Sophorolipids (SLs), produced by Candida bombicola, are of interest as potential replacements for hazardous commercial surfactants. For the first time, a series of molecularly edited SLs with ethyl (EE), n-hexyl (HE), and n-decyl (DE) esters were evaluated at an oil (almond oil)-water interface for their ability to reduce interfacial tension (IFT) and generate stable emulsions. An increase in the n-alkyl ester chain length from ethyl to hexyl resulted in a maximum % decrease in the IFT from 86.1 to 95.3, respectively. Furthermore, the critical aggregation concentrations (CACs) decreased from 0.035 to 0.011 and 0.006 mg/mL as the ester chain length was increased from ethyl to n-hexyl and n-decyl, respectively. In contrast, the CAC of natural SL, composed of 50/50 acidic and LSL, is 0.142 mg/mL. Dynamic IFT analysis showed significant differences in diffusion coefficients for all SLs studied. Almond oil emulsions with up to 200:1 (by weight) oil/SL-DE were stable against oil separation for up to 1 week with average droplet sizes below 5 μm. Emulsions of almond oil with natural SLs showed consistent oil separation 24 h after emulsification. A unique connection between IFT and emulsification was found as SL-DE has both the lowest CAC and the best emulsification performance of all natural and modified SLs studied herein. This connection between CAC and emulsification may be generally applicable, providing a tool for the prediction of optimal surfactants in other oil-water interfacial applications.

Abstract

Lactonic sophorolipids (LSL) are glycolipid biosurfactants produced in large quantity by yeast fermentation. Chemical or enzymatic modification of naturally produced sophorolipids is an effective route to improve their functional properties. For the first time, ring-opening cross metathesis (RO/CM) was used to convert natural LSL to a unique family of modified sophorolipids. Reaction conditions for RO/CM of LSL with n-alkyl acrylates, trans-3-hexene, 1-hexene and ethylene were investigated. For the RO/CM of n-alkyl acrylates with LSL, %-conversions greater than 95% within 1 h resulted from conducting reactions in THF at 60°C using 5 mol% of Grubbs second generation ruthenium based catalyst (M2). The RO/CM reaction of LSL with ethylene performed at 3 bar under an ethylene atmosphere using Grubbs first generation (G1) (10 mol%) as the catalyst in dichloromethane (room temperature, 5 h) gave complete conversion of LSL to the corresponding ring-opened product. Ethanolysis of LSL RO/CM products generated a series of medium chain (C10-C14) SL-surfactants and fatty acid co-products. Values of surface tension reduction at the air-water interface versus Log (C) for modified SLs were measured by the Wilhelmy plate method. Minimum surface tension values varied as a function of the hydrophobic character of modified SLs. The modified SL from RO/CM with 1-hexene (SL-14) gave the largest surface tension reduction and lowest CMC (to 34 mN/m and 0.15 mM, respectively) and showed a similar surface tension reduction behavior as n-dodecyl-b-D-maltoside (Mal-C12). Increasing the number of carbons in the hydrophobic segment for the homologous series of n-alkyl sophorosides results in an almost linear decrease in log(CMC), with B ¼ 0.18 AE 0.03. This number is smaller than that of other related surfactants such as alkyl-b-D-glycosides and alkyl-b-D-maltosides.Practical applications: Modified sophorolipids can be used in a wide variety of applications such as stabilization of oil-in-water dispersions, antimicrobials and various cleaning operations.

Surfactants are ubiquitous constituents of commercial and biological systems that function based on complex structure-dependent interactions. Sophorolipid (SL) n-alkyl esters (SL-esters) comprise a group of modified naturally derived glycolipids from Candida bombicola. Herein, micellar self-assembly behavior as a function of SL-ester chain length was studied. Surface tensions as low as 31.2 mN/m and critical micelle concentrations (CMCs) as low as 1.1 μM were attained for diacetylated SL-decyl ester (dASL-DE) and SL-octyl ester, respectively. For deacetylated SL-esters, CMC values reach a lower limit at SL-ester chains above n-butyl (SL-BE, 1-3 μM). This behavior of SL-esters with increasing hydrophobic tail length is unlike other known surfactants. Diffusion-ordered spectroscopy (DOSY) and T relaxation NMR experiments indicate this behavior is due to a change in intramolecular interactions, which impedes the self-assembly of SL-esters with chain lengths above SL-BE. This hypothesis is supported by micellar thermodynamics where a disruption in trends occurs at n-alkyl ester chain lengths above those of SL-BE and SL-hexyl ester (SL-HE). Diacetylated (dA) SL-esters exhibit an even more unusual trend in that CMC increases from 1.75 to 815 μM for SL-ester chain lengths of dASL-BE and dASL-DE, respectively. Foaming studies, performed to reveal the macroscopic implications of SL-ester micellar behavior, show that the observed instability in foams formed using SL-esters are due to coalescence, which highlights the importance of understanding intermicellar interactions. This work reveals that SL-esters are an important new family of green high-performing surfactants with unique structure-property relationships that can be tuned to optimize micellar characteristics.

In this manuscript, we approach the production of biosurfactants as a cleaner alternative to the chemically-produced surfactants currently used in a wide range of industries. Sophorolipids are microbially produced biosurfactants of the glycolipid type that have entered the market in select applications such as detergent or cosmetic formulation ingredients. This study focuses on sophorolipid production by the yeast Starmerella bombicola from stearic acid (C18:0), a low-cost carbon source that is difficult to work with in submerged fermentation since it remains a solid due to its high melting temperature. Consequently, optimizations of solidstate fermentation inoculated with Starmerella bombicola were studied for conversions of stearic acid and molasses to sophorolipids. Polyurethane foam functioned as the inert support. The effect of polyurethane foam density and water holding capacity was assessed and the process was optimized in terms of substrate and inoculum ratio. The best conditions were: foam with a density of 32 kg m-3 at 75% water holding capacity, 1.17:1 molasses/stearic acid (w/w) and 5% (v/w) inoculum, to obtain a yield of 0.211 g sophorolipids per g of substrates. Mass spectrometry revealed that the sophorolipids produced herein had high concentrations of diacetylated acidic and lactonic C18:0 forms. The results of interfacial properties studies revealed that C18:0 sophorolipids had promising surface tension lowering capacity and emulsification behavior. This study describes a new strategy to produce biosurfactants using low environmental impact technologies as an alternative to traditional ways to produce chemical detergents.

Sophorolipids (SLs) are biosurfactants produced by Candida bombicola from renewable feedstocks in yields > 400 g/L. Molecular editing of natural SLs gave a series of n-alkyl SLesters that, along with natural SLs, were interrogated to determine how structural changes alter SL interfacial tension (IFT), diffusion, adsorption, and emulsification at the lemon oil/water interface. SL-ethyl ester (SL-EE) reduced the IFT by 95.1% and has a critical aggregation concentration (CAC) of 0.026mg/mL while SL-hexyl ester (SL-HE) had a lower CAC, 0.02mg/mL, but a lower IFT reduction (87.5%). Adsorption behavior further highlighted differences between the SL-esters as SL-decyl ester (SL-DE) had the lowest surface excess concentration while SL-HE had the highest adsorption coefficient. The competing effects of these interfacial parameters were manifested in the relative performance of SL-esters in forming lemon oil-in-water emulsions. SL-EE had the largest average emulsion droplet size but showed no oil separation up to 200:1 wt/wt oil/surfactant. Microscopy provided information on macroscopic emulsion morphologies. For example, flocculation was observed for all 1wt% SL 20wt% lemon oil emulsions. These results were corroborated by shear thinning rheological behavior. Studies were conducted with a natural SL mixture consisting of 1:1 wt/wt lactonic and

Sophorolipids (SLs) offer an “environmentally friendly” alternative to chemically produced surfactants currently used in formulations for crude oil extraction, processing, and reclamation. Studies herein describe how sophorolipid structure influences its interfacial properties for environmentally and industrially relevant oil–water systems where the oil phase is Arabian light crude oil, paraffin oil, decane, hexadecane, a 1:1 vol/vol mixture of o‐xylene and 1,2‐dimethylcyclohexane, or a mixture of paraffin oil, o‐xylene, and 1,2‐dimethylcyclohexane (synthetic crude oil). SL‐hexyl ester (SL‐HE) reduces the crude oil–water interfacial tension (IFT) by 57 and 91% at 0.001 and 0.5 mg/mL, respectively. Crude oil displacement tests reveal that SL‐ethyl ester (SL‐EE) and SL‐HE contract a crude oil slick on water to about 20% of its starting volume allowing for easier burning of spilled crude oil on marine surfaces. Water retention and emulsion phase (e.g., o/w vs. w/o) are determined by SL‐structure/concentration, oil concentration, and oil composition to understand their performance for crude oil transportation and clean‐up. For the first time, w/o emulsions were obtained using SLs and their formation occurred after homogenization when the oil phase consisted of a 1:1 mixture of o‐xylene and 1,2‐dimethylcyclohexane. Generally, the performance of SL‐esters in the above studies was superior to that using Triton X‐100, a comparison nonionic surfactant. Hence, SL‐esters offer a valuable platform for tuning interfacial properties to optimize surfactant performance.

Magnetorheological fluids (MRFs) are functional materials, prepared by dispersing magnetic particles in a nonmagnetic carrier fluid, that exhibit a change in mechanical properties (e.g., increase in viscosity and yield stress) when subjected to a magnetic field. Change in mechanical properties is demonstrated by a fast (i.e., fraction of milliseconds) and reversible transition from a liquid‐ to a solid‐like state. This transition, due to a structural reorganization of magnetic particles in the carrier fluid, makes MRFs a valuable material for damping, breaking systems, medical and prosthetics, and robotics. Since the discovery of MRFs in 1948, developing MRF preparation methods that result in improved performance and the storage without oxidation and settling is paramount. This article presents a review on recent developments in the preparation process of MRFs with a special emphasis on the state of the art in additives and coatings used in enhancing MRF chemical, colloidal, and thermal stability. Recent advances in MRF materials and formulations that have increased yield stress and magnetic properties are discussed. Finally, this review analyses the present‐day challenges in MRF research and makes suggestions for the field to improve MRF stability and performance drawing on previous MRF work and work outside of the fields.