Article pubs.acs.org/Langmuir
Dispersion Behavior and Aqueous Foams in Mixtures of a VesicleForming Surfactant and Edible Nanoparticles Bernard P. Binks,*,† Shawn Campbell,‡ Saeed Mashinchi,† and Michael P. Piatko‡ †
Surfactant & Colloid Group, Department of Chemistry, University of Hull, Hull HU6 7RX, United Kingdom Rich Products Corporation, One Robert Rich Way, Buffalo, New York 14213, United States
‡
S Supporting Information *
ABSTRACT: In an attempt to prepare ultrastable aqueous foams composed entirely of food-grade ingredients, we describe the foamability and foam stability of aqueous phases containing either calcium carbonate particles (CaCO3), sodium stearoyl lactylate surfactant (SSL), or their mixtures. Techniques including zeta potential measurements, adsorption isotherm determination, contact angles and optical and cryo-scanning electron microscopy are used to probe the interaction between particles and surfactant molecules. Aqueous dispersions of inherently hydrophilic cationic CaCO3 nanoparticles do not foam to any great extent. By contrast, aqueous dispersions of anionic SSL, which forms a lamellar phase/vesicles, foam progressively on increasing the concentration. Despite their foamability being low compared to that of micelle-forming surfactant sodium dodecyl sulfate, they are much more stable to collapse with half-lives (of up to 40 days) of around 2 orders of magnitude higher above the respective aggregation concentrations. We believe that, in addition to surfactant lamellae around bubbles, the bilayers within vesicles contain surfactant chains in a solidlike state yielding indestructible aggregates that jam the aqueous films between bubbles, reducing the drainage rate and both bubble coalescence and gas-transfer between bubbles. In mixtures of particles and surfactant, the adsorption of SSL monomers occurs on particle surfaces, leading to an increase in their hydrophobicity, promoting particle adsorption to bubble surfaces. Ultrastable foams result with half-lives of around an order of magnitude higher again at low concentrations and foams which lose only around 30% of their volume within a year at high concentrations. In the latter case, we evidence a high surface density of discrete surfactant-coated particles at bubble surfaces, rendering them stable to coalescence and disproportionation.
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INTRODUCTION An aqueous foam is a dispersion of gas bubbles in water and can be stabilized by surface-active agents such as surfactants, proteins, polymers, or solid particles. Foams are thermodynamically unstable, with destabilization occurring through a combination of drainage, disproportionation, and coalescence.1 Many surfactant-stabilized foams are inherently unstable and collapse within a day or so. Foams stabilized by particles alone can be ultrastable, whereby all three instability processes are halted to a large extent.2−6 However, particles must be partially hydrophobic to enable them to adsorb at the air−water surface, and this is not always simply achieved. Examples exist in which particles are modified ex situ2 or quite exotic particles are synthesized7 that happen to possess the required wettability characteristics for creating stable foams. A second approach used to enhance the surface activity of, say, hydrophilic particles is to mix them with suitable surfactants by which the adsorption of surfactant onto particle surfaces renders particles increasingly hydrophobic, at least at relatively low surfactant concentrations.4,8−14 Such surfactant-coated particles give rise to foams exhibiting long-term stability. The interaction between the two species is frequently electrostatic, although hydrogen bonding is implicated if the surfactant is uncharged. © 2015 American Chemical Society
Examples of such in situ activation include silica and cationic surfactants (alkylamine and alkyltrimethylammonium bromides),10,14 laponite clay and cationic surfactants,8,11,13 laponite and nonionic surfactants,9 calcium carbonate and anionic surfactants (sodium dodecyl sulfate, SDS, and sodium-2ethylhexylsulfosuccinate, AOT),12 and alumina with shortchain carboxylic acids.4 However, if such systems are to be of use in food formulations, then the ingredients must be foodgrade. This represents a challenge in the case of particles as the preparation, properties, and surface modification of such particles are in their infancy. We have chosen to investigate a widely used food-grade surfactant, both alone and in combination with edible calcium carbonate particles, as a stabilizer of aqueous foams. Sodium stearoyl lactylate, SSL, with chemical formula CH3·(CH2)16· COO·CH(CH3)·COO·CH(CH3)·COO−Na+, is prepared by the esterification of stearic acid with lactic acid in the presence of sodium hydroxide. It is used extensively in the food industry, particularly as an additive in bread, biscuits, and cookies.15 Received: December 9, 2014 Revised: February 24, 2015 Published: March 3, 2015 2967
DOI: 10.1021/la504761x Langmuir 2015, 31, 2967−2978
Langmuir
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Although produced in thousands of tons and used by companies worldwide, surprisingly little information has been reported on its behavior in the bulk and at surfaces. A limited foam study by Bezelgues et al.16 at one concentration (0.1 wt %) shows that SSL forms a lamellar liquid-crystalline phase in water as opposed to micelles, and although surfactant adsorption to air−water surfaces is slow (monitored via dynamic surface tension measurements), very stable foams form that are argued to be due to the “compact, tight and ordered surface layers around bubbles”. Neat SSL is normally composed of α crystals (hexagonal subcell) which melt at around 45 °C. The crystals have a short spacing of 4.1 Å and a long spacing of 38 Å, showing that the hydrocarbon chains intercalate strongly. The surfactant molecules are orientated in bilayers with the polar groups headto-head separated by layers of solid hydrocarbon chains. A schematic phase diagram of this surfactant in mixtures with water shows that up to around 45 °C, hydrated crystals are formed.15 Upon heating above this temperature and up to 40 wt % surfactant, the crystals melt and a dispersion of lamellar structures (including vesicles) is formed, known as the Lα phase in which water penetrates between opposing layers of headgroups and a transition from a solid state to a liquid state takes place in the hydrocarbon chain region. If the system is now cooled to below the chain-melting temperature, a gel may form, known as the Lβ phase, whose structure is similar to that of the Lα phase but in which the hydrocarbon chains are solid again and oriented parallel to each other. Similar phase behavior has been discussed in detail recently for a polyglycerol fatty acid (C16/18) ester surfactant in water.17 The surface rheology of SSL−water mixtures was studied by Kokelaar et al.18 using the sinusoidal oscillation of a glass cylinder through the enclosed air−water surface. For a 0.1 wt % dispersion made at room temperature, the surface dilational modulus E =dγ/d ln A was 25−50 mN/m, in which γ is the surface tension and A is the liquid surface area. However, if the dispersion was first heated to 60 °C and then cooled such that the Lβ phase appeared, then E increased progressively on reducing the temperature to a very high value of around 1800 mN/m at room temperature. This imparts a high stiffness to the surface with important implications for the process of disproportionation occurring between air bubbles in foams.19 The latter is the growth of large bubbles at the expense of smaller ones caused by a difference in the Laplace pressure due to the difference in size. The solubility of gas is higher around smaller bubbles than around larger ones, causing gas to diffuse toward the larger bubbles; this process is self-accelerating. As formulated by Lucassen,20 a requirement to retard disproportionation is that E > γ/2, a condition well met by SSL at water surfaces after heat treatment. It is thus anticipated that aqueous foams of SSL should be relatively stable to disproportionation. We investigate here some aspects of the aqueous phase behavior of SSL in water and prepare and characterize aqueous foams it stabilizes. To increase the stability of these foams, we investigate the effects of adding CaCO3 particles of opposite charge to the surfactant. This involves us studying the aqueous dispersion behavior of the mixtures prior to foaming as well as characterizing the foamability and subsequent stability of these mixed dispersions. Evidence is given of superstable foams due partly to the attachment of surfactant-coated particles to air bubble surfaces.
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EXPERIMENTAL SECTION
Materials. Commercially available SSL as Emplex 268 is a paleyellow powder and was purchased from Caravan Ingredients (USA) and used as received. It contained mainly SSL2 in which two lactate moieties comprise the headgroup (earlier formula). Anionic surfactant sodium dodecyl sulfate (SDS, > 99%, Sigma-Aldrich) was also used as received. Precipitated calcium carbonate (PCC) may be made by the hydration of calcium oxide to calcium hydroxide, followed by direct carbonation with carbon dioxide.21 It is of greater purity compared to naturally available calcium carbonate as impurities are removed during the production process. The sample used in this study, Calofort U (Specialty Minerals Inc.), is a white microcrystalline powder containing particles of controlled shape and exhibiting a high degree of whiteness. The primary particles are quasi-spherical with diameters of no more than 150 nm. It is uncoated, and its hydrophilic nature makes it a suitable ingredient in water-based systems. Water was passed through an Elgastat Prima reverse osmosis unit and then a Millipore Milli-Q reagent water system. It had an electrical resistivity of 16 MΩ cm and a pH of 6.5. NaOH and HCl were AnalaR reagents from Sigma. Methods. Scanning Electron Microscopy (SEM) and Specific Surface Area Determination of Calcium Carbonate Particles. Calofort U nanoparticles were imaged using SEM. A carbonimpregnated sticky disk was applied to the surface of a standard 12mm-diameter aluminum SEM mount. This was then brought into contact with a small mass of the particle powder that adhered to the sticky surface. The mounted sample was coated with a thin film (0.5 mM) as we evidenced vesicles also sedimenting with particles after centrifugation invalidating this method. For good accuracy, conditions were such that depletion amounted to around 50% of the initial concentration. Particle Size Distributions. The size distributions of Calofort U particles in dispersions were determined by light diffraction using a Malvern Instruments MasterSizer 2000 droplet size analyzer equipped with a small-volume sample dispersion unit (Hydro 2000SM). A few 2969
DOI: 10.1021/la504761x Langmuir 2015, 31, 2967−2978
Article
Langmuir
Figure 2. Zeta potential versus pH of a 1 wt % Calofort U aqueous dispersion. where f 0 is the initial foam volume (mL) and l0 is the solution volume (mL) prior to foaming. Optical Microscopy and Cryo-SEM. The foams were observed using an Olympus BX51 optical microscope. A thin layer of foam was gently spread on a glass slide without destroying the bubbles. Digital micrographs of the foam were taken using a 12-bit Olympus DP70 camera. Cryo-SEM analysis was carried out for selected systems in two laboratories. Samples were prepared by mounting a small volume of foam onto a copper holder. The holder was placed in liquid nitrogen (−208 °C) where the sample was initially frozen by conduction followed by complete submersion. Frozen samples were transferred under vacuum into either the chamber of the SEM (Emscope 1250X) or the PP3010T cryogenic preparation chamber (Quorum Technologies Ltd.) where they were fractured, sublimated for 30−60 min at −90 °C, coated with a thin layer (
