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Metallosurfactants, a "Novel Portmanteau": A Holistic Insight into the Structural-Physiognomies Relationships, Synthesis Stratagems, and Characterization

Ankush Parmar1, Shilpee Sachar2, and Shweta Sharma1

1Panjab University, Institute of Forensic Science and Criminology, Chandigarh, 160014, India

2University of Mumbai, Department of Chemistry, Vidyanagari, Santacruz (E), Mumbai, 400098, India

1.1 Introduction

Recently, "surface chemistry" has gained enormous amount of attention in bringing the vivid paradigm of synthetic chemistry [1]. Owing to the splendid attributes, being offered by it, a vast majority of the leading researchers, scientists, and physical chemists across the globe are putting their exertions to envisage noble advancements in this illustrious domain [2]. The inception of this field can be dated back to the late seventeenth century. It was through the numerous experiments (to study the phenomenon responsible for spontaneous spreading of oil-on-water) conducted by the famous physician Sir Benjamin Franklin that the early breakthroughs in this domain were achieved [3]. This innovation later on proved to be a souvenir and became the underlying fundamental basis for the upcoming research and development in this field [4].

In subsequent decades, a colossal growth has been witnessed in the dominion of surface chemistry, and the corresponding systematic findings have actually transformed variegated scientific disciplines [5]. It is because of this particular realm that the eminent fields such as colloid chemistry, interface science and surface engineering came into existence.

Several other eminent and impending disciplines viz. nanoscience, energy conversion, and catalysis have also momentously gained from it [6]. In other words, surface chemistry has essentially laid down the underpinning stone for voluminous superlative fields [7]. Henceforth, it can be rightly said that "surfactants, a portmanteau of surface-active agents" as we call them have proven to be highly potent agents (Figure 1.1) [8-11]. Amidst, all the molecules investigated in the realm of chemistry, surfactants are most exclusively studied. Owing to their exceptional solution and interfacial properties, they are currently being utilized on a widespread level in several fields.

Surfactants can be defined as the "materials which when present at low concentrations, alters the interfacial free energies of the interfaces via adsorbing themselves onto the interface/surface of the system" [8,12-20]. They generally fall under the category of organic compounds and are truly amphiphilic in nature [21].

Figure 1.1 Schematic representation of surfactant.

When it comes to structural organization, surfactants usually comprises of two vital components the first one being the head (polar) group, which is hydrophilic in nature [22,23], and the second part often referred to as the tail (non-polar) group is hydrophobic (lipophilic) in nature [24]. It is mostly observed that the polar head group varies greatly in structure and can be multifaceted, while the tail group demonstrates less diversity and as a rule it primarily comprises of a hydrocarbon (alkyl) chain with 8-20 carbon atoms. Broadly speaking, the tail can be branched/linear hydrocarbon, aliphatic, alkyl/aryl, short/long whereas; the head group can be ionic/non-ionic [21-23]. These chemical moieties have an innate ability to affect the air-water interface in an articulate fashion. This might be attributed to the reduction in the surface tension (interfacial tension) of water and formation of assorted assemblies (micelles) at the interface [10,25]. Upon adsorption at the interface, the two characteristic portions of the surfactants viz. head and tail group align themselves according to their polarities/preferential solubilities. The polar head group align itself toward the aqueous (water) part, while the tail group orient itself away from the water (outward direction) thereby resulting in the formation of micelles [26].

When dispersed in aqueous phase, at low concentration these surfactants generally exist in monomeric/dimeric state. An increase in their concentration beyond a designated threshold (viz. critical micellar concentration [CMC]) leads to the spontaneous accumulation of surfactant monomers. This further results in the formation of assemblies/colloidal-sized clusters also commonly referred to as "micelles" [4,7,26,27]. A major share of this potential breakthrough goes to James William McBain, whose discovery inevitably changed the dynamics and lead to significant advancements in the field of surface/surfactant chemistry [12,27-29]. It was in the year 1916, while conducting his research experiments, he observed infrequent alteration in the electro-conductive physiognomies as a function of soap concentration and he coined the term "micelles" [30].

Nowadays, a dire need is felt for the upgradation and development of novel technologies, which will play an intricate role in improvising the varied aspects of human lives. This upsurge calls for a quick modification of the surfaces in order to perfectly align with these rapid advancements [31]. Herein, the innate potential to alter the interfaces as per the requisite demand plays a pivotal role in envisaging and devising innovative technological advancements, ranging from energy production to biomedical implants, which will offer promising outcomes [12,27]. Incessant strides are being made in the field of science and technology on a regular basis to attain the aforementioned goal.

Irrespective of the fact that these chemical moieties possess such dynamic properties yet, they have not been able to secure a place for themselves among the "catalogue of advanced materials" [32]. In lieu of this, unique complexes with pre-selected functionalities (viz. proteins, carbohydrates, and metal ions) have been developed, which comprehensively aided in triumphing the lacunas associated with conventional surfactant systems. Additionally, incorporation of such modalities resulted in the fabrication of novel surfactants with engineered interfacial attributes [31]. Among all functional groups/modalities, metal ions have played a pivotal role in escalating the physicochemical attributes of the conventional surfactants exponentially. This in particular has lately led to the emergence of remarkable, and idiosyncratic complexes commonly referred to as metallosurfactants (MTS) [33].

1.2 Intrinsic Physiognomies of Metallosurfactants

Self-aggregation/association/assembly above CMC is an intrinsic property, which makes these surfactants and amphiphilic structures an invigorated tool. Length and volume of lipophilic component, size and charge of the polar head group, type of interaction with the solvent system, ionic strength, and molecular framework of the system are some of domineering factors, which tend to govern the hydrodynamic radii (particle size), and surface charge (zeta potential) of these singular entities [34-36]. The whole credit for this fundamental concept of aggregation goes to Zhulina et al. [35]. It was because of their incessant attempts that a comprehensive overview of this distinguishing phenomenon could be deciphered more than 30?years ago [37,38].

With an advent in time, several triumphant advancements have been accomplished in the field of surfactant chemistry. These progressions have broadened up the horizons of our understanding in lieu of surfactants. Similarly, the underlying concepts providing an inclusive aftermath of the phenomenon have also upgraded in recent years. This upgradation vindicated the fact that it is the metal ion, which plays a pivotal role in regulating the self-aggregation behavior of MTS/MTSC, respectively [34]. Whilst, the positioning of the metal ion component (counter ion/integral component) in the base matrix does not seem to have an obligatory effect on the assembling phenomenon.

When dispersed in aqueous media, these MTS tends to "diminish the characteristic qualities of metalloenzymes" [39]. Therefore, the MTSC felicitates a self-aggregation/association of distinct metal-complex-based aggregates reciprocating the aggregation number equivalent to their counterparts viz. surfactants [15,39]. This distinguishing property to self-aggregate, and formulate idiosyncratic complexes bestow these MTS with unique potential applications, which can prove to be handy in real-time scenario [39]. Additionally, the complexes so generated possess multiphasic properties, and can solubilize an array of diverse chemical moieties i.e. lipophilic/hydrophilic, and ionic components [13]. Depending upon the CMC value, distinctive structures illustrating assorted geometrical facets viz. micelles (spherical, oblate/prolate/ellipsoid, hybrid crystals with layered perovskite structure), and aggregates (vesicles, bilayers, helixes, tubules) are customarily formed by this special class of...