Composition of Surface Layer at the Water–Air Interface and Micelles of Triton X-100 + Rhamnolipid Mixtures

Measurements of the surface tensions, densities and viscosities of aqueous solutions of Triton X-100 (TX-100) and rhamnolipid (RL) mixtures, at constant concentration of RL or TX-100, were carried out. The measured values of the surface tension were compared to those determined using different theoretical models and on the basis of the surface tension of aqueous solutions of individual surfactants. From the surface tension isotherms, the Gibbs surface excess concentration of TX-100 and RL, the composition of surface layer and the standard Gibbs free energy of adsorption at the water–air interface were determined. Moreover, on the basis of surface tension, density and viscosity isotherms, the CMC of surfactants mixtures were evaluated. From the density isotherms, apparent and partial molar volumes of TX-100 and RL were also determined. These volumes were compared to those calculated from the sizes of TX-100 and RL molecules. There was observed a synergetic effect in the reduction of water surface tension and micelle formation, which was confirmed by the intermolecular interactions parameter. In the case of micelle formation, this effect was discussed based on the standard Gibbs free energy of micellization as well as of TX-100 and RL mixing in the micelles. The synergism of TX-100 and RL mixtures in the reduction of water surface tension and micelle formation was explained on the basis of electrostatic interactions between the hydrophilic part of TX-100 and RL molecules; this was supported by pH measurements.     

Volumetric and Surface Properties of Short Chain Alcohols in Aqueous Solution–Air Systems at 293?K

From measurements of the surface tension, density, viscosity and light scattering of aqueous solutions of methanol, ethanol and propanol at 293?K, their activity in the surface monolayer, surface excess concentration, and apparent and partial molar volume were determined. The surface excess concentration of alcohols at the water?Cair interface was determined from the Gibbs equation by using both the alcohol's activity and their molar fraction in the bulk phase and recalculated by using the Guggenheim?CAdam equation. The values of the surface excess concentration determined from the Gibbs equation were also applied to determine the standard Gibbs energy of alcohol adsorption at the water?Cair interface from Langmuir??s equation and compared to those determined from that of Aronson and Rosen.     

Mutual influence of cetyltrimethylammonium bromide and Triton X-100 on their adsorption at the water–air interface

On the basis of surface tension values of the aqueous solution of cetyltrimethylammonium bromide (CTAB) and Triton X-100 (TX-100) mixtures measured at 293 K as a function of CTAB or TX-100 concentration at constant TX-100 or CTAB concentration, respectively, the real surface area occupied by these surfactants at the water–air interface was established which is inaccessible in the literature. It appeared that at the concentration of the CTAB and TX-100 mixture in the bulk phase corresponding to the unsaturated monolayer at the water air-interface this area is the same as in the monolayer formed by the single surfactant at the same concentration as in the mixture. In the saturated mixed monolayer at this interface the area occupied by both surfactants is lower than that in the single surfactant monolayer corresponding to the same concentration in the aqueous solution. However, the decrease of the CTAB adsorption is lower than that of TX-100 and the total area occupied by the mixture of surfactants is also lower than that of the single one. The area of particular surfactants in the mixed saturated monolayer changes as a function of TX-100 and CTAB mixture concentration and at the concentrations close to CMC or higher the area occupied by both surfactants is the same. The changes of the composition of the mixed surface monolayer are connected with the synergetic effect in the reduction of the water surface tension by the adsorption of CTAB and TX-100 at the water–air interface. This effect was confirmed by the values of the standard Gibbs free energy of adsorption of both individual surfactants and their mixtures with different compositions in the bulk phase determined by using the Langmuir equation if RT instead of nRT was applied in this equation.     

Adsorption of alkyl trimethylammonium bromides at the air/water interface

A number of features of the adsorption of alkyl trimethylammonium bromides with nc=10,12,14, and 16 at the air/water interface were studied. First, the adsorption isotherms were calculated from experimental surface tension vs concentration curves by means of the Gibbs equation. Second, a novel method was used to estimate the adsorption free energy change. From the analysis of these data it was concluded that the hydrophobic driving force for the adsorption first increases with increasing adsorbed amount and then levels off in a plateau, which holds true for all four homologues. This peculiar behavior was interpreted by the formation of a thin liquid-like alkane film at the air/water interface once a certain adsorbed amount is exceeded. The hydrophobic contribution to the standard free energy change of adsorption was compared with those values previously determined for alkyl sulfate homologues. This comparison suggests that the alkyl trimethylammonium type surfactants behave as if their alkyl chain was approximately one methylene group shorter than those of the corresponding alkyl sulfates.     

The adsorption process of some acetic acid derivatives at the water/air interface

The adsorption of trifluoro-, trichloro-, tribromo-, and trimethylacetic acid at the water/air interface is discussed on the basis of surface tension measurements. The process of adsorption is described by Henry's and Langmuir's isotherm equations. The obtained results allow calculation of the standard free energy of adsorption of investigated molecules and the contribution to this energy of hydrophobic groups of these molecules.     

Effect of molecular packing on adsorption and micelle formation of a homologous cationic surfactant mixture of hexadecyltrimethylammonium bromide and dodecyltrimethylammonium bromide

The surface tension of an aqueous solution of a hexadecyltrimethylammonium bromide (HTAB) and dodecyltrimethylammonium bromide (DTAB) mixture was measured as a function of the total molality and the composition of DTAB at 298.15 K under atmospheric pressure. The phase diagrams of adsorption and micelle formation were constructed and the excess Gibbs energy was evaluated by analyzing the phase diagrams thermodynamically. Both the excess Gibbs energy in the adsorbed film and the excess surface area are negative; therefore the mutual interaction between HTAB and DTAB is said to be stronger than that between the same species and is enhanced with increasing adsorption. By combining the results with those obtained in previous studies, we claimed that DTAB molecules can use effectively the space among the hydrocarbon chains of HTAB molecules and their polar head groups take a staggered arrangement at the surface so as to reduce the electrostatic repulsion. Consequently the dispersion force between hydrophobic chains becomes stronger. Furthermore, the comparison of the excess Gibbs energy in the adsorbed film with that in the micelle shows that the staggered arrangement of molecules is not necessary in the spherical micelle.     

Adsorption Properties of Hydrocarbon and Fluorocarbon Surfactants Ternary Mixture at the Water-Air Interface

Measurements were made of the surface tension of the aqueous solutions of p-(1,1,3,3-tetramethylbutyl) phenoxypoly(ethylene glycols) having 10 oxyethylene groups in the molecule (Triton X-100, TX100) and cetyltrimethylammonium bromide (CTAB) with Zonyl FSN-100 (FC6EO14, FC1) as well as with Zonyl FSO-100 (FC5EO10, FC2) ternary mixtures. The obtained results were compared to those provided by the Fainerman and Miller equation and to the values of the solution surface tension calculated, based on the contribution of a particular surfactant in the mixture to the reduction of water surface tension. The changes of the aqueous solution ternary surfactants mixture surface tension at the constant concentration of TX100 and CTAB mixture at which the water surface tension was reduced to 60 and 50 mN/m as a function of fluorocarbon surfactant concentration, were considered with regard to the composition of the mixed monolayer at the water-air interface. Next, this composition was applied for the calculation of the concentration of the particular surfactants in the monolayer using the Frumkin equation. On the other hand, the Gibbs surface excess concentration was determined only for the fluorocarbon surfactants. The tendency of the particular surfactants to adsorb at the water-air interface was discussed, based on the Gibbs standard free energy of adsorption which was determined using different methods. This energy was also deduced, based on the surfactant tail surface tension and tail-water interface tension.     

Adsorption and Micellization in Solutions of Dodecylpyridinium Bromide–Nonionic Surfactant Mixtures

Dependences of the surface tension of aqueous solutions of cationic (dodecylpyridinium bromide) and nonionic (Tween 80, Triton X-100) surfactants and their mixtures on total surfactant concentration and solution composition were studied. The values of critical micellization concentration (CMC) and excess free energy of adsorption were determined from tensiometric measurements. Based on Rubingh–Rosen model (approximation of the theory of regular solutions), the compositions of micelles and adsorption layers at the solution–air interface as well as parameters of interaction between the molecules of cationic and nonionic surfactants were calculated for the systems indicated above. It was established that, in the case of surfactant mixtures with considerable difference in the CMCs, the micelles of individual surfactant with lower CMC value are formed. The effect of negative deviation from the ideality during the adsorption of surfactants from mixed solutions at the solution–air interface was disclosed. It was shown that the interaction energy depends significantly on the composition of mixed systems.     

Interfacial and micellar properties of binary mixtures of surfactant and phospholipid in an aqueous medium

Mixed micelles formed by zwitterionic surfactant dimethyldodecylammniopropane sulfonate and short-chain phospholipid 1,2-diheptanoyl-sn-glycero-3-phosphocholine in different proportions in an aqueous medium have been studied physicochemically at an air/water interface and in the bulk by using interfacial tension and pyrene fluorescence intensity measurements, respectively. The critical micellar concentration and free energies of micellization and of interfacial adsorption have been determined. The interfacial study reveals that a mixed monolayer is formed at the air/water interface by the adsorption of surfactant and phospholipid monomers. This has been confirmed by evaluating the interfacial parameters; the maximum surface excess, the minimum area per molecule of a surface-active compound, and the Gibbs surface excess related to surface pressure. The nonideality of mixing, expressed in the terms of the regular solution interaction parameter, #, has negative values over the whole mole fraction range. The negative # values indicate the mutual synergism between the surfactant and phospholipid monomers. The equilibrium distribution of components between micelle and monomer phases was evaluated using a theoretical treatment based on excess thermodynamics quantities evaluated by Motomura's formulation.     

Adsorption and solvation of ethanol at the water liquid-vapor interface: a molecular dynamics study

The free energy profiles of methanol and ethanol at the water liquid-vapor interface at 310K were calculated using molecular dynamics computer simulations. Both alcohols exhibit a pronounced free energy minimum at the interface and, therefore, have positive adsorption at this interface. The surface excess was computed from the Gibbs adsorption isotherm and was found to be in good agreement with experimental results. Neither compound exhibits a free energy barrier between the bulk and the surface adsorbed state. Scattering calculations of ethanol molecules from a gas phase thermal distribution indicate that the mass accommodation coefficient is 0.98, and the molecules become thermalized within 10 ps of striking the interface. It was determined that the formation of the solvation structure around the ethanol molecule at the interface is not the rate-determining step in its uptake into water droplets. The motion of an ethanol molecule in a water lamella was followed for 30 ns. The time evolution of the probability distribution of finding an ethanol molecule that was initially located at the interface is very well described by the diffusion equation on the free energy surface.     

Mixed micellization and surface properties of non-ionic/cationic surfactants

We investigate the surface properties of aqueous binary mixtures of our cationic surfactant O-dodecyl-N,N′-diisopropylisourea hydrochloride (ISO-DIC C₁₂) with commercially available nonionic surfactant polyoxyethylene p-(1,1,3,3-tetramethylbutyl)phenyl ether (TritonX-100) at different temperatures (288 to 303?K). The micellization behavior of the binary systems is studied by determining the surface tension and other important physicochemical parameters, such as the critical micelle concentration (CMC), surface tension at the CMC(γ_cmc), Krafft Temperature (T_K), maximum excess concentration (Γ_max), minimum surface area per molecule (A_min), surface pressure at the CMC (П_cmc), and the adsorption efficiency (pC20) at the air/water interface. The study has additionally covered the calculation of thermodynamic parameters of micellization, including the standard Gibbs free energy, the standard enthalpy, the standard entropy, the free energy, and the Gibbs free energy of adsorption at air/water interface. The CMC values of the binary systems determined by experimental data are used to evaluate the micellar composition in the mixed micelle, the interaction parameter β and the activity coefficients f₁(ISO-DIC C₁₂) and f₂ (polyoxyethylene p-(1,1,3,3-tetramethylbutyl)phenyl ether) using the theoretical treatment proposed by Clint and Rubingh. Our results reveal that the proposed binary systems possess enhanced surface activity compared to those of the individual surfactants.     

Surface adsorption and micelle formation of aqueous solutions of polyethyleneglycol and sugar surfactants

The surface tension of aqueous solutions of tetraethyleneglycol octyl ether (C8E4) and octyl-β-d-maltopyranoside (OM) mixture was measured as a function of the total molality of surfactants and the composition of OM under atmospheric pressure at 298.15 K by drop volume technique. The results of surface tension measurements were analyzed by originally developed thermodynamic equations, then phase diagrams of adsorption and micelle formation were constructed. From the analysis of the surface tension data, it was found that the C8E4 and OM molecules interact attractively in the adsorbed film and the excess Gibbs energy of adsorption can be compared with those observed in typical cationic–nonionic surfactant systems; nevertheless, they are mixed almost ideally in the mixed micelle. Judging from a negative excess surface area calculated by differentiating the excess Gibbs energy by the surface tension, we concluded that the attraction between C8E4 and OM molecules is a short-range one originated in the hydrogen bonding between them which favors the planar configuration.     

Micellization and Adsorption Parameters of Poly(Propylene Oxide)/Poly(Ethylene Glycol) Block and Graft Copolymers in Aqueous Medium

This work aims to measure the adsorption and micellization parameters of new water soluble nonionic amphiphilic block and graft copolymers based on hydrophilic poly (ethylene glycol) (PEG) and hydrophobic poly (propylene oxide) (PPO) at ambient temperature and normal atmospheric pressure. The chemical composition and molecular weights of the prepared copolymers were determined from ¹H NMR analyses. The surface properties of the prepared surfactants were determined by measuring the surface tension at different temperatures. The surface tension, critical micelle concentration, and surface activities were determined at different temperatures. Surface parameters such as surface excess concentration, the area per molecule at interface and the effectiveness of surface tension reduction were determined from the adsorption isotherms of the prepared surfactants. Some thermodynamic data for the adsorption and micellization process were calculated and are discussed.     

多支链烷基苯磺酸钠水溶液的表面性质

用自制的四种高纯度多支链烷基苯磺酸钠,研究了支链结构对其表面性质的影响.结果表明,随支链烷基碳数增加,临界胶束浓度降低,标准吸附自由能DGadӨ更负;但是,饱和吸附量Γmax却随支链烷基碳数增加而减小,且临界胶束浓度时的表面张力γcmc随吸附量减小而降低,表现出与一般表面活性剂不同的变化趋势.从多支链烷基苯磺酸钠的分子结构特点,解释了随支链烷基碳数增加Γmax和γcmc的变化规律,探讨了分子的独占面积(as)对Γmax及γcmc的影响.     

X型与线型嵌段聚醚在空气/水和正庚烷/水界面上聚集行为的比较研究

通过阴离子聚合方法合成了环氧乙烷(EO)含量和分子量均相同的线型聚氧丙烯(PEO)-聚氧乙烯(PPO) (LPE)和X型聚氧丙烯-聚氧乙烯(TPE)嵌段聚醚,考察了它们在空气/水及正庚烷/水界面上聚集行为的差异. 界面活性的研究结果表明,TPE降低水、正庚烷界面张力的效率和效能均低于LPE的. 聚醚分子在正庚烷/水界面达到吸附平衡的时间比在空气/水表面短. 由于正庚烷分子插入到聚醚吸附层中,聚醚分子可以在正庚烷/水界面上采取更为直立的状态,因此聚醚分子在正庚烷/水界面扩散较快. 聚醚在正庚烷/水界面的扩张弹性高于空气/水表面的.     

Adsorption of cetyltrimethylammonium bromide and propanol mixtures with regard to wettability of polytetrafluoroethylene. I. Adsorption at aqueous solution-air interface

Measurements of surface tension of aqueous solutions of cetyltrimethylammonium bromide (CTAB) and propanol mixtures (gamma(L)) for 1 x1 0(-5), 1 x 10(-4), 6 x 10(-4), and 1 x 10(-3) M concentrations of CTAB as a function of propanol concentration in the range from 0 to 6.67 M at 293 K were carried out. The obtained results indicate that there is first-order exponential relationship between the surface tension and propanol concentration in the solution at constant CTAB concentration. These results were compared with those calculated from the equations derived by von Szyszkowski, Joos, Miller et al. From the comparison it resulted that the values of gamma(L) determined by the Szyszkowski equation are correlated with those measured only in a limited propanol concentration range because of changes of the constant related to the specific capillary activity in this equation as a function of propanol concentration, particularly in the range of its high concentration. In the case of the modified Joos equation there is a correlation between the calculated and measured values of gamma(L) only at a very low concentration of propanol. The values of the surface tension of aqueous solutions of CTAB and propanol mixtures determined by the relationships of Miller et al. at CTAB concentration, corresponding to unsaturated surface layer in the absence of propanol, are close to those measured, but there are bigger differences between the calculated and measured values of the surface tension for solutions at a constant value of CTAB concentration close to CMC. However, the values of the surface tension of aqueous solution of CTAB and propanol mixtures calculated from the modified Miller et al. equation, in which the aggregation process of alcohol molecules at water-air interface was taken into account, are in excellent agreement with those measured. The measured values of the surface tension and the Gibbs equations were used for determination of the surface excess of CTAB and propanol concentration at solution-air interface. The obtained results indicate that at the constant concentration of CTAB equal to 1 x 10(-5) and 1 x 10(-4) M there is a maximum of excess concentration of propanol in the surface region at its bulk concentration close to 1 M. Using the calculated values of the surface excess concentration of propanol and CTAB at solution-air interface and assuming the proper thickness of the interface region, the total values of their concentration in this region were evaluated. Next, the standard surface free energy of CTAB and propanol mixtures adsorption was calculated. The calculated values of this energy indicate that the tendency to adsorb molecules of CTAB and propanol decreases with increasing propanol concentration probably because of entropy of adsorption decrease resulting from water structure destruction by propanol molecules.     

Synthesis and study of nonionic surfactants based on propylene oxide and lauric acid

Nonionic surfactants were synthesized at various molar ratios of propylene oxide and lauric acid. Propoxylation products were identified by a number of physico-chemical analytical methods. The tensiometric method was used to measure the surface tension of aqueous solutions of the products at interface with air, find their high surface activity, and determine certain colloidal-chemical parameters (critical micelle concentration, surface pressure, maximum adsorption, minimum cross-sectional area of a molecule, change in the Gibbs free energy in micelle-formation and adsorption processes). Laboratory studies of the oil-collecting capacity of these substances were carried out for the example of a thin oil film on the surface of water with varied degree of mineralization.     

固-液界面上的吸附膜

利用Gibbs吸附公式处理了硅胶自四氯化碳和环己烷中吸附脂肪醇、环己醇、苯甲醛、苯甲醚、乙酸丙酯的实验结果,得到吸附膜的表面压(π)和分子面积(A)的关系曲线,这些曲线均可用描述不溶物液态扩张膜的Smith方程描述。文中对所得结果给出了初步的解释。     

Preferential adsorption of cationic surfactant mixture studied by bromide ion selective total-reflection XAFS measurement

The total-reflection XAFS measurement possessing bromide ion selectivity at the interfacial region was applied to the adsorbed film of hexadecyltrimethylammonium chloride (HTAC) and dodecyltrimethylammonium bromide (DTAB) mixture. The surface compositions XjH of individual ions j ( j = HTA+, Cl(-), DTA+, and Br (-)) were evaluated by combining the surface excess concentration of Br(-) estimated from the XAFS with the surface composition of the respective surfactants from the surface tension results. It is clearly shown that HTA+ and Br(-) are preferentially adsorbed to DTA+ and Cl(-) at the air/water interface. The preferential adsorption was estimated numerically in terms of activity coefficient fi+/-(H,p) of component i and excess Gibbs energy of adsorption ?prH,E. Then, the magnitude of ?prH,E was compared with that of ?prH,E attributable to intrinsic interaction between ions.     

Adsorption of poly(ethylene oxide) at the air/water interface: a dynamic and static surface tension study

The adsorption of polyethylene oxide (PEO) homologues in a wide range of molecular weight (from M(PEO)=200 to 10(6)) at the air/aqueous solution interface was investigated by dynamic and static surface tension measurements. An approximate estimate for the lower limit of PEO concentration was given at which reliable equilibrium surface tension can be determined from static surface tension measurements. It was shown that the observed jump in the earlier published sigma-lg(c(PEO)) curves is attributable to the nonequilibrium surface tension values at low PEO concentrations. The adsorption behavior of short chain PEO molecules (M(PEO)1000) is similar to that of the ordinary surfactants. The estimated standard free energy of PEO adsorption, DeltaG(0), increases linearly with the PEO molecular weight until M(PEO)=1000. In this molecular weight range, DeltaG(0) was found to be approximately the fifth of the hydrophobic driving force related to the adsorption of a surfactant with the same number of methylene groups. In the case of the longer chain PEOs the driving force of adsorption is so high that the adsorption isotherm is near saturation in the experimentally available polymer concentration range. Above a critical molecular weight the PEO adsorption reveals universal features, e.g., the surface tension and the surface density of segments do not depend on the polymer molecular weight.