Novel calibration of a dynamic surface tension detector: flow injection analysis of kinetically-hindered surface active analytes

First, a novel technique for calibration of a dynamic surface tension detector (DSTD) is described. The DSTD measures the differential pressure as a function of time across the liquid-air interface of growing drops that repeatedly form and detach at the end of a capillary tip. The calibration technique utilizes the ratio of pressure signals acquired from the drop growth of two separate solutions, i.e. a standard solution and a corresponding mobile phase, such as water, both of which have a known surface tension. Once calibrated, the dynamic surface tension of an analyte is obtained from the ratio of the pressure signals from the analyte solution to that of the mobile phase solution. Thus, this calibration technique eliminates the need to optically image the radius of the expanding drop of liquid. Accurate dynamic surface tension determinations were achieved for aqueous sodium dodecyl sulfate (SDS) solutions over a concentration range of 0.5-5.4 mM. The measured surface tensions for these SDS solutions range from 70.3 to 46.8 dyne/cm and were in excellent agreement with the literature. A precision of 0.2 dyne/cm (1 S.D.) was routinely obtained. Second, the DSTD with this calibration technique was combined with flow injection analysis (FIA) for the study of model protein solutions and polymer solutions. The kinetic surface tension behavior of aqueous bovine serum albumin (BSA) solutions as a function of concentration and flow rate is presented. Evaluation of the dynamic surface tension data illustrates that a protein such as BSA initially exhibits kinetically-hindered surface tension lowering, i.e. a time dependence, as BSA interacts with the liquid-air interface of an expanding drop. FIA/DSTD is then shown to be an effective tool for the rapid study of kinetically-hindered surfactant mixtures. It was found that mixtures of SDS and the polymeric surfactant Brij(R)-35 (lauryl polyoxyethylene ether with an average molecular weight of 1200 g/mol) result in essentially an additive lowering of the surface tension. Mixtures of polyethylene glycol (PEG), with an average molecular weight of 1470 g/mol, and Brij(R)-35, however, result in a competitive (non-additive) surface tension with the Brij(R)-35 dominating the response.     

The measurement of dynamic surface tension by the maximum bubble pressure method

The principle of maximum pressure in a bubble for measurements of dynamic surface tension is realized in a fully automatically operating apparatus. The set-up yields data in the time interval from 1 ms up to several seconds and can be temperature controlled from 5° to 80°C. Experimental data obtained for different surfactants and gelatine in water and/or water/glycerine mixtures at different temperatures are discussed. A direct comparison with results from oscillating jet and inclined plate experiments shows excellent agreement.     

Surface tension of some liquid crystals in the cyanobiphenyl series

The temperature dependence of the surface tension was measured by the pendant drop method for four compounds from the homologous series of alkylcyanobiphenyls (nCB), in the nematic liquid crystal and isotropic phases. For 8CB (octylcyanobiphenyl) the temperature dependence was also measured in the smectic range. Not very close to the isotropic transition temperature, and with the exception of 8CB, the surface tension decreases with increase in temperature in the nematic range. A downward jump at the transition temperature was observed for all liquid crystals studied. The shape of the drop in the smectic A phase of 8CB gives indications of stratification in a system of terraces.     

A small molecular size system giving unexpected surface effects: alpha-Cyclodextrin + sodium dodecyl sulfate in water

Maximum drop volumes (MDV) and the resultant surface tension values (sigma) of alpha-cyclodextrin (alpha-CD) + sodium dodecyl sulfate (SDS) aqueous mixtures have been determined over a broad concentration range of both solutes at 283.15, 293.15, 303.15, 313.15, and 323.15 K. Drops significantly larger than those of pure water (up to approximately 25% larger) were observed at low temperatures for solutions with [alpha-CD]/[SDS] concentration ratios, approximately > 2, producing unexpectedly high surface tension values. Our results indicate that at certain solute concentration ratios and temperatures, the drop volume method provides wrong values for equilibrium surface tensions. This is due to the high viscoelasticity of the surface film whose effect is important even though the injection rate of the drops was slow and the solutes molecular sizes are small.     

Surface behavior of the 1-bromobutane with isomeric butanol mixtures

Surface tensions of the 1-bromobutane with isomeric butanol mixtures were measured in the temperature range 283.15 K (or 298.15 K for 2-methyl-2-propanol) to 313.15 K with a drop volume tensiometer, and the corresponding surface tension deviations were calculated. Using this information together with bulk thermodynamic properties a thermodynamic study on surface formation was presented. This study includes the calculation of excess surface compositions and excess properties of surface formation.     

Development of a new methodology to study drop shape and surface tension in electric fields

Development of a new methodology for the study of both shape and surface tension of conducting drops in an electric field is presented. This methodology, called axisymmetric drop shape analysis-electric fields (ADSA-EF), generates numerical drop profiles in an electrostatic field, for a given surface tension. Then, it calculates the true value of the surface tension by matching theoretical profiles to the shape of experimental drops, using the surface tension as an adjustable parameter. ADSA-EF can be employed to simulate and study drop shapes in the electric field and to determine its effect on liquid surface tension. The method can also be used to measure surface tension in microgravity, where current drop-shape techniques are not applicable. The axisymmetric shape of the drop is the only assumption made in the development of ADSA-EF. The new scheme is applicable when both gravity and electrostatic forces are present. Preliminary measurements using ADSA-EF suggest that the surface tension of water increases by about 2% when an electric field with the magnitude of 10(6) V/m is applied.     

Do polysaccharides such as dextran and their monomers really increase the surface tension of water?

It has been reported in the literature that sugars such as dextrose and sucrose increase the surface tension of water. The effect was interpreted as a depletion of the solute molecules from the water-air interface. This paper presents accurate measurements of the surface tension of different concentrations of dextrose solution as well as its polymer (i.e., dextran). An automated drop shape technique called axisymmetric drop shape analysis (ADSA) was used for the surface tension determination. The surface tension measurement is presented as a function of a shape parameter, P(s), which has been used to quantify the range of the applicability of ADSA. The results of the above study show that dextrose solutions decrease the surface tension of water in contradiction to the results obtained from the weight drop method in the literature. The surface tension decreases continuously with increasing concentration. A similar effect was observed for the dextran solutions. To verify that the setup and the methodology are capable of accurately measuring increases in surface tension, a similar experiment was conducted with a sodium chloride solution with a concentration of 1 M. It is well-known that electrolyte solutions, e.g., sodium chloride, increase the surface tension of water. The results obtained from ADSA verify that the sodium chloride increases the surface tension of water by 1.6 mJ/m(2). It is concluded that dextrose and dextran decrease the surface tension of water. Thus, there is no evidence of depletion. To identify the sources of discrepancy between the results of ADSA and those reported in the literature, the experiments were repeated for different concentrations and the rate of drop formation using the drop weight method. It was found that the rate of drop formation is most likely the source of error in the results reported in the literature.     

近红外光谱用于低温水结构的分析

采用近红外光谱分析并结合拉曼光谱和分子动力学模拟研究了二甲基亚砜(DMSO)-水混合物中水在低温时的结构, 对DMSO降低水冰点的机理进行了讨论. 通过对不同DMSO含量混合物的近红外和拉曼光谱分析, 发现了DMSO与水相互作用的光谱信息, 表明DMSO与水在混合物中主要以摩尔比1∶2和2∶1的氢键结构(DW2和D2W)的形式存在, 结构形式主要取决于DMSO和水在混合物中的比例. 通过对水和30%(摩尔分数)DMSO-水混合物的温控近红外光谱分析, 发现DW2结构抑制四面体水结构的形成是混合物冰点降低的主要原因. 采用分子动力学模拟对DMSO-水混合物体系进行的模拟进一步证明了结论的可靠性.     

Effect of anionic surfactant and short-chain alcohol mixtures on adsorption at quartz/water and water/air interfaces and the wettability of quartz

Measurements of the advancing contact angles for aqueous solutions of sodium dodecyl sulfate (SDDS) or sodium hexadecyl sulfonate (SHS) in mixtures with methanol, ethanol, or propanol on a quartz surface were carried out. On the basis of the obtained results and Young and Gibbs equations the critical surface tension of quartz wetting, the composition of the surface layer at the quartz-water interface, and the activity coefficients of the anionic surfactants and alcohols in this layer as well as the work of adhesion of aqueous solutions of anionic surfactant and alcohol mixtures to the quartz surface were determined. The analysis of the contact angle data showed that the wettability of quartz changed visibly only in the range of alcohol and anionic surfactant concentration at which these surface-active agents were present in the solution in the monomeric form. The analysis also showed that there was a linear dependence between the adhesion and the surface tension of aqueous solutions of anionic surfactant and alcohol mixtures. This dependence can be described by linear equations for which the constants depend on the anionic surfactant and alcohol concentrations. The slope of all linear dependence between adhesion and surface tension was positive. The critical surface tension of quartz wetting determined from this dependence by extrapolating the adhesion tension to the value equal to the surface tension (for contact angle equal zero) depends on the assumption whether the concentration of anionic surfactant or alcohol was constant. Its average value is equal to 29.95mN/m and it is considerably lower than the quartz surface tension. The positive slope of the adhesion-surface tension curves was explained by the possibility of the presence of liquid vapor film beyond the solution drop which settled on the quartz surface and the adsorption of surface-active agents at the quartz/monolayer water film-water interface. This conclusion was confirmed by the work of adhesion of aqueous solutions of anionic surfactants and short-chain alcohol mixtures to the quartz surface determined on the basis of the contact angle data and molar fraction of anionic surfactants and alcohols and their activity coefficient in the surface layer.     

The thermodynamic properties of solutions of L-ascorbic acid in dimethyl sulfoxide and water-dimethyl sulfoxide mixtures

Solutions of L-ascorbic acid (AA) in dimethyl sulfoxide (DMSO) and DMSO-water mixtures were studied by the densitometry, surface tension, and calorimetry methods. The apparent and partial molar volumes of AA in solutions at 298.15 K were calculated. Surface tension insignificantly increased as the concentration of AA in DMSO grew. The enthalpies of solution of AA in the solvents and the enthalpies of transfer of AA from water into DMSO and DMSO-water mixed solvents were calculated. The results obtained were explained by the existence of H-bonds between AA and DMSO molecules.     

Surface tension of some liquid crystals in the cyanobiphenyl series

The temperature dependence of the surface tension was measured by the pendant drop method for four compounds from the homologous series of alkylcyanobiphenyls (nCB), in the nematic liquid crystal and isotropic phases. For 8CB (octylcyanobiphenyl) the temperature dependence was also measured in the smectic range. Not very close to the isotropic transition temperature, and with the exception of 8CB, the surface tension decreases with increase in temperature in the nematic range. A downward jump at the transition temperature was observed for all liquid crystals studied. The shape of the drop in the smectic A phase of 8CB gives indications of stratification in a system of terraces.     

Effect of solution viscosity on dynamic surface tension detection

A dynamic surface tension detector (DSTD) was used to examine the molecular diffusion and surface adsorption characteristics of surface-active analytes as a function of solution viscosity. Dynamic surface tension is determined by measuring the differential pressure across the air/liquid interface of repeatedly growing and detaching drops. Continuous surface tension measurement throughout the entire drop growth is achieved for each eluting drop (at a rate of 30 drops/min for 2 μl drops), providing insight into the kinetic behavior of molecular diffusion and orientation processes at the air/liquid interface. Three-dimensional data are obtained through a calibration procedure previously developed, but extended herein for viscous solutions, with surface tension first converted to surface pressure, which is plotted as a function of elution time axis versus drop time axis. Thus, an analyte that lowers the surface tension results in an increase in surface pressure. The calibration procedure derived for the pressure-based DSTD was successfully extended and implemented in this report to experimentally determine standard surface pressures in solutions of varied viscosity. Analysis of analytes in viscous solution was performed at low analyte concentration, where the observed analyte surface activity indicates that the surface concentration is at or near equilibrium when in a water mobile phase (viscosity of 1.0 Cp). Two surface-active analytes, sodium dodecyl sulfate (SDS) and polyethylene glycol (MW 1470 g/mol, PEG 1470), were analyzed in solutions ranging from 0 to 60% (v/v) glycerol in water, corresponding to a viscosity range of 1.0-15.0 Cp. Finally, the diffusion-limited surface activity of SDS and PEG 1470 were observed in viscous solution, whereby an increase in viscosity resulted in a decreased surface pressure early in drop growth. The dynamic surface pressure results reported for SDS and PEG 1470 are found to correlate with solution viscosity and analyte diffusion coefficient via the Stokes-Einstein equation.     

Surface tensions for isomeric chlorobutanes with isomeric butanols

A drop volume tensiometer was employed to measure the surface tensions for the binary mixtures of each of the isomers of chlorobutane with each of the isomers of butanol at a temperature of 298.15 K. From these data the surface tension deviations were calculated. The results have been compared with the predictions obtained by the group contribution method proposed by Suarez.     

A liquid drop RC filter apparatus for detection

A new analytical detector based on a liquid drop resistor–capacitor (RC) filter is described, in which transformed gain vs. frequency curves are used to analyze compounds. This detector can be used to detect either charged or neutral species (that are dielectrically different) which are dissolved in a liquid (e.g., water, alcohol, solvent mixtures, etc.). This device was fabricated by modifying an electrowetting on dielectric (EWOD)-based experimental setup. When a liquid drop is placed on a dielectric surface, the system acts as a RC filter. At a given frequency, gain is a function of conductivity, surface tension, dielectric constant, double-layer thickness of the solid–liquid drop interface, as well as the applied voltage. Since different liquids and solutions have different physical properties, each liquid/solution has a unique curve (peak) in gain vs. frequency plot. This is the basic principle behind the detector. Different amounts of zinc chloride dissolved in water, benzalkonium chloride in water, 1-methylimidazole in water, cetyltrimethyl-ammonium chloride (CTAC) in water, and CTAC dissolved in ethylene glycol solutions were tested with the detector as proof of principle. The device can be used as a stand-alone detector or can easily be coupled with droplet based microfluidic lab-on-a-chip systems such as EWOD-based microfluidic chips.     

水合物生成条件下甲烷在水中的吸附

采用悬滴法系统地测定了温度274.2 ~ 282.2 K、压力0.1 ~ 10.1 MPa下甲烷/纯水间界面张力。实验结果表明在恒定温度下界面张力随压力的增加而增大。在高压条件下,压力对界面张力有很大的影响。不同温度和压力下计算出的甲烷在水中的表面过剩浓度结果表明,压力越高,温度越低,甲烷在水溶液中的吸附浓度越高。同时,计算出的甲烷在水溶液中的表面吸附自由能结果表明,在水合物生成条件下,甲烷在水中的吸附比298.2 K更容易。     

Synthesis and Characterization of Poly(ether-block-amide) Membranes

Poly(ether-block-amide) membranes were made via casting a solution on a nonsolvent (water) surface. In this research, effects of different parameters such as ratio of solvent mixture (n-butanol/isopropanol), temperature, composition of coagulation bath (water) and polymer concentration, on quality of the thin film membranes were studied. The mechanism of membrane formation involves solution spreading, solvent–nonsolvent exchange, and partial evaporation of the solvent steps. Solvent- nonsolvent exchange is the main step in membrane formation and determines membrane morphology. However, at higher temperature of polymeric solution greater portion of solvent evaporates. The results showed that type of demixing process (mutual affinity between solvent and nonsolvent) has important role in film formation. Also, addition of solvent to the nonsolvent bath is effective on membrane morphology. The film quality enhances with increasing isopropanol ratio in the solvent mixture. This behavior can be related to increasing of solution surface tension, reduction of interfacial tension between solution and nonsolvent and delayed solvent-nonsolvent demixing. Uniform films were made at a temperature rang of 60–80 °C and a polymer concentration of 4–7 wt%. Morphology of the membranes was investigated with scanning electron micrograph (SEM). Pervaporation of ethyl butyrate/water mixtures was studied using these membranes and high separation performance was achieved. For ethyl butyrate/water mixtures, It was observed that both permeation flux and separation factor increase with increasing ethyl butyrate content in the feed. Increasing temperature in limited range studied resulted in decreasing separation factor and increasing permeation flux.     

Effect of boron on the surface tension of molten silicon and its temperature coefficient

The influence of boron concentration (C(B)/mass%) on the surface tension of molten silicon has been investigated with the sessile drop method under oxygen partial pressure P(O(2))=1.62x10(-25)-2.63x10(-22) MPa, and the results can be summarized as follows. The surface tension increases with C(B) in the range below 2.09 mass%, and the maximum increase rate of the surface tension is about 30 mN m(-1)(mass% C(B))(-1). The temperature coefficient of the surface tension, ( partial differential sigma/ partial differential T)C(B), was found to increase with the boron concentration in molten silicon. At the interface between molten silicon and the BN substrate, a discontinuous Si(3)N(4) layer was reckoned to form and the layer might prevent BN from dissolving into the molten silicon. Since dissolved boron from the BN substrate into the molten silicon is below 0.054 mass% and the associated increase in surface tension is below 1.5 mN m(-1), the contamination from the BN substrate on the surface tension can be ignored. The relation between the surface tension and C(B) indicates negative adsorption of boron and can be well described by combining the Gibbs adsorption isotherm with the Langmuir isotherm.     

Wetting behavior of mixtures of water and nonionic polyoxyethylene alcohol

Five binary water + C4Ej mixtures, water + n-C4E0, water + 2-C4E0, water + iso-C4E0, water + n-C4E1, and water + iso-C4E1, were chosen to perform the surface/interfacial tension measurements over the experimental temperature range from 10 to 85 degrees C at the normal pressure by using a homemade pendent drop/bubble tensiometer. The symbol CiEj is the abbreviation of a nonionic polyoxyethylene alcohol CiH(2i+1)(OCH2CH2)jOH. The wetting behavior of the CiEj-rich phase at the interface separating gas and the aqueous phase is systematically examined according to the wetting coefficient resulting from the experimental data of surface/interfacial tensions measurements. For those systems with a lower critical solution temperature, for example, water + n-C6E2, water + n-C4E1, and water + iso-C4E1, a wetting transition from partial wetting to nonwetting is always observed when the system is brought to close to its lower critical solution temperature. On the other hand, to start with a partial wetting CiEj-rich phase, a wetting transition from partial wetting to complete wetting is always observed when the system is driven to approach its upper critical solution temperature. The effect of hydrophobicity of CiEj on the wetting behavior of the CiEj-rich phase at the interface separating gas and the aqueous phase was carefully investigated by using five sets of mixtures: (1) water + n-C4E0, water + n-C5E0, and water + n-C6E0; (2) water + 2-C4E0 and water + 2-C5E0; (3) water + 2-C4E0 and water + n-C4E0; (4) water + n-C4E1, water + n-C5E1, and water + n-C6E1; (5) water + n-C4E0 and water + n-C4E1. The CiEj-rich phase would tend to drive away from complete wetting (or nonwetting) to partial wetting with an increase in the hydrophobicity of CiEj in the binary water + CiEj system. All the wetting behavior observed in the water + CiEj mixtures is consistent with the prediction of the critical point wetting theory of Cahn.     

Surface tension of binary and two ternary mixtures containing water–acetonitrile–methanol/ethanol at 298.15 K: experimental data,correlation and prediction by various models

Surface tension of two ternary mixtures of water/acetonitrile/methanol and water/acetonitrile/ethanol, and their constituent binaries, were measured over the whole range of composition at 298.15 K and ambient pressure. The experimental data were used to calculate in the surface tension deviations (Δσ). The negative values of Δσ for the binary and ternary systems indicate the strong hydrogen bonding between unlike molecules of mixtures (particularly in the high concentration of water). Surface tension data of the binary systems were correlated with Fu et al., Wang–Chen, Redlich–Kister and Myers–Scott models. The mean standard deviation obtained from the comparison of experimental and calculated surface tension values for binary systems with four models is less than 0.42. Finally, the concentration dependence of the surface tension deviation of the ternary mixtures at 298.15 K was correlated using Pando et al. and Ku et al. models, with satisfactory results.