Mechanisms, performance optimization and new developments in demulsification processes for oil and gas applications.

The present review discusses new developments and optimization of demulsification processes in oil and gas applications, and highlights the critical parameters. Discussed are the primary mechanisms of demulsification, as well as the strategies for developing optimum demulsifiers. Demulsification mechanisms are presented in the context of emulsion stability principles which are equally applicable to the destabilization of crude oil-water emulsions. The present paper is a concise overview of the various surfactant classes and their structure-activity relationship. It correlates demulsification optimization with surfactant properties and their applications. These classes include, but are not limited to pluronic block co-polymers, as well as amine- and siloxane based nonionic surfactants. The emphasis is on providing some strategies for achieving optimum crude oil-water separation efficiency by tuning the demulsifier to the intended application and crude oil properties. A brief overview of unconventional analytical techniques, which reach beyond the standard demulsifier evaluation methods, i.e., Near Infrared Spectroscopy (NIR), and in particular, low resolution NMR relaxometry, highlights their role in monitoring demulsification processes.

Self-assembly of linactants: micelles and lyotropic liquid crystals in two dimensions.

We report observations of phenomena within two-component Langmuir monolayers that are analogous to surfactant self-assembly in 3D solutions. A partially fluorinated fatty phosphonic acid played the role of 2D surfactant (linactant), and a perfluorinated fatty acid acted as the 2D solvent. Langmuir-Blodgett monolayers were prepared from mixtures of these compounds and examined using atomic force microscopy. Above a critical linactant mole fraction of approximately 0.013, distinctive monodisperse structural features were observed with a characteristic diameter of approximately 30 nm and a relative height of 1.4 nm. As the linactant concentration was further increased, the number density of these features increased linearly with concentration, whereas the size remained approximately the same. A quantitative analysis of these observations suggested that the features corresponded to self-limiting clusters composed of approximately 2000 linactant molecules and that the dispersed clusters represented a 2D micellar phase. Above a linactant mole fraction of 0.63, the clusters organized into a local hexagonal structure with short-range positional order indicative of a 2D "lyotropic" liquid crystalline phase; the correlation length increased systematically with increasing linactant concentration.

Correlating linactant efficiency and self-assembly: structural basis of line activity in molecular monolayers.

Surfactants exhibit characteristic phenomena, including the reduction of interfacial free energy, self-assembly into aggregates, and even the formation of lyotropic liquid crystalline phases at high concentrations. Our research has shown that a semifluorinated phosphonic acid can act as the two-dimensional analogue of a surfactant-a linactant-by reducing the line tension between hydrocarbon-rich and fluorocarbon-rich phases in a Langmuir monolayer. This linactant can also self-assemble into nanoscale clusters in a monolayer. Here, we explore the dependence of linactant behavior on molecular structure. Members of a homologous series of partially fluorinated phosphonic acids were synthesized and tested as linactants: CF(3)(CF(2))(n-1)(CH(2))(m)PO(3)H (abbreviated as FnHmPO(3)). The tests revealed that linactants with longer hydrophobic chains were most efficient in lowering line tension. For linactants with the same overall chain length, the length of the fluorocarbon block was correlated with efficiency. Thus, the linactant efficiency was ranked in the order F8H8PO(3) < F10H6PO(3) < F8H11PO(3) < F10H9PO(3). In all cases, linactant-containing Langmuir-Blodgett monolayers exhibited nanoscale molecular clusters with characteristic dimensions of 20-30 nm; enhanced linactant efficiency was systematically correlated with larger clusters.

Linactants: surfactant analogues in two dimensions.

We report a new class of molecules, linactants, that partition at phase boundaries and reduce the line tension between coexisting two-dimensional phases in molecular monolayers. The line tension between hydrocarbon-rich and fluorocarbon-rich phases was determined by monitoring the relaxation kinetics of deformed domains. Two partially fluorinated linactant molecules (with one and two tails, respectively) were synthesized and tested; the more efficient single-tail variant reduced the line tension by more than 20% at a mole fraction of only 8 x 10(-4).

X-ray diffraction study of the structure of carboxymethylcellulose-cationic surfactant complexes.

We have investigated dilute aqueous solutions of an anionic polymer (carboxymethylcellulose) mixed with cationic surfactants of different chain lengths (dodecyl to octadecyl trimethylammonium bromides: DTAB, TTAB, CTAB and OTAB). The structures of the concentrated phases formed above the precipitation threshold were studied by X-ray diffraction. Different body-centred cubic structures with space groups Pm3n were observed in the presence of surfactant with a short aliphatic chain (DTAB), despite the fact that the polymer persistence length is comparable to the repeat distance of the structure (5 nm). For larger surfactant chain lengths (TTAB and CTAB), the structure of the precipitates can be either cubic (Pm3n) or 2D hexagonal depending on the initial surfactant and polymer concentrations. For still larger chain length (OTAB), the structure becomes lamellar. This structural evolution from micellar cubic towards 2D hexagonal and lamellar is attributed to the decrease of the local curvature of the surfactant aggregates, as observed for flexible synthetic polymers and short DNA fragments under similar conditions. Furthermore, the structure of the bulk complexes formed just below the precipitation threshold anticipates the structure seen in the precipitated phases.

Aggregate formation in aqueous solutions of carboxymethylcellulose and cationic surfactants.

The addition of cationic surfactants to an aqueous solution of an anionic polymer, carboxymethylcellulose (carboxyMC), causes the spontaneous formation of aggregates in a certain range of concentrations. Here we studied two surfactants, dodecyl and hexadecyl trimethylammonium bromide (DTAB and CTAB, respectively). Using different techniques (light scattering, potentiometry, viscosimetry, and zetametry), we found that a simple lengthening of the surfactant tail length by four CH2 groups drastically changes the aggregate morphology, size, and charge. We explored in detail how the surfactant and polymer concentrations act on these systems.

Swelling of a cluster phase in Langmuir monolayers containing semi-fluorinated phosphonic acids.

Langmuir monolayers of semi-fluorinated nonadecylphosphonic acid (F8H11PO), hexadecylphosphonic acid (H16PO), and their mixtures were investigated by Brewster angle microscopy (BAM), atomic force microscopy (AFM) and surface-pressure measurements. Nanometre-scale two-dimensional clusters were observed by AFM in a spread monolayer of pure F8H11PO transferred to mica. Two different organized arrangements of clusters were observed. AFM and BAM observations showed that the mixture exhibits a solid phase over a large range of mole fraction and surface pressure, sometimes in coexistence with clusters. With increasing mole fraction of H16PO, the lateral shape of these clusters remains the same while their organization and their height change.

Critical aggregation concentration in mixed solutions of anionic polyelectrolytes and cationic surfactants.

We have examined the polymer/surfactant interaction in mixed aqueous solutions of cationic surfactants and anionic polyelectrolytes combining various techniques: tensiometry, potentiometry with surfactant-selective electrodes, and viscosimetry. We have investigated the role of varying polymer charge density, polymer concentration, surfactant chain length, polymer backbone rigidity, and molecular weight on the critical aggregation concentration (Cac) of mixed polymer/surfactant systems. The Cac of these systems, estimated from tensiometry and potentiometry, is found to be in close agreement. Different Cac variations with polymer charge density and surfactant chain length were observed with polymers having persistence lengths either smaller or larger than surfactant micelle size, which might reflect a different type of molecular organization in the polymer/surfactant complexes. The surfactant concentration at which the viscosity starts to decrease sharply is different from the Cac and probably reflects the polymer chain shrinkage due to surfactant binding.