A ‘touch me not’ plant folding up rapidly upon being attacked or microbes depositing on teeth or ocean vessels even under hostile conditions are examples in nature that provide inspiration for developing new classes of personal care release or deposition systems. In this paper, development of such systems based on polymer/surfactant colloid chemistry is explored for achieving transport and release of cosmetic and pharmaceutical molecules at desired rates at desired sites. The successful development of products depends upon understanding and utilizing key interactions among surfactants, polymers and hybrid polymers that are relevant to personal care products. Thus, the absorbed layers or tethers on the particulates can be manipulated for desired dispersion of actives or depositions on substrate under any and all conditions. New hybrid polymers and nanogels have been synthesized for tuning up nanodomains that can extract and deliver at will cosmetics/drugs/toxins by perturbing pH, temperature or ionic strength of the system. Particularly, hydrophobically modified polymers have features of both polymers and surfactants and due to the associative nature of the hydrophobic groups, such polymers can form intramolecular nanodomains for performing carrier functions. Nanogels developed recently include that of polyacrylamide, poly(acrylic acid) and starch nanogels modified for extraction and subsequent slow release of fragrances and overdosed toxic drugs. Binding and release processes were investigated using surface plasmon resonance and fluorescence spectroscopies, powerful techniques for monitoring short term and long term changes.
The interaction of poly(vinyl caprolactam) (PVCAP) with sodium dodecyl sulfate (SDS) and dodecyltrimethylammonium bromide (DTAB) in aqueous solutions has been studied systematically by measuring the phase separation temperature, hydrodynamic radius, pyrene solubility, and surfactant binding isotherms. Both surfactants were observed to elevate the solution cloud point of PVCAP and cause the polymer to undergo a coil to globule transition. This transition occurs at a concentration about 1/10 of the critical micelle concentration (cmc) for SDS in the absence of polymer but at the cmc with DTAB. The results indicate that PVCAP interacts with SDS monomers but only with micelles in the case of DTAB. The phase behavior of the PVCAP/SDS/DTAB ternary system shows that the binding of SDS to PVCAP is reversible on changing the concentration of the free surfactant monomer in solution. Potentiometric titration of PVCAP and measurements of pyrene solubility in its mixtures with surfactants suggest that complexation of PVCAP and SDS is due to a combination of ion−dipole and hydrophobic effects. PVCAP and DTAB micelles interact through hydrophobic inclusion of polymer segments into the DTAB micelles. Pyrene is not solubilized by PVCAP in solution alone. Addition of SDS to PVCAP solutions induces marked pyrene solubilization well below the cmc, characterized by a region indicating saturation adsorption of the pyrene to the PVCAP/SDS complex. Above the SDS cmc, solubilization of pyrene increases linearly with SDS concentration, corresponding to inclusion of the pyrene into the SDS micelles. In contrast to SDS, the addition of DTAB to a solution of PVCAP shows no pyrene solubilization until the cmc is reached.
The properties of aqueous solutions of poly(maleic acid/octyl vinyl ether), PMAOVE, were studied by pyrene solubilization and fluorescence, solution viscosity, and phase separation induced by inorganic salts. The results indicate the existence of hydrophobic intramolecular microdomains formed by the octyl side groups. Pyrene solubilization and intramolecular alkyl chain association diminish sharply as the polymer is neutralized and unfolds with increasing ionization. The saturation solubilization of pyrene by PMAOVE corresponds to a ratio of 35 octyl side chains per pyrene at higher polymer concentrations for the un-neutralized polymer, and 42 at constant pH 4.0. Some hydrophobic solubilization averaging one pyrene per polymer molecule persists up to complete neutralization. Besides pH, the polymer conformation is also sensitive to solution ionic strength. A compact conformation is induced by the addition of 0.01 M NaCl, as indicated by the low reduced viscosity at low pH. Further increase to about 0.1 M leads to the precipitation of PMAOVE at concentrations of 500 ppm and above. Precipitation or phase separation of PMAOVE solutions at pH 4 by bivalent cations occurs at much lower salt concentrations than with single-valent cations. With barium in particular, the phase separation is not a simple precipitation. A very weak gel phase separates from the solution. Only minor differences are detected with a range of anions of sodium salts in precipitating PMAOVE at pH 11. In contrast to PMAOVE, the less hydrophobic lower homologue poly(maleic acid/methyl vinyl ether), PMAMVE, shows no hydrophobic solubilization of pyrene and is not precipitated by addition of NaCl up to saturation of the salt.
The colloid stability of silica dispersions in water in the presence of poly(vinyl caprolactam) (PVCAP) has been studied below and above the lower consolute temperature (LCT) of its solutions. The dispersion sediments slowly without PVCAP in the temperature range studied (26-40 degrees C) or with PVCAP below the LCT ( approximately 30 degrees C). In contrast, with PVCAP above the LCT, rapid flocculation occurs at acid pH, with re-dispersal on cooling. Reversible flocculation is also obtained above the LCT by cycling the pH from alkaline to acid and back. The flocculation observed above the LCT may also be regarded as heterocoagulation between the silica particles and the aggregates of the polymer.
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