Adsorption isotherms

Similar surface tension measurement has applied to the rest of KHIs. The measurement was carried in NaCl 3.5 wt% so as to correlate the results to the observations in Chapter 3. Figure 4.10 shows the surface tension of NaCl 3.5 wt%

containing various KHIs of varying concentration at 278 K, all taken 30 minutes after formation of the interface. The bulk polymer concentration, cb is expressed this time in weight percentage for an easier comparison among all the polymers.

0.0 0.1 0.2 0.3 0.4 0.5 50

55 60 65 70

75 Gaffix VC713

Luvicap EG PVP40 PVP360 PEO-VCap

PVCap homemade

 (mN/ m)

c

(wt%)

Figure 4.10 Surface tension of different KHIs at the air–sodium chloride solution interface at 278 K.

All tensions are reported at 30 min

For Gaffix^ VC713 the surface tension decreased rapidly at low polymer concentrations, revealing the adsorption of the polymer molecules at the air–liquid interface. Above 0.25 wt% of polymer, the surface tension did not change significantly, due to the saturation of the air–liquid interface. Similarly, strong adsorption behaviour was observed for Luvicap^ EG.

For PEO-VCap and PVCap further reductions of the surface tension were observed in Figure 4.10 compared to Gaffix^ VC713 and Luvicap^ EG. However, contrary to Gaffix^ VC713 and Luvicap^ EG, the surface tension was still reducing at concentrations of 0.5 wt% of the polymer. This could be due to the presence of polydispersity, which creates a competitive adsorption, producing the continuous displacement of the smaller molecules by the slower larger ones. Therefore, the surface tension will exhibit reductions as a function of the polymer concentration.

It is interesting to see, from Figure 4.10, that the surface tension reduced from 74.70 mN/m (for NaCl 3.5%) to about 71 mN/m when different concentrations of PVP40 and PVP360 were used in the system, indicating no significant surface activities of

these two polymers at the air–liquid interface. Neither the polymers concentration effect nor the molecular weight dependence, was observed from the surface tension measurements. In the PVP–water solution, a strong hydrogen-bonding interaction exists between the electronegative oxygen atom in the carbonyl group and the surrounding water molecules (Huang and Wanga, 1996). These interactions can also result in intra- and inter-chain associations, and may lead to the formation of polymer aggregates in the bulk solution (Huang and Wanga, 1996). Therefore, the explanation as to why PVPs do not show surface activity could be attributed to PVP being mostly bound to water molecules in the interior of the polymer solution, and therefore, there is negligible adsorption of the PVP molecules at the air–liquid interface. This indicates that adsorption layers of PVP are not formed at the interface for the period of time and concentrations evaluated in this study.

We also see from Figure 4.10, that when the polymer concentration is below 0.25 wt%, the surface tension follows a trend with the order of PEO-VCap, ~ PVCap <

Luvicap^ EG ~ Gaffix^ VC713 << PVP40 ~ PVP360. When the polymer concentration becomes greater than 0.25wt%, the trend is altered as Gaffix^ VC713

< Luvicap^ EG < PEO-VCap ~PVCap << PVP40, PVP360. At 0.2 wt%, Gaffix^ VC713 ~ PVCap < Luvicap^ EG ~ PEO-VCap << PVP40, PVP360. This means that depending on the concentration evaluated, some of the inhibitors are more effective than others at reducing surface tension. A recent study has investigated the concentration effect and electrolyte dependence of some of these inhibitors on the inhibition efficiency of THF hydrates (Ding et al., 2010). The authors demonstrated that the performance of KHIs is affected significantly by the concentration of the inhibitors and electrolyte strength, and reported a specific critical concentration for each inhibitor in different environments (Ding et al., 2010).

Polymer molecules adsorbed at the air–water interface appear as trains, loops, and tails. Trains are sequences of polymer segments in actual contact with the surface;

whereas loops and tails are sequences of polymer segments in the solution. Loops have both ends connected to trains, whereas a tail is at one or both ends of the polymer chain (Nahringbauer, 1995). According to Lankveld and Lyklema (1972), the time dependence of the reduction in surface tension by a polymer molecule must

involve an increase in the number of adsorbed segments per unit area with time. This means that the surface properties of a polymer solution depend on the length and distribution of trains, loops, and tails. A change in the conformation of the adsorbed macromolecules can cause a drastic effect, both on the fraction of the segments directly in contact with the surface, i.e., on the surface tension, and on the thickness of the adsorbed polymer layer (Nahringbauer, 1995). This different surface activity observed for the KHIs can be attributed to differences in the fundamental properties of the polymer molecules, including the flexibility of the polymer chain, which leads to different conformations of the adsorbed macromolecules and interactions between and within the polymer chain, and molecular weight.

Particularly for Gaffix^ VC713, the nature and conformation of the side groups and the specific interactions between these side groups and the solvent, seem to play an important role in the preferential adsorption behaviour of this polymer. The presence of three different monomer units results in a more irregular chain structure, than the rest of the polymers whose structure consists of generally only one basic monomeric unit (excepting PEO-VCap, which has 2 monomeric units). This allows Gaffix^ VC713 a higher flexibility in the polymer chain. Furthermore, the steric factor induced by the size of the hydrophilic pendant groups (7-membered lactam ring) of Gaffix^ VC713, enable them to adopt a fairly extended conformation in liquid water, as opposed to a tight coil. Consequently this also allows the polymer a high degree of versatility in adopting various conformations, in comparison to the other polymers, and also enhances the level of interaction between the terpolymer and the water solution (Koh et al., 2002). For this polymer, the charge groups could also be contributing substantially to the chain’s stiffness, and the chain’s conformational degrees of freedom when coupled with the electrostatic ones (Netz and Andelman, 2003).

Some authors (Kashchiev, Firoozabadi and Anklam) have proposed that the inhibiting efficiency of KHIs is higher when they adsorb strongly at the solution–gas interface or onto the surfaces of nucleation–active microparticles and solid substrates present in the solution. They propose a model where the adsorption of inhibitor

molecules leads to a lowering of interfacial tension or edge energy on the crystal surface (Kashchiev and Firoozabadi, 2002) (Anklam and Firoozabadi, 2005).

If the adsorption behaviour of these polymers is compared to its effectiveness inhibiting THF hydrates showed in the previous chapter (compared in the presence of 3.5 wt% of NaCl), for a polymer concentration of 0.1 wt%, the trend observed in terms of To was PEO-VCap < PVP360 < Gaffix^ VC713 < Luvicap^ EG ~ PVP40.

For a concentration of 0.25 wt% of polymer, the trend observed in To was Gaffix^ VC713 < PVP40 < Luvicap^ EG < PVP360 < PEO-VCap. This means that the polymers with the highest inhibition efficiency observed in terms of To were the ones that produced lower surface tension values in Figure 4.10 (excepting for PVP40 and PVP360 which did not show any significant reduction of the surface tension). In this case, PEO-VCap for 0.1 wt% of polymer concentration, and Luvicap^ EG ~ Gaffix^ VC713 for 0.25 wt% of polymer.