An Impossible Dream?

It used to be thought that hydrocarbon ionic surfactants were incompatible with dense liquid or supercritical CO2, and fluorination of the chains was necessary to achieve any appreciable solubility. Here it is shown that branching of pendant hydrocarbon chains, especially methylation of the tips, can lead to substantial solubility enhancements in CO2 over normal linear chain hydrocarbon surfactants. In addition, increasing the number of these t-butyl-tipped groups from single-chained, through twin-tailed, to triple-chain surfactants has a profound effect on CO2 compatibility. High-pressure interfacial tension, small-angle neutron scattering, and near infrared spectroscopy have been applied to reveal the effects of surfactant structure on surface activity, aggregation, and solubility in CO2.

Interfacial tension of the aqueous two-phase systems of cationic-anionic surfactant mixtures

Interfacial tensions of the aqueous two-phase systems formed by cationic-anionic surfactant mixtures were measured using spinning drop method. The effects of surfactant structure, molar ratio of cationic to anionic surfactants, surfactant concentration, salt, and temperature on the interfacial tensions were investigated. It was shown that the values of the interfacial tensions of the aqueous two-phase were in the scale of ultra-low interfacial tensions at certain molar ratios of cationic to anionic surfactants. Three types of interfacial tension curves were observed. The first curve comprised two curves that were located on either side of 1:1 molar ratio, and the interfacial tension decreased with the increase of excessive surfactant components. The second one was a saddle-shaped curve that strode over the 1:1 molar ratio. The third type was a saddle-shaped curve that was located beside the 1:1 molar ratio. The types of interfacial tensions depended on the molecular structure of the surfactants such as the hydrophilic groups and the lengths and symmetry of hydrophobic chains.

Sphere‐to‐Rod Transitions in Cationic Micelles: Quaternary Ammonium Halides+Phenol+Water Systems

Viscosity measurements on aqueous micellar solutions of cationic surfactants containing phenol with and without sodium bromide were made to study the sphere‐to‐rod transitions. Effect of surfactant structure (nonpolar tail, polar head group sizes and counterion), temperature, and phenol and sodium bromide concentration on the viscosity behavior of the surfactant solution is discussed. The sphere‐to‐rod transition is usually indicated by a marked increase in viscosity. While low temperature, high surfactant concentration, presence of salt, and hydrophobic nature of surfactant all favor the formation of rod‐like micelles, the presence of phenol showed peculiar behavior. Initial additions of phenol (up to about 2.5 wt%) showed a marked increase in viscosity, independent of the nature and concentration of surfactant and temperature; lower viscosities were observed at higher phenol concentration. Conductance and sound velocity results support the viscosity results.