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],... 

Instead of measuring the weight directly we may calculate it from the volume and the density the drop volume method has been applied by Harkins chiefly to the measurement of the tension between two liquid phases, and it probably falls little short in accuracy from the previous method. More frequently it has been j modified, especially for biochemical purposes, as a drop number method that is, a known volume of liquid iFallbwed oo nov. of a tube, and the number of drops formed is compared with that formed by a standard fluid. This method is necessarily very rough. 

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. 

The interfacial tension methods are described in ISO 6889 [20], ISO 9101 [21] and ASTM D1331 -89 (2001) [ 18 ]. The method described in ISO 6889 is a simple method and applicable for the systems if the interfacial values arebetween 4 and 50 dyne cm-1, the immiscible liquids are water and organic liquids and the systems contain non-ionic or anionic surfactants but not cationic surfactants. The repeatability is within about 2 dyne cm-1. On the other hand, the drop volume method as described in ISO 9101 can be used for viscous liquids and liquids containing all types of surfactants. This method can measure the interfacial tension as low as 1 dyne cm-1 with 0.5 dyne cm-1 accuracy. If the interfacial tension is lower than 1 dyne cm-1, the spinning drop will be the suitable method. 

ISO 9101. Determination of interfacial tension - drop volume method. 

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 ). 

The drop weight, or drop volume method (sec. 1.6) is intrinsically dynamic the time scale can be varied by applying a variable pressure on the capillary. The volume of the drop is measured as a function of time, emd theory is needed to dafve y(t). Practically speaking, this technique is convenient although the interpretation may offer problems temperature control Is simple, the accuracy is = 0.1 mN m and LG and LL Interfaces can both be studied. 

Figure 1.31 gives one example of a comparison between results from two different techniques, applied to one and the same system. In this case the (rton-ionic) surfactant was specially synthesized and well-defined p-tert.butylphenol with 10 EO groups. It is seen that the results of the (faster) maximum bubble technique and the (slower) drop volume method, connect well. Many more... 

Figure 1.32 deals with adsorption of palmitic acid from hexane to the oil-water interface, using the drop volume method. As the drop volume method is relatively slow, the initial decay from the pristine hexane-water interfacial tension to the first reported data cannot be given. Otherwise stated, the data refer to the later stages of diffusion. The trend is that equilibration is somewhat slower than the adsorption of surfactants from aqueous solution. 

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 interfacial tension between a swollen particle and the aqueous phase can be approximated by the interfacial tension between the monomer phase and the swollen latex dispersion. The latter was measured by the drop volume method. The swollen latex dispersion was dropped into the monomer phase using a microsyringe with a thin-wall needle the end of which had been filed flat. The drop volume and density data were then converted to interfacial tension. 

Figure 3 contains dynamic data for ff-LG received by three methods the maximum bubble pressure method in the time range 0.001 s to 100 s, the drop volume method for times in the range 5 s to 500 s, and the profile analysis tensiometer PAT l in the time range from 10 s up to several hours. 

The aim of this chapter is to present the fundamentals of adsorption at liquid interfaces and a selection of techniques, for their experimental investigation. The chapter will summarise the theoretical models that describe the dynamics of adsorption of surfactants, surfactant mixtures, polymers and polymer/surfactant mixtures. Besides analytical solutions, which are in part very complex and difficult to apply, approximate and asymptotic solutions are given and their range of application is demonstrated. For methods like the dynamic drop volume method, the maximum bubble pressure method, and harmonic or transient relaxation methods, specific initial and boundary conditions have to be considered in the theories. The chapter will end with the description of the background of several experimental technique and the discussion of data obtained with different methods. 

The easy sample handling, the simple temperature control over a broad temperature interval, and the direct applicability without any modifications to liquid/air and liquid/liquid interfaces are reasons for the fi equent use of the drop volume method. The accuracy and reproducibility are as high as those of other methods, 0.1 mN/m. In addition, it has the advantage that only small amounts of solute and solvent are needed for a series of measurements. 

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. 

Under certain conditions, the hydrodynamics of growing drops can cause flow pattern inside and outside the drop, dependent on the liquid flow rate. Such situations were described above in connection with the drop volume method. 

In a recent paper Miller et al. (1994d) discussed parallel experiments with a maximum bubble pressure apparatus and a drop volume method (MPTl and TVTl from LAUDA, respectively), and oscillating jet and inclined plate instruments, performed with the same surfactant solutions. As shown in Fig. 5.27, these methods have different time windows. While the drop volume and bubble pressure methods show only a small overlap, the time windows of the inclined plate and oscillating jet methods are localised completely within that of the bubble pressure instrument. 

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)...

A different type of experiments was performed by Van Hunsel Joos (1987b). They studied the steady state of adsorption of various alkanols at the alkane/water interface by means of the drop volume method (Fig. 5.35). The steady states differ remarkably from the equilibrium state. A description of the adsorption process has therefore to allow for a transfer of hexanol molecules across the hexane/water interface. The difference of the two studied steady states is... 

Speed Dependence. All experiments done with the controlled drop volume method are inherently accompanied by a steadily varying speed of lateral movement of the three-phase interline. As the rate of volume change remains constant throughout each experiment, the lateral movement of the three-phase interline decreases with increasing drop volume and increases with decreasing drop volume. For our experiments with a maximal drop volume of 3.4 cu. mm., a drop volume change of 0.0189 cu. mm. per second, and a 0 of 119°, the speed (computed over 6 seconds) varied from0.3 x lO to 0.1 mm. per second. 

Lohnstein founded the theoretical basis of the drop volume method already at the beginning of the last century [180, 181]. This theory is still the basis for all further refinements, which has... 

Fig. 4.43 Dependence of y as a function of ffor DPPC in chloroform/water systems as measured by the drop volume method at different lipid concentrations Co = 2 ( ), 3 ( ), 4 ( ), 5 (O), 6 ( ) 10 mol/cm external aqueous phase in the cuvette contains 5.0 10 mol/1 p-Casein, according to [239]...

The results demonstrate that in the time period covered by the drop volume method the lipid has reached the equilibrium adsorption before the proteins starts to adsorb significantly. This is seen from the y-values at short times (large values of which are significantly lower than... 

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. 

Interfacial Behavior of Food Proteins Studied by the Drop Volume Method... 

In order to have an initial clean interface for adsorption studies with this technique, one usually injects the protein solution into the subphase. In the drop volume method the cleaning of the interface is performed by forming a drop rapidly in between those to be measured, in such a way that the volume detached corresponds toYg, i.e. the clean interface. The main difference between the drop volume and the Wilhelmy plate method, when using the injection technique, is that the fresh interface is exposed to an initially uniform concentration of protein in the former method, whereas this is not the case in the latter. This difference promotes the existence of more unfolded proteins in the adsorbed films formed with the Wilhelmy plate method as opposed to the drop volume method. This could be the explanation for the higher surface activity shown by the proteins at the very low concentrations and the less marked difference in behaviour at the A/W- and 0/W-interfaces as obtained by measurements with the Wilhelmy plate method. 

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