Volume of the drop

Vapor Pressure Osmometry. VPO is a very practical method for determining Mn values in a wide range of solvents and temperatures. Recently, results obtained with classical pendant-drop instruments showed a significant dependence of the calibration constant upon the molecular weight of the standards (8,9). On the other hand, with an apparatus equipped with thermistors allowing the volume of the drops to be kept constant, this anomaly is not observed (10,11). 

The continuous formation of drops, however, can lead to substantial errors in obtained adsorption kinetic data. For short drop formation times, hydrodynamic effects have to be taken into account. At large flow rates, the measured drop volume at the moment of detachment must be corrected. This is because a finite time is required for the drop meniscus to be disrupted and the drop to detach. Even though the volume has already reached its critical value, fluid may still flow from the reservoir into the drop. The volume of the drop is thus larger than its measured value, which leads to larger calculated interfacial tension values. The shorter the drop formation time is, the larger the error w i 11 be. K1 oubek et al. (1976) were the first to quantify this effect by introducing a corrected critical drop volume, Vc ... 

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 ... 

Unfortunately, the change in surface area depends on two variables a and h (we do not have to consider a change in 0, it is of second order). However, these two variables are not independent because the volume of the drop is constant. The volume of a spherical cap is... 

Reduce the volume of the drop to 0.3 pL and slowly bring it in contact with the hydrophobic surface. 

Before use the droppers must be calibrated, i.e. the volume of the drop delivered must be known. Introduce some distilled water into the clean dropper by dipping the capillary end into some distilled water in a beaker and compressing and then releasing the rubber teat or bulb. Hold the dropper vertically over a clean dry 5 ml measuring cylinder, and gently press the rubber bulb. Count the number of drops until the meniscus reaches the 2 ml mark. Repeat the calibration until two results are obtained which do not differ by more than 2 drops. Calculate the volume of a single drop. The dropper should deliver between 30 and 40 drops per ml. Attach a small label to the upper part of the dropper giving the number of drops per ml. 

It must be remembered that the volume of the drop delivered by a dropper pipette depends upon the density, surface tension, etc., of the liquid. If the dropper delivers 20 drops of distilled water, the number of drops per ml of other liquids will be very approximately as follows dilute aqueous solutions, 20-22 concentrated hydrochloric acid, 23-24 concentrated nitric acid, 36-37 concentrated sulphuric acid, 36-37 acetic acid, 63 and concentrated ammonia solution, 24-25. 

A question of interest here is the origin of the DMPC molecules building up the bilayer, considering the low monomer concentration in the DMPC suspension and the small volume of the drop in the cell. However, as indicated in Section 3.4.3, NBF can be formed only at close packing at the interface (r ). A possible mechanism is the vesicle degradation at the surfaces, i.e. at the solution/air interface. An evidence of this mechanism are the kinetic studies of insoluble phospholipid monolayer of Ivanova et al. [291]. Nevertheless, NBF formation from vesicle suspensions needs further research. 

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. 

Make sure that the three trials produce data that are similar to each other. K one is greatly different from the others, perform steps 3-5 over again. If you re still waiting for the eggshell in the drying oven, calculate and record in the first data table the total volume of the drops and the average volume per drop. 

Since surface forces depend on the magnimde of the area, the drops tend to be as spherical as possible. Distortions due to gravitational forces depend on the volume of the drop. In principle, it is however possible to determine the surface tension by measurement of the shape of the drop, when gravitational and surface tension forces are comparable. Two principally different methods must be taken into account. There are methods based on the shape of a static drop lying on a solid surface or a bubble adhering underneath a solid plate, and dynamic methods, based on continuously forming and falling drops. It should be noted that all the principles described here for drops are valid also for bubbles. 

The latter condition is the statement that the total volume of the drop is fixed. The integral is taken over the total area of the drop in the horizontal plane, i.e., for all x.v < R(t), where R(t) = R (t)/Ro is the position of the outer edge of the drop in the horizontal plane nondimensionalized by the characteristic length scale Ro, and q is the volume of the drop scaled by either I to R t for d = 2, or H0Ro per unit length ford = 1. 

The monolayers in the cell were deposited on doubly distilled water. The method of deposition consisted of running out a small drop of spreading solution and allowing it to touch the surface. The volume of the drop was 1 /xl and was delivered to the surface with an accuracy of 1%. The spreading solvent was cyclohexane which was chosen for its... 

We assume that the volume of the drop is constant, that the drop always meets the surface with a constant, intrinsic angle, Q (at equilibrium), and that gravitational forces are absent. For simplicity we also assume that the liquid consists of a single component and that the solid is insoluble in the liquid. The radius of the drop is assumed to be much greater than the separation of asperities. 

Figure 5, A and B, shows in sequence the behavior of a water drop at pH 14. In Figure 5, A, the abscissas are the successive volumes of the drop, but in Figure 5, B, the abscissas represent the diameter of the three-phase interline. The ordinates in both A and B are the observed contact angles. Figure 5, B, shows that the drop expands with constant 0 = to an interfacial diameter of approximately 1.0 mm. At this point the volume increase is stopped. During the 18 seconds that elapsed before the drop volume reduction was started, the system showed a slight relaxation phenomenon, which increased the three-phase interline diameter, and slightly decreased 0, while the drop volume remained constant (square dots in 5, B). When the drop volume was reduced, steadily decreased to zero while the interfacial area, Hg-H20, remained constant. When 0 reached zero, a thin film remained on the mercury surface.

---