Structure and Interaction of Lithium Dodecyl Sulfate Micelles in the Presence of Li-Specific Macrocyclic Cage: A Study by SANS

Complex Fluids Confined into Semi-interpenetrated Chemical Hydrogels for the Cleaning of Classic Art: A Rheological and SAXS Study

The removal of aged varnishes from the surface of easel paintings using the common conservation practice (i.e., by means of organic solvents) often causes pigment leaching, paint loss, and varnish redeposition. Recently, we proposed an innovative cleaning system based on semi-interpenetrated polymer networks (SIPNs), where a covalently cross-linked poly(hydroxyethyl methacrylate), pHEMA, network is interpenetrated by linear chains of poly(vinylpyrrolidone), PVP. This chemical gel, simply loaded with water, was designed to safely remove surface dirt from water-sensitive artifacts. Here, we modified the SIPN to confine complex cleaning fluids, able to remove aged varnishes. These complex fluids are 5-component water-based nanostructured systems, where organic solvents are partially dispersed as nanosized droplets in a continuous aqueous phase, using surfactants. The rheological behavior of the SIPN and the nanostructure of the fluids loaded into the gel were investigated, and the mechanical behavior of the gel was optimized by varying both the cross-linking density and the polymer concentration. Once loaded with the complex fluids, the hydrogels maintained their structural and mechanical features, while the complex fluids showed a decrease in the size of the dispersed solvent droplets. Two challenging case studies have been selected to evaluate the applicability of the SIPN hydrogels loaded with the complex fluids. The first case study concerns the removal of a surface layer composed by an aged brown resinous patina from a wood panel, the second case study concerns the removal of a homogeneous layer of yellowed varnish from a watercolor on paper. The results show that the confinement of complex fluids into gels allowed unprecedented removal of varnishes from artifacts overcoming the limitations of traditional cleaning methods.

An amine-oxide surfactant-based microemulsion for the cleaning of works of art

Surfactant-based aqueous fluids, such as micellar solutions and microemulsions, are effective, safe and selective media for cleaning operations in conservation of cultural heritage. The search for better-performing systems and eco-friendly cleaning systems is currently a major goal in conservation science. We report here on a ternary o/w microemulsion, composed of diethyl carbonate (DC) as the oil phase and N,N-Dimethyldodecan-1-amine oxide (DDAO) as the surfactant. DDAO is a well known and widely used detergent and solubilizing agent, selected here for its degradability and eco-compatibility. Due to its nonionic/cationic nature, it can be used also when nonionic-based formulations become ineffective because of clouding and phase separation. Moreover, DDAO is insensitive to the presence of divalent metal ions, usually abundant in wall paintings substrates. Small-Angle Neutron Scattering (SANS) provided detailed information about the nanostructure of the surfactant aggregates. Finally, the cleaning effectiveness of the nanofluid was assessed both on fresco mock-ups and on real wall paintings conserved in the archeological site of Tulum, Mexico. Here, conservators successfully used the microemulsion to remove naturally aged films of complex polymer mixtures from the works of art surface.

Nanostructured surfactant-based systems for the removal of polymers from wall paintings: a small-angle neutron scattering study

Nanostructured soft matter systems represent effective and long-lasting solutions with respect to traditional and often obsolete methodologies for the conservation of works of art. In particular, complex fluids such as micelles and microemulsions are the most performing media for the removal of organic materials from porous supports, like wall paintings or stones. In this Article, we report on the characterization of two systems, EAPC and XYL, which have shown good to optimal performances in the removal of organic polymers from wall paintings. EAPC is a five-components fluid composed of water, sodium dodecylsulfate (SDS), 1-pentanol (PeOH), propylene carbonate (PC), and ethyl acetate (EA), while XYL is a "classical" o/w microemulsion, where p-xylene droplets are stabilized in water by SDS and PeOH. Small-angle neutron scattering (SANS) with contrast variation is used to infer a detailed picture of the structure of these complex fluids, with a particular focus on the partition of the components between the bulk phase and the nanocompartments. We found that, differently from XYL, the EAPC system is neither a microemulsion nor a simple micellar solution, with the cosolvents partitioned between the dispersing phase and the disperse droplets. These different structural features play a key role in defining the cleaning effectiveness and specifically the kinetics of interaction between the nanofluid and the polymeric coating to be removed, which is of paramount importance for the application in the field. Both of these nanofluids are effective in polymer removal, but EAPC is considerably more efficient and versatile. The composition and the structure at the nanoscale determine the capability of removing a broad range of different polymer coatings from porous materials. A representative case study is here described, addressing a particularly challenging conservative issue, which is the removal of a multilayered aged coating that was irreversibly damaging the pictorial layer of the Annunciation Basilica in Nazareth.

Acrylamide-based magnetic nanosponges: a new smart nanocomposite material

Nanocomposite materials consisting of CoFe2O4 magnetic nanoparticles and a polyethylene glycol-acrylamide gel matrix have been synthesized. The structure of such materials was studied by means of small-angle scattering of X-rays and polarized neutrons, showing that the CoFe2O4 nanoparticles were successfully and homogeneously embedded in the gel structure. Magnetic, viscoelastic, and water retention properties of the nanocomposite gel confirm that the properties of both nanoparticles and gel are combined in the resulting nanomagnetic gel. Scanning electron microscopy highlights the nanocomposite nature of the material, showing the presence of a gel structure with different pore size distributions (pores with micron and nano-size distributions) that can be used as active sponge-like nanomagnetic container for water-based formulations as oil-in-water microemulsions.

Amphiphilic Self-Assemblies Decorated by Nucleobases

Phosphatidyl-nucleosides are a class of functional amphiphiles, where a nucleic acid monomer is conjugated to a lipid skeleton. These derivatives self-organize in aqueous solution as assemblies of various size, shape, and interfacial curvature. This paper presents a comparison of the aggregation behavior of different 1-R,2-R-sn-glycero-3-phosphatidyl-nucleosides, where R = 8 (DiC8PN) or R = 12 (DLPN) and N is either adenosine (a purine) or uridine (a pyrimidine), a complementary pair in RNA. Surface tension, small angle neutron scattering, cryo-TEM, and circular dichroism are used to highlight and distinguish the impact of the hydrophobic assembler and of the base substitution on the solution phase behavior. Our main conclusion is that the nucleic functionalization provides an additional parameter to control self-assembly through specific interactions among the polar heads. Further nonideal effects are induced by mixing nucleolipids with complementary base substitution. We show that these contributions alter the aggregation thresholds and modulate properties of the aggregates on the mesoscale.

Structure of SDS Micelles with Propylene Carbonate as Cosolvent: a PGSE−NMR and SAXS Study

The effect of propylene carbonate on SDS micelles was investigated by means of pulsed gradient spin-echo (PGSE) NMR, small-angle X-ray scattering (SAXS), conductivity and ion-selective electrode (ISE) measurements. The knowledge of the cosolvent partition between continuous phase and micelles (obtained by means of PGSE-NMR) allowed the identification of relevant dilution paths. Along these paths the system is composed of identical micelles that become more and more diluted. The extrapolation of measured self-diffusion coefficient to infinite dilution (where direct and hydrodynamic interactions are negligible) permits the determination of hydrodynamic size of the micelles. Moreover, the micelle ionization degree (measured by means of ISE) combined with PGSE-NMR and conductivity data furnishes an estimate of the aggregation number without any assumptions on micellar shape. On the other hand, troublesome hydrodynamic interactions are irrelevant to SAXS, and scattering data collected at fixed composition can be analyzed according to a reasonable model by exploiting the insight on the propylene carbonate partition gained through PGSE-NMR. By means of these approaches, we have found that propylene carbonate acts mainly as cosurfactant for the SDS micelles, decreasing their size and aggregation number by increasing the mean headgroup area of SDS.

Small-Angle Neutron Scattering of Mixed Ionic Perfluoropolyether Micellar Solutions

Aqueous mixed micellar solutions of perfluoropolyether carboxylic salts with ammonium counterions have been studied by small-angle neutron scattering. Two surfactants differing in the tail length were mixed in proportions n2/n3 = 60/40 w/w, where n2 and n3 are the surfactants with two and three perfluoroisopropoxy units in the tail, respectively. The tails are chlorine-terminated. The mixed micellar solutions, in the concentration range 0.1-0.2 M and thermal interval 20-40 degrees C, show structural characteristics of the interfacial shell that are very similar to ammonium n2 micellar solutions previously investigated; thus, the physics of the interfacial region is dominated by the polar head and counterion. The shape and dimensions of the micelles are influenced by the presence of the n3 surfactant, whose chain length in the micelle is 2 A longer than that of the n2 surfactant. The n3 surfactant favors the ellipsoidal shape in the concentration range 0.1-0.2 M with a 1/2 ionization degree of n2 micelles. The very low surface charge of the mixed micelles is attributed to the increase in hydrophobic interactions between the surfactant tails, due to the longer n3 surfactant molecules in micelles. The closer packing of the tails decreases the micellar curvature and the repulsions between the polar heads, by surface charge neutralization of counterions migrating from the Gouy-Chapman diffuse layer, leading to micellar growth in ellipsoids with greater axial ratios.

Small-Angle Neutron Scattering of Ionic Perfluoropolyether Micellar Solutions: Role of Counterions and Temperature

This paper reports a small-angle neutron scattering (SANS) characterization of perfluoropolyether (PFPE) aqueous micellar solutions with lithium, sodium, cesium and diethanol ammonium salts obtained from a chlorine terminated carboxylic acid and with two perfluoroisopropoxy units in the tail (n(2)). The counterion and temperature effects on the micelle formation and micellar growth extend our previous work on ammonium and potassium salts n(2) micellar solutions. Lithium, sodium, cesium and diethanol ammonium salts are studied at 0.1 and 0.2 M surfactant concentration in the temperature interval 28-67 degrees C. SANS spectra have been analyzed by a two-shell model for the micellar form factor and a screened Coulombic plus steric repulsion potential for the structure factor in the frame of the mean spherical approximation of a multicomponent system reduced to a generalized one component macroions system (GOCM). At 28 degrees C, for all the salts, the micelles are ellipsoidal with an axial ratio that increases from 1.6 to 4.2 as the counterion volume increases. The micellar core short axis is 13 A and the shell thickness 4.0 A for the alkali micelles, and 14 and 5.1 A for the diethanol ammonium micelles. Therefore, the core short axis mainly depends on the surfactant tail length and the shell thickness on the carboxylate polar head. The bulky diethanol ammonium counterion solely influences the shell thickness. Micellar charge and average aggregation number depend on concentration, temperature and counterion. At 28 degrees C, the fractional ionization decreases vs the counterion volume (or molecular weight) increase at constant concentration for both C = 0.1 M and C = 0.2 M. The increase of the counterion volume leads also to more ellipsoidal shapes. At C = 0.2 M, at 67 degrees C, for sodium and cesium micelles the axial ratio changes significantly, leading to spherical micelles with a core radius of 15 A, lower average aggregation number, and larger fractional ionization.