Dynamic Surface Tension Measurements Research Articles

Influence of thermally induced structure changes in diluted β-lactoglobulin solutions on their surface activity and behavior in foam fractionation

Surface activity is an intrinsic protein feature, leading to the capability of aqueous protein solutions to form foam. This feature provides opportunities for downstream processing, such as usage of foam fractionation for purification. In order to investigate the impact of the surface activity on the performance of the foam fractionation process, protein solutions with different surface activity were produced by different thermal denaturation of aqueous β-lactoglobulin solutions. The effectiveness of the denaturation procedure was verified with circular dichroic spectroscopy, and the impact on surface activity was determined via dynamic surface tension measurement. The increased surface activity resulted in higher foamate flow rates. Furthermore, the effects could be correlated with secondary structure changes and with the dynamic surface pressure. The new result of this study is that the effect of the denaturation of a protein on foam fractionation depends on the protein concentration. At the lower feed concentration, effects became visible, which could not be observed at the higher concentration.

Protective effects of aerosolized pulmonary surfactant powder in a model of ventilator-induced lung injury.

Mechanical ventilation may contribute to the impairment of the pulmonary surfactant system, which is one of the mechanisms leading to the progression of acute lung injury. To investigate the potential protective effects of pulmonary surfactant in a rat model of ventilator-induced lung injury, the surfactant powder was aerosolized using an in-house made device designed to deliver the aerosolized powder to the inspiratory line of a rodent ventilator circuit. Rats were randomized to (i) administration of aerosolized recombinant surfactant protein C based pulmonary surfactant, (ii) intratracheally instillation of the same surfactant re-constituted in saline, and (iii) no treatment. Animals were monitored during 2h of high-tidal volume mechanical ventilation, after which rats were sacrificed, and further analysis of lung mechanics and surfactant function were completed. Blood gas measurements during ventilation showed extended maintenance of oxygen levels above 400mmHg in aerosol treated animals over non-treated and instilled groups, while total protein analysis showed reduced levels in the aerosol compared to non-treated groups. Dynamic captive bubble surface tension measurements showed the activity of surfactant recovered from aerosol treated animals is maintained below 1 mN/m. The prophylactic treatment of aerosolized surfactant powder reduced the severity of lung injury in this model.

Salt Effects on Formation and Stability of Colloidal Gas Aphrons Produced by Anionic and Zwitterionic Surfactants in Xanthan Gum Solution

This work is devoted to the influence of NaCl salt concentration on the formation and stability of colloidal gas aphrons (CGA) produced by the anionic surfactant sodium dodecyl sulfate (SDS) and zwitterionic surfactant coco amido propyl betaine (CAPB) in the presence of xanthan gum (XG) as a stabilizer. Dynamic surface tension measurements as well as volume and half-life time of the produced foams are considered for stability analysis. A sharp decrease of the half-life time and volume of the CGAs at NaCl concentrations larger than 20,000 ppm was observed, which was attributed to the precipitation of SDS in the solution. The mentioned SDS precipitation altered the dynamic surface tension behavior, dilational surface elasticity, and turbidity of the solution. The main reason for the precipitation of SDS is the increased Krafft point caused by the addition of salt. However, for the zwitterionic surfactant CAPB, the effects of added NaCl on the interfacial properties required for CGAs production was negligible due to the simultaneous effects on the cationic and anionic head groups in the CAPB leading to negligible changes in the net repulsion forces. Yet, an overall reduction in the half-life time of CGAs with increasing salt concentration, even when we have no precipitation, was observed for both surfactants, which could be explained by the reduction in the ability of XG to increase the viscosity with increasing salt concentration.

The dynamics of surface adsorption and foam formation of carbonate modified nonionic surfactants

For a group of nonionic surfactants with polar head groups synthesized by copolymerization of ethylene oxide and carbon dioxide, the adsorption dynamics at the water-air surface were investigated by dynamic surface tension measurements using the maximum bubble pressure technique. From the apparent diffusion coefficients at different concentrations, the diffusion coefficients of surfactant monomers and micelles were deduced and compared to conventional nonionic surfactants. These transport properties were correlated with results from foamability studies in Bartsch tests as well as sparging tests. The obtained foam volumes show a good correlation with the diffusion coefficients of the surfactant monomers multiplied with the concentration of available monomers, the cmc. The same relation is found for the foam stability, expressed in the foam half lifetime. The correlations show for surfactant solutions well above the critical micelle concentration that the transport of surfactant monomers to the interface rather than the micelles is the essential step in both, the foam formation as well as the foam stabilization, following a simple Fickian diffusion law.

Mechanism of Time-Dependent Adsorption for Phosphatidylcholine onto a Clean Air-Water Interface from a Dispersion of Vesicles: Effect of Temperature and Acyl Chain Length.

Dynamic surface tension measurements were used to track adsorption kinetics for dilauroylphosphatidylcholine (DLPC) or dimyristoylphosphatidylcholine (DMPC) from monodisperse vesicle dispersions to an air-water interface at elevated temperatures ≥30 °C. Effects of vesicle concentration, aqueous solubility of the lipids, and temperature T on the adsorption kinetics were determined, and the controlling transport pathway was identified. Adsorption dynamics were tracked for 0.1-10 mM DLPC at 30 and 38 °C and for 1-10 mM DMPC at 30, 50, and 58 °C. Experimental results were compared to theoretical predictions for a reaction-enhanced, molecular transport mechanism, which was previously shown to effectively predict DLPC adsorption kinetics at 22 °C. At higher temperatures, for DLPC concentrations ≥0.25 mM or DMPC concentrations ≥1 mM, a weak dependence of adsorption time on concentration was observed, again consistent with the reaction-enhanced molecular pathway. Molecular release rates from vesicles increased with increasing temperature or decreasing acyl chain length. At equivalent ratios T/Tm of the dispersion temperature to the lipid chain phase transition temperature Tm, measured adsorption times for DLPC were approximately 10-fold shorter than those for DMPC, suggesting that the fluidity of the acyl tails is not the only lipid property determining adsorption rates. Despite the significant difference in aqueous solubility and chain phase transition temperature between DLPC and DMPC, the results provide further evidence for an adsorption mechanism that is well described by diffusion of molecular lipid, with rates of molecular diffusion near the interface enhanced by release from nearby vesicles.

Synthesis and Properties of Novel Catanionic Surfactant Phosphonium Benzene Sulfonate

Abstract A new type of catanionic surfactant phosphonium benzene sulfonate was synthesized by quaternization of triphenyl phosphine with dimethyl carbonate and followed by anion exchange with alkyl benzene sulfonic acid. The molecular structure was characterized by FT-IR, 1H-NMR, and 31P-NMR. The thermal stability of phosphonium benzene sulfonate was evaluated by thermogravimetric analysis (TGA). Its surface properties were studied systematically through equilibrium surface tension, electrical conductivity, and dynamic surface tension measurements. The wettability, foam properties, and emulsification of phosphonium benzene sulfonate were estimated in this paper. TGA results revealed that it has an excellent thermostability and could be used below 350 °C. Equilibrium surface tension results indicated that it has a low critical micelle concentration (CMC, about 0.10 mmol/L), lower than that of ammonium benzene sulfonate and sodium dodecyl benzene sulfonate. Furthermore, the micellization of phosphonium benzene sulfonate in aqueous solution is an entropy-driven spontaneous process. The adsorption process of phosphonium benzenesulfonate at the air-liquid interface is controlled by hybrid kinetic adsorption. Moreover, it has excellent wetting and emulsifying properties and low foam properties.

Interfacial Dynamics and Its Relations with ?Negative? Surface Viscosities Measured at Water?Air Interfaces Covered with a Cationic Surfactant.

We studied the dynamics of a cationic surfactant monolayer, Gemini 12-2-12, at the air?water interface for surfactant aqueous solutions at concentrations below the critical micelle concentration. We present surface rheology experiments performed in a Langmuir trough by the oscillatory barrier technique. From these, we found negative surface viscosities at certain frequencies. We demonstrate that this unphysical result is a consequence of an unconsidered surfactant dynamics within the interfacial region. By surface pressure relaxation experiments, after a sudden modification of the interfacial area and by dynamic surface tension and surface potential measurements, several relaxation phenomena and relaxation times were identified. We found that surfactant adsorption and desorption processes are asymmetric: the characteristic times and the number of processes involved in the mechanisms of adsorption and desorption are different. This asymmetry invalidates the usual data analysis procedure that leads to the negative viscosities. Similar mechanisms could be at the origin of the negative viscosities reported in other systems, a possibility that remains to be explored.

Comparative studies on the surface/interface properties and aggregation behavior of mono-rhamnolipid and di-rhamnolipid

The surface properties of mono-rhamnolipid (Rha-C10-C10, R1) and di-rhamnolipid (Rha-Rha-C10-C10, R2) were investigated after separation and purification. The effects of environmental factors on equilibrium surface tension of these surfactants were studied by changing the temperature, salinity, and pH. Results show that R1 possesses a better surface activity than R2, but both are stable at low pH values and high temperature. Moreover, the diffusion and adsorption processes of R1 and R2 were studied by dynamic surface tension measurements. The initial adsorption processes of R1 and R2 were diffusion-controlled, and the effective diffusion coefficient of R1 was higher than that of R2 at the same concentration. We also monitored the dynamic interfacial tension curves of R1 and R2 with or without aging at high temperature, revealing that both feature high temperature resistance, but R1 exhibited a better interfacial activity than R2. For aggregation behavior in the bulk phase, dynamic light scattering and UV ˗ vis spectrophotometry were used to measure and observe the aggregation of rhamnolipids R1 and R2 at different temperatures and pH values. Results show the vesicle-to-micelle transformation of R1 and R2 aggregates with decreasing pH. This result is attributed to the considerable influence of solution pH to the dissociation degree of rhamnolipids. Thus, pH values significantly influence particle size distribution.

Adsorption of β-lactoglobulin A on gold surface determined in situ by QCM-D measurements

In this work, we described in detail the physicochemical properties of β-lactoglobulin isoform A (LGB-A) in a bulk solution and on a gold surface. DLS studies confirmed that the LGB trend for aggregation and the formation of larger oligomeric forms are dependent on environmental conditions. The isoelectric point of the system was determined using dynamic surface tension measurements and electrophoretic mobility, which are located at pH 5.45 and 4.8, respectively. The use of these methods has provided valuable information about the conformation stability of the LGB molecule in the electrolyte solution. The properties of self-assembling the LGB-A layer adsorbed on the gold surface were estimated in situ using a quartz crystal microbalance with dissipation monitoring (QCM-D) and Fourier transform infrared spectroscopy (FTIR) as a function of protein concentration, pH, and ionic strength. Based on the measurement of changes in the surface tension at the gas/liquid interface as well as the efficiency of adsorption at the liquid/solid interface using the QCM-D method, we estimated the preferred side-on orientation of the LGB-A molecules. The effectiveness of LGB-A adsorption on the gold surface was mostly controlled by electrostatic interactions that were confirmed by a very strong correlation with the zeta potential of the protein molecule and the gold surface. FTIR spectra shows significant changes in the secondary structure of the adsorbed protein. Particular changes concern the content in the structure β-sheet and random coil forms, with the α-helix content unchanged. The behaviour of the LGB molecule at the interface is in accordance with the direction of the dipole moment of the molecule as determined by molecular dynamics (MD).

Interaction of salicylic acid analogues with Pluronic® micelles: Investigations on micellar growth and morphological transition

The aqueous solutions of PEO-PPO-PEO type triblock copolymers (Pluronic® P123, P103 and P105) with variations in their total molecular weight and block contribution are investigated for encapsulation of drugs. Two drugs, namely Aspirin (AS) and 5-Methyl Salicylate (MS) having different partition coefficient was studied. The changes in size and micellar dynamics due to formation of drug-polymer complexes are investigated using dynamic light scattering (DLS), dynamic surface tension (DST), Nuclear Magnetic Resonance (NMR) and Small angle neutron scattering (SANS) measurements. Increase in apparent hydrodynamic diameter (Dh) measured by DLS suggests the growth of micelles, which is equally supported by calculated micellar parameters from SANS measurements. The solubilization of drugs within micelles alters the diffusivity of the monomers due to stronger hydrophobic forces leading to highly stable aggregates as concluded by dynamic surface tension measurements. The significant increase in micellar size was noticed in presence of MS, while AS had less prominent effect on micellar growth. The preferential partitioning of drug into the hydrophobic micellar core is the driving force for the possible formation of higher morphologies. The hydrophobic interactions between MS and P123 are confirmed by 1H NMR. The differences in molecular interactions leading to distinct morphological behavior are explained by the localization of drug within nanocarriers.

The Effect of Particles on Surface Tension and Flotation Froth Stability

It is widely accepted that particles stabilise flotation froths and that stable froths result in improved flotation performance. Predicting the effect of particle addition on froth stability is, however, challenging. Dynamic surface tension measurement using maximum bubble pressure presents an attractive technique to investigate the effect of surfactant and particles at the air-water interface. The range of bubble lifetimes that can be studied (typically 0.1 to 60 s) is analogous to variations in air rate in flotation cells, and the corresponding changes in surface tension give an indication to the diffusion and adsorption rates of particles at the interface. In this paper, we use dynamic surface tension measurements to investigate the effect of particles on bubble surfaces at the microscale and link this to bulk froth stability measurements carried out using a froth column. Using the maximum bubble pressure method, the results show that the addition of particles results in lower surface tension, both at the dynamic (i.e. short) bubble lifetimes and towards equilibrium (i.e. 60 s bubble lifetime). This corresponds with the bulk froth stability measurements, where the three-phase system yielded more stable froths than the surfactant only system. Furthermore, increased particle loading at the air-water interface, whether through higher surfactant concentrations or lower air rates (longer bubble lifetimes), gave lower surface tension and higher froth stability. This demonstrates the link between bubble loading and froth stability. It is proposed that the maximum bubble pressure technique can be used to predict froth stability for two- and three-phase systems, enabling the effect of particle loading to be accounted for and quantified. Moreover, the technique has the potential to allow rapid determination of particle and surfactant diffusion at the air-water interface and prediction of the corresponding effect on bulk froth behaviour.

Formation and stability of colloidal gas aphron based drilling fluid considering dynamic surface properties

The dynamic surface behavior of surfactant in polymer solution is essential information to understand the formation and stability of Colloidal Gas Aphrons (CGAs). Measurements of dynamic surface tension, volume and half-life time of CGAs as well as size distribution of bubbles have been performed in this work. Three different surfactants of anionic sodium dodecyl sulfate (SDS), nonionic Triton X-100 (4-(1,1,3,3-tetramethylbutyl)phenyl-polyethylene glycol) and zwitterionic cocamidopropyl betaine (CAPB) were used for producing the CGAs in presence of the polymer Xanthan Gum (XG) as stabilizer. In addition, the influence of stirring time and speed as well as polymer and surfactant concentrations were investigated with respect to CGA volume and half-life time. The results showed an approximately linear relationship between half-life time and volume of CGAs for different stirring times and speeds. This reveals that at fixed polymer and surfactant concentrations, the half-life time mostly depends on the CGA volume. The XG concentration has also a strong influence on CGAs formation and stability. The increase in XG concentration reduces the CGA volume but significantly increases the half-life time. The dynamic surface tension of the applied surfactant has a major influence on CGA formation as only surfactants of a certain type and at a respective concentration provides the conditions for proper CGA formation. However, the stability of CGAs is rather complex and can be affected by the electrostatic nature of the surfactant and possible interactions with other compounds. Ionic SDS and CAPB surfactants allow producing CGAs of significant stabilities in contrast to the poor stability of CGAs made by the nonionic Triton X-100. The increase of the surfactant concentration increases the CGA volume as well as the half-life time, even at concentrations above the CMC. However, for the determination of optimum surfactant concentrations, economic issues should also be considered.

Synergism and Interfacial Property Study of Sodium Lauryl Ether Sulfate and Polysorbate 80 at the Air–Water Interface

AbstractAdsorption and micellization behavior of sodium lauryl ether sulfate (SLES) and Polysorbate 80 (PS80) was investigated using equilibrium and dynamic surface tension and foaming measurements. Critical micelle concentration (CMC) was determined using surface tension and dye (Sudan red III) solubilization method with the help of the Wilhelmy plate method and ultraviolet–visible spectroscopic method, respectively. The strength of the interaction for micellization and adsorption between the mixtures are calculated using the β interaction parameter. The SLES‐PS80 mixture shows synergistic interaction with β = −8.0. The dynamic and foaming behavior at constant bulk concentration showed a higher t * value with lowest foaming at 50% mole fraction of SLES. The emulsification index (EI) of pure SLES, pure PS 80, and 1:1 SLES‐PS 80 composition was studied using neem, karanja, and sunflower oils. Karanja and sunflower oils show a higher EI for mixture than the pure surfactants, whereas neem shows a lower EI for mixture than pure surfactants.

Molecular insights into sodium dodecyl sulphate mediated control of size for silver nanoparticles

A specific condition directing the formation of silver nanoparticles (AgNPs) with narrow size-distribution in aqueous sodium dodecyl sulphate (SDS) solutions, using trisodium citrate (TSC) is investigated. The peculiar control in distribution of size for AgNPs in presence of 35 mM (~1 wt%) aqueous SDS solution as stabilizing agent is evident. Water penetration through packed PTFE powder and dynamic surface tension measurements demonstrate formation of stable aggregates at the given condition and accordingly reduced numbers of free SDS monomers diffusing to the growing AgNP interface. The smaller size of AgNPs was also confirmed by TEM images, which illustrates mixed morphologies for the nanoparticles in an aggregated state. The NMR experiments reveal strong hydrophobic interactions between the alkyl chains of SDS molecules adsorbed on AgNPs. These results also indicate that TSC is molecularly distributed in bulk phase without binding to the SDS micellar aggregates. A plausible molecular mechanism is proposed based on diffusion of Ag+ ions to the growing AgNP surface along with SDS monomers and submicellar aggregates, subsequently forming bigger size AgNPs. The narrow distribution of size for AgNP in presence of 35 mM SDS is arguably driven by the capping of stable SDS micelles or monomers around growing nuclei and restricted number of SDS monomers and submicellar aggregates responsible for supply of Ag+ at the growing AgNP nuclei.

Mechanism of Time-Dependent Adsorption for Dilauroyl Phosphatidylcholine onto a Clean Air-Water Interface from a Dispersion of Vesicles.

This study focused on mechanisms of adsorption for dilauroyl phosphatidylcholine (DLPC) from a dispersion of large, unilamellar vesicles (LUVs) onto a clean air-water interface. The adsorption kinetics were tracked using dynamic surface tension measurements for 0.01-10 mM concentrations of DLPC, contained within monodisperse LUVs with mean diameters between 100 and 300 nm. Any lipid in excess of the solubility limit, determined to be 1.1(±0.7) × 10-5 mM (1.1 × 10-8 M), was assumed to be in vesicle form. The adsorption rate was found to increase with increasing lipid concentration and decreasing vesicle diameter, indicating a clear mechanistic role for the vesicles. An induction regime was observed, during which lipid adsorption occurred without significantly changing the surface tension. Pressure-area isotherm data suggested that the surface concentration at the end of this induction period was ∼50% of the concentration at saturation, with the latter estimated as 4.2(±0.7) × 10-6 mol/m2. Convection was also introduced into these experiments to probe the importance of bulk transport mechanisms to the overall kinetics. Theoretical expressions for possible contributing mechanisms and pathways, via molecular and/or vesicle transport, were developed and used to predict associated transport time scales for different scenarios. These theoretical time scales were compared to experimentally measured characteristic times for a variety of DLPC concentrations, vesicle diameters, and convection rates. For DLPC concentrations ≥0.25 mM, our results were consistent with the monolayer formation arising from a molecular transport mechanism that is enhanced by vesicle-to-monomer exchange beneath the interface. At lower concentrations, experimental rates of adsorption increased with increasing convection, and a strong effect of lipid concentration was also observed. For DLPC ≤0.25 mM, transport controlled by direct interfacial vesicle adsorption reasonably captured the observed effect of lipid concentration; however, neither monomer nor vesicle pathway mechanisms captured the influence of convection. Understanding the adsorption kinetics for such nearly insoluble surfactant systems is important in several areas, including food emulsification, foam or microbubble formulation, spray drying techniques, and therapeutics.

Dynamic Surface Tension Measurements for Animal Blood Analysis and Correlations with Related Biochemical Parameters

The dynamic surface tension (DST) data and biochemical parameters (BCP) of animal blood have been obtained. A strong positive correlation was found for goats between σ1, σ2, σ3 (DST) and sodium levels (BCP), λz (DST)—with the levels of lipid and sodium (BCP); whereas a strong negative correlation was found for goats between σ3 (DST) and the level of total protein and chloride (BCP), λz (DST)—with the level of albumin (BCP). A strong positive correlation was found for horses between σ1, σ2, σ3 (DST) and lipid levels (BCP); λ0 (DST)—the level of chloride (BCP); λz (DST)—the level of albumin and chloride (BCP). A strong negative correlation was found for horses between σ1 (DST) and sodium level (BCP); σ2, σ3 (DST) and chloride level (BCP); λ0 (DST) and lipid level (BCP). Some moderate and weak correlations of different types were also found, but these have less importance for practical usage. The particular correlations between BCP and DST data of the same serum samples allowed DST data to be checked directly at the farm, and requiring the use of more expensive and time-consuming biochemical analysis only in cases of high necessity for a particular animal.

Thixotropic bulk elasticity versus interfacial elasticity in Xanthan Gum surfactant mixed solutions

Dynamic surface behavior and interfacial rheology of Xanthan Gum (XG) polymer and surfactant mixed aqueous solutions have been investigated in this work using Profile Analysis Tensiometry (PAT). The interactions of XG with anionic sodium dodecyl sulfate (SDS) and nonionic triton X100 surfactants were studied. The results demonstrate a significant influence of the XG thixotropic bulk rheology behavior on dynamic surface tension measurements. The time dependent thixotropic force (due to the movement and rearrangement of XG molecules under applied shear and relaxation time afterwards) was evaluated using a high precision buoyancy force measurement system. This extra thixotropic force can affect the expected balance of gravity and surface forces in the Gauss-Laplace equations and causes an abnormal increasing dynamic surface tension for aqueous XG solutions versus time in absence of surfactants. It is shown that the thixotropic property of this polymer can also induce an additional surface elasticity which can be measured by drop oscillation experiments via PAT. Therefore, surface elasticity measurements can be proposed as a novel method for determining the extent of fluid thixotropy. The results also showed strong interaction between XG and SDS molecules. At SDS concentrations below critical micelle concentration (CMC), the equilibrium surface tension of XG-SDS solution decreased considerably with increasing XG concentration. This interaction originates from the anionic nature of both XG and SDS molecules. The surface tension behavior of mixed solutions of XG and Triton X100 molecules did not show significant interactions. The obtained results allow improving the ability of the Gauss-Laplace Equation to calculate more accurately the dynamic surface tension of thixotropic fluids.

The colloidal properties of alkaline‐soluble waterborne polymers

ABSTRACTWaterborne polymer dispersions are widely used in coatings and graphic arts markets as environmentally friendly and more sustainable alternatives to solvent borne binders. Traditionally, waterborne (meth)acrylic dispersions are prepared by emulsion polymerization using low molar mass surfactants as a key ingredient to control particle size. However, these surfactants can have a negative influence on the performance of coatings such as reduced water resistance and adhesion. To mitigate the negative effects of surfactants, polymer latexes have been developed that employ alkaline‐soluble polymers as the sole stabilizer for a subsequent emulsion polymerization step. In this way surfactant‐free polymer dispersions are obtained. Despite the high commercial impact and relevance of this technology, fundamental studies regarding the physicochemical properties of the alkaline‐soluble polymers are lacking. In this article, the synthesis and colloidal properties of alkaline‐soluble waterborne methacrylic copolymers are reported. The dissolution behavior and colloidal properties of these alkaline‐soluble polymers were studied as function of molar mass, acid content, and pH. The dissolving polymer particles were characterized using static and dynamic light scattering, static and dynamic surface tension measurements, and cryogenic‐transmission electron microscopy analysis. It is concluded that the dissolution mechanism of alkaline‐soluble polymers follows a gradual process. As the pH increases deprotonation of the carboxylic acid groups swells the particle enhancing the further swelling with water. At a certain amount of base, the particles disintegrate into small polymer aggregates while the most water‐soluble polymer chains are dissolved in the water phase. An important learning is that part of the alkaline‐soluble polymer resides in very small particles (<5 nm). The formation of these polymer particles below 5 nm was not reported previously and offers a new insight into the dissolution mechanism of alkaline soluble polymers. The most important parameter steering this process is the acid value of the polymer, while the molar mass plays a modest role. The understanding gained in this study can be used to further advance alkaline‐soluble polymers as stabilizer in surfactant‐free emulsion polymerization technology, improving the performance of a wide range of industrially relevant coatings. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018, 135, 46168.

Ion Pairing and Adsorption of Azo Dye/C16TAB Surfactants at the Air-Water Interface.

Mixed layers of 6-hydroxy-5-[(4-sulfophenyl)azo]-2-naphthalenesulfonate (Sunset Yellow, SSY) and cetyltrimethylammonium bromide (C16TAB) at the air–water interface were studied using vibrational sum-frequency generation (SFG) and dynamic surface tension measurements. In the bulk, addition of C16TAB to SSY aqueous solution causes substantial changes in UV/vis absorption spectra, which originate from strong electrostatic interactions between the anionic SSY azo dye with the cationic C16TAB surfactant. These interactions are a driving force for the formation of SSY/C16TAB ion pairs. The latter are found to be highly surface active while free SSY molecules show no surface activity. Dynamic SFG as well as surface tension measurements at low SSY concentrations reveal that free C16TAB surfactants adsorb at the air–water interface on time scales <1 s where they initially form the dominating surface species, but on longer time scales free C16TAB is exchanged by SSY/C16TAB ion pairs. This causes a dramatic reduction of the surface tension to 35 mN/m but also in foam stability. These changes are accompanied by a substantial loss in SFG intensity from O–H stretching bands around 3200 and 3450 cm–1, which we relate to a decrease in surface charging due to adsorption of ion pairs with no or negligible net charges. For SSY/C16TAB molar ratios >0.5, the O–H bands in SFG spectra are reduced to very low intensities and are indicative to electrically neutral SSY/C16TAB ion pairs. This conclusion is corroborated by an analysis of macroscopic foams, which become highly instable in the presence of neutral SSY/C16TAB ion pairs. From an analysis of SFG spectra of air–water interfaces, we show that the electrostatic repulsion forces inside the ubiquitous foam films are reduced and thus remove the major stabilization mechanism within macroscopic foam.

Adsorption behavior of branched polyoxyethylene ether carboxylate surfactants

In our work, branched anionic surfactant (3, 5, 7-trimethyl decanol polyoxyethylene ether carboxylate (A13EC5-Na)) has been successfully synthesized and characterized by fourier transform infrared (FT-IR) spectra. Static equilibrium surface tension, contact angle and dynamic surface tension of alcohol ether carboxylic acid (A13EC5-H), alcohol ether carboxylate (A13EC5-Na) and linear chain carboxylate (A12EC5-Na, as comparison) were investigated to study their spreading and adsorption behaviors. Electrolyte tolerance of anionic surfactants (A13EC5-Na and A12EC5-Na) was also studied to explore the application in enhanced oil recovery. The results indicated that critical micelle concentration (CMC) and minimum area per molecule (A0) are the lowest and the surface excess concentration (Γm) is the highest for nonionic surfactants (A13EC5-H). A13EC5-Na can decrease the surface tension of water to 27.43mN/m. Due to the introduction of branched chains, Γm of A13EC5-Na is smaller than that of A12EC5-Na. And the adsorption efficiency and the efficiency of forming micelles of A13EC5-Na molecules are superior to that of A12EC5-Na. By contact angle and dynamic surface tension measurements, we can know that for branched products, the wetting ability is stronger, diffusion rate to the interface is faster and the time of surface tension reaching equilibrium is shorter. The adsorption process of A13EC5-Na is diffusion controlled, while the adsorption process of A12EC5-Na is mixed diffusion-kinetic adsorption mechanism. CaCl2 tolerance of A13EC5-Na is stronger than that of A12EC5-Na but NaCl tolerance of A12EC5-Na is stronger.