Anomalous pH-Dependent Enhancement of p-Methyl Benzoic Acid Sum-Frequency Intensities: Cooperative Surface Adsorption Effects.

Vibrational sum-frequency generation (SFG) spectroscopy is used to determine the surface pKa of p-methyl benzoic acid (pMBA) at the air-water interface by monitoring the carbonyl and carboxylate stretching modes over the pH range of 2 to 12. The SFG intensities of pMBA and its conjugate base, p-methyl benzoate (pMBA-), exhibit an anomalously large enhancement over a narrow pH range (∼0.5) centered at pH 6.3 near the SFG-determined surface pKa, 5.9 ± 0.1. The increase in the surface pKa relative to the bulk value of 4.34 is consistent with the trend previously observed for long chain carboxylic acids in which the surface pKa is higher than the bulk solution pKa. SFG polarization studies help distinguish the orientation and number density contributions to this observed anomalous surface phenomenon. The large SFG intensity increase is attributed to an increase in the pMBA and pMBA- surface concentrations in this narrow pH range due to a cooperative adsorption effect between pMBA and pMBA-. This cooperativity is manifested only on the 2D air-water interface, where the interactions between the acid and base are not as dielectrically screened as in the aqueous bulk phase. Surface effects are critical to understanding and controlling the reactivity, solubility, and behavior of organic acids at interfaces and can have an impact on biomedical applications.

Spectroscopic evidence for the origin of odd-even effects in self-assembled monolayers and effects of substrate roughness.

This paper reports the effects of substrate roughness on the odd-even effect in n-alkanethiolate self-assembled monolayers (SAMs) probed by vibrational sum frequency generation (SFG) spectroscopy. By fabricating SAMs on surfaces across the so-called odd-even limit, we demonstrate that differentiation of the vibrational frequencies of CH3 from SAMs derived from alkyl thiols with either odd (SAMO) or even (SAME) numbers of carbons depends on the roughness of the substrate on which they are formed. Odd-even oscillation in SFG susceptibility amplitudes was observed for spectra derived from SAME and SAMO fabricated on flat surfaces (RMS roughness = 0.4 nm) but not on rougher surfaces (RMS roughness = 2.38 nm). In addition, we discovered that local chemical environments for the terminal CH3 group have a chain-length dependence. There seems to be a transition at around C13, beyond which SAMs become "solid-like".

Identifying characteristics and outcomes in youth with obesity and developmental disabilities.

BACKGROUND: Youth with developmental disability are at increased risk of obesity; literature focusing on the two is rare. OBJECTIVE: To identify characteristics and outcomes of youth presenting for obesity care having a disability as compared to without. METHODS: A medical record review of youth aged 2-18 years initiating obesity care 2013-2015 at a tertiary care obesity management program. Youth were grouped by disability status to identify differences in presenting characteristics and factors associated with a reduction in body mass index (BMI) percent of the 95th BMI percentile (BMIp95) over 12 months. Logistic regression (LR) models examined associations with BMIp95 drop (<5-points versus ≥5-points) for each disability group. RESULTS: Of 887 subjects, 253 (28.5%) had a disability. At presentation, youth with disability were more often (p < 0.01) male (58.5% versus 47.9%), had birth weight <2500 g (14.1% versus 8.4%), had a father who was not obese (61.6% versus 47.4%), and were on weight influencing medications. Overall, 182 subjects (20.5%) completed 12-month follow-up. At follow-up, the with disability group (n = 63) had mean -2.3 (SD 10.7) BMIp95 change (p = 0.679); youth having a motor disability less often had ≥5-point BMIp95 drop (odds ratio 0.15, 95% confidence interval 0.04-0.59). At follow-up, the no disability group (n = 119) had mean -2.9 (SD 8.5) BMIp95 change; youth identified as having initial severe obesity status and not having a parent with diabetes more often had ≥5-point BMIp95 drop. CONCLUSION: Youth with developmental disabilities were as successful in obesity care as those without disabilities. Predictors of success differed between the groups.