Shi-Yow Lin

Temperature dependence of critical micelle concentration of polyoxyethylenated non-ionic surfactants

Abstract The critical micelle concentrations (CMCs) of an n-dodecyl polyoxyethylene glycol monoether (C12H25O(C2H4O)jH) with three different oxyethlyene chain length (j=4, 6 and 8) are experimentally determined over the temperature ranging from 10°C to 80°C using the Wihelmy plate technique. It is found that there exists a minimum CMC in the CMC-temperature curve. The temperature of the minimum CMC for three systems is around 50°C. The enthalpy and entropy of micelle formation are evaluated. The correlation of enthalpy and entropy of micelle formation exhibits an excellent linearity, and the compensation temperature is 321 K, dramatically larger than previous findings for non-ionic surfactants in aqueous solutions.

Pretreatment and conversion of lignocellulose biomass into valuable chemicals

In the past three decades, many studies on the production of biofuels and other chemicals have been conducted using renewable sources such as lignocellulosic biomass. Lignocellulosic biomasses are abundantly available in most countries and furthermore they are carbon neutral. However, the main problem in utilizing lignocellulosic materials lies in the recalcitrance of its bonding. This review provides a comprehensive overview and a brief discussion on producing biofuel and valuable chemicals from lignocellulose biomass. Various aspects of the physical, chemical, thermophysical, thermochemical and biological pretreatment of lignocellulosic materials are discussed in this review. The success in biofuel and chemical production strongly depends on the pretreatment method used. Overall, pretreatment is the major step in the successful production of valuable products from lignocellulosic biomass.

A method for correcting the contact angle from the θ/2 method

Abstract The θ /2 method, a widely used technique on measuring the contact angle of a sessile drop, assumes that the drop profile is part of a sphere. However, the shape profile of a sessile drop is governed by the Young–Laplace equation and is different from a sphere, especially for drops with a large bound number (e.g. large volume or small surface tension). The spherical assumption, therefore, causes errors on evaluating the contact angles. The deviation of contact angle from the θ /2 method is evaluated from a theoretical calculation in this work. A simple means is given for correcting the measurement error. The corrected angle results from the drop volume, surface tension, liquid density and the contact angle from θ /2 method. An algorithm for finding the correct contact angle without knowing the density and surface tension is also given. At the end, two examples of pendant drops are given for the illustration.

Determination of adsorption of an ionic surfactant on latex from surface tension measurements

Abstract The adsorption isotherm of sodium dodecyl sulfate (SDS) on polystyrene (PSt) latex is evaluated from surface tension (γ) measurements of aqueous SDS solutions and PSt latex solutions. A video-enhanced pendant bubble/drop tensiometer is applied for the equilibrium and dynamic surface tension measurement of SDS aqueous solutions and PSt latex solutions. The bulk concentrations of un-absorbed SDS molecules (CSDS, SDS in water bulk phase) in PSt latex solutions are evaluated from equilibrium surface tension data, which is uniquely related between the equilibrium γ and bulk concentration CSDS for SDS aqueous solutions. From the mass balance calculation, mtotal=CSDS×Vwater+ΓSL×A (here A is the surface area of pendant drop of latex solution), the surface concentration of SDS (ΓSL) on PSt latex is then determined for aqueous latex solutions with different SDS concentrations. A unique relationship between ΓSL and CSDS is resulted and the Langmuir adsorption isotherm, with maximum surface concentration of SDS at PSt latex (Γ∞=5.75×10−10 mol cm−2) and equilibrium constant a=6.49×10−6 mol cm−3, describes this dependence well.

Dynamic behaviors of droplet impact and spreading: water on five different substrates.

The dynamic wetting behaviors, especially the droplet morphology, of a water droplet impinging on five substrate surfaces were investigated. A water drop was released from 13.6 mm above a solid surface and impinged on substrates. The images (the silhouette and 45 degrees top view) were sequentially recorded from the moment that the droplet impacted the solid surface until it reached equilibrium. The entire profile of each of the water droplets during spreading was obtained from the digitized recorded images. The digitized droplets were then used to detail the spreading mechanism, including information on the relaxations of the wetting diameter, droplet height, contact angle, and spreading velocity. A comparison of the full droplet profiles allows us to clarify the independent motion of two related but independent components, the central region and rim, of an impinging droplet. An interesting plateau region in the droplet height relaxation curve was observed in the first cycle for all substrate surfaces. For hydrophobic surfaces (paraffin and Teflon), three particular growth modes in the droplet height relaxation curve were detected in every oscillation cycle during the early spreading stages. It only took three and four oscillation cycles for a water droplet on the glass and quartz substrates, respectively, to dissipate its energy and reach its equilibrium state. However, it took 72 and 28 oscillation cycles for a water droplet on the Teflon and paraffin substrates, respectively. Moreover, several other new phenomena were also observed.

Adsorption kinetics of C10E4 at the air–water interface: consider molecular interaction or reorientation

Abstract The adsorption kinetics of C10E4 was studied using a video-enhanced pendant bubble tensiometer. Two dynamic processes were studied. The bubble was suddenly created and C10E4 adsorbs onto a freshly created interface. After it had reached the equilibrium, the bubble was then impulsively shrunk and the surface was compressed with some change of area large enough to appreciably enrich the surface concentration and change the surface tension. Two sets of equilibrium data, equilibrium surface tension γ(C∞) and surface equation of state γ(Γ) were measured and utilized for the determination of model parameters. The data were analyzed by either considering the molecular interaction between the adsorbed molecules or assuming that surfactant molecules adsorb at interface in two orientation states. Both models fit the equilibrium γ(C∞) and dynamic γ(t) data well. There exists a shift on the control mechanism for C10E4 onto a freshly created air–water interface: diffusion-control at dilute C∞ and mixed-control at higher C∞. The re-equilibration for C10E4 out of a suddenly compressed surface at C∞=1.0×10−8 mol cm−3 is also a mixed controlled process. An average diffusivity of (7.2±1.0)×10−6 cm2 s−1 and adsorption/desorption rate constants (β1=3.0×107 cm3 (mol s)−1 and α1=5.1×10−2 s)−1 are obtained using the Frumkin model. A discussion on the effect of interfacial curvature of fluid/liquid interface was also provided.

On the uniqueness of the receding contact angle: effects of substrate roughness and humidity on evaporation of water drops.

Could a unique receding contact angle be indicated for describing the wetting properties of a real gas-liquid-solid system? Could a receding contact angle be defined if the triple line of a sessile drop is not moving at all during the whole measurement process? To what extent is the receding contact angle influenced by the intrinsic properties of the system or the measurement procedures? In order to answer these questions, a systematic investigation was conducted in this study on the effects of substrate roughness and relative humidity on the behavior of pure water drops spreading and evaporating on polycarbonate (PC) surfaces characterized by different morphologies. Dynamic, advancing, and receding contact angles were found to be strongly affected by substrate roughness. Specifically, a receding contact angle could not be measured at all for drops evaporating on the more rugged PC surfaces, since the drops were observed strongly pinning to the substrate almost until their complete disappearance. Substrate roughness and system relative humidity were also found responsible for drastic changes in the depinning time (from ∼10 to ∼60 min). Thus, for measurement observations not sufficiently long, no movement of the triple line could be noted, with, again, the failure to find a receding contact angle. Therefore, to keep using concepts such as the receding contact angle as meaningful specifications of a given gas-liquid-solid system, the imperative to carefully investigate and report the inner characteristics of the system (substrate roughness, topography, impurities, defects, chemical properties, etc.) is pointed out in this study. The necessity of establishing methodological standards (drop size, measurement method, system history, observation interval, relative humidity, etc.) is also suggested.

Adsorption and desorption kinetics of CmE8 on impulsively expanded or compressed air–water interfaces

Abstract The adsorption kinetics of C m E 8 ( 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 C 14 E 8 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 C m E 8 ( m =10 and 12). For all of these molecules, the adsorption rate constant is similar ( β 1 =5.6±1.0×10 −6 cm 3 (mol s) −1 ). However, the desorption rate constant ( α 1 ) varies strongly. Increasing m by 2 lowers the desorption rate constant α 1 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.

A simple method for measuring the superhydrophobic contact angle with high accuracy

A modified selected-plane method for contact angle (theta) measurement is proposed in this study that avoids the difficulty of finding the real contact point and image-distortion effects adjacent to the contact point. This method is particularly suitable for superhydrophobic surfaces. The sessile-drop method coupled with the tangent line is the most popular method to find the contact angle in literature, but it entails unavoidable errors in determining the air-solid base line due to the smoothness problem and substrate tilting. In addition, the tangent-line technique requires finding the actual contact point. The measurement error due to the base line problem becomes more profound for superhydrophobic surfaces. A larger theta deviation results from a more superhydrophobic surface with a fixed base line error. The proposed modified selected-plane method requires only four data points (droplet apex, droplet height, and two interfacial loci close to the air-solid interface), avoiding the problem of the sessile-drop-tangent method in finding the contact point and saving the trouble of the sessile-drop-fitting method for best fitting of the numerous edge points with the theoretical profile. A careful error analysis was performed, and a user-friendly program was provided in this work. This method resulted in an accurate theta measurement and a method that was much improved over the classical selected plane and the sessile-drop-tangent methods. The theta difference between this method and the sessile-drop-fitting method was found to be less than three degrees.

A video‐enhanced plate method for simultaneous measurements of surface tension and contact angle

The assumption of zero contact angle of the Wilhelmy plate method causes error for the surface pressure‐area characteristics of some spread monolayers. To avoid this problem, a video‐enhanced plate method is developed to monitor the dynamic surface tension and contact angle simultaneously. According to this method, the profile of the air–water interface adjacent to a flat plate is captured by an image digitization technique. The surface tension and contact angle are obtained from the best fit between the edge coordinates of the captured image and a theoretical equation derived from the Young–Laplace equation. Preliminary results show satisfactory agreement between the curve of the air–water interface obtained from experimental data and the theoretical curve for obtained values of surface tension and contact angle. Twenty tests for pure water at 25 °C give an average surface tension 72.0±0.2 mN/m, which confirms the accuracy of this method. A preliminary dynamic test also indicates that the effect of the c...