Healthy human skin has an acidc pH of about 4 to 6, the so-called "acid mantle". An increase in pH (more basic or alkaline) can disrupt the activity of enzymes involved in barrier function and anti-microbial protection, resulting in bacterial overgrowth and a weakened barrier.
Many of the moisturizers we apply to our skin help in multiple ways: they add water, seal the skin, and can even add fats and nutrients to help the skin heal. However, they probably can alter the pH of the skin, especially if they are applied frequently and in large amounts as recommended.
Ideally, those with a pH near the ideal range--or perhaps even a bit more acidic--would be selected. Though much more work needs to be done on this topic as it is more complex than the measured pH alone, here are some pH values for common over-the-counter moisturizers from a recent study.
Name: pH value
Vanicream Light Moisturizing Lotion: 3.73
Vanicream Moisturizing Skin Cream: 4.27
Cetaphil Cream: 4.71
Eucerin Calming Cream: 5.41
CeraVe Cream: 5.49
Aveeno Daily Moisturizing Lotion: 5.62
Cetaphil Restoraderm: 5.94
Dove Day Lotion: 6.47
Aquaphor Ointment: 6.82
Eucerin Original Dry Skin Therapy Cream: 8.01
Selected data from:
Shi VY, Tran K, Lio PA. A comparison of physicochemical properties of a selection of modern moisturizers: hydrophilic index and pH. J Drugs Dermatol. 2012 May;11(5):633-6.