Molecular interactions of CPC, CPB, CTAB, and EPC biosurfactants in aqueous olive oil mixtures analyzed with physicochemical data and SEM micrographs

3.1 Densities

Limiting density ρ⁰ is a function of interacting strength of solute and the ρ⁰ data of the surfactants with olive oil + water mixture are as CTAB > CPC > CPB > EPC (Table 1). The higher values of the CTAB inferred stronger molecular interactions and the lower of the EPC the weaker than those of the CPC and CPB. The CPC and CPB had similar cetyl chains and pyridine ring with the Cl⁻ and Br⁻ anions, respectively. The ionic effect of smaller sized Cl⁻ with the CPC produced higher ρ⁰ than those of the CPB. The CPC and CPB with pyridine rings caused stronger intermolecular force (IMF) due to π-conjugation. The CTAB without pyridine ring and with a quaternary nitrogen atom (N⁺) and cetyl chain alike CPC and CPB produced the higher ρ⁰ values than those of the CPC and CPB. Hence the activities of π-conjugation in hydrophobic–hydrophilic interactions where the surfactants have multiple centers of forces such as hydrophobic forces along alkyl chain and hydrophilic forces along hydrophilic parts. The water interacts differently with these configurations of the surfactants. Similarly, the EPC with an N⁺ atom alike CPC, CPB, and CTAB and additional $\text{PO}_4{}^{-3}$ and 2 cetyl chains (Fig. 6) produced lowest ρ⁰ data with almost two times higher hydrophobic interactions than of other the BS. The EPC with different molecular configuration interacted differently due to intramolecular multiple forces [IMMF] and model is noted as intramolecular multiple forces theory [IMMFT]. Since the EPC is centered on phosphate P atom with three alkyl chains which surround the P (Fig. 6). The molecule has steric and induction effects with covalent bonding force (CBF) which with double bond with each alkyl chains with sp² configuration that conceptually weakened the CBF. Each alkyl chains attached with O atoms with different electronic configuration and CBF. With EPC different force centers with single molecule are operational and caused different activities because of that the EPC molecule caused effects weaker on surface activities. Similarly the olive oil with different force centers induced different alignments such as benzene ring with π-conjugation center developed a shifting charge centers. Also alkyl chains broken with O₂ caused different forces within single molecule and a double within terminal chain also caused many operational forces centers. The IIMFT theory offers a better solution to structurally explain molecular forces responsible for emulsification action of the EPC as it has not largely reduced the surface tension and does not tend toward interface. The IMMFT is valid as the EPC molecule does not behave like CPC, CPB, CTAB where their alkyl chain caused hydrophobic but the head caused hydrophilic which reduces the surface tensions by more than 43% as compared to the EPC. The higher values of the CTAB are due to an effective reorientation caused by alkyl chain and N⁺ ion and the higher IMF exerted by CTAB–OOW interactions. The EPC was not as effective as others with weaker disruption of water structure due to exertion of weaker IMMF and the EPC produced the lowest ρ⁰ values. The EPC is a mild structure breaker with weaker IMF due to comparatively larger hydrophobic interaction developing units (HIDU). The EPC behaved as an excellent emulsifier only because of many forces centers are operational in developing interactions at separate points within a molecule. Hence it could not pushed to interface but remained suspended in hydrophilic water while the CPC, CPB, and CTAB reduced almost 43% ST as they tend to surface and saturate the same by reducing surface energy or tension due to integrated hydrophobic CBF but the EPC does not have integrated CBF along the alkyl chain because of the O atoms are in chains. Hence comparatively the EPC behaves as the best emulsifier while the CPC, CPB, and CTAB as the best cationic surfactants. This observation is also supported by an SEM pattern, with stronger intermolecular forces with CTAB and olive oil which produced a higher compatibility and density (Fig. 5).

A decrease in ρ⁰ data of the CPC and EPC with olive oil–water from 0.99190 to 0.99150 × 10³ kg m⁻³ is attributed to a swelling effect similar to sol gel formation with weaker internal forces (Fig. 1). The pyridine ring and the Cl⁻ ion of the CPC caused stronger hydrophilic interaction while the $\text{PO}_4{}^{-3}$ of the EPC induced weaker forces. The EPC as BS showed a weaker ability to disrupt the water structure in comparison to others due to an emulsion formation with weaker cohesivity and densities. The slopes values with EPC are lower than of those of the others and inferred emulsification or homogenous dispersion in bulk water phase and not at surface contrary to CTAB, CPC, and CPB due to surface accumulation. Since with concentration the CTAB, CPC, and CPB move to surface and strongly weaken tension. Thus the slope values with composition are higher than those of the EPC where the IMMFT explained a maximum dissolution of the EPC. Thus the EPC is a suitable medium for transportation of bio-molecules with a weaker friction. The S_d values are as CPB > CPC > CTAB > EPC with the olive oil–water (Table 2). The CPB showed higher IMF with higher values while the $\text{PO}_4{}^{-3}$ group with quaternary ammonium ion showed the lowest S_d values due to its effect on pair wise interactions. The CPB and CPC with olive oil–water forces exerted comparatively higher activity (Fig. 1).

3.2 Viscosities

The viscosity of water as a function intermolecular force is lower than of the BS–olive oil–water due to stronger Newtonian forces with water–BS interactions due to a destabilization of water structure. The CPC, CPB, and CTAB produced higher viscosities by 1.1 kg m⁻¹ s⁻¹ and the EPC had the 0.2 kg m⁻¹ s⁻¹ times than of the water which ensured their stronger water structure breaking effects (Table 1). The (η_rel − 1)/c vs c analysis was polynomial (Eq. (3)) with positive B values (Table 2) as EPC > CPB > CPC > CTAB with 110.01, 38.72, 37.55, and 37.21 kg mol⁻¹, respectively. The highest values of EPC with olive oil–water inferred comparatively stronger hydrophobic–hydrophobic interactions along emulsification supported with SEM analysis (Fig. 5). The EPC showed a stronger dispersion network with stronger solute–solute interactions while the CTAB as along dispersions and disrupted molecular surface. The molecular binding with EPC was less endothermic (Table 2) but increased with increments in alkyl-chain length and used its internal disorders as a driving force for binding. Thus the higher EPC viscosities inferred it as comparatively stronger water structure breaker. The ρ and V_ϕ data resolved Newtonian flow with lower water density than of the mixtures (Table 1) and dipolar interactions of BS are responsible for disrupting water structures. The stronger structural interactions occurred due to partially charged centers with higher values than the water. Resultants water monomers enter multiple force centers due to induction in alkyl chains and positively charged quaternary nitrogen atoms. The B coefficient is as CPC > CTAB > CPB > EPC inferred slightly weaker hydrophilic interactions with OOW over the hydrophobic. (Table 2).

3.3 Surface tension

The surface tensions of BS–olive oil–water are lower than of the water by 1.5 mN m⁻¹ except the EPC, with slightly lower values (Table 1). With OOW, the higher surface forces than of the water are attributed to the stronger hydration spheres with the CPC, CPB, and CTAB. The γ⁰ are as EPC > CTAB > CPC > CPB (Table 1) and are different than those of the ρ⁰ and η⁰ trends. The γ⁰ data inferred a role of cohesive and adhesive forces due to activities in addition to electrostatic and Newtonian forces (Table 3).

A stronger ionic effect of a large sized Br⁻ anion enhanced hydrophobic–hydrophilic interactions between the CPB and OOW and produced a lowest γ⁰ value with CPB. The CPB and CPC reoriented the water structures, with weaker surface forces due to a pyridine ring while the CTAB without π-conjugation did not show similar mechanism (Fig. 6). The higher γ⁰ value with the EPC exerted a stronger IMF with stronger hydrophobic–hydrophobic interactions of longer alkyl chain. The IMMFT model explained an action mechanism of the EPC due to multiple force centers such as two alkyl chain, oxygen atoms, $\text{PO}_4{}^{-3}$ group, and quaternary nitrogen atoms. The $\text{PO}_4{}^{-3}$ group weakly disrupted the water, and highly asymmetricity in the EPC structure initiated the stronger cohesive forces. The higher entropic changes due to multiple force centers for interactions caused stronger hydrophobic–hydrophobic interactions (Singh and Matsuoka, 2009). The IMMFT predicted that the EPC molecules are not able to disrupt the hydrogen bonded water and the multiple forces of electrostatical points EPC exert higher tension. With such mixtures the hydrophobic interactions dominate by further attracting surface forces downwards with composition of the EPC. For example, from 8 mm kg⁻¹ BS, the γ⁰ values became linear with no further change in surface forces, due to a complete kind of the reorientation of water along little monomer formation (Fig. 3) which further increased with increasing concentration. The 8 mm kg⁻¹ BS, initiated micelles formation, and is a critical micelle concentration point (Fig. 3). The S_γ values are negative, due to weaker cohesive forces with increase in compositions (Table 2). The SEM illustrated dispersed structures of biosurfactants with higher intermolecular forces between them in aqueous olive oil mixtures (Fig. 5). The IMMFT model excellently explained geometry of microstructures as a function of frictional and cohesive forces at multiple points, especially with the EPC. The molecular dispersion has maximum surface forces but an olive oil brought them together causing intermolecular forces which reduced an exposed area with reduction surface tension as EPC > CTAB > CPC > CPB. Their comparative study illustrated higher surface tension and lowest dispersion with EPC (Fig. 5). The stronger intermolecular forces between the EPC and EPC with higher cohesive and adhesive forces in olive oil–water are responsible for such a behavior. The SEM illustrated comparatively uniform dispersion with the EPC with weaker Van der Waals and London dispersion forces and is attributed to IMMFT.

3.4 Apparent molal volume

Apparent molal volume explained solute–solvent interactions with increments in concentrations. The molal volumes depicted an exertion of internal pressure caused by intermolecular forces where weaker are the forces higher are the volumes. The $V_{\varphi }^0$ data are EPC > CPC > CPB > CTAB with 1808.9 > 1078.2 > 983.7 > 751.4 × 10⁻⁶ m³mol⁻¹, respectively (Table 3), inferred weaker IMF with EPC + OOW. Of course, molecular size of the EPC is larger still higher volume showed weaker IMF. Positive $V_{\varphi }^0$ values inferred weaker internal pressure caused by weaker heteromolecular interactions. It is also true in cases of the ρ⁰ and η⁰, the V_ϕ data with similar trends as transport and static properties respectively, with exceptional behavior of phosphate of the EPC. The V_ϕ values of the EPC decreased from 17450.0 to 976.8 × 10⁻⁶ m³ mol⁻¹ from 2 to 10 mm kg⁻¹ with OOW, while the V_ϕ of the CPC, CPB, and CTAB decreased from 2 to 6 mm kg⁻¹ but increased from 6 onwards (Fig. 4). It suspected micelles formation and surface saturation with CPC, CPB, and CTAB with two behaviors with lower and higher compositions while EPC did not show such a dual behavior with compositions which is explained due to IMMFT. As the multiple force centers inhibit surface saturation with EPC. In dilute concentration region, the interactions are weaker than those of the comparatively concentrated regions. The S_v and $S_{\text{v}}^{\prime }$ data inferred stronger solute–solute interactions of EPC than that of CPC, CPB, and CTAB (Table 3), with a crucial role of activation energy of phosphate–water interactions. The $\text{PO}_4{}^{-3}$ caused an asymmetry in the EPC molecules with a maximum optimization as per a flickering model, the water molecules surround and adhered to the $\text{PO}_4{}^{-3}$ with a stable conformation (Table 1).

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Figure 4

Apparent molal volume (V_ϕ)/10⁻⁶ m³ mol⁻¹ with a sharp decrease from 0.002 to 0.01 mol kg⁻¹ with distinct behavior of egg-phosphatidylcholine (EPC).

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3.5 SEM

With SEM, primary electrons are thermionically or field emitted by a cathode filament (W or LaB₆) or a field emission gun (W-tip) gets accelerated with high energy 1–30 KeV. The electron beam is steered with scanning coils over an area of interest, upon interaction with material, the primary electrons decelerated by transferring energy inelastically to other atomic electron and to the lattice of the sample. Due to continuous scattering events, the emitted radiation from the specimen are collected and used for imaging a topography of surface while characteristic X-rays emitted from the sample are analyzed to get information (quantitative and qualitative) about the elements present in the sample. Usually oil develops colloidal solution with diffuse interface forces while smaller biosurfactants are well defined hydrated units depicted by SEM. The biosurfactants breakdown the water structure while olive oil is not able to do so but it shifts the bulk structure of water around itself with a cage formation. The dispersion of the olive oil in water and of CPC, CPB, CTAB, and EPC molecules with olive oil–water mixture is depicted by the SEM as CTAB > CPC > CPB > EPC > OOW (Fig. 5). Here, the SEM studies illustrate that the olive oil structure in water does not get disrupted due to stronger intermolecular forces but it remains in its original structure. The thread like structure inferred the C backbone of olive oil (Fig. 5). The EPC has caused least disruption into n-fragmentations due to stronger solute–solute interactions, resulting with higher surface forces (Fig. 5). The CTAB though weakened the frictional forces but also caused some integrated or packed patches or the smaller globules which is dewetting (Fig. 5).

The CPC disrupted the structure of a surface alike the CPC molecule in clusters by weakening the cohesive forces (Fig. 5). The CPB further weakened IMF and strongly disrupted even clusters of olive oil, may be in a form of monomolecular layer and results into an effective structural reorientation (Fig. 5). The micrographs are of same concentration (0.1 m) varying their size from 5 to 10 μm in aqueous olive oil mixture (Fig. 5). The biosurfactants showed higher IMF in aqueous olive oil mixture due to stronger solute–solute and weaker solute–solvent interactions. The surfactants are highly soluble in water with stronger hydrogen bonding in water. Hence a molecular structure of the surface gets dispersed with weakening in cohesive and adhesive forces with olive oil–water mixture. The SEM depicted that the biosurfactants formed an integrated molecular solution due to hydrated hydrophilic forces with water while with olive oil get scattered due to the rising hydrophobic forces.