Adsorption of surfactant molecules at the gas-liquid interface

2023-03-11 16:01:02

Surfactant molecules are adsorbed at the gas-liquid interface. Figure 7 is a schematic diagram of surfactant molecules in the adsorption process. When the concentration is very dilute, the adsorption amount is small, the surface adsorbed molecules are less, and can lie flat on the gas-liquid interface; as the concentration increases, at medium concentrations, the orientation of surface adsorbed molecules on the solution surface has greater randomness, and there are There are three orientations: flat lying, oblique and upright; when the concentration continues to increase to saturated adsorption, the surface adsorbed molecules gradually move closer to the interface, the hydrophobic group points to the gas phase, and the hydrophilic group points to the bulk phase to form a relatively tight directional arrangement in the upright state; Continue to increase the concentration to a high concentration. If the concentration of the surfactant solution is too high, the effect of the hydrogen bond with the water molecules will be weakened, the driving force for hydrate formation will be reduced, and the hydrate formation rate will be reduced. At the same time, if the concentration is too high, a large number of surfactant molecules will gather at the gas-liquid interface in the initial stage of nucleation, which hinders the further formation of hydrates. This is why there is a short induction period in the initial stage in high-concentration systems. .

When the saturated adsorption concentration is reached, the interfacial tension of the gas-liquid interface layer can be effectively reduced, so that gas molecules quickly enter the interface layer and reach saturation to form bubbles, and form hydrate crystal nuclei with water molecules on the interface layer. Under the action of water molecules and gas molecules, a thin layer of hydrate begins to form on the surface of the crystal nucleus, thus adding two interfaces: gas-hydrate interface and hydrate-water interface. When interfacial tension is minimized, surfactant molecules form micelles in solution. The micelles of long carbon chain surfactant molecules have more aggregates, and the formed micelles are larger. These molecules put the hydrophobic groups together, and the hydrophilic groups point to the outside to form a sphere. The micellar model is shown in the figure.

The water molecules in the solution combine with the hydrophilic groups forming the spherical surface, and the gas molecules are wrapped in the cavity, which plays a good role in solubilization. As the surfactant molecules move to the gas-hydrate interface and contact with gas molecules, the gas-liquid contact area increases, thereby increasing the growth rate of hydrates. When the carbon chain length of surfactant molecules is short, the number of micelles aggregates is small, and spherical micelles cannot be formed, and the contact between water molecules and gas molecules cannot be effectively promoted.

At this time, the solution concentration is just the saturated adsorption concentration, and the adsorbed molecules are arranged on the interface in a relatively compact upright state, forming a dense monomolecular film on the liquid surface, so that hydrates can only be formed on the gas-liquid interface. . With the gradual increase of hydrate at the interface, it will play a role of "immobilization", blocking further mass transfer and heat transfer between methane gas molecules and water molecules, so that the rate of hydrate formation gradually decreases until the formation stops.