Equilibrium Surface Tension Values Research Articles

Surface activity of herniariasaponin and non-saponin fractions of Herniaria L. species

The study investigated the surface activity of aqueous extracts from three Herniaria L. species, namely H. glabra, H. polygama, and H. incana. The three crude extracts exhibited similar chromatographic saponin profiles and equilibrium surface tension values (46-47 mN/m at 1% concentration). The adsorbed layers were predominantly elastic, with a high compression storage modulus, E'. However, they also showed the ability to dissipate mechanical energy over the aqueous-air interface, as evidenced by fairly high values of the surface compression loss modulus, E'' (45-50 mN/m). The extract of H. glabra exhibited a particularly high E' value (>150 mN/m). To analyze its complex chemical composition, the crude extract was fractionated using liquid chromatography. The sugar fraction was found to be dominant (~65% of the extract mass), but exhibited the lowest surface activity. As the acetonitrile content in the eluent increased, the subsequently eluted fractions showed an increasing ability to lower surface tension. The HGW30-70 fraction, which constitutes approximately 20% of the crude extract mass, was the main contributor to the surface properties of the extract due to its herniariasaponins content. The HGW20-25-A fraction, which showed the second highest surface activity, was further separated into two sub-fractions: non-flavonoid and pure-flavonoid. The former components were primarily responsible for the surface activity and their composition resembled that of a more polar HGW10 fraction (terpene glycosides and C9-C12 fatty acids). The non-flavonoid subfraction was also responsible for solubilization of otherwise poorly water-soluble components, like flavonoids.

Synthesis and surface properties of branched-chain tertiary fatty alcohol sulfate surfactants

Abstract In this paper, anionic branched-chain tertiary fatty alcohol sulfate surfactants were synthesized from n-hexene and n-octene using selective olefin dimerization, hydration reaction (hydroxylation), and sulfur trioxide sulfation. The direct hydration reaction of the α-olefin dimer 2-butyl-1-octene with water as a model reaction was explored for the synthesis of branched-chain tertiary fatty alcohols. Two branched-chain tertiary fatty alcohol sulfate surfactants, namely C12-SBTAS and C16-SBTAS, with different carbon chain lengths, were synthesized by the sulfur trioxide sulfation method. Their structures were confirmed by various analytical techniques, including HPLC, FT-IR, HR-MS, and 1H NMR. Equilibrium and dynamic surface tension, foaming, wetting, and emulsifying properties were compared with those of Guerbet cetyl alcohol sulfate (C16-SGAS). C12-SBTAS and C16-SBTAS exhibited good surface activity with equilibrium surface tension (γ CMC) values of 27.41 mN m−1 and 26.69 mN m−1, respectively. They also had low foaming and rapid defoaming abilities, as well as good wetting and emulsifying properties, which match the application characteristics of typical branched-chain surfactants.

Surface tensions at elevated pressure depend strongly on bulk phase saturation

HypothesisUnderstanding interfacial phenomena at elevated pressure is crucial to the design of a variety of processes, modeling important systems, and interpreting interfacial thermodynamics. While many previous studies have offered insight into these areas, current techniques have inherent drawbacks that limit equilibrium measurements. ExperimentsIn this work, we adapt the ambient microtensiometer of Alvarez and co-workers into a high pressure microtensiometer (HPMT) capable of experimentally quantifying a wide range of interfacial phenomena at elevated pressures. Particularly, the HPMT uses a microscale spherical interface pinned to the tip of a capillary to directly measure surface tension via the Laplace equation. The stream of microscale bubbles used to pressurize the system ensures quick saturation of the bulk phases prior to conducting measurements. The HPMT is validated by measuring the surface tension of air–water as a function of pressure. We then measure the surface tension of CO2 vapor and water as a function of pressure, finding lower equilibrium surface tension values than originally reported in the literature. FindingsThis work both introduces further development of a useful experimental technique for probing interfacial phenomena at elevated pressures and demonstrates the importance of establishing bulk equilibrium to measure surface tension. The true equilibrium state of the CO2-water surface has a lower tension than previously reported. We hypothesize that this discrepancy is likely due to the long diffusion timescales required to ensure saturation of the bulk fluids using traditional tensiometry. Thus we argue that previously reported elevated pressure measurements were performed at non-equilibrium conditions, putting to rest a long standing discrepancy in the literature. Our measurements establish an equilibrium pressure isotherm for the pure CO2-water surface that will be essential in analyzing surfactant transport at elevated pressures.

Interaction of Lipase with Lipid Model Systems

The aim of this work was to study the interaction of lipases (as an important biopolymer) with models of biomembranes based on the phospholipid and cholesterol. Lipases (triacylglycerolacyl hydrolases) are widely distributed enzymes and well-known by their hydrolytic activity. The study of the lipase interactions with lipid vesicles in aqueous dispersions is of fundamental and practical interest. The pure phosphatidylcholine from egg yolk (ePC) and cholesterol (Chol) were obtained from Sigma-Aldrich. Lipase was obtained from hog pancreas. Measurements of the current and equilibrium surface tension (ST and eST) values were carried out using a BPA-1P device and ADSA program. The particle sizes in the prepared colloidal solutions were determined by the method of dynamic light scattering. An addition of lipase led to some decrease both, of ST and eST for the samples of ePC:Chol (in the ratios from19:1 to 1:1). The mean particle diameter (MPD) and effective particle diameter (EPD) values for the samples of ePC:Chol changed drastically by lipase addition. The EPD/MPD ratios increased from 1.7 to 2.0, from 1.8 to 2.6, from 2.3 to 6.5, from 1.5 to 2.9 for the samples of ePC:Chol at the ratios of 19:1, 14:1, 9:1, 7:1, respectively by lipase concentration increase. This general tendency can be explained by strong interaction of lipase with lipid membrane that leads to the formation of the mixed particles ePC:Chol:lipase with more narrow particle size distribution as compared to the initial EPD/MPD ratio (for the ePC:Chol mixture without lipase).

Equilibrium, dynamic surface tensions and application properties of fatty alcohol ether carboxylic ester surfactants

A series of fatty alcohol ether carboxylic ester (AECE) surfactants were mainly synthesized from succinic anhydride and different fatty alcohol ethers (AEO) with green and atom economic process. The chemical constitutions of AECE were characterized by Fourier transform infrared (FTIR), 1H nuclear magnetic resonance (1HNMR) and matrix-assisted laser desorption/ionization time of flight mass spectra (MALDI–TOF–MS). The equilibrium and dynamic surface tensions (DST) of AECE surfactants were investigated, and Ward–Tordai equation was employed to analyze the adsorption and diffusion process of surfactants by calculating the relevant parameters (n, t*, ti , tm , R 1/2) as well as the effective diffusion coefficients (D long and D short) from DST curves. Equilibrium surface tension results showed that AECE surfactants have strong surface activities, and their equilibrium surface tension and critical micelle concentration values are less than 30 mN/m and 0.4 mmol/L, respectively. According to DST parameters, nonionic surfactant AEC5E-H is more favorable to diffuse and adsorb to the air/water interface than others. In addition, the movement of surfactant molecules is also related to the concentration of solutions, and a reasonable concentration can be selected in practical applications based on the experimental results. The outcomes of the diffusion coefficients illustrated that the adsorption mechanism is the mixed diffusion–kinetic adsorption. Moreover, the application performances results showed that the ionic surfactants (AEC3E-Na and AEC5E-Na) are superior to the nonionic surfactants (AEC3E-H and AEC5E-H) in terms of wetting, foam and detergency, while the emulsifying ability is different for various oil phases.

Accurate Determination of the Equilibrium Surface Tension Values with Area Perturbation Tests.

We demonstrate two robust protocols for determining the equilibrium surface tension (EST) values with area perturbation tests. The EST values should be indirectly determined from the dynamic surface tension (DST) values when surface tension (ST) values are at steady-state and stable against perturbations. The emerging bubble method (EBM) and the spinning bubble method (SBM) were chosen, because, with these methods, it is simple to introduce area perturbations while continuing dynamic tension measurements. Abrupt expansion or compression of an air bubble was used as a source of area perturbation for the EBM. For the SBM, changes in the rotation frequency of the sample solution were used to produce area perturbations. A Triton X-100 aqueous solution of a fixed concentration above its critical micelle concentration (CMC) was used as a model surfactant solution. The determined EST value of the model air/water interface from the EBM was 31.5 ± 0.1 mN·m-1 and that from the SBM was 30.8 ± 0.2 mN·m-1. The two protocols described in the article provide robust criteria for establishing the EST values.

Accurate Determination of the Equilibrium Surface Tension Values with Area Perturbation Tests

Accurate Determination of the Equilibrium Surface Tension Values with Area Perturbation Tests

Ovalbumin and guar gum foam and its surface properties as influenced by sucrose and sorbitol

The surface properties and foaming of ovalbumin and guar gum aqueous solutions was investigated in the presence of sucrose or sorbitol. All solutions had a broad particle size distribution (395.60 at 1137.50 nm). Higher ovalbumin concentrations had lower equilibrium surface tension and higher absolute values of the zeta potential, regardless the presence of sucrose or sorbitol. Mixtures containing ovalbumin and guar gum resulted in a predominantly elastic character of the air-water interface probably due to the formation of a complex (hydrogen bonding and/or hydrophobic interactions) between ovalbumin and guar gum. Besides, the increase in guar gum concentration enhanced the elasticity of the surface film. Higher concentrations of both polymers were required to provide higher kinetic stability to the system, although the increase in guar gum concentration reduced foam capacity due to the increase in the apparent viscosity. Foams formed in the presence of sucrose or sorbitol showed similar half-lives.

Cationic silicon-based gemini surfactants: Effect of hydrophobic chains on surface activity, physic-chemical properties and aggregation behaviors

Abstract A series of cationic silicon-based gemini surfactants (C m -PSi-C m ) having different hydrophobic chains length (m = 8, 10, 12, 14, 16, 18) were synthesized. Subsequently, surface parameters including equilibrium surface tension (γ CMC ), critical micelle concentration (CMC), surface pressure at CMC ( Π CMC ) and maximum surface excess ( Γ max ) were calculated according to the surface tension measurements. The importance of hydrophobic chains length on the surface activity of C m -PSi-C m was revealed in detail. With the increasing hydrophobic chains length, the equilibrium surface tension (γ CMC ) values follow the order of C 8 -PSi-C 8 > C 10 -PSi-C 10 > C 12 -PSi-C 12 14 -PSi-C 14 16 -PSi-C 16 18 -PSi-C 18 . The thermodynamic parameters of micellization process, namely, standard Gibbs free energy (ΔG 0 m ), enthalpy (ΔH 0 m ) and entropy (ΔS 0 m ), were derived from conductivity measurements at different temperatures. The aggregation of the cationic silicon-based gemini surfactants (C 12 -PSi-C 12 ) in aqueous solution at different concentrations was studied by DLS and TEM measurements. The formation of micelles and vesicles in C 12 -PSi-C 12 solutions was observed at a concentration well above the CMC. Moreover, the morphology of vesicles changed from sphere to rod-like, dumbble-like and finally string dumbbell-like with the increasing of the surfactant concentration.

Dynamic surface tension measurement for the screening of biosurfactants produced by Lactobacillus plantarum subsp. plantarum PTCC 1896.

Currently, screening of microbial biosurfactants (BSs) is based on their equilibrium surface tension values obtained using static surface tension measurement. However, a good surfactant should not only have a low equilibrium surface tension, but its dynamic surface tension (DST) should also decrease rapidly with time. In this study, screening of BSs produced by Lactobacillus plantarum subsp. plantarum PTCC 1896 (probiotic) was performed based on their DST values measured by Wilhelmy plate tensiometry. The relationship between DST and structural and functional properties (anti-adhesive activity) of the BSs was investigated. The results showed that the changes in the yield, productivity and structure of the BSs were growth medium and incubation time dependent (p<0.05). Structurally different BSs produced exhibited identical equilibrium surface tension values. However, differences among the structure/yield of the BSs were observed through the measurement of their DST. The considerable dependence of DST on the concentration and composition of the BS proteins was observed (p<0.05). Moreover, the anti-adhesive activity of the BS was found to be positively correlated with its DST. The results suggest that the DST measurement could serve as an efficient method for the clever screening of BSs producer/production condition, and consequently, for the investigation of probiotic features of bacteria, since the anti-adhesive activity is an important criterion of probiotics.

Interactions between a triblock copolymer and hydroxyethyl cellulose in aqueous solution and their use in the solubilization of Amiodarone

A physicochemical characterization of the mixtures of triblock copolymer PEO20-PPO70-PEO20, (P123) and hydroxyethyl cellulose (HEC) in aqueous solutions, with and without the hydrophobic drug amiodarone (ADA), was performed by pendant drop tensiometry, differential scanning calorimetry (DSC) and dynamic light scattering (DLS). The results allowed us to propose a detailed diagram for the formation of the different aggregates in water. The equilibrium surface tension values showed that adding HEC to P123 solutions promotes the aggregation of the P123 monomers on the HEC polymer until saturation. By adding ADA into the P123 micelles, the same equilibrium surface tension values were obtained as the ones found in the pure solution of P123. This suggests that the drug is mostly encapsulated. The DSC experiments showed that the critical micelle temperature and the enthalpy of aggregation of P123 were not modified by the presence of HEC. But the incorporation of ADA to the micelles showed a reduction of 40% in the enthalpy of aggregation, indicating a strong interaction between P123 and ADA. In contrast to the empty micelles, when HEC is added to the loaded micelles, the enthalpy of aggregation increases and the interaction of HEC-P123 becomes evident. The DLS results support that the hydrodynamic diameter of the ADA-loaded micelles is 17% smaller than the diameter of the empty P123 micelles. In the presence of HEC, the empty or loaded micelles form a larger complex. This suggests that HEC surrounds the micelles and provides the drug with a second protecting shell. This system could be a good candidate for the hydrophobic drug delivery.

Synthesis and characterization of cationic colloidal unimolecular polymer (CUP) particles

Cationic colloidal unimolecular polymer (CUP) particles were prepared by using a lower concentration of the quaternary ammonium functional copolymers during the process of water reduction. True nanoscale (diameter 3–9 nm), zero-volatile organic content (VOC), spheroidal CUP particles, and self-stabilized via electro-repulsion of surface cationic groups were obtained. The viscosity of the cationic CUP systems was influenced by the electroviscous effects arising from the surface charge and the associated surface water layer. The density of surface water was 1.6 % greater than the bulk water density which was attributed to the structuring of water around charged quaternary ammonium groups. The equilibrium surface tension values decreased linearly with increasing concentration and surface charge density of CUP particles due to a greater reduction in surface energy. The rate of surface tension reduction determined by maximum bubble pressure method decreased with increasing concentration and the molecular weight of the CUP due to diffusion effects.

Interaction of four-antennary oligoglycines and lipopolysaccharides in aqueous media

Aqueous solutions containing both four-tailed oligoglycine [Gly7–NHCH2–]4C and lipopolysaccharides (Ra-LPS from Escherichia coli EH100) are studied in the present work. The experimental procedure comprises examination of the properties of interfacial layers at the solution/air boundary and the drainage characteristics of microscopic foam films. Significant decrease of the dynamic and equilibrium surface tension values is established upon raising the content of the oligoglycine in the mixture. This effect is related to the plausible formation of amphiphilic LPS/tectomer complexes. The drainage properties of the films go in line with the notion for the possible onset of amphiphilic entities in the solutions. The reported data may be regarded as experimental evidences for the existence of surface active LPS/tectomer complexes in the investigated systems. The results suggest that aqueous solutions of four-antennary olygoglycines could be used for registration of trace quantities and efficient capture of LPS in aqueous media.

Investigação Bioquímica e Biofísica do Surfactante em Aspirado Gástrico Neonatal ao Nascimento

The optimal approach to detection of surfactant deficiency in the prematurely born infants at birth remains unclear and the decision to apply exogenous surfactant is based mainly on the development of clinical and radiological signs of neonatal respiratory distress syndrome (NRDS). We studied the biochemical and biophysical properties of gastric aspirates (GA) from prematurely born infants with NRDS and healthy full term infants with an aim to find an approachable method for assessment of surfactant maturity at birth. Forty-seven newborn infants divided into two groups were enrolled in the study. The first group comprised 34 healthy infants born at term (after 37 weeks of gestation). The second group included 13 premature infants (aged from 26 to 32 weeks of gestation) developing clinical signs of NRDS for which they were treated by assisted ventilation and exogenous surfactant. A biochemical analysis of the protein and lipid content of GA collected at birth was performed. The fatty acid composition of the GA samples was determined by Gas Chromatography-Mass Selective Detector (GS-MSD) analysis. The surface characteristics (equilibrium, maximal and minimal surface tension values) of the GA samples were measured by using the pending drop method. Data were compared between the groups by using Student's t test or Mann-Whitney analysis. Values were considered significantly different if the p value was = 0.05. The mean phospholipids' concentration in GA of the premature infants was lower (295.7 µg / ml vs. 374.5 µg / ml) than in the term infants and the mean protein content was less in GA of the premature babies than the term newborns (574.5 µg / ml vs. 641.5 µg / ml). The measurement of dynamic surface characteristics of GA showed significantly higher mean values of the minimal surface tension (γmin) in the premature infants - 20.5 m / Nm compared to the term babies - 12.3 mN/m (p < 0,01). There was no difference between the equilibrium surface tensions (38 mN/m vs. 38 mN/m) of both groups; The mean values of maximal surface tension (?max) in GA did not differ significantly between the groups (50.1 mN/m vs. 48.5 mN/m). Our findings revealed lower phospholipids' and protein concentrations in the GA at birth from premature infants as compared to the healthy term infants. The dynamic surface characteristics of GA had significant differences between the two groups, the minimal surface tension being the most important parameter for evaluation of surfactant maturity. It could be used in the clinical practice for fast surfactant's assessment in the premature infants in regard to administration of exogenous surfactant.

Dynamic surface tension, critical micelle concentration, and activity coefficients of aqueous solutions of nonyl phenol ethoxylates

Dynamic surface tensions, σ( t) for aqueous solutions of nonyl phenol ethoxylates (NPEOs) at the temperature 298.15 K were measured using a Lauda drop volume tensiometer. The non-ionic surfactants analyzed in this work were Tergitol NP-9, NP-35 and NP-40. By using the classical Ward and Torday equation, the diffusion coefficient for each bulk surfactant concentration was calculated. The equilibrium surface tension values were determined by extrapolating the dynamic surface tension to t → ∞ on the σ( t) vs. t −1/2 curves. These values were used to determine the critical micelle concentrations (CMC) of the surfactant aqueous solutions as well as to calculate the infinite dilution activity coefficient of the surfactant, following a model that combines the Volmer surface equation of state and the Gibbs adsorption equation.

Possible Existence of Convective Currents in Surfactant Bulk Solution in Experimental Pendant-Bubble Dynamic Surface Tension Measurements

Traditionally, surfactant bulk solutions in which dynamic surface tension (DST) measurements are conducted using the pendant-bubble apparatus are assumed to be quiescent. Consequently, the transport of surfactant molecules in the bulk solution is often modeled as being purely diffusive when analyzing the experimental pendant-bubble DST data. In this Article, we analyze the experimental pendant-bubble DST data of the alkyl poly (ethylene oxide) nonionic surfactants, C12E4 and C12E6, and demonstrate that both surfactants exhibit "superdiffusive" adsorption kinetics behavior with characteristics that challenge the traditional assumption of a quiescent surfactant bulk solution. In other words, the observed superdiffusive adsorption behavior points to the possible existence of convection currents in the surfactant bulk solution. The analysis presented here involves the following steps: (1) constructing an adsorption kinetics model that corresponds to the fastest rate at which surfactant molecules adsorb onto the actual pendant-bubble surface from a quiescent solution, (2) predicting the DST behaviors of C12E4 and C12E6 at several surfactant bulk solution concentrations using the model constructed in step 1, and (3) comparing the predicted DST profiles with the experimental DST profiles. This comparison reveals systematic deviations for both C12E4 and C12E6 with the following characteristics: (a) the experimental DST profiles exhibit adsorption kinetics behavior, which is faster than the predicted fastest rate of surfactant adsorption from a quiescent surfactant bulk solution at time scales greater than 100 s, and (b) the experimental DST profiles and the predicted DST behaviors approach the same equilibrium surface tension values. Characteristic (b) indicates that the cause of the observed systematic deviations may be associated with the adsorption kinetics mechanism adopted in the model used rather than with the equilibrium behavior. Characteristic (a) indicates that the actual surfactant bulk solution in which the DST measurement was conducted, most likely, cannot be considered to be quiescent at time scales greater than 100 s. Accordingly, the observed superdiffusive adsorption behavior is interpreted as resulting from convection currents present in a nonquiescent surfactant bulk solution. Convection currents accelerate the surfactant adsorption process by increasing the rate of surfactant transport in the bulk solution. The systematic nature of the deviations observed between the predicted DST profiles and the experimental DST behavior for C12E4 and C12E6 suggests that the nonquiescent nature of the surfactant bulk solution may be intrinsic to the experimental pendant-bubble DST measurement approach. To validate this possibility, we identified generic features in the experimental DST data when DST measurements are conducted in a nonquiescent surfactant bulk solution, and the DST measurements are analyzed assuming that the surfactant bulk solution is quiescent. An examination of the DST literature reveals that these identified generic features are quite general and are observed in the experimental DST data of several other surfactants (decanol, nonanol, C10E8, C14E8, C12E8, and C10E4) measured using the pendant-bubble apparatus.

Synthesis and Properties of Surface Chemically Pure Alkylamidoamine‐N‐oxides at the Air/Water Interface

AbstractA homologous series of surface active 2‐(alkanoylamino)ethyldimethylamine‐N‐oxides, EDA‐p(O), and 3‐(alkanoylamino)propyldimethylamine‐N‐oxides, PDA‐p(O), were synthesized. Their aqueous stock solutions were processed by the automatically operating apparatus to remove surface‐active contamination and provide chemical purity at the air/water interface. In case of 3‐(tetradecanoylamino)propyldimethylamine‐N‐oxide, PDA‐14(O), the difference between equilibrium surface tension values of the purified surfactant solutions and the corresponding values of the solutions prepared from the “as received” compounds amounts to 15 mN m−1. Moreover, in presence of the surface‐active contaminants time needed to reach equilibrium surface tension values is over 2 h. For the solution of the “surface‐chemically pure” grade the change of the surface tension within adsorption time is negligible and the equilibrium is reached instantaneously. Physicochemical properties of obtained surface‐chemically pure aqueous solutions of N‐oxides of alkylamidoamines and adsorption parameters (standard free energy of adsorption, ΔG°ads, surface excess by saturation surface concentration, Γ∞, minimum surface area demand per molecule adsorbed, Amin) were evaluated from the equilibrium surface tension versus concentration isotherms at the air/water interface using Gibbs's equation. The introduction of the CH2 moiety into the intermediate part of molecule causes a slight decrease of the hydrophobic character of surfactant. Also the minimum surface area demand, Amin, is slightly greater for PDA series than for the corresponding EDA derivatives. Surface potential measurements were performed in addition to surface tension studies. Electric surface potential versus concentration isotherms was determined. Surface potential increases with increasing surfactant's bulk concentration for all investigated compounds. At highest concentrations, where interface is almost saturated, changes of surface potential become almost negligible.

On the influence of surfactants in electrowetting systems

Buffer liquids currently used in biotechnology contain surfactants. These surfactants modify the wetting properties of the liquid and affect the behavior of droplets in electrowetting on dielectric (EWOD) microsystems. An analysis of the effect of surfactants in EWOD-based microsystems, at equilibrium and in a non-equilibrium state, is given here. First, a reference for the value of the surface tension and its time variation is obtained by the use of the pendant drop method, showing that the time variation of the surface tension follows a decreasing exponential law. Next, equilibrium experiments have been performed on a specially designed EWOD experimental microsystem. It has been observed that the Lippmann–Young theoretical equation for electrowetting is satisfied at equilibrium, even in the presence of surfactants. However, at the CMC, equilibrium surface tension values are different from those obtained by the pendant drop method. This difference is explained by an internal convective motion that remixes the surfactants. This internal motion is observed by following the motion of fluorophores. Consequently, surfactant concentrations about ten times the CMC are required to obtain the same values of the surface tension in the pendant drop method and by EWOD measurements. In a second section, it is shown that the behavior of surfactant-loaded droplets in EWOD-based microsystems can be explained by considering that the surface tension relaxes with time according to the exponential law found in the first section.

Adsorption-Equilibrium Surface Tension of Surfactant Solutions; Examination by the Drop Volume Method

Abstract The surface tension of aqueous solutions of cationic, nonionic, and anionic surfactants was measured as a function of the size of drop formed under the tip of a glass capillary and the time for which the drop is allowed to stand by a previously developed apparatus based on the drop volume method. The surface tension of cationic and nonionic surfactant solutions was slightly dependent on the drop size and the adsorption time at low concentrations, and we could easily obtain the adsorption-equilibrium surface tension values by use of an intact capillary. However, the surface tension of anionic surfactant solutions showed a strong dependence on these factors, and the equilibrium surface tension values could not be obtained by use of the intact capillary at all. This situation was drastically improved by use of the water-repellent capillary coated with a silane-coupling reagent. We proposed that the equilibrium surface tension value of surfactant solutions can be determined by adopting the adsorption time at which the surface tension value becomes constant on the surface tension vs the drop size curves. Further, the conditions under which the equilibrium surface tension value of surfactant solutions could be acquired by means of the drop volume method were quite different than those of the Wilhelmy plate method.

Effect of surfactant structure on the adsorption of carboxybetaines at the air–water interface

In order to study the effect of charge on the adsorption of surfactants at the air–water interface, two carboxybetaines have been synthesized with different number of separation methylenes between their charged groups. After purification and structure confirmation, the equilibrium and dynamic surface tensions were measured as a function of surfactant concentration for both the cationic and neutral forms of the surfactant molecules. The effect of ionic strength on the adsorption process was also studied. The equilibrium surface tension values were interpreted according to the Langmuir model and the dynamic surface tension data, converted to surface concentration by the Langmuir parameters, are consistent with the assumption of diffusion control over the range of surfactant concentrations studied. The diffusion coefficients show a progressive decrease in the rate of adsorption when the number of methylene units between the betaine charged groups increase.