The protocol is applicable in studying the effect of surfactant inhibitors on hydrate crystals. It provides information on both the type of the crystal and the mechanism for inhibition. Attach a 19 gauge needle to a one milliliter glass syringe.
Rinse the needle and syringe three times with deionized water and then fill the syringe with deionized water. Next, fill the hydrate visualization cell with 25 milliliters of cyclopentane. Using the syringe, insert a droplet of deionized water at the bottom of the hydrate visualization cell.
This water droplet is the seed hydrate. Then, place the temperature sensor inside the hydrate visualization cell close to the bottom of the cell. To prevent evaporation of the cyclopentane, put the acrylic cover on the cell and screw the cover in place.
Adjust the lights and the camera to focus on the seed hydrate. Using the temperature control device, set the temperature of the Peltier plate to negative five degrees Celsius. Monitor the values reported from the temperature sensor.
When the temperature reaches negative five degrees Celsius, make sure the seed hydrate at the bottom of the hydrate visualization chamber turns to ice. Set the temperature of the Peltier plate to two degrees Celsius in increments of 0.5 degrees Celsius. When the temperature reaches two degrees Celsius, fill the plumbing with water using the syringe.
Then, lower the brass hook into the cyclopentane and allow it to equilibrate for five minutes. Using the software for the pressure transducer, press the Start button to start the digital transducer recordings. Connect the syringe to the syringe pump, set the syringe pump to inject a volume of two microliters and activate it.
The syringe will plunge the water into the cyclopentane bath to form a submerged water droplet. Use a needle tip to remove a small piece of the seed hydrate. Bring the needle tip with the piece of seed hydrate into brief contact with the water droplet to initiate the formation of the hydrate.
Press Start Recording on the camera capture software. Record images of the crystallization process at one Hertz. To find the critical micelle concentration, begin by preparing standard solutions as described in the manuscript.
To measure the surface tension of each surfactant solution using the stalagmometry method, program the pump to expel one milliliter of solution at a rate of 0.5 milliliters per minute. Place the syringe pump and syringe vertically and release the drops into the air. Count the number of drops and divide one milliliter by the number of drops to find the drop volume.
For each solution, calculate the surface tension as described in the manuscript and plot the surface tension as a function of surfactant concentration. The concentration where the surface tension curve flattens is the CMC, the critical micelle concentration. Repeat the procedure used to measure hydrate formation on a water droplet, but use surfactant solutions of various concentrations.
Use image processing software to open the first image in the sequence of the crystallization process. Use the length tool in the software to measure the diameter of the brass tube in the image. Set the scale in the image based on the known diameter of the brass tube, one sixteenth of an inch.
Select 10 equally spaced images, which capture the process from nucleation to droplet conversion. For each image, use the software to manually detect the contour of the drop and mark the contour in red. Then, manually trace the contour of the hydrate and fill the contour with black.
The camera only captures the 2D projection of the spherical droplet. Use mathematical modeling software to form a 3D reconstruction of the drop and the surface area covered by the hydrate. Using this experimental system, one can examine hydrate formation at the oil water interface and measure the interfacial stress associated with the crystallization process.
In pure water and low surfactant concentrations, the hydrate formed a planar shell morphology, growing at a constant rate from the two poles towards the equator. As the hydrate grew, the same number of surfactant molecules occupied a smaller area, resulting in decreased interfacial stress over time. In high surfactant concentrations, the hydrate grew as a conical crystal.
When the crystal became large enough, a portion of the cone broke free from the droplet surface. This growth pattern happened over and over again in an oscillatory manner. After the conical crystal reached a critical size and detached from the droplet's surface, the sudden increase in the available surface for surfactant molecules caused an increase in the interfacial stress.
A crystal then started growing again, yielding an oscillatory pattern. Most surfactant solutions inhibited hydrate growth compared to pure water. A high concentration of polyoxyethylene sorbitan tristereate was the most effective inhibitor.
This system can provide information on why some surfactants inhibit hydrates better than others. The system can also be used to study the general formation of crystals at interfaces.
