To understand the structural factors that control the adsorption of acid dyes onto cotton (cellulose), the adsorption of 15 acid dyes from water has been studied. An equilibrium exists between dye adsorbed in the cotton and dye in solution, and by measurement of the temperature dependence (277-333K) of the equilibrium constant, the enthalpy and entropy of binding are obtained. For most dyes adsorption is driven by the binding enthalpy, which is ascribed to van der Waals forces. Acid dyes consist of an aromatic core with peripheral solubilizing groups (generally sulfonates). Dyes in which the sulfonates are on one side of the molecule have the largest binding enthalpy. Assuming a binding geometry where the sulfonates protrude into water pools in the bulk amorphous regions and as much as possible of the dye core touches the surface, then a good correlation exists between the binding enthalpy and the summation of 1/r(6) over all the C, N, and O dye atoms, where r is the distance of each atom from the cellulose surface.
that approximately 20 to 30 NMEs were approved each year from 2000 through 2013. Topical NMEs are developed at a much lower rate (0.9 NMEs per year). These findings are consistent with prior studies 5 illustrating the underrepresentation of all drugs developed for primarily dermatological uses. Dermatological illnesses are not often fatal and may be considered lower priority by policymakers. Priority designations are assigned by the US Food and Drug Administration to new drugs that represent significant improvements over existing options and command more resource investment and regulatory attention. Only 5 total topical applications and 1 topical NME were given the priority review designation compared with 45% of all pharmaceutical and biological NMEs from 2000 through 2009. 6 Beyond being considered lower priority, traditional metrics of blood and urine drug levels often do not apply to topical therapies. The lack of accepted surrogate end points may explain why the median approval time for topical medications is comparable with oral and intravenous therapies despite representing a lower systemic risk. Given these regulatory challenges, the low number of active companies with the necessary expertise to develop topical therapeutics likely contributes to the continued high cost and low availability of these drugs. Prior studies on medical innovation of devices, small molecules, and biotechnology suggest a multifaceted approach to spur future development. 7 Strategies most likely to succeed include continued research funding to study diseases that are likely to be responsive to topical therapeutics and increased collaboration between academic and industry professionals. In the short term, adoption of more surrogate end points reduces the regulatory burden and may encourage companies to invest more resources in this underserved area.
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