# Surface Coatings and Treatments for Controlled Hydrate Formation: A Mini Review
## 3 Hydrate-Public Surface Treatments and Coatings
### Functionalized Hydrate-Phobic Surfaces  
[PERSON] et al. [40] reported that by functionalizing the solid surfaces systematically (varying hydrophobicity), the adhesion between the hydrates and the solid surfaces can be reduced. This will not allow the hydrate to deposit on the pipeline wall, which may lead to hydrate plug formation. This study was carried out using THF as hydrate former at atmospheric pressure and was the first of its kind where the role of surface energy and wettability on the adhesion force of hydrate particles in solid/solid contact were measured. The authors used glass/steel substrates and several functionalized coatings (Table 1). They demonstrated that treated surfaces compared to bare steel can reduce the adhesion strength of THF hydrates by a factor of four (Figure 2A). Such reduction is possible on low Lewis acid/base and van der Waals characterized surfaces. Later, [PERSON] et al. [41] studied, using a MMF apparatus, the adhesive forces between the cyclopentane hydrates and five distinctly modified steel surfaces in both dry (solid/solid) and water-wet (solid/liquid) conditions. The graphite-functionalized surface reduced the solid/solid adhesion force by 79% compared to the untreated steel surface due to a reduction in surface wettability, proved by contact angle measurements. Citric acid ester functionalization not only reduced the adhesive force by 98% but was also responsible for morphological changes in crystals that resulted in hydrate growth. However, the effectiveness of both surfaces decreased in water-wet conditions due to 7% and 55% reductions in adhesive forces, respectively. Nonanedithiol modification resulted in a 49% increase in adhesive forces due to the formation of specific cylindrical morphology at the hydrate (solid)/surface (solid) interface. Interfacial studies indicated qualitatively that citric acid ester adsorbed at the hydrate/steel interface easily since hydratephilic molecules may adsorb preferentially at the hydrogen bonding sites on the hydrate surface as observed by the adsorption isotherms for water/oil and hydrate/oil interfaces. Figure 1: (**Left**) Schematic diagram of the top view of a cell of the micromechanical force (MMF) apparatus. (**Right**) Working mechanism of the MMF: The cantilever placed at the left is hand operated and holds the hydrate particle, whereas the right-hand one, where the solid substrate is placed, is highly precise. (**a**) Initial position of the cantilevers. (**b**) By moving down the precise cantilever, a preload can be applied to the hydrate particle, (**c**) the precise cantilever can be moved back until the particle is removed from the solid surface, and (**d**) the required distance (\(\delta\)) to remove the particle from the solid sample can be registered. Following [PERSON]’s law, the distance is multiplied to the spring constant of the cantilever to obtain the force of adhesion. Reprinted with permission from ref. [68]. © 2021 Elsevier (**left**) and reprinted with permission from ref. [65]. © 2021 Elsevier (**right**). [PERSON] and [PERSON] [45] functionalized the glass test tube with glycerol in different configurations and tested their effects compared to polyvinylpyrrolidone (PVP) on THF hydrate nucleation. The conjugation of glycerol on the glass surfaces was achieved in different configurations: (1) through employment of two distinct silane spacers, (2) by attaching to glycerol at the 1 or 2 position covalently, and (3) by attaching a succinic acid spacer. Contact angle measurements were used to ascertain the desired functionalization of the surface. Induction times were measured for 1:15 THF solutions at 0 \({}^{\circ}\)C, and at least 50 data points were gathered on each modified surface. Among all the functionalized surfaces, the best hydrate inhibition results were shown by the surface modified by APTES and glycerol coupled at the 1 position (leaving a free 1,2-diol).