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Eco‐friendly surfactant alkyl polyglycosides (APGs) have garnered ample interest in the literature. However, the adsorption kinetics of APGs at an air‐water interface has not been studied to date. Consequently, this study aimed at investigating the adsorption kinetics of the APG n‐decyl‐β‐D‐maltopyranoside (β‐C10G2) at a freshly created air‐water interface. The dynamic and equilibrium surface tension (ST) of the aqueous β‐C10G2 solutions were experimentally measured at varying concentrations using a pendant bubble tensiometer and the ST data were examined via theoretical simulations using the Langmuir and Frumkin models. The fitting results revealed that the Frumkin model (with K = −2.9) well‐predicted the equilibrium ST data; signifying a considerable cohesive intermolecular force amidst the adsorbed β‐C10G2 molecules. The theoretical mixed‐controlled ST profiles (predicted by the Frumkin model) described the dynamic ST data reasonably well, but the adsorption rate constant increased significantly at increasing concentration. A comparison on the ST data of β‐C10G2 and β‐C12G2 (n‐dodecyl‐β‐D‐maltopyranoside) was also conducted and a relatively greater surface‐activity and lower cmc of the latter alkyl glycoside was observed. [ABSTRACT FROM AUTHOR]

• Dynamic/equilibrium surface tension of Aerosol-OT in 10 mM NaCl solution were measured. • The partially ionized Langmuir model was applied to fit the ST data. • The Langmuir model with α = 0.15±0.05 described dynamic/equilibrium ST data very well. • The adsorption mechanism was found to be either diffusion-controlled or mixed controlled. • Surfactant depletion was observed for AOT in 10 mM NaCl solution at c ≤ 5 × 10−9 mol/cm3. The adsorption kinetics of anionic surfactant Aerosol-OT (AOT) in 10 mM NaCl solution was investigated by measuring and simulating the dynamic and equilibrium surface tension (ST) data. The relaxation of ST for AOT molecules adsorbing onto a freshly created bubble surface was measured using a video-enhanced pendant bubble tensiometer at eighteen AOT concentrations. The partially ionized Langmuir model was applied to best-fit the dynamic/equilibrium ST data. The subsequent findings revealed that the model with a dissociation degree α of 0.1 − 0.2 described the dynamic and equilibrium ST data very well. This implied that AOT behaved as a partially ionized surfactant in 10 mM NaCl solution. The adsorption for AOT onto a clean air–water interface was found to be either (i) diffusion-controlled with a diffusivity value that varied [between 2.5 and 4.6 (10−6 cm2/s)] depending on the bulk concentration or (ii) mixed-controlled with an adsorption rate constant of 2.4 ± 0.8 (107 cm3/mol.s). [ABSTRACT FROM AUTHOR]

• The CA relaxation profile was used to determine the advancing CA of sessile drop. • Sessile drop of surfactant solution was formed from an equilibrated pendant drop. • Two advancing contact angles were detected for TX-100 and C 12 E 6 solution drops. • Only one advancing CA was detected for the sessile drop of SDS solution. • Pre-equilibration of the drop before deposition affected the CA relaxation profile. This study investigated the uniqueness of advancing contact angle (CA) by examining the evaporation process of a sessile drop of surfactant solution [sodium dodecyl sulfate (SDS), hexaethylene glycol monododecyl ether (C 12 E 6) and Triton X-100 (TX-100)] on parafilm. The sessile drop was formed from a pendant drop that had reached its equilibrium surface tension in an atmosphere with ∼100% humidity. The relaxation of the drop profile and wetting diameter were recorded from the moment the drop touched the parafilm, and the data were then analyzed to determine the advancing CA. The results showed that for SDS solution drops, only one advancing CA was observed at each SDS concentration. However, the experimental data of TX-100 and C 12 E 6 solution drops indicated the presence of two distinct regions that showed a near-constant CA with a quasi-equilibrium advance of the triple line; hence matching the definition of an advancing CA. The possibility of the existence of two advancing CA was further explored and the presence of two advancing CA for the sessile drops of TX-100 and C 12 E 6 solution was verified by closely examining the advance of the triple line. [ABSTRACT FROM AUTHOR]

A pendant bubble tensiometer was modified for the dynamic surface tension (ST) measurement of a dodecylamine (DDA) surfactant solution. The dynamic STs of DDA solutions showed a peculiar relaxation: the ST relaxed from the ST of the solvent, reached a minimum, and then increased to ∼72 mN/m. The pH values of aqueous DDA solutions indicated that the DDA was partially ionized, with an ionization degree between 2 and 7%. The partially protonated DDA molecules probably prefer to adsorbed on the quartz surface of the sample cell and the metal needle surface, thereby depleting the bulk concentration and causing the desorption of the previously adsorbed DDA molecules at the air–solution interface. A coated quartz cell and a continuous pumping system were implemented in the pendant bubble tensiometer. The dynamic STs of DDA in the 0, 50, and 500 mM NaCl solutions smoothly relaxed from the ST of the solvent, reached the equilibrium ST (γ e ) and remained at γ e for a long time without any increase. This pendant bubble tensiometer with modified cell system is capable of measuring the dynamic ST of the partially ionized dodecylamine surfactant solution. [ABSTRACT FROM AUTHOR]

The short-time approximation method is known as a simple way to determine the adsorption mechanisms and to estimate the surfactant diffusivity. Three assumptions are needed in order to obtain the linear short-time approximation relationship between γ( t ) and t 1/2 to fit the experimental dynamic surface tension data: the surfactant backward diffusion process is negligible, the forward adsorption contributed from the spherical geometry is much smaller than that to a planar surface, and surfactant follows Henry's equation of state. Recently, a general criterion, in terms of adsorption time and surface pressures for short-time approximation applicability on solution systems assuming a planar gas–liquid interface, has been explored by Casandra et al. (2015). How are the criteria applicable for spherical surfaces? Surface tension relaxes faster and reaches its equilibrium earlier for surfactant molecules mass-transporting onto a spherical interface than onto a planar interface due to the geometrical effect. Ignoring this geometrical effect may cause a serious error in estimating diffusivity and in determining the adsorption mechanism when applying the short-time approximation technique. In this study, a theoretical numerical simulation of the short-time approximation method was conducted, and the general criteria for accurately utilizing the short-time approximation were investigated. [ABSTRACT FROM AUTHOR]

Abstract: The dynamic surface properties of mixed solutions of polyacrylic acid (PAA) and alkyltrimethylammonium bromides (CnTAB) were measured by the oscillating ring method as a function of surface age and surfactant concentration. The concentration dependencies of the surface tension and dilational dynamic surface elasticity were similar to the corresponding results for other polyelectrolyte/surfactant solutions, which have been investigated earlier. The kinetic dependencies of the surface elasticity proved to be non-monotonic for all the investigated surfactants at n =8, 10, 12, 14, 16 and displayed two local maxima at surfactant concentrations close to the critical micellar concentration (CMC). The first maximum was caused by an aggregate formation in the surface layer while the second maximum occurred simultaneously with a multilayer formation at the gas–liquid interface. The surface tension corresponding to the beginning of the aggregate formation decreased and the surface elasticity in the region of the first local maximum increased with the hydrophobicity of CnTAB/PAA complexes caused by the increasing hydrocarbon chain length of the surfactant. The hydrophobic interactions influenced also the height and position of the second local elasticity maximum. These effects were connected with changes in the rate of adsorption layer and aggregate structure formation. Comparable values of the real and imaginary parts of the dynamic surface elasticity at concentrations for aggregate formation in the surface layer indicated that the relaxation time of the mass exchange between the aggregates and the surrounding adsorption layer is comparable with the period of oscillations, i.e. ≤10s. [Copyright &y& Elsevier]

Abstract: The bulk phase of nonionic surfactant C10E4 solution was monitored by a dynamic light scattering (DLS) system at 20°C in a narrow range of concentration near the cmc. Two particle aggregations were observed. The DLS data show (i) there exist premicellar multimers (or called sub-micelles) and (ii) micelles coexist with multimers. The C10E4 sub-micelles have a narrow size distribution with an averaged hydrodynamic diameter (D h) of 1.35nm. The D h of the micelles is around 10.5nm at 1.0×10−6 mol/mL and increases slightly with C10E4 concentration. It is illustrated from the DLS data that (i) at C =0.78–0.82μmol/mL, monomers and premicellar multimers coexist and (ii) at C =0.84–0.92μmol/mL, monomers+submicellar multimers+micelles coexist. At more elevated concentrations, only the signals from the micelles are detected by DLS. [Copyright &y& Elsevier]

Surfactant molecules adsorb onto the interfaces of moving fluid particles and are convected to regions in which the surface flow converges. Accumulation of surfactant in these regions creates interfacial tension gradients that retard the surface flow. In this study it is argued theoretically and demonstrated experimentally that fluid movement on the surface of a drop or bubble can remain unhindered in the presence of a single adsorbed surfactant if, relative to the convective rate of transport of adsorbed surfactant along the surface, desorption is fast, and the bulk concentration is high enough so that diffusion away from the particle is fast. For this circumstance, a uniform surface concentration of surfactant is maintained, and no gradients in surface tension arise to retard the surface velocity. The fluid particle flow behaves as it would in the absence of surfactant save that it has a reduced, uniform surface tension. The remobilization of surfactant-laden interfaces of fluid particles is demonstrated experimentally in a three-phase periodic slug flow in a capillary tube in which a train of alternating air and aqueous slugs ride on an annular wetting film of fluorocarbon oil. Surfactant, dissolved in the aqueous slug phase, adsorbs onto and retards the aqueous–oil interface. The hydrodynamics of this flow is such that small changes in the mobility of this interface create large shear rates in the oil layer. This significantly increases the pressure drop required to drive the slug train at constant velocity. Three surface adsorbers are used to demonstrate surface remobilization: The polyethoxy, nonionic surfactants Triton X-100 and Brij-35, which have fast desorption kinetics and do not retard the surface flow at high concentrations and, as a counter example, the desorption hindered protein bovine serum albumin, which is shown to be unable to remobilize an interface even at high concentration. [ABSTRACT FROM AUTHOR]

Kinetic dependences of the dynamic dilatational surface elasticity of solutions of the complexes of b-lactoglobulin and bovine serum albumin with dodecyltrimethylammonium bromide show that the destruction of the tertiary protein structure and the partial destruction of the secondary structure in the surface layer occur at lower surfactant concentrations than in the bulk solution. [Copyright &y& Elsevier]

Measurements of the dynamic elasticity of an adsorption film of poly(vinylpyridinium chloride)/sodium dodecylsulfate (PVP/SDS) complexes indicate a structure transition in the film when the surfactant concentration approaches the concentration of PVP monomers and show that the rate of relaxation processes in polyelectrolyte/surfactant adsorption films is determined by the properties of the polymer chain. [Copyright &y& Elsevier]

Abstract: The dynamic surface elasticity of mixed solutions of β-casein and surfactants of different chemical nature was measured as a function of surface age and surfactant concentration with the aim to elucidate the influence of non-ionic interactions between the components on the dynamic surface properties. Small admixtures of the amphiphilic non-ionic polymer poly(N-isopropylacrylamide) (PNIPAM) did not influence the dynamic surface elasticity and the surface properties of the solution were entirely determined by the adsorbed protein. The corresponding kinetic dependencies of the dynamic surface elasticity have two local maxima. The increase of the polymer concentration resulted in the appearance of a third local maximum in the surface elasticity corresponding to the polymer adsorption. In this case the height and position of the second peak in the kinetic curve of the surface elasticity were changed noticeably indicating interactions between the components. The increase of the surfactant hydrophobic chain length in the case of small additions of cationic surfactants of low molecular weight increased the hydrophobicity of the formed protein/surfactant complex and also led to changes of the shape of the dynamic surface elasticity. The application of the scanning probe microscopy indicated different mechanisms of the β-casein displacement from the liquid surface by the surfactants of low and high molecular weights, correspondingly. [Copyright &y& Elsevier]

[Display omitted] • The new linear-approximation γ( t ) for mixed-controlled adsorption was examined. • Uniqueness and applicability of this approximation (planar surface) were studied. • Linear regions in a γ( t ) profile could be well-described by this approximation. • Smaller error in D & β values was detected by using the latter part of γ( t ) profile. • Multiple sets of D & β values may be resulted using this approximation. A new linear approximation for a mixed-controlled adsorption process had been designed for estimating the surfactant diffusivity (D f) and adsorption rate constant (β f) by purely using the dynamic surface tension (ST) data (Moorkanikkara and Blankschtein, 2006). However, the uniqueness and applicability of this linear approximation are yet to be ascertained. Subsequently, a theoretical simulation was conducted to study the uniqueness and robustness of this linear approximation. An adsorption process of nonionic surfactant onto a freshly created planar air-water interface was considered and its mixed-controlled dynamic ST profiles were simulated using the Langmuir model. Linear regions in the simulated ST vs t1/2 profiles do exist, which could be described well by a straight line with an intercept >72 mN/m (representing the linear approximation). Values of D f and β f could then be estimated from the slope and the intercept of this straight line. While the applicability of this linear approximation was confirmed, the results did show a distinct possibility of obtaining multiple sets of D f and β f if different linear regions of the γ vs t1/2 profiles were selected for the fitting. Additionally, the applicability of this linear approximation on experimental dynamic ST data was tested. [ABSTRACT FROM AUTHOR]

Abstract: The assumptions commonly made in dynamic surface tension (DST) measurement using the pendant bubble method are examined. A pendant bubble changes its shape in response to DST due to surfactant adsorption. The shape change induces motion in the bulk fluid during the relaxation of surface tension. Three assumptions motivated by which are: (i) negligible influence of the bulk fluid motion, (ii) uniform surface concentration along the bubble surface, and (iii) negligible convective effects in surfactant mass transfer. The first two are related to the validity of the use of the Young–Laplace equation for DST measurement, so that DST measurement can be conducted under the conditions of keeping a bubble nearly hydrostatic and of a constant surface tension at each instant in time. The last assures the transport process to be diffusion-dominated, so that genuine kinetics can be examined without being mediated by the bulk convection. This study invokes a dimensional analysis to inspect these assumptions via identifying relative importance among relevant effects. Experimental data are provided for justification. Criteria are not only established in line for validating these assumptions, but also guide in the choice of appropriate experimental conditions for conducting DST measurement. [Copyright &y& Elsevier]

The dynamic surface tension (ST) for Aerosol-OT (AOT) molecules adsorbing onto a freshly created bubble surface in a 10 mM NaCl (aq) solution was measured at extremely dilute concentrations using a pendant bubble tensiometer. An air bubble was formed one after another inside the same uniform solution at two different states of the free surface on the top of the solution in quartz cell: (i, bubble #1) a nearly clean air-water interface and (ii, bubble #2) a thermodynamic equilibrated air-water interface. Whilst the dynamic ST of bubble #1 decreased noticeably faster than that of bubble #2, both these bubbles reached the same equilibrium ST. AOT molecules kept adsorbing onto the free surface during the ST measurement of bubble #1, which caused the concentration of AOT in bulk to decrease significantly at dilute concentrations. During the measurement of bubble #2, the bulk concentration remained constant. Subsequently, the dynamic ST data of bubble #2 precisely described the adsorption of AOT molecules onto a freshly created air-water interface. Consequently, the adsorption onto the free surface played an important role in the dynamic ST measurement for surfactant solutions at extremely dilute concentrations. This new experimental procedure was capable of precisely measuring the dynamic ST of surfactant solution at extremely low concentrations. Unlabelled Image • Dynamic/equilibrium ST for Aerosol-OT in 10 mM NaCl (aq) solution were measured. • Depletion for AOT in bulk occurred due to the adsorption onto free surface. • Bubble #1 and bubble #2 were formed one after another inside the same AOT solution. • The dynamic ST of bubble #1 relaxed much faster than that of bubble #2. • Bubble #1 and bubble #2 always reached same equilibrium ST. [ABSTRACT FROM AUTHOR]

The critical micelle concentration (cmc) is an important parameter in colloidal and surface chemistry and a wide range of techniques have been devised for cmc determination. In this study, the determination of the cmc of surfactant solutions from the unique advancing and receding contact angle (CA) data was investigated. The sessile drop was formed from a pendant drop which had reached its equilibrium surface tension (ST) in a saturated atmosphere (~100% humidity). The CA was determined from the best-fit between the edge locations of drop and the Young-Laplace theoretical profile. The advancing and receding CAs were determined from the CA relaxation profiles of the sessile drops. It was observed that the breakpoints of the advancing and receding CAs of SDS solution drop coincided with the cmc determined from the ST data. However, the comparatively slower adsorption and bulk diffusion rates of TX-100 and C 12 E 6 made the CA breakpoints deviate from that of ST. Thus, it was concluded that the approach of using the unique CA data for the determination of cmc could be applied only to those surfactants with a relatively high mass transport rate, such as SDS. Unlabelled Image • The unique advancing and receding CA of SDS, TX-100 and C 12 E 6 solution drops were determined from the CA relaxation profiles. • The sessile drop was formed from a pendant drop which had reached its equilibrium surface tension in a saturated atmosphere. • The values θ a and θ r decreased with an increasing bulk concentration of the surfactant solutions at C < cmc. • Breakpoints of θ a and θ r matched well with the cmc from surface tension for SDS solution. • Breakpoints of θ a and θ r did not coincide with the cmc for TX-100 and C 12 E 6 solutions. [ABSTRACT FROM AUTHOR]

The study of surfactant adsorption kinetics at a gas-liquid interface has essentially been built with the Gibbs adsorption equation. A simple one-parameter adsorption equation without using the Gibbs adsorption equation was proposed and tested on surfactants C 14 TAB and C 16 TAB [Phan et al., Colloids Surfaces A 406 (2012) 24 and Langmuir 29 (2013) 4743] to describe equilibrium surface tension. The applicability of this one-parameter adsorption equation was examined in this study. Theoretical equilibrium surface tension (ST) of nonionic and ionic surfactants generated from the Langmuir and Frumkin isotherms and several sets of experimental ST data were compared with the one-parameter adsorption equation. This study demonstrates that this one-model adsorption equation predicts the equilibrium ST of the aqueous surfactant solutions well following: (i) the nonionic Langmuir isotherm with Γ ∞ ∼ 20 × 10 −10 mol/cm 2 or (ii) the nonionic Frumkin isotherm with a specific correlation between maximum surface concentration and intermolecular interaction, K = 0.12 (Γ ∞ × 10 10 ) − 2.6. It is concluded from this study that the one-parameter adsorption equation is not a general predictive model and cannot describe general equilibrium ST data except under the above conditions. [ABSTRACT FROM AUTHOR]

In past studies, estimating the dynamic surface tension in processes involving significant surface deformation relied heavily on empirical relations. Despite its importance, the detailed dynamics of surfactant adsorption onto a dilating surface have not been investigated extensively. In this study, we modified the pendant bubble tensiometer to provide direct observation on the dynamics of surfactant adsorption during rapid surface dilation. The relaxation of dynamic surface tension for bubbles rapidly expanding in the solution of four different bulk concentrations of Triton-X 100 was studied. It was found that surface tension rose with increasing surface area due to the decrease in surface concentration. Moreover, when surface dilational rate was increased, it was found that the relation between surface tension and surface area came to resemble the equation of state of two dimensional surfactant phase when dilational rates were larger than a certain threshold value, indicating that the adsorption of surfactant became negligible. This threshold value was found to increase with surfactant concentration. The results showed that the procedure established in this study could serve as a simple guideline to determine whether adsorption can be neglected in a process involving dilating surfaces. In addition, the detailed dynamical data reported in this work can serve as an experimental reference for multiphase flow simulations involving species that adsorb on interfaces. [ABSTRACT FROM AUTHOR]

The asymptotic approximation equation was derived as a simple means for determining the adsorption mechanism and estimating surfactant diffusivity for surfactant adsorption onto a clean, spherical air-water interface (Makievski et al., 1999). However, the applicability of this approximation is yet to be examined. In this study, a theoretical simulation was conducted to better understand its applicability and validity. The diffusion-controlled adsorption of a nonionic surfactant onto a clean spherical interface (with radius = 0.03, 0.1, 0.5, and ∞ cm) was considered, and theoretical dynamic surface tension (γ) profiles were generated using the Langmuir adsorption isotherm. The initial region of γ(t1/2) profiles was best-fitted with asymptotic approximation with only one unknown parameter, the surfactant diffusivity. The initial region of the γ(t1/2) profiles could be well-described by the asymptotic approximation (a parabolic equation), but a notable deviation was observed on the diffusivity obtained from the fitting. This deviation was dependent on the bulk concentration and the duration of the initial region fitted; whilst being only slightly dependent on bubble radius. Although the results demonstrated the viability of this approximation, there existed a possibility of obtaining multiple values of diffusivity by choosing different initial regions of a γ(t1/2) profile. [Display omitted] • Applicability of asymptotic approximation for a spherical interface was examined. • Initial regions of the γ(t1/2) profiles could be well-described by this approximation. • This approximation is helpful for finding the diffusivity and adsorption mechanism. • These diffusivities depend on bulk concentration and initial γ(t1/2) region fitted. • The time interval where the spherical effect could be neglected was also estimated. [ABSTRACT FROM AUTHOR]

Highlights: [•] The adsorption mechanism of a fluorinated surfactant is investigated. [•] The adsorption of the fluoroalcohol and its hydrocarbon-type analog were compared. [•] Dynamic surface tensions were measured at five different concentrations. [•] The dominant mechanism was found to be mixed diffusive-kinetic control. [•] The desorption rate constant of the fluorinated surfactant was determined. [ABSTRACT FROM AUTHOR]

Abstract: A notable universal relationship has been proposed in the literature for the evolution of dimensionless droplet height and wetting diameter during the initial spreading stage of droplet impingement. In this study, this universal relationship was investigated by employing three sets of measurements. Sequential images were recorded, and the whole droplet profile ensembles were plotted to facilitate this study. These sets of experiments were designed by changing impact velocity, surface hydrophobicity, or solution property. The experimental results illustrate that the importance of parameters causing the data variation is in the order of surface hydrophobicity>initial impact velocity>surfactant on wetting diameter, and surface hydrophobicity≈initial impact velocity>surfactant on droplet height. No universal relationship was observed for dimensionless droplet height and wetting diameter. [Copyright &y& Elsevier]

Abstract: The adsorption of C12E4 onto a fresh air–water interface was reported to be diffusion-controlled. Hsu et al. applied a video-enhanced pendant bubble tensiometer and obtained the dynamic surface tension (DST) for different C12E4 concentrations [C.T. Hsu, M.J. Shao, S.Y. Lin, Langmuir 16 (2000) 3187]. The entire DST profiles were compared with the theoretical DST from a classical spherical model (CSM). CSM assumes that the effect of pendant bubble shape and the existence of needle on DST were negligible. In this study, the actual shape of pendant bubble and the existence of a needle are considered on generating the theoretical DST profiles. The diffusion-controlled simulation profiles in this work predict the DST data well, but a slight different diffusivity may be resulted. Without considering the existence of needle and bubble shape, CSM underestimates a diffusivity 10–12%. An alternative way for correcting the effect of bubble shape in order to use the simple CSM is also given at the end of this work. [Copyright &y& Elsevier]

The adsorption kinetics of CmE8 (m=10, 12, and 14) at an air–water interface are investigated. A pendant bubble is formed in aqueous surfactant solution and allowed to attain equilibrium. The bubble is then impulsively expanded or compressed with some change of area large enough to appreciably deplete or enrich the surface concentration and change the surface tension. The surfactant is then allowed to re-equilibrate. The surface tension evolution during this process is measured using video images of the pendant drop. The surface tension evolution is compared to mass transfer arguments. First, the re-equilibration of interfaces laden with C14E8 are studied. For compressed interfaces, surfactant must desorb to restore equilibrium. The surface tension rises more slowly than predicted by a diffusion-controlled evolution, implying that the re-equilibration is mixed diffusive-kinetic controlled. By analyzing the surface tension evolution in terms of a mixed kinetic-diffusive model, values for the kinetic constants for adsorption and desorption are found. These results are compared to those obtained previously for CmE8 (m=10 and 12). For all of these molecules, the adsorption rate constant is similar (β1=5.6±1.0×10−6 cm3 (mol s)−1). However, the desorption rate constant (α1) varies strongly. Increasing m by 2 lowers the desorption rate constant α1 by nearly a factor of 15. This is consistent with an increased resistance to re-immersion into water with the length of a hydrocarbon chain. [Copyright &y& Elsevier]