Surface tension drop volume method

Since the drop volume method involves creation of surface, it is frequently used as a dynamic technique to study adsorption processes occurring over intervals of seconds to minutes. A commercial instrument delivers computer-controlled drops over intervals from 0.5 sec to several hours [38, 39]. Accurate determination of the surface tension is limited to drop times of a second or greater due to hydrodynamic instabilities on the liquid bridge between the detaching and residing drops [40],... 

Surface tension (30 C) of the solution was determined by the drop-volume method (12). The density of the solution needed for calculating the surface tension was measured by a U-tube pycnometer. 

Surface tension measurement. Adsorption titration, also called soap titration, (2.3) was carried out by the drop volume method at different polymer concentrations. The equivalent concentration of salt was held constant. The amount of emulsifier necessary to reach the critical micelle concentration (CMC) in the latex was determined by each titration. The total weight of emulsifier present in the latex is the weight of emulsifier in the water plus the weight of emulsifier adsorbed. The linear plot of emulsifier concentration (total amount of emulsifier corresponding to the end-point of each titration) versus polymer concentration gives the CMC as the intercept and the slope determines the amount of emulsifier adsorbed on the polymer surface in equilibrium with emulsifier in solution at the CMC (E ). 

Figure 4.12 is taken from a study by Neam and Spaull ). Surface tensions were measured by the drop-volume method as a function of chain length (C40H-Cg0H)... 

The principle of the drop volume method is of dynamic character and therefore, it can be used for studies of adsorption processes in the time interval of seconds up to some minutes. At small drop times a so-called hydrodynamic effect has to be considered, as discussed in many papers (Davies Rideal 1969, Kloubek 1976, Jho Burke 1983, Van Hunsel et al. 1986, Van Hunsel 1987, Miller et al. 1994a). This hydrodynamic effect appears at small drop times under the condition of constant liquid flow into the drop and gives rise to apparently higher surface tensions. Davies Rideal (1969) discussed two factors influencing the drop formation at and its detachment from the tip of a capillary the so-called "blow up" effect and a "circular current" effect inside the drop. The first effect increases the detaching drop volume and simulates a higher surface tension while the second process leads to an earlier break-off of the drop and results in an opposite effect. A schematic of these two effects on measured drop volumes is shown in Fig. 5.10. 

Fig. 5.30 Dynamic surface tension of a 0.025 mol/l pt-BPh-EOlO solution measured using the maximum bubble pressure ( ) and drop volume ( ) methods original data ( - ), corrected data ( ) according to Miller et al. (1994d)...

It has been already indicated (Fig. 7) that micelles can lead to an essential acceleration of the adsorption process. Therefore, special experimental techniques are necessary for its investigation, allowing measurements of the dynamic surface tension in a time interval of milliseconds. The maximum bubble pressure method [78, 81, 83, 89,90,93] and the oscillating jet method [77, 82, 86, 87, 88, 90, 92, 93, 156] are most frequently used for these purposes. The inclined plate method [83, 89, 90, 93], the method of constant surface dilation [85] and the drop volume method [84] have been used also for slow adsorbing surfactants. 

As mentioned above, the drop volume method is of dynamic character and it can be used for adsorption processes in the time interval of seconds up to some minutes. At small drop time, the sohydrodynamic effect has to be considered [27]. This gives rise to apparently higher surface tension. Kloubek et al. [28] used an empirical equation to account for this effect. 

Figure 12.7. Dynamic surface tension data of two diethylde-cylphosphine oxide solutions, as measured by the drop volume method (TVT2, LAUDA, Germany) in the dynamic mode , Co = 10 mol/cm O, Co = 10 mol/cm ...

Abstract The effect of interaction between proteins and amphiphiles on the surface tension reduction have been measured by the drop-volume method. The equilibrium surface tension reduction isotherms at the air/water interface of serum albumin and of ovalbumin in sodium dodecylsulphate and of ovalbumin in 1-monocaproin are reported. The surface tension reduction isotherms of the proteins in the anionic amphiphile solutions exhibit plateau regions, which have been interpreted in terms of different states of protein-amphiphile interaction in the bulk solution. Any interaction between ovalbumin and the monoglyceride is not reflected in the surface tension isotherm. At increased amphiphile concentration the protein seems to be replaced by 1-monocaproin in the surface. 

In many experimental techniques used for dynamic surface-tension measurements (such as the MBP method and the drop-volume method [14,76,82]), the surface expands gradually with time. In such a case, the convective terms in Eqs. (24) and (25) carmot be neglected. Nevertheless, it can be demonstrated that with the help of the new independent variables. 

This is a fairly accurate and convenient method for measuring the surface tension of a liquid-vapor or liquid-liquid interface. The procedure, in its simpli-est form, is to form drops of the liquid at the end of a tube, allowing them to fall into a container until enough have been collected to accurately determine the weight per drop. Recently developed computer-controlled devices track individual drop volumes to = 0.1 p [32]. 

To investigate the influence of swelhng of the substrate by the contacting liquid, the contact angle 6 of sessile drops of tricresylphosphate, TCP (drop volume 2 p,L, viscosity t = 70 cP, surface tension = 40.9 mN m ), has been measured as a function of time after deposition, t, on flat, smooth, horizontal surfaces of soft and rigid solids at 20°C. The method of measurement of contact angle is the same as in Section Ill.A. 

Recently, Samec et al. [38] have investigated the same system by the video-image pendant drop method. Surface tension data from the two studies are compared in Fig. 2, where the potential scale from the study [36] was shifted so that the positions of the electrocapillary maxima coincide. The systematic difference in the surface tension data of ca. 3%, cf. the dotted line in Fig. 2, was ascribed to the inaccurate determination of the drop volume, which was calculated from the shape of the drop image and used further in the evaluation of the surface tension [38]. A point of interest is the inner-layer potential difference A (pj, which can be evaluated relative to the zero-charge potential difference A cpp c by using Eq. 

If one considers a system consisting of water (with or without added electrolyte) + oil + surfactant (with or without a cosurfactant) at equilibrium, there will most likely be present more than two phases (due to the formation of emulsion or microemulsion). The determination of the interfacial tension, Yij> between the two liquid phases is, therefore, of much importance, in order to understand the forces which stabilize these emulsions or microemulsions. The interfacial tension can be measured by using a variety of methods, as described in detail in surface chemistry text-books (1-3). If the magnitude of yij is of the order of few mN/m (=dyne/ cm), then the methods generally used are Wilhelmy plate method or the drop volume (or weight) method (1-4). However, in certain systems ultra-low (or low) interfacial tensions have been reported. Since these low values are reported to be essential in order to mo-... 

Figure D3.5.6 Adsorption kinetics of a small molecule surfactant. Surface tension of polyoxyethylene (10) lauryl ether (Brij) at the air-water interface decreases as time of adsorption increases. Brij concentration is 0.1 g/liter, as measured by the drop volume technique and the maximum bubble pressure method (UNITD3.6).

Joos, P. and Rillaerts, E. 1981. Theory on the determination of dynamic surface tension with the drop volume and maximum bubble pressure method. J. Colloid Interface Sci. 79 96-100. 

This unit will introduce two fundamental protocols—the Wilhelmy plate method (see Basic Protocol 1 and Alternate Protocol 1) and the du Noiiy ring method (see Alternate Protocol 2)—that can be used to determine static interfacial tension (Dukhin et al., 1995). Since the two methods use the same experimental setup, they will be discussed together. Two advanced protocols that have the capability to determine dynamic interfacial tension—the drop volume technique (see Basic Protocol 2) and the drop shape method (see Alternate Protocol 3)—will also be presented. The basic principles of each of these techniques will be briefly outlined in the Background Information. Critical Parameters as well as Time Considerations for the different tests will be discussed. References and Internet Resources are listed to provide a more in-depth understanding of each of these techniques and allow the reader to contact commercial vendors to obtain information about costs and availability of surface science instrumentation. 

Modem drop volume tensiometers are connected to a computer with sophisticated software that can be used to automatically record the surface tension as a function of the true interfacial age. Adsorption kinetics experiments with the drop volume technique can be conducted using either the constant drop formation method or the quasistatic method (for details, see Commentary). The choice of the dynamic measurement method depends primarily on the time range over which the adsorption kinetics needs to be measured. 

The theoretical foundation of the drop volume technique (DVT) was developed by Lohnstein (1908, 1913). Originally, this method was only intended to determine static interfacial tension values. Over the past 20 years, the technique has received increasing attention because of its extended ability to determine dynamic interfacial tension. DVT is suitable for both liquid/liquid and liquid/gas systems. Adsorption kinetics of surface-active substances at liquid/liquid or liquid/gas interfaces can be determined between 0.1 sec and several hours (see Fig. D3.6.5). 

The time required to conduct an interfacial tension experiment depends largely on the properties of the surfactants and less on the chosen measurement method. A notable exception is the drop volume technique, which, due to the measurement principle, requires substantial ly more time than the drop shape analysis method. Regardless of the method used, 1 day or more may be required to accurately determine, e.g., the adsorption isotherm (unit D3.s) of a protein. This is because, at low protein concentrations, it can take several hours to reach full equilibrium between proteins in the bulk phase and those at the surface due to structural rearrangement processes. This is especially important for static interfacial tension measurements (see Basic Protocol 1 and Alternate Protocols 1 and 2). If the interfacial tension is measured before the exchange of molecules... 

Provides measuring techniques of contact angle, surface tension, interfacial tension, and bubble pressure. Suitable methods for both static and dynamic inteifacial tension of liquids include du Nous ring, Wilhelmy plate, spinning drop, pendant drop, bubble pressure, and drop volume techniques. Methods for solids include sessile drop, dynamic Wilhelmy, single fiber, and powder contact angle techniques. 

Drop-weight method. To determine the surface tension of a hexadecane (Ci6H34) you let it drop out of a capillary with 4 mm outer and 40 /. m inner diameter. Hexadecane wets the capillary. Its density is 773 kg/m3. 100 drops weigh 2.2 g. Calculate the surface tension of hexadecane using the simple Eq. (2.15) and the correction factor /. It was concluded that / should be a function of rc/V 1/3, with V being the volume of the drop. Values for the correction factor are listed in the following table (from Ref. [1], p. 19). Is it necessary to use the correction ... 

For foams, it is the surface tension of the foaming solution that is usually of most interest. For this, the most commonly used methods are the du Noiiy ring, Wilhelmy plate, drop weight or volume, pendant drop, and the maximum bubble pressure method. For suspensions it is again usually the surface tension of the continuous phase that is of most interest, with the same methods being used in most cases. Some work has also been done on the surface tension of the overall suspension itself using, for example, the du Noiiy ring and maximum bubble pressure methods (see Section 3.2.4). 

Figure 3.9 Illustration of the drop weight/volume surface tension method (shown here for a wetting contact angle).

Hogness,1 Burdon,2 Bircumshaw, and Sauerwald have done a great deal to render accurate measurements possible the best method is probably the maximum bubble pressure method, but the measurement of sessile drops (see Chap. IX), and of drop volumes, are also useful. Metals always have a very high surface tension. Table X gives typical results. 

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