**9. Requirements for CO2-philic surfactants for CO2-philicity**

Maximum attraction with the CO2 and minimum intermolecular attractions are the basic requirements involved in designing a CO2-philic molecule. Few of the Carbon dioxide philic appearance are splitting, less molecular weight hydrophobes, tip tail, and presence of groups of carbonyl, methyl, propylene oxide (PO), tertbutyl tip and a minimum number of methylene groups [22, 27, 33, 34]. A detailed discussion of the important factors that favour the carbon dioxide philicity of a surfactant will be followed in the next few sections.

#### **9.1 Branches**

*Analytical Chemistry - Advancement, Perspectives and Applications*

problems.

**7. CO2-philic surfactants**

is illustrated in **Figure 4**.

**8. CO2-philic surfactants for foam**

*Structure of a CO2-philic surfactant.*

collaboration in their phenomena.

stability at high temperatures. Usually, a foam that absorbs on the rock matrix becomes worse over time, and has a higher decay at increased temperatures if present with oil. These problems become worse when the CO2 gas is present. This phenomena was particularly created to produce fresh surfactants with an affinity for CO2 gas under controlled conditions and to defeat the problems linked with surfactants that is conventional. The surfactants of novel can produce much balance foam at an increased temperature and in the existence of oil with lesser adaption

Like conventional surfactants, the surfactants of carbon dioxide phallic are pure amphophilic compounds but rather than lipophilic and hydrophilic parts, they are made up of carbon dioxide-phobic fragments and carbon dioxide philic. Usually the ends of these surfactants are attracted toward CO2, and are known as the surfactants of CO2-philic segments, and CO2-phobic parts are head group of the surfactant [22]. Phobic segments of carbon dioxide are usually selected as a traditional group of hydrophilic, formerly we have identified the segment of tail of the carbon dioxide-philic surfactant. The distinctive form of a carbon dioxide philic surfactant

The leading group of researchers have informed that non-fluorinated, hydrocarbon-based systems can be created in a way that they are CO2-philic in nature. These surfactants resolve the above mentioned foam problems [23–26]. Since CO2 is a weak solvent, the polar and high molecular weight substances are only partially soluble, but CO2 can dissolve in other few volatile and low molecular weight solvents. The carbon dioxide is a Lewis acid because it has an accepting electron nature, in spite of the fact that it has low polarisable properties [27]. The CO2 can take part in Lewis acid–base interactions because of CO2 having an electron accepting property. Many researchers have proved this kind of carbon dioxide bonding with other stuff like polymers and surfactants, etc. Fin and Lei Hong have expressed this kind of

Fin also stated in his work that ab initio molecular simulation research have shown that the O2 (ester or ether) in the side chain play an major role in promoting philicity of carbon dioxide because of the carbonyl oxygen. They recognised the three different ways the CO2 linked with a hydrocarbon end of the molecule.

**144**

**Figure 4.**

The surfactants branching *is* a key factor for the carbon dioxide philicity of the hydrophobic part. It is because of the effect that when chain length decreases, the CMC (aqueous) increases; while, an increase in branching increases the solubility in CO2. According to Ben Tan branching in the diacid as well as diol moiety has the increasing effect on solubility, and acyl chains branching increases the solubility up to 20 times.

#### **9.2 Number of tails**

It is observed that in case of CO2-philic compounds the solubility is greatly affected by the tail number. With the increase in the number of tails the dissolved surfactants in the carbon dioxide increases. A huge and emergent part of literature is fixated on the phenomena of interfacial of the carbon dioxide/H2O interface and proposes that there should be more contact with the interface for a double tail surfactant and as a result offers more stability for the micro emulsion. When a third chain is added in the surfactant structure the surfactant's solubility is increased in carbon dioxide even more [35–38].

#### **9.3 Tail length and tip**

In the past the relationship of CO2-philicity with the tail length has been widely studied and the interdependency of CO2-philic properties and surfactant tail structure was observed. At various temperature and pressure values, different double tail fluorinated surfactants have been studied for the phase behaviour [6, 7, 39]. This study leads to specifications for the optimization of tail length which is suitable for the maximum water/carbon dioxide emulsion formation at micro level. The phase behaviour for the oligomers is altered by the end-group modification of the oligomer PVAc-OH. Audrey DuPont examined P and T phase stability, chain structure effect and the aggregation structure. Surfactant free volume and surfactant packing are the parameters to view the effects of the chain lengths. Carbon dioxide solubility in esters is significantly affected by small structural changes in them. Depending upon the number of carbon atoms we can observe even/odd effects on solubility of carbon dioxide. According to observations made by Bray Christopher the acyl chain length is important with the carbon dioxide solubility for the molecule. With the increase in length of the chain by 10 carbon atoms the solubility increases in a systematic way. The influence of minor structural changes to the solubility of CO2

molecule proposes that a qualitative as well as quantitative study of property– structure relationship is possible, that leads to the ability to predict properties associated with carbon dioxide solubility of molecules [8, 16].

#### **9.4 Methyl groups, PO groups and methylene groups**

CO2-philicity is also favoured by the increase in methyl group number. Other factors that are considered in surfactant development are a smaller chain length, low molecular weight, lower no. of methylene and the propylene oxide groups.

#### *9.4.1 Carbonyl groups*

Beckman and Styranec have formulated CO2-philic compounds by the use of only oxygen, hydrogen and carbon comprising precursors. They observed the polyether solubility was significantly influenced by the side chain or by adding the carbonyl group in its backbone. Addition of acetate group in the side chain gives an increase in solubility to a certain limit after that limit the solubility tends to decrease. According to the studies of Fink et al., the solubility of CO2-philic compounds in carbon dioxide was significantly influenced by the addition of a good number of ester-functional side chains. The effect of numerous fluorine and vinyl-acetate groups in the side chain was studied by Bilal Baradie. He observed the polyether solubility was strongly changed with the side chain or with the addition of the carbonyl group [9, 40].

#### *9.4.2 Molecular weight*

A lot of research studies have claimed that carbon dioxide solubility greatly depends on the MW (weight of molecular) of the compound, as PVAc. At low pressures oligomer PVAc-OH (Mw < 3000 g/mol) is found to be dissolved in carbon dioxide but as the molecular weight increases it decreases in solubility. It was determined by Tan et al. Polymer's solubility such as PEC and PEE is significantly dependent upon their molecular weight as well as chemical structure of those polymers. A series of trials was conducted by Matthew B. Miller\* by the mixing of different solvents with various extents of carbon dioxide to measure the compatibility for both mixture components having bubble point as basis. It is observed that species with low MM (molar mass) having minimum one atom of O2 in ether, acetate groups/or carbonyl would have most favourable interaction with carbon dioxide through Lewis base/Lewis acid interactions [17].
